Research On 6061Al/AZ31Mg Dissimilar Alloy FSW Process And IMC Characteristic Of Joint

As high strength and lighter metals,aluminum alloys and magnesium alloys have been widely used in the aerospace field.However the problem of joining Al alloy and Mg alloy has become the focus of the industry.Cracks,porosity and a large number of intermetallic compound?IMC?are easily found in the fusion welding joint,which severely reduces the strength of the joints.Friction stir welding is a solid welding method and the low welding heat input can reduce the number of IMC,which has great potential for development in industrial application.This paper used the friction stir welding to join the Al and Mg dissimilar metals.Based on the optimization of parameters,the types,distribution and formation mechanism of IMC and the influence of IMC on the performance of weld were investigated using SEM,EDS,XRD,TEM and DSC and established the relationship between IMC and the joint performance.In order to improve the properties of the Al/Mg welding joint,the transition layer of zinc was added into the interface between Al and Mg plates.Moreover,the formation mechanism and the impaction of IMC on the performance in the joint were studied detaily.Firstly,the FSW process of tools offset in the 6061Al/AZ31Mg was studied.Results show that when the Al alloy was placed on advancing side?AS?and the tool offset to the Al side 0.5 mm,the welding process window was bigger.Defect-free Al/Mg FSW joint with80MPa tensile strength were obtained at rotating speed and welding speed of 1000 rpm and250 mm/min,respectively.Although at low welding speed of 50 mm/min,the strength of the Al/Mg dissimilar joint was higher and reach to 93MPa,tunnel and grooves defects were easy to form at the ending of welding position.When Mg alloy was placed at AS or the tool offset to the Al side,the welding process parameter range of sound-formed joints was very narrow and even the weld was difficult to form,and the joint strength is generally at about20 MPa.Fracture location of joints are the combination interface between Al and Mg plates,and the fracture model is brittle fracture.Secondly,the formation mechanism of IMC and its effects on the joint performance were studied.The combination interface between Al and Mg dissimilar metals of FSW joints occurs stratification phenomenon.Through the EDS analysis found that from Mg side to Al side,the IMC layer mainly consisted of these three layers:??Mg?+Al₁₂Mg₁₇,Al₁₂Mg₁₇+Al₃Mg₂,Al₃Mg₂+??Al?,respectively.The structure in severely mixed zone and the banded structure are mainly composed of Al₃Mg₂ and Al solid solution,although there are some Mg-rich regions,no Al₁₂Mg₁₇ intermetallic phase is found.Petal-like Al₁₂Mg₁₇gets together in the weld,and every petal shape structure has tip by means of TEM.The effects of the thickness and count of IMC on the joint tensile strength do not show obvious regularity.But usually the tensile strength of joints is higher,the IMC count is more and thickness is bigger,this is mainly related to fully mixing between dissimilar metals.Once the defects of holes and tunnel form in the weld,the joint tensile strength will rapidly descend.By placing Al alloy on AS and tool offsetting to aluminum side,IMC amount is relatively small and the joint performance is also higher relative to by placing the magnesium alloy on AS and tool offsetting to magnesium side.Finally,a transition layer of zinc was added between Al plate and Mg plate to research the effect mechanism of zinc interlayer on FSW dissimilar metal joints.If the zinc layer is added under the same welding parameters,the welding crack will not easy to appear in joints,and the tensile strength can reach up to 102 MPa.The joint without addition of Zn interlayer is easy to crack after welding when the tool offset to Al side 1.9mm,however the joint strength can reach 84MPa with the addition of Zn layer.When the zinc interlayer is added,the zinc element easily to accumulate in the interface and its distribution is very uneven.The zinc layer which was not completely smashed up will influence the mixing between Al and Mg materials,and reduce the performance of the joint,which will cause the appearance of holes and crack defects.The joint of Mg-zinc-Al reduces the amount of Al-Mg intermetallic compound,and the interface is mainly composed of MgZn,MgZn₂,Al₅Mg₁₁Zn₄ eutectic organization and so on.