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. In the case of laser welded planar ring structure, the case of the solidified hot crack in the overlapped welding section at the end and the tail is taken as an example. By means of the combination of experiment and numerical simulation, the occurrence of the hot crack in the annular weld is studied synthetically from two aspects of metallurgy and mechanics The microstructure of the weld near the hot crack was observed and the grain size near the crack was measured by electron backscatter diffraction. It was found that the grains in the center of the overlap weld were coarse, while the coarse grain caused the toughness of the weld metal to decrease. The temperature field and strain field in laser welding of planar ring structure are calculated by finite element numerical simulation. Compared with single pass welding, the weld metal of overlap welding has a lower cooling rate in the solidification temperature range, the rate of change of strain with temperature The mechanical driving force of generating hot cracks is relatively weak, therefore, the low toughness caused by the coarsening of the overlapped welding parts is the main reason for the occurrence of the hot cracking in the flat annular laser weld.