A review on solid-state recycling of aluminum machining chips and their morphology effect on recycled part quality

Abstract

The increasing demand for sustainable manufacturing has revived the interest in solid-state recycling (SSR) as a promising alternative method for aluminum waste. In this context, chips generated during machining processes constitute a substantial portion of aluminum waste, offering significant potential for recycling and mitigating waste. However, the machining chip morphology significantly impacts the properties of chip-based recycled parts. This review paper examines the current state-of-the-art solid-state recycling methods, focusing on hot forging, extrusion, equal channel angular pressing, friction stir extrusion and field-assisted sintering. It investigates the impact of aluminum chip morphology on the properties of the directly recycled material, emphasizing the chip machining consequence on the final quality of the product. Several studies reported that the strain and operating temperature are the most influential factors in SSR processes, followed by chip size with an average length of less than 4 mm. Yet, the heating time up to 3 h also had a major impact on chip weld strength. The findings highlighted the significance of aluminum chip morphology in improving the quality of recycled material. The properties of direct recycled samples primarily depend on chip weld strength and microstructure. Overall, this study presented a comprehensive overview of the current state of solid-state recycling and emphasized the significance of chip morphology in advancing the recycling process. Consequently, it equips researchers with a valuable resource for developing effective strategies for sustainable recycling of aluminum chips with high quality.

1. Introduction

Aluminum alloys occupy the top position among nonferrous materials in terms of production and consumption owing to their lightweight, corrosion resistance, and outstanding mechanical and physical properties. It is widely employed in a variety of applications, such as aerospace and aeronautical, due to its superior strength-to-weight ratio. However, global demand for aluminum alloy caused huge waste in the form of scrap and machining chips, as this alloy is among the most popular machining applications due to its low density and high machinability [[1], [2], [3]]. It is the most promising material as hard-strength machinable materials [4]. The good machinability is attributed to the ability to form small chips during machining, which reduces the possibility of chip clogging and tool wear [5,6]. Aluminum swarf and chips or flakes originated from product machining processes, such as milling, turning, drilling and grinding. These processes produce substantial aluminum chip waste as a by-product, which can be recycled through various methods.

Among the commonly recyclable materials found in landfills, including plastics, glass, metals, and paper, aluminum stands out as the sole material that is 100 % recyclable while also paying for itself [7]. To date, two principal approaches have been proposed for aluminum waste recycling; conventional remelting and the recently developed solid-state recycling (SSR) or direct conversion method [6]. Recycling aluminum using the conventional route involves re-melting aluminum scrap and casting [8]. Nevertheless, this technique consumes substantial energy and poses challenges in achieving complete aluminum recovery due to metal losses during melting and other process phases [[9], [10], [11]]. Roughly 45 % of the aluminum alloy is wasted and converted into a new scrap phase [12]. These losses result from metal oxidation, slag removal, jumbled metal on the molten material surface, and scrap production during aluminum ingot casting and further processing [13]. Many studies have found that conventional aluminum recovering (melting-based method) is less cost-effective, consumes more energy, and has a more significant environmental impact than SSR which is known for its low energy usage and minimal loss of metal [[14], [15], [16], [17], [18]].

Solid-state recycling of aluminum chip waste involves consolidating the aluminum waste through cold compaction followed by high-pressure torsion, hot extrusion, forging, rolling, equal channel angular pressing (ECAP), or other process such as sintering into semi-finished or finished products without melting [1,19,20]. SSR technique can be categorized into mechanical deformation-based and sintering-based processes. The choice of technique depends on the type of aluminum alloy, the desired product, and the desired mechanical properties of the finished product. The technique can produce homogeneous and high-density parts with relatively similar properties to the original materials, making them convenient for various industrial applications [21]. The compaction force, temperature, and duration of heating are significant factors in improving the performance of the SSR recycled product, where hot deformation refines the microstructure and enhances the mechanical properties [22,23].

The direct recycling of aluminum chip waste presents several opportunities, such as reducing the adverse environmental impact of primary production and conventional recycling, improving the resource efficiency of aluminum, and developing sustainable recycling processes [24]. The economic and environmental advantages of solid-state recycling of aluminum waste are garnering increased attention. The technique has emerged as a promising approach for producing recycled aluminum parts with outstanding mechanical and physical properties. However, solid-state recycling of aluminum chips presents several challenges, including low yield strength and fatigue life [25]. In addition, identifying optimal processing conditions and characterizing the recycled products are challenging due to the complexity of the microstructure and the presence of defects and porosity [26]. Another challenge lies in attaining full chip-to-chip bonding due to the chip morphology variation, leading to the deterioration of recycled material's quality.

Aluminum chips, generated during machining processes used to form aluminum components, constitute a significant portion of aluminum waste. This chip typically comprises high-purity aluminum and alloying elements, and its disposal can pose a significant environmental challenge owing to large volumes and potential ecological impact [27]. Aluminum machining chip formation is significant within the context of solid-state recycling. The characteristics of solid-state recycled materials are influenced by chip morphology [28,29]. Chip morphology refers to physical characteristics such as shape, size, geometry, structure, and surface topography. The properties of the chips depend on several factors, including the type of machining process, the material being machined, the cutting parameters, and the machining coolant [[30], [31], [32]]. The chip dimensions and configuration including serrated, segmented, continuous, and chips with built-up edges are formed depending on machining parameters used [[33], [34], [35], [36]]. The chip's properties, including chemical composition, density, surface area, and surface roughness, can significantly influence the final product's quality including the microstructure and surface characteristics [37]. The size, shape, and aspect ratio of aluminum chips can influence the distribution of pores, fissures, and inclusions in solid-state recycled aluminum. Surface roughness and oxide layer thickness lead to poor chip welding and corrosion resistance [[29], [30], [31], [32], [33], [34], [35], [36], [37], [38]]. Characterizing chip morphology is essential for comprehending its impact on final products' mechanical and physical properties. Various techniques, including Scanning Electron Microscopy (SEM), X-ray diffraction, and optical microscopy are used to evaluate the surface characteristics of the chips. These techniques provide valuable information on the morphology, defects, and other properties of chips.

The challenge in the direct recycling of aluminum chip waste is achieving comparable mechanical properties as the original material. This challenge arises from material failures attributed to poor chip bonding, causing micro-voids and micro-cracks development in the recycled profile. A review of the pertinent literature reveals a scarcity of studies dedicated to investigating the ideal chip formation necessary to achieve the desired quality of chip-based recycled products. Most previous studies lacked a distinct focus on elucidating the effect of machining chip morphology on the properties of chip-based recycled material. Consequently, a comprehensive review of aluminum chip recycling and its morphology's impact on recycled part quality becomes imperative. This work critically reviews the literature on solid-state recycling of aluminum machining chips, emphasizing the effect of their morphology on microstructural evolution and mechanical and physical properties. Besides that, this study aims to investigate the characteristics of chip morphology in the machining process and the impact of different chip sizes, shapes, and surface roughness, on the welding mechanism during recycling.

In summary, this review paper aims to provide a comprehensive analysis of the current state of research progress on the solid-state recycling of aluminum machining chips, with a particular emphasis on the effect of chip morphology on the performance of the recycled parts and evaluate their direct recyclability with desired quality. The review will also highlight the gaps in knowledge and future research direction in this important field. Understanding the relationship between chip morphology and recycled part performance is crucial for developing effective strategies to optimize chip recycling and improve the overall quality of recycled aluminum. The findings could be valuable for researchers and industry involved in developing and implementing solid-state recycling processes for aluminum machined chips.

2. Literature methodology

The methodology selection for the current study is crucial, as it aims to thoroughly review the most relevant published research on solid-state recycling of aluminum chip waste. The selection of literature articles was based on the following keywords: (1) solid-state recycling of aluminum chip (2) hot press forging (3) hot extrusion, friction stir extrusion, ECAP and field-assisted sintering (4) aluminum chip morphology (5) characterization of aluminum chip-based recycled material. The literature selection method prioritized research articles published by IEEE, Elsevier, Springer, ACM, IOS press, United States Patent, and MDPI. The recently published articles from peer-reviewed sources that are closely related to this study were also considered. The current review is divided into two main sections; the first section focuses on hot deformation processes including hot extrusion, hot press forging, and equal channel angular pressing of direct recycling. The second section reviews the effect of aluminum chip morphology on the characterization of the recycled parts to provide valuable insights into the chip consolidation process and improved chip bonding.

3. Solid-state recycling technique (conversion method)

Forming aluminum components to meet modern technological requirements necessitates multiple machining processes. In recent years, the adaptability of machining processes has significantly enhanced productivity and reduced costs but caused an immense quantity of aluminum waste in swarf, shavings, and chip form [39]. This waste affects the environment in multiple direct and indirect ways. Therefore, recycling of aluminum machining chips is crucial for environmental and economic reasons. The contemporary world endeavors to reduce air pollution in every sector through the most efficient conversion of waste to wealth. The direct recycling of aluminum chips has gained attention as an energy-efficient and environmentally friendly method. The generated chips are typically characterized by irregular shapes and varying sizes, with the presence of cutting fluids and contaminants [40]. Recycling these types of chips requires efficient methods.

The process of extracting aluminum from virgin ore demands a significant quantity of energy [41]. Subsequently, aluminum waste recycling is increasingly important for economic and environmental reasons. At present, two primary routes have been proposed for recycling aluminum waste; conventional remelting and the recently developed direct conversion or meltless recycling. In conventional recycling, metal waste including scrap and machining chips is remelted and reformed into die-casting products or ingots. While in solid-state recycling plastic deformation-based, aluminum waste is directly recycled through recently proposed techniques such as hot extrusion, friction stir processing (FSP), equal channel angular pressing (ECAP), high-pressure torsion (HPT) and hot press forging or sintering without melting. Severe plastic deformation (SPD) processes have been effectively applied in aluminum chip consolidation by inducing substantial shear deformation. The chip-based consolidated parts with a finer microstructure demonstrate better mechanical properties compared to the as-cast specimen. In the SPD process, scrap and chips are initially pulverized into small particles and subsequently recycled into semi-finished and finished aluminum products.

