a b s t r a c tMaterial flow during friction stir welding 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 plates. Cross-sections and longitudinal sections of welds were observed with and without the use of material marker (MM) to investigate the material flow. 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. However, the analysis revealed a too low vertical motion towards the bottom of the weld, attributed to the lack of threads. The product of the plunge force and the rotational speed was found to affect the size of the shoulder dominated zone. This effect is reduced using the cylindrical tapered pin with flats.

Various modelling approaches using commercial numerical software have been proposed in the friction stir welding literature. We initiate a comparative analysis of such modellings, involving aspects such as the constitutive laws, the representation of the continuum medium and the contact between medium and tool, and the definition of the heat sources. Numerical problems are also considered: contact definition, mass scaling, and the use of the arbitrary Lagrangian Eulerian in ABAQUS/Explicit. Finally, we propose numerical tests to explore the ability of a Lagrangian code with an ALE option to simulate the process.