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. Author manuscript; available in PMC: 2020 Oct 1.
Published in final edited form as: Addit Manuf. 2019 Jul 8;29:100783. doi: 10.1016/j.addma.2019.100783

Direct Fabrication of Bimetallic Ti6Al4V+Al12Si Structures via Additive Manufacturing

Yanning Zhang 1, Amit Bandyopadhyay 1,*
PMCID: PMC6690621  NIHMSID: NIHMS1536279  PMID: 31406684

Abstract

Ti6Al4V+Al12Si compositionally graded cylindrical structures were fabricated on a Ti6Al4V substrate using laser engineered net shaping (LENS™) process. LENS™ fabricated materials had two regions of Ti6Al4V+Al12Si compositions, a pure Al12Si, and a pure Ti6Al4V area. Microstructural changes were affected by both laser power and compositional variations. In addition, TiSi2 and Ti3Al phase formations were also identified in low and high laser power processed Ti6Al4V+Al12Si sections, respectively. Moreover, the high laser power processed Ti6Al4V+Al12Si section showed the highest hardness value of 685.6 ± 10.6 HV0.1, which was caused due to the formation of new intermetallic phases. This high hardness section exhibited brittle failure modes during compression tests, while the pure Al12Si sections showed ductile deformation. The maximum compressive strengths of Ti6Al4V+Al12Si compositionally graded material was 507.8 ± 52.0 MPa. Our results show that compositionally gradient bulk structures of Ti6Al4V and Al12Si can be directly manufactured using additive manufacturing, however, performances can vary significantly based on process parameters and compositional variations.

Graphical Abstract

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1.0. Introduction

Additive manufacturing (AM) technologies have shifted the manufacturing exemplification to a more practical method that utilized in many applications including aerospace, biomedical and automotive industries [1–5]. The concept of AM is fabricating 3D objects layer-by-layer based on designed digital file. AM provides a way of fabricating complex and highly customized 3D parts which traditional manufacturing methods could not achieve. Additionally, compared to traditional manufacturing methods, the production time of making a 3D part by AM can be significantly reduced particularly for low volume production due to direct fabrication without needing any part-specific tooling. Moreover, the unused materials for AM could be recycled, particularly for metal powders, which could reduce the cost of manufacturing further. With the development of AM technologies, researchers are now able to design and fabricate novel structures comprising of bi-metallic and metal-ceramic compositionally graded materials [6–12]. Directed energy deposition and powder-bed fusion based AM technologies are the two main methods to fabricate metallic materials. Selective laser melting (SLM), electron beam melting and selective laser sintering (SLS) are the popular powder-bed based AM technologies. However, powder-bad based AM technologies are unsuitable for making compositionally graded materials, especially as welding of incompatible materials leads in poor bonding strength at the interface between two materials [6]. Fox et al. have conducted a study of using SLM to fabricate Ti coatings on a CoCr alloy [13]. Based on their results, a poor interfacial bond with brittle cracks was found between Ti and CoCr. Normally, the interface of the compositionally graded materials fabricated by SLM has a sharp transition, which means a direct transition from 100% of material A to 100% of material B. Such sharp transition could lead to thermal property mismatches at the interface and results in high residual stresses and defects. Although using pre-mixed powder with multiple compositions for SLM could fabricate a smooth transition with graded composition at the interface, the redundant procedures such as changing powders during fabrication are very inefficient. Moreover, the mixed powder cannot be separated from the build-chamber causing more material loss. Laser engineered net shaping (LENS™) technology is a directed energy deposition-based AM method, however, is ideally suited for the production of the compositionally graded materials. The laser beam can be focused at substrate, and melt the fed powders carried by argon gas. Additionally, the multiple powder hoppers’ feature allows different types of powder mixing in situ during the fabrication process. Previous studies have shown the successful fabrication of Inconel 718/Cu [11], Inconel 718/Ti6Al4V [12], and CoCrMo/Ti FGMs [14] via LENS™ technology. These studies have demonstrated that LENS™ is a viable method to fabricate bi-metallic and compositionally-graded materials.

Ti6Al4V is an α-β titanium alloy with both alpha- (Al) and beta- stabilizers (V). It is one of the most extensively used materials in aerospace and biomedical applications due to its properties such as high strength-to-weight ratio, excellent fatigue resistance, good biocompatibility, and high corrosion resistance [12]. Al alloys have low density and low melting point. Like Ti-alloys, Al-alloys also show good strength-to-weight ratio, good ductility, and good corrosion resistance [6]. In addition, the cost of Al alloys is significantly lower than many other alloys. Therefore, Al alloys are the primary materials prior to others in most industrial applications. Table 1 lists the thermal properties of both Ti6Al4V and one of the Al alloys, Al 6061.

Table 1.

