Mechanical Performance of Laser Welded and Resistance Spot Welded Overlap Joints from Magnesium AZ31B Sheets

Mg is already used as a cast alloy in the automotive industry. Using Mg wrought alloys - e.g., in the form of sheets - will allow for further weight reduction potential. In automobile manufacturing, laser beam welding is concurring to resistance spot welding due to its high process flexibility and efficiency. The rule of thumb is that a joint can be laser welded if it can be seen, which allows for complex structures to be realised. Nevertheless, studies concerning the mechanical performance of laser welded Mg overlap joints are scarce. The aim of this work was to develop a process for laser welding single overlap joints of 2.5 mm thick Mg sheets, AZ31B, and investigate the mechanical performance of these joints relative to resistance spot welded joints. An Nd:YAG laser was used for the welding experiments, and a design of experiment approach was applied. Both mechanical testing and advanced metallurgical examinations were used to establish the process-property-performance relationship of the overlap joints. The micro-hardness, shear tensile and fatigue tests were implemented for the mechanical characterisation. To improve the fatigue behaviour, different joint geometries (linear, circular and C-shape) were investigated. The developed surface preparation provided welds with optimal bead geometry and without cracks and pores. Compared with conventional resistance spot welds, laser welded joints showed improved fatigue behaviour with comparable performance for the tensile shear strength. The developed laser process can also be implemented via remote scanner welding. For example, it is expected that using a scanner optic increases the throughput and reduces the cost of automotive production