Formation And Control Of Solidification Defects In High Strength Steel And Aluminum Alloy Resistance Spot Welded Joint

With the development of lightweight automobiles,aluminum alloys and advanced high-strength steels are widely used.The dissimilar structure composed of aluminum alloy and advanced high-strength steel can not only meet the structural strength and safety requirements but also achieve the purpose of weight reduction.However,due to the differences in physical,chemical and mechanical properties between aluminum and steel,the effective joining of aluminum alloy and high-strength steel is a challenge.Since RSW is particularly suitable for large-scale automated production of automobile bodies.It is an important process method for the connection of modern automobile bodies.Many scholars have conducted a lot of research on aluminum/steel dissimilar metal resistance spot welding.These studies include the composition of intermediate layer,microstructure,forming mechanism,mechanical properties and tensile fracture mode of the welded joints;and the effect of welding parameters,electrode morphology,alloy elements,intermediate layers and welding defects on microstructure and mechanical properties of the welded joints.During the process of aluminum / steel dissimilar metal RSW,there are many spatter,electrode adhesion,nugget shift,cracks,shrinkage cavities and brittle intermetallic welding defects,which deteriorate the mechanical properties of the joint.Therefore,it is difficult to achieve effective connection of aluminum and steel by RSW.The Al6016 aluminum alloy and HC420 LA low-alloy high-strength steel dissimilar metals resistance spot welding and the effect of different cover plates on the microstructure and mechanical properties of the joints were studied,to solve the problem of nugget shift and adjust the thermal balance of the joints,different materials and thickness of cover plates were used on aluminum alloy side.The results show that due to the difference in weldability between Al6016 aluminum alloy and HC420 LA high-strength steel,there is an unmelted plastic zone between the high-strength steel and the aluminum/steel interface of the joints,and two nuggets formed on aluminum alloy and high strength steel side respectively.The aluminum alloy solution wetting,spreading,and interface reactions on the plastic zone of the steel,and the effective connection were formed.During the Al6016 / HC420 LA RSW process without cover plate,the welding defects such as external spatter,internal spatter,electrode adhesion,burnout,shrinkage and cracks were appeared.The cover plate can not only compensate for heat and change the welding temperature field,increase the nugget diameter,but also reduce the aluminum alloy splash and electrode adhesion in some extent,and improve the surface quality of the spot welding joint.When the 0.3mm thick cover plates were used,the macro quality of the joint is relatively good.When the 0.3mm thick HC420 LA or 0Cr25Ni20 cover plates were used,the mechanical properties of the joint are also relatively good.Due to the different weldability of cover plates,when the 0.3mm thick HC420 LA or 0Cr25Ni20 cover plates were used,the thickness of the aluminum nugget increases and the depth of the indentation decreases with the electrode pressure increases.when the 0.3mm thick Q235 cover plates were used,the indentation depth of the spot welding joints were increases,and the thickness of the aluminum alloy nugget were decreases with the electrode pressure increases.As to the cover plates,the microstructure of aluminum alloy nugget was delaminated.The structure of the steel-side nugget were obvious different with different welding processes.When the HC420 LA and 0Cr25Ni20 were used,the steel-side nugget is pearlite or martensite iron phase structure.when the Q235 cover plates were used,the aluminum phase structure appears on the steel nugget,due to the aluminum alloy solution breaks through the plastic zone on the surface of the high-strength steel because of the not matched temperature field and the stress field at the aluminum / steel interface.The spot welded joints without cover plate mainly fail in the form of interfacial fractures,but when the cover plates was used,the joints fail mainly in the form of button fractures,half-interface fractures,and interface fractures.The forming characteristics,microstructure and mechanical properties of the joints with three cover plates were compared,it was found that the forming characteristics and mechanical properties of the joint with 0.3mm cover plate were relatively good.When the 0.3mm 0Cr25Ni20 cover plate was used,The maximum tensile shear of the joint reaches 7150N.