| The application of advanced high strength steel can ensure the strength of automobile body and realize the lightweight of automobile at the same time,and achieve the purpose of energy saving and emission reduction.The third generation of advanced high strength steel Q&P,namely quenched ductile steel,is especially suitable for complex automotive parts because of its high strength and good elongation.It has broad application prospects in automotive body.Hot stamping steel is widely used in automotive body safety structures due to its good formability and high strength plastic area.Welding performance of materials is one of the key factors to measure whether they can be used in automobile production.Resistance spot welding is the most important way to connect automobile body components.Therefore,it is of great significance to study the resistance spot welding performance of materials for its application in automobiles.In this paper,two kinds of ultra-high strength steels,Q&P1180 developed by a steel plant and 2.0GPa new type hot stamping forming steels independently developed,are studied.Through the measurement of weld nugget size,static and dynamic mechanical properties test,microhardness and microstructure evolution analysis and failure mode evaluation,two ultra-high strengths are studied.The resistance spot welding performance of steel lays a theoretical foundation for the application of a new generation of high-strength steel in automobiles.The main contents of the paper are as follows:(1)Evaluating the resistance spot welding performance of Q&P steel with the same thickness and strength of 1.2GPa,in the absence of off welding,welding cracks and splashes and other and measuring the nugget size of the solder joint under the premise of no welding defects such as desoldering,welding crack and splash.The mechanical properties and failure modes were obtained,and the optimal welding window was obtained.The microstructure,microhardness and typical failure modes of the joints were analyzed.(2)Studying the resistance spot welding performance of 1.2GPa grade ultra-high strength Q&P steel with high strength steel of different thickness,different strength and different surface quality.By conducting quasi-static cross-stretching and shear-stretching performance test of the solder joint and dynamic impact performance test evaluation to determine the best process window.(3)Studying the resistance spot welding weldability of a new hot stamping forming steel with 2.0GPa,simulate the coating baking in the assembly process of automobile,weld the tempering after welding,and improve the mechanical properties of the solder joint.(4)Analyzing the failure form of hot stamped steel joints that all the nugget removal failure positions of the spot welds are at the nugget line,because the spot welds are not penetrated due to insufficient welding heat input.In the mechanical properties test,the initial crack propagates along the incompletely penetrated area between the sheets,resulting in failure of the nugget pull-out fracture.Dual pulse welding process was used to improve the fracture mode of the new hot stamping forming steel spot welding joint.The mechanical properties of the joint obtained by dual pulse welding process were significantly improved,but the fracture mode was not improved.Further research is needed in the future. |
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