Fig. 1 depicts the conventional and convection recycling flowchart of aluminum waste recycling. The solid-state recycling method is characterized by its minimal material loss, higher recovery, and fewer steps. The diagram depicts the sequence of solid-state recycling steps. The steps involve shaping the aluminum chip material into new semi-finished or finished products through cold working and elevated temperature heating. The conventional method of recycling is unfavorable owing to high metal loss. During the conventional recycling via casting process, a consequential reduction in material of approximately 8 % has been observed. Further losses, approximately 18 %, have also been recorded during the subsequent processing of aluminum ingots [42]. Thus, the net recovery of the metal is limited to a maximum of 54 %, as shown in Fig. 2 (a). The percentage of aluminum metal yield during conventional and conversion recycling is shown in Fig. 2 (a) and (b). The metal losses are attributed to casting and extrusion scraps, melting losses, dross, and butt. While the direct conversion method boasts a remarkably low material loss of just 4 %, as illustrated in Fig. 2 (b) [43].

Fig. 1.

Flow chart of conversion (Solid-state Recycling) and Conventional Approaches [44].

Fig. 2.

Comparison between metal loss in (a) conventional and (b) direct conversion methods [44].

These factors have prompted the development of an alternative approach to mitigate material loss in recycling endeavors. The approach involves solid state recycling of aluminum waste into semi-finished and finished products through mechanical pressing and heating operations. The approach was initially patented by Stern in 1945 [45]. It significantly limits energy consumption and facilitates the direct recycling of aluminum waste through plastic deformations at heating temperatures below solidus phase [46]. Conversion recycling is a more efficient way of recycling aluminum alloys than conventional recycling, as it requires less energy and produces less material loss. Solid-state or direct conversion of aluminum waste, including scrap and machining chips, is a promising way to address the challenges associated with conventional aluminum recycling techniques.

The various mechanical techniques, such as hot extrusion, forging, and rolling are achieved along with plastic deformation by high pressure, while powder metallurgy is directly used for sintering [47,48]. Hot forging and hot extrusion typically use higher temperatures and pressure to form the material, while cold forging and cold extrusion use lower temperatures and less pressure. The choice of method depends on the type of aluminum waste and the desired mechanical properties of the finished product [1].

3.1. Experimental studies of aluminum waste recycling by solid-state techniques

Several research studies have investigated solid-state recycling techniques that directly convert aluminum scrap and machining swarf into semi-finished and finished products without remelting to enhance recycling efficiency, energy consumption, and cost. The significance of the direct conversion of light metals to meet the rising demand for these commodities while simultaneously addressing the environmental issues related to their manufacturing is emphasized by several research [[49], [50], [51], [52]]. Wan et al. [18] presented the advantages of solid-state techniques over traditional remelting method for recycling aluminum chips. The study concluded that the prospects for solid-state recycling include optimizing processing parameters, die geometry redesign, and enhancing the mechanical properties of recycled products.

The flow chart in Fig. 3 provides an understanding of the different steps and methods involved in solid-state recycling through plastic deformation and powder metallurgy methods. The solid-state recycling techniques including hot extrusion, hot forging, friction stir extrusion, ECAP, and spark plasma sintering are discussed in the following subsections.

Fig. 3.

Diagram of various solid-state recycling SPD-based techniques and powder metallurgy method [24].

3.1.1. Hot extrusion

Hot extrusion solid-state recycling involves subjecting a heated metal to high pressure within a die in order to shape it into a desired form. The technique is commonly used to produce precise and complex metal profiles with outstanding mechanical properties, such as strength and microstructure. High shear stress is required in this technique as the process is conducted at a relatively low temperature. The expression for the theoretical pressure in the solid-state extrusion process of a fully dense material is given in Eq (1):

$$P=\sigma \ln \left(r\right)$$ (1)

Where P, σ, and r are pressure, yield strength, and extrusion ratio, respectively.

Evaluating extrusion process parameters is significant for quality control, cost-effectiveness, and environmental sustainability. Studying the effect of the parameters enables manufacturers to adapt their production methods to produce profiles with consistent mechanical properties. Lela et al. [53] developed a mathematical model to investigate the relationship between extrusion process parameters and the mechanical performance of the final profile recycled from EN AW 2011 aluminum turning chips. The Box-Behnken experimental design was employed to investigate the impact of compressing force, extrusion temperature and chip size on the mechanical properties. The findings revealed that extrusion temperature was the most significant factor influencing UTS and yield strength, while chip size had no effect. The effect of compaction force on ultimate tensile strength and percent elongation was minor. The study determined that the optimal parameters for attaining maximal ultimate tensile strength (UTS) and yield strength responses are 358 °C extrusion temperature and 200 kN compaction force according to the performed optimization. However, the finest microstructure was obtained with the highest extrusion temperature (500 °C).

Ab Rahim et al. [54] evaluated the direct hot extrusion for recycling aluminum chip, focusing on the finished product, characteristics, and processing method. They reported that extrudate characteristics depend on aluminum particle alloying elements and process parameters. The oxide layer on the chip surfaces is dispersed by high shear strain, improving bonding and consolidating between exposed aluminum chips [55], as presented in Fig. 4 (a). Yet, porosity may still occur, concentrated mainly in the center of the billet, as depicted in Fig. 4 (b).

Fig. 4.

Compacted billets made with differing dies, (a) Stationary and (b) Floating [55].

In a different study, Ab Rahim et al. [56] compared the mechanical characteristics of hot-extruded Al6061 chip-based recycled parts to as-received billets. The extrusion process involved preheating temperatures and times of 450–550 °C and 1–3 h, respectively, while the RAM speed was kept at 1 mm/s. The tensile result of material extruded at 550 °C and preheated for 3 h was better than that extruded at 400 °C, with a trade-off in ductility. The extruded profile exhibited comparable elongation-to-failure (ETF) and UTS properties to those of the as-received billets.

Tekkaya et al. [57] investigated the characteristics and microstructure of the profile produced by direct hot extrusion of AA6060 alloy chips produced by milling and turning processes with varying sizes and shapes. The study found that billets produced by AA6060 chips have similar mechanical and microstructural properties as conventionally cast aluminum billets. The yield strength of chip-based extruded billet was comparable to that of extruded solid billets with a yield stress reduction of less than 10 %. In other research studies [43,58,59], aluminum chip-based composites were developed through the hot extrusion route. Aluminum oxide, ferrochromium, carbon, and tungsten were used as reinforcing phases. The findings indicated that the relative density of the extruded composite is over 98 % identical to solid material made from aluminum powder with hardening additives.

Abd El Aal et al. [60] presented the recycling of AA6061 aluminum alloy turning chips via cold compaction and hot extrusion. The research examined how extrusion temperature (ET) and extrusion ratio (ER) influence the evolution of microstructure and the resulting physical and mechanical properties. Dense samples with satisfactory tensile properties and microhardness were obtained. Increasing ER and ET improved chip bonding and subsequently enhanced tensile properties. The highest mechanical properties were obtained with ER of 12.8 and ET of 500 °C, while fracture surfaces exhibited an equiaxed dimples with micro-cracks, reflecting the degree of chip bonding. This was agreed by Krolo et al. [53], that higher extrusion temperature (500 °C) enhanced extruded sample mechanical characteristics compared to 400 °C and 450 °C.

Badarulzaman et al. [61] investigated solid-state direct conversion through cold compaction and sintering process to recycle 6061 aluminum alloy chips (2 mm in length) for producing metal-metal composites. The composite's surface integrity and tensile strength were tested using different volume fractions of stannum (Sn) matrix. Constant pressure and sintering temperature were used for cold forging. The optimal yield and ultimate tensile strength results with 20 vol% of Sn composition were 3 Pa and 8.3 Pa, respectively. Analysis revealed that the tensile strength of composite with more than 20 vol% Sn is diminished.

Güley et al. [62] investigated the influence of ER on the mechanical characteristics of hot-extruded 6060 aluminum chip profiles. The extruded profiles at 68:1 ER demonstrated approximately 20 % improved ductility and strength than those of an ER of 34:1. In contrast, an ER of 10:1 resulted in insufficient pressure for chip welding. The mechanical properties were found to be nearly identical to the conventional extruded profile.

Chiba et al. [63] investigated the mechanical and physical properties of C-shaped recycled swarf profiles by hot extrusion. Except for the front-end region, the investigation demonstrated that the ER of 18 led to a straight extrudate without warping. The recycled material at this ratio exhibited a similar density to the original material, without coarse voids due to the large strain introduced. Compared to the original ingot, the dense recycled material demonstrated higher ductility in uniaxial tensile tests, but 30 % lower UTS. Overall, the paper reported that hot extrusion could effectively recycle aluminum alloy swarf into C-channels with improved mechanical and physical properties.

Noga et al. [64] examined the impact of solid-state recycling using hot co-extrusion, and arc welding methods on the microstructure and mechanical characteristics of AlSi11 chip-based recycled flat bar while maintaining the mechanical properties observed in the original cast material. The study's findings indicated that the recycled flat bars exhibited more favorable mechanical properties than the castings, with a yield strength and UTS of 155 MPa and 212 MPa, respectively. The weldability evaluation tests indicated that porosity reduced the tensile strength of up to 20 MPa and cross-section by up to 60 %.

