Friction Stir Welding (FSW) between commercially pure iron and nickel was carried out using polycrystalline cubic boron nitride (PCBN) tool. The bond interface region revealed unusual microstructural features. The Inter-diffusion zone width was about 1.5μm in single pass weld and 1.8μm in double pass joints. Diffusion distances at iron-nickel interface conformed with calculations based on static diffusion rates.
Friction stir welding (FSW) is a solid state materials joining process invented by TWI in 1991[Thomas, 1991]. In FSW, a hard cylindrical rotating tool is used to soften the materials by frictional heating of the work pieces. Once the rotating tool is fully plunged into the mating interface, it is moved along the joint. As the tool moves along the joint, softened material flows from the front of the tool to the rear leaving behind a solid-state joint. FSW is currently being widely investigated especially in the aerospace industry for joining high strength Al alloys. [Lohwasser, 2003].
Recently there has been considerable interest in exploring the use of FSW for joining steels and other higher melting temperature materials[Ozekcin, 2004; Park, 2003]. However, many fundamental aspects of the process, e.g. bonding nature of joint and materials flow pattern during the welding, remain unanswered yet.
The overall objective of present work was to develop a mechanistic understanding of the 3-D flow pattern and interface bonding in FSW by utilizing dissimilar materials joint where flow patterns are easily visualized by differential etching and chemistry. Choice of FSW of dissimilar metals was made based on the following criteria,
Similar melting points and comparable flow stresses at the joining temperature and
same crystal structure at joining temperature but differ in their crystal structures at room temperature.
In the present study Fe-Ni was the system of choice.
