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.

Comparison of the Microstructural, Mechanical and Corrosion Resistance Properties of Ti6Al4V Samples Manufactured by LENS and Subjected to Various Heat Treatments

“…Our microstructural observations were confirmed by XRD ( Figure 7 ), where the XRD pattern for the commercial implant is characterized by the presence of peaks originating from the α and β phases. For additively manufactured materials, only o peaks originating from the α’ phases are observed, which is typical for additively manufactured Ti6Al4V alloys [ 13 , 14 , 15 , 16 , 17 , 42 , 43 ]. For the remaining samples, both annealed and HIP-processed, peaks originating from the α and β phases can be observed.…”

Ti6Al4V-ELI Alloy Manufactured via Laser Powder-Bed Fusion and Heat-Treated below and above the β-Transus: Effects of Sample Thickness and Sandblasting Post-Process

“…In particular, the as-built UTS and YS always decrease for each heat treatment performed on the as-built samples [12]. As a matter of fact, also considering a fully martensitic microstructure obtained in solution heat-treated and water-quenched samples, the UTS and YS are lower than 1.0 GPa and 950 MPa, respectively, due to the presence of recrystallized equiaxed β-grains [32][33][34]. At the same time, the elongation values do not show continuous growth with increasing heat-treating temperatures and time due to the following reasons: firstly, the presence of columnar β-grains, which significantly influence the fracture mechanisms, remains even after the heat treatments performed below the β-transus; secondly, defects generated during the L-PBF process play a fundamental role in the premature failure of samples [12,35].…”

Direct Powder Forging—A New Approach for near Net Shape Processing of Titanium Powders

“…Although the sample was annealed at 800 • C the preferred orientation remains. Regarding the β-phase, no clear β peak was identified in the as-DPF sample, it is important to point out a slight broadening of the (1 0 1) alpha-peak which could be evidence of some remaining β-phase that did not transform into martensite during the rapid cooling from the forging [31], see detailed view in Figure 7's upper right corner. Although we cannot affirm the existence nor absence of the β-phase by XRD analysis, a further microstructural examination was needed to correlate the XRD evidence.…”