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. 2026 Feb 4;16:7308. doi: 10.1038/s41598-026-35741-w

Effect of Ti addition on microstructure and mechanical properties of Co–Cr–Mo alloy developed by µ-plasma arc metal powder additive manufacturing process

Balbir Singh Negi 1, Pradyumn Kumar Arya 2, Neelesh Kumar Jain 1,✉, Sharad Gupta 3, Ankur Srivastava 4,✉
PMCID: PMC12923809  PMID: 41639143

Abstract

This paper presents effects of addition of 4wt.% of Ti to Co-Cr-Mo alloy by µ-plasma arc metal powder additive manufacturing (µ-PAMPAM) process on microstructure, phase evolution, bending strength, tensile and compressive yield and ultimate strength, % elongation, microhardness, porosity, and density of the resultant alloy and their comparison with Co-Cr-Mo alloy. Microstructure and phase evolution study of Co-Cr-Mo alloy showed Co-rich matrix comprising of γ-Co and ε-Co phases, formation of Cr7C3 and Cr23C6 carbides due to presence of carbon and affinity of Cr towards it, and micro-cracks. Addition of 4 wt% Ti to Co-Cr-Mo alloy refined its grains, minimized formation of micro-cracks, led to formation of β-Ti phase and Co-Ti intermetallic compound along with the chromium carbides. It also reduced porosity and density of the resultant Co-Cr-Mo-4Ti alloy. Grain refinement increased flexural strength of Co-Cr-Mo-4Ti alloy. Solid solution effect of Ti increased tensile and compressive yield strength, ultimate compressive and tensile strength, and percentage elongation of Co-Cr-Mo-4Ti alloy as compared to Co-Cr-Mo alloy. Microhardness of Co-Cr-Mo-4Ti alloy increased to 473 MPa from 382 MPa of Co-Cr-Mo alloy due to formation of Co-Ti intermetallic compound and β-Ti phase. All these improvements enhance durability and strength of Co-Cr-Mo-4Ti alloy along with a reduction in stress shielding effect as compared to the Co-Cr-Mo, making it more suitable for knee prothesis applications.

Keywords: Ti addition, Β–Ti, Co–Cr–Mo alloy, Co–Cr–Mo–4Ti alloy, Multilayer deposition, µ-PAMPAM process

Subject terms: Engineering, Materials science

Introduction

Metallic materials for biomedical applications can be categorized as biodegradable or non-biodegradable based on their degradation behaviour inside the human body. Among the non-biodegradable materials, the most prominent materials are titanium and its alloys, cobalt-based alloys, and stainless steel1. These materials have superior resistance in a corrosive environment and the implants made of these materials have life expectancy of 10 to 20 years. Titanium and its alloys are extensively used for orthopaedic and denture implants, and cardiovascular stents owing to their outstanding biocompatibility and high strength2. But titanium exhibits limited resistance to wear, which can lead to failure of an implant over a time. Whereas austenitic stainless steel 316 L is not suitable for long-term use due to its associated risk to allergic reactions in many patients3. Cobalt-based Co-Cr-Mo alloy is used for femoral component of knee implant due to its good biocompatibility, high resistance to corrosion and wear, and superior mechanical properties4–6. It is also used for denture applications7.

Researchers are increasingly exploring different additive manufacturing (AM) processes to develop a new generation of biomedical implant material and for their manufacturing with high accuracy and precision according to the needs of a patient. Most often, these implants have complicated shapes and geometries, such as curved surfaces with porous and hollow structures.

