电辅助成形工艺对TC4钛合金微观组织及拉伸性能的影响

Influence of Electric-Assisted Forming Process on Microstructure and Tensile Properties of TC4 Titanium Alloy

摘要:

通过对航空材料TC4钛合金在拉伸过程中通入不同频率的电流，以此来增加材料的流动应力并降低拉伸过程中的最大屈服强度。采用SEM观察通电后材料组织的演变及断口形貌。结合拉伸实验结果来分析电辅助成形工艺在拉伸过程中的影响。结果表明，随着脉冲电流密度的增加，α相的含量大大降低，β相含量大大增加，其晶粒尺寸开始增大，在冷却过程中存在少量的马氏体相变生成的细小的针状α′相；随着电流密度的进一步增加，初生α相完全消失，β相进一步的长大，转变生成的α′相尺寸有所增大。在拉伸过程中，施加电流的瞬间试样的温度急剧上升，随着拉伸过程的进行，温度持续上升，且上升的速度不断加快，在断裂的瞬间达到峰值；峰值温度随着电流密度和脉冲频率的增加而增加，随着电流密度的升高，TC4钛合金的流动应力逐渐下降，塑性有所提升。SEM及TEM结果表明，随着电流密度的增加，TC4钛合金板材拉伸断口中的韧窝显著加深，微观断口表面呈现出蜂窝状，韧窝周围出现撕裂棱，呈现出明显的韧性断裂的特征。与高温和常温拉伸对比，电辅助拉伸后材料内部的位错密度显著下降，位错较为平直且部分位错沿某一方向有序排列，这表明了脉冲电流对位错运动的促进作用。

Abstract:

By applying different current frequencies during the tensile process of the aerospace TC4 titanium alloy, the flow stress of the material is increased and its maximum yield strength is reduced. The microstructural evolution of the material after electrification and the fracture morphology of the samples were observed. The influence of the electric-assisted forming process on the tensile process was analyzed in combination with the tensile test results. The experimental results show that with the increase in pulse current density, the content of the α phase decreases significantly, while the β phase content increases substantially, and the grain size begins to increase. A small amount of martensitic phase suffers transformation during cooling, resulting in fine acicular α′ phase. As the current density further increases, the primary α phase disappears completely, the β phase grows further, and the size of the transformed α′ phase increases. During tensile deformation, the sample temperature rises sharply at the moment when current is applied. It continues to increase during the tensile process, with rising increment until it reaches a peak value at the moment of fracture. The peak temperature increases with the rise in current density and pulse frequency. As the current density increases, the flow stress of TC4 titanium alloy gradually decreases, and its ductility improves. SEM and TEM results show that with the increase in current density, the dimples in the tensile fracture surface of TC4 titanium alloy sheets become significantly deeper, presenting a honeycomb-like appearance, with tear ridges around the dimples, indicating a typical ductile fracture feature. Compared with that after high-temperature and room-temperature tensile tests, the dislocation density inside the material after electric-assisted tensile tests is significantly reduced, with dislocations appearing more straight and some dislocations orderly aligned in a certain direction, indicating that pulse current promotes dislocation motion.