Transfer Behavior Study In Weld Thickness Direction Of The Welding Plastic Metal In Friction Stir Welding

The friction stir welding of 8mm Al alloy LY12-M and LY12-CZ was conducted by using four kinds of welding tools. The influence of welding tools configuration and welding parameters on plastic metal transfer behavior in weld thickness direction was analyzed by observing the morphology of weld cross sections. The model for the three-dimensional flow field of the plastic metal was set up, and according to the definite condition of experimental parameter, the three dimensional velocity and flow line trajectory of plastic metal were analyzed by using software FLUENT.The results shown that the driving force of plastic metal flowing in weld thickness direction is from the skew threads on the pin surface. When the pin has left-hand skew thread on its surface, the pressure from the surface of thread to plastic metal and the friction between plastic metal and the surface of thread are main reasons for driving plastic metal moving to the bottom of the weld, then the plastic metal enters into the thread from the root of the pin, finally slough off from the thread at the tip of the pin and squeezes into the surrounding area, forms a extrusion zone. The nugget with onion ring is actually the projecting picture of the extrusion zone cut at the cross section of the weld.The quantitative analysis of the plastic metal transfer behavior in weld thickness direction shown that, the plastic metal transfer volume in unit time depends on welding parameters,the increase of the plunge deepness and n/v in a certain range causes the plastic metal transfer volume in weld thickness direction to increase, the sucking-extruding effect in weld thickness direction boost up, the macro performance is the area of nugget cross section increasing.With the thread number of the pin increasing, the plastic metal transfer volume in weld thickness direction is increased, the sucking-extruding effect boost up, and the area of nugget cross section is increased. Besides, the rule of the area of nugget cross section changed with the thread number of the pin is independent of welding parameters.Simulation results indicated that, there is circular transfer of the plastic metal in the weld thickness direction. The transfer speed of plastic metal in the thread is increased with the increasing distance of the thread, and more plastic metal moves to the bottom of the weld with the thread number increasing of the stirring pin, the sucking-extruding effect boost up gradually during welding process.