High-speed observation of visible and infrared radiation was performed to measure the molten poolgeometry, velocity of the solid–liquid interface and temperature profile during laser spot welding of alu-minum. Hot cracking occurred at a late stage of solidification for the investigated laser pulse shapes. Hotcracking could be minimized by using a pulse shape with two distinct power levels and a final coolingslope to shut down the laser power. The drop of the laser power from the first to the second powerlevel led to a high cooling rate and high interface velocity at the beginning of solidification. This drop intemperature and molten pool diameter released strains originating from thermal contraction and solid-ification shrinkage at the beginning of solidification, where spot welding is expected to have a higherductility. After this initial high solidification rate, low interface velocities were observed during solidi-fication within the second power level. The final solidification rate increased again as a function of thecooling time of the last laser pulse section. This type of solidification process was also found in metallo-graphic microstructures with greater dendritic structures during solidification within the second powerlevel. The strain release at the beginning of solidification minimized residual strains for the remainingsolidification, so that crack-free and full penetration bead-on-plate seam welding with overlapping spotwelds was possible.