Multiple pass friction stir processing for high strain rate superplasticity and the addition of nanotubes into aluminum through friction stir processing

Friction stir welding (FSW) is a recently developed process for welding aluminum alloys that has expanded into applications other than welding by the concept of friction stir processing (FSP). Through FSP, it has been found that the properties of materials can be enhanced whether by inserting forged microstructure into a casting, consolidating regions of a part produced by powder metallurgy, or by producing materials with properties which allow them to be formed more easily to name just a few of the applications. The ability of FSP to alter the microstructure of material has lead to its expansion as a method for creating superplastic material. While some studies have been performed to determine the enhancement of tensile elongations in the nugget region, the understanding is limited and an understanding of the effects of using multiple passes to cover large areas is still needed. This study examines the use of FSP to make commercial alloys superplastic by altering their microstructure. Not only is the understanding of the single pass process studied in more detail but the effects of using multiple passes of FSP are examined. In the single pass study it was found that the formability of 6061 A1 alloy was enhanced with FSP. Multiple pass work on 7075 A1 alloy found that the process could be expanded to multiple, overlapping passes of FSP and create material which was superplastic with properties with elongations just slightly less than those of the single pass regions. Multiple passes were also performed on 5083 A1 followed by a rolling schedule to increase the achievable elongations possible in the material. In addition to the examination of FSP for superplasticity, the use of FSP for the addition of carbon nanotubes into aluminum was also evaluated. It was found that the nanotubes were able to survive the harsh conditions of FSP but ideal increases in properties were not seen in the new composites --Abstract, page iv