Research on Resistance Spot Welding Process of DP780 Cold Rolled Sheet

In this paper,the spot welding performance of DP780 cold rolled sheet was evaluated by measuring the nugget diameter and penetration rate,detecting the shear force,texting the hardness of the welded joints,amd analyzing the microstructure and morphology,in order to propose the weldable process scope and the optimized technological parameters under test conditions. The result showed that,the spot welding performance of DP780 cold rolled sheet was good. When the electrode pressure was 3 k N,the welding current could be selected from 8 to 10 k A,and the welding time could be selected from 200 ms to 400 ms,while the welding current was 10 k A,then welding time 400 ms could not be selected. When the electrode pressure was 4 k N,the welding current could be selected from 8 to 9 k A,the welding time could be selected from 200 ms to 400 ms,while the welding current was 9 k A,then welding time 400 ms could not be selected.The optimized technological parameters under test conditions are as follows: welding current 9 k A,welding time 300 ms,electrode pressure 4 k N.Microstructural observation showed that; nugget zone was composed of coarse lath martensite and ferrite,the martensite presented columnar morphology,the microstructure of HAZ was blocky martensite and ferrite; the hardness in base metal and nugget zone were 240 HV and 420 HV,respectively.