The A356 alloy has been widely used in automotive components, such as wheels and brake disks, because it is an excellent lightweight material with high corrosion resistance and good mechanical properties. Recently, to reduce the weight of brake disks, the Fe-A356 hybrid brake disk has been suggested. Because brake disk quality is directly related to driving safety, the T4/T6 heat treatment of centrifugally cast A356 alloys were performed to enhance the mechanical properties and reduce micro-segregation. The solid-solution heat treatment followed by annealing caused the formation of Mg-rich intermetallic compounds on the grain boundaries of the Al matrix, decreasing the average hardness of the alloys by 13 HV. In contrast, the solid solution followed by water quenching (T4) reduced the area fractions of the intermetallic compounds and increased the average hardness by 11 HV. The T6 heat-treated A356 alloys, which were influenced by the formation of the Guinier–Preston zone exhibited a relatively higher average hardness, by 18 HV, compared to T4 heat-treated A356 alloys.

As environmental regulations have been tightened around the world, domestic and foreign automobile industries are increasing the proportion of hydrogen and electric automotive components using lightweight materials to reduce exhaust gas emissions and improve fuel efficiency. In particular, hot-stamped boron steel is being used as an alternative material for frame weight reduction due to thickness reduction, excellent workability, and good elongation. However, surface coating is essential to prevent surface decarburization phenomena and oxidation scale formation between hot stamping processes of boron steel. The coating layer causes many problems in the weldability such as a narrow weldable current range and deterioration of mechanical properties during resistance welding. Therefore, in this study, we introduce research performed to improve the weldability of Al-Si coated hot-stamped boron steel in terms of the material and process.

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.