Thermal properties of Ti6Al4V [36] and Al 6061 [37].

Materials Melting Temperature (°C) Thermal Conductivity (W/mK) Specific Heat Capacity (J/g°C) Coefficient of Thermal Expansion, linear 20–300 °C (μm/m°C)
Ti6Al4V 1604–1660 6.7 0.5263 8.6–9.7
Al 6061 582–652 167 0.896 23.6–25.2
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There are demands for structural materials which has sufficiently high strength with light weight. In practice, both Ti and Al alloys are extensively used as structural materials in aerospace and automobile industries [14]. Some applications require joining Ti alloys with Al alloys to make the structures. However, due to very different thermal properties, welding of Ti to Al is still challenging. In addition, recent studies have shown that using 3D printed Ti6Al4V bar with fractal geometry as reinforcements for cement mortars can enhance mechanical properties [15]. Based on Table 1, Ti6Al4V and Al alloys have significant differences in thermal properties, which makes Ti6Al4V and Al alloys incompatible for welding [16]. Moreover, brittle intermetallic phases such as TixAly could be formed during the welding process between Al and Ti, which can embrittle the interphase and cause premature failure. Additionally, both Ti6Al4V and Al alloys tend to form oxides during welding process. These oxides can also cause defects, primarily pores and inclusions at welded joints, and prevent forming leak tight structures [17]. If it is possible to fabricate Ti-Al bimetallic joints using additive manufacturing, the welding related challenges can be minimized while maintaining structural reliability.

Researchers have demonstrated using a SLM and cold spraying hybrid additive manufacturing methods to fabricate Al-Ti6Al4V compositionally graded materials [6] and shown that the interface of the Al-Ti6Al4V bi-metallic structures had no intermetallic phase formation. However, the transition between the two metallic materials was sharp, which could lead to microstructure mismatch when the material is applied at high temperature environment and result failure. LENS™ technology offers multiple powder hoppers and real time processing parameters control, which has potential to be an efficient way of bonding two metallic materials, which have significant difference in thermal properties with a smooth transition interface. In this study, processing of Ti6Al4V-Al12Si compositionally graded material was carried out using LENS™ by dynamically adjusting processing parameters. The goal of this research is to understand directed energy deposition based additive manufacturing to fabricate Ti6Al4V-Al12Si compositionally graded materials with good bonding strength at the interfaces while varying the composition from 100% Ti6Al4V to 100% Al12Si.

2.0. Materials and methods

2.1. Laser engineered net shaping (LENS™)

LENS™ is a directed energy deposition based AM method, which uses a continuous wave Nd:YAG high-powered laser as main power source. The generated laser beam which comes from a laser head can be focused on the surface of a selected metal substrate with a focal point diameter of 1.65 mm. The metal substrate is fixed on a stage which can move in X and Y directions in a raster scanning motion. Once the laser is focused on the metal substrate, a molten metal pool is created, and fresh powder is carried by argon gas and fed into that molten metal pool. As the stage moves in the X and the Y direction per the designed file, liquid metal is solidified rapidly creating the first layer. After one layer is printed, the laser head moves up in Z direction to print another layer on top of the previous one. Through this layer by layer fabrication, the entire 3D object is printed. Moreover, the LENS™ system is assembled in an enclosed glove box to maintain the build environment that is low in oxygen to avoid oxidation of liquid metals. An oxygen sensor is installed inside of the chamber to monitor the oxygen level all the time. Furthermore, multiple powder hoppers are attached to the LENS™ system which gives the capability of fabricating compositionally graded materials in one operation [8,18,19]. The processing parameters such as laser power, laser scan speed and powder feed rate can be adjusted during the printing process based on print quality. Since this work was a first-generation study, multiple attempts were conducted to optimize different LENS™ processing parameters.

