Microstructural evolution and microhardness of a selective-laser-melted Ti-6Al-4V alloy after post heat treatments

Microstructure and hardness of a powder-bed-type selective laser melted Ti-6Al-4V alloy after post heat treatments at from 300 degrees C to 1020 degrees C were systematically investigated by using optical microscope (OM), scanning electron microscope (SEM), transmission electron microscope (TEM) and Vickers hardness (HV) tester. Long columnar original beta grains together with the inside dominated parallel acicular martensite in the side view, and chessboard pattern in the top view, were found in the as-received specimen. The subtransus heat treatment does not enable modification of the morphology of the original columnar beta grain, only leading to the acicular alpha' martensite decomposition into the alpha platelet and whether surrounded beta phase or transformed alpha' phase depending on the heating temperature; while the supertransus heat treatment would thoroughly break up the original long columnar beta grain, leaving only big original equiaxial beta grain filled with the new forming weave-type acicular alpha' martensite like the supertransus heat treated wrought specimen. Vickers hardness evolution strictly follows the trend of the microstructural change as the heating temperature increasing, and the double peak phenomenon of the Hardness-Temperature plot should be attributed to substructural refinement effect at around 500 degrees C, martensitic refinement effect at around 1000 degrees C, and softening effect resulting from the completely decomposition of the martensite at around 875 degrees C. (C) 2016 Elsevier B.V. All rights reserved.