Densification behavior, microstructure evolution, and wear performance of selective laser melting processed commercially pure titanium

This work presents a comprehensive study of the densification behavior, phase and microstructure development, hardness and wear performance of commercially pure Ti parts processed by selective laser melting (SLM). An in-depth relationship between SLM process, microstructures, properties, and metallurgical mechanisms has been established. A combination of a low scan speed and attendant high laser energy density resulted in the formation of microscopic balling phenomenon and interlayer thermal microcracks, caused by a low liquid viscosity, a long liquid lifetime, and resultant elevated thermal stress. In contrast, using a high scan speed produced the disorderly liquid solidification front and considerably large balling, due to an elevated instability of the liquid induced by Marangoni convection. A narrow, feasible process window was accordingly determined to eliminate process defects and result in full densification. The phase constitutions and microstructural characteristics of SLM-processed Ti parts experienced a successive change on increasing the applied scan speeds: relatively coarsened lath-shaped alpha -> refined acicular-shaped martensitic alpha' -> further refined zigzag-structured martensitic alpha', due to the elevated thermal and kinetic undercooling and attendant solidification rate. The optimally prepared fully dense Ti parts had a very high hardness of 3.89 GPa, a reduced coefficient of friction of 0.98 and wear rate of 8.43 x 10(-4) mm(3) N-1 m(-1) in dry sliding wear tests. The formation of an adherent, plastically smeared tribolayer on the worn surface contributed to the enhancement of wear performance. (C) 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.