2.2. LENS™ processing of Ti6Al4V-Al12Si compositionally graded materials

Spherical Ti6Al4V powder (ASTM B348–13, Grade 23, Oxygen < 0.10%, TEKNA™, Québec, Canada) with powder size range of 45–105 μm and spherical Al-12Si powder (Grade S20, Valimet Inc., Stockton, CA) with powder particle size range of 48–105 μm were utilized in this study. Each type of powder was further sieved by mechanical sieve shaker for 15 min, and then collected the powders with the size range from 45 μm to 150 μm (−100/+325 mesh) to achieve the best print results. The sieved Ti6Al4V and Al-12Si powders were loaded in two different powder hoppers separately. A 3.5 mm thick Ti6Al4V metallic plate (Grade 5, Tiger Metals Group, Los Angeles, CA) was used as substrate. The Ti6Al4V-Al12Si compositionally graded material had a cylindrical shape design with a diameter of 12.7 mm. Additionally, the cylindrical structure was composed of a pure Al12Si section, pure Ti6Al4V section, and two Ti6Al4V+Al12Si sections. All sections had a designed hatch distance of 0.43 mm and a designed layer thickness of 0.18 mm. For the laser scanning path, a contour was first scanned for each outer layer followed by linear in-fill scans. The in-fill scan orientations were set as 0, 60, and 120 degrees. A LENS™ 750 (OPTOMEC Inc., Albuquerque, NM) system was employed to fabricate the Ti6Al4V-Al12Si compositionally graded cylinders. The chamber of the LENS™ unit was purged with argon gas to reduce the O2 level < 10 ppm. Multiple attempts were used to optimize the processing parameters. Table 2 shows the LENS™ processing parameters of each section after optimization. A Ti6Al4V+Al12Si section was fabricated first with an initial laser power of 390 W, then reduced 10 W of laser power by each layer and kept the laser power at 300 W for the rest of this section. Additionally, both powder feeder 1 (loaded with Al12Si powder) and powder feeder 2 (loaded with Ti6Al4V powder) were opened. However, the powder feed rate of both powder feeders was changed dynamically during the fabrication. Specifically, the initial powder feed rates of powder feeder 1 and 2 were set as 6 g/min and 17.3 g/min, respectively. Then, the powder feed rate of powder feeder 1 increased to 9.5 g/min, and the powder feed rate of powder feed 2 decreased to 11.3 g/min. A 97.5 cm/min hatch scan speed and a 91.4 cm/min contour scan speed were applied to make Ti6Al4V+Al12Si section. A pure Al12Si section was built on top of the previous bi-metallic section. Only powder feeder 1 was opened and the powder feed rate was 9.5 g/min. The laser power was set at 300 W with a hatch scan speed of 89.4 cm/min and a contour scan speed of 83.8 cm/min. Another Ti6Al4V+Al12Si section was printed after the pure Al12Si section was fully built. Like previous Ti6Al4V+Al12Si, the laser power and the powder feed rate of both powder feeder 1 and 2 were dynamically adjusted during the printing. But, in this section, the laser power was initially set at 300 W then increased to 390 W with 10 W increments per layer. The powder feed rate of powder feed 1 was 9.5 g/min initially, and then dynamically decreased to 6 g/min. The powder feed rate of powder feed 2 was dynamically adjusted from 11.3 g/min to 17.3 g/min. Both hatch scan speed and contour scan speed used as the same as previous Ti6Al4V+Al12Si section which were 97.5 cm/min and 91.4 cm/min, respectively. The final section was pure Ti6Al4V section and a laser power of 425 W was applied. Additionally, only powder feeder 2 was opened with a powder feed rate of 17.3 g/min. The hatch scan speed and the contour scan speed were 65 cm/min and 61 cm/min were used, respectively.

Table 2.

Processing parameters of LENS™ fabricated Ti6Al4V-Al12Si compositionally graded structures.

Processing Schematic
Inline graphicTi6AI4V
Inline graphic AI12Si		 Section Laser Power (W) Powder Feed Rate (g/min) Hatch Scan Speed (cm/min) Contour Scan Speed (cm/min) Specific Energy Input per Scan (J/mm3)
Powder Feeder 1 (Al12Si) Powder Feeder 2 (Ti6Al4V)
graphic file with name nihms-1536279-t0009.jpg (Ti64+Al12Si)1 390 → 300 6 → 9.5 17.3 → 11.3 97.5 91.4 3.98 → 5.17
graphic file with name nihms-1536279-t0010.jpg Pure Al12Si 300 9.5 0 89.4 83.8 4.34
graphic file with name nihms-1536279-t0011.jpg (Ti64+Al12Si)2 300 → 390 9.5 → 6 11.3 → 17.3 97.5 91.4 3.98 → 5.17
graphic file with name nihms-1536279-t0012.jpg Pure Ti64 425 0 17.3 65 61 8.45
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2.3. Characterization

The LENS™ made Ti6Al4V-Al12Si samples were cut transversely by utilizing a low speed diamond saw (MTI, Richmond, CA) to evaluate cross-sectional microstructure. To achieve the best surface finishing, sectioned specimens were mounted with phenolic molding powder, then ground by sandpaper with grits from 200 to 1000. Finally, the cross-section specimens were further polished by using 1 μm, 0.3 μm and 0.05 μm aluminum oxide suspension. The specimens were ultrasonicated with 50% ethanol solution for 30 minutes to clean the surface. An air blow gun was utilized to dry the specimens after ultrasonication.

