Three-dimensional numerical modeling of the friction stir welding of dissimilar steels

Steady- and transient-state solutions are developed to predict temperature, streamlines, stress, and viscosity distributions during the friction stir welding of dissimilar carbon steels AISI/SAE 1008 and 1078. The mass, momentum, and energy transport equations are solved using a Eulerian formulation within the computational fluid dynamics package FLUENT in the plates of dissimilar steels being joined. Viscoplastic behavior in the stir zone is supposed. Both Medina and Hernandez flow stress and Perzyna viscoplastic models are used to model this behavior. The Medina and Hernandez model calculates the flow stress based on the Zener–Hollomon parameter, which is valid for large deformations and high strain rates and takes into account the steel chemical composition of any structural steels having low or medium carbon contents. The present work couples this flow stress model to the continuity, momentum, and energy transport equations. Temperature-dependent thermal properties are taken from the literature. A comparison of temperature cycles taken from published experimental data and those obtained by simulation is presented. Results indicate that the steady-state simulation can provide solutions more quickly than simulation using the transient model. However, the latter simulation provides temperature, stress, viscosity, and volume fraction distributions that are more detailed than those that the former simulation provides, with the distributions being asymmetric at welding times longer than 30 s. Volume fraction distributions are more symmetric for a rotational speed ? = 450 rpm and a welding velocity U = 0.31 mm s -1. The shape of the stir region is strongly related to the temperatures on the advancing side (AISI 1008 steel) and retreating side (AISI 1078 steel) and the thermo-mechanical properties of the steels; the lobular shapes correspond to those found experimentally.