While friction stir processing (FSP) has been used to refine the grain structure in sheet metals, this work explores the potentials of refining the grain structure of bulk material using the friction stirring phenomenon via the novel concept of spiral friction stir processing (SFSP). With this concept, the rotating stirring tool is plunged into the material, rather than being traversed across it as in FSP; this imposes severe plastic deformation on the material while pushing it radially outwards in complex spiral paths. By confining the material within a closed cylindrical die, the processed material is microstructurally-refined while forming a tube via a special form of SFSP called “friction stir back extrusion” (FSBE). The hypothesised concept was investigated using samples from the AA6063-T52 aluminium alloy and the AZ31B-F magnesium alloy. The preliminary results presented here demonstrate the viability of SFSP, and the special form of FSBE, in producing tubular samples that are structurally sound, with no signs of voids or internal channels. Optical microscopy was performed at key locations within selected tube specimens, and the obtained micrographs clearly show the presence of a stir zone with a fine grain structure; grain size measurements demonstrate the effectiveness of the processing technique in refining the microstructure of the starting material.

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