X-ray diffraction (XRD) analysis was performed on the sectioned specimen for phase identification. A Siemens D 500 Kristalloflex diffractometer was utilized with a Cu-Kα radiation source, a 2θ range from 20 to 80 degrees, at a 0.05 degree step size. Additionally, the cross-sections of the specimen were etched by full immersion in Keller’s reagent (95 mL of deionized water, 2.5 mL of HNO3, 1.5 mL of HCl and 1 mL of HF) for 30 seconds. After the specimens were dried by air, the morphology and elemental distribution of the cross-sections of the specimen were studied using a Scanning Electron Microscope (SEM) and Energy-dispersive X-ray spectroscopy (EDS).

The microhardness profile of the sample’s cross-section was studied using a microhardness tester (Phase II, NJ). A testing load of 0.9807 N (HV0.1) and a dwell time of 15 seconds were used for the hardness testing. Additionally, five groups of diamond indentations were performed on each LENS™ fabricated section, and each group of indentations had five single indentations located at different depth in each fabricated section to obtain complete hardness profile.

A SHIMADZU AG-1S (50 KN) screw driven universal testing machines was utilized for compression tests of the LENS™ made Ti6Al4V-Al12Si compositionally graded samples. Five specimens were prepared based on ASTM E9–09 [20] with an initial length (L0) to diameter (D) ratio of L0/D ≈ 3. The speed of the compression tests was set at 0.33 mm/min for all tests. All tests were done until failure. Compression tested samples were cut in transverse direction and the cross-sections were surface finished for microstructural analysis using SEM.

3.0. Results

Fig. 1 shows the schematic and a typical LENS™ processed Ti6Al4V-Al12Si compositionally graded sample on Ti6Al4V substrate. Specifically, Fig. 1a demonstrates the design of the LENS™ fabricated Ti6Al4V-Al12Si compositionally graded cylindrical structure on a Ti6Al4V metallic substrate with a diameter of 12.7 mm. This cylinder was designed with four sections and the building direction Z is also shown in Fig. 1a. Fig. 1b shows the LENS™ processed Ti6Al4V-Al12Si cylinder after minor surface finishing. Cracks were mainly found at the interface of (Ti6Al4V+Al12Si)1 section to pure Al12Si section, and (Ti6Al4V+Al12Si)2 section to pure Ti6Al4V section. Pores were also found on the body of the cylindrical structure. In addition, Fig. 1c illustrates the laser scanning path for Ti6Al4V-Al12Si bimetallic cylindrical structure.

Figure 1.

Figure 1.

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LENS™ fabricated Ti6Al4V-Al12Si compositionally graded structure. (a) Design of the LENS™ fabricated Ti6Al4V-Al12Si compositionally graded cylindrical structure with a diameter of 12.7 mm. (b) Image of the Ti6Al4V-Al12Si compositionally graded cylindrical structure after surface finishing. (c) Schematic of the laser scanning path (0°, 60°, 120°).

3.1. Microstructure of Ti6Al4V-Al12Si compositionally graded materials

The micro-morphology of each section of LENS™ processed Ti6Al4V-Al12Si were studied. Fig. 2 demonstrates the SEM images of each section of the LENS™ processed Ti6Al4V-Al12Si sample. Specifically, Fig. 2a shows the interface between Ti6Al4V substrate and (Ti6Al4V+Al12Si)1 section. Strong bonding along the interface with no defects can be seen. Both α-Ti and β-Ti were found in Ti6Al4V substrate. In addition, unmelted Ti6Al4V particles were embedded in Al12Si matrix. Acicular microstructures were revealed inside of the Ti6Al4V particles. Furthermore, micropores and microcracks were found in (Ti6Al4V+Al12Si)1 section. Fig. 2b illustrates a smooth transition from (Ti6Al4V+Al12Si)1 section to pure Al12Si section without cracks and delamination. Micropores could be seen in the pure Al12Si section. Fig. 2c exposes the fine particle-shape microstructures in the pure Al12Si section. In Fig. 2d and Fig. 2e, smooth transitions from pure Al12Si section to (Ti6Al4V+Al12Si)2 section, and from the (Ti6Al4V+Al12Si)2 section to pure Ti6Al4V section are shown. Based on Fig. 2d, pores were found in both pure Al12Si and the (Ti6Al4V+Al12Si)2 sections. Moreover, in Fig. 2e, only acicular shape microstructures were obtained in the pure Ti6Al4V section and no unmelted particles can be seen. Coarse equiaxed grains with fine acicular microstructures were found near the interface of the (Ti6Al4V+Al12Si)2 and the pure Ti6Al4V sections (Fig. 2f). Fig. 2g–2i show the microstructural variation in Ti6Al4V+Al12Si sections. In Fig. 2g, fine Si particles are seen in low laser power processed Ti6Al4V+Al12Si section (laser power: 300 W). In addition, reactions occurred at the boundary of unmelted Ti6Al4V particles. Fig. 2h shows the graded microstructure in high laser power processed Ti6Al4V+Al12Si. Specifically, Fig. 2i demonstrates dendritic microstructures in Ti6Al4V+Al12Si section processed with a laser power of 350 W. Reactions were obtained along the boundary of unmelted Ti6Al4V particles. Fig. 2j shows the Ti6Al4V+Al12Si section made with an increased laser power of 370 W. The dendritic microstructures disappeared, and finer particle-shape microstructures were formed. Additionally, new reaction occurred near the boundary of unmetled Ti6Al4V particles. Fig. 2k reveals the Ti6Al4V+Al12Si section fabricated with a laser power of 390 W. Coarse equiaxed microstructures can be seen in this section.

