The effect of Zn coating layer on the microstructure and mechanical properties of friction stir spot welded galvanized DP590 high-strength steel plates

Abstract

Friction stir spot welding was successfully applied to the 1.4 mm thick galvanized DP590 dual-phase steel plates using a W-Re alloy rotating tool. During welding, a constant rotation speed of 800 rpm and a dwell time of 3 s were used and the tool penetration depth was ranged from 0 to 0.9 mm in order to study the flowing behavior and distribution of the Zn element in the joints. It was revealed that different forms of Zn were present along the interface. For example, there were Zn aggregates at the edge of the bonded interface, and there were droplet-like Zn particles along the partially bonded interface. Layers of accumulated Zn also existed outside the weld nugget. No obvious Zn element was detected inside the entire stir zone away from the interface. With the increase of penetration depth, the size of martensite in the stir zone increased from 8 to 14.2 μm. When the penetration depth was less than 0.6 mm, the shear tensile tests of the FSSW joints showed that the existence of Zn at the hook-like interfaces reduced the mechanical properties of the joints. However, when the penetration depth was larger than 0.6 mm, no noticeable effect of Zn was found for the joints with straight interfaces and a higher shear tensile load could be reached. The maximum shear tensile load was about 21.35 kN through the plug failure mode when a bonded interface of 1.9 mm was formed at a penetration depth of 0.8 mm.