Research On Microstructure And Properties Of Mg/Al Dissimilar Alloy Joint By Friction Stir Welding

The Friction Stir Welding?FSW?technology for magnesium/aluminum alloy was studied in order to solve the technical problem of connection between magnesium alloy and aluminum alloy.The FSW process experiments were carried out on the AZ31B magnesium alloy and 6061-T6 aluminum alloy by abutting joint.According to the welding characteristics of the dissimilar alloy,the stir welding joint with different material and structure was designed.The formability of the weld under different process parameters was studied.The microstructure of the joint was characterized by OM,SEM,XRD,EDS and other analytical methods.The effects of process parameters on the mechanical properties and corrosion properties of the FSW joint were obtained by microhardness test,tensile test,electrochemical corrosion test and immersion corrosion test.The process parameters of mg/Al dissimilar alloy FSW can be optimized by the above research.The main results of this paper are as follows:When the rotating speed of the stirring head is 750 rpm,the welding speed is 23.5⁷5mm/min,and the offset of the stirring needle is 0.3 mm to the magnesium alloy side,the defect free of FSW joint of magnesium alloy/aluminum alloy material can be successfully obtained.In the FSW joint zone of magnesium/aluminum alloy,strip interlayer is obvious,fine grain of equiaxed grain is formed by grain refinement,and a small amount of intermetallic compound of Al₃Mg₂ and Al₁₂Mg₁₇ are formed at the interface of magnesium/aluminum alloy.The grain is elongated in the thermo mechanical zone,and grain coarsening in the heat affected zone.Because of the existence of intermetallic compounds,the abrupt increase of hardness at the interface of magnesium/aluminum dissimilar alloy in FSW joint nugget area is observed.With the decrease of welding speed,the uniformity of grain mixing is enhanced,and the low gradient effect makes the tensile strength increase.When the rotating speed of the stir-welding head is750 rpm,the welding speed is 23.5 mm/min and the offset of the stirring needle is 0.3mm,the joint performance is the best,the tensile strength is 188 MPa,and it is 70%of the strength of magnesium alloy.Fracture occurs in the thermal mechanical influence zone at the advancing side of the joint,and the fracture mode is brittle fracture.In the 3.5 wt.%NaCl solution medium,the electrochemical corrosion behavior of magnesium alloy parent material,aluminum alloy base material and FSW sample did not appear obvious passivation.The corrosion current density of FSW samples is 6.4546 A cm^-2,which is 28.2%and 43.7%higher than that of the base metal and the base metal respectively.In the immersion corrosion test of 3.5 wt.%NaCl solution medium,the corrosion mode of FSW sample is mainly pitting,and the corrosion degree of the partial advancing side is higher than that on the retreating side.The formation of galvanic pair between magnesium alloy and aluminum alloy was formed,and the micro battery composed of magnesium alloy as anode accelerated the corrosion of the joint,which made the corrosion resistance of the FSW sample lower than that of the two kinds.