Girardin et al.8 used the direct metal laser sintering (DMLS) process to develop Co-Cr-Mo alloy for biomedical applications. They observed that the developed alloy samples have higher ultimate tensile strength (UTS), % elongation, and microhardness as compared to those developed by the traditional manufacturing processes due to the formation of ε-Co lamellae dispersed in γ-Co phase, which is confirmed by their XRD graphs, images from SEM, and transmission electron microscope (TEM). Takashima et al.9 used the electron beam melting (EBM) process to fabricate cylindrical rods of Co-Cr-Mo alloy. Their SEM results revealed the development of different phases in different parts of the cylindrical rods. They observed that the central part of the rod had been exposed to a higher temperature, causing more development of the γ-Co phase than the ε-Co phase, while the peripheral regions consisted of more ε-Co phase than γ-Co phase. This led to variations in cooling rates in different parts of the same rod, which directly affect grain structure, mechanical properties, phase composition, reliability, and durability of final parts. Wang et al10. used the SLM process to develop a predominantly martensitic Co-Cr-Mo alloy that consists of 95 vol% of ε-Co and 5 vol% of γ-Co phases. Their study revealed that non-equilibrium solidification results in the formation of internal defects (such as dislocations and stacking faults) and showed higher strength and good ductility compared with EBM and cast-produced Co-Cr-Mo alloys. Latecola et al11. fabricated Co-28Cr-6Mo alloy by the SLM process, and anodized surfaces of the fabricated samples by the plasma immersion ion implantation (PIII) technique. They evaluated osseointegration of following three types of samples of Co-28Cr-6Mo alloy in rat femurs: (i) simply fabricated samples, (ii) samples after anodization by PIII, and (iii) samples coated with titanium (Ti) followed anodization by PIII. The third type of samples yielded the best outcomes in terms of bone quantity and biomechanical quality. Kumar et al.12 developed Co-Cr-Mo-xTi alloys (having x as 2, 4, 6 wt%) by the micro-plasma-based 3D-printing process and assessed their in-vitro biocompatibility using cell viability, metallic ion release, and corrosion behaviour study. They found that the Co-Cr-Mo-4Ti alloy yielded maximum cell viability, minimum corrosion rate, and reduced metallic ion release. They concluded that these values are within the acceptable physiological limits for human beings. Oukati et al13. investigated the effect of Ti addition on microstructure and mechanical properties of cast Co-Cr-Mo alloy by varying wt% of Ti as 0.2, 0.4, 0.6, and 0.8 wt%. They found that an increase in Ti amount increased ultimate tensile strength and yield strength and reduced release of Co and Cr ions. They claim that these improvements enhance biocompatibility of Co-Cr-Mo alloy making it more suitable for biomedical applications. It can be concluded from the review of the past work that (a) Co-Cr-Mo alloy has been developed using the DMLS, SLM, EBM and cast processes, (b) the SLM developed Co-Cr-Mo alloy requires post-processing due to the development of residual stresses, (c) varying heating and cooling effects in the EBM developed Co-Cr-Mo alloy led to some undesirable mechanical properties. Moreover, it requires an extremely high vacuum environment, which significantly increases the cost of alloy development, and (d) Ti addition in Co-Cr-Mo alloy can increase its ultimate tensile strength, yield strength, reduce the metallic release ions, and enhance osseointegration, thereby improving its suitability for biomedical applications. In this context µ-plasma arc metal powder additive manufacturing (µ-PAMPAM) process can play a vital role in development of Co-Cr-Mo alloy due to its following unique advantages: (i) use of a controlled environment employing argon as a shielding gas which helps in reducing thermal gradients and residual stresses that typically develop during SLM process, (ii) accurate control of both deposition rate and temperature during the process to achieve the desired microstructure and mechanical properties which is difficult in the EBM process, and (iii) cost-effective process, unlike the EBM process, due its energy-efficient nature (uses maximum direct current (DC) power of 440 W with max. current of 20 A only) and environment-friendly nature due to absence of use and emission of any harmful gases. But no work is available on the development of Co-Cr-Mo alloy by the µ-PAMPAM process, and studying enhancements in microstructure and mechanical properties via the addition of Ti to it. Therefore, present work aims to fulfil this gap with the following specific objectives: (i) to develop Co-Cr-Mo alloy by the µ-PAMPAM process, (ii) to develop a better metallic material for knee prothesis that has high strength, good ductility, moderate flexibility, and minimum defects by adding 4 wt% Ti to Co-Cr-Mo alloy by µ-PAMPAM process, and (iii) to evaluate enhancements in porosity, density, microstructure, phase evolution, microhardness, tensile and compressive yield and ultimate strength, % elongation, flexural yield and ultimate strength that can be achieved through addition of 4 wt% Ti to Co-Cr-Mo alloy and compare these characteristics of the developed Co-Cr-Mo-4Ti alloy with that of Co-Cr-Mo alloy.

Materials and methods

Preparation of powders and study of their morphology

Powders of cobalt, chromium, molybdenum, and titanium having 99.5% purity, and particle size ranging from 45 to 105 μm (procured from Orion Chemical Pvt Ltd. Pune, India) were used to prepare powders for Co-Cr-Mo and Co-Cr-Mo-4Ti alloys. These powders were taken according to their wt% as presented in Table 1 and were mixed a planetary ball milling machine (Pulverisette 6 from Fritsch, Germany) for 4 h at 350 rpm using 6 mm diameter tungsten carbide balls and maintaining 2:1 weight ratio of the balls to powders. The prepared powders were de-moisturized in an oven at 120 °C for 40 min before their depositions by the µ-PAMPAM process. Figure 1a and c depict SEM images showing morphology of the prepared powders of Co-Cr-Mo and Co-Cr-Mo-4Ti respectively while, Fig. 1b and d present their elemental composition by wt% through Energy Dispersive X-ray Spectroscopy (EDX) images.

Table 1.

Chemical composition of the prepared powders of Co-Cr-Mo-4Ti and Co-Cr-Mo alloy.

Alloy name Wt.% of the constituents
Co Cr Mo Ti
Co-Cr-Mo 65 30 5 -
Co-Cr-Mo-4Ti 63 29 4 4
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Fig. 1.

Fig. 1

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Morphology and chemical composition of the prepared powder of: (a,b) Co-Cr-Mo alloy; and (c,d) Co-Cr-Mo-4Ti alloy.

It can be seen in Fig. 1a and c that morphologies of the ball milling prepared powders of Co-Cr-Mo alloy (Fig. 1a) Co-Cr-Mo-4Ti alloy (Fig. 1c) are different. It is due to presence of Ti in Co-Cr-Mo alloy powder which makes easier it to fracture thus producing finer and sharp-edged particles whereas Co–Cr–Mo alloy powder tends to deform plastically thus producing more lamellar particles.

