Research ArticleEnhancing work hardening and ductility in additively manufactured β Ti: roles played by grain orientation, morphology and substructure
Introduction
Epitaxial growth during additive manufacturing (AM) of titanium (Ti) alloys leads to the formation of columnar bcc-structured β grains along the preferred direction of <100> [[1], [2], [3]]. In α-β Ti alloys, these prior β columns transform into intersecting hcp-structured martensitic α' plates upon rapid cooling or α/β lamellae after α' decomposition [[4], [5], [6], [7], [8]]. On the other hand, their bcc structure is largely preserved in β Ti alloys thanks to high contents of β stabilising elements which suppress the β to hcp-α'/orthorhombic-α'' martensitic transformations although nano-sized athermal omega (ωath) particles can form in near β Ti [1,3,9].
In both types of alloys, the as-AM microstructure is strongly textured, leading to anisotropic mechanical properties. In particular, the best ductility in α-β alloys such as Ti-6Al-4V was achieved parallel to the build direction [[10], [11], [12], [13], [14]]. This has been attributed to various factors, including grain boundary α (GBA) along the prior β columns which brings about intergranular fracture [11,12], Schmid factor differences [13] and more significant pore growth when the tensile axis is perpendicular to the build direction [15]. It has been shown that the anisotropy can be significantly reduced by transforming columnar β grains into equiaxed using super transus heat treatments [[16], [17], [18]], tuning printing parameters [8,19] and vibrating the melt pool by high intensity ultrasound [20]. Some recent studies [21,22] tackled this issue by designing new Ti alloys capable of solidifying into equiaxed, fine structures thanks to peritectic [21] and eutectic [22] transformations. The anisotropy of mechanical properties in β alloys is expected to be different since the as-AM microstructure is free of GBA and consists only of bcc-β with totally different deformation mechanisms. Research on AM of β Ti alloys has been growing in the last decade especially to make use of their great biocompatibility. Many α-β Ti alloys commonly used for medical implants (e.g. Ti-6Al-4V) have high elastic moduli of > 100 GPa much larger than 10−30 GPa in bone [23] and contain toxic elements (e.g. Al and V), leading to adverse tissue reactions, long-term Alzheimer diseases and stress-shielding induced bone resorption [24,25]. Al and V in β Ti alloys can be replaced with non-toxic β stabilising elements such as Ta, Nb and Mo to produce biocompatible alloys with significantly lower elastic moduli [26]. Laser powder bed fusion (LPBF) of β Ti alloys consisting of such alloy elements as Ta, Nb, Mo and Zr have been extensively studied [[27], [28], [29], [30], [31], [32], [33]]. In addition to low elastic moduli of ∼50−80 GPa, these alloys have shown a good combination of strength (∼600−1000 MPa) and ductility (total elongation of > 15%). The high elongation particularly results from their microstructures dominated by bcc-β phase formed upon rapid cooling during LPBF [1,30,32]. Besides biocompatible β Ti alloys, some studies have been dedicated to the printability of β Ti alloys designed for aerospace applications, such as Ti-5Al-5V-5Mo-3Cr [3,34] and Ti-5Al-5V-5Mo-1Cr-1Fe [35]. However, these studies on AM of β alloys have so far investigated tensile properties exclusively along the build direction [3,30,34,36,37]. The assumption is that the best properties are achieved parallel to the columnar β grains, based on results from α-β alloys without considering the differences between the two types of alloys.
One of the most important concepts neglected is the low work hardening rate, often seen to cause a reduction in strength immediately after yielding (work softening) in β Ti alloys produced by LPBF [28,37,38]. Work hardening rate in β Ti is particularly low when plastic deformation is dominated by dislocation slip [39,40], although those with low β stability undergoing significant twinning and stress-induced martensitic transformation (SIMT) at early stages of deformation show considerable work hardenability [[40], [41], [42]]. Whether slip dominates is largely dependent on β stability. The deformation mechanism gradually changes from SIMT/twinning to slip with increasing β stability [43,44] although at large strains, slip is normally the main deformation mechanism regardless of β stability [45]. While multiple deformation mechanisms including twinning, SIMT and slip can be operative [[46], [47], [48]], one of the main deformation characteristics in β alloys is slip banding. Slip bands are areas where dislocation slip is localised [49]. It has been found that grains with soft orientations (i.e. with high Schmid factor or low Taylor factor) are susceptible to such slip localisation [50,51]. In near β Ti alloys, in particular, the formation of the metastable ω phase can trigger slip banding. Nano-sized ω particles on slip planes can be sheared and destroyed during deformation, giving rise to ω-free bands and slip banding [[52], [53], [54], [55]]. These slip bands are channels of easy glide, causing work softening. More importantly, the texture in AM-fabricated β alloys (i.e. long columnar grains along <100>) is completely different from the conventionally processed ones (randomly oriented, equiaxed grains). As will be shown later, this specific texture increases the propensity for slip banding on {110} (i.e. planes of maximum shear stress), leading to work softening immediately after yielding. Such tensile behavior was previously observed in AM-fabricated β Ti alloys, although no discussion on the work softening mechanism was provided [28,37]. It is desirable to enhance the work hardening ability of AM processed β alloys to improve ductility.
