Resistance Spot Welding Process,Microstructures And Properties Of DP980 High Strength Steel Joints

With the development of science and technology and the improvement of social civilization,the requirement of automobile is higher and higher,which requires not only good comfort and safety,but also energy saving and environmental protection.As a lightweight material with great application prospect,high strength steel has been widely used in automobile manufacturing industry because of its advantages of high strength,good formability,high energy absorption rate and good crashworthiness.Resistance spot welding has the advantages of short welding process,high production efficiency and high degree of automation.It is one of the most widely used welding technologies in automobile production.The process window of resistance spot welding of high strength steel is narrow,and it is easy to produce defects such as virtual welding,spatter and shrinkage hole in the process of resistance spot welding,which has become the technical bottleneck restricting the wide application of high strength steel.Aiming at the difficulty of resistance spot welding of high strength steel,the window of DP980 high strength steel spot welding process are studied in this paper,and the influence of welding current,welding time and electrode pressure on the microstructure and properties of resistance spot welding joint are.analyzed.The conclusions of the experiment are as follows:DP980 high strength steel spot welding has good weldability in the process window and has narrow process window,but it still meets the requirements of GWS-5A standard,and can obtain the required nugget size in the narrowest current width from 8.6 kA to 10.3kA.The microstructure of the nugget zone of spot welding joint of DP980 high strength steel is mainly columnar crystal structure,which is coarse lath martensite structure,and the grain size is larger than that of base metal and heat affected zone.Compared with the key points in the left side of the process window,the microstructure of the nugget zone of the joints is larger and the mechanical properties of the joints are better than those of the key points in the right side of the process window.Generally speaking,the resistance spot welding of high strength steel DP980 has excellent performance in the process window.The spot welding joint welded outside the process window will appear shrinkage hole,spatter and other defects,so the weldability is poor.The indentation depth of spot welding joint will change under different spot weldingprocess parameters,which will affect the beauty and quality of joint.The greater the welding current,the longer the welding time,the greater the indentation depth of spot welding joint,and the smaller the indentation depth is,but the less obvious the change is.With the increase of welding current,the prolongation of welding time,the decrease of electrode pressure,that is,the increasing of welding heat input,the microstructure of nugget zone and heat affected zone is not changed,but the grains of martensite are grown and roughened.The process parameters of spot welding have a significant effect on the mechanical properties of joints.When the welding current varies from 8.0 kA to 10.0 kA,with the increase of welding current,the size of solder joint nugget increases,the shear force increases,and the strength of welded joint increases.When the welding current is 10.5 kA,the welding heat input is too large,which results in the spatter of liquid metal inside the nugget.When the welding time was increased from 150 ms to 400 ms,with the increase of heat input,the nugget size of the solder joint gradually increased and then tended to stabilize,but the strength of the spot welding joint decreased with the increase of time.If the electrode pressure is too small,the joint will produce shrinkage cavity defects,which will lead to the decrease of nugget size and joint strength.Then,with the increase of pressure,the size and shear strength of nugget increase first and then decrease.The hardness of nugget zone of spot welding joint of DP980 high strength steel is the highest.The hardness of the heat-affected zone decreases gradually with the increase of the distance from the center.The hardness of the base metal area is slightly higher than that of the heat affected zone.With the increase of the current,the welding time becomes longer,the electrode pressure becomes smaller,and the welding heat input increases.The hardness of the joint decreases.In the shear condition,the resistance spot welding head of DP980 high strength steel mainly has two fracture modes,namely,the interface fracture belonging to brittle fracture and the fracture of the ductile fracture which belong to ductile fracture.