Fabrication of multilayer depositions

Multilayer deposition (consisting of 8 layers) of Co-Cr-Mo alloy and Co-Cr-Mo-4Ti alloy were fabricated on a Ti base plate (having 120 mm length and width, and 15 mm thickness) by the µ-PAMPAM process using its in-house custom-built 5-axis computer numeric control (CNC) machine as shown in Fig. 2.

Fig. 2.

Fig. 2

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Photograph of in-house customized 5-axis CNC machine for µ-PAMPAM process.

Surface of the base plate was polished by emery papers and subsequently cleaned by acetone. The depositions were fabricated using the following optimum parameters identified by Kumar et al.12 for multilayer deposition of Co-Cr-Mo-2Ti powder on Co-Cr-Mo base plate: 264 W as µ-plasma power, 2.5 g/minute as powder mass flow rate of feedstock or deposition material, and 50 mm/min as travel speed of the deposition head.

Evaluation of density and porosity

A sample was cut by the CNC machine for wire spark erosion machining (WSEM) process (model: Ecocut from Electronica India Ltd. Pune, India) from multilayer deposition of Co-Cr-Mo and Co-Cr-Mo-4Ti alloy each to measure their experimental and theoretical densities and porosity. The cut sample had 5 mm length along the deposition length, 5 mm width along the build direction, and 2 mm thickness along the deposition width as shown in Fig. 3. Experimental density ‘ρe’ of a sample was computed by Eq. (1) using Archimedes principle, and its theoretical density ‘ρt’ was computed by Eq. (2) using the rule of mixtures.

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Fig. 3.

Fig. 3

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Location of different samples from multilayer deposition of Co-Cr-Mo-4Ti alloy.

where, ‘mair’ and ‘mwater’ are mass (g) of a multilayer deposition sample in air and deionized (DI) water respectively; ‘ρdi’ is DI water density (g/cm3); ‘Ai’, ‘ρi’ and ‘Ci’ are respectively atomic weight (g); density (g/cm3) and at% of ith element of a deposition sample. Mass of each sample in DI water and air was measured using a weighing scale having a least count of 0.1 mg (model: ME204 from Mettler Toledo). Each measurement was repeated 5 times, and its average value was used in the analysis. Relative density ‘ρr’ (%) of a sample is the ratio of its experimental density ‘ρe’ to its theoretical density ‘ρt’, and its porosity (%) is obtained by subtracting ‘ρr’ from 100.

Acquiring microstructure, chemical composition, and phase evolution data

A cuboid sample was cut transversely by the CNC WSEM machine from a multilayer deposition of Co-Cr-Mo and Co-Cr-Mo-4Ti alloy, each to study their microstructure, chemical composition, elemental mapping, and evolution of phases. The cut sample had 10 mm length along deposition length, 10 mm width along the build direction, and 2 mm thickness along the deposition width as shown in Fig. 3. Each sample was polished using 2500 grit size silicon carbide emery paper followed by polishing by alumina liquid of 1–1.5 μm fineness, and finally polishing by diamond spray spread over a velvet cloth. The polished samples were kept at 50 °C for 5 min in a freshly prepared etchant solution consisting of 50 ml of HCl, 5 ml of HNO3, and 50 ml of H2O. Microstructure of each prepared sample was obtained by field emission SEM (Gemini 360 from ZEISS Germany), and the elemental composition was acquired using its EDX facility. Data related to phase evolution were obtained by XRD equipment (D2-Phaser from Bruker, USA) using 1.5418 A0 for Cu-Kα radiation, 2θ range of 10–100 ° with 0.02 step size, and dwell time of 1 s. The observed peaks in an XRD graph were matched with the database of the International Centre for Diffraction Data (ICDD) using X’Pert HighScore plus software to identify the presence of major phases and their crystal structures in it.

Microhardness evaluation

One sample was cut by a CNC WSEM machine from a multilayer deposition of Co-Cr-Mo and Co-Cr-Mo-4Ti alloy, each to evaluate their microhardness using a Vickers microhardness measuring machine (UHL VMH-002 V from Walter UHL, Germany). Each cut sample has 30 mm length along the build direction of a multilayer deposition, 5 mm width along the deposition length, and 1.5 mm thickness along the deposition width as shown in Fig. 3. Microhardness profile of each cut sample was obtained by making 10 indentations at interval of 3 mm through application of 300 g load for a duration of 15 s as per ASTM E92-82 standard. An average value of Vickers’ microhardness of each sample was also computed for a comparative study.

Tensile testing

Three samples were cut by a CNC WSEM machine from a multilayer deposition of Co-Cr-Mo and Co-Cr-Mo-4Ti alloy, each following the ASTM E8M standard. Each cut sample has 25.4 mm overall length and 7.62 mm gauge length along the build direction, 5 mm width along the deposition length, and 1.2 mm thickness along the deposition width as depicted in Fig. 3, schematically shown in Fig. 4a, and through photographs in Fig. 4b. Tensile test of each prepared sample was conducted on universal tensile testing machine (H50KL from Tinius Olsen, USA) using a strain rate of 0.001 per second to find its yield strength, ultimate tensile strength, and % elongation. The average value of 3 samples of an alloy was used to analyse its tensile properties.

