采用搅拌摩擦焊(friction stir welding, FSW)实现了2 mm厚TC4钛合金连接, 结合数值模拟结果研究了温度分布对焊缝沿厚度方向显微组织特征和接头力学性能的影响规律. 结果表明, 当焊接速率为50 mm/min且转速为300 r/min时, 靠近焊缝表面的材料温度峰值超过b相变温度. 随着到焊缝表面距离的增加温度峰值逐渐变小, 靠近焊缝底部的材料未超过b相变温度. 在温度峰值超过b相变的焊缝区域, 显微组织是由初生a相、板条状a相和剩余转变b相组成, 且焊缝内部的板条状a相尺寸大于近表面区域. 焊缝底部受到动态再结晶作用, 呈现尺寸较小的a和b双相组织, 且b相在a相基体上分布更均匀. 当转速提高到350 r/min时, 沿焊缝厚度方向上的超过b相变温度的区域变宽, 板条状a相所占面积和尺寸增大, 组织中出现板条状a相丛. 取向不同的板条状a相散乱分布于组织中, 阻碍裂纹扩展, 利于接头的抗拉强度.

As a solid state technology, friction stir welding (FSW) has been used to join titanium alloys for avoiding the fusion welding defects. So far, many previous studies have attempted to elucidate the microstructure characteristics and evolution during the FSW process of titanium alloy, but few are about the mechanism of microstructure transformation along the thickness direction of joint. For solving this problem, in this work, 2 mm thick TC4 titanium alloy is successfully welded by FSW. On the basis of numerical simulation, the effects of temperature distribution on the microstructure along the weld thickness direction and the tensile strength of welding joint were investigated. The results show that the peak temperatures of material close to weld surface exceed b phase transus temperature under the rotational speed of 300 r/min and the welding speed of 50 mm/min. With the increase of distance away from the weld surface, the peak temperature decreases. The peak temperature of weld bottom near the backing board is difficult to be higher than b phase transus temperature owing to quick heat radiation. The region, where the peak temperature is higher than b phase transus temperature, consists of primary a, lath-shape a and residual b phases. The size of lath-shape a inside the weld is larger than that near the weld surface. Primary a and b phases with smaller size are attained in the weld bottom owing to the dynamic recrystallization, and the distribution of b phase on primary a matrix is more homogeneous. When the rotational speed reaches 350 r/min, the area where the peak temperature is higher than b phase transus temperature becomes wider along the thickness direction, which makes the size and quantity of lath-shape a phase increase and then the lath-shape a clump appears. Lath-shape a phase with different orientations hinder the propagation of crack and be beneficial for the tensile strength of FSW joint.