以激光焊接平面环形结构时, 在首尾段重叠焊接部分发生的凝固热裂纹为例,采用实验和数值模拟相结合的手段, 从冶金和力学2个方面综合研究了环形焊缝凝固热裂纹的发生机理. 对凝固热裂纹附近的微观组织进行观察,并用电子背散射衍射对裂纹附近晶粒尺寸进行测量, 发现重叠焊缝中心部位晶粒粗大, 而粗大晶粒导致焊缝金属凝固时韧性下降. 利用有限元数值模拟对平面环形结构激光焊接过程的温度场和应变场进行计算, 与单道焊接相比,重叠焊接的焊缝金属在凝固温度区间内冷却速率较低, 应变随温度的变化速率较低,产生热裂纹的力学驱动力相应较弱. 因此, 重叠焊接部分晶粒粗大化导致的低韧性是平面环形激光焊缝凝固热裂纹发生的主要原因.

The combined influence of metallurgical and mechanical factors on welding solidification cracking structure in laser welded ring was investigated by using both experiments and numerical simulation. It is found that solidification cracking is directly related with the overlap weld part. The microstructure was observed under OM, and the grain size was measured by EBSD observation. The grains in overlap weld part are much bigger than those in single weld part, which results in falling of ductility during solidification. The temperature and strain changes of weld metal in solidification temperature range were calculated out by using 3D finite element analysis. Compared with the single weld part, the strain rate with temperature drop is lower in overlap weld part, i.e., the mechanical driving force to solidification cracking is weaker in overlap weld part. Consequently, it is concluded that the low ductility caused by coarse grain results in the occurrence of solidification cracking in overlap weld part during laser welding of ring structure. This research not only helps to understand the mechanism of solidification cracking in more details, but also guides the study on prediction of solidification cracking occurrence.