Fig. 4.

Fig. 4

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Sample for the tensile testing: (a) dimensions as per standard ASTM E8M, and (b) photographs of the prepared tensile samples of Co-Cr-Mo alloy and Co-Cr-Mo-4Ti alloy.

Compression testing

Four cylindrical samples were cut by a CNC WSEM machine from a multilayer deposition of Co-Cr-Mo and Co-Cr-Mo-4Ti alloy. Each cut sample has a 6 mm length along the build direction and a 3 mm diameter along the deposition length and width as per the ASTM E9 standard. The sample was then ground and finely polished. Compression test of each prepared sample was performed at room temperature on the compression testing machine (AGX-10kN from SHIMADZU, CHINA) using a 0.001/s strain rate to evaluate its compressive yield strength, ultimate compressive strength, and % increase in cross-section area. The average value of 4 samples of an alloy was used to analyse its compression properties.

Bending testing

Two samples were cut by the CNC WSEM machine from a multilayer deposition of Co-Cr-Mo alloy and Co-Cr-Mo-4Ti alloy, each as per ASTM D790 standard. Each cut sample has 50 mm length along the deposition length, 10 mm width along the build direction, and 3 mm thickness along the deposition width as depicted in Fig. 3. Figure 5a and b depict a schematic and a photograph of a sample for the 3-point bending test. The 3-point bending test was conducted on the universal testing machine (AGX-10kN from SHIMADZU, CHINA) to find its flexural yield strength, flexural ultimate strength, and % elongation. The average value of both samples of an alloy was used to analyse its bending properties.

Fig. 5.

Fig. 5

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Sample prepared for the bending test: (a) dimensions as per ASTM D790 standard and (b) photograph of prepared sample from multilayer deposition of Co-Cr-Mo-4Ti alloy.

Results and discussion

Analysis of density and porosity

Figure 6 presents SEM images showing porosity distribution in multilayer deposition samples of Co-Cr-Mo alloy (Fig. 6a) and Co-Cr-Mo-4Ti alloy (Fig. 6b), and Table 2 presents values of their experimental measured density (Eq. 1), theoretical computed density (Eq. 2), relative density, and porosity.

Fig. 6.

Fig. 6

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Porosity distribution in multilayer deposition samples of (a) Co-Cr-Mo alloy, and (b) Co-Cr-Mo-4Ti alloy.

Table 2.

Density and porosity of Co-Cr-Mo and Co-Cr-Mo-4Ti alloys.

Alloy name Experimental
density (g/cm3)		 Theoretical density (g/cm3) Relative density Porosity (%)
Co-Cr-Mo 7.78 8.68 89.6 10.4
Co-Cr-Mo-4Ti 7.56 8.11 93.2 6.8
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It can be observed from Fig. 6; Table 2 that the Co-Cr-Mo-4Ti alloy has smaller values of experimental density (7.56 g/cm3), theoretical density (8.11 g/cm3), and porosity (6.8%) than that of the Co-Cr-Mo alloy (7.78 g/cm3, 8.68 g/cm3, 10.4%). Decrease in experimental and theoretical densities of Co-Cr-Mo-4Ti alloy is due to the smaller density of Ti as compared to Co, Cr, and Mo. Whereas, a decrease in its porosity is due to the formation of titanium oxide, which prevent further oxidation of its other metallic constituents, thus reducing oxide-induced porosity. Corrosion behavior results of Kumar et al.12 for Co-Cr-Mo-2/4/6Ti alloys have also confirmed formation of titanium oxide layer due to presence of Ti in them.

Analysis of microstructure, chemical composition, and evolution of phases

Figure 7 presents SEM and EDX images showing microstructure and chemical composition for multilayer deposition of Co-Cr-Mo alloy (Fig. 7a and b) and Co-Cr-Mo-4Ti alloy (Fig. 7c and d). Figure 7a and c also include XRD plots whose peaks confirm formation of chromium carbides i.e., Cr7C3 and Cr23C6, in both Co-Cr-Mo alloy (Fig. 7a) and Co-Cr-Mo-4Ti alloy (Fig. 7c) and formation of Co-Ti intermetallic in Co-Cr-Mo-4Ti alloy (Fig. 7c). Figure 8a and b, present XRD plots showing evolution of different phases in these alloys.

Fig. 7.

Fig. 7

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Microstructure along with XRD plots for Cr7C3, Cr23C6 phases formed in both the alloys and Co-Ti intermetallic formed in Co-Cr-Mo-4Ti alloy, and elemental distribution for: (a,b) Co-Cr-Mo alloy, and (c,d) Co-Cr-Mo-4Ti alloy.

Fig. 8.

Fig. 8

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Evolution of phases for multilayer deposition of (a) Co-Cr-Mo alloy, and (b) Co-Cr-Mo-4Ti alloy.

