Additive manufacturing of strong and ductile Ti–6Al–4V by selective laser melting via in situ martensite decomposition
Introduction
Selective laser melting (SLM) is a powder-bed-based additive manufacturing (AM) technology [1], [2], [3]. As with other AM processes [1], [2], [3], [4], [5], [6], [7], SLM offers distinct advantages over conventional manufacturing, such as design freedom, free of tooling, near-net or net shape production, efficient use of materials, short lead time, and substantial cost savings in many cases. A wide variety of metallic materials have been processed to date using SLM, and Ti–6Al–4V (wt.%) has received prime attention as the benchmark titanium alloy because of its broad applications in industry and the associated high cost of manufacturing and long lead time [2], [8].
An important performance benchmark for additively manufactured metallic structural components is to achieve mechanical properties comparable or even superior to their wrought counterparts. For additively manufactured Ti–6Al–4V, this requires that its microstructure must be essentially pore free and comprise proper phases that can offer strong and ductile properties. This poses great challenges to the AM of Ti–6Al–4V by SLM.
Table 1 summarizes the literature data on the tensile properties of SLM-fabricated Ti–6Al–4V, Ti–6Al–4V additively manufactured by selective electron beam melting (SEBM), mill-annealed (MA) Ti–6Al–4V (the most commonly used state for wrought Ti–6Al–4V) and solution-treated and aged (STA) Ti–6Al–4V [1], [4], [9], [10], [11], [12]. SLM-fabricated Ti–6Al–4V can achieve yield strength over 1300 MPa, but the tensile elongation is noticeably below the minimum threshold of 10% suggested for critical structural applications [13]. This disqualifies the SLM-fabricated Ti–6Al–4V and, accordingly, post-SLM heat treatment is often applied for ductility improvement [3], [9].
The mechanical properties of the SLM-fabricated Ti–6Al–4V depend largely on its constituent phases and their morphology and characteristic length scales, as well as the size and orientation (texture) of the prior-β grains. In view of the current standard SLM practice, which is conducted at powder-bed temperatures less than 230 °C, the resulting microstructure often features columnar prior-β grains filled with acicular α′ martensite [1], [2], [3], [11]. The elongated prior-β grain boundaries in conjunction with the presence of acicular α′ martensite favor intergranular failure [14]. In addition, such strongly textured structures lead to significant anisotropic mechanical behavior, causing large discrepancies in mechanical response when subject to external loading along different sample orientations [3]. Post-heat treatment is thus regarded as a must-do process to transform the acicular α′ martensite into equilibrium (α + β) microstructures, while reducing thermal stresses at the same time [3], [9].
In contrast, the fabrication of Ti–6Al–4V by SEBM is often performed at powder-bed temperatures above the α′ martensitic transformation temperature (Ms, 575 °C [15]), which favors the formation of α and β instead of α′ martensitic transformation [16]. The occurrence of both α and β not only leads to a near-equilibrium lamellar (α + β) structure, but also breaks up the textured columnar prior-β grains alleviating the texture [14]. As a result, the SEBM-fabricated Ti–6Al–4V often exhibits better ductility than the SLM-fabricated Ti–6Al–4V, as well as improved isotropic mechanical behavior. However, the better ductility of the SEBM-fabricated Ti–6Al–4V usually corresponds to lower strength (Table 1), falling into the so-called strength–ductility trade-off dilemma [17]. STA Ti–6Al–4V seems to be able to evade the strength–ductility dilemma, standing out with high yield strength (⩾1100 MPa) at no ductility trade-off. The STA approach involves solution treatment above β transus, followed by water quenching and low-temperature aging [18], [19], [20]. The resultant microstructure is composed of fine lamellar α and β with or without primary α, thanks to the decomposition of α′ martensite during aging (4–8 h or longer) [20]. This raises an intriguing question: Can such strong and ductile Ti–6Al–4V be produced by SLM in the as-fabricated state?
This study focuses on addressing this challenge. It is shown that, through novel fabrication design based on both phase transformation and processing optimization, in situ α′ martensite decomposition can be realized during SLM to produce an ultrafine (α + β) lamellar structure, analogous to that obtained by STA. Consequently, better than forged tensile mechanical properties are achieved in the SLM-fabricated Ti–6Al–4V.
Section snippets
Experimental procedures
Gas atomized Ti–6Al–4V powder (ASTM Grade 23, ELI, 0.1 wt.% O, 0.009 wt.% N, 0.008 wt.% C, 0.17 wt.% Fe,<0.002 wt.% H; TLS Technik GmbH & Co.) in the size range 25–45 μm was used. Specimen cubes (10 mm each dimension) and cylindrical bars (10 mm in diameter) were fabricated vertically onto support structures using an SLM facility (SLM Solutions GmbH, SLM 250 HL) with a maximum power of 400 W. There, different layer thicknesses, 30, 60 and 90 μm, were applied. The powder bed was pre-heated to 200 °C and
Dependence of microstructure on SLM processing conditions
Table 3 summarizes the microstructure and constituent phases present in the SLM-fabricated Ti–6Al–4V, corresponding to the samples listed in Table 2. Columnar prior-β grains formed in all samples (Fig. 1) owing to the steep temperature gradients encountered during SLM. The use of 30 μm layer thickness resulted in the finest columnar prior-β grains (Fig. 1a; average width: 15 μm) vs. the use of 60 μm and 90 μm layer thicknesses (Fig. 1b and c; average width, 70 μm), indicative of its fastest cooling
Selection of SLM processing variables in favor of in situ martensite decomposition
In the realm of SLM of Ti–6Al–4V, it is often recognized that the ultimate microstructure is dominated by α′ martensite as a result of rapid cooling. Post-SLM heat treatment is thus needed to transform the α′ martensite into α and β. This study has shown that tuning the SLM processing variables can result in substantial variations in the constituent phases, owing to the changed thermal profiles. In particular, through processing optimization, the SLM process can allow in situ decomposition of
Conclusions
Ti–6Al–4V fabricated by SLM often consists of columnar prior-β grains filled with acicular α′ martensite, and usually displays high yield strength, but limited ductility. Enlightened by STA Ti–6Al–4V, which consists of fine lamellar α and β arising from the decomposition of α′ martensite and exhibits outstanding mechanical properties, this study was motivated to produce a novel ultrafine lamellar α and β microstructure in the SLM-fabricated Ti–6Al–4V to offer mechanical properties comparable
Acknowledgements
The RMIT Centre for Additive Manufacturing is created by the RMIT University to generate critical research mass in advanced manufacturing technology and design to serve both the Victorian and Australian manufacturing industry. WX thanks the SAMME Emerging Researcher Seed Fund at RMIT University for financial support. The authors thank RMIT Microscopy and Microanalysis Facility (RMMF) for providing access to advanced electron microscopy.
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