It is known that interactions of slip bands with each other and with grain boundaries can contribute significantly to work hardening since slip band/slip band interactions hinder easy glide and slip band/grain boundary interactions lead to deformation transfer between grains [53,56]. It is tempting to manipulate the AM β alloys to increase these interactions, in particular those with grain boundaries. One obvious solution is to increase the number of grain boundaries involved in deformation. This can be simply achieved by tensile testing perpendicular to the build direction. Additionally, we have shown that β columns in a β alloy fabricated by LPBF, also known as selective laser melting (SLM), can be morphologically transformed into equiaxed β grains by conducting a super transus heat treatment (HT) [1]. This is attributed to the breakage of high aspect ratio β columns and grain growth through a deformation-free geometric recrystallisation. Such a transformation in grain shape is expected to increase the extent of interaction between slip bands and grain boundaries, leading to enhanced work hardening.
It is therefore the objective of the present study to test AM β Ti both parallel and perpendicular to the build direction to investigate anisotropy and to achieve appreciable work hardening. The AM material is also heat-treated to generate an equiaxed grain structure to prove the hypothesis proposed above. The results have demonstrated that, unlike in α-β alloys, the best tensile properties are obtained perpendicular to the build direction, with substantial work hardening and uniform elongation. The transformation of the β columns to equiaxed grains by HT also leads to work hardening with enhanced ductility. The substructures featured in the as-AM microstructure (e.g. the internal cells, unique dislocation structures and melt pool boundaries) have been removed after HT, revealing their effects on mechanical properties, particularly yield strength (YS).
Section snippets
Experimental materials and procedures
A plasma atomised Ti-5Al-5V-5Mo-3Cr-0.5Fe (wt.%) powder (15−45 µm) supplied by AP&C was used. The Renishaw AM250 SLM system was employed for printing rods of 8 mm in diameter and 52 mm in height (Fig. 1(a, b)) using stripe scanning strategy (Fig. 1(c)) on a titanium substrate at room temperature (no preheat was applied) in an atmosphere set at the maximum oxygen content of 100 ppm. Other printing parameters are listed in Table 1. Nearly full density of 99.7% relative to the density of 4.65 g cm
As-LPBF and LPBF-0°+HT microstructures
Fig. 2(a−c) (LPBF-0°) and Fig. 2(d−f) (LPBF-0°+HT) show inverse pole figure (IPF) maps along the build direction, band contrast (BC) maps, and grain misorientation angle distribution respectively. The epitaxial growth during LPBF of Ti-5553 created columns of bcc-β along <100> (Fig. 2(a, b)), as their preferred growth direction, parallel to the build direction [1]. Although the columnar β grains dominated the microstructure, there existed many grains whose growth was hindered by the
Discussion
Slip banding in β Ti-5553 studied here is very similar to that in β Ti gum metals produced conventionally [53,56], in which the interactions between slip bands and with GBs govern the deformation behavior. However, unlike gum metals which can be uniformly deformed at a constant stress to large strains (i.e. plateau curve), LPBF-0° showed significant strain softening with a high rate of necking immediately after yielding (Fig. 5(a) and (b)). This can be attributed to its much stronger texture
Summary and conclusions
(1) Tensile tests were conducted on as-LPBF Ti-5553 along (LPBF-0°) and perpendicular to (LPBF-90°) the build direction, respectively, as well as after a super-transus heat treatment along the build direction (LPBF-0°+HT), revealing dramatically different behaviors.
(2) While LPBF-0° work softened and developed necking immediately following yielding, LPBF-90° and LPBF-0°+HT displayed appreciable work hardening, leading to substantial uniform deformation (UE of ∼6% in LPBF-0°+HT and ∼10% in
Acknowledgements
This work was supported by the Australian Research Council (No. DP190103557) and the University of Melbourne (No. ECRG20). We appreciate the facilities and technical assistance provided by the Bio21 Ian Holmes Imaging Centre, the University of Melbourne.
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