TC4钛合金筒形收口件超塑胀形数值模拟及试验研究

Numerical Simulation and Experimental Study on Superplastic Bulging of TC4 Alloy Necking Cup

对TC4钛合金进行超塑拉伸试验,研究了变形温度为800~950℃、应变速率为0.0002~0.005 s^(-1)下的超塑变形特性并建立相应的Backofen本构方程。研究表明:最佳超塑温度变形为850℃,应变速率为0.0005 s^(-1),在该参数下,TC4钛合金的延伸率为788%,应变速率敏感系数m为0.60。将Backofen方程导入ABAQUS软件并在最佳超塑参数下进行超塑胀形数值模拟。超塑胀形后筒形收口件的应力、应变和厚度均呈对称分布,顶部区域最先贴模,中部收口段和顶部的过渡区域最后贴模,应力、应变最大值和厚度最小值均出现在过渡区域;优化后的保压胀形的最大压强为1.90 MPa。超塑胀形试验结果与数值模拟结果基本一致,厚度相对误差为6.98%。超塑胀形后不同部位的微观组织观察结果表明:超塑胀形后晶粒明显长大,其中顶部粗化程度最大,为17.9μm,中部收口段粗化程度最小为14.7μm。由于不同部位变形方向不同,晶粒的取向发生改变,端口部和顶部的晶粒在[0001]方向具有取向,中部晶粒在[2110]方向具有明显取向。变形量较小的端口处α晶粒内部发生大量的位错增殖,在应变诱导下,变形量较大的中部α晶内产生针状马氏体,变形量最大的顶部有板条状马氏体生成。与原始管坯相比,在薄弱区的顶部直筒段,抗拉强度得到提高,屈服强度和延伸率略有下降。

T hrough superplastic tensile test,the superplastic deformation characteristics at the deformation temperature of 800—950℃and strain rate of 0.0002~0.005 s^(-1) were studied and the corresponding Backofen constitutive equation was established.The optimum superplastic bulging temperature of TC4 alloy is 850℃,and the strain rate is 0.0005 s^(-1).Under this parameter,the elongation of TC4 alloy is 788%,and the strain rate sensitivity coefficient m is 0.60.The superplastic rheological constitutive model of TC4 alloy Backofen established by true stress-strain curve was applied to the superplastic bulging numerical model established by ABAQUS numerical simulation software.The results show that stress,strain and thickness are symmetrically distributed during superplastic bulging.Due to the difference of die sticking sequence,the top area of the tube piece is stuck first,while the middle necking section and the transition area at the top are stuck last,and the maximum value of stress and strain and minimum value of thickness all appear in the transition area.The maximum pressure of pressure-keeping bulging is 1.90 MPa.The experimental results are basically consistent with the numerical simulation results,and the relative error is 6.98%.The microstructure after superplastic bulging is observed.The results show that the grain size of TC4 titanium alloy tube necking part grows obviously after superplastic bulging.The grain size is 17.9μm at the top and 14.7μm at the middle.Due to the different deformation directions in different parts,the orientation of grains changes.The grains at the port and top have orientation in the[0001]direction,while the grains in the middle have obvious orientation in the[2110]direction.A large number of dislocations proliferate in theαgrain at the port with small deformation.Under strain induction,needle martensite is produced in theαgrain at the middle necking section with the largest deformation,and lath martensite is produced at the top with the largest deformation.Mechanical tests show that the tensile strength of the straight tube section at the top of the weak zone is improved,while the yield strength and elongation are slightly decreased,compared with the original tube blank.