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

Abstract

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 (α′) martensitic structure. Annealing and solution treatments as post-heat treatments of α′ 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 °C and 1020 °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 °C followed by furnace cooling resulted in the best possible combination of ductility (13%) and tensile strength (σy = 932, σu = 986 MPa) with a microstructure consisting mainly of 78.71% α and 21.29% β. Heat treatment at 850 °C followed by water cooling was characterized by a reduction in hardness and the formation of predominantly α plus α′′ and a small amount of β. 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 °C and subsequent cooling in the furnace was characterized by the formation of an α + β lamellar microstructure. In contrast, heat treatment at 1020 °C and subsequent water cooling formed semi-equiaxial β grains of about 170 µm in diameter with longer elongated α grains and basket-weave α′. Post-treatment at 1020 °C followed by furnace cooling showed high ductility with an elongation of 14.5% but low tensile strength (σy = 748, σu = 833 MPa). In contrast, post-treatment at 1020 °C followed by water cooling showed poor ductility with elongation of 8.6% but high tensile strength (σy = 878, σu = 990 MPa). The effect of aging at 550 °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 β, α, and α″ precipitation and fragmentation or globularization of elongated α grains. The aging process at 550 °C leads to an increase in tensile strength and a decrease in ductility.