Influence Of Tool Pin-tip Profiles On Flow Behavior Of FSW Joint-root Metal

During friction stir welding, incomplete penetration, kissing-bond and upward migration of alclad often occur in the root of the butt joint, and the formation of such root flaws is closely related to the flow behavior of joint-root metal. In this paper, tracer material method is used for analyzing the flow behavior of joint-root metal, and three kinds of pin-tip profiles are designed to studying the influence of pin-tip profiles on flow behavior of FSW joint-root metal. In addition, the influence laws of soft constraint layer thickness and welding parameters on the flow behavior of joint-root metal are investigated.The results show that the flow of joint-root metal can be divided into three modes: one is the root metal flow down as it has a downward velocity when divorced from pin thread; Second is the flow owing to extrusion, this mode of flow is formed by the metal breakaway from pin thread and the pin-tip extruding the root metal; The third is flow owing to friction provided by pin-tip, and the flow belongs to laminar flow caused by friction.Based on three kinds of flow behavior of metal in the weld root, respectively, set up the corresponding flow physical model, namely penetrating flow physical model, friction flow physical model and extrusion flow physical model. Thus it can be seen, with the increase of these process variables, such as the initial speed of metal that breakaway from pin thread, welding speed, rotational speed, the pin-tip taper or the pin-tip-face diameter, the flow velocity of weld-root metal increases, and the penetration thickness of plate increases.With the increase of pin-tip taper, the distance of the metal breakaway from pin thread downward migration increases, but the distance of the metal in nugget zone extrusion upward migration decreases. With the pin-tip- face diameter increases, the thickness of pin-tip affected zone is increases, thus thicker plate will be well-welded.Soft constraint layer thickness and welding parameters will affect the initial state of metal breakaway from pin thread. With the increase of soft constraint layer, the volume of downward migration caused by pin thread and the depth of stir zone are decrease, and the turbulent area is more vulnerable to form the loose defect. Welding parameters affect the volume of downward migration caused by pin thread and its velocity when divorced from pin thread simultaneously.With the rotation speed increases, the volume per revolution of metal downward migration caused by the thread is decrease and the initial velocity of metal breakaway from pin thread increase, and the height of upward migration of alclad increases at first and then decreases while the thickness of incomplete penetration decreases at first and then increases. With the welding speed increases, the volume per revolution of metal downward migration caused by thread and the height of upward migration of alclad increases at first and then decreases, and the thickness of incomplete penetratio n increases gradually.