Study On Microstructure And Mechanical Properties Of Friction Stir Welding Joint For Aluminum/Magnesium Dissimilar Metal

Aluminum alloy and magnesium alloy with low density and high specific strength,and magnesium alloy also has excellent damping property and electromagnetic shielding effect are the two most commonly used lightweight metal structural materials,and which widely used in aerospace,national defense military industry and vehicles and other fields.In order to give full play to their performance advantages,it is necessary to weld them to form a composite structure.Mg-Al brittle intermetallic compounds(IMCs)are easily formed in the welding process of magnesium/aluminum dissimilar metals,which severely weaken the strength of joint and limit its application in the industry of equipment manufacturing.Friction stir welding(FSW)is a solid-phase bonding technique,which can minimize the formation of brittle Mg-Al series IMCs due to low heat input.It is one of the most promising welding technologies for Al/Mg dissimilar metals.In this paper,the finite element software ANSYS was firstly used to establish the three-dimensional thermodynamic coupling finite element model of 6061 aluminum alloy(T6)and AZ31B magnesium alloy FSW flat joint with a thickness of 4 mm.The temperature field and residual stress field in the joint are calculated,and the evolution law of process parameters on the temperature field and residual stress field was explored,and the appropriate range of process parameters was found out.Based on the results of finite element simulation,the plate butt tests of 6061 aluminum alloy and AZ31B magnesium alloy with thickness of 4 mm were carried out by using FSW equipment.The surface formation and microstructure of Al/Mg dissimilar metal FSW joint were observed by using type microscope,optical microscope(OM),scanning electron microscope(SEM),transmission electron microscopy(TEM),X-ray diffraction(XRD)and energy dispersive spectrometer(EDS),and the WNZ banded structure characterization of intermetallic compounds(IMCs)were characterized.The microhardness tester and electronic universal testing machine were used to test the mechanical properties of the joint,and the evolution law of process parameters on microstructure and mechanical properties of Al/Mg dissimilar metal FSW joint was explored.The results of the study are as following.(1)The temperature field and residual stress field of aluminum/magnesium dissimilar metal FSW joint are asymmetrically distributed,the peak temperature is about 465?at about 5mm of the magnesium side of WNZ.The residual stress is mainly longitudinal residual stress(?_x)and the peak value is about 230MPa on the aluminum-biased side of WNZ.With the increasing of rotating speed,the peak temperature increases and?_xincreases.With the increasing of welding speed,the peak temperature decreases slightly and?_xincreases slightly.(2)The bias behavior of the stirring head has a great influence on the macroscopic forming of the joint.When the stirring tool is offset towards magnesium or in the middle,the badly deformed area at the bottom of the weld nugget presents a chaotic sandwich structure.The welding joint with smooth surface and no defect can be obtained by mixing needle with 0.3mm aluminum offset and proper process parameters.Aluminum/magnesium dissimilar metal friction stir welding joints are mainly divided into weld core zone(WNZ),thermal mechanical influence zone(TMAZ)and heat influence zone(HAZ).FSW enables metallurgical bonding of aluminum/magnesium dissimilar metal joints without the addition of an intermediate layer.However,adding the intermediate layer of Zn cannot make the joint form an effective connection.(3)There is an obvious banded structure in the WNZ of Aluminum/magnesium dissimilar metal FSW joint.The banded structure is composed of Mg matrix,Al alloy strips inserted into the Mg matrix and IMCs dispersed on the banded structure.The IMCs are mainly small grain Al₁₂Mg₁₇and large grain Al₃Mg₂.Crack nucleation and propagation of Aluminum/magnesium dissimilar metal FSW joint occurred in banded structure.With the increasing of the rotational speed(?)or the decreasing of the welding speed(?),the banded structure is curved,the length is relatively short and discontinuous distribution.When the speed(?)is too high or the welding speed(?)is too low,the banded structure becomes thinner,but the number and size of IMCs increase.The strength of Aluminum/magnesium dissimilar metal FSW joints is mainly determined by the banded structure shape and the size and number of IMCs.The optimal welding parameters of Aluminum/magnesium dissimilar metal FSW are as follows:rotating speed is 750r/min,welding speed is 20mm/mim,joint strength reaches 131MPa,which reaches 61%of Magnesium alloy base material and 42%of Aluminum alloy base material.The micro vickers hardness distribution of Aluminum/magnesium dissimilar metal FSW joint is not uniform,micro vickers hardness of HAZ began to decrease gradually,micro vickers hardness value of TMAZ decreased to the lowest,and the micro vickers hardness of WNZ reached the peak due to fine grain strengthening and IMCs.