It can be observed from Fig. 7a and c that the microstructure of Co-Cr-Mo alloy and Co-Cr-Mo-4Ti alloy that both consist of a Co-rich matrix formed due to the presence of 64.6 wt% and 62.5 wt% of Co respectively in them as shown by their elemental distribution (Fig. 7b and d). It is confirmed by their XRD plots shown in Fig. 8a and b respectively which reveal that this Co-rich matrix consists of γ-Co phase having face centered cubic (FCC) crystal structure, and relatively smaller amount of ε-Co phase having hexagonal close-packed (HCP) crystal structure. It is supported by the findings of Gupta14 who found formation of γ-Co and ε-Co phases in Co-Cr-Mo alloy. He reported that γ-Co phase (also known as austenitic phase) is stable at high temperature. However, Kilner et al.15 reported that slow and incomplete transformation of γ-Co to ε-Co phase in Co-Cr alloys can make γ-Co phase to remain stable even after cooling to room temperature. The γ-Co phase provides good ductility, toughness, and corrosion resistance and addition of nickel and iron stabilize it by decreasing its transformation temperature. Whereas, ε-Co phase increases hardness and wear resistance of Co-based alloys despite its inherent brittle behaviour16,17 and addition of Mo stabilizes it by increasing its transformation temperature18. Both Co-Cr-Mo and Co-Cr-Mo-4Ti alloys also show formation of Cr23C6 phase which is an inter-dendritic light grey blocky phase resembling a fish bone precipitate due to its richness in Cr19. They also show formation of Cr7C3 phase which is typically an irregular polygonal or hollow hexagonal morphology and is a eutectic with Co solid solution. It is a hard carbide phase formed in Co-Cr alloys, nucleates at lower temperatures, and important for mechanical strengthening and wear resistance. Formation of these carbide phases is enabled by the presence of 29.4 wt% and 28.3 wt% of Cr respectively, and 1.8 wt% and 1.9 wt% of carbon respectively in them (Fig. 7b and d), and due to high affinity of Cr towards carbon1. Presence of Cr23C6 phase is confirmed by the intensity peak located at 44.7 with the (400) plane in the XRD plot for Co-Cr-Mo alloy (Fig. 7a) and by the intensity peak located at 42.1̊ with (511) plane for Co-Cr-Mo-4Ti alloy (Fig. 7c). Presence Cr7C3 phase is confirmed by intensity peaks located at 41.7̊ with (002) plane in the XRD plot of Co-Cr-Mo alloy (Fig. 7a) and by the intensity peaks located at 49.3̊ with the (004) plane and 39.5̊ with the (131) plane in the XRD plot of Co-Cr-Mo-4Ti alloy (Fig. 7c). Use of ImageJ software estimated volume fraction of Cr23C6 and Cr7C3 phases in Co-Cr-Mo alloy and Co-Cr-Mo-4Ti alloy as 27–30% and 15–20% respectively. Figure 7a also reveals formation of cracks due to voids.

The observed peaks in the XRD plot of Co-Cr-Mo-4Ti alloy (Fig. 8b) confirm formation of β-Ti phase, and Co-Ti as an intermetallic compound (IMC) in it due to presence of Ti in it. Formation of Co-Ti IMC phase is also confirmed by intensity peaks located at 42.4̊ for (110) plane and 61.5̊ for (200) plane in the XRD plot of Co-Cr-Mo-4Ti alloy (Fig. 7c). Formation of β-Ti phase and Co-Ti IMC is supported by the findings of Kumar et al.20 for Co-Cr-Mo-4Ti alloy and formation of Co-Ti IMC is supported by observations by Gupta21 in a ternary Co-Cr-Ti alloy through a eutectic reaction at 1397 °C. The Co-Ti IMC resembles a cuboidal shape and often appears as blocky or angular particles embedded within the Co-rich matrix, contributing to higher hardness, strength, stability, and good corrosion resistance, thus making it a promising material for high-temperature applications22. However, it can be seen in Fig. 8b that peak intensity of CoTi IMC is very small as compared to γ-Co, ε-Co, β-Ti, Cr23C6 and Cr7C3 phases. This implies that CoTi IMC is present only in trace amounts which is far below any threshold value where it can significantly affect plasticity of Co-Cr-Mo-4Ti alloy. Volume fraction of Co-Ti IMC is estimated to be 10–12%.

Titanium at room temperature primarily exists in α-Ti phase having HCP crystal structure, but as temperature increases beyond 883 °C then it transforms into β-Ti phase having body-centered cubic (BCC) crystal structure, implying that it is stable at high temperatures23. The β-Ti contributes to higher hardness, ductility, formability, corrosion resistance, and strength-to-weight ratio24. It appears as a distinct phase embedded within the Co-rich matrix during solidification and remains structurally stable within it. It is due to significant solubility exhibited by Ti in γ-Co phase (FCC) at high temperature (i.e., 1100–1250 °C). Upon cooling, a portion of the dissolved Ti remains in the Co-rich matrix enabling solid-solution strengthening while, remaining Ti segregates to inter-dendritic region to form trace Co-Ti IMC. The XRD plot in Fig. 8b confirms this explanation where intensity peak of γ-Co occurs at slightly lower 2θ value as compared to pure Co (FCC) indicating lattice expansion of Co lattice when alloyed with larger atomic-radius elements such as Ti, Mo, and Cr. Intensity peak of ε-Co (HCP) also appears slightly shifted as compared to pure Co (HCP) indicating that Cr, Mo, and Ti are partly dissolved in it.

