Effect Of Heat Treatment Process On Microstructure And Properties Of Ti-6Al-4V Titanium Alloy

In the paper, the effects of solution and aging treatment and double heat treatment process on microstructure and mechanical properties of Ti-6A1-4V alloy were investigated through heat treatment experiment, mechanical properties test, optical microscope observation(OM), scanning electronic microscope analysis(SEM), X-ray difraction analysis(XRD). The results show that the microstructure of Ti-6A1-4V alloy was consisted of a phase and undissolved martensite phase after solution treated at 900～920℃ and aging at 500℃. When solution treated above 940℃ and aging at 500℃, primary a phase was disappeared, we could get a’ phase and reacted β phase. With the increase of temperature, a’phase became coarse, the microstructure of the alloy became bimodal structure. The Ti-6A1-4V alloy exhibited best strength and better plasticity when solution treated at 920℃. After solution treated above 920℃, the strength and plasticity both decreased gradually. After solution treated at 920℃ and aging at 500℃, the tensile strength of Ti-6A1-4V alloy was 1166MPa, the elongation was 8.5%, the reduction of area was 26.9%, the impact energy was 42.8J.When solution treated at 920℃ and holding time increased, primary a phase was dissolved and the size decreased gradually, the content of a’phase increased. The Ti-6A1-4V alloy had better mechanical properties after holding 1h, the tensile strength was 1053MPa, the elongation was 9.2%, the reduction of area was 25.5%, the impact energy was 43.8J. With different cooling rate after solution treated at 920℃, duplex microstructure could be obtained by air cooling and commercial quenching liquid cooling, the microstructure was composed of a phase and undissolved a’phase by water cooling and oil cooling. The content of a phase gradually increased with the decrease of cooling rate, the content of P phase decreased. The Ti-6A1-4V alloy got the better comprehensive mechanical properties with water cooling, the hardness was 36.4HRC, the tensile strength was 1070MPa, the elongation was 9.5%, the reduction of area was 32.5%, the impact energy was 39.5J.The tensile strength and hardness of Ti-6A1-4V alloy increased with aging temperature below 500℃, then decreased above 500℃ aging, with the increase of aging temperature after solution treated at 920℃. The strength was max of 1166MPa at 400～500℃ aging, the plasticity increased with the increase of aging temperature. The a phase hardly decomposed after solution treatment under 500℃ aging, the a’phase decomposed obviously above 600℃ aging, and the a’phase broke down into lamina α+β phase with the increase of aging temperature.When solution treated at 920℃ and aging at 650℃, the tensile strength decreased gradually with aging time increasing, but the extent was not large, the strength was max of 1031MPa at 1h aging, and the elongation and reduction of area gradually increased. When solution treated at 920℃ and aging at 650℃, the quenched structure broke down insignificantly with aging time below 3h, the microstructure significantly broke down over 3h aging, and the martensite completely broke down into strip α+β structure over 8h aging.When double heat treatment process of the Ti-6A1-4V alloy was 920℃×1h, WC+650℃× 3h, AC×3h+920℃×1h, WC+650℃×3h,AC, under which the alloy was consisted of the fine α+ β structure and had better comprehensive mechanical properties, especially the plastic toughness significantly increased.