Effects of Annealing and Solution Treatments on the Microstructure and Mechanical Properties of Ti6Al4V Manufactured by Selective Laser Melting

Ti6Al4V (Ti64) alloys manufactured by selective laser melting (SLM) are well known for their susceptibility to failure at a low ductility of less than 10% due to the formation of an (alpha ') martensitic structure. Annealing and solution treatments as post-heat treatments of alpha ' are considered a good way to improve the mechanical performance of SLM-manufactured Ti64 parts. In this research, the effect of heat treatment parameters such as temperature (850 degrees C and 1020 degrees C) and cooling rate (furnace and water cooling) on the microstructure and mechanical properties of the SLM Ti64 structure was investigated. It was shown that the tensile strength/ductility of the Ti64 alloy produced by SLM was determined by the post-heat treatment. The experimental results revealed that heat treatment at 850 degrees C followed by furnace cooling resulted in the best possible combination of ductility (13%) and tensile strength (sigma(y) = 932, sigma(u) = 986 MPa) with a microstructure consisting mainly of 78.71% alpha and 21.29% beta. Heat treatment at 850 degrees C followed by water cooling was characterized by a reduction in hardness and the formation of predominantly alpha plus alpha '' and a small amount of beta. HT850WC exhibited yield and tensile strengths of about 870 and 930 MPa, respectively, and an elongation at fracture of 10.4%. Heat treatment at 1020 degrees C and subsequent cooling in the furnace was characterized by the formation of an alpha + beta lamellar microstructure. In contrast, heat treatment at 1020 degrees C and subsequent water cooling formed semi-equiaxial beta grains of about 170 mu m in diameter with longer elongated alpha grains and basket-weave alpha '. Post-treatment at 1020 degrees C followed by furnace cooling showed high ductility with an elongation of 14.5% but low tensile strength (sigma(y) = 748, sigma(u) = 833 MPa). In contrast, post-treatment at 1020 degrees C followed by water cooling showed poor ductility with elongation of 8.6% but high tensile strength (sigma(y) = 878, sigma u = 990 MPa). The effect of aging at 550 degrees C for 3 h and cooling in a furnace on the microstructure and mechanical properties of the specimens cooled with water was also studied. It was found that aging influenced the microstructure of the Ti6Al4V parts, including beta, alpha, and alpha '' precipitation and fragmentation or globularization of elongated alpha grains. The aging process at 550 degrees C leads to an increase in tensile strength and a decrease in ductility.