Results of microhardness evaluation

Figure 9a shows microhardness profiles of multilayer deposition of Co-Cr-Mo and Co-Cr-Mo-4Ti alloy along their build direction along with photographs of their indentation and their average value of microhardness are presented through bar diagram in Fig. 9b.

Fig. 9.

Fig. 9

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Microhardness of multilayer deposition of Co-Cr-Mo and Co-Cr-Mo-4Ti alloy: (a) microhardness profile along their build direction with their indentation photographs, and (b) average value of microhardness.

It can be observed from Fig. 9a that multilayer deposition of Co-Cr-Mo-4Ti alloy continuously exhibits higher microhardness along its build direction than the multilayer deposition of Co-Cr-Mo alloy. Consequently, the Co-Cr-Mo-4Ti alloy possessed a higher average value of microhardness of 473 HV than 382 HV of the Co-Cr-Mo alloy, as shown in Fig. 9b. These findings are due to the formation of β-Ti phase and Co-Ti IMC which contribute to its higher microhardness, and Mo and Cr act as stabilizing elements for the β-Ti phase. Lower microhardness of Co-Cr-Mo alloy can also be attributed to its higher porosity (as shown in Fig. 6a; Table 2), which acts as a stress concentrator, thus leading to its fracture at lower loads. Nova et al.1 have also found microhardness values of 240 HV and 510 HV, respectively, for Co-Cr-Mo and Co-Cr-Mo-Ti alloys developed by the powder metallurgy process.

Results of tensile testing

Figure 10 presents engineering tensile stress and tensile strain graphs for multilayer deposition of Co-Cr-Mo and Co-Cr-Mo-4Ti alloy, and Table 3 summarizes values of their tensile yield strength, UTS, and % elongation.

Fig. 10.

Fig. 10

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Engineering tensile stress and tensile strain graphs for multilayer deposition samples of Co-Cr-Mo and Co-Cr-Mo-4Ti alloy.

Table 3.

Ultimate tensile strength, yield strength, and % elongation along with their standard deviations for multilayer deposition of Co-Cr-Mo and Co-Cr-Mo-4Ti alloy.

Alloy name Tensile yield strength (MPa) Ultimate tensile strength (MPa) Elongation (%)
Co-Cr-Mo 740 ± 8.5 758 ± 9.7 12.6 ± 1.3
Co-Cr-Mo-4Ti 789 ± 7.2 817 ± 10.1 15.8 ± 1.4
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It can be observed from Fig. 10 that for tensile strain up to 6%, Co-Cr-Mo alloy has higher tensile stress than that of Co-Cr-Mo-4Ti alloy, but beyond 6% tensile strain, Co-Cr-Mo-4Ti alloy has higher tensile stress. It is due to the higher amount of Cr present in Co-Cr-Mo alloy (29.4 wt% from Fig. 7b) than Co-Cr-Mo-4Ti alloy (28.3 wt% from Fig. 7d) which results in formation of more amount of Cr23C6 and Cr7C3 phases (estimated volume fraction as 27–30%) in it than Co-Cr-Mo-4Ti alloy (estimated volume fraction as 15–20%). These chromium carbides act as obstacles in grain boundaries this hindering movement of dislocations and thereby imparting higher tensile strength to Co-Cr-Mo alloy up to 6% tensile strain. Addition of 4 wt% Ti stabilizes some FCC phase of Co (at temperatures from 417 to 1495 °C), which makes slip easier at the beginning thus lowering tensile strength of Co-Cr-Mo-4Ti alloy. But, this trend reverses after 6% tensile strain because of local lattice distortions created by larger atomic size of Ti atoms in Co-Cr-Mo-4Ti alloy. These distortions obstruct movement of the dislocations thereby imparting it higher tensile strength. It is evident from Table 3 that the Co-Cr-Mo-4Ti alloy has higher values of tensile yield strength (i.e., 789 MPa), UTS (i.e., 817 MPa), and % elongation (i.e., 15.8%) than those for the Co-Cr-Mo alloy (740 MPa tensile yield strength, 758 MPa UTS, and 12.6% elongation). It can be attributed to the following factors: (a) Dissolution of Ti atoms in Co-rich matrix thus creating a solid-solution effect which hinders movement of the dislocations, thereby increasing UTS and yield strength of Co-Cr-Mo-4Ti alloy, (b) Presence of Ti in Co-Cr-Mo-4Ti alloy leads to formation of β-Ti phase, which contributes to its higher strength and ductility22. Elements such as Cr and Mo acts as phase stabilizers β-Ti phase enabling its retention even at lower temperatures. Kumar et al.25 also observed that the presence of Ti in Co-Cr-Mo-2/4/6Ti alloy increased their yield strength, UTS, and % elongation, (c) Interface between hard carbide (i.e., Cr23C6 and Cr7C3) phases and softer γ-Co phase in Co-Cr-Mo alloy acts as a site for initiation and propagation of cracks during tensile testing, thus reducing its UTS and yield strength26, and (d) Higher porosity of Co-Cr-Mo alloy (Fig. 6a; Table 2) also reduces its strength and ductility by creating structural weaknesses.

