Process And Mechanism Of Friction Stir Welding On 6063 Aluminum Alloy Sheet

Aluminum alloy is first widely used in aircrafts, ships, weapons, trains, automobiles, because of its light weight and high strength. But there are many problems in traditional melting welding of aluminum, such as weld warpage, surface oxidation, hot cracking, weld porosity, burning-out of alloying elements, softening of the weld metal, etc.. So it is necessary to research the technology of Friction Stir Welding (FSW), which is a high-efficiency, high-quality, non-polluting emerging technology of the solid welding. FSW is a patented at TWI seventeen years ago.The aim of the research project is to get the welding heat cycle, the flow of the weld material, the weld for the 6063-T6 aluminum alloy with 5mm thickness and to know how we can weld better and repair the spot defects for the industry application of the aluminum alloy and the FSW.The contents of the study including the following:Firstly, the practical welding technology for aluminum alloy is summarized after a larger number of references were read. Comprehensive summary of the development and research of FSW is provided in domestic and abroad.Many optimization experimentations were done for the technological parameter, the shape of the welding pin and the experimentation condition. The sheets of 6063-T6 aluminum alloy with 5mm thickness were welded very well efficiently. And the tensile strength of that weld seam metal was about 93 percent of the tensile strength of the base metal. The critical factors of the defects, such as the groove, the flash, the cavity, the tunnel hole, the kiss bonding and the incomplete penetration, were investigated. The effect of key parameters of the welding pin on the shape factor of the friction heat model was studied. It's found that the small butt-joint interval has little effect on the flow forming and the quality of the weld.The microstructure of the weld, the movement characteristics and the grain growth of the recrystallization of the weld nugget matel was observed. It's studied that the cause of the the onion rings in the bottom section of the weld and the half-cirque feature in the longitudinal section. It's found that the flow layer following the surface of the welding tool is about 19～103 microns, where the grain is stired and broken completely. The hardness on the interface of the zones by testing drops, and the thermal-mechanical affected zone (TMAZ) in retreating side is not the position ruptured mostly in testing while the thermal-mechanical affected zone in advancing side is. Secondly, the sliding friction on contact surface of the welding tool and the weld was described based on the fixed Coulomb-Amontons Low. The heat generation mathematic model was established for the welding pin with complex shape. The equivalent strain rate on the contact surface is estimated. It's analysid and simulated that the initial conditions and the boundary conditions for the heat conduction of the welding process.The temperature of the welding process was measured by eight thermocouples with different distances from the center line of the weld and the heat cycle of these measuring points was obtained. The results of the measurement and simulation show that the peek of the welding temperature is not only over the temperature range of initial recrystallization, but closed to the initial melting temperature of 6063-T6 aluminum alloy. The temperature with 3.5mm distance from the center line of the weld is higher for 9～110 seconds than the temperature of initial recrystallization. And a process of preheat and heat transfer in time exists at the beginning of the welding.The similar flow model of the flow layer following the surface of the welding tool was established based on the fluid particle model and the continuum theory. The flow analysis using finite volume method shows that the accumulation of the material occurred at the back of the retreating side, but a vacancy trend occurred at the symmetrical position in advacing side. The spiral movement of the forside material stired into the bottom of the welding pin is obvious, which is the course of the onin ring. The movement of the material in the upper part of the weld focuses on laminar flow while the movement of the material at lower part focuses on the turbulence or turbulent flow.Finally, the critical factor of the wear of the welding tool was analysed, and the wear coefficient was estimated. The experiments of repairing the punctuate defects were done, and the welding defects were investigated. The welding temperature and the flow of the material is determined by the rotating speed and the welding speed downwards of the welding tool. The insufficient weld depth is the main reason of the ecavity defect at the bottom of the joint.