Wagiman et al. [65] examined the effect of annealing and extrusion on chip-based billets produced from AA6061-T6 aluminum, to determine the role of processes prior to hot extrusion in achieving defect-free micro-void extrudates. The relative density of the billet produced from the annealed chip was 96.9 %, while that of the non-annealed billet was lower. The annealed chip had an average hardness of 49.5 HV0.1, which is an estimated 1.8 times lower than the non-annealed chip's average hardness of 89.5 HV0.1. The differential force caused voids to be unevenly distributed along the radial and vertical, making the peripheral area denser and more rigid than the center. However, annealing treatment before compaction improved the homogeneity of void distribution and resulted in a higher billet density, indicating that annealing can be beneficial for solid-state recycling through hot extrusion of AA6061-T6 aluminum scrap.

Wagiman et al. [66] discussed the effect of hot extrusion on the quality of recycled aluminum, focusing particularly on the effect of thermal treatment on extrudate density. The chips were heat treated to enhance the alumina layer and alleviate compaction pressure before recycling. The density of the recycled extrudate varied between 2.724 and 2.998 kg/m³. Analysis of Variance (ANOVA) revealed that extrusion temperature was the most significant factor, followed by extrusion ratio, chip treatment temperature and time. The proposed ANOVA-based predictive model was capable of estimating the density with less than 1 % error. Upon microstructure examination, alumina entrapped was observed in the hot-extruded profile. The result indicated that thermally treated chips contain more alumina than untreated chips and as-received material. The shear strain exerted by the extrusion process influences the consolidation and porosity between the chips. A high strain breaks up the chip oxide layer leading to better consolidation and density, as agreed by Refs. [67,68]. The article concluded that the density of the extruded profile could be higher than the as-received aluminum alloy.

Wang et al. [69] examined the impact of a residual emulsion and its in-situ decomposition on the mechanical properties and microstructure of an Al–Si–Cu–Fe aluminum chip recycled by hot extrusion in the solid-state. The findings showed that the in-situ formation of a carbon-rich phase was finely and homogeneously dispersed in the recycled billets using hot extrusion, resulting in better interfacial bonding between the matrix and alumina. The recycled specimen prepared using chips pre-heated at 623 K for 1 h had the highest mechanical properties, including a yield strength of 130 MPa, UTS of 305 MPa, and an elongation of 11.8 %. In comparison to recycled specimens prepared with clean chips, the yield and tensile strength increased by more than 1.2 times, and the elongation increased by roughly 3 %.

Gebhard et al. [70] evaluated the microstructure and mechanical properties of recycled aluminum extruded with and without a stationary valve using Becks' plasticine as a model material. The study characterized the mechanical properties of Becks' plasticine and used AA7075 as a reference material. The direct extrusion process involved inserting a billet of AA1050 into container-1, placing a circular blank of AA6016 underneath the billet, and extruding them through the valve into container-2. A stationary valve substantially reduced the porosity of the extruded material and resulted in a more consistent distribution of intermetallic particles. The extruded material's ultimate tensile strength and elongation also improved by approximately 20 %.

Sidelnikov et al. [71] investigated the recycling of Al–Mg alloy chips, specifically alloys 01570 and 1580, alloyed with scandium through a combination of rolling and extrusion, along with heat treatment, to produce semi-finished products. The research emphasized the significance of controlling parameters and sustaining high-quality billets in order to reduce recycled wire defects. The recycled samples exhibited similar mechanical properties to those prepared by casting.

Sherafat et al. [72] studied feasibility of recycling 7075 aluminum alloy chips combined with pure Al powders via powders metallurgy using hot extrusion. The study examined the impact of aluminum powders and their respective quantities on microstructure and mechanical properties of recycled chips. The findings showed that extrusion temperature above 500 °C results in better bonding between chips and powder. Samples with 60 wt% chips exhibited double the strength compared to those with zero chips, although they have reduced ductility.

In another study, Sherafat et al. [73] investigated the effect of the pure Al powder to Al7075 chips ratio on deformation behavior and mechanical properties of a hot extruded two-phase material. Yield strength (Sy) and UTS were higher with low Al powder wt% addition, but uniform strain (eu%) and elongation to failure (ef%) decreased, as shown in Fig. 5.

Fig. 5.

Variation of yield strength at 0.2 percentage of strain, UTS, uniform strain (eu%), and elongation to failure (ef%) with respect to Al powder wt% [73].

Hot extrusion provides accurate control over the geometry of the recycled profile, potentially improving the mechanical properties through grain refinement. However, the technique requires high equipment cost and energy consumption. In addition, hot extrusion is limited to producing continuous cross-section components, restricting its applicability for complex designs or dimension variations. While hot extrusion ensures outstanding material utilization and quality control, its adoption depends on particular requirements of the recycling process.

3.1.2. Equal channel angular pressing (ECAP)

In the hot extrusion process, low temperatures and extrusion ratio cause grain growth and other defects that can result in poor material strength and toughness. To resolve this issue, some researchers have incorporated ECAP into the extrusion process. This technique passes a recycled billet through a channel with a particular die angle to achieve severe plastic deformation. Abbas et al. [74] examined the impact of solid-state recycling of Aluminum Alloy 6061 chips through hot extrusion followed by ECAP to enhance mechanical properties. The chip was cold compacted and then extruded under a 5.2 extrusion ratio and 425 °C extrusion temperature. The extrudate pressed three times in ECAP with a 90° channel die angle exhibited the lowest surface roughness and the maximum hardness.

Similarly, Taha et al. [75] investigated the influence of ECAP on the microstructure of AA6061 chip-based recycled samples and their corrosive behavior in saline solution. The electrochemical impedance spectroscopy (EIS) and linear polarization (LP) were used to conduct the analysis. The chip was compacted and extruded at 500 °C, followed by six passes through ECAP, with the aim of studying the resultant microstructure. The properties of the recycled profile were significantly improved by the hot extrusion and ECAP processes. The six passes ECAPed sample exhibited top-notch UTS and microhardness as a result of the attained refined microstructure. Following extrusion, the recycled billet ability to self-passivate was enhanced, and subsequent ECAP passes resulted in a reduced corrosion rate and an increase in film thickness.

Selmy et al. [76] investigated the impact of hot extrusion and ECAP on the Al6061 chip-based recycled profile's mechanical properties and microstructure. The study found that hot extrusion effectively produced high density recycled samples, better than cold ECAP process. However, the ECAP plastic strain was able to enhance the quality of solid bonding by breaking chip oxide layers as the number of passes increased. Additionally, combining hot extrusion with ECAP enhanced the hardness, strength, and ductility of the samples. The microstructure characterization of the ECAPed samples demonstrated uniform fine grains and enhanced chip welding, consistent with the superior mechanical properties observed.

Krolo et al. [77] studied the solid-state recycling (SSR) hot extrusion followed by ECAP of aluminum alloy EN AW 6082, aiming to reduce greenhouse gas emissions and increase energy savings compared to conventional recycling. The study found that the extruded-ECAPed samples had mechanical properties similar to conventionally produced material. Samples ECAPed at 20 °C had maximum UTS, YS, and Elongation of 287 MPa, 254 MPa, and 6.9 %, respectively. The article concluded that SSR could be a viable alternative to conventional recycling production.

In another study, Krolo et al. [78] recycled EN AW 6082 aluminum chips using a combined direct extrusion and ECAP-heat treatment processes. The design of experiments and response surface methodology were used to perform a comprehensive number of experiments. The combination of ECAP and heat treatment following direct extrusion (DE) significantly enhanced the mechanical properties of the recycled samples compared to those processed only with DE and a single ECAP pass. The recycled samples met the EN 755-2: 2016 standard for EN AW 6082 extruded bars in T6 condition. In a separate study, Krolo et al. [79] conducted a further investigation on the fatigue and corrosion behavior of recycled EN AW 6082 aluminum. The findings revealed that the fatigue life of recycled specimens was comparable to that of reference specimens (conventionally produced) but with crack propagation mechanisms than reference material. The ensuing plastic deformation and heat treatment enhanced recycled materials' performance and corrosion resistance.

Mohammed et al. [80] recycled Al-6061 alloy chips using hot extrusion and SPD at ambient temperature to study the material's mechanical properties and microstructures. The mechanical properties of recycled specimens were examined before and after passing through ECAP for 1, 2, and 4 cycles at 90° and 20° angles. The extrudates samples underwent four ECAP passes at 90° and 250 °C recorded 67 % increase in UTS and 81 % in micro-hardness, with significant reduction in grain size. ECAPed specimens had better mechanical properties than extruded ones. However, Krolo et al. [77] reported that the number of ECAP passes had a negligible influence on the mechanical properties of the extruded samples when 160 °C was used.

Sabbar et al. [81] examined the effect of hot extrusion, equal channel angular pressing (ECAP), and heat treatment on recycled AA7075 aluminum chip waste reinforced with ZrO₂ nanoparticles. RSM was used to optimize volume fraction (VF), preheating temperature (T), and preheating time (t). The study found that T and VF are crucial to maximizing UTS and microhardness. According to the RSM, 550 °C, 1.58 h, and 1 % ZrO₂ are the optimum conditions to achieve the highest UTS, density, and hardness of 487 MPa, 2.85 g/cm³ and 95 HV, respectively. The application of ECAP and heat treatment resulted in a notable increase in microhardness and UTS by 22 % and 29 %, respectively. Furthermore, a slight elevation in density of 3 % was observed. In a comparable investigation [75], turning machined chips form AA6061 were hot extruded at 500 °C, followed by ECAP die channeled at a 90° angle. At ECAP-6 passes, the recycled chip-based profile had a maximum UTS of approximately 400 MPa, while microhardness was 110 HV. The dramatic rise in mechanical properties was attributed to the grain refinement and high dislocation density caused by the ECAP-induced significant deformation. The excessive ECAP passes resulted in an increase in film thickness and a reduction in the corrosion rate. However, the number of replicates used in the corrosion testing was not specified.

