Suppression Of Intermetallic Compounds And Enhancement Of Joint Quality In Friction Stir Welding Of AA6061-T6 And AZ31B

Aluminum(Al)and its alloys(AA6061-T6)have been widely used in numerous structural parts by a number of transportation and electronics industries due to their high specific strength and low cost.Meanwhile,the magnesium(Mg)and its alloys(AZ31B)are also drawing special attention as alternative lightweight material due to their lightweight and impressive specific stiffness.In order to gain the advantages of both,a successful joining of both the metals is of prime concern.Various fusion welding processes have been attempted to join AA6061-T6 to AZ31B in past,however,most of them proven unattractive due to compromised weld strength.Besides,the massive formation of brittle intermetallic compounds(IMCs)into the fusion weld zone has further degraded the joint strength and hence limited their industrial applications.Though a solid-state joining technique,friction stir welding(FSW)can produce satisfactory joints of AA6061-T6 and AZ31B Mg alloy,however,the IMCs are still found in the weld zone and hence the joint properties are compromised.Besides,the requirement of a large amount of axial downward force and torque have further limited its industrial application.In order to enhance the joint strength by suppressing the IMCs and reduce the tool wear by decreasing the welding loads during FSW of AA6061-T6 and AZ31B Mg alloy,the addition of ultrasonic-assistance in the FSW process has been proposed in this study.The addition of ultrasonic assistance into the stirred zone(SZ)can also be beneficial to escalate the material mixing and suppress the defects formation.In order to successful and effective implementation of ultrasonic vibration assistance in the FSW process,the appropriate addition of various ultrasonic components with the FSW machine is vital to ensure the fulfillment of its objectives.The design and development of the ultrasonic vibration system are carried out as per the endpoint requirement.It includes the design of customized ultrasonic horn and the selection of other ultrasonic components.To facilitate rigid contact between the stationary ultrasonic horn and rotating FSW tool,novel horn attachments(horn front part,roller bearings,etc.)have been added at the horn output end.The design features of customized components have been further supported via numerical simulation and experimental validation.Modal and harmonic analysis of ultrasonic horn has been carried out in Solidworks to examine its resonance frequency,amplitude,and von Mises stresses.A special hollow FSW tool geometry has also been designed and developed to amplify the endpoint amplitude and subsequently compared with solid tool geometry via analytical and experimental methods.In order to understand the fundamental mechanism and insights of the ultrasonic-assisted FSW process(UVaFSW),force-torque-power measurement,IMCs and microstructure analysis,and mechanical testing,etc.are made and subsequently compared with respect to their counterparts.The butt and lap configurations of AA6061-T6 and AZ31B Mg alloy have been taken into account and analyzed for a wide range of parameters for both conventional FSW and UVaFSW processes.It is observed that the ultrasonic addition with the FSW tool can improve the material flow and suppress the welding defects.With acoustic addition,a significant diminution in axial downward force,tool torque,and power requirement have been observed.During the butt welding,the ultrasonic assistance has been found effective to shrink the IMCs layer,and facilitate their infant fragmentation,and interruption across the dissimilar interfaces.A diminution in IMCs layer thickness as high as 57.9%has been achieved with acoustic addition.In most of the parameters studied,the maximum effect of ultrasonic addition has been experienced at the weld top region and minimum at the weld bottom.The joint efficiency as high as 71.3%(based on UTS of Mg BM)has been achieved for UVaFSW joints compared to 58.4%of its counterpart.In the case of lap joints,the acoustic assistance has effectively improved the material flow and intermixing across the SZ of the AZ31B Mg and AA6061-T6 layered structures.The SZ of AA6061-T6 side has reported higher micro-hardness compared to that of the AZ31B Mg side which is due to the formation of brittle Al3Mg2 IMCs phase at Al-rich region.The conventional and ultrasonic joints confirm the presence of dual IMCs of nature Al3Mg2 and Al12Mg17.However,XRD spectrum analysis shows a sharp reduction in IMCs peaks and their distribution across the SZ that may be due to enhanced material plasticization in the shear layer during ultrasonic assistance.The diminution in IMCs layer thickness as high as 56.52%has been achieved with acoustic addition.The shrinkage and elimination of IMCs phases with ultrasonic addition can favor a significant rise in weld shear load,equals to 2456.7 N(when AA6061-T6 is kept at the top)and 3667.5 N(when AZ31B Mg is kept at the top).The ultrasonic joints portray several dimple shape features and substantial necking which can replicate the ductile fracture mode contrary to the conventional joints which have brittle fracture failure.With Ni interlayer addition across the AZ31B Mg and AA6061-T6 lapped interface,the joints with higher strength can be produced.The Ni interlayer addition can cease the Mg/Al brittle IMCs while favored the formation of Al/Ni and Mg/Ni intermetallic layers across the Al/Ni and Mg/Ni interfaces respectively.With Ni interlayer,the maximum weld shear load comes out 4390 N which is 920 N higher compared to conventional joint(when AZ31B Mg plate is kept at the top).