Results of compression testing

Figure 11 presents engineering compressive stress and compressive strain graphs for the compressive samples of multilayer deposition of Co-Cr-Mo and Co-Cr-Mo-4Ti alloy obtained from their compressive testing and Table 4 presents values of their compressive yield strength, ultimate compressive strength (UCS), and % increase in cross-sectional area.

Fig. 11.

Fig. 11

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Compressive stress and strain diagram for multilayer depositions of Co-Cr-Mo and Co-Cr-Mo-4Ti alloy.

Table 4.

Ultimate compressive strength, compressive yield strength, and % increase in cross-section area along with their standard deviations for multilayer deposition of Co-Cr-Mo and Co-Cr-Mo-4Ti alloy.

Alloy name Compressive yield strength (MPa) Ultimate compressive strength (MPa) Increase in cross-section area (%)
Co-Cr-Mo 593 ± 6.5 712 ± 8.3 8.9 ± 1.2
Co-Cr-Mo-4Ti 786 ± 5.8 877 ± 9.2 13.8 ± 0.8
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It can be seen from Fig. 11 that for compressive strain up to 6%, the Co-Cr-Mo alloy has higher compressive stress than that of the Co-Cr-Mo-4Ti alloy, but beyond 6% compressive strain, the Co-Cr-Mo-4Ti alloy has higher compressive stress. It can also be observed from Table 4 that the Co-Cr-Mo-4Ti alloy has much higher values of its compressive yield strength (i.e., 786 MPa), UCS (i.e., 877 MPa), and % increase in cross-section area (i.e., 13.8%) than the corresponding values for Co-Cr-Mo alloy (i.e., 593 MPa compressive yield strength, 712 MPa UCS, and 8.9% increase in cross-section area). It is due to the small size difference between Ti grains and grains of Co, Cr, and Mo in Co-Cr-Mo-4Ti alloy. It increases its grain boundary, which impedes the movement of the dislocations, thus enhancing its compressive yield strength and UCS27. Also, precipitation strengthening is the mechanism for strength enhancement of Co-Cr-Mo-4Ti alloy, where Co-Ti IMC serves as a hard secondary precipitate within the Co-rich matrix. This precipitate hinders the movement of the dislocations, thereby increasing resistance to plastic deformation, thus enhancing compressive yield and ultimate strength of Co-Cr-Mo-4Ti alloy28. More pores in Co-Cr-Mo alloy reduce the actual cross-sectional area available to carry the compressive load; therefore, for the same applied force, compressive stress on the remaining solid material becomes higher, thus causing it to fail at a lower applied load, which reduces its compressive yield strength and UCS.

Results of 3-point bending testing

The 3-point bending test helps in a better understanding of a material in terms of mechanical strength when subjected to out-of-plane loading. Figure 12 presents flexural stress-flexural strain curves for the bending test samples of multilayer deposition of Co-Cr-Mo and Co-Cr-Mo-4Ti alloys obtained from their three-point bending tests. Table 5 presents values of their flexural yield strength, flexural strength, and % elongation.

Fig. 12.

Fig. 12

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Flexural stress-flexural strain diagram for multilayer deposition of Co-Cr-Mo alloy and Co-Cr-Mo-4Ti alloy.

Table 5.

Values of flexural yield strength, flexural strength, and % elongation along with their standard deviations for multilayer deposition of Co-Cr-Mo and Co-Cr-Mo-4Ti alloy.

Alloy name Flexural yield strength (MPa) Flexural strength (MPa) Elongation (%)
Co-Cr-Mo 450 ± 4.7 485 ± 5.8 9.1 ± 0.7
Co-Cr-Mo-4Ti 563 ± 3.2 645 ± 4.1 15.2 ± 1.3
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It can be observed from Fig. 12 that the Co-Cr-Mo-4Ti alloy has higher flexural stress than the Co-Cr-Mo alloy for all values of flexural strain. Table 5 also confirms that the Co-Cr-Mo-4Ti alloy has higher values of flexural yield strength (i.e., 563 MPa), flexural strength (i.e., 645 MPa), and % elongation (i.e., 15.2%) than the corresponding values for Co-Cr-Mo alloy (i.e., 450 MPa, 485 MPa, and 9.1%). It is due to the development of more concentrated carbides in Co-Cr-Mo alloy, which act as stress concentrators and become preferential sites for initiation of micro-cracks, especially on the side experiencing tensile stress during the 3-point bending test. These microcracks are developed either within the carbide particle itself or at the interface between carbide and the Co-based matrix. These microcracks will propagate upon loading and fail the Co-Cr-Mo alloy when a load is applied to the transverse side during the 3-point bending test. But, addition of 4wt.% Ti to Co-Cr-Mo alloy promotes grain refinement, and finer grains typically enhance resistance of the resultant Co-Cr-Mo-4Ti alloy to plastic deformation, thereby increasing its flexural yield strength, flexural strength, and % elongation. This is consistent with the findings of Zhang and Zhang29 who reported that varying Ti content from 0 to 4.62 wt% in Mo-Cu-xTi alloy reduced its grain size from 5 to 4.1 μm, which increased its flexural yield strength and flexural strength.