Lacour-Gogny-Goubert et al. [82] reported the impact of Equal-Channel Angular Pressing (ECAP) and Spark Plasma Sintering (SPS) methods on the microstructural, mechanical, and thermal characteristics of consolidated milled powders of aluminum and alumina Al–Al₂O₃ nanocomposites. The ECAP process produced a more uniform microstructure and a higher hardness value of 133 HV, whereas the SPS process produced a greater compressive yield strength of 568 MPa. The study revealed that these methods could directly recycle aluminum waste to produce high-quality profiles. Fig. 6 (a), (b) and (c) of schematic recycling process illustrate the powder compaction, extrusion and sever deformation of extruded billet by ECAP [79].

Fig. 6.

Schematic of the solid-state recycling process: (a) compaction stage of chip, (b) hot extrusion and (c) 90° ECAP route [79].

ECAP excels in grain structure refinement, leading to improved recycled profile's mechanical properties. This technique is particularly favorable for research purposes as it can handle small quantities of aluminum chips. However, ECAP's complex die design and limited manufacturing capacity make it less suitable for large-scale industrial applications. Despite these limitations, ECAP can still be a valuable tool for investigating the impact of chip formation on the microstructure of recovered material, which can provide insights for optimizing alternative recycling processes.

3.1.3. Friction stir extrusion (FSE)

FSE is an SSR process that generates heat through axial force friction by operating a rotating tool. The heat softens aluminum chips, which are then extruded through a die to produce profiles. The technique has been developed over the past two decades, making tremendous progress. Mehtedi et al. [83] studied the feasibility of recycling AA1090 aluminum machining chips using FSE. The universal MTS compaction machine and chip-based compacted billet are shown in Fig. 7 (a), and (b), respectively. Initially, the extrusion rate and plunge rotational speed were optimized using Deformd-3D. The turning-off machining chips were compacted under 150 KN load into chip billet measuring 40 mm in diameter and 30 mm in height. The FSE parameters used were 1000 rpm rotational speed with 0.8 mm/s of plunge displacement. Overall, the study showcases the potential of FSE for recycling aluminum chips; however, non-homogeneity and minor internal voids were present.

Fig. 7.

(a) Universal MTS machine of compaction process; (b) Cold-compacted AA1090 aluminum part [83].

Mehtedi et al. [84] in another study, examined the roll bonding and accumulative roll bonding (ARB) of AA3105 aluminum alloy at 300 °C to produce sheets with a final thickness of 1.2 mm. Tensile and three-point bending tests were performed to assess the mechanical properties of the sheets at different stages based on the number of cycles. The maximum strength was achieved after three ARB cycles. Following four cycles, the bonding interfaces were uniformly distributed through the sheet thickness, and no significant improvement in strength was observed. Micro-cracks were observed on the outer surface of samples after two ARB cycles at ≥ 90° bending angles. However, each ARBed sample failed at 180°, except the one-cycle ARBed sample.

Baffari et al. [85] studied the energy effectiveness of utilizing the FSE process to recycle aluminum alloy scraps into wires compared to the traditional remelting-based method. The FSE recycling method was found to require significantly less primary energy than the traditional, resulting in energy savings of up to 74 %. Good performance is primarily attributable to the absence of material wastes in the remelting process and the lack of energy-intensive steps, such as wire drawing. However, the authors noted that SSR processes restrict compositional changes, making closed-loop recycling the only viable alternative.

FSE is a heat-sensitive technique that offers lower energy consumption compared to a conventional method. However, the application of this technique is restricted to specific shapes and sizes, and initial apparatus investment may be substantial. Further research should focus on optimizing processing speed and parameters and expanding the applicability of the technique to various materials and shapes, despite its potential for energy efficiency and improvement in material properties.

3.1.4. Hot press forging

Hot press forging (HPF) is a solid-state recycling technique in which a metal is shaped by heating and high pressure applied directly without melting. It is a deformation process where the aluminum scrap/chip is compressed between two dies under high plastic strain. Fig. 8 (a), (b), and (c) display the HPF top plunger used for pressing the chip, the cavity die where the chip is placed, and a schematic diagram of both parts, respectively. When high temperatures are applied to metals, their microstructure becomes more ductile and easily forms into the desired shape through the dies [86]. In comparison to other recycling SSR methods, it offers several advantages, such as enhanced toughness, ductility and strength [87].

Fig. 8.

Drawing for (a) top plunger (moving), (b) bottom die (stationary), and (c) diagram assembly of compression process [88].

Research has demonstrated that HPF has a significant positive impact on the mechanical properties of recycled aluminum materials [26,[89], [90], [91], [92]]. In comparison to as-cast material, the forged parts at 500–550 °C temperature and a strain rate of 1 s⁻¹ had a remarkable increase in yield strength and tensile strength by 60 % and 70 %, respectively [93]. Higher strain results in an increase in formability [94].

Ahmad et al. [95] studied the feasibility of HPF to directly recycle AA6061 aluminum chips waste. The surface integrity and mechanical properties recycled billet were examined at forging temperatures of 430 °C, 480 °C, and 530 °C and 60-min, 90-min, and 120-min holding times. Forging temperature had a significant effect on UTS, in which the increase from 430 °C to 530 °C resulted in an increase in UTS by 89.51–93.35 % (14.97–266.78 MPa), elongation to failure (ETF) by 86.32–98.67 % (0.091–16.12 %), and microhardness by 7.88–10.28 HV, as shown in Fig. 9. The optimal UTS, ETF, and microhardness of 266.78 MPa, 16.129 %, and 81.744 HV were obtained at 530 °C operating temperature and 120 min holding time. Similarly, Yusuf et al. [96] investigated the impact of the same processing parameters on the microhardness and density of AA6061 aluminum chip and aluminum oxide (Al₂O₃) composite recycled by HPF technique. The findings showed that both temperature and holding time had a positive effect on the microhardness and density. However, the temperature had a greater impact than the holding time. The microhardness increased by 4.94–10.45 %, while the density improved by about 0.95–12.71 % whenever operating temperature was increased from 430 to 530 °C. The findings provide insights into the industrial development of SSR processes as eco-friendly solution to melting-based practices.

Fig. 9.

Tensile and Elongation to break results of recycled samples at different holding times and forging temperatures [95].

Ahmad et al. [88] employed finite element simulation to analyze the behavior of a recycled aluminum-based metal matrix composite fabricated through the HPF process. Their simulation revealed that the effective stress-strain increased with each step due to the repeated cycle, which made the workpiece more rigid and required additional stress to deform. The temperature distributions were found to be delineated manner, with lower forming velocities decreasing material temperature while raising forming stress. A 10 % reduction in thickness was observed due to the high stress exerted on the workpiece and the workpiece and its confinement.

Cislo et al. [97] examined the effect of pulsed electric current sintering (PECS) on the bonding quality and material properties of recycled EN AW 6082 aluminum chips. PECS positively influenced aluminum chip consolidation, and pre-compacted specimens above 320 MPa exhibited comparable densities following sintering, regardless of the system. Chip bonding improved with higher pressure, temperature, and deformation. Notwithstanding, the chips partially delaminated during heat treatment due to inhomogeneity caused by chip morphology and pre-compression. The authors recommend more research to ensure uniform bonding and prevent delamination.

Ho et al. [98] investigated the effects of high velocity on recycled profiles from AA6061 aluminum chips. The study examined the anisotropic damage characteristics of recycled parts through Taylor Cylinder Impact testing. The authors analyzed deformation behavior of the specimens, including digitized footprint, side profile, and fracture modes. The microstructural behavior of a region located 0.5 cm from the impact area was analyzed using SEM to assess the progression of damage. The study revealed that the deformed specimen of recycled AA6061 displayed anisotropic behavior, manifesting as a non-symmetric ellipse-shaped footprint. The recycled material had a critical impact velocity less than 212.35 m/s. The material undergoes localized plastic strain deformation at impact velocities below the critical threshold, forming a mushrooming shape. At values exceeding the critical threshold, the tensile splitting and petalling fracture modes exhibited severe characteristics. The recycled AA6061 demonstrates a notable strain rate dependence, whereby damage progression is amplified with escalating impact velocity, leading to pronounced localized plastic strain deformation. This article provides valuable insights into recycled aluminum alloys' deformation behavior and fracture mode under high-velocity impact. The research may help identify advanced engineering applications. The authors suggested adding particle reinforcement to refine the microstructure.

In separate research, Ho et al. [99] studied the strain rate effect on mechanical behavior of Al6061 chip extrudate using a tensile strength test. The material's microstructure was analyzed for micro-cracks and micro-voids using SEM. The results indicated that mechanical behavior of chip-based recycled material is affected by microcracks and microvoids, which propagated significantly during finite strain deformation. Crack formation and micro-void formation evolved faster as the extrusion speed increased. The mechanical performance of the extrudate is comparable with primary AA6061-T1. However, the microstructure showed degradation in the UTS value compared to AA6061-T1. Further improvements are needed to optimize the performance of recycled materials in order to match that of primary materials, due to the damage parameters developed within the recycled materials.

Song et al. [100] examined the impact of Accumulative Roll Bonding (ARB) and cryorolling (CR) with subsequent aging treatment on the microstructure and mechanical properties of the AA1050/AA6061 multilayer composite. The findings indicated that the composites' tensile strength and hardness were enhanced by ARB and CR; however, CR was more effective. The tensile strength of ARBed samples increased from 73 to 156 MPa and to 192 MPa after processed through CR. The composites' microstructure was considerably refined, and the phases were distributed uniformly after ARB and CR. The composites' mechanical properties were further enhanced by aging, with a maximal hardness of 124 Hv and tensile strength of 550 MPa after 1 h at 170 °C processing temperature. The work illuminates direct processing and the possibilities for high-strength multilayer composites.

