Study On Failure Mode Of Three-sheet Resistance Spot Welding Joints Of Advanced High Strength Steel

With the global energy crisis and environmental pollution intensified,lightweight car has become the current trend of the automotive industry.The use of advanced high-strength steel instead of the original low-carbon steel is gradually one of the trend of automobile lightweighting,to reduce weight on the basis of ensuring the same strength.Resistance spot welding,which has high welding speed,simple operation,low cost,high degree of automation,good adaptability and other advantages,has already occupied an irreplaceable position in the automotive industry.Compared with the traditional resistance spot welding on low carbon steel,the advanced high strength steel resistance spot welding has some problems such as complex microstructure of the joint,prone to interface failure,nugget migration and asymmetry,easy to splash in the welding defects.Among the above problems,the failure mode of advanced high-strength steel resistance spot welding joints is one of the key issues to be studied.The research on the failure mode of advanced high strength steel three-sheet resistance spot welded joints has important theoretical and practical significance because different failure modes will affect the fracture mechanism,bearing capacity and fracture location of the spot welded joints.In this paper,the influence of thickness factor and strength factor on the failure mode of two-sheet spot welded joint is analyzed.Based on the force analysis of the two-sheet spot welded joint,the critical nugget size is predicted by formula.Then,the results from the prediction formula were verified and compared with the previous test results and industry standards recommended values.For advanced high strength steel three-sheet spot welded joints,the influences of thickness factor,strength factor and stacking order factors on the failure mode of spot welded joints have been studied.Based on the prediction formula of the critical nugget size of the two-sheet spot welded joints,the prediction formula of the critical nugget size of the three-sheet spot welded joint was revised and established by comparing the experiments results between the two-sheet and the three-sheet.On this basis,the methods to improve the interface failure have been proposed.For the advanced high strength steel two-sheet spot welded joints,the strength factor will directly affect the failure mode,failure location and mechanical properties,but has no effect the shape of the nugget and hardly produce nugget migration.The thickness factor will cause the changes of the failure mode greatly,but it does not affect the failure location and the peak load.Besides,the shape of the nugget is asymmetrical and there exsists the nugget migration phenomenon.The critical nugget size prediction formula of the two-sheet welded joints can be established through the force analysis and experiment results.The critical nugget size predicted by the formula is in good agreement with the test results,also it can be directly used to judge the initial failure location where the fracture is pulled out.However,the recommended standard by automobile industry is only valid for mild steel spot welded joints.In the case of the spot welded joint of the advanced high strength steel three-sheet,the thickness factor of the three-sheet spot welded joint does not affect the failure mode but affects the failure location,the mechanical propertties and the feature of pulling out from the joint with unequal thickness of the symmetrical structure.When the thickness of the middle sheet is less than that of the outside sheet,the whole middle sheet breaks out along the base metal directly during the failure process.In the unequal thickness three-sheet spot welded joint with asymmetric structure,the thickness factor does not affect the failure mode,failure location and mechanical properties of thinner sheet,but will affect the pull-out failure feartures of the thicker sheet.When the thickness of one thicker sheet is greater than or equal to the total thickness of the other two sheets,pull-out failure occurs simultaneously on the two sheets together.The strength factor of the outside sheet of the spot welded joint can directly affect the failure mode of the interface near the outside sheet and the peak loads of both the upper interface and the lower interface.However,it have no effect on the failure mode and failure location of the other interface.The little change of the middle sheet strength of spot welded joints will not influence the failure mode,the failure location and peak load,but the great change of the middle sheet strength of spot welded joints will directly change the failure mode.When thickness factor and strength remain constant,the stacking order exchange between middle sheet and the outside sheet will directly affect the peak load and failure mode.Under the conditions of the same other factors,the failure modes of the two-sheet and three-sheet plates are not identical due to the existence of the third sheet.Also,the initial fracture positions of the same failure mode in pull-out fracture are different.On the basis of the two-sheet spot welded joints,the critical nugget size prediction formula of the three-sheet spot welded joint can be revised and established.The predictive values of this formula is in good agreement with the experimental results.Similarly,the formula can be used to directly judge the initial fracture failure location.Combining with the critical nugget size prediction formula and the experimental results,it is found that increasing the strength of the middle plate of the three-sheeet joint and avoiding the use of two equally thicker sheet combination of joints can suppress the interface failure tendency.