Role of Martensite Decomposition in Mechanical Property of Selective Laser Melted Ti-6Al-4V

In literatures of Selective Laser Melted (SLMed) Ti-6Al-4V, the popular post-SLM annealing scheme is 2h at temperatures from 700 °C to 850 °C, and all the reported ductility values are below 15%. In this work, TEM results showed 2h annealed samples at 700 and 800 °C still have retained martensite containing twins and a high density of dislocations. The retained twin structure was examined by TEM and determined as {10-11} <10-12> type twin, which normally formed during martensitic transformation in Ti alloys. To the authors’ knowledge, this is the first {10-11} twin reported in SLMed Ti-6Al-4V.

The incomplete martensite decomposition was identified to result in an inadequate ductility. After a prolonged post-SLM annealing treatment at 800 °C for 6h, an equilibrium microstructure of fine α + β lamellae was obtained, which provided a ductile SLMed Ti-6Al-4V with an elongation at approximate 18% without sacrificing the strength. Experiments were systematically conducted on the influences of annealing temperatures and time on martensite decomposition, microstructure evolution, and resultant mechanical property by XRD, SEM, and TEM.