| In this paper,the effective connection between ?8mm plug and 45° through hole was successfully realized by friction plug welding(FPW)method.The structure matching between the plug and the through hole was optimized by the control variable method,and the parameters of FPW welding were optimized by the mathematical model established by BBD method.Temperature field,metal fluidity,microstructure,second phase distribution and corrosion resistance near FPW joint were studied by temperature recorder,OM,SEM,EDS,XRD and sensors.The mechanical properties of the joint were studied,including tensile fracture morphology,axial force measurement of plug,bending performance and microhardness.Finally,the failure rule and fracture mechanism of FPW joint are analyzed.In FPW welding,for 5mm plate thickness,the welding effect is the best under the matching of ?8mm plug and 45° through hole.The response surface analysis and variance analysis show that the most important influence on the tensile properties of FPW joint is the spindle speed,the next is the upsetting speed,and the worst is the welding time.In the range of 2160~2360rpm of spindle speed and within 25±1s of welding time,increasing upsetting speed can continuously improve the tensile performance of FPW joint.When the welding parameters are spindle speed 2254 rpm,welding time 25 s and upsetting speed 2mm/s,the best fit is obtained.The tensile strength of FPW joint is 262.34 MPa,which can reach 82% of the base metal strength.When the spindle speed is 2200 rpm,the highest temperature on the upper surface at 5mm from the outer edge of the shoulder can reach 461.66?,and the temperature difference between the upper and lower plates at the same position is 49.49 ?.The temperature of the lower layer of the FPW weld is the lowest,which makes the root of the lower layer of the FPW weld become the weak area of the whole FPW joint due to the insufficient heat input.With the increase of spindle speed,the microstructure of FPW joint becomes better and better.From the upper layer to the lower layer of FPW joint,the range of thermal effect and mechanical effect on the weld is gradually reduced,and the phenomenon of “multi-ring bonding line” appears in PTMAZ area.With the increase of spindle speed,the size of ? phase in each region of FPW joint becomes larger and its distribution range is wider.The FPW joint has good corrosion resistance in 3.5%NaCl solution.The corrosion resistance of the FIZ zone was the strongest,followed by the BTMAZ zone,the PM zone and the BM zone,and the HAZ zone was the worst.When the Al-Si primary battery system is formed in the solution,the ?-Al matrix is dissolved as an anode,and the elemental Si is protected as a cathode;when the Mg2Si-Al primary battery system is formed,the ?-Al matrix is protected as a cathode,and the Mg2 Si phase is dissolved as an anode.The size of the isoaxial dimple at the contact part of the shoulder of the FPW joint is larger and the tearing edge is obvious than that of the base metal.The dimple in the BTMAZ area is sharp and the tearing edge is relatively strong.The fract ure at the root of the weld is laminar and the dimple is flat and shallow.The tensile crack initiated along the weak connection defect at the root of the weld and then propagated rapidly to the weld surface along the FIZ zone until fracture,which belongs to the ductile fracture mode.At the spindle speed of 1800 rpm,the axial force of the plug is the highest on the whole,and the top forging force can reach up to 767.36 kg.At 2mm/s,the axial force suffered by the plug can reach up to 960.84 kg.With the increase of the forging speed,the axial force of the plug increases obviously,which can enhance the quality of FPW joint by improving the forging speed.With the increase of spindle speed,the bending performance of FPW joint shows a trend of decreasing slightly after increasing.At 2200 rpm,the maximum bending strength of the joint can reach 1203.75 MPa,the bending elongation can reach 8.86%,and the bending Angle can reach 135°.All the bending cracks start from the junction line of one side of the spec imen in the length direction and propagate rapidly until fracture.In the vertical and horizontal directions of FPW joints,the hardness of the joints presents a “W” shape distribution.In the horizontal direction,the lowest hardness values of the upper,middle and lower layers were 55.65,66.62 and 71.81 HV,showing an increasing trend.In the vertical direction,the change trend of hardness is: the microhardness of BM area is the highest,and it gradually decreases after crossing into HAZ area,and it decreases to the lowest value of 60±5HV at the junction of HAZ area and BTMAZ area,and then gradually rises to the stable stage. |
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