Optimization Of DP780 Dual Phase Steel Weld-bonded Process And Joint Failure Analysis

In this paper,the 1.2mm thick DP780 dual-phase steel sheet material was used to carry out the weld-bonded process test research.The regression models of the static strength index of the weld-bonded joint were established,and the process parameters were optimized.The fracture failure characteristics of the joint were analyzed.The research content and conclusions include:Based on the BBD response surface method,the welding time,welding current and supply pressure were taken as the process influencing factors,and the test design and parameter optimization of the weld-bonded were carried out.The results show that the welding time has the most significant effect on the failure load and energy absorption of the joint.In order to improve the tensile strength of the weld-bonded joint,a larger welding time and welding current should be selected.The optimal process parameters of 1.2 mm thick DP780 weld-bonded are obtained by response surface analysis: welding time 150 ms,welding current 8.3 kA,supply pressure 0.3 MPa.The maximum failure load of the joint is 16369 N in the process test,and the error calculated from the regression equation is within 5.7%.By analyzing the ultrasonic C-scan image and the A-scan signal characteristics of the weld-bonded joints,it is found that there is a gasification phenomenon of the adhesive layer in the weld nugget near the weld joint.The width of the burning zone is between 2 mm and 3 mm.The joints structure can be divided into: a weld nugget zone,a fusion zone,the heat affected zone,the adhesive layer zone and the burning zone;secondly,the C-scan image of the weld-bonded joint exhibits the splash and the over burning defect.The uniaxial tensile test was carried out on the weld-bonded joints of the shear test specimen and the peel test specimen by means of the MTS mechanical property testing machine.The results show that the load-displacement curves of the joints have doublepeak characteristics,and the adhesive layer fails before the weld nugget;the adhesive layer bearing capacity of the peel test specimen is relatively poor.The shear test specimen failure joint has two typical failure modes: " pull-out failure" and " interface failure",and the failure mode of the joint of the spatter defect corresponding to the partial pull-out failure.The SEM fracture analysis of the failure joint shows that the fracture of the interface failure exhibits brittle fracture as a whole,and a large number of dimples appear in the fracture of pull-out failure,showing ductile fracture characteristics.The GTN/cohesion simulation model of the shearing process of the weld-bonded joint was established,and the fracture failure characteristics of the joint were analyzed.The results show that,when the load reaches the tensile strength of the adhesive layer,the adhesive layer at the edge of the joint is first broken and the stress peak of the adhesive layer is toward the center of the nugget.Gradually expanding,after the adhesive layer is completely failed,the external load is only carried by the nugget alone.As the displacement continues to increase,cracks occur at the stress concentration boundary between the weld nugget and the sheets between the two plates,and the crack propagates along the weld nugget of the circumferential direction,and then the nugget failure occurred.Compared with the spot welded and bonded joints,it is found that the opening displacement of the weld-bonded joints is slower than that of the spot welded,the Mises stress of the adhesive layer is smaller than that of the bonded joint.Therefore,the weld-bonded joint of the joint has a superior structural bearing capacity.