Figure 2.

Figure 2.

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SEM microphotographs of each section of LENS™ processed Ti6Al4V-Al12Si compositionally graded material. (a) SEM image of the interface between Ti6Al4V substrate and (Ti6Al4V+Al12Si)1 section. (b) SEM image of the interface between (Ti6Al4V+Al12Si)1 section and pure Al12Si section. (c) SEM image of pure Al12Si section. (d) SEM image of the interface between made pure Al12Si section and (Ti6Al4V+Al12Si)2 section. (e) SEM image of the interface between (Ti6Al4V+Al12Si)2 section and pure Ti6Al4V section. (f) SEM image of pure Ti6Al4V section. (g) SEM image of low laser power processed Ti6Al4V+Al12Si section at 2000x magnification (laser power: 300 W). (h) SEM image of high laser power processed Ti6Al4V+Al12Si section at 500x magnification. (i) SEM image of LENS™ processed Ti6Al4V+Al12Si section with a laser power of 350 W. (j) SEM image of LENS™ processed Ti6Al4V+Al12Si section with a laser power of 370 W. (k) SEM image of LENS™ processed Ti6Al4V+Al12Si section with a laser power of 390 W.

3.2. Elemental distribution and phase analysis

Fig. 3 displays the EDS mapping of Ti6Al4V+Al12Si at different locations. Various of laser powers were applied to fabricate each region. Specifically, Fig. 3(a) shows the elemental distribution at Ti6Al4V+Al12Si section processed with a laser power of 300 W. The dark region was Al rich region, and the fine equiaxed microstructures were Si rich region. Furthermore, a large amount of Si was found at the boundary of unmelted Ti6Al4V particles. Based on the results from Fig. 3(b1), the elemental distributions show the compositional variation through the Ti6Al4V+Al12Si section processed by high laser power. The ratio of Ti6Al4V to Al12Si increased along the build direction Z. The Si distribution mapping reveals high Si intensity at the boundary of unmelted Ti6Al4V particles in lower laser power region, but not the higher laser power region. Fig. 3(b2) shows the EDS mapping at the lower half region of Fig. 3(b1). This section was fabricated with a laser power of 350 W and low amount of Ti6Al4V to Al. The dendritic microstructure is Al rich. Moreover, higher amount of Si was found at the boundary of Ti6Al4V particles. The location shown in Fig. 3(b3) was fabricated with a laser power of 390 W and higher amount of Ti6Al4V to Al12Si ratio. Coarse equiaxed microstructure is Ti rich region without any Si rich boundary of Ti6Al4V particles. Fig. 4 demonstrates the XRD spectrums of LENS™ processed Ti6Al4V+Al12Si at both Ti6Al4V+Al12Si and pure Al12Si sections. According to the XRD analysis results, Al and Si phases can be seen in pure Al12Si region. In Ti6Al4V+Al12Si zone, both TiSi2 and Ti3Al phases can also be seen. In addition, α-Ti, β-Ti, Al and Si phases are found in Ti6Al4V+Al12Si section.

Figure 3.

Figure 3.

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EDS mapping in Ti6Al4V+Al12Si section at different locations. (a) EDS mapping at Ti6Al4V-Al12Si section processed with low laser power. (b1) EDS mapping at high laser power processed Ti6Al4V-Al12Si section. (b2) EDS mapping at Ti6Al4V-Al12Si section processed with a laser power of 350 W. (b3) EDS mapping at Ti6Al4V-Al12Si section processed with a laser power of 390 W.

Figure 4.

Figure 4.

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XRD profiles of Ti6Al4V+Al12Si and pure Al12Si sections by LENS™.

