A 3D Finite Element Model (FEM) is built for the thermal-mechanical analysis of the Friction Stir Welding (FSW) joint of dissimilar material. Modeling and simulations are carried out by using the FEM package ABAQUS^®. In this model, the welding nugget is modeled as Functionally Graded Material (FGM) area.  The FGM model, initially verified by using available friction stir welding experimental data, has been further used to study thermal and residual stresses of dissimilar materials joints. The heat generated between the welding tool and the specimen's surfaces is modeled as a moving heat source, and ABAQUS^® user subroutine DFLUX is used for such purpose. The proposed model can be also used in conjunction with experimental results of welding processes for the development of an updating finite element scheme to improve correlations among numerical simulations and available data.

Figure 1 shows a 2-D section view of the model; pure materials A and B regions and FGM region, with % volume fraction of A and B, are indicated. Numerical simulations use as variables the following set of parameters: d (distributed heat flux  diameter), h (thickness of the specimen), w (2 times width of the specimen), l (length of the specimen), and p (parameter indicating the % of material volume fraction in the FGM region). To verify the proposed modeling approach numerical investigations are compared with FSW experimental data gathered by the Reynolds et al. (2003) who conducted experiments for the FSW of 304L stainless steel with two pin-rotating speeds, 300 rpm and 500 rpm, respectively. Figure 2 shows the predicted residual stress and the measured data by neutron diffraction. The results show good agreement. Preliminary parametric studies provides a prediction of the PEEQ (Equivalent plastic strain) as illustrated in Figure 3; the following parameters have been used in the numerical simulations: A= Aluminum, B= stainless steel, r = 10 mm, d = 40 mm, w = 60 mm, h = 5 mm, p = 1,2,3 l=300mm. As verified by experiments, the trend in Figure 3 shows that higher strains are reached in the region A for x < 0 with higher % of Aluminum, with a corresponding maximum strain that does not occur in the weld centerline (at x = 0 mm).