Yusuf et al. [26] recycled the AA6061 aluminum chip using HPF method. The investigation concentrated on multiresponse optimization and environmental analysis of the HPF process. The results indicated that the process could produce AA6061 aluminum with outstanding mechanical properties. Using response surface methodology (RSM), the optimal parameters for the HPF process were identified as 530 °C and 82.19 min holding time, with a desirability of 64.2 %. The predicted UTS, ETF, and GWP were 241 MPa, 10.21 %, and 35.21 kg CO₂-equivalent/kg, respectively. A confirmation test was carried out to validate the predicted values. The variations between predicted and confirmed values were 2.28 % for UTS, 13.03 % for ETF, and 0.68 % for GWP, respectively. The results were close to the predicted values of 241 MPa, 10.21 %, and 35.21 (kg CO₂-equivalent/kg). The HPF direct recycling process has a lower carbon footprint and energy consumption than conventional manufacturing processes, according to environmental analysis.

In a different experimental study, the recycling conditions of AA6061 chips reinforced by Al₂O₃ particulate were optimized utilizing RSM and the desirability function (DF) to enhance the performance of HPFed composite. The effects of forging temperature different levels (430–530 °C) and holding duration ranging from 60 min to 120 min were analyzed through ANOVA. RSM results suggested a temperature of 530 °C and a holding time of 120 min are the optimum conditions to attain maximum UTS of 317.99 MPa, microhardness (MH) of 86.656 HV, and Elongation to failure (ETF) of 20.45 %. Three confirmation experiments were conducted using the optimal parameters suggested by RSM. The observed error in these experiments was less than 2.5 %. Fig. 10 demonstrates the predicted and actual values for MH, ETF and UTS in each confirmation run, with hardly noticeable residuals. Temperature and holding time influenced all responses, according to ANOVA analysis. However, the interaction between temperature and holding duration resulted in higher responses exclusively for UTS and ETF. The pressing force and high heat caused Al₂O₃ to accumulate in the chips' gaps by relaxing the materials. Void-filling decreased porosity and increased composite density [101]. Lajis et al. [102] reported the environmental impact of HPF in aluminum recycling, focusing on forging time and temperature. The study emphasized the significance of optimizing parametric settings in the HPF process to reduce energy consumption and environmental effects. An eco-friendly aluminum recycling process can be achieved by carefully adjusting the forging temperature and time.

Fig. 10.

The percentage error between actual and predicted values of UTS, ETF, and MH confirmation tests [101].

The effect of HPF direct recycling with fewer steps on surface integrity and mechanical properties of AA7075 aluminum chip-based recycled profiles was studied by Ruhaizat et al. [90]. The experiment was conducted at different levels of forging temperatures (380, 430, and 480 °C) and forging times of 0, 60, and 120 min. The recycled parts exhibited the maximum UTS and ETF of 245.62 MPa and 6.91 % at the highest parameters (480 °C and 120 min). The surface integrity was emphasized by the measured density and micro-hardness of 2.795 g/cm³ and 69.02 HV, respectively. According to the study, the forging temperature had a major effect on enhancing mechanical properties. The article provides insights into the effects of HPF recycling approach on the mechanical properties and surface integrity of recovered aluminum alloys.

The microstructure development and flow stress of AA6099 aluminum alloy during hot compression was studied by Chamanfar et al. [103]. The study conducted isothermal hot compression tests at 350–500 °C temperatures ranging and 0.01–0.1 strain rates, and 1 s⁻¹ to a 0.8 true strain. The observed flow-softening behavior of deformation was attributed to the dynamic recovery (DRV) and partial dynamic recrystallization (DRX) occurrence. The empirical model expressed in Eq (2), was derived as a constitutive equation to link flow stress to deformation temperature and strain rates, based on the power-law relationship. The microstructural examination was analyzed using light optical microscopy (LOM) and electron backscatter diffraction. The study found that dislocations sliding over solute atoms and precipitates decreased yield under specific hot deformation conditions. The amount of flow softening was proportional to the DRX volume fraction. The findings illuminated AA6099 alloy deformation and microstructural evolution during hot compression. However, there is a lack of investigation into the effect of strain rate or temperature on the microstructure and flow stress of the alloy.

$$\mathit{Ln}\sigma =0.1235\mathit{ln}\left[\acute{\epsilon }\mathit{exp}\left(-\frac{162224}{RT}\right)\right]+0.5558$$ (2)

Zhang et al. [104] studied the effect of hot pressing and hot extrusion on the performance of recycled AA6061 tubes. The tensile mechanical properties were examined in relation to the microstructural evolution and formation of oxide particles. The findings revealed that the breaking of Al₂O₃ layers on the chips caused fine oxide particles, which inhibited the formation of certain recrystallized grains and formed a microstructure of fine equiaxed grains and coarse elongated grains. The tensile results indicated that the heat-treated samples exhibited an excellent combination of strength and ductility, with 296 MPa and 7.6 %, respectively. The investigation analyzed the impact of oxide layers on anisotropic microstructure and inter-chip bonding.

In another Al6061 chip recycling study [105], the Cyclic Extrusion Compression Back Pressure (CECBP) was employed in the hot forging process. The procedure entailed the compression of specimens made from CECBP by HPF closed die machine. In comparison to the direct compact technique, this method increased billet density by up to 12.11 %. Additionally, the hardness was enhanced in highly compacted billet. The most significant factor affecting billet density and cyclic hardness was compaction pressure. The technique effectively enhanced the directly recycled forged part from Al6061 chips.

This technique is highly favored for rapid processing, achieving high material utilization, and producing near-net shape components with good mechanical characterization. Nevertheless, it has notable drawbacks, such as considerable energy demands and the difficulty of shaping intricate geometries. While it can be effective for certain applications, these limitations raise questions about its long-term viability and cost-effectiveness.

3.1.5. Field-assisted sintering techniques (FAST)

FAST is a sintering-based solid-state recycling method that uses pressure-assisted with pulsed DC electric current. The basic concept underlying the FAST processes, including Spark Plus Sintering (SPS) and Pulsed Electric Current Sintering (PECS), is that of directly heated pressure-assisted sintering [106]. The process can achieve full densification at relatively lower temperatures and shorter processing cycles compared to conventional sintering methods [107]. It can transform aluminum scrap into high-density products with higher strength and hardness through a combination of uniaxial pressure and a pulsed direct current, as depicted in Fig. 11. FAST has several advantages such as energy efficiency, waste reduction, and outstanding physical and mechanical properties due to well-refined microstructure [108].

Fig. 11.

Schematic diagram of basic design of a FAST/SPS device [109].

Several studies have demonstrated the technical feasibility of FAST in recycling aluminum waste.

Research by URSINUS et al. [110] investigated the FAST and impact extrusion for recycling aluminium chips of EN AW-6082 and EN AW-7075 alloys, to develop multi-material parts. The varying of sintering temperatures from 400 °C to 500 °C influenced the inter-chip diffusion and microstructure, resulting in less defects and improved fatigue properties and formability. The approach proved the feasibility of producing nearly defect-free hybrid parts directly from waste material, with implications for improving part performance under certain loading conditions.

In [111], PECS was used to consolidate EN AW 6082-T6 aluminum chips through a combination of heating and pressure. PECS demonstrated significant improvements in density and hardness at 640 MPa pre-compaction pressure, suggesting stronger bonding between chips. However, the chip morphology and uneven distribution of current can cause inhomogeneity in the sintered specimens. The authors proposed adopting double-action pressing to improve pre-compaction uniformity. The authors found that PECS can be a successful method for consolidating aluminum alloy chips.

The implementation of FAST in aluminum composite recycling presents an innovative method for producing heterogeneous composite materials with well-developed interfaces and continuous diffusion layers. Kozlik et al. [112] discussed the FAST application for sintering Al6061 alloy powder with Ti Grade 2 wire bulk at 560 °C and 10 MPa pressure for 10 min. Al6061 and Ti Grade 2 exhibited a well-developed interface with no residual porosity. The study concluded that FAST can produce heterogeneous composites from dissimilar materials. Paraskevas et al. [113] examined SPS as a SSR method for consolidating AA6061 and AA6082 aluminum chips. Al chip successfully consolidated into near-full density samples at 490 °C and 200 MPa pressure with no physical material losses.

Existing research shows potential for surmounting the challenges associated with scalability and economic viability, thereby facilitating the broader implementation of FAST within the aluminum recycling sector. The technique has emerged as a promising solution for recycling aluminum scrap, providing a more sustainable approach. However, further development is required to address inhomogeneity issues, optimize operation parameters for various chip sizes, and widen the applicability in aluminum recycling industry.

In general, each recycling method has benefits and limitations, and the selection of the technique depends on factors such as the desired geometry of the component, properties, and energy efficiency. Further research and development are needed to enhance these approaches and broaden their applicability in the recycling of aluminum waste.

4. Effect of chip morphology on the characterization of directly recycled material

Despite the paucity of available literature, the research on the effect of aluminum chip morphology on the quality of recycled products appears to be promising. Existing research articles offer insights into the relationship between aluminum chip morphology and recycled material's mechanical and physical properties by analyzing various factors, including size, shape, surface area, and characteristics. These studies used different experimental techniques, including conventional and conversion recycling methods, to investigate the influence of chip formation on the final material properties. Recent publications have cited previous studies, demonstrating the development of a research base and the recognition of the significance of this topic. This upward trend suggests that interest in this field is growing and that researchers are building upon existing knowledge to further explore the subject. Analyzing the shape, size, surface area, and roughness of the chips can provide insights into the preparation of chip waste for the consolidation process in order to produce high-quality recycled products. Analyzing chips' shape, size, surface area, and roughness may assist the preparation of chips to achieve high-quality recycled products. In the direct recycling approach, different types and sizes of chips are hydraulically compacted under certain force and temperature to achieve the desired consolidation in a hot process [114]. However, the direct recyclability of machining chips can be influenced by morphological characteristics including geometry, structure, surface texture, size, and shape. Ragab et al. [115], subjected the pre-compacted irregular chips to a specific semi solid temperature prior to the ultimate extrusion process to investigate the chip weld strength. Similarly, Hu et al. [116] employed the strain-induced melt activation technique in their efforts to produce semi-solid billets of AZ91D magnesium alloy using segmented chips as starting material. ECAP was utilized to recycle both Titanium and aluminum chips [82]. In the study conducted by Shi et al. [117], the authors utilized the ECAP technique to produce totally dense as-recycled materials from Ti–6Al–4V machining chips. Subsequently, using heat treatments within the 700–1000 °C range. It was noted that the boundaries of the recycled large chips were eliminated. Additionally, studies [[118], [119], [120]], reported that a totally dense bulk aluminum chip with ultra-fine grains was successfully produced in recycled segmented machining chip developed by hot extrusion. The resulting material exhibited comparable performance to the initial ingot, reaching up to 350 MPa with 16 % ductility. Kanani et al. [121] used the orthogonal cutting technique as an SPD method to produce good mechanical properties chips with an ultra-fine-grained microstructure. They reported that high-travel seizure machining could produce continuous, ultra-fine-grained chips.

