Study On Fine-grain Size Titanium6Al-4V Alloy Material For Low Temperature Superplasticity

Abstract:Fine-grained titanium6Al-4V alloy, which typically has a grain size of about2μm, can be made to superplastic form at around800℃. The normal temperature for superplastic forming (SPF) with conventional titanium6A1-4V sheet material is900℃. The lower temperature performance is of interest to scientists because it can be exploited to achieve significant cost savings in processing by reducing the high temperature oxidation of the SPF dies, improving the heater rod life for the hot presses and increasing operator safety.Superplastic tensile deformation behavior was studied on fine-grained TC4alloy sheets which are achieved from annealing treatment at temperature of700～850℃and strain rate of3×10-4s-1～5×10-3s-1. And the effect of the factors such as grain size and deformation temperature on the properties of the alloy were analyzed on superplastic deformation capacity. The microstructure before and after superplastic tensile deformation was observed and compared through SEM, TEM, EBSD, etc. Changes of microstructure after superplastic deformation and different deformation mechanisms at different temperatures were analyzed deeply. Based on superplastic deformation mechanisms, the superplastic deformation models of TC4alloys were established and conditions for low temperature superplasicity were analyzed and discussed.The results show that, fine-grained Ti-6A1-4V alloys has excellent superplasticity. When the TC4alloy with grain size of2.5μm deform at the temperature of800℃and strain rate of1×10-3s-1, the highest elongation of862%can be obtained with the m value of0.6. Even at700℃, the elongation was found to be as high as516%. Moderate annealing temperature can guarantee the superplastic deformation act smoothly, as excessive annealing temperature would coarsen the grains. With the increase of deformation temperature, the elongation of TC4alloys increase first and the decrease, different microstructures of TC4 alloys has different optimum deformation temperatures. In addition, moderate strain rate is the key to achieve high elongations.Moreover, through the analysis of deformation mechanisms at different temperatures, it indicates that at deformaion tempertures below800℃, the deformation activation energy is much more than the grain boundary self-diffusion energy. The deformation mechanism is the movements of dislocations and dynamic recovery, with isostrain-rate model. At deformation temperatures of800℃and above, the deformation activation energy is similar to the grain boundary self-diffusion energy. The deformation mechanism is mainly GBS and dynamic recrystallization, with isostress model. Stress-induced phase transformation can change the volume fraction of β phase significantly, thereby affects the superplasticity of TC4alloys. Fine grains, narrow grain size distribution, low grain aspect ratio and high volume fraction of β phase can contribute to low-temperature superplasticity. For the present TC4alloy, The optimum superplastic deformation temperature is50℃-100℃lower than the regular optimum temperature.