Development of Friction Stir Welding for Aerospace Manufacturing: Derivative Processes and Novel Applications

Abstract

Since its invention over 30 years ago, friction stir welding (FSW) has matured into a capable and versatile joining method for many materials and applications. The process enables the assembly of advanced materials that are challenging to join with other methods, allows for weight savings over mechanical fasteners, and improves material properties over fusion welding methods. The benefits and versatility of FSW have led to novel process derivatives and corresponding novel applications in the aerospace industry. The first research detailed in this dissertation focused on graphite as an ablative material. With a derivative FSW process called friction stir extrusion (FSE), isostatically molded graphite sections attach to aluminum alloy plates through a dovetail extrusion-groove mechanical interlock. The work is a proof of concept for furthering advanced ablative material joining with FSW-based processes. Another FSW derivative process, butted friction stir forming (BFSF), can weld aluminum alloys to structural steel members as an alternative to fasteners or fusion welding. Research validated both flat plates and curved surfaces in a butted configuration. The FSW process displaced material from the weld zone into prepared threaded holes. The aluminum alloy extruded into the screw threads to create a mechanical interlock. The developed methodology enables the joining of three workpieces in one operation. Another aerospace application of FSW comes from its unique operating conditions and compatibility with in-space manufacturing. Experimental methods approximating lunar conditions permitted a preliminary validation study for FSW on potential off-world locations. Material property analysis of the welds showed that only a minimal effect was found on the strength, suggesting that lunar FSW is viable. The research presented in this dissertation validated proof of concept FSW technologies for several aerospace applications.