Control of the microstructural evolution and toughness of welded joints by preheating and subsequent cooling in hybrid friction stir welding of Q960 high strength steel

Friction stir welding (FSW) is a solid-state welding technology that can join high-strength steel with high quality. However, due to the high melting point and hardness of the steel, the expensive stirring tool suffers severe abrasion during FSW. In this study, Q960 high-strength steel was subjected to conventional FSW, FSW with 150degree celsius and 300degree celsius preheat assist, and FSW with 300degree celsius preheat followed by water and high-pressure air cooling, respectively. The experimental results show that martensite with fine blocks was formed in conventional FSW and low temperature preheat (150degree celsius) welded joint, resulting in good impact toughness. However, the cooling rate decreased significantly at high preheating temperature (300degree celsius), resulting in considerable coarsening of martensite blocks and carbides. The density of high angle boundaries (> 45 degrees) also decreased significantly due to the occurrence of martensitic variant selection, leading to a severe deterioration in impact toughness. Therefore, subsequent cooling with water or high-pressure air was carried out under high-temperature preheating conditions. Subsequent cooling successfully suppressed martensitic variant selection and coarsening of the martensite blocks, preventing significant toughness degradation. However, the water-cooled welded joint exhibited relatively low impact toughness with the formation of typical quenched martensite. With high pressure air cooling, the welded joint was cooled rapidly at high temperature, but the cooling rate at low temperature was low. This facilitated the tempering process, resulting in the formation of small block tempered martensite and an increase in impact toughness. In conclusion, high temperature preheating followed by cooling can reduce wear of expensive stirring tool, manage microstructure evolution and improve impact toughness of welded joint simultaneously.