3.3. Microhardness and compression strength

Microhardness results are shown in Fig. 5(a) where the Ti6Al4V substrate had a hardness of 320.9 ± 12.4 HV0.1. Hardness increased near the interface between the Ti6Al4V substrate and the (Ti6Al4V+Al12Si)1 section. Hardness of the (Ti6Al4V+Al12Si)1 section initially was 442.9 ± 33.3 HV0.1, then dropped to 96.6 9 ± 6.0 HV0.1. The average hardness value of LENS™ processed pure Al12Si section was 59.3 9 ± 4.9 HV0.1. The (Ti6Al4V+Al12Si)2 section had a hardness value of 123.9 ± 30.2 HV0.1 at first, then the increased to 487.1 ± 24.3 HV0.1 with increasing of depth, and finally reached at a hardness value of 685.6 ± 10.6 HV0.1. The average hardness value of LENS™ processed pure Ti6Al4V section was 395.4 ± 8.3 HV0.1, a 24% increase compared to Ti6Al4V substrate. Furthermore, the (Ti6Al4V+Al12Si)2 section had the highest hardness value of 685.6 ± 10.6 HV0.1.

Figure 5.

Figure 5.

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(a) Microhardness profiles of LENS™ processed Ti6Al4V+Al12Si compositionally graded structure. (b) Images of LENS™ processed pure Al12Si section after compression test. (c) Failed sample after compression test, vertical cracks are indicated by arrows.

According to the results of compression tests, the maximum compressive stress of LENS™ processed Ti6Al4V+Al12Si material was 507.8 ± 52.0 MPa. Fig. 5b shows the SEM image of pure Al12Si section after compression test. Micropores can be seen compressed along the applied force direction. Microcracks could be observed and propagated in transvers direction. Expanded gaps in transverse direction are found at the unmelted Ti6Al4V particle’s boundary. No significant deformation in shape was found on the unmelted Ti6Al4V particle. Fig. 5c demonstrates a failed cylindrical shape specimen after compression test. Multiple vertical cracks went through the Ti6Al4V+Al12Si section while the LENS™ processed pure Al12Si section was fully compressed.

4.0. Discussion

The goal of this study was to explore the possibility of fabricating compositionally graded bi-metallic structure by utilizing metallic powders via LENS™ technology, which have significantly different thermal properties and are challenging to join using traditional methods. A LENS™ system with multiple powder feeders has the capability to manufacture bi-metallic materials with dynamic processing parameter adjustment ability. Specifically, the multiple powder feeders’ setup allows powder mixing during processing by controlling each powder hopper’s feed rate individually. This function can minimize the work of pre-mixing powder preparation. Moreover, the laser power and scan speeds can be adjusted in real-time based on manufacturing needs. The flexible processing parameter controls is helpful to avoid defects during fabrication, especially in multi-material fabrication.

There were two main challenges related to AM processing of these bimetallic structures. The first challenge was fabrication of pure Al alloy via LENS™ technology. Since Al alloys have high laser reflectivity [6,21], understanding the laser-Al alloy interaction behavior along with LENS™ processing parameters for printing Al12Si were critical. Al12Si metallic powder was chosen in this research because the Si contents could help in laser absorption to melt Al, which is also widely used in industry [22,23]. To fabricate pure Al12Si by LENS™ technology, a laser power range from 250 W to 300 W were tested. Additionally, a range of powder feed rates from 5 g/min to 12 g/min, and laser scan speeds from 48.8 cm/min to 97.5 cm/min were also tried to optimize LENS™ processing parameters. However, the Al12Si could not be fully melted below 300 W laser power. LENS™ fabricated pure Al12Si parts had minimum defects at a powder feed rate of 9.5 g/min and laser scan speed of 89.4 cm/min. The second challenge was optimization of processing parameters at the transition zones. Because of Ti6Al4V and Al12Si have such significant differences in thermal properties, the compositional ratio was dynamically changed at the transition zone during laser processing where the processing parameters had to be adjusted on each layer. Many attempts were made to optimize the processing parameters to minimize defects at the interface region. Specifically, the minimum powder feed rate of Al12Si and Ti6Al4V were 6 g/min and 11.3 g/min, respectively. Additionally, previous studies have shown to fabricate Ti6Al4V using LENS™ [7,19,24–27] in which processing parameters were 425 W laser power, 17.3 g/min powder feed rate and 61–65 cm/min laser scan speed. Therefore, for making the (Ti6Al4V+Al12Si)1 section, the powder feed rates of Al12Si powder and Ti6Al4V powder were set at 6 g/min and 17.3 g/min. A 390 W laser power was used to process the first layer. This laser power was chosen based on experimental runs from 425 W to 380 W. When the laser power was above 390 W, non-flat layer with “bubbles” formed; if the laser power was below 390 W, the first layer could not bond with the substrate. A 97.5 cm/min hatch scan speed and a 91.4 cm/min contour scan speed were utilized to make the first layer of the (Ti6Al4V+Al12Si)1 section to help avoid “bubble” formation. After the first layer of the (Ti6Al4V+Al12Si)1 section was successfully deposited, the rest layers in the (Ti6Al4V+Al12Si)1 section were fabricated with a laser power decrement of 10 W with an Al12Si powder feed rate increment of 0.35 g/min, and Ti6Al4V powder feed rate decrement of 0.6 g/min. Ten layers in total were fabricated in the (Ti6Al4V+Al12Si)1 section. The method of fabrication of the (Ti6Al4V+Al12Si)2 section was similar to the (Ti6Al4V+Al12Si)1 section but using a laser power increment of 10 W, Al12Si powder feed rate decrement of 0.35 g/min, and Ti6Al4V powder feed rate increment of 0.6 g/min. With this knowledge, Ti6Al4V+Al12Si compositionally graded cylindrical structures were successfully fabricated with minimum defects. Table 2 shows the final LENS™ processing parameters after optimization. The processing parameters of making Ti6Al4V+Al12Si sections were based on the parameters of making pure Ti6Al4V and pure Al12Si. Fig. 1b illustrates the LENS™ processed Ti6Al4V+Al12Si compositionally graded cylindrical structure after surface finishing. Major cracks were observed at the surface of the Ti6Al4V+Al12Si cylinders after surface finishing. These defects suggest that further processing parameters optimization is required. Moreover, the SEM microphotographs show that the cracks were mainly present at the outer surface. The specific energy input (E) of each scanned track was calculated by [28],

