Thermal Deformation Behavior Of Diffusion-Connected Titanium Alloys At High Temperature

In this paper,TC4 titanium alloy material as the test object,under the test temperature of 920,950,980,1010℃and strain rate of 0.01,0.1,1,10 s^-1,TC4 titanium alloy diffusion joint region of high temperature compression test.The influence law of deformation temperature and strain rate on the flow stress and microstructure of TC4 titanium alloy diffusion joint zone was studied.In order to more accurately analyze the high temperature deformation characteristics of TC4 titanium alloy diffusion joint zone,the thermal working diagram of TC4 titanium alloy diffusion joint zone was established according to the dynamic material model and the theory of dissipative structure.The forging test of diffusion-connected TC4 titanium alloy was carried out in the safe zone of the hot working diagram to explore the effect of forging deformation on the microstructure and mechanical properties of the diffusion-connected interface.The results showed that:The microstructure at the diffusion connection interface of TC4 titanium alloy is consistent with that of the original bar,and is composed of equiaxedαphase and a small amount ofβphase.No obvious defects such as holes were found at the interface,indicating that the TC4 titanium alloy bar interface obtained by the diffusion connection process of furnace cooling with temperature of 950℃,pressure of 140 MPa and holding time of 4 h was well combined,and complete metallurgical bonding was achieved.The alloy-element distribution in the region where the diffusion connection interface was located was uniform.No segregation of alloying elements was found,but an oxide film was formed on the surface of the diffusion end of the bar due to the high temperature activity of titanium elements and oxygen.The diffusion joint zone of TC4 titanium alloy has obvious dynamic softening characteristics during high temperature compression.The flow stress decreases with the increase of deformation temperature and increases with the increase of deformation rate.After high temperature deformation,the diffusion interface disappeared,the volume fraction of equiaxedαphase decreased with the increase of deformation temperature,and the secondaryαphase appeared with short rod and strip.When the deformation temperature reached 1010℃,the martensiteα’phase appeared.The volume fraction of secondaryαphase decreases with the increase of strain rate.Based on the hyperbolic sinusoidal modified Arrhenius equation,the hyperbolic sinusoidal constitutive equation was established.According to the basic theory of irreversible thermodynamics and dissipative structure as well as the Prasad stability zone criterion,the power dissipation diagram and instability diagram of TC4 titanium alloy diffusion joint zone were successfully drawn.Finally,the thermal working diagram of TC4 titanium alloy diffusion joint zone was established.The optimum deformation temperature and deformation rate of TC4 titanium alloy are determined to be 920～950℃and 0.01～0.1 s^-1.After the high temperature forging of TC4 titanium alloy with diffusion connection,the appearance quality of the forging is good,which meets the shape requirements of the cylindrical component after upsetting deformation,and there is no cracking phenomenon at the diffusion connection interface of the forging.The microstructure of forging is composed of equiaxedαphase,secondaryαphase and a small amount ofβphase.When the forging deformation increases,the size of the equiaxedαphase decreases gradually,the volume fraction of the secondaryαphase increases gradually,and the tensile strength at room temperature increases.When the forging deformation is 40%,the content of equiaxedαphase and secondaryαphase reaches the best matching degree.The tensile strength at room temperature reaches 950 MPa and the elongation reaches 17.5%.After forging,the fracture mode changes to ductile fracture.