Effect Of Ultrasonic Vibration On Microstructure And Properties Of Friction Stir Welding Joint Of Aluminum/Magnesium Dissimilar Alloys

Due to the higher and higher requirements for energy conservation,emission reduction and environmental protection,lightweight structures are increasingly used.Composite structures of aluminum(Al)alloys and magnesium(Mg)alloys have the advantages of light weight and good comprehensive performance,and have broad application prospects in the automotive,aerospace,high-speed train,energy,shipbuilding industries,etc.When Al/Mg dissimilar alloys are joined,it inevitably forms hard and brittle Al-Mg intermetallic compound layers(IMCLs),which deteriorate the mechanical properties of the weld joint.Compared with other joining processes,friction stir welding(FSW)has a big advantage for joining Al/Mg dissimilar alloys.However,conventional FSW still has following problems:large welding load,low welding speed,narrow range of optimum process parameters and large thickness of IMCLs at the Al/Mg interface.To solve these problems,the ultrasonic vibration enhanced friction stir welding(UVeFSW)was developed and applied to the dissimilar welding of Al and Mg alloys.By comparing and analyzing the microstructures and properties of FSW and UVeFSW joints of Al/Mg dissimilar alloys,the effects of ultrasonic vibration on weld formation,material flow and IMCLs in joining of Al/Mg disimilar alloys were investigated in this work.It is of great theoretical significance and engineering practical value to reveal the mechanism of UVeFSW process,improve the microstructure and properties of Al/Mg dissimilar alloys joints and promote the wide application of Al/Mg composite structures in the automotive industry.The butt welding of Al/Mg alloy sheets were carried out by FSW and UVeFSW with varied conditions,such as different relative positions(the Al plate and the Mg plate were respectively on the advancing side),pin offset values(the pin was located on the Al plate or the Mg plate),welding process parameters and ultrasonic output power.The weld formation,the macrostructure of the weld at traverse cross section and the morphology of the Al/Mg bonding interface were analyzed using a metallographic microscope and a scanning electron microscope(SEM).The results indicate that ultrasonic vibration could obviously improve the weld surface appearance and eliminate some of the weld defects,such as voids,tunnels and unbonding,which expands the process parameters-window for sound weld formation.In addition,ultrasonic vibration facilitates the mechanical interlock features around the Al/Mg bonding interface.On the other hand,real time monitoring the welding load during FSW and UVeFSW processes shows that the ultrasonic vibration could significantly decrease the traverse force,tool torque and spindle power,and reduce the axial force in most cases.The types,quantity,morphology and distribution of IMCs in weld nugget zone(WNZ)of Al/Mg joints with and without ultrasonic vibration were examined and characterized by SEM equipped with energy dispersive spectroscopy(EDS).Application of ultrasonic vibration could diminish IMC bi-layer(Al12Mg17 +Al3Mg2)at Al/Mg bonding interface into Al12Mg17 or Al3Mg2 mono-layer.and greatly reduce the thickness of the continuous IMC single-layer or bi-layer existing at the Al/Mg bonding interface.When the ultrasonic output power reaches 340W,UVeFSW even completely removes the IMC layer,but it still retains the good characteristics of metallurgical bonding at the interface.Investigation on the brittle Al-Mg IMCs inside the mechanically mixed region of Al and Mg dissimilar alloys in the nugget zone suggests that as the ultrasonic amplitude(output power)was increased the number of IMC bands that severely deteriorate the mechanical properties of the joints remarkably decreases,the IMC bands become shorter,slimmer and eventually disappeared due to ultrasonic shattering,and the distribution of IMC particles becomes uniform and dispersed while the number of IMC particles is gradually reduced.Thermal and material flow characteristics in friction stir welding with and without ultrasonic vibration were comprehensively analyzed to reveal the physical mechanism of inhibiting IMCs by ultrasonic vibration.Ultrasonic vibrations eliminate the thick insulation layer in shoulder affected zone(SAZ)and onion-ring-like structure in pin affected zone(PAZ)during UVeFSW,and promotes the upward flow behavior of the plastic material in the inner shear layer of the weld nugget zone,and causes the plastic material in the outer shear layer to be transferred from the weld bottom to the shoulder affected zone(SAZ).The application of ultrasonic vibration promotes the flow of material in the shear layer to reduce the heat generation,and improves the flow and transmission of plastic material over the weld nugget zone to accelerates the dissipation of heat,and thus inhibits the partial formation of the IMCs during the welding process and reduces the thickness of the IMCLs on the Al/Mg bonding interface.Tensile test shows that the tensile strength of the UVeFSW joints can be increased by up to 34%.This is due to the thickness reduction of the IMC layer at Al/Mg bonding interface,the increase of the mechanical interlocks around the Al/Mg joining interface and the improvement of the morphology and distribution of IMCs in the nugget zone,which could enhance the strength of the AI-Mg bonding interface,and even the strength of the whole joints.The measurement of the microhardness distribution shows that the fluctuations of the microhardness in UVeFSW joints can be significantly reduced due to the improvement of the distribution of IMCs in weld nugget zone,which ensures the stability of the joint performance.Finally,it should be noted that when IMC layers are present at the Al-Mg bonding surface,the overall thickness of the IMC layers which is most beneficial to the tensile strength of the joints is 1.3 ?m.Thinner or thicker than this level can decrease the tensile strength of the joints.