Research On Microstructure And Properties Of Al/Mg Friction Stir Lap Welded Joints

Aluminum and magnesium alloys are widely used in the field of traffic transportation due to their low density,high specific strength,specific rigidity and other excellent properties,especially in the lightweight production of automobiles.However,when the traditional fusion welding method is used to connect Al/Mg dissimilar material components,the weldment is deformed seriously,and cracks are easily formed.Al-Mg intermetallic oxides are formed at the Al/Mg interface,which will increase the brittleness of the joint and the welded component.The life span is greatly reduced.Friction stir welding is a new type of solid-phase joining method,which can effectively control the welding thermal cycle process,and effectively alleviate the deterioration of Al/Mg dissimilar joints,eventually improving the welding quality.The research uses friction stir welding to carry out the systematic research of Al/Mg heterogeneous joining technology.This subject firstly studied the influence of friction stir welding process parameters on microstructure and mechanical properties.A 3mm-long cylindrical thread stirring head was used to explore the dissimilar friction stir welding for joining 5052aluminum alloy/AZ31 magnesium alloy.The results show the thickness of the IMCs layer and the Hook size of the advancing side(AS)increase with the decrease of the travel speed,and the increase of both rotational speed and heel plunge depth.The hardness distribution of the joint shows a typical“W”shape in the horizontal direction.The maximum value the hardness value in the vertical direction is 230HV and the maximum tensile shear force is 3.97KN.The thickness of the IMCs layer and the size of the Hook on the forward side are the main factors governing the tensile shear force.Based on the optimal parameters of dissimilar friction stir welding,this research explored the influence of different pin shapes(cylindrical thread,conical thread)on the weld quality and interface charaterization of aluminum/magnesium dissimilar lap joints.The results show that optimal parameters of friction stir welding include:the rotating speed of 950rpm,the traveling speed of 30mm/min,and the plunge depth of0.6mm.Compared with the tool of cylindrical threaded,the IMCs layer at the Al/Mg interface distribute on the AS,the retreating side(RS),the bottom of the pin,and the junction of the two plates,mainly composed of grid-like Al₁₂Mg₁₇IMCs and magnesium matrix.The hardness of the joint is up to 230-240HV,and the hardness in the center of the mixing zone is significantly reduced to about 150HV.Using optimal parameters,the effective lap width is up to 2.5mm,and the effective thickness is1.6mm.The tensile-shear force test results show that the maximum tensile-shear force of the joint under the optimal process conditions is 2.52KN.When the increase of the plunge depth,the effective width and thickness firstly increase,but then significantly reduced,eventually largely reduce the level of tensile shear force.The main reason is that the effective connection at the joint interface is greatly reduced and the distribution of IMCs layers is more dispersed.Finally,dissimilar Al/Mg friction stir welding with the Zn interlayer added between Al/Mg was carried out using a cylindrical threaded pin to explore the effect of thickness of Zn interlayer on the microstructure and properties of aluminum/magnesium joints.The results show that defect-free Al/Mg can be obtained joint using the Zn interlayer with the thickness of 0.05mm.The thickness of the IMCs layer increases with the increase of the thickness of the Zn interlayer,and its distribution characteristics are similar to those without the Zn interlayer.The IMCs layer is mainly composed of Al₁₂Mg₁₇,and Mg-Zn and Al-Mg-Zn appear inside the grid.In the horizontal direction of the weld cross-section hardness,the hardness of the stirring zone is about 160V,and the hardness value in the vertical direction is up to210HV.When the thickness of the Zn interlayer is 0.05mm,the tensile shear force reaches the maximum value of 4.79KN.The generation of Mg-Zn and Al-Mg-Zn IMCs are the main reason that the tensile shear force is increased by 20%compared with that without the Zn layer.