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 °C to 1020 °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 β 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 β grain, only leading to the acicular α′ martensite decomposition into the α platelet and whether surrounded β phase or transformed α′ phase depending on the heating temperature; while the supertransus heat treatment would thoroughly break up the original long columnar β grain, leaving only big original equiaxial β grain filled with the new forming weave-type acicular α′ 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 °C, martensitic refinement effect at around 1000 °C, and softening effect resulting from the completely decomposition of the martensite at around 875 °C.