Quality Analysis of Friction Stir Welded Butt Joints by Means of Experiments and Simulations

Abstract

Friction Stir Welding (FSW) is a solid-state welding process introduced and developed in last decades. In this process a rotating tool is pressed on the two parts to be welded (mainly two plates), driven into the material and then translated along the parts interface. Academic and industrial interest is focused on the characteristics of the joined part in terms of mechanical resistance and fatigue resistance of the joints. These characteristics are heavily related to the process parameters chosen since the material stirring and the material temperature greatly depend on the pin rotating and translating speed. In fact, the stirring phenomena and the friction acting between the shoulder of the pin and the sheets, greatly increase the part temperature so that the material greatly changes its structural characteristics due to softening effect: grain dimensions, local hardness, grain orientation. Moreover, due to the physical material movement different types of defects (mainly voids) can be present in the welded zone (nugget). In particular three different areas can be identified: the heat affected zone (HAZ), the thermo-mechanical affect zone (TMAZ) and the nugget. The extension and the characteristics of these zone are very important in order to define the joint quality. These investigations are very important especially when FSW is applied in industrial fields such as aerospace, automotive and naval. To cut and to investigate an experimentally obtained joint is interesting for understanding the weld quality, but FEM simulation of the process can add very useful information in defining how the process parameter influence the joint behaviour and the three different zone extensions. As an example the heat flux, and consequently the temperature distribution inside the material, depend on the combination of rotation and welding speeds. For this reason, in the last years several efforts were oriented to the numerical simulation of the process, in order to investigate thermo-mechanical aspects, stress and strain distributions, thermal flow, residual stresses. The present paper deals with the set up of a FE model for the simulation of the FSW process whose results are correlated with the experimental observations carried out when joining AA6060-T6 aluminium alloy plates 5mm thick with a cylindrical tool with flat shoulder. The experimental campaign was performed under different welding conditions varying the tool rotational speed and the welding speed. A three-dimensional piezoelectric load cell was used to measure the welding forces in the main directions. The numerical model was developed and set up in DEFORM 3D environment. The information obtained from the model helped in the understanding of the welding phenomena.