3D thermo-mechanical finite element modeling of friction stir welding of similar and dissimilar metallic joints with cooling

Abstract

Friction stir welding (FSW), a solid-state metal joining technique, was introduced in 1991 and since then it has become the standard method of welding in many applications. The resulting weld’s microstructure and thus weld quality and strength varies greatly depending on input process variables such as tool feed and speed, tilt angle, and tool geometry. Therefore, it is imperative that accurate modeling of this complex process is available to provide proper guidance for selecting the suitable combination of input parameters. Researchers have succeeded in thermo-mechanically modeling of FSW similar aluminum joints but none of the developed models fully simulated the FSW of magnesium, steel or dissimilar aluminum -magnesium joints. This work presents the results of four thermo-mechanical finite element simulations (utilizing a commercial FEM software, DEFORM) for friction stir welded similar and dissimilar metallic joints. Using proper material’s flow stress constitutive equations, four FE models were developed for: (1) similar aluminum joints, (2) similar magnesium joints, (3) similar mild carbon steel joints, and (4) dissimilar aluminum-magnesium joints. Once the boundary and contact conditions of the models and were systematically calibrated, it was possible to numerically determine the relevant state variables of interest (strains, strain rates, temperatures, and microstructural phase transformations). These results and other results related to the material flow were successfully validated against experimental results found in the literature for all four cases. Also, determined from the numerical simulations were estimates of the resulting grain size at the weld. For example, it was found that in-process cooling during FSW of similar magnesium and aluminum joints would result in the decrease of grain size. However, decreasing the cooling rate during FSW of carbon steels would decrease the formation of brittle martensite in the weld center.