Effect of Tool Geometry on The Microstructure of Friction Stir Welding of Copper and 316L Stainless Steel Studied by SEM, TEM and EBSD

Welding of materials is an essential step in manufacturing process in many industries. The process of joining two or more dissimilar or similar materials requires the supply of a source of energy as driven force for the welding. Blacksmith had used for millennia the heating and hammering procedure to perform solid-state welding. Technology diversified rapidly during the 20th century, and nowadays, to perform solid-state welding, the sources of heating are broad: pressure, diffusion, electricity, friction.<br/>The Friction Stir Welding (FSW) is a solid-state welding process patented in 1991 at The Welding Institute. In this process the driven force for the welding is the frictional heat. A tool (fabricated with an inert material) penetrates into the workpieces, moves along the welding line generating friction and plastic deformation in order to perform the joint. This process suppresses defects present in others welding technics as cracking, distortions, and at the same time improve dimensional stability.<br/>In this work, the microstructure evolution during dissimilar Friction Stir Welding of copper with stainless steel 316L was studied. The welding was performed in transparence configuration using tools of two different geometries. The joints were characterized by optical and electron microscopy methods (SEM and TEM), electron backscattered diffraction (EBSD), and microhardness. It was shown that the microstructure and crystallographic texture of base metals are significantly affected by the geometry of the tool. The joint produced using a shoulder of 8 mm in diameter is characterized by a copper nugget with refined grains. The texture of copper, which is for the first time reported for this hybrid welding, is dominated by B/-B component of the simple shear texture. The steel beneath the tool also shows a grain refinement with grains of sub-micron size and a texture which is close to that of ideal simple shear texture. For the joint produced by the 16 mm in diameter tool shoulder the nugget is composed of large grains of copper while the steel beneath the tool shows small recrystallized grains of micrometric size. The texture in the steel is dominated by B/-B component of the simple shear texture. The Cu/316L interface in both types of joint is of very good quality, free from microcracks or voids. A detailed study of the interface reveals that the welding between the two dissimilar metals is achieved by their mutual intense mechanical interlocking without any formation of intermetallic compounds.