Conclusions

This paper presented the development of Co-Cr-Mo alloy along with the benefits of Ti addition to it by µ-PAMPAM process, and a comparative study of microstructure and mechanical properties of Co-Cr-Mo and resultant Co-Cr-Mo-4Ti alloy. The following conclusions can be inferred from this work:

  • Addition of 4 wt% Ti to Co-Cr-Mo alloy decreased porosity of resultant Co-Cr-Mo-4Ti alloy due to formation of protective titanium oxide, which prevents further oxidation of its other metallic constituents, thus reducing its oxide-induced porosity. It decreased the density of the resultant alloy due to the smaller density of Ti than that of Co, Cr, and Mo.

  • Microstructure and phase evolution revealed the formation of Co-rich matrix comprising γ-Co and ε-Co, and Cr23C6 and Cr7C3 phases in both the alloys. The γ-Co phase enhances ductility while the ε-Co phase increases its microhardness. The estimated volume fraction of Cr23C6 and Cr7C3 phases in Co-Cr-Mo alloy and Co-Cr-Mo-4Ti alloy is 27–30% and 15–20%.

  • Presence of Ti in Co-Cr-Mo-4Ti alloy led to the formation of β-Ti phase and Co-Ti IMC, which contributed to its increased microhardness along its build direction than Co-Cr-Mo alloy and its higher average microhardness of 473 HV than 382 HV of Co-Cr-Mo alloy. The estimated volume fraction of Co-Ti IMC is to be 10–12%.

  • Co-Cr-Mo-4Ti alloy has higher tensile yield strength, UTS, and elongation % than Co-Cr-Mo alloy due to the solid solutioning effect of Ti. The interface between Cr23C6 and Cr7C3 particles and softer γ-Co phase in Co-Cr-Mo alloy acts as a site for initiation and propagation of cracks, thus reducing its UTS and yield strength. Its higher porosity also reduced its strength and ductility.

  • Co-Cr-Mo-4Ti alloy has higher compressive yield strength, UCS, and % increase in cross-section area than Co-Cr-Mo alloy due to the small size difference between grains of Ti and Co, Cr, and Mo, and precipitation strengthening mechanism where Co-Ti IMC serves as a hard precipitate within the Co-rich matrix. Both hinder the movement of dislocations, thereby increasing resistance to plastic deformation.

  • Co-Cr-Mo-4Ti alloy has higher flexural yield strength, flexural strength, and % elongation than Co-Cr-Mo alloy due to grain refinement enabled by Ti. Whereas the development of more concentrated carbides in Co-Cr-Mo alloy acts as a stress concentrator and becomes a preferential site for the initiation of micro-cracks, which are developed either within the carbide particle itself or at the interface between carbide and Co-rich matrix.

Acknowledgements

Authors express their gratitude to Additive and Micromanufacturing Lab (AMAL) and other labs of IIT Indore for providing research facilities to complete this work. Authors also express gratitude towards MTech student Kartik Chaudhary for his help in conducting the experiments. Authors also express their gratitude to Manipal University Jaipur for providing financial support for the article processing charges (APC).

Author contributions

Balbir Singh Negi: Visualization, Methodology, Investigation, Formal analysis, Data Curation, Validation, Writing - Original Draft.Neelesh Kumar Jain: Visualization, Resources, Funding acquisition, Project administration, Supervision, Writing - Review & Editing.Sharad Gupta: Visualization, Resources, Funding acquisition, Supervision.Pradyumn Kumar Arya: Visualization, Formal analysis, Data Curation, Funding acquisitionAnkur Srivastava: Visualization, Formal analysis, Data Curation, Funding acquisition.

Funding

Open access funding provided by Manipal University Jaipur.

Data availability

The datasets generated or analyzed during the current study are available from Balbir Singh Negi ( [phd2201203004@iiti.ac.in](mailto: phd2201203004@iiti.ac.in) ) upon reasonable request.

Declarations

Competing interests

The authors declare no competing interests.

Footnotes

Publisher’s note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Contributor Information

Neelesh Kumar Jain, Email: nkjain@iiti.ac.in.

Ankur Srivastava, Email: ankur.srivastava@jaipur.manipal.edu.

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Data Availability Statement

The datasets generated or analyzed during the current study are available from Balbir Singh Negi ( [phd2201203004@iiti.ac.in](mailto: phd2201203004@iiti.ac.in) ) upon reasonable request.

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