E=Pv×h×t

where P is laser power, v is scan speed, h is hatch distance, and t is layer thickness. Based on Table 2, the experimental specific energy input of pure Ti6Al4V and pure Al12Si were 8.45 J/mm3 and 4.34 J/mm3, respectively. The specific energy input of pure Ti6Al4V is almost double the specific energy input of pure Al12Si, which illustrates significant difference of laser-materials interactions between these two materials. In addition, the specific energy input at interface section had a range from 3.98 J/mm3 to 5.17 J/mm3 due to the laser power variation, which also contributed to the microstructural variations at the interfaces.

The SEM images, EDS mapping and XRD analysis revealed the microstructures, elemental distribution and phase formations in each section of LENS™ processed Ti6Al4V+Al12Si compositionally graded materials. In general, the microstructural variations in each section was a combined effect by both laser power and compositional variations. The acicular microstructures was found in Ti6Al4V substrate (Fig. 2a); unmelted Ti6Al4V particles (Fig. 2(a, g–k)) along with Widmanstätten α-Ti laths were seen in LENS™ processed pure Ti6Al4V (Fig. 2f) [9,29,30]. From Fig. 2a, no defects were found along the boundary between Ti6Al4V substrate and (Ti6Al4V+Al12Si)1 section. The appearance of unmelted Ti6Al4V particles in Ti6Al4V+Al12Si sections were due to insufficient laser power to melt Ti6Al4V. Fig. 2b illustrates a smooth transition from the (Ti6Al4V+Al12Si)1 section to LENS™ processed pure Al12Si section with no defects. The formation of micropores in LENS™ processed pure Al12Si section was influenced by input energy density and surface roughness [6,31]. The white regions in Fig. 2c were resulted due to Si and it was confirmed by EDS analysis (Fig. 3a) and XRD spectrum (Fig. 4). Fig. 2d demonstrates another smooth interface from LENS™ processed pure Al12Si section to the (Ti6Al4V+Al12Si)2 section. The formations of micropores were mainly found between the adjacent unmetled Ti6Al4V particles in (Ti6Al4V+Al12Si)2 section. These unmelted particles were obstacles and blocked the molten Al12Si powders filling in, then resulted in formation of micropores [31]. Widmanstätten α-Ti laths were the dominated microstructures in Fig. 2e and Fig. 2f. The indicated coarse equiaxed microstructures shown in Fig. 2e were barely melted Ti6Al4V powders. Fig. 2g and Fig. 3a shows the micro-morphology and elemental distribution of the Ti6Al4V+Al12Si section processed with 300 W laser power and low Ti6Al4V/Al12Si ratio. The EDS results confirmed that the white particle-shape microstructures were the Si rich microstructures and the dark matrix was Al rich region. Additionally, reactions occurred around the boundary of unmelted Ti6Al4V particles. These reacted regions were dominated by Si element. Based on XRD results, these reactions resulted in the formation of TiSi2 phase. Previous studies have shown that using Ti and Si particles to synthesize titanium silicide [32,33]. The sequence of synthesis follows TiSi2 → TiSi → Ti5Si4 → Ti5Si3 with increasing thermal input. Based on Ti-Si phase diagram, the phase formation temperature of TiSi2 is 1332°C. In this case, the presented unmelted Ti6Al4V particles have high surface-to-volume ratio, which allows relatively more intimate contact to Si. More contact area leads to diffusion flux and resulted in decreasing the formation temperature of TiSi2 phase. Significant microstructural variation affected by both laser power and compositional variations as seen in Fig. 2h and Fig. 3(b1). Specifically, Al dendritic structures were appeared in 350 W laser power processed Ti6Al4V+Al12Si section (Fig. 2i) with low Ti6Al4V/Al12Si ratio and confirmed by EDS (Fig. 3(b2)). The formation of Al dendrites was due to the high melt temperature and faster cooling rate [34]. The formation of TiSi2 was also found near the boundary of unmelted Ti6Al4V particles. With increasing laser power and amount of Ti6Al4V/Al12Si ratio, the Al dendrites disappeared (Fig. 2j) and formation of Ti-Al intermetallic phase formation was noticed. The EDS (Fig. 3(b3)) and XRD (Fig. 4) results confirmed the coarse equiaxed microstructures of Ti3Al intermetallic phases, which were shown in Fig. 2k. Xiao et al. have conducted a study of analyzing the microstructures of spark plasma sintered TiAl alloy [35]. A significant increase in the size of Ti-Al intermetallics from 1000°C to 1100°C was reported [35], which is similar to Fig. 2j and Fig. 2k. In addition, the formation of Ti3Al was also occurred near the boundary of unmetled Ti6Al4V particle in high laser power processed Ti6Al4V+Al12Si section with high Ti6Al4V/Al12Si ratio.

