| Magnesium and aluminum alloys have been widely used in aerospace industry owing to small density,good strength and stiff ratios.In order to utilize the advantages of two materials and perform optimal allocation,joining of Al and Mg alloys has become necessary,which has been the hotspot and difficulty of welding.As a solid state welding technology,friction stir welding(FSW)can easily obtain higher quality Al/Mg joint than that based on the fusion welding.However,intermetallic compounds(IMCs)always appear at the interface of Al and Mg alloys,which significantly affects mechanical properties.In order to reduce the influence of IMCs on Al/Mg joint of FSW,stationary shoulder and ultrasonic assisted friction stir welding methods were proposed.Chosen 6061-T6 aluminum alloy and AZ31 B magnesium alloy as research object,adhesion of pin,surface formation,microstructures and fracture mode of joint were mainly investigated,while the welding mechanism of FSW of Al/Mg alloys was deeply discussed.The main research contents and achievements can be extracted in the following.Aiming at the problems that big flashes and thickness reduction of stir zone(SZ)appeared in FSW,stationary shoulder assisted friction stir welding was proposed based on the viewpoint of reducing the formation of IMCs.Physical model of material flow of stationary shoulder FSW was established,while the mechanisms which stationary shoulder can improve material flow and broaden Al/Mg welding parameter range were explained from the viewpoints of quick absorption of heat,preventing plasticized material overflowing and increasing upsetting force.When the rotational velocity and welding speed are respectively 1000 rpm and 60mm/min,the maximum tensile strength of Al/Mg joint reaches 137 MPa,which is increased by 28.0% compared with that of traditional FSW for 3mm thick plates.In order to release adhesion of pin,broaden welding parameter range and enhance joint quality,applying the vibration and coupling effect of flow and ultrasonic,ultrasonic aided FSW based on stationary shoulder was put forward.Influences of ultrasonic on adhesion of pin,joint formation,morphology of IMCs,mechanical properties and fracture location of joint were analyzed in details.Physical models of ultrasonic vibration at the interfaces of mixing IMCs layer closely to thermo-mechanically affected zone were established and the ultrasonic aided FSW mechanism was revealed.At higher heat input,high peak temperature can easily results in liquid phase of Al/Mg and then lead to formation of IMCs,in which smash continuous IMCs layer of Al/Mg interface near the TMAZ,delaying formation and propagation of crack.At lower heat input,ultrasonic vibration mainly promotes material flow and degree of atom diffusion,avoiding formation of groove defect resulted from insufficient material flow without ultrasonic.Under the actions of both ultrasonic and stationary shoulder,the 3mm thick and high quali ty Al/Mg joint with tensile strength of 152 MPa is obtained,which is improved by 42.1% compared with conventional FSW.Aiming at the problem that the adhesion of pin is difficult to avoid,pinless friction stir welding of Al/Mg alloys was proposed.Effect of shoulder morphology on joint formation was studied.Material flow behavior of pinless tool was established and optimized six-groove tool which can obtain small flash and improve vertical material flow.During pinless friction stir welding of Al/Mg alloys,the additions of stationary shoulder and ultrasonic can effectively broaden welding parameter range and improve material flow.Compared with traditional proc ess,the welding depth was increased from 0.7mm to 1mm during the combined action of ultrasonic and stationary shoulder aided pinless friction stir welding and then maximum tensile strength of Al/Mg joint reaches 130 MPa. |
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