TEM Study of Friction Stir Welding Joints of 316L Steel and 5083 Al Alloy

Welding of materials is an important manufacturing process by which two or more similar or dissimilar materials are joined. The result of the process, i.e. the consolidation of two parts into a single one, has been known since iron-age. The most rudimentary and very well-known method, the forge welding, have been mastered by the blacksmith for millennia. Forge welding is a solid-state welding process in which iron and steel are welded by heating and hammering.<br/>Today, the type of joining process can be classified according to the presence or absence of a liquid phase. During the so called fusion welding, there is a state change, from solid to liquid and once again from liquid to solid. On the other hand, during the solid state welding process there is no state change and welding is performed without fusion of the materials. The later process suppresses the solidification-related defects and leads to low internal stresses compared to conventional welding. The Friction Stir Welding (FSW) is a solid-state welding process, it was initially conceived to join metals and alloys and today is also used to produce nanostructured materials.<br/>In this work, FSW was used to join stainless steel 316L and aluminum alloy 5083. During the process, the materials undergo intense plastic deformation at elevated temperatures. The structure of the material at the welded region is strongly affected: the microstructure changes and new phases may form to join the pieces. The conditions of the welding are severe, diffusion processes are fast, leading sometimes to the formation of intermetallic compounds (IMC), which can exist on the nanometer scale. The formation of IMC at the interface and in the welded region is an important factor, affecting physical and chemical properties of the piece. In this work, the combined analysis by scanning and transmission electron microscopy shows that the intermetallic compounds have different size and composition, depending on the region in which they are located with respect to the interface. The size varies from few microns to submicrometric size, the composition is variable, beside Fe and Al some alloy elements like Cr, Mn, Mg and/or Ni can be present. The identification of such compounds would be difficult without using electron microscopy techniques, in particular TEM. To the best knowledge of the authors, only the formation of binary IMC, appearing in the Fe-Al phase diagram, have been reported for similar systems. We show here that the presence of more alloying elements in the new compound, besides Fe and Al, has to be carefully taken into account in order to identify correctly the phase responsible for the integrity of the welded material. In the studied case, some ternary intermetallics compounds, which has no equivalent in the binary Fe-Al system, were identified by TEM.