The microstructure transformation of selective laser melted Ti-6Al-4V alloy

The microstructure transformation of Ti-6Al-4V was closely related to the complex thermal history in the selective laser melting (SLM) process. In this paper, the surface morphology and microstructure of SLM fabricated Ti-6Al-4V were characterized to explore the densification behavior and microstructure formation mechanism. The in-depth relationship between the bulk energy density, thermal cycle and microstructure transformation was explored. The results revealed that the low bulk energy density of 25 J/mm(3) resulted in disordered solidification front and considerably terrible densification behavior. The microstructure characteristics and phase constitutions experienced a successive transformation as the bulk energy density increased: ultrafine Z-zigzag martensite alpha'- fine acicular martensite alpha'- coarse lamellar alpha. Moreover, the hierarchical martensite phase was differentiated into primary alpha', secondary alpha'2, tertiary alpha'3, quartic alpha'4 and the dots or rods of alpha phase according to the morphology and transformation sequence. Furthermore, the hierarchical substructure formation and transformation mechanism based on the thermal cycle were proposed. Due to the present work, the control of phase constitution and martensitic morphology of fabricated Ti-6Al-4V can be realized by regulating the bulk energy density in SLM process.