| Friction stir welding(FSW)of steel-aluminum is an important research direction to realize lightweight development in automotive industry,aviation and other fields.At present,there is a lack of research report on the development and welding performance of ceramic welding tool for steel-aluminum FSW.In the thesis,the distribution law of temperature field and stress field in steel-aluminum lap welding was studied by hydrodynamics simulation to optimize the welding process and geometric parameters of welding tool.According to the thermochemical matching principle,the material composition of tool was designed,and the preparation of ceramic welding tool by one-time hot-pressing sintering through experimental optimization was studied.The feasibility of the new fabricated ceramic welding tool for the welding of steel-aluminum dissimilar materials was preliminarily verified by welding experiments,and the failure mechanism of ceramic tool was studied by welding experiments.It is benificial to provide theoretical guidance for the development of new ceramic welding tools and the application of ceramic materials in FSW.The distribution of temperature field and stress field in steel-aluminum FSW with the ceramic tool was simulated and analyzed.The results showed that the distribution of temperature field along the thickness direction of the weldment was in the shape of an hourglass,and the temperature at shoulder and bottom of the tool was higher.The highest temperature distribution was in the contact zone between the tool and steel,and the high stress zone of steel-aluminum workpiece was distributed in the forward side of the tool.The welding process and geometric parameters of the welding tool were optimized.The results showed that the welding surface quality was optimized in the condition of the rotation speed(600 r/min～800 r/min),welding speed(60 mm/min～70 mm/min),shoulder diameter(18mm～22mm)and pin diameter(6mm).The welding mechanism was of the transient cavity and vortex-like steel particles at the steel and aluminum interface.The welding mechanism was that the steel in the steel-aluminum interface was hooked into the aluminum alloy.The material composition of ceramic tool was determined.The results showed that Ti(C,N)cermet with Co,Ni and Mo as sintering additives was designed according to steel-aluminum thermochemical matching principle.The optimum fabrication process was as follow:a heating rate of 30℃/min,a sintering temperature of 1475℃,holding time of 30 min under a pressure of 32MPa.The flexural strength,Vickers hardness,fracture toughness of the ceramic welding tool at room temperature were 1543.80 MPa,15.28 GPa and 9.10 MPa·m1/2.The Vickers hardness,fracture toughness at weldding temperature obtained by simulation were above 12.0 GPa and 5.0 MPa·m1/2.A method of preparing ceramic tool by vacuum hot-press and one-time sintering was studied.The sintering effects of the mold assembly with common and convex pressing blocks were compared.The results showed that the welding tool with common pressing block had the radial shrinkage and was easy to fracture at the pin root,and its maximum size error was more than 2.0 mm.The welding tool with convex pressing block had a better shape,and its maximum size error was less than 0.1 mm.Therefore the tool with convex pressing block was appliable for steel-aluminum FSW.The sintering mechanism with convex pressing block was as follow:the protruding part of the convex block increased the height from the bottom of the inner cavity of the forming block to convex block,and it reduced the degree of uneven pressure of each layer in the inner cavity.According to the structure of the welding tool and mechine,a well-matched fixture was developed.The preliminary welding experiment was studied by using a self-developed welding tool to verify the feasibility of steel-aluminum FSW with a ceramic tool.The tool shape,the pin length and rotation speed were optimized by the welding experiment,and the welding quality and failure of welding tool were analysed.The results showed the number of spot welding before fracture failure was the lowest in the condition of rotation speed(800 r/min)and pin length(3mm).Based on the optimization of welding process and geometric parameters of the ceramic tool by simulation calculation,the cylindrical and tapered welding tools were designed to study the performence of steel-aluminum FSW and failure mode of welding tools.The results showed that the welding surface by the cylindrical tool was smooth,and its width was uniform.There were a few defects,and the main failure mode of cylindrical tool was brittle fracture.The welding mechanisms were mechanical occlusion and chemical reaction.the welding surface by the tapered tool had depressions,grooves and other defects.The main failure mode of tapered tool was wear,and the wear mechanisms were adhesive wear and oxidation wear. |
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