Experimental Study On Ultrasonic Assisted FSW Of 2A12/AZ31B Dissimilar Alloys

Nowadays,hybrid structures of 2A12 aluminum alloy and AZ31B magnesium alloy which have the advantages of light weight and high specific strength,are in great demand in aerospace industry.Due to their great differences in crystal structure and physical and chemical properties,high quality welding of dissimilar 2A12 and AZ31B alloys is a key problem to be solved urgently.In this study,friction stir welding(FSW)and ultrasonic assisted friction stir welding(UVaFSW)experiments were carried out to butt weld the 2A12 and AZ31B plates with 3 mm thickness.Through comparison,the effects of ultrasonic vibration on the weld formation,macroscopic metallograph and mechanical properties of the joints were investigated.The results show that when the aluminum alloy was placed on the advancing side and the offset of the tool was 0 mm,the weld formation was statisfactory and the tensile strength of the joint was the highest.Applying ultrasonic vibration enlarged the window of welding process parameters,improved the weld surface morphology,increased the mechanical properties of joints,and reduced the tendency of cracks and cavities at the boundary between thermos-mechanic ally affected zone and weld nugget zone(WNZ)on the advancing side.When ultrasonic vibration was applied,the mixing of Al/Mg dissimilar materials was more uniform,and the mechanical interlocking degree of Al/Mg dissimilar materials in the WNZ was higher.Due to the presence of intermetallic compounds(IMCs),there were high hardness regions in the WNZ,mainly at the Al/Mg interface and the edge of the WNZ on the advancing side.With application of ultrasonic vibration,the hardness distribution in the weld nugget zone was more uniform.The fracture mode of 2A12/AZ31B joints was brittle fracture with obvious river pattern and cleavage fracture characteristics.The existence of brittle intermetallic compounds was the main factor leading to the fracture of the joint.The effect of ultrasonic field on the grain structure evolution in WNZ of 2A12/AZ31B dissimilar joints was investigated by EBSD experiments on both sides of the Al/Mg interface.The recrystallization mechanisms of the aluminum alloy side grains in the WNZ were continuous dynamic recrystallization(CDRX)and geometric dynamic recrystallization(GDRX),while the recrystallization mechanisms of the magnesium alloy side grains were continuous dynamic recrystallization(CDRX),discontinuous dynamic recrystallization(DDRX)and geometric dynamic recrystallization(GDRX).The average grain sizes of aluminum alloy and magnesium alloy in the WNZ were about 0.7 μm and 2.5 μm,respectively.The proportion of low-angle grain boundaries was reduced and the recrystallization degree of grain was increased by applying ultrasonic.The intermetallic compounds(IMCs)were characterized by scanning electron microscope(SEM),electron probe(EPMA)and transmission electron microscope(TEM).The effects of ultrasonic vibration on the morphology,thickness and distribution ofIMCs in WNZ were investigated,and the factors influencing the IMCs formation were analyzed.It is found that the thickness of IMCs was larger in the top and middle part of the WNZ and thinner in the bottom part.The thickness of IMCs was decreased when ultrasonic vibration was applied.The thickness of IMCs in dissimilar FSW of 2A12/AZ31B alloys was larger,and the maximum thickness could exceed 100 μm.The temperature measurement experiments shown that there was little difference in welding temperature between 2A12/AZ31B FSW and AA6061/AZ31B FSW,and the difference in welding temperature was not the main reason for the larger thickness of IMCs in 2A12/AZ31B weld.However,the chemical composition of 2A12 aluminum alloy differs greatly from that of AA6061.2A12 aluminum alloy contains more copper element.In the 2A12/AZ31B welding process,the grain boundaries of magnesium grain protruded and nucleated,aluminum atoms diffused into the magnesium grain,γ phase(Al12Mg17)nucleated at the interface first,and then β phase(Al3Mg2)precipitated at the grain boundary of γ phase and grew into the grain of γ phase first.The continuous growth of the grain toward the aluminum side led to the concentration of copper on the β phase side,which resulted in the decrease of the nucleation power of the recrystallization and the growth of IMCs grains.This is the main reason for the thicker IMCs at 2A12/AZ31B FSW.