Effect of heat treatment on microstructure and mechanical properties of TC16 titanium alloy bar

TC16(Ti-3Al-5Mo-4.5V) titanium alloy is a special titanium alloy for fasteners, which has the advantages of high specific strength, small sensitivity to stress concentration such as notch and torsion, good corrosion resistance and non-magnetic, etc., and is widely used in the aerospace field. TC16 titanium alloy fasteners are mainly hot heading and cold heading two preparation methods, the use of cold heading technology does not need to use heating equipment and protective media, high production efficiency, and the production of TC16 titanium alloy fasteners good mechanical properties, high precision, good surface quality, especially suitable for mass production. The results show that the cold heading deformation ability of the material is related to plasticity, shear strength and microstructure. Both the TC16 titanium alloy net basket structure and the equiaxial structure can be cold headed according to the cold heading ratio of 1:4 under certain conditions. Because the medium axis structure is easier to control than the net basket structure in industrial production, this work mainly optimizes the microstructure and mechanical properties of TC16 titanium alloy bars with equiaxial structure by adjusting the heat treatment regime, and provides reference for industrial production.

TC16 titanium alloy bars prepared by hot rolling usually need to be heat treated to eliminate residual stress, soften the alloy and improve the comprehensive properties. The results show that the plasticity of TC16 titanium alloy increases with the increase of recrystallization degree during annealing, and then decreases with the growth of grain size. During heat treatment at low temperature in the two-phase zone, because only recovery and recrystallization occur, the grain is still very fine, its plasticity is good, and the strength is also high. Therefore, the heat treatment temperature of TC16 titanium alloy bars is generally selected at about 780°C.

Comparison of mechanical properties of TC16 titanium alloy bar at room temperature after processing and double heat treatment at different temperatures. As can be seen from FIG. 5b, after double heat treatment, the room temperature strength of TC16 titanium alloy bar increases with the decrease of the second stage annealing temperature, while the plasticity decreases. It can be seen from the microstructure in Figure 3 that after double heat treatment, the lower the second stage temperature, the smaller the grain size, resulting in an increase in strength, but due to poor homogeneity of the organization, resulting in a decrease in plasticity. By comparing the performance data of TC16 titanium alloy bars after double heat treatment and furnace cold treatment, it can be found that the strength of TC16 titanium alloy bars after double heat treatment is obviously higher than that after furnace cold treatment, but the plasticity is reduced, especially when the second stage annealing temperature is 550°C. This is related to the proportion of the primary a phase and the degree of equiaxialization. Comparison of mechanical properties of TC16 titanium alloy bar at room temperature after processing and 780°C holding for 2h and cooling by different methods. It can be seen that the influence of different cooling methods on the tensile strength Rm and shear strength t of TC16 titanium alloy bar is not obvious, but the plastic index 4 and Z appear minimum value when water cooling. This is related to the decomposition of martensite and the precipitation of secondary phase in the cooling process. Martensite did not decompose during water cooling, and the secondary phase was unable to precipitate (see Figure 4), resulting in a decline in plasticity. TC16 titanium alloy bar is kept at 780C for 2h, and the comprehensive performance of air cooling is better after furnace cooling to 530°C.