Investigation of Tribological Performance of Ti-6Al-4V Alloy Processed by Selective Laser Melting and Electron Beam Melting Processes
Powder bed fusion (PBF) additive manufacturing (AM) or 3D printing, either laser based or electron beam based, is a recent advanced manufacturing process and is being more widely applied industrially. Ti-6Al-4V alloy is a printable alloy and is the most commonly used alloy in PBF-AM for a wide range of applications. However, microstructures of the alloy manufactured by different processes are different and properties including tribological properties relating to the different microstructures need to be well understood. Furthermore, relevant to wear resistance of PBF proceed Ti-6Al-4V alloy, applying physical vapor deposition (PVD) coating and ion implantation surface treatment commonly applied to conventional processed Ti-6Al-4V parts need to be evaluated for AM parts. Thus, through a comprehensive series of wear testing and analysis, this study aims to understand the tribological behaviors of the Ti-6Al-4V samples processed by PBF and further by surface treatments.
Dry linear reciprocating (sliding) wear tests were conducted to evaluate how wear rate (WR) of the alloy samples processed differently by selective laser melting (SLM), electron beam melting (EBM) and processed conventionally (CP) may differ under room temperature or elevated temperatures. The tests were conducted with a wide range of test conditions using WC-Co as counter material. Furthermore, dry sliding wear tests were conducted on PVD TiN coated and nitrogen ion implanted samples processed either by SLM or by CP, also using WC-Co as counter material. After wear testing, wear rates were measured and surface and cross-sectional features of wear tracks were examined in detail.
Results have shown the expected increase in WR as applied normal load increases, but the dependence of WR on sliding frequency has been found very weak. The major finding is that WR is not affected by the alloy processed in different manufacturing routes having different microstructures and hardness values. It will be shown that under a same wear test condition for the three different samples, their deformation behaviors leading to fracture and thus wear loss differ significantly. It will be reasoned that WR does not only depend on strength and thus hardness, but also ductility of the alloy.
It has been observed that test temperature has not affected WR significantly, although it appears that the wear debris layer has become richer in oxygen content as test temperature increases. Thus, it will be suggested that although a higher testing temperature may soften the sample, a more stable oxide layer as temperature increases is more wear resistant.
TiN PVD coating has provided a significant protection of wear for both SLM and CP samples. It has been found that local fracturing of TiN coating after being thinned to 0.4-0.5mm in both coated SLM and CP samples is similar. The major wear mechanism leading to a gradually rapid increase in wear rate of the coated samples will be demonstrated through a wear process as thinning and local fracturing of the coating before the widening of the wear track, while substrate deformation underneath the coating for the case of the low hardness substrate will be illustrated affecting little the overall wear rate.
Surface treatment by nitrogen implantation however has not provided a significant effect on increasing wear resistance of SLM and CP samples. Nanoindentation test data have revealed that, overall, the implantation has not resulted in a significant increase in hardness even in the top layer of 200-300nm. Thus, it is suggested that the implantation treatment used in this study has not resulted in forming a wear resistant layer and thus has an insignificant effect on WR.