Study on the microstructure and failure mechanisms for flat resistance element welding-bonding aluminium alloy/Q235 steel joints

A novel method called flat resistance element welding-bonding (FREWB) is proposed to join aluminium alloy to Q235 steel. The microstructure and mechanical performance of FREWB and flat resistance element welding (FREW) were investigated. The microstructure of the nugget was lath martensite. The heat-affected zone of the rivet could be divided into two zones according to the microstructure: the upper-critical heat-affected zone (UCHAZ) and the inter-critical heat-affected zone (ICHAZ). The microstructure of UCHAZ consists of martensite, and the microstructure of ICHAZ consists of martensite and ferrite. The failure mechanisms of the FREW and FREWB joints were analysed. Moreover, an analytical model was established to predict the variation of critical nugget sizes, which provided a relatively good prediction.Abbreviations AB: Adhesive bonding; ABZ: Adhesive bonding zone; BM: Base material; CFRP: carbon fibre reinforced plastic; FREW: Flat resistance element welding; FREWB: Flat resistance element welding-bonding; HAZ: Heat-affected zone; ICHAZ: Inter-critical heat-affected zone; IF: Interface failure; PO: Pull-outfailure; REW: Resistance element welding; RSW: Resistance spot welding; RSWB: Resistance spot welding-bonding; SPR: Self-pierce riveting; UCHAZ: Upper-critical heat-affected zone; UPO: Upper plate poll off failure; WZ: Weldingzone.