Study On The Mechanism Of Interface Bonding Between Steel And Magnesium By Laser Melting Assisted By Adding Foil

Magnesium alloy is an important lightweight material for automobile,which is known as"the most promising lightweight material for automobile".With the improvement of lightweight requirement of automobile,there must be the connection problem between steel and magnesium alloy.However,there is a big difference in the thermophysical and chemical properties between steel and magnesium,and the bonding strength of the lap joint interface is weak.The laser fusion welding experiment and the related mechanism research are carried out for the lap joint of"steel upper magnesium lower"and the metal foil added between steel and magnesium.The topic has theoretical significance and practical application value.In this paper,the DP590 dual phase steel plate with a thickness of 1.4mm and AZ31 magnesium plate with a thickness of 1.5mm are welded by laser fusion welding,and the aluminum foil and titanium foil interlayer are added between the steel and magnesium for auxiliary welding.The microstructure,element distribution,phase composition and mechanical properties of steel/magnesium welded joint were studied by means of metallographic microscope,scanning electron microscope,X-ray diffraction(XRD)and universal material testing machine.The temperature field and flow field in the process of steel/magnesium fusion welding were simulated by COMSOL finite element software.The main conclusions are as follows:(1)The parameters of laser welding with aluminum foil as the middle layer are optimized as follows:laser power 2100W,welding speed 1800mm/min,aluminum foil thickness 0.3mm,and the strength of joint line is 52n/mm.It is found that the molten pool is divided into two parts:the upper part and the lower part.(2)The parameters of laser welding with titanium foil as the middle layer are optimized as follows:laser power 2100W,welding speed 1800mm/min,thickness of titanium foil 0.1mm,and the strength of joint line is 41N/mm.It is found that partial melting of titanium interlayer is the main reason for improving the properties of welded joints.In the joint,magnesium and steel attach well to the titanium intermediate layer,and the molten pool on the steel side and magnesium side contact and mix with each other through the small holes melted on the titanium foil.There is no new phase in the base metal and titanium interlayer,but the aluminum component in the base metal magnesium alloy and the iron component in the steel form new phases Fe Al₂,Al Fe₃and Al Fe.(3)The influence of Al and Ti interlayer on the energy transfer,temperature field distribution and microstructure transformation in laser welding of steel/magnesium lap joint was studied by numerical simulation.The results show that aluminum and titanium interlayer can control the energy transfer in the welding process and realize the simultaneous melting of steel and magnesium in the welding process;the temperature rise and fall of the molten pool are very fast in the welding process,and the cooling rate is faster than that of the formation of martensite,so the martensite in the dual phase steel will not be tempered and softened;the molten metal flow in the molten pool is slow,and the molten pool is relatively small It is stable and the flow direction of molten metal is opposite to the welding direction,which is beneficial to the feeding of molten metal during solidification.The simulation results are consistent with the experimental results.