Research On Process And Mechanism Of Refill Friction Stir Spot Welding For AZ91D Magnesium Alloy

Refill Friction Stir Spot Welding(RFSSW)is a solid-state joining technology developed on the basis of friction stir spot welding(FSSW).Compared with conventional FSSW,the keyhole formed can be filled,therefore,the stress concentration can be reduced and the effective bearing area of the joint can be improved.Compared with the Resistance Spot Welding(RSW),this method has a smaller heat input and has no melting welding defects.It is a suitable spot welding method for light metals such as magnesium and aluminum alloy.In this paper,the numerical simulation,the joint formation,the microstructure and the mechanical properties of RFSSW for the 2 mm thick AZ91D-H24 magnesium alloy were studied,the formation mechanism of joint was revealed,which provided a theoretical basis for the application of RFSSW for magnesium alloy at the industry level.3D coupled thermo-mechanical finite element model was established by Abaqus,the welding temperature field,flow field and material deform behavior for the RFSSW were studied.The results indicate that the temperature field w hich was axial symmetrical with respect to the weld axis presented unsteady.In Stir Zone(SZ),the temperature gradient was markedly higher in thickness direction with the welding process.The peak temperature during the welding process increased with the decrease of welding speed,the increase of plunging depth and rotation speed,wherein the influence of welding speed was greatest.Under different process paramete condition,the maximum temperature in Thermal Mechanically Affected Zone(TMAZ)and Heat Affected Zone(HAZ)were both lower than the solution temperature;only when the welding speed was highest the maximum temperature in SZ was lower than the solution temperature.The material flowed rotationally and symmetrically with respect to the weld axis.In the plunging stage,the material entered the cavity formed by the rising pin.In the retreating stage,the material in the cavity was pressed to move outward to fill the blank area formed by the rise of the sleeve.Effective strain of welded joint on the cross section was axisymmetrically distributed with respect to the central axis,Sleeve-SZ(S-SZ)had the largest effective strain,followed by P-PZ(Pin-SZ)and TMAZ.The effective strain of each region increased with the decrease of welding speed,the increase of plunging depth and rotation speed.The weld morphology characteristics and microstructure evolutions of RFSSWed 2 mm AZ91D-H24 magnesium alloy by using single variable method in the range of 0.3-1.2 mm/s welding speed,2.0-3.5 mm plunging depth and 800-1700 rpm rotation speed were researched.With the increase of welding speed,the decrease of plunging depth and rotation speed,the better the joint surface quality was.Joints can be divided into four zones: Base Metal(BM),HAZ,TMAZ and SZ,the SZ can be subdivided into S-SZ and P-SZ.With the increase of welding speed and the decrease of rotation speed,the grains in HAZ and P-SZ became coarser,while the grains size in TMAZ and S-SZ decreased.TMAZ shows relatively strong texture,while which was weak in HAZ and SZ.In addition,the precipitated phase of AZ91 D magnesium alloy consists of Mg17Al12 and Al8(Mn,Fe)5.After the thermal cycle,the content of dissolved aluminium in ?-Mg increased,Mg17Al12 was small and dispersed,while Al8(Mn,Fe)5 was large and fewer.The density of precipitated phase in TMAZ and HAZ increased with the ascend of heat input,and in SZ,with the increase of heat input,the size of Al8(Mn,Fe)5 phase was larger,and the density of Mg17Al12 phase decreased.The hardness of joints was affected by grain size and precipitated phase density.The hardness of HAZ was relatively low,and the hardness of S-SZ was the highest.The tensile shear testings showed that the RFSSWed AZ91 D magnesium alloy joints demonstrated three failure modes: through-the-weld failure,non-circumenferential failure and pull-out failure.The main factor influencing the failure mode was the plunging depth.The maximum tensile shear load that the joint can bear when non-circumenferential failure occured.Response surface optimization method was used to establish the optimization model,and the optimal process parameters were: 0.672 mm/s,2.345 mm and 1368 rpm,and the corresponding tensile shear load of joint was 6456.97 N.