Full length articleSelective laser melting-fabricated Ti-6Al-4V alloy: Microstructural inhomogeneity, consequent variations in elastic modulus and implications
Introduction
Selective laser melting (SLM) is an established powder-bed-fusion-based additive manufacturing (AM) process today [1], [2], [3], [4], [5]. A wide variety of metallic materials have been processed to date by this process. In particular, AM of Ti-6Al-4V (wt.%) has received extensive attention due to both its broad applications in various industry sectors and its high cost of manufacture and long lead time by the conventional manufacturing processes [2], [5]. AM provides an important solution to the manufacture of Ti-6Al-4V components, especially for the manufacture of intricate bone implants.
In a typical SLM process, the metal substrate or build platform is preheated to 200 °C and fine metal powder is spread on the platform at a layer thickness of 30–60 μm. Areas selected according to the digital design are then scanned by laser, which can quickly heat up the selected area to above 2000 °C for melting, followed by solidification at cooling rates in the order of 104–106 °C/s [5]. The process is repeated layer by layer until the part is completed. However, the AM of each new layer undergoes an ever-changing thermal environment, where specimen or part geometry and dimensions can add further complexity to the process [6], [7]. Consequently, the microstructure and therefore the mechanical performance of each additively manufactured layer can be different in principle. Many studies on SLM of Ti-6Al-4V have discussed variations in both microstructure and mechanical properties [8], [9], [10], [11]. They all contributed to the understanding of SLM of Ti-6Al-4V.
This study aims to provide a detailed quantitative evaluation of the microstructural inhomogeneity in SLM-fabricated Ti-6Al-4V using micro-X-ray diffraction (μ-XRD) together with scanning electron microscopy (SEM) and the consequent elastic modulus variations. The motivation is for the manufacture of long-lasting and better performing Ti-6Al-4V bone implants. Ti-6Al-4V is a premier implant alloy. In particular, additively manufactured extra low interstitial (ELI) Ti-6Al-4V implants has found significant applications since 2007 [12], [13], because of the unrivalled advantage of AM in design freedom and much reduced lead time. However, as with conventionally manufactured Ti-6Al-4V implants, additively manufactured Ti-6Al-4V implants continue to face two issues, i.e. the excessively high elastic modulus compared with that of human cortical bone, which can cause stress shielding, and the toxicity of both V and Al. Attaining an in-depth understanding of the microstructural inhomogeneity including the distribution of V and Al and the resultant elastic modulus variations in SLM-fabricated Ti-6Al-4V is essential to capitalize on the SLM process for the manufactured of desired Ti-6Al-4V bone implants.
Section snippets
Materials and selective laser melting
Gas atomized ELI Ti-6Al-4V powder produced by TLS Technik GmbH & Co. (ASTM Grade 23, 0.1 wt% O, 0.009 wt% N, 0.008 wt% C, 0.17 wt% Fe, <0.002 wt% H; size range: 25–45 μm) was used to fabricate Ti-6Al-4V specimens using a SLM 250HL system (SLM solution GmbH). The SLM parameters used include laser power of 375 W, layer thickness of 60 μm, hatch spacing of 120 μm, scan speed of 1020 mm/s and focal offset distance of 2 mm. Since the section thickness of most SLM-manufactured intricate parts of
Constituent phases and microstructures at different build heights
Fig. 2 shows the μ-XRD line diffraction patterns obtained from different build heights along the central position shown in Fig. 1a. The hexagonal close-packed (HCP) α-phase or α′-phase is prominent at each build height. In order to identify the diffraction peaks for the β-phase, the patterns over the 2θ range from 38° to 42° are zoomed up and shown in Fig. 2b. The β-phase was detected only at Build Heights 1–3, i.e. at the first 10 mm from the bottom. The absence of the β-phase at Build Heights
Microstructural inhomogeneity in SLM-fabricated Ti-6Al-4V
The distinct difference in the constituent phase and microstructure with increasing build height is a result of the complex thermal effect experienced by each build height. In principle, the basic phase transformation processes are clear during SLM of Ti-6Al-4V. The small melt pool in each layer solidifies as part of parallel long columnar β-grains in the build direction due to epitaxial growth. These as-solidified β-grains inhibit the composition of Ti-6Al-4V and are stable before the
Summary
This focused experimental study has revealed that even though the SLM-fabricated Ti-6Al-4V specimens were small (ϕ12 mm × 20 mm), distinct changes in constituent phases were observed under specified SLM conditions from the mid-build height to the top both along the build direction (vertical) and the horizontal direction. In general, below the mid-build height the constituent phases were α and β while above that α′ was predominant. Similarly, the constituent phase changes at the same build
Acknowledgements
This project was funded by the Australian Research Council (ARC) through ARC DP150104719. The authors wish to thank the reviewers of this paper for their detailed comments and value suggestions.
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