Study On Microstructures And Mechanical Properties Of Friction Plug Welds Repairing For Aluminum Friction Stir Welded Joints

Friction plug welding(FPW)is a new solid phase friction welding method.In this paper,friction plug welding test repairing for FSW is carried out for 2219 aluminum alloy material used in large launch rocket storage tank.Meanwhile,the friction plug welding for FSW of dissimilar aluminum alloy has been carried out.The results provide important experimental basis for widely applying friction plug welding technology.The main conclusions of this paper include:(1)The size of the backing plate has an important influence on the formation of the weld.The optimum size of the backing plate is taper hole,the top is 20 mm and the bottom is 16 mm.The optimization parameter of friction plug welding of 2219&2219,6061&6061,2024&2219 and 7075&2219 same and dissimilar alloy are respectively as follows: 7500rpm&40-55 kN,7500rpm&30-40 kN,6500-7500rpm&45-50 kN and 6500-7500rpm&45-50 kN.(2)The weld formation and microstructure of friction plug welding of aluminum alloy were analyzed,friction plug welding can be divided into 6 areas: plug metal(PM),plug thermo-mechanically affected zone(PTMAZ),recrystallized zone(RZ),thermo-mechanically affected zone(TMAZ),heat affected zone(HAZ),nugget zone(NZ)or base metal(BM).(3)Because of the difference of aluminum alloy and heat treatment,the hardness will decrease in the process of friction plug welding.PM are the most severely affected areas,so the hardness decreases most.The hardness of 2219 aluminum alloy plate also decrease,therefore bonding interface hardness of 2219 aluminum alloy friction plug welding also low.The hardness of the 6061,2024 and 7075 plates decreases little,and the hardness of the bonding interface is higher than that of PM.(4)When using 7500 rpm welding speed and 45 kN welding force,the ultimate tensile strength(UTS)and elongation of 2219 FPW joint in perpendicular direction can reach 336.3MPa and 8%,respectively,approximately 73.9% and 66.7% being equivalent to that of base metal.When using 7500 rpm welding speed and 40 kN welding force,the ultimate tensile strength(UTS)of 6061 FPW joint in perpendicular direction can reach 228.8Mpa,approximately 73.8% being equivalent to that of base metal.When using 7500 rpm welding speed and 50 kN welding force,the ultimate tensile strength(UTS)of 2024&2219 FPW joint in perpendicular direction can reach 338.5Mpa,approximately 72% being equivalent to that of base metal.When using 6500 rpm welding speed and 45 kN welding force,the ultimate tensile strength(UTS)of 7075&2219 FPW joint in perpendicular direction can reach 345.1Mpa,approximately 64.1% being equivalent to that of base metal.(5)When the connection quality is good,the fracture of FPW is micro porous ductile fracture,there are many dimples on the fracture surface and undergo a lot of plastic deformation.When connection quality is not good,the part of joint fracture surface has no dimples.Such as the bottom bonding surface of the 2219&2219 aluminum alloy friction plug welding joint and the top bonding interface of the 7075&2219 aluminum alloy friction plug welding.These poor areas can become the source of the cracks and reduce tensile properties.