Microstructure and mechanical properties of Q&P 1180 steel joint processed by friction stir mechanical alloying

• A novel friction stir mechanical alloying (FSMA) technique proposed to weld Q&P 1180 steel. • Higher content retained austenite achieved in the stir zone than in base material. • FSMA joint exhibits an excellent strength-ductility product of 19213 MPa·%, reaching 110 % of base material. • Transformation-induced plasticity and dislocations across martensite and austenite interface effects mediated high strength and ductility of FSMA joint. Conventional fusion welding of Q&P 1180 steel typically results in the decomposition of retained austenite and poor strength-ductility synergy. To overcome these limitations, this study proposes a novel friction stir mechanical alloying (FSMA) technique for joining Q&P 1180 steel. The results showed that the stir zone of the FSMA joint consisted of martensite, cementite, and a higher fraction of retained austenite than the base material (BM). The inter-critical and sub-critical heat-affected zones also contained martensite and retained austenite, along with minor cementite. The FSMA joint exhibited a significant increase in microhardness (458 HV) and ultimate tensile strength (1264 MPa) while maintaining an elongation of 15.2 %. This resulted in a strength-elongation product of 19213 MPa·%, reaching 110 % of that of the BM (17465 MPa·%). This study demonstrates that FSMA is an effective strategy for fabricating high-performance Q&P steel joints.