| FSW tool is the core of FSW technology,and its characteristics are crucial to the microstructure and performance of the joint.The influence of shoulder motion state and the pin characteristics on the welding temperature field and material flow field has been studied in this paper.On this basis,the mechanism of regulating the microstructure and performance of 6061 aluminum alloy FSW joints by the characteristic of the tool was systematically investigated.Combining the failure reason of the tool,Hybrid Shoulder Friction Stir Welding（HS-FSW）were proposed and the fatigue performance and corrosion behavior mechanism of the HS-FSW joint were deeply studied.The engineering application of HS-FSW technology in the new energy battery box is realized.The results are summarized as follows:（1）The mathematical models of Rotational Shoulder Friction Stir Welding（RS-FSW）and Non-rotational Shoulder Friction Stir Welding（NRS-FSW）were established,and the effects of RS-FSW tool with a taper-screwed pin（S1）,NRS-FSW tool with a taper-screwed pin（S2）,NRS-FSW tool consisted of the pin with triple facets（S21）and NRS-FSW tool consisted of a taper-screwed pin with triple facets（S3）on welding temperature field and material flow field were studied.The mechanism of the frictional heat produced by the shoulder and the pin and the influence mechanism of the characteristic of the pin during RS-FSW and NRS-FSW were deeply analyzed.Non-rotational shoulder significantly reduced the welding peak temperature and HAZ zone affected by welding thermal cycle.In terms of these four tools,the peak temperature on the retreating side（RS）was approximate 20℃lower than that on the advancing side（AS）.Material flow was significantly reduced by NRS-FSW,and the tool consisted of a taper-screwed pin with triple facets enhanced the material flow in the direction of the welding thickness and perpendicular to the welding direction.The flow rate and welding temperature field of the joint were related to the viscosity of the material.（2）The mechanism of different shoulders and the pin characteristic regulating the microstructure and mechanical properties of joint was discussed.The Triangular zone（TZ）was discovered in RS-FSW joint,meanwhile,the NRS-FSW joint didn’t exit Shoulder and Pin Affected Zone（SPAZ）and TZ.The characteristic of the“updraft phenomenon”on TMAZ caused by the right screw on the pin,and the triple facets on the pin also improved the uniformity of the joints.The distribution of average grain size in each zone of the S1,S2 and S3 and the changes in the proportion of HAGBs present“M”and“W”shapes,respectively.The average grain size in PAZ is less than 9.0μm with more than 50%of grains fully dynamic recrystallized,and the microstructure homogeneity on TMAZ was the worst.The change of micro grain size and the proportion of HAGBs of the joint cause the“W”shape of microhardness distribution,and the non-rotational shoulder significantly reduced the softening area of the joint.The tensile strength and fatigue performance of the NRS-FSW joint were better than that of the RS-FSW joint,and the microstructure of the NRS-FSW joint welded by the tool consisted of a taper-screwed pin with triple facets were finer and the uniformity was significantly improved.Based on the analysis of the failure and the heating and the Mises stress of the tools,the HS-FSW was proposed.The joint efficiency of the joints welded by HS-FSW and NRS-FSW were reach more than 76%,and the life time of the HS-FSW tools was significantly improved.However,the increased size of auxiliary shoulder will decrease the joint performance and stability.Under the optimal welding parameters of 800 mm/min and1800 rpm,the joint coefficient of the HS-FSW joint can reach 82%.（3）The fatigue strength of the HS-FSW joint was 160MPa,exceeding 53.1%of the quasi-static tensile strength of the BM,and the weak position of the joint appeared on RS.The fatigue crack expansion was related to the stress strength factors.When theΔK value was lower than 8MPa*m0.5（the stress concentration was low）,AS exhibited a larger plastic deformation located on the crack tip during the stable expansion stage of the crack,which led to an increased stress concentration,and the trend of fatigue crack expansion on RS increased.The fracture of the fatigue crack showed obvious fatigue strip,and the width was about100nm.However,the fracture showed obvious dimples in the middle and late stages of crack expansion.The residual stress of the HS-FSW joint showed“M”type,and the maximum stress was located at HAZ of the AS.The non-rotational shoulder reduced the welding heat input and the forging force,leading to a little compression stress,which benefited the fatigue performance of the joint.（4）The corrosion characteristics of HS-FSW joints were studied by open circuit voltage,polarization curve,electrochemical corrosion and stress corrosion.The results showed that PAZ presented as a significant deactivation state with the highest corrosion resistance.The BM zone showed better corrosion resistance during the starting stage,while the AS and RS side had higher corrosion activity,and the difference during the starting stage caused the overall corrosion rate of the joint,which will affect the corrosion evolution of the subsequent corrosion behavior.With the increase of corrosion time,the electrochemical corrosion behavior of the joint presents the“pitting-passivation”cycle corrosion behavior.ISSRT of the HS-FSW joint was 2.36%,and the fracture position with a typical toughness morphology located on the AS side,where was the weakest area of the joint.（5）The high performance of the HS-FSW joint was obtained based on the above research,and the HS-FSW was successfully applied in the manufacturing of aluminum alloy battery box for new energy vehicles.The tensile strength,welding coefficient,extensibility,fatigue strength（107 cycles）of the HS-FSW joint were 238MPa,78.9%,9.7%,154.2MPa.During the squeezing test of battery box,the HS-FSW box showed better squeezing resistance,with a maximum squeezing force of 113k N. |
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