Friction stir welding using unthreaded tools: analysis of the flow

Friction stir welding (FSW) is a solid-phase welding process. Material flow during FSW is very complex and not fully understood. Most of studies in literature used threaded pins since most industrial applications currently use threaded pins. However initially threaded tools may become unthreaded because of the tool wear when used for high melting point alloys or reinforced aluminium alloys. In this study, FSW experiments were performed using two different pin profiles. Both pins are unthreaded but have or do not have flat faces. The primary goal is to analyse the flow when unthreaded pins are used to weld thin (4 mm) plates. Material flow with unthreaded pin was found to have the same features as material flow using classical threaded pins: material is deposited in the advancing side (AS) in the upper part of the weld and in the retreating side (RS) in the lower part of the weld; a rotating layer appears around the tool. The plunge force and the rotational speed were found to affect the thickness of the shoulder dominated zone. This effect is reduced using the cylindrical tapered pin with flats.