The microhardness profile shows hardness variation in each section. The LENS™ fabricated pure Ti6Al4V had higher hardness value compared to Ti6Al4V substrate due to grain refinement and higher residual stresses. Moreover, the hardness value of low laser power processed Ti6Al4V+Al12Si section with low amount of Ti6Al4V/Al12Si ratio was greater than pure Al12Si section mainly due to compositional variations. Moreover, unmetled Ti6Al4V particles could generate internal stress during rapid solidification and influence the hardness. The high laser power processed Ti6Al4V+Al12Si section had the highest hardness value. The increase in hardness was caused by both residual stresses and new phases formation.

Based on the results of compression testing, the LENS™ fabricated pure Al12Si section was the primary deformed section. Fig. 5b demonstrates the direction of cracks’ propagation in pure Al12Si section when the specimen experienced compression stress. In addition, the vertical cracks shown in Fig. 5c illustrates brittle failure in Ti6Al12Si+Al12Si section of the tested specimen, which matches the results from the microhardness data. Furthermore, the maximum compressive stress of LENS™ processed Ti6Al4V-Al12Si compositionally graded materials is compared with ultimate stress of other Al alloys and reported in Table 3. Based on the comparison, the LENS™ processed Ti6Al4V-Al12Si compositionally graded material showed a similar ultimate stress to Al AA2024 T4 alloy. Moreover, the ultimate stress of the LENS™ processed Ti6Al4V-Al12Si compositionally graded material is 70% and 90% greater than reported compression strength values for Al AA6082 T6 and Al AA6111 alloys, respectively.

Table 3.

Comparison of maximum compressive stress.

Material Maximum Compressive Stress (MPa)
LENS™ processed Ti6Al4V-Al12Si compositionally graded material 507.8 ± 52.0
Al AA6082 T6 [38] 290
Al AA2024 T4 [39] 476
Al AA6111 [40] 272
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5.0. Conclusions

The goal of this study was to fabricate a 100% Ti alloy → 100% Al alloy compositionally graded structure via LENS™, a DED based additive manufacturing technique. LENS™ fabricated materials were built on a Ti6Al4V substrate, and had two Ti6Al4V+Al12Si sections, a LENS™ processed pure Al12Si section, and a LENS™ processed pure Ti6Al4V section. The SEM characterization showed unique microstructures in each section. The microstructural variation was affected by both laser power and composition. In addition, TiSi2 and Ti3Al phases were formed in low and high laser power processed Ti6Al4V+Al12Si sections, respectively. Based on the microhardness tests, the high laser power processed Ti6Al4V+Al12Si section had the highest hardness value of 685.6 ± 10.6 HV0.1, which was caused by both the formation of new phases and existence of residual stresses. This high hardness section experienced brittle failures during compression tests. During compression tests, LENS™ processed pure Al12Si section was deformed mainly, and the maximum compressive strength of Ti6Al4V+Al12Si structures was 507.8 ± 52.0 MPa.

Acknowledgements

The authors acknowledge financial support from the National Science Foundation under the grant # CMMI 1538851 (PI- Bandyopadhyay). Authors also acknowledge financial support from the National Institute of Arthritis and Musculoskeletal and Skin Diseases of the National Institutes of Health under Award Number R01 AR067306–01A1. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health. Authors would like to acknowledge support from Professor Thomas Williams from University of Idaho for assistance with XRD analysis. Authors would also like to acknowledge help from Mr. Bryan Heer for SEM operations.

Footnotes

Conflict of interest

None.

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