In other studies [122,123], high-pressure torsion and hot extrusion methods were employed to consolidate milled aluminum chips. The resulting consolidated material demonstrated notable microhardness and remarkable fine grain microstructures. In Ref. [95], the AA6061 milled chip was processed through hot press forging. The forged chip-based parts exhibited improved properties compared to the as-received material. In a similar study [26], AA6061 aluminum chips were fabricated using a hot forging technique at three different temperatures and holding times. At 530 °C and 120 min, microhardness and maximum tensile strength improved by 9.27 % and 20.48 %, respectively, compared to 430 °C and 60 min possessed billet from small chips. Numerous researchers have reported that direct chip-based recycled is significantly affected by temperature levels, pressure factors and chip formations [124,125,126,127].

The significance of aluminum chip morphology in direct recycling is crucial, as it profoundly affects the bonding mechanisms and, subsequently, the overall quality of recycled materials [128]. Studying the effect of various characteristics such as size, shape, surface area, and surface topology, on the quality of recycled parts is a substantial and valuable research endeavor. It is an important aspect of understanding chip behavior during the recycling process and optimizing the influential parameters to achieve improved chip bonding and material properties [129]. Some researchers observed that certain chip formations improve strength and decrease porosity [130]. Alternatively, certain chip geometries were found to be associated with diminished performance and an increased occurrence of defects [131]. This theme's discussion focuses on the trade-offs between recycling efficiency and material quality, as well as the optimization of chip morphology for the desired material properties.

The shape of aluminum particles plays a significant role in strain localization and, as a result, influences the damage mechanism [1,132]. Chiba et al. [63] reported that the recycled profiles from the turning curled swarf with a length of 5–30 mm, width of 1.5 mm, and thickness of 0.5 mm chips exhibited poor bonding among chips compared to that recycled from the milling flakes under the same conditions, resulting in inferior mechanical properties. The material recycled from the milling swarf at an ER of 10 exhibited substantial voids, while the chip recycled at 18 had a similar density to the original ingot. The dense recycled material from side milling swarf and lathe turning swarf at 600 K temperature had 30 % lower ultimate tensile strength than the original ingot but better ductility [63]. However, this study only focused on one type of chip without identifying the size or form.

Krolo et al. [79], presented recycling of turning machined EN AW 6082 aluminum chips by hot extrusion and equal channel angular pressing (ECAP). The fatigue and corrosion behavior of recycled billets was investigated. At specified stress levels, the endurance life of recycled specimens was compared with that of reference specimens. The as-extruded recycled samples had similar corrosion resistance to the reference specimen, while ECAPed samples had higher corrosion resistance. The study concluded that contamination control is essential to prevent galvanic corrosion during solid-state recycling of machining chips. This work suggested that solid-state recycling of aluminum machining chips can improve fatigue and corrosion behavior in aluminum alloys. Yet, the corrosion and fatigue behavior of recycled specimens were not investigated for various processing parameters such as extrusion ratio or ECAP passes. Krolo et al. [78], reported that chip morphology affects the quality of chip-based extrudate if inadequate pressure and strain are applied during plastic deformation. ECAP chip-based recycled materials can achieve high density, however, the microstructures, including grain size and chip boundary spacing are contingent upon the initial chip morphologies and dimensions, as reported by Ref. [37].

In a study by Noga et al. [133], aluminum 6082 chips with varying sizes and morphologies resulting from milling and turning operations were hot extruded at 400 °C with a ram speed of 2 mm/s. The study examined the quality variation of the recycled materials from coarse and fine chips. The chip-based extruded material exhibited similar density, elongation, and electrical conductivity to initial cast ingots. The billet from coarse chips exhibited a 12.44 % reduction in tensile strength compared to the original material, while that from fine chips exhibited a 5.41 % decrease. The as-received sample exhibited a tensile strength of 185 MPa, while the large chip and fine chip recycled samples showed 162 MPa and 173 MPa, respectively, as shown in Fig. 12. Still, the grain size and grain boundary were not examined.

Fig. 12.

Tension curves of as-received material, coarse and fine chips recycled samples

In a similar study [57], the re-use of aluminum AA6060 waste by direct hot extrusion from milling and turning chips was presented. The increasing area of the surface oxide layer with decreasing chip size was observed. The milling process chips revealed a rough surface torn open in many spots. No pores or inclusions could be seen at the grain boundaries of the turning chip profile. The study concluded that the different types of chips could result in a homogeneous extrudate if a critical value of pressure, strain, and temperature is surpassed.

An investigation [134] utilized hot extrusion to recycle AA6060 aluminum alloy turning chips. The oxygen contamination during the recycling process was quantified and localized through transmission electron microscopy (TEM) and X-ray hotoelectron spectroscopy (XPS). During annealing and extrusion, the chip-thin Al₂O₃ layer gets covered by MgO layer, resulting in a tenfold increase in the oxygen amount in the extrudate with an average thickness of 290 nm compared to the initial chips. XPS surface examination revealed chip oxide layer evolution during machining and homogenization, confirming atmospheric oxide formation during these processes. Furthermore, the presence of alloying elements, such as magnesium, is involved in oxidation. On the basis of the findings, tuning processing parameters could limit oxidation and enhance chip adhesion, depending on alloying materials. However, the paper did not examine the mechanical properties of recycled aluminum chips or the potential impact of plastic deformation during extrusion on oxide growth, which could shed light on the oxidation mechanism.

Hu et al. [38], recycled AZ91D magnesium alloy by hot extrusion based on different chip sizes. The concentration of accumulated oxygen increased linearly with the total surface area of the machined pieces. Ambient oxide in recycled specimens improves ultimate tensile strength and elongation to failure, while excessive oxide may adversely affect elongation. The chip total surface area-to-volume ratio was measured using Eq (3).

$$S=\frac{2\left(ab+bc+ac\right)}{abc}$$ (3)

where S is the total surface area to volume ratio, where a, b, and c are the length, width, and thickness of chip, respectively.

In the investigation [38], recycled specimens from fragment dimensions of 4–6 mm in length, 3.5–4.5 mm in width, and thickness of 1.45–1.55 mm outperformed as-cast specimens. This is shown by its greater ultimate tensile strength of 340 MPa and 10.5 % elongation to failure. However, Gronostajski et al. [58] recommended that the average length of the machined chip be less than 4 mm.

In related research [29], the effect of chip surface area on the microstructure and mechanical characteristics of recycled AZ31B was examined. The oxide content on the total surface area and the thickness of the oxide film on the chip surface were estimated using Eq (4).

$$O=Sl=\frac{2\left(ab+bc+ac\right)}{abc}l$$ (4)

where O is the oxide percentage, S is the total surface area of chips, l is the oxide film thickness, where a, b, and c are the length, width, and thickness of the chip, respectively.

The strength of the recycled specimens exhibited an increase proportionate to the total surface area of the chips, suggesting that various grain sizes contributed to the enhancement of strength. However, recycled specimens from medium-area chips exhibited the highest elongation. The recycled specimens had lower elongation than the reference specimens due to oxide precipitation, and grain size, oxide quantity, and billet density affected their ductility. However, the used equation for estimating the oxide amount on chip surface area is applicable only to chips with a cubical shape. Schulze et al. [135] reported the importance of the turning parameters and their direct effect on the chip geometry. The geometry of chips influenced the amount of oxide contamination in recycled samples. In addition, the shape of the chips affects the quality of the chip-based extrudate if insufficient stress and strain are applied during plastic processing.

Koch et al. [136] have studied the influence of high-pressure torsion (HPT) processing on hardness, wear properties, and the microstructure evolution of Al 1080A turning continuous chips with a 0.25 mm thickness ratio. The HPTed samples had fully dense ultrafine-grained (UFG) microstructure with improved hardness and wear resistance. The refinement and fragmentation of Al swarf and reinforcement particles contributed to a hardening. The study recommended a new recycling method that can preserve chip ultrafine-grained size microstructures or introduce a further refinement of the chip grain size. In another study [137], high-pressure torsion (HPT) processing was used to recycle Al7075 machining chips with pure Al powder. The findings revealed that the inclusion of pure aluminum powder enhanced mechanical properties by minimizing porosity and improving chip welding. The HPFed composite with 50 wt% annealed Al 7075 chips had the highest UTS and fracture strain, as well as a fine microstructure with uniform chip distribution. However, the weight fraction of Al 7075 powder in the composite was not optimized, which could achieve better mechanical properties.

In a comparable investigation [138], AA6061 milled chips and powder were recycled through uniaxial cold compaction for 20 min at 9 tons and 552 °C sintering temperature. The size of the aluminum powder particles was 25 μm. The pure chip-based samples demonstrated the maximum compression strength and hardness value at 307.7 MPa and 65.6 HV, respectively. Nevertheless, However, they reduced gradually as the aluminum powder ratio increased. Additional Al powder led to less pore formation and smaller average pore size. The void caused by large chips developed large pores on the surface of chip-based samples. Al powders added incrementally to chips reduced pores with grain boundary formation.

