Cracking in spot welding aluminum alloy AA5754

The phenomenon of cracking was observed during resistance spot welding a co mmercial aluminum alloy AA5754, and mechanisms of cracking and healing are discussed in this paper. Metallographic study of welded coupons revealed cr acks located on only one side of a weldment in the heat-affected zone (HAZ) , with respect to the welding sequence. Cracks are visible from longitudina l cross sections only. Some of them are partially or fully filled. Crack ap pearance and orientation are fairly repeatable and their intergranular char acteristics and dendritic fracture surface morphology prove they were forme d at elevated temperatures in the presence of liquid metal. The discussion of metallurgical factors considering the AI-Mg equilibrium phase diagram an d the possible temperature histories of various zones in a weldment during spot welding elucidated the approximate conditions for cracking during spot welding and for mending the structure. A thermomechanical analysis reveale d a high possibility for tensile stress buildup on the cracked side of the weldments as a result of material flow, thermal stress development and loca lized straining.