Effects Of Stirring Needle Morphology And Auxiliary Heating On The Formation Of Aluminum/Magnesium FSW Weld

In this paper,Friction stir welding tests were carried out on 5A06 aluminum alloy and AZ31B magnesium alloy with 20mm thickness by using three different shapes of stirring needles and adding auxiliary heating device.By changing the number of mixing pin thread,the influence on weld forming was studied,and the physical model of defect formation was established.By changing the auxiliary heating temperature,the change rule of temperature field was studied,and the mechanical properties,microstructure,phase composition and fracture morphology of the joint were analyzed.It provides a reference for optimizing the design of mixing needle and FSW of aluminum/magnesium heterogeneous metals.The increase of the number of mixing pin threads will increase the driving force of the plastic metal on the weld and increase the flow of the plastic metal to the bottom of the weld,so as to improve the weld defects such as holes and cracks.The welding joint with good surface formation and no obvious defects can be obtained by using three-head thread stirring needle.For single-head and double-head thread stirring needles,three-head thread stirring has a stronger effect on the flow of plastic metal along the thickness of the plate.With the increase of the number of mixing pin threads,the driving force of the plastic metal of the weld is greater,and the amount of plastic metal flowing to the bottom of the weld is more,which is reflected in the larger area of the weld core.The problem of large temperature difference along the thickness of the plate can be reduced by preheating the 20mm aluminum/magnesium alloy before welding with an auxiliary heating device.When the auxiliary heating temperature from room temperature to 280℃,the minimum weld,the lower temperature difference only 30℃,the highest degree of softening,metal weld plasticizing metal quantity increase,through stirring needle thread surface and the interaction of friction between the metal,the plasticizing of suction at the bottom of the weld metal quantity increase,for achieving nuclear area of welding.SEM analysis was carried out on the upper,middle and lower layers along the thickness of the plate.The fracture forms of the joints were all brittle fractures,and the fracture locations occurred at the interface of Mg side.Compared with the middle part of the interface on Mg side,more serious liquefaction occurred on the upper part,while no liquefaction occurred on the lower part of the interface on Mg side.EDS and XRD analysis on the interface reaction layer showed that Al₁₂Mg₁₇7 and Al₃Mg₂ intermetallic compounds existed,and the continuous existence of brittle and rigid intermetallic compounds was the main factor for the brittle fracture of the joint.