Rojas-Díaz et al. [139] investigated the recyclability of aluminum chips from sawing processes using powder metallurgy, focusing on grinding time effect on the properties of aluminum powder and sintered materials. The saw chip recycled had a high aluminum percentage and low oxygen content, according to energy-dispersive X-ray spectrum analysis. Alumina formed a shell around the aluminum core in the powders. The surface area of the aluminum powders was changed based on the grinding duration. The specific surface area grew when the particles flattened, but subsequently reduced as they formed rounded agglomerates. The study found that powder metallurgy can effectively recycle sawing process aluminum chips. However, this study primarily considers the grinding time effect on aluminum powder and sintered materials, not other factors that may affect aluminum chip recyclability and performance. Prior to direct recycling, pulverizing the chip into smaller sizes or powder forms depending on its initial size is favorable [114].

Based on the study by Wagiman et al. [66], AA6061-T650 chips from milling machining were thermally treated before hot extruding. The milling was performed using a 10 mm diameter tool at cutting speed of 345 mm/s, and 1 mm for both feed per tooth and depth of cut. The average length of the produced chip was 2 mm. The study findings revealed that the thermal treatment-induced presence of alumina on the surface of the chip had a substantial effect on the density of the recycled extrudate. The chip's large surface area tends to have higher alumina formation. However, excessive alumina addition can cause particle agglomeration and impede chip bonding.

In another work [140], large and fine Al6060 chips generated by turning and CNC machining processes, respectively were extruded at temperatures of 400 °C and 450 °C. The average size of large chips was 97.4 x 3.15 × 0.72 mm, whereas the estimated range of fine chip size based on sieve analysis was 0.40–0.315 mm. The outcomes indicated that the mechanical properties of fine chip-based extrudate were comparable to cast ingot. Samples prepared from large chips recorded inferior mechanical properties with characteristic delamination along the boundary of individual chips due to poor bonding, while fine chip samples showed a well-compacted fracture surface with a high density of dimple-like features. However, the microstructure of both large and fine chip-based extrudate was not examined. A similar study by Ref. [141], investigated the hot extrusion of turning process chips (large chips) with an average length of 20 mm and milling process chips (fine chips) with typical dimensions of 0.1 mm thickness and 0.3 mm diameter. The UTS of extruded profile from coarse chips was 180 MPa, while extrudate from fine chips recorded 200 MPa. This can be attributed to a refined microstructure containing fine Si particles and Fe-rich intermetallic phases. Chip morphology significantly impacted the final strength of the profile, with smaller-sized chips expected to possess superior properties.

In a similar investigation, Noga et al. [64] presented co-extruding to determine the influence of AlSi11 chip sizes on the mechanical properties of extruded bars and their weldability. The fine chips with a fraction of 0.16–0.4 mm were produced by the milling machine, as shown in Fig. 13 (a). The coarse spiral chips with average dimensions of 22 mm × 4 mm × 0.5 mm were produced by the turning process, as depicted in Fig. 13 (b). The chip size affected the mechanical properties of the co-extruded profile, with thicker chips providing more desirable mechanical properties. The distribution of pores in the cross-section was irregular and 60 % higher in fine chip specimens, while the porosity of welded junctions was lower in thick chip samples. Although these findings were inconsistent with [141], the exact outcome may vary depending on the aluminum chip's composition.

Fig. 13.

(a) Milling chips, (b) turning chips [64].

Similar to other studies [60], AA6061 turning chips with dimensions of 30–50 mm in length, 2 mm in width, and 1 mm in thickness were extruded at 500 °C. The extrudate had superior mechanical properties compared to as-received samples. With a higher extrusion ratio and temperature, chip welding quality improved, which had a considerable effect on the mechanical characteristics of recycled samples.

Lela et al. [53] have developed empirical model to optimize the depth of cut (chip size) in the turning process, and its influence on the mechanical properties of solid-state recycled aluminum alloy EN AW2011 chips. Three different chip sizes and cross-sectional areas were investigated. The optimal cut of depth 0.5 mm (smallest chip size) yielded the maximum UTS and ideal microstructure at the highest extrusion temperature of 358 °C. However, yield strength was increased with a large chip size and low extrusion temperature.

According to Ref. [96], medium-sized AA6061-T6 chips prepared by a milling machine with average dimensions of 5.20 mm × 1.097 mm x 0.010 mm were processed by hot forging. The forging temperatures selected were 430, 480, and 530 °C with 120 min heating time. The increase in microhardness and UTS was 20.48 % and 9.27 %, respectively, compared to the theoretical AA6061-T4 material when medium-sized chips were utilized. In comparison to Ref. [95], medium-sized chips presented better specimen performance, with a notable rise in density and microhardness for entire composites.

Khamis et al. [126], investigated the impact of varying sizes of AA6061 aluminum chips (small, medium, and large) produced through high-speed milling on the mechanical properties of the recycled parts. The higher UTS value was obtained using a large chip size and prolonged holding time with 4 times pre-compaction cycle. Larger chips may retain heat more efficiently due to their greater mass, which can facilitate adhesion between aluminum chips. In contrast [142], reported the specimen forged from a small-sized AA6061 chip at 520 °C temperature showed the highest UTS of 117.58 MPa, while the large-sized chip specimen exhibited the lowest UTS value of 30.83 MPa. Recycled specimens with large chip surfaces demonstrated higher UTS regardless of operating temperature. The material is more likely to develop a finer grain structure during recycling in the presence of oxides [143]. However, the achieved UTS in the study remained significantly lower than that of the original ingot.

In [69], an Al–Si–Cu–Fe alloy chip was emulsion and emulsionless machined in a lathe. The generated chip was short-curved and curled with dimensions of 4.3 mm in length,1.8 mm in width, and 0.5 mm in thickness. The extruded specimen prepared from emulsified chips showed the highest mechanical properties. The results revealed that at a preheating temperature of 623 K, the emulsion decomposed in-situ, forming a carbon-rich phase that filled the interface between the matrix and alumina. This process significantly improves interfacial weld strength and could inhibit micro-cracks formation.

In summary, the body of literature addressing the impact of aluminum chip morphology on the quality of directly recycled material is still relatively limited in terms of extent and depth. Nevertheless, the existing literature suggests a promising level of research depth, unveiling various facets of chip morphology that impact aluminum recycling. The rising rate of publications and citations indicates a growing interest in this field and the possibility of future advancements and contributions. A further summary of the effect of chip morphology on recycled materials characteristics is presented in Table 2.

Table 2.

Summary of the influence of various chip morphologies on the characteristics of recycled materials.

Chip Morphology	Characterization	Impact
Irregular chip: complex shapes, sharp edges with small size and higher surface area [69,96,115,[118], [119], [120]]	•Chip weld strength •Hardness, •Microstructure •Yield strength	•Higher hardness •Microstructural variation •Higher yield strength with larger chips and lower extrusion temperatures
Spherical and curled chip: rounded surfaces with medium size moderate surface area [60,63,140]	•Density, •Ductility •Yield strength	•billet exhibited improved density comparable to the original ingot. •lower UTS than the original ingot but better ductility •Low hardness and poor chip bonding when processing at less than 430 °C extrusion temperature.
Fragmentary or discontinuous chip chip: Thin-planar structure with large size and very high surface area [29,79,126,134,141]	•Fatigue Resistance, Surface Roughness •Tensile strength	•Recycled and reference specimens fatigue endurance were compared. •Increased surface roughness. •Improve corrosion behaviour. •Excessive formation of oxide layer on chip surface area negatively influences complete chip bonding.
Angular chip: sharp corners and angles with medium-sized and moderate surface area [57,136,133]	•Wear Resistance, Adhesion •Elongation	•Angular chips exhibited enhanced wear resistance and improved adhesion. •Similar to the original ingot at extrusion ratio up to 18 •Similar elongation to initial cast ingots
Dendritic chip: tree-like branched structures with large size and very high surface area [66,114]	•Thermal Conductivity •Porosity	•Billet showed higher thermal conductivity. •Increased porosity

Furthermore, the cutting fluid in machining processes can potentially affect the chip welding during recycling processes due to chip contamination. Reducing impurities is crucial in order to achieve strong chip bonding. The contamination affects the quality of the chip-based extrudate if insufficient strain is applied during plastic processing [24]. Seliger et al. [142] reported that the microstructure of AA6061 aluminum chip-based recycled profile showed more chip boundaries than the cleaned chip-based specimen due to the presence of the precipitation. Many techniques for cleaning and degreasing machining chips to remove cutting fluid contaminants were reported. The cleaning methods include Acetone A solution for 30 min can mitigate impurities and enable metallic bonding as recommended by Refs. [102,124,[151], [152], [153], [154]].

5. Discussion

Numerous studies have investigated the solid-state recycling of aluminum chips and their morphology effect on the quality of recycled materials in various methods. The summarized findings in Table 1 present a concise outline of various solid-state recycling methods, specifically hot extrusion, FSE, ECAP, HPT, and HPF. Each technique is distinguished by its prominent processing characteristics and corresponding advantages. The integration of solid-state techniques with other processes can achieve outstanding material properties. To ensure industrial viability, the scale-up challenges need to be addressed by developing tool efficiency using computational modeling for precise control. Additionally, prioritizing sustainability through energy efficiency and recycled materials are all prospective developments in solid-state recycling. These future development trends emphasize the dynamic progression in solid-state recycling, focusing on efficiency, adaptability, and environmental consciousness in material processing.

Table 1.

Summary of various solid-state recycling techniques with processing features.

