Fatigue And Fracture Behavior Of Novel Aluminum To Aluminum And Aluminum To Steel Resistance Spot Welds

Multi-material solutions,particularly the mixed use of aluminum alloys and steels,effectively reduces automotive body structure weight to meet increasing fuel economy demands.However,the low melting point,high thermal and electrical conductivity and the formation of surface oxide layers of aluminum alloys lead to difficulties in resistance spot welding(RSW)process.In the meantime,the large differences in electrical resistivity and thermal conductivity in aluminum alloys and steels pose unique challenges for the RSW process.A brittle intermetallic layer forms at the aluminum to steel interface,which can be detrimental to the strength of the RSWs.In the present study,a variety of aluminum to steel RSWs were created using a ringed welding electrode,the Multi-Ring Domed(MRD)electrode,and multiple solidification weld schedules to systematically study the effect of intermetallic compounds on fatigue and fracture properties of the RSWs.Firstly,the 0.8 mm thick X626-0.8 mm thick X626 RSWs were welded using the MRD electrode were made and exhibited acceptable strengths that achieved the static load strength requirements of the American Welding Society(AWS)standard.The tensile stress and strain curves of the weld nugget and heat affected zone(HAZ)in aluminum-aluminum RSWs were measured using modified tensile shear tests.These results serve as the baseline to compare with the aluminum to steel RSWs in the present study.In aluminum-steel RSWs,the intermetallic layer was comprised of Fe Al₃ and Fe₂Al₅intermetallic compounds.A mini-shear specimen was developed to directly measure the shear strength of the intermetallic layers.Results show that more uniform intermetallic structures were formed when using the symmetrical MRD electrode on both the aluminum and steel sheets.In addition,satisfactory shear strength was achieved when the thickness of the intermetallic layer at the weld nugget periphery of the weld nugget was less than 2?m.New formulae to determine the critical weld nugget diameters of aluminum to aluminum and aluminum to steel RSWs were derived taking consideration of the material properties of weld nugget and heat affected zone(HAZ)and successfully predict the fracture modes of aluminum to aluminum and aluminum to steel RSWs.The dominant mechanism to induce interfacial fracture in coach peel specimens was the shear stress in the interfacial compound layer.Reducing the aluminum-to-steel thickness ratio,ensuring the aluminum sheet was thinner than the steel sheet,was demonstrated to be a beneficial mitigator for interfacial fracture.The effect of the steel coating type,notch root angle,and steel sheet thickness on the fatigue behavior of the RSWs was studied.The mechanism of fatigue fracture was revealed using a modified coach peel model through in-situ observation of crack propagation.Finally,the principal strain in the aluminum alloy sheet near the faying interface and the notch root was calculated and used to assess fatigue life of a variety of stack-ups with different materials and thickness.It was discovered that the principal strain of the aluminum alloy sheet near the faying interface and the notch root of aluminum-steel RSWs was smaller than that of the aluminum alloy RSWs,and further decreased with increasing steel sheet thickness.The master fatigue life curve based on the principal strain was superior to that using structural stress.