| Friction stir welding(FSW)is considered to be one of the best welding processes for aluminium alloys and has received great attention in automotive lightweighting technology,providing a viable technical solution for forming hollow chassis cast aluminium alloy components.In this paper,a two-piece A356 aluminium alloy wheel was investigated by numerical simulation,microstructure observation and mechanical property testing to study the influence of the base welding process and the pin retraction process on the wheel joint in retractable pin tool friction stir welding(RPT-FSW).A reasonable process interval for base welding and an optimum pin retraction process were derived,and the keyhole defect was successfully eliminated,resulting in a defect-free circular weld on the two-piece wheel.The influence of different process parameters on the microstructure and mechanical properties of the various areas of the base weld part and the pin retraction part of the weld was also analysed.In this paper,the effects of rotation speed and welding speed on joint formation,microstructure and mechanical properties in the base welding of A356 aluminium alloy wheels were firstly systematically investigated.The results show that the defect-free weld can be obtained at RP values(Revolutionary pitch,welding speed/rotation speed)of 0.15 to 0.23 mm/rev.The numerical simulation results show that the tunnel holes were caused by insufficient metal flow at high RP values and insufficient metal replenishment at the bottom of the advancing side of the stirred zone.The microstructure results show that the cross-section of the RPT-FSW base welded joint was divided into four zones: the stirred zone(SZ),the thermomechanically affected zone(TMAZ),the heat affected zone(HAZ)and the base material zone(BM).The microscopic hardness of the base welded joint shows an overall W-shape,where the mean hardness of SZ,TMAZ and HAZ all rise with the increase in RP value.The tensile test results follow a similar pattern to the hardness results,with both tensile strength and elongation increasing with increasing RP values from 0.15 to 0.23mm/rev,but at an RP value of 0.25 mm/rev,there is a sharp decrease in tensile strength and elongation due to the presence of micro-porosity in the SZ.In order to complete a weld without keyhole defects,the effect of the pin retraction speed on the microstructure and mechanical properties of the welds in the retraction part was systematically studied during the retraction process.The results show that the SZ of the pin retraction part can be divided into the double stirred zone(DSZ)and the single stirred zone(SSZ).At pin retraction speeds of 6 to 25 mm/min,a defect-free weld was obtained in the retraction part;when the pin retraction speed reached 37.5mm/min,a hole defect appeared below the DSZ.The microstructure results show a gradual decrease in grain size and a gradual increase in geometrically necessary dislocation(GND)density in the DSZ and SSZ due to reduced heat input as the pin retraction speed increases.The hardness map shows a clear delamination in the SZ,with the overall microhardness of the DSZ being greater than that of the SSZ,and the upper layer of the weld showing a W-shaped hardness distribution;the lower layer of the weld showing a W-shaped microhardness distribution at high pin retraction speeds of 12.5-25 mm/min,and a U-shaped hardness distribution at low pin retraction speeds of 6 mm/min,with the lowest point of the weld hardness being in the SSZ.The tensile test results show that the tensile strength and elongation of DSZ and SSZ increased with the increase of the pin retraction speed.Finally,the welded hubs were subjected to dynamic radial fatigue texts.All fatigue cracks were located in the pin retraction section,and higher pin retraction speeds resulted in longer fatigue life,and the fatigue life data pattern was consistent with the tensile strength of the corresponding RPT-FSW samples. |
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