Research On The Crafts And Auto-Control Technique Of The Friction Stir Welding For Large-Scale Launch-Vehicle-Structures

As a solid-phase welding technique,the friction stir welding(FSW)does not produce blowholes,solidification cracks or other defects which are very common in the fusion welding of aluminum(Al)alloy and dissimilar metals,the once-welding rate is correspondingly very high.So in the aerospace industry,FSW has been widely used for the welding of large-scale launch-vehicle structures,and the general techniques of Al structures welding have been very mature.Based on this,the FSW techniques of launch-vehicle structures are developing toward the next stage and have formed three major trends:(1)developing the dissimilar FSW technology of Al/steel for expanding the application of Al/steel connecting structures,which can reduce the structure weight while still keep the structure strength;(2)achieving the high-quality welding of the large-thin-wall structures,simplifying the expensive welding equipment and improving the productive efficiency by the bobbin-tool friction stir welding(BT-FSW)technology;(3)developing the forging-force feedback control technique,which can achieve the shape-following control,keep the tool plunge depth stable and improve welding qualities,for the FSW of large-scale low-stiffness structures.The above trends have significant influences on the developing prospects of launch-vehicle FSW and aerospace manufacturing technologies,so the following important problems of the trends must be solved:(1)for the dissimilar FSW of Al alloy and advanced high strength steel(AHSS,an ideal kind of steel for lightweight launchvehicle structures),no literature has provide any study about the effects of welding parameters on the welding qualities,especially about the effects of the tool size;(2)for the BT-FSW of 2219 Al alloy,which is widely used in the launch-vehicle structures,the studies about the relationships between welding qualities and welding parameters such as welding speed,tool rotational speed,and tool offset,are quite limited;(3)The huge heavy-load CNC equipments,which are widely employed in the FSW of largescale launch-vehicle structures,can produce significant damping and time-delay in the force control system,and in turn deteriorate the system dynamic properties,controlling qualities,and finally welding qualities.However,in the field of FSW forging force control,there is still a lack of huge heavy-load welding equipments.Based on the above analysis,this thesis focuses on the FSW crafts and auto-control technologies for large-scale launch-vehicle structures,the research works and major results are as follows:(1)By comparing the welding temperatures,joint microstructures,intermetallic compounds(IMCs),joint fracture modes and tensile strengths,this study analyzed the effects of tool size and tool offset on the joint defects,the effects of tool size and rotational speed on the Al/steel metallurgical bonding,and the effects of welding parameters on the welding forces.The main experimental results include: firstly,the defect-free joint with a strength of 89% base metal was gotten;secondly,the relationships between welding parameters and welding loads were inspected,which could provide the design basis for the FSW equipment of Al/steel rocket structures(2)By the comprehensive application of statistical tools,temperature measurements and joint-structure inspections,this study analyzed the effects of welding speed,tool rotational speed and shoulder pinching-gap on the BT-FSW of 2219 Al alloy.The main experimental results include: firstly,the welding parameters were optimized and the defect-free joint,which had the strength and elongation of 78% and 56.9% of the base metal respectively,were gotten;secondly,a defect-free launch-vehicle-tank circle-seam of diameter 2.25 m was produced by the optimized parameters.(3)To solve the difficulties in the forging-force control for the FSW of large-scale launch-vehicle structures,this study discreted the time-delay system based on a variable substitution method,and optimized the controller parameters based on the linearquadratic-regulator(LQR)technique.The main experimental results include: firstly,when the significant interference and time-delay existed,the tracking error of the forging force was lower than 1% of the reference value,the welding process was stable and smooth,the strength of the defect-free joint could reach 79.6% of the base metal;secondly,a defect-free launch-vehicle-tank bottom-seam of diameter 3.35 m was produced by the control system.