Pulsed laser beam welding of thin aluminium alloy sheets is increasingly employed in lightweight structures, including prismatic battery casings for e-mobility applications. However, defects such as hot cracking remain a critical challenge during solidification, with different underlying mechanisms, such as tensile strain development. In the current study, in-situ high-speed synchrotron X-ray diffraction and digital imaging were used to study time-dependent strain evolution around the melt pool region and its effects on hot crack initiation under varying laser welding pulse shapes. The results reveal a strong agreement between the temporal strain evolution determined by time-resolved X-ray diffraction measurements and the temporal evolution of hot cracks at the surface while using the rectangular pulse shape, indicating an internal stress drop about 1 ms after the end of the laser pulse and the appearance of surface hot cracks directly thereafter. In contrast, the use of a rampdown pulse shape effectively suppressed hot cracks despite its much higher energy input and much higher strain maxima in the welded sample by delaying the peak strain and reducing the strain rate afterwards. The present study establishes a practical approach for understanding and monitoring the hot crack formation mechanism during pulsed laser welding and enabling calibration of physics-based models.