| In the current world,the amount of automobile manufacturing and automobile possession increases rapidly,the automobile industry in the new era takes "environmental protection,safety,energy saving" as the three main themes,the use of reducing the weight of the car body to achieve the car's lightweight is one of the ways to work effectively in the carbon emission reduction and energy-saving.To achieve the requirements of the carbon emission reduction and energy-saving,Using the hot stamping high strength steel can increase the level of the collision resistance,and reduce the self-weight of the car on the other hand,so it replaces the materials in common specifications for automobile manufacturing gradually,for example,the low alloy steel and the like.Using the high strength steel structures with different thicknesses and different materials to design automotive load-bearing structural parts is the key condition to reduce the weight of the automobile body and ensure the strong collision resistance and high safety of the automobile.However,the differences in mechanical properties,physical properties,chemical composition,structures and manufacturing process of the dissimilar thickness high strength steels results in that the more complex and nonuniform condition of force,electricity,and heat distribution of the resistance spot welding joints in being welded,the range of the resistance spot welding process designed for application gets to be narrowed,and it is more difficult to control the welding.When the high strength steel is heated,its internal chemical composition and microstructures change will be affected very easily.The ultra-fast cooling rate during the resistance spot welding process can break the original balance of the welded basematerials thoroughly,Usually,the joints after the resistance spot welding will have performance degradation.The automobile manufacturing industry adopts the resistance spot welding as a universal connection method for connecting the structural parts of the automobile sheets,mainly based on the advantages of easy-to-operate,convenient use,good working conditions,low input cost in welding and easy automatic production.In the complete vehicle connection engineering of the current automobile industry's processing,the resistance spot welding is used widely,and the performance of the various aspects of the automobile body is directly determined by the quality of the connection of the resistance spot welding.In this paper,the welding parameters of the 1.8mm 22MnB5 and 1.5mm TRIP590 dissimilar thickness high strength steels are not easy to realize the correct welding.The localized softening phenomena will appear in heat-affected zone promoting the resistance spot welding joints producing cross-section fracture,the resistance spot welding joints appeared splashing,and other problems.The resistance spot welding experiments and the post-weld microstructure performance tests have been carried out for 1.8mm 22MnB5 and 1.5mm TRIP590.The microstructure and mechanical properties of the resistance spot welding joints were studied.The resistance spot welding process was analyzed on the influence of parameters on the microstructures and mechanical properties,and the fracture mode under the external forces and mechanism of the resistance spot welding joints.The main conclusions of this paper are as follows:1.The fusion zone,the heat affected zone and the base materials are the three regions that make up the dissimilar thickness high strength steels 22MnB5/TRIP590 resistance spot welding joints.The microstructures of the fusion zone on the plates from the both sides of the spot welding joints are mainly composed of the coarse lath martensite,and also contain a small amount of ferrite structures.On the 22MnB5 side of the resistance spot welding joints,the microstructures of the heat affected zone region are mainly composed of the coarse lath martensite and a small amount of ferrite.The microstructures of the heat affected zone on the TRIP590 side arecomposed of martensite,ferrite and high temperature tempered bainite.The closer the fusion zone is,the larger the microstructures become and the proportion of martensite increases.2.The microhardness of the fusion zone on the dissimilar thickness high strength steels 22MnB5/TRIP590 resistance spot welding joints is higher than the heat affected zone and the base material zone on the spot welding joints.The softening phenomenon occurs in the heat affected zone being adjacent to the base material zone on the 22MnB5 side,but the softening phenomenon does not appear in the heat affected zone being adjacent to the base material zone on the TRIP590 side.The mechanical properties of the 22MnB5/TRIP590 resistance spot welding joints are between the two base materials.The toughness is far worse than the two base materials using in the resistance spot welding.3.In the tensile test of the dissimilar thickness high strength steels22MnB5/TRIP590 resistance spot welding joints,the form of the pullout fracture,partial interface fracture and interfacial fracture occurred.In the three failure modes,the pullout fracture is ductile fracture.The ductile fracture and brittle fracture are partial interface fracture's characteristics.The brittle fracture is the characteristic of the interfacial fracture.4.The main welding defects existing in the dissimilar thickness high strength steels 22MnB5/TRIP590 resistance spot welding joints are shrinkage cavities,and the shrinkage cavities of the resistance spot welding joints have a certain negative impact on the connect strength of the resistance spot welding joints.5.Increasing the welding current and welding time,the grains in the fusion and the heat affected zone of the 22MnB5/TRIP590 resistance spot welding joints become thicker,and the proportion of the martensite increases gradually.The continuous application of the electrode pressure,the microstructures of the fusion zone and the heat affected zone exhibits grains refinement characteristics.6.When the welding current value increases,the strength of the resistance spot welding joints begins to rise,until it rises to a certain extent,it will decrease.When the welding current is 8.5 kA,the strength of the resistance spot welding jointsreaches a maximum value,and the welding current is too large,causing severe splashing.Extending the welding time gradually and the strength of the resistance spot welding joints increases.When the electrode pressure is 4.0kN,the strength of the 22MnB5/TRIP590 resistance spot welding joints is maximized,it's value is16.02 kN.7.The melting rate,indentation rate,and indentation diameter on both sides of the resistance spot welding joints increase as the welding current increases.When the welding time increases,the melting rate of the resistance spot welding joints increases gradually and then decreases,and the indentation rate,the indentation diameter,and the fusion zone diameter increase gradually.The effect of the electrode pressure on the cross-sectional morphology of the resistance spot welding joints is less than that of the welding current and welding time. |
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