Study On Microstructure And Mechanical Properties Of Usw Joints Of Magnesium/Aluminum Dissimilar Metal

Magnesium alloys and aluminum alloys are widely used light metals in the 21 st century.The use of magnesium alloys and aluminum alloys instead of steel plate materials reduces the economic cost and meets the objectives of lightweight,green environmental protection,energy conservation and material reduction advocated by industrial development.However,the weldability of magnesium alloys and aluminum alloys is poor,and the physicochemical properties of these two alloys are quite different.It is difficult to obtain Mg/Al joints with excellent performance by the general fusion welding method.Ultrasonic spot welding has the advantages of short welding time,high production efficiency,good stability,convenient operation and low requirement on the surface of the workpiece.It is expected to achieve reliable connection of Mg/Al dissimilar metals.In this paper,the ultrasonic spot welding test of AZ31 B magnesium alloy and 6082-T6 aluminum alloy was carried out to study the effects of welding energy,welding amplitude and welding static pressure on interface forming,microstructure and mechanical properties of joints.The Zn foil and Sn foil were used as the intermediate layer in the test.The interfacial diffusion of Mg and Al was blocked and the compound type and distribution state of the microstructure in the joint interface were changed.Thus,the mechanical properties of the joints could be improved to some extent and the welding energy could be reduced at the same time.Experimental results show that the Mg/Al connected interface was relatively straight with no obvious vortex-shaped plastic deformation and mechanical occlusion.No intermetallic compounds were formed in the joint interface when the welding energy was 200 J.But a thinner layer of Mg-Al intermetallic compounds were formed in the interface when the welding energy reached 500 J.The thickness of intermetallic compounds layer increased gradually with the further increase of welding energy.Moreover,the intermetallic compounds layer became more uniform at the same time.When the welding energy reached 1700 J,the crack was present in the interface.The intermetallic compounds layer was thicker and the crack defects were obvious when the welding energy reached 2500 J,which seriously deteriorated the joint quality.The shear force of joints increased first and then decreased with the increase of welding energy or welding static pressure,while gradually increased with the increase of welding amplitude.The shear force of joints reached the maximum of 1232.3 N(the shear strength was about 30.38 MPa)when the welding energy was 1400 J,the welding amplitude was 95%(9.5 ?m)and the welding static pressure was 0.4 MPa.The joint interface was well formed and the area of interfacial bonding zone increased after the Zn foil was selected as the intermediate layer.And the flat connection interface was transformed into a wavy interface with uneven thickness,eddy current characteristics and network diffusion characteristics.According to the forming characteristics,it could be divided into four regions of a vortex-shaped plastic deformation zone,a thinner flat zone,a grid-like diffusion zone and a thicker flat zone.The joint interface was mainly composed of a Mg-Zn liquid layer,a residual Zn intermediate layer and a Zn-Al solid solution layer,without the formation of Mg-Al intermetallic compounds.The Mg-Zn system structure and Zn-Al solid solution structure with relatively good performances improved the mechanical properties of joints and reduced the welding energy.The energy saving effect was remarkable.The shear force of joints increased first and then decreased with the increase of welding energy.The shear force reached the maximum of 2337 N(the shear strength was about 57.62 MPa)when the welding energy was 1700 J,which was increased by nearly 89.66% compared with the joints without any intermediate layer.And the fracture mode of the joints was changed from brittle fracture to the ductile-brittle mixed fracture dominated by brittle fracture.When the Sn foil was selected as the intermediate layer,the joint interface was well formed and the interfacial bonding zone was larger than that of the joints with the Zn intermediate layer.The connected interface was undulating with obvious vortex-shaped plastic deformation and mechanical occlusion.According to the forming characteristics,it could be divided into four regions of an undulating zone,a thinner flat zone,a vortex-shaped plastic deformation zone and a thicker flat zone.The joint interface was mainly composed of a Mg-Sn eutectic structure layer and a residual Sn intermediate layer without any Mg-Al intermetallic compounds.And a small amount of Mg-Sn granular eutectic structure was mixed inside the residual Sn intermediate layer at the undulating zone.The presence of Mg-Sn system structure with relatively good performance reduced the brittleness tendency of joints and improved the mechanical properties of joints successfully.The welding energy was reduced while obtaining high-quality joints.And the energy saving effect was more remarkable.The shear force of joints increased first and then decreased with the increase of welding energy.The shear force of joints reached the highest level when the welding energy was 1100 J,which was 3465.5 N(the shear strength was about 85.22 MPa).The shear strength increased by 47.90% compared with that of joints with a Zn intermediate layer and by 180.51% compared with that of joints without any intermediate layer.