Intrinsic heat treatment induced graded surficial microstructure and tribological properties of selective laser melted titanium

Abstract

In the course of the selective laser melting (SLM) process, the part is built layer by layer involving partial re-melting/heating of the previous layer, called as intrinsic heat treatment. Therefore, superficial properties of as-built parts are somewhat different from that of the inside of the part. In this work, the nano-tribological behavior of commercial pure Ti (CP-Ti) sample built using SLM was investigated considering the near-surface regions with focus on the effect of intrinsic heat treatment. Microstructure and nano- testing allow identifying specific sliding behaviors in three attitude regions: (I) surface (0 to 50 µm); (II) subsurface (50 to 200 µm); (III) inner-part (below 200 µm). The average hardness drops slightly when moving from Regions I and II (3.35 GPa) to Region III (3.09 GPa). The friction coefficient (COF) values vary from 0.18 to 0.45 for all three regions, and the region III presents highest worn trace width of 5.8 μm. Regions (I) and (III) promote a stick-and-slip behavior while sliding is smooth and continuous in Region (II). This gradient microstructural characterization enables associating the behavior of region (II) to large-sized lath martensite α’ morphology, which is different from the region I and III with finer microstructure. A FEA thermal model suggests that the existence of the three identified regions is the consequence of the intrinsic heat treatment induced by the SLM process, in which the re-melting/heating and recrystallization have been considered as the main reasons for microstructure coarsening and refinement between region I and II, region II and III, respectively.