Friction Stir Processing: An Emerging Surface Engineering Technique

Abstract

Surface modification technologies impart improved surface properties without affecting the bulk properties of the material. The properties could be mechanical, electrical or thermal properties. Until recent past, thermal spray techniques, namely, plasma spraying, high-velocity oxy-fuel coatings and many others widely used for these applications. Friction stir processing (FSP) is a relatively newer technique that uses friction (between two surfaces) as a heat source to form a surface composite on the base alloy. This solid-state process not only refines the given structure but also disperses the reinforcements well within matrix alloy to enhance the surface properties. FSP was earlier employed to low melting point alloys such as aluminum and magnesium-base alloys, but now, with the recent development in tool geometry and tool materials, it can even be effectively used for high melting point alloys like steel and titanium-based alloys. Several process parameters seem to affect temperature and dispersion of reinforcements at the surface. They include rotational speed and traverse speed of the tool, number of passes, cooling medium and the tool geometry. Among these, rotation speed and traversing speed of tool seem to greatly affect the temperature distribution in the plasticized zone formed at the surface. This temperature, in turn, affects the grain refinement and dispersion of reinforcement particles. The present chapter summarizes the effect of these parameters. This chapter also reviews the latest developments in the tool material and its design. Further, their role in augmenting the base alloy properties is also discussed. High hardness, high fracture toughness, chemical inertness and high-temperature strength are few desirable properties of a tool to be used for FSP. In the end, the applicability of FSP as a surface modification technique has been assessed. © 2020, Springer Nature Switzerland AG.