This paper investigates the change in solidification microcrack susceptibility under the influence of thermal-shock-induced effects for pulsed laser spot welding molten pools with different waveforms, powers, frequencies, and pulse widths. During the welding process, the temperature of the molten pool under the effect of thermal shock changes sharply, triggering pressure waves, creating cavities in the molten pool paste area, and forming crack sources during solidification. The microstructure near the cracks was analyzed using a SEM (scanning electron microscope) and EDS (electronic differential system), and it was found that bias precipitation occurred during the rapid solidification of the melt pool, and a large amount of Nb elements were enriched in the interdendritic and grain boundaries, which eventually formed a liquid film with a low melting point, known as a Laves phase. When cavities appear in the liquid film, the chance of crack source formation is further increased. Using a slow rise and slow fall waveform is good for reducing cracks; reducing the peak laser power to 1000 w is good for reducing cracks in the solder joint; increasing the pulse width to 20 ms reduces the degree of crack damage; reducing the pulse frequency to 10 hz reduces the degree of crack damage.
Keywords: heat-affected zone; microcrack; pulsed laser spot welding; thermal shock.