Direct Observation of Weld Pool Formation and Cracking in Laser Welding of Al Alloy by Using X-ray Phase Contrast Method

摘要

Aluminum alloy is lightweight material used in automobiles, batteries, and airplanes, etc, to reduce greenhouse gases. A highly efficient and stable laser welding method has been sought for Al alloys for a long time because of its poor weldability due to its high heat conductivity and high reflectivity to 1 μm wavelength laser light. There is still a lack of understanding of the welding phenomena and the defect formation mechanism because of the complexity of those phenomena. The objective of this research was to investigate laser welding phenomena of Al alloy, especially crack formation and weld pool behavior in butt welding. To observe the cracking and weld pool dynamics during welding, the weld pool was observed with the X-ray phase-contrast method. The observations of the keyhole and molten pool in butt welding indicated that, unlike in bead-on-plate welding, the front part of the molten pool is unstable, and as a result, a huge pore forms at the bottom of the weld. The reason is assumed to be fluctuation of heat transfer based on the deviation of the laser radiation position and the gap between the specimens. By using the X-ray phase contrast method, solidification cracking was observed in spot welding of 5000 series aluminum alloy. Cracks occurred immediately after solidification at a speed of 320 mm/s or more in the laser irradiation direction. The solidification speed was around 190 mm/s.