Research On Laser Welding Technology And Mechanism Of Mg/Steel Dissimilar Metals

In order to achieve the lightweight of automobile,it is an inevitable trend that more and more light alloy is applied to the auto body,so the connection between light alloy and steel becomes the research focus.In this paper,AZ31 B Mg alloy and DP590 galvanized dual phase steel were used as base materials,and fiber laser was selected to explore the influence of intermediate layer,laser parameters and magnetic field on the process and mechanism of laser welding of Mg alloy/Steel.The effect of pure Cu powder and different mass ratio of Cu-Si composite powder on laser deep penetration welding of Mg alloy/Steel was studied by using the overlapping form of steel top and magnesium bottom.The test results show that: when the laser power was 800 W,the middle layer was added to improve the stability of the welding process,and the continuous,uniform and beautiful weld was obtained,which shows silvery white metallic luster,no spatter and pits and other defects.The addition of Cu improves the stability of the molten pool,promotes the continuous and stable formation of Fe-Al interlayer at the Mg/Fe interface,and inhibits the residual Zn on the surface of galvanized steel at the Mg/Fe interface of the weld,which improves the tensile shear force of the welded joint.The addition of Si makes the Fe-Al phase of FeAl reaction layer transform into Fe-Si-Al phase,which further improves the mechanical properties of the joint.When Si accounts for 5%,the maximum tensile shear force of the joint reaches 81.9 N/mm,which is 30% higher than that of Mg alloy/Steel direct welding.The fracture mode is brittle fracture.After the Cu-Si composite powder was determined to be Cu-5%Si with a thickness of 130?m,the effects of laser power,welding speed and defocusing amount on the laser welding of Mg alloy/Steel were further investigated.The results show that under the appropriate process parameters,the weld is continuous,beautiful and well formed.When the heat input was too low,the defects such as pits appear on the front of the weld;when the heat input was too high,the defects such as overlap appear on the front of the back of the weld,and the back of the weld is penetrated.When the heat input was low,the density of Cu-Mg IMCs in the weld on the magnesium side increases and becomes sparse with the increase of heat input.In addition,the defocusing amount affects the thickness of the reaction layer at the Mg/Fe interface.When the defocusing amount was-1 mm,the reaction layer was the thickest.With the defocusing amount increasing to 0 mm and positive defocusing,the thickness of the reaction layer gradually decreases.When the laser power was 800 W,the welding speed was 0.05m/s and the defocusing distance is 0 mm,the maximum tensile shear force of the joint is81.9 N/mm,and the fracture mode of the joint is brittle fracture.In the electromagnetic field assisted welding,the lapping form of magnesium upper and steel lower was adopted,and the process parameters were reoptimized as follows: the laser power was 600 W,the welding speed was 0.07m/s,the defocusing distance was 0 mm,the external current of electromagnetic field was 5A,and the voltage was 3V.The results show that the electromagnetic field assisted welding can increase the absorption of laser energy,make the internal structure of the magnesium side weld more uniform,promote the formation of reaction layer at the Mg/Fe interface in the weld,and improve the mechanical properties of the joint.The tensile shear force of the joint reaches 96.5 N/mm,which is 13% higher than that without increasing the electromagnetic field.The fracture occurs at the base metal,and the fracture mode is brittle fracture Split.