Study On Pinless Friction Stir Spot Welding And Interface Strengthening Mechanism Of AZ31 Magnesium Alloy/22MnB5 High Strength Steel

As a key industry in the‘14 th Five-Year Plan’,the application of lightweight materials in key components such as new energy vehicles and high-speed railways has become an inevitable development trend.As a common lightweight alloy,magnesium alloy has the advantages of high specific strength,good mechanical properties and thermal conductivity.As the most widely used metal material in automobile body,high strength steel has the advantages of high strength,good plasticity and toughness and simple manufacturing process.The joining of magnesium alloy and high strength steel heterostructures is an important research direction to achieve lightweight manufacturing.The key to obtain high-performance welded joints of magnesium alloy/steel is to promote the interface reaction and realize the metallurgical bonding between the interface layer and both sides.However,during the welding process,the change of welding method,process parameters and alloying elements will lead to a great change in the type and thickness of the magnesium alloy/steel interface layer,which makes it difficult to directly control the performance of magnesium alloy/steel joints.In this study,a combination of magnetron sputtering and needleless friction stir welding was proposed to obtain high-performance joints with controllable interface layer.In this study,the lap spot welding of AZ31 magnesium alloy/22Mn B5 high strength steel was compared with that of AZ31 magnesium alloy/22Mn B5 high strength steel with preset Fe₂Al₅ interface layer.The macroscopic morphology,shear strength,fracture morphology and microstructure of the interface layer were analyzed.The effect of Fe₂Al₅ interface layer thickness on the properties of welded joints was simulated by molecular dynamics simulation,and the interface temperature field of welded joints was simulated by ABAQUS.Firstly,the orthogonal test of friction stir lap spot welding process of AZ31 magnesium alloy/22Mn B5 high strength steel was carried out to study the shear strength of welded joints obtained by different parameters.The influence of factors on the properties of joints is:press amount>residence time>rotation speed.The optimal welding parameters are:rotation speed of 2000 rpm,welding time of 60 s,press amount of 0.5 mm.With the increase of welding heat input,the thickness of the interface layer decreases first and then increases.The maximum thickness of the interface layer is 14μm and the minimum is 5μm.The thinner the interface layer is,the higher the shear performance of the joint is.The maximum shear resistance is 1.3 k N at 5μm.The interface structure isα-Mg/Fe₂Al₅/α-Fe.The fracture of the joint is mainly brittle fracture along the magnesium alloy/steel interface.Secondly,ABAQUS software was used to simulate the temperature field of magnesium alloy/steel joints and thermocouple measurements were carried out to verify.The results show that the interface temperature of the joint does not reach 80%of the melting point of magnesium alloy,which belongs to solid phase welding.With the increase of rotational speed and pressing amount,the temperature of the joint increases,and the temperature field of the longitudinal section of the joint is bowl-shaped.Combined with the calculation results of temperature field,the Gibbs free energy change of Fe-Al compound that may be generated at the interface is calculated,indicating that Fe₂Al₅ is formed first in the welding temperature range.Finally,the Fe₂Al₅ interface layer was preset on the surface of high strength steel by magnetron sputtering method.The forming,microstructure and interface structure of the friction stir lap welding joint of AZ31 magnesium alloy/22Mn B5 high strength steel with preset Fe₂Al₅ interface layer were analyzed.When the heat input is small,the magnesium alloy/steel joint cannot achieve metallurgical connection.With the increase of heat input,the shear strength of magnesium alloy/steel joint increases gradually.When the experimental parameters are rotational speed 2000 rpm,welding time 60 s,and downward pressure 0.5 mm,the maximum shear performance is 2.45 k N,and the interface structure isα-Mg/nanoscale Fe₂Al₅/α-Fe.When the heat input continues to increase,the shear strength of the joint decreases.The shear strength of the joint is 1.56 k N,and the interface structure changes toα-Mg/（α-Mg+Mg₂Al₃）/Fe₂Al₅/α-Fe.Transmission electron microscopy analysis of the best joints showed that the interface layer was mainly amorphous and nanocrystalline Fe₂Al₅mixed structure.The molecular dynamics simulation of Fe₂Al₅ interface layer with different thickness and structure shows that with the decrease of the thickness ofη-Fe₂Al₅ interface layer,the stress ofα-Mg/η-Fe₂Al₅/α-Fe structure increases first and then decreases,which is consistent with the experimental results.