Microstructure And Properties For Weld Of Friction Stir Welding For AZ31B Mg/5052Al Dissimilar Materials

With the continuous development of green industry,more and more attention has been paid to the research of automobile lightweight.As high-quality lightweight materials:magnesium alloys and aluminum alloys,they have the characteristics of light weight and high specific strength,and have a wide range of applications in automobile manufacturing,rail transportation and other fields.Therefore,achieving a reliable connection of magnesium/aluminum dissimilar alloys will have advantageous advantages in future industrial development.In this paper,the friction stir butt welding and overlap welding of 3mm AZ31B magnesium alloy and 5052 aluminum alloy are studied respectively.The effects of different welding process parameters（rotation speed,welding speed）on the formation,microstructure and mechanical properties of the joint were studied.Based on this,the effect of Sn foil interlayer on friction stir welding of magnesium/aluminum dissimilar alloys was conducted.In addition,a new welding process was attempted in this paper.The microparticle thermal reaction-assisted magnesium/aluminum dissimilar alloy friction butt welding was performed,and the microstructure and properties of the welded joint were analyzed.Conclusion as below:⑴In the welding process,with a feed speed of 50mm/min and a rotation speed of700-1500 rpm,friction stir lap welding of 5052 aluminum alloy and AZ31B magnesium alloy with or without Sn interlayer can be obtained without obvious welding defects.The joint interface of the Sn-containing interlayer contains Mg₂Sn intermetallic compounds（IMC）instead of the brittle Mg-Al IMC in the direct solder joint.The principal role of Sn is to increase metal fluidity.The maximum lap shear failure load of the direct weld head was achieved at 1300 rpm.The maximum lap shear failure load with the addition of the Sn interlayer was reached at 900 rpm.The addition of the Sn interlayer further saved energy.In Mg/Al welded joints,the speed required to achieve maximum lap shear strength was reduced from 1300 rpm to 900 rpm.For joints without Sn interlayer,the micro-morphology is river-like,and they are mainly brittle fractures.For joints with Sn interlayers,the fracture topography contains a small number of dimples,and there are hardly point phases in the dimples,which appear as mixed fractures.⑵In the welding speed range of 6-45mm/min and rotation speed of 700-1100 rpm,the friction stir butt welding of magnesium/aluminum dissimilar alloy can obtain good welding morphology.At a feed speed of 25mm/min,as the rotation speed increases,the performance of the welded joint increases first and then decreases.At a rotation speed of 900 rpm,as the feed speed continues to grow,the performance of the welded joint first increases and then decreases.The performance of the welded joint can obtain the maximum tensile strength at a rotation speed of 25mm/min and a rotation speed of 900rpm.The performance reaches 154.63 MPa.When the rotation speed is 25mm/min and the rotation speed is 900 rpm,the thickness of 0.03mm Sn foil is added,and the joint performance reaches 165.42 MPa,which is slightly improved compared with direct welding.⑶At a welding speed of 900 rpm and a feed speed of 6mm/min,a welded joint without obvious welding defects such as cracks was obtained.For the friction stir welding of magnesium/aluminum dissimilar alloys assisted by the thermal reaction of particulates,the weld joint has a small flash.The maximum tensile strength of the magnesium/aluminum dissimilar alloy welded joint assisted by particle thermal reaction can reach 149 Mpa,which is 67%higher than that of direct welding.In addition,after adding the microparticle thermal reaction mixed powder,the fracture is indicated by quasi-cleavage fracture,and the fracture of the directly welded joint is shown as brittle fracture.After adding the mixed powder,Mg₁₇Al₁₂,Mg₂Al₃ and AlNi phases were found in the welded joint.The width of the transition layer in the mixed region of the Mg-Al interface reaches 150μm,which is much larger than the 25μm of direct welding.The hardness distribution shows that the maximum hardness of self-reaction-assisted friction stir welding joints of magnesium/aluminum dissimilar alloy is lower than that of direct welded joints.At the same time,the width of the sudden hardness increase zone is increased compared to the width of the direct weld.