Study Of Solid-State Friction Welding Technologies In The Manufacturing Of Rocket Propellant Storage Tank

The study focuses on the research of stationary shoulder friction stir welding(SSFSW)and friction plug welding(FPW)technology in the Manufacturing of rocket propellant storage tank.The 8.5 mm thick 2A14-T4 aluminum alloy commonly used in rocket propellant storage tank is used to study the welding process,weld structure characteristics and mechanical properties of SSFSW in detail.Using 2219-T87 and7075-T651 aluminum alloys,based on the structure matching of plug and plug hole and optimization of welding process,the metallurgical characteristics,microstructure,welding defects,heterogeneity and mechanical properties of the materials in FPW joints have been thoroughly and systematically studied and explained.The main conclusions are as follows:The SSFSW tool was designed for 8.5 mm thick 2A14 aluminum alloy and successfully realized the SSFSW process in the low-speed range of 400～600 rpm.SSFSW weld seam with smooth surface,almost no flash and thinning can be obtained in the range of the low-speed process parameters,but it is difficult to obtain a defect-free weld seam surface at a high-speed above 800 rpm.The SSFSW weld zone is mainly composed of the weld nugget zone(NZ),the surrounding thermo-mechanical affected zone(TMAZ)and the heat affected zone(HAZ).It is found that the NZ shape is basically uniformly distributed along the thickness of the plate,and is composed of two scales of fine equiaxed crystal structure interlaced and mixed,the NZ grains on the advancing side are relatively smaller than on the receding side.The TMAZ grains undergo flow bending deformation,and the advancing side deforms more severely than the receding side.The grain shape of HAZ is similar to that of the base material.The NZ hardness value is higher than 80～90 % of the base metal,the hardness of the HAZ and TMAZ softening zone reaches 72 % of the base metal,and the hardness of the weld zone has a large dispersion.Finally,the tensile strength of SSFSW welded joints is obtained between 80～87 %.The process window is 400～600 rpm&60～120 mm/min.Through the study of the mutual influence law of FPW process parameters and weld formation,found that the spindle speed and friction pressure are the most important factors affecting the forming quality of FPW welds.In the given spindle speed in the range of 5500～7500 rpm,as the friction pressure increases from 30 k N to50 k N,the weld flash and plug deformation increase and the shape becomes better.In the friction pressure range of 30～50 k N,the higher the spindle speed,the better the weld formation.When the process parameters are not selected properly(spindle speed is less than 5500 rpm,friction pressure is less than 35 k N),welding defects will occur in the FPW joints,analyzing the generation mechanism of various defects,it is concluded that: when the heat input is insufficient,it will produce unwelded defects;when the friction pressure and upsetting pressure are insufficient,it will produce hole-type defects and loose structure defects.when the heat input is too large,it will cause structure over burning and plug breakage defects.When the consumption of the plug is insufficient,the defect of insufficient filling of the upper surface will occur.Due to the short time of temperature and pressure in the FPW process,sufficient metallurgical diffusion is not obtained at the friction interface of the joint,and the original interface connection line is retained.Due to the difference in the distance between the plastic material and the friction interface in the FPW process,different positions are subjected to the dual effects of uneven friction heat and force,in order to reveal the evolution of the microstructure of the FPW joint,the microstructure characteristics and material flow behavior of the FPW joints for FSW welds have been studied and analyzed in detail.The FPW joint is composed of the recrystallization zone(RZ),the thermo-mechanical affected zone(TMAZ)and the heat-affected zone(HAZ).The structure of the 2219-T87 and7075-T651 aluminum alloy FPW joints is similar,the RZ grains are very small.The TMAZ grains have grown and deformed,the grain structure of HAZ is similar to that of the base material.The study of the flow behavior of the material found that in the axial direction,the TMAZ plastic metal on the base material side uses the plug transition arc as the demarcation point,the material above the demarcation point flows to the upper surface,forming upper surface flash under the action of the plug rotation force,the material below the demarcation point flows downward forming back flash.In the circumferential direction,the material of the base material TMAZ forms elongated crystal grains parallel to the tangential direction of the plug.In order to fully understand the crystal grain morphology,crystalline phase and precipitation phase distribution characteristics and local mechanical properties of each area of the FPW joint,improve the welding process,the heterogeneity of the FPW joint was deeply studied.Found that the FPW joints of 2219-T87 and 7075-T651 aluminum alloy FSW welds are similar.The heterogeneity is mainly reflected in the small equiaxed crystals in RZ,the crystalline phase is crushed and almost completely dissolved with the precipitated phase.Only a small amount of the precipitated phase is dispersed and accompanied by a large number of dislocations,2219-T87 aluminum alloy FPW joint has higher mechanical properties in RZ,and the ultimate tensile strength and elongation reach 88.7 % and 145.8 % of the base metal,respectively.In HAZ,the grains close to RZ have grown and deformed,and only a small amount of precipitated phase is precipitated.The ultimate tensile strength and elongation of the plug close to RZ are the lowest,reaching 58 % and 55.8 % of the base material,respectively.As the distance to RZ increases,the number of precipitate phases increases.The ultimate tensile strength and elongation also gradually increased.The mechanical properties of FPW joints were evaluated in detail.Finally,the process window suitable for 8 mm thick 2219-T87 aluminum alloy FPW is: 5500 ～7500 rpm & 40 ～ 50 k N,when the parameters is 7500 rpm & 40 k N,the maximum ultimate tensile strength of the joint is 363 MPa,which is 78.9 % of the base material.The process window of 7075-T651 aluminum alloy FPW is: 6500～7500 rpm&45～50k N,when the parameters is 6500 rpm&45 k N,the ultimate tensile strength is the largest,reaching 63.8 % of the base material.Using the optimized process parameters,the FPW engineering verification of FSW weld seam was carried out on the wall plate and 1:1 test storage tank of the Rocket Propellant.It is proved that FPW technology has good reproducibility and provides strong support for the realization of FSW welding repair of the tank circumferential seam.