Friction stir back extrusion has recently been identified as a method for manufacturing stronger, more ductile seamless tubes. The long term goal of the project described here is to miniaturize the process in order to produce highly ductile microscale tubes for biomedical, microscale heat exchanger, and fuel cell manufacturing applications. The process is similar to friction stir welding and processing in that the end of a non-cutting tool rotates against a metal workpiece, heats the workpiece, and creates an ultrafine grain structure. Conventional microtube manufacturing is done by hot direct extrusion using dies with mandrels. After the workpiece passes the mandrels, the tube segments weld together from residual heat. The work described here considers macroscale tooling design prior to down scaling to the multi and microscale. The immediate objective was to develop tooling that can produce 50mm long tubes with 12.5mm outside diameters and 6.35mm inside diameters. Design considerations such as strength, fatigue, buckling, and vibration were considered. This paper documents the development of the tooling design process that was used in order to overcome the various design issues. Tooling failure and analysis is presented as part of the evolution of the tooling design. While majority of the paper discusses the tooling design process, a final design was developed and preliminary results for friction stir back extrusion tests are presented for tubes that are 25 and 50mm long.

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