Effect Of Heat Treatment On The Mechanical Behavior Of Selective Laser Melted Ti-6Al-4V By In Situ X-ray Characterization

Ti-6Al-4V alloys prepared by selective laser melting(SLM)generally have high yield strength and low ductility.Heat treatment is a popular way to improve the low ductility of this material.However,the mechanism of this ductility improvement has not been clearly explained and understood.In order to understand the effect of heat treatment,in situ high energy X-ray diffraction(HEXRD)is performed.Selective Laser Melted Ti-6Al-4V(as-SLMed)exhibits decreased yield strength,increased work hardening,and increased ductility after heat treatment at 730?(HT-730)or 900?(HT-900)for 2h.The as-SLMed specimen is dominated by acicular martensitic??.After heat treatment,??partly or fully decomposes into?+?phase,reducing the yield strength.In HT-730,?precipitates with confined size show much higher lattice strain than the??/?matrix during plastic deformation;in HT-900,the lattice strain difference is mostly eliminated.This is the main reason for the increased ductility in HT-900.From the lattice strain development,basal slip is identified as the easiest slip system in hexagonal close packed(HCP)??/?.Using an elasto-plastic self-consistent(EPSC)model,the critical resolved shear stress ratio between prismatic slip and basal slip(CRSS_prismatic/CRSS_basal)is estimated to be 1.31 and 1.16 in the as-SLMed and the HT-900 specimens,respectively.The?phase in HT-900is able to activate multiple slip systems and accumulate more dislocations during plastic deformation.This explains why the sample after heat treatment at 900°C has better ductility and higher work hardening rate than the other two samples.The result in this work sheds light on how to optimize the mechanical properties by adjusting microstructure of Ti-6Al-4V fabricated by selective laser melting.