| The hybrid structures of Al alloy and Mg alloy in manufacturing of lightweight vehicles can not only make full use of the respective advantages of Al and Mg alloys,but also meet the increasingly stringent requirements of economic and social development for energy conservation and emission reduction.It is a challenge to realize high quality welding of Al/Mg alloys.Friction stir welding(FSW),as a solid-state join process,has the potential to achieve high-quality welding of Al/Mg alloys.To further improve the microstructure and properties of dissimilar Al/Mg FSW joints,our research group has developed a new process——ultrasonic vibration enhanced friction stir welding(UVeFSW),which has good process effectiveness in welding of Al/Mg alloys.Previous studies showed that compared with Al/Mg FSW welds,the thickness of hard and brittle intermetallic compounds(IMCs)in Al/Mg UVeFSW welds was thinner,and the amount is less.How does ultrasonic vibration suppress IMCs and improve the mechanical properties of dissimilar Al/Mg joints?Up to now,no convincing explanation has been given.Therefore,it is essential to continue deep study on the comparative experiments of dissimilar Al/Mg FSW and UVeFSW,to observe and characterize the dynamic evolution behaviors of weld microstructure and IMCs,and reveal the influence and action mechanism of ultrasonic vibration.It has very important academic significance and engineering application value for realizing the optimization of dissimilar Al/Mg UVeFSW process.Butt FSW and UVeFSW of AA 6061-T6/AZ31B-H24 plates,with thickness of 3 mm are carried out.Based on the observation and characterization of the weld microstructure,the mensurement of the joint tensile strength,and the analysis of the fracture location and fracture surface,the welding process conditions of AA 6061-T6/AZ31B-H24 were optimized.Electron backscatter diffraction(EBSD)technique was used to characterize the evolution of grain structure at different depths in the nugget zone(WNZ)on both sides of the bonding interface,and the effect of ultrasonic vibration on the evolution of grain structure was analyzed.The results show that the recrystallization mechanism in Al alloy side is continuous dynamic recrystallization(CDRX)and geometric dynamic recrystallization(GDRX),while the recrystallization mechanism in Mg alloy side is CDRX and discontinuous dynamic recrystallization(DDRX).CDRX plays a major role on both sides.At the transverse crosssection of welds,the average grain size on both sides increases first and then decreases from top to bottom.Ultrasonic vibration has no effect on the recrystallization mechanism in the WNZ,but it can refine the grain size slightly and improve the recrystallization degree in the whole WNZ.A few observation locations were selected at transverse cross-section of FSW/UVeFSW welds,and the thickness of IMCs at the Al/Mg bonding interface was determined by scanning electron microscopy(SEM)and energy dispersive spectroscopy(EDS).The results show that with the increase of the distance from the upper surface of the weld,the thickness of IMCs first increases and then decreases,and the peak value of IMCs thickness appears at the position about 1 mm from the upper surface of the weld.With the application of ultrasonic vibration,the variation trend of IMCs thickness along the bonding interface is not changed,but the IMCs thickness on the whole bonding interface is decreased significantly.Two horizontal cross-sections HP-1(2.2 mm from the bottom of the plate)and HP-2(1 mm from the bottom of the plate)were selected to observe the average thickness of IMCs along the Al/Mg bonding interface behind the exit hole at the end of the weld.With the increase of the distance from the exit hole,on HP-1,the average thickness of IMCs first increases,then decreases,and finally stabilizes at the edge of the shoulder,while on HP-2,the average thickness of IMCs gradually increases,and then reaches a stable value.With the application of ultrasonic vibration,the thickness variation trend of IMCs on HP-1 remains unchanged,but the peak value of IMCs thickness occurs at a position closer to the exit hole,and the thickness of IMCs(including the peak value and the thickness at stable stage)decreases.On HP-2,the average thickness of IMCs still increases first,then decreases,and finally stabilizes.For the formation and growth of IMCs in the banded zone,ultrasonic vibration also plays an inhibitory role.The microstructure of IMCs was observed by transmission electron microscope(TEM).No eutectic structure was found in dissimilar Al/Mg butt weld.The wireless temperature measurement system was used to measure the maximum temperature in the WNZ during the process of FSW and UVeFSW,which is about 370℃.It is far lower than the eutectic reaction point of Al-Mg.Based on these two points,it is clear that the formation mechanism of IMCs is complete solid-state atomic diffusion under the process conditions adopted in this study.The metallographic structure around the pin in the welding process was obtained by"emergency stop"+liquid nitrogen cooling technology.Based on the horizontal cross-section of the weld,the evolution of the interface microstructure of Al/Mg material was observed during the whole process from early stage of plastical deformation at the leading side,material flow to the retreating side,and deposition at the trailing side of the pin.In the welding process,under the shear driving action of the tool,the Mg alloy material on the advancing side flows into the retreating side.At the same time,the oxide layer on the original faying surfaces are gradually broken.Then,the width of Mg alloy layer decreases with the increase of strain/strain rate.After that,IMCs/matrix sandwich structure is formed on the Al/Mg interface,and metallurgical bonding is realized between Al/Mg materials.When the interface material is deposited behind the pin,a zigzag interface is formed between Al/Mg materials,and there is a simple IMCs layer on the interface.At this time,metallurgical bonding and mechanical interlocking are realized between Al/Mg materials.With the application of ultrasonic vibration,the volume of the deformed material driven by the pin increases,but the material flow and evolution process is similar to that in FSW.By near in-situ observing the evolution of Al/Mg interface microstructure in the welding process,it is found that Al/Mg materials can interpenetrate and intermix with each other when flowing around the pin,forming fine strips on or near the interface.These strips react with the surrounding matrix to form IMCs,and the type of IMCs is related to the strip width.When the material flows in the retreating side of the pin,the Al/Mg interface is a complex multilayer structure(IMC+Mg+IMC+Al).When the material is deposited at the trailing side,the interface becomes a sample IMCs with two sublayers(β+γ).With the application of ultrasonic vibration,the multi-layer structure only appears in the middle stage of the material flow around the pin.When the material begins to deposit on the trailing side of the pin,the Al/Mg interface is the same as that in FSW.But at this time,the ultrasonic vibration has begun to play a role in inhibiting the growth of IMCs,so that the thickness of IMCs in UVeFSW is significantly reduced.Considering the effects of ultrasonic vibration on the evolution of grain structure,dislocation density and precipitation phase in A1 alloy matrix,the mechanism of ultrasonic vibration in dissimilar Al/Mg FSW was analyzed.EBSD characterization and XRD microanalysis show that ultrasonic vibration can reduce the dislocation density at the initial position of deposition behind the pin and in the final weld.Therefore,ultrasonic vibration can reduce the dislocation density in the WNZ,and the atomic diffusion rate is reduced.Thereby,the formation and growth of IMCs are suppressed. |
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