Improvement of fatigue strength of lap fillet joints by using tandem MAG welding in a 590-MPa-grade galvannealed steel sheet

This investigation is for the lap joint of a 590-MPa-grade steel sheet. The effect of weld bead shape on weld fatigue strength using the tandem metal active gas (MAG) welding process was investigated by comparing with the direct current (DC) and DC pulse welding processes. For the same amount of deposited metal, the wire feeding speed was fixed at 8 m/min and the welding speed at 80 cm/min. The fatigue test was carried out at intervals of 10% from 10 to 60% of the tensile strength of the base material, and an S-N curve was obtained. In the DC and DC pulse processes, the fatigue strengths of the welded parts were 61 and 92 MPa, respectively. In the case of the tandem MAG process, the fatigue strength of the welded part was 122 MPa. The fatigue test was carried out at a frequency of 15 Hz and a load stress ratio of 0.1, with a fatigue life of 2 x 10(6) cycles. To investigate the cause of different fatigue strengths according to the welding process, the geometry of weld toe, microstructure, and hardness of the welds were checked. Because of the larger weld toe angle and toe radius of the tandem MAG welding process, the stress concentration in the toe part was reduced under applied repeated loads, and the fatigue strength and fatigue life were greater than those of the DC and DC pulse welding processes.