Investigating delayed cracking behaviour in laser welds of high strength steel sheets using an X-ray transmission in-situ observation system

We have developed a technique for observing the weld cracking behaviour in laser welds of high strength steel sheets, using an X-ray transmission in-situ observation system. Weld cracking in the lap joints of this type of steel could be clearly seen, demonstrating that this was, in fact, delayed cracking initiated from the solidification crack at the weld end. It was also discovered that the delayed cracking behaviour such as cracking modes, incubation periods and crack growth rates, varied according to the carbon content in the steel and welding length. The former influenced the hardness of the weld bead, while the latter affected solidification cracking sizes and residual stress. The results of this crack propagation are useful not only for developing laser welding methods of high strength steel but also for additive manufacturing, aimed at mass production.