Effect of Heat Treatment Conditions on Resistance Spot Weldability of 1.8 GPa-grade Al-Si Coated Hot Stamping Boron Steel

Abstract

This study entailed resistance spot welding conducted using 1.8 GPa-grade hot stamping boron steel heat-treated under varied conditions. The relationship between weldability and melting behavior of the intermetallic layer during resistance spot welding was examined. As regards the four heat treatment conditions beyond the austenitic temperature, it was found that the intermetallic layer thickened with increased time and temperature. Furthermore, the contact resistance also increased with increased time and temperature of the heat treatment. This was mainly because of the expansion of the area of Al-Fe-based intermetallic phases (e.g., FeAl2 and Fe2Al5) within the intermetallic layer. Contact resistance induces the thickening of the intermetallic layer and results, even with a low current, in the occurrence of expulsion due to the high heat generation in the faying surface. Subsequently, the weldable current range became narrow, and satisfying the property requirements was challenging. The welding using the heat treatment condition of 900 °C and 5 min showed sufficient current path area. This can be attributed to the phenomenon that the intermetallic layer was forced out to the rim of the corona bond area at an early stage of the welding. However, the welding using other conditions showed that the intermetallic layer remained on the border of the nugget and corona bond areas it was observed and verified via electron probe micro analysis. Consequently, the nugget was insufficiently formed and the fracture mode was a partial interfacial fracture. On the contrary, it was found that the adoption of the pre-pulse could enhance the weldability of all conditions by obtaining a larger contact area.