Numerical Simulation And Experimental Investigation On Friction Stir Welding Of 6061-T6 Aluminum Alloys And Dissimilar Aluminum To Copper

The research on friction stir butt welding of 6061-T6 aluminum alloy was performed to investigate the distribution of temperature and vectors via numerical simulation and experimental method,as well as the dissimilar friction stir butt welding between 6061-T6 aluminum alloy and T2 copper wihch was performed to investigate the influence of temperature distribution on microstructure and properties of the welded joint.At first,a modified two-dimensional model for friction stir welding was established.The model included the mass conservation equation and momentum conservation equations,as well as the additional equation for energy conservation,which solved the flows involving heat generation by friction between the tool and workpieces and material plastic deformation,as well as the heat transfer and heat exchange between the workpieces and air.The volume around the tool in the model was filled with triangular cells,and the disappearance and generation of the triangular cells around the tool provided a proper environment for dynamic mesh model.The tool in the model moved along the welding line as experimental.The Eularian model was coupled to the momentum and continuity equations,which were solved for each phase.The aluminum was defined as the first phase and the zinc or copper was defined as the second phase.The material properity including the material viscosity and heat dissipation coefficient were also coupled to the model by setting boundry conditions and editing user defined function codes.All of these are only for more accurate simulation results.A small piece of zinc stock was embedded into the advancing plate during the friction stir butt welding of two pieces of 6061-T6 aluminum alloy,which was regarded as the trace elements in the experment.The zinc stock melted during the experiment according to the temperature recording,which caused limited fluctuation of the temperature distribution and little effect on material flow behavior.The basic motion trajectory of the material around the tool was concluded by analyzing the vector field and streamtraces diagram of the nodes aroud the tool wall in the model.The big clockwise whirlpool around the tool and the small anticlockwise whirlpool in the retreating side,as well as the small clockwise whirlpool behind the tool,have an important contribution to the uniformed welded joint.The thermocouples recorded the temperature change during the friction stir butt welding of 6061-T6 aluminum alloy and T2 copper,but the numerical simulation provided a more precise temperature distribution around the tool by the contour graphics of temperature.It had a lower temperature around the tool but a higher temperature far away from the tool in the copper side compared with the aluminum side because of lower energy generation and higher thermal conductivity in copper side,which caused a poor fluidity in advancing side.Addition energy source in copper side improved the fluidity of the welding joint.Intermetallic compounds generated during the FSW process and deposited in the adcancing side when the copper had a poor fluidity,which caused that the hardness value of the region increased,while the tensile strength decreased dramatically.Fewer intermetallic compounds generated when the copper had a better fluidity,wihch improved the comprehensive mechanical properties of the welded joint.