Processing technique	Prominent processing characteristics	Advantages
Direct hot extrusion [54,60,64,99,144]	•A billet is produced by cold/hot chip, powder, and reinforcement material mixture compaction. •The preheated billet is subjected to deformation by shear force in a die with reduced cross-sectional area. •Capability to form complex shapes through a shaped die.	•Efficient consolidation of aluminum chips into extrudate profile as result of SPD •Improved material homogeneity and product quality •Reduction in defects and porosity
Friction stir Processing (FSP) [83,[129], [145], [146]]	•A billet is formed by a compaction process from powder or chips. •Frictional heat and plastic deformation are produced by rotating the tool throughout friction between the tool and workpiece (billet) by stirring and compaction. •By employing a rotating tool, aluminum waste can be joined and refined structurally without melting. •FSP localizes heating only on the material near the tool. •Versatility: FSP can join aluminum, magnesium, and titanium.	•Improved material homogeneity. •Fine-grained microstructure is produced. •Reduce distortion and residual stresses by minimizing the heat-affected zone (HAZ). •Heat-sensitive alloys can be easily recycled with this feature.
Equal channel angular pressing (ECAP) [74,76,82,147]	•A billet undergoes repetitive deformation by passing through a die channel with different angles usually varying from 90 to 120° with intense high shear stress and strain rates. •Continuous process. •Cold working process •This causes grain boundaries to be broken and grains to be refined. •Multidirectional deformation improves grain structure and mechanical properties	•Outstanding grain refinement. •Enhanced mechanical properties. •Improved recycled material formability. •Significant material recovery rate •Energy Efficiency: cold working process does not require heating
High-pressure torsion (HPT) [[136], [148], [149], [150]]	•The sample is twisted between two anvils at room temperature. The torsional shear stress of high pressure resulted in sample consolidation and microstructure refining which contributes to enhanced strength. •SPD is caused by high pressure and torsional shear. •Continuous Deformation: HPT typically deforms material continuously.	•HPT significantly refines grain structure to ultrafine microstructures. •Fully dense ultrafine grained (UFG) microstructure with increased hardness and wear resistance •HPT-induced SPD improves material strength and hardness.
Hot press forging (HPF) operation [26,[88], [89], [90], [91],124,125]	•The billet is subjected to heating in conjunction with axial compressive stress between two dies by hydraulic loading. •The chip-based billet is shaped by plastic deformation under high pressure and temperature. •Densification: high pressure and temperature convert aluminum chips into a dense and homogenous product. •High dimensional accuracy: HPF produces precise and complex parts. •Closed press die reduces aluminum chip oxidation during processing. •Quick processing: HPF takes short processing time.	•High material recovery rate •Ability to recycle a variety of aluminum waste, including swarf, chips and turnings. •Enhanced fatigue resistance. •Minimizes degradation of wrought aluminum alloy recycling.

The reviewed literature has shown a perceptible variation in the number of studies in each approach. The review emphasized the significance of solid-state techniques for recycling aluminum waste and its potential for substantial improvements in the quality of recycled materials. Solid-state recycling (SSR) has emerged as an advantageous methodology for addressing the sustainability challenges and metallic recycling. Applying statistical analysis to evaluate the processing parameters and their impact on the mechanical properties of SSR recycled samples presents a valuable tool for optimizing recycled quality. This research possesses practical significance within academic circles and also aluminum recycling industry. By adopting sustainable manufacturing practices, industry can reduce the environmental footprint and conserve raw materials. By virtue of its ecological friendliness and resource efficiency, SSR is a significant contributor to the shift towards a more responsible and sustainable aluminum recycling sector on a global scale.

The operating temperature between 500 °C and 550 °C is the most influential parameter in SSR processes, followed by chip size with an average length of less than 4 mm. Yet, the heating time of up to 3 h also had a major impact on chip weld strength along with high strain rate. SSR is a proven beneficial and eco-friendly recycling technique. The statistical modeling and optimization approach had the capability to enhance the processing parameters of SSR and output properties of recycled profiles. The literature review provided an understanding of the present status of research concerning the relationship between the morphology of aluminum machining chips and the quality of directly recycled material. It is apparent that, at this juncture, the current body of literature pertaining to this particular subject remains relatively limited. This limitation might be attributed to the specialized nature of the topic, in addition to the field's emerging status in the wider domains of materials science sustainable recycling. However, the available literature exhibits a noteworthy degree of scholarly investigation, suggesting that the current studies are committed to comprehensively examining diverse facets of chip morphology and its impact on the process of aluminum recycling. Analyzing the shape, size, surface area, and roughness of the chips provides insights into determining the interlocking and interdiffusion of particles during the plastic deformation process. The thorough analysis presented here demonstrates the commitment of academics to further explore the intricacies of the correlation between chip morphology and the manufacturing process of recycled aluminum products of superior quality. Further investigation is advisable to comprehensively grasp the effect of time-varying cutting conditions on surface topography, as well as to optimize cutting parameters that can preserve surface integrity and chip morphology to enhance recyclability.

In light of the comprehensive review of relevant literature, it is evident that the morphology-related characteristics aluminum machining chips are an intricate consequence of numerous interrelated factors. The fine-tuning and control of machining parameters is paramount in attaining the desired chip morphology, hence enhancing bonding in solid-state recycling and the overall product quality. The literature review highlights the practical significance of this research through its illuminating the multifaceted effect of chip morphology on recycling process and resultant material quality. The observed chip formation is crucial in determining various aspects of alloy design, encompassing processing parameters, structural integrity, and material selection. Understanding the implications of chip morphology is essential for optimizing the performance of chip-based recycled alloys.

The type of chip morphology affects the recycled chips' correlated processing parameters, including extrusion, forging, or sintering. More specifically, uniform size and shape chips promote processing operations and enable material flow and homogeneity. Conversely, irregular and heterogeneous chips may introduce defects in recycled material, necessitating adjustments to the associated processing parameters, including strain rate and temperature. Such modifications are required to achieve the desired microstructural features and material properties. Optimizing processing parameters based on chip morphology can improve recycling efficiency and uniformity, producing high-quality recycled aluminum products. Chip morphology also affects the structural integrity of aluminum recycled components. The surface defects or internal inclusions can be stress concentration sites, potentially degrading the final product's fatigue resistance. Therefore, chip morphology implications on structural integrity must be considered when designing components or structures using chip-based recycled aluminum alloys. Strategy for minimizing the impact of chip morphology on structural integrity may include refining microstructure through heat treatment. The review revealed the critical influence of chip morphology on the fatigue performance of chip-based recycled aluminum as well. Comprehending the implications is crucial for assuring the reliability of parts in fatigue-sensitive applications. The reinforcement material addition and processing optimization techniques can enhance fatigue resistance. These insights can contribute to producing more durable and reliable recycled aluminum parts for various engineering applications.

Understanding the complexities associated with chip morphology is essential to optimize the efficiency, sustainability, and overall quality of aluminum recycling procedures within the industry. The upward trend in publications and citations within this field is indicative of a growing interest and recognition among researchers. The increased academic activity in this area suggests a favorable prospect for the potential for further advancements and meaningful contributions on the horizon. Consequently, the literature review encourages the expectation that this field will continue to evolve and potentially yield innovative solutions for aluminum chip recycling aluminum. In summary, findings in this literature review pave the way for more efforts to improve the sustainability and efficiency of aluminum recycling practices.

6. Conclusion

This review paper provides an overview of the current state of research on the recycling and treatment of aluminum machined chips and their morphology impact on recycled part quality, aiming to analyze the characteristics of aluminum machined chips to evaluate their solid-state recyclability with desired mechanical and physical properties. The review also covered the recent developments in recycling aluminum machined chips, including cleaning, separation, and alloying techniques. The second part of the review focuses on the effect of the morphology of aluminum machined chips on recycled part quality. The morphology of chips, including size, shape, and surface quality, are significant factors affecting the properties of chip-based recycled parts. The review of various solid-state recycling techniques, including hot extrusion, FSP, ECAP, HPT, and HPF, has emphasized the vital significance of processing temperature along with heating time, chip morphology, and compaction force in chip weld strength, which contribute to the recycled product performance. Operating temperatures between 500 and 550 °C, chip lengths less than 4 mm, and heating times of up to 3 h are the key parameters in attaining good chip welding and high-quality recycled products. The chip bonding is a primary factor dictating the strength of SSR recycled samples. Overall, this review paper provides a thorough comprehension of the current studies on the recycling and treatment of aluminum machined chips and their effect on recycled part quality. The comprehension of the chip morphology effect on the bonding mechanism is fundamental for the progression of aluminum chip recycling and for contributing to sustainable and resource-efficient practices. The findings of this review can help researchers and industry professionals develop effective and sustainable strategies for recycling aluminum machined chips and producing high-quality recycled parts. As the need of sustainable manufacturing continues to grow, optimizing the role of chip morphology in SSR processes is crucial for a greener, more profitable future.

Funding and acknowledgment

This research was supported by Universiti Tun Hussein Onn Malaysia (UTHM) through Tier 1 (Q390).

(Communication of this research is made possible through monetary assistance by Universiti Tun Hussein Onn Malaysia and the UTHM Publisher's Office via Publication Fund E15216. The authors would also like to express the most profound appreciation for supplementary provisions provided by Sustainable Manufacturing and Recycling Technology, Advanced Manufacturing and Materials Center (SMART-AMMC), Universiti Tun Hussein Onn Malaysia.)

Data availability statement

No data was used for the research described in the article.

CRediT authorship contribution statement

Yahya M. Altharan: Writing – original draft. S. Shamsudin: Supervision. Sam Al-Alimi: Conceptualization. Yazid Saif: Writing – review & editing. Wenbin Zhou: Conceptualization, Methodology, Writing – review & editing.

Declaration of competing interest

The author W.Z. is an Associate Editor for Heliyon and was not involved in the editorial review or the decision to publish this article.