| In the fast-growing automobile industry,lightweight vehicles is one of the most effective methods for energy saving and emission reduction.Duplex steel(DP)is widely used in car body manufacturing because of its light weight,high strength and good plasticity.In the welding process of automobile body,resistance spot welding is favored because of its low cost and high efficiency.However,compared with ordinary cold-rolled steel,dual-phase steel is prone to defects due to its high content of alloy elements during spot welding,resulting in poor performance of spot welded joints.Therefore,studying the spot welding process of dual-phase steel is of great significance to improve its welding quality and ensure production efficiency.To this end,this article takes DP980 dual-phase steel as the research object,through the combination of numerical simulation and experiment to study the macro/microstructure,mechanical properties and fracture mode of DP980 dual-phase steel spot welded joint.The results show that:The difference between the nugget size of the spot welded joint obtained by numerical simulation and the experimental result is within 10%.The peak temperature of the joint appears at the center of the nugget.The structure of the center of the nugget is martensite.The transition from the nugget to the base metal gradually reduces the martensite content.There is a large compressive stress in the nugget area,and a large radial tensile stress is formed in the heat-affected zone on both sides of the nugget.In the experimental results,the nugget zone of the spot welded joint consists of coarse lath martensite,and the microhardness is high,reaching an average of 400 HV;the microstructure of the coarse grain zone is coarse massive martensite,about 380 HV;the microstructure of the fine grain area is fine martensite,and the microhardness reaches the maximum value of 430 HV of the spot weld joint;the microstructure of the softening area is a mixture of ferrite and fine martensite,in which the martensite is tempered.The precipitated carbides cause obvious softening zone in the spot welded joint,the hardness reaches a minimum of 230 HV;the microstructure of the base material is ferrite matrix with martensite distributed on it,and the microhardness is 330 HV.The experimental analysis of the fracture mode and tensile-shear performance of DP980dual-phase steel spot welded joints found that the tensile shear force of spot welded joints gradually increased with the increase of welding current,welding pressure,and welding time.After reaching the peak,welding current and welding the pressure and welding time continue to increase,and the tensile shear force of the spot welded joint gradually decreases;when the nugget size of the spot welded joint is less than 4.695 mm,the fracture mode is interface fracture,and the joint performance is poor,while when the nugget size is greater than 4.695 mm,the fracture mode is the nugget.Pull-out fracture,excellent joint performance.The F test results of analysis of variance show that:welding current has the greatest influence on the spot welding head nugget size and the average temperature of the electrode tip,welding pressure is second,and welding time has no significant effect on the nugget size and the average temperature of the electrode tip;for the nugget radius,the F value of the welding current12.365 is much greater than the critical valueF0.01(4,12),and the welding pressure FF=7.874is slightly greater than the critical valueF0.01(4,12),indicating that the welding current and welding pressure have a very significant effect on the nugget radius;for the average temperature of the electrode tip,welding currentIF=47.807and welding time TF=6 4.081are much greater than the critical valueF0.01(4,12).The welding current and welding time have a very significant effect on the nugget radius;the recommended welding process window for DP980 dual-phase steel spot welding is welding pressure 2.6 k N,welding current width is 7.9?9.8 k A,the welding time width is 110?245 ms. |
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