Friction stir welding of commercially available superplastic aluminium

Abstract

A series of Friction Stir Welds (FSW) has been produced in order to optimise tool designs and weld/process variables to minimise flaws in the weld and obtain the best possible microstructure for superplastic Forming (SPF). Therefore the main goal is to produce friction stir welds which do not fail during subsequent SPF processes. The friction stir welds have been created using novel tools which are oversized for the material thickness used; this creates a wider weld region of fine equiaxed grains which are suitable for SPF. These original tools have been compared to tools which are already in mainstream FSW production. The welds created for this investigation also represent an evolution of friction stir welding by starting with a milling machine; a very basic piece of engineering workshop equipment. This was then replaced by a modified milling machine with force monitoring capabilities and finally using a state of the art dedicated FSW machine for the final welds. Room temperature properties are not usually a good indicator of high temperature response; however in this thesis the room temperature properties are closely linked to the FSW microstructure and have been used to assess the suitability of the weld structure for subsequent superplastic forming operations. The welds created for this thesis have been completed using hot and cold welding conditions, evaluated for room temperature properties and microstructural stability. The results have then been used to assess the welds and select the most suitable structure for cone testing, which is used to test the welds‘ performance during SPF. Friction stir welds were then recreated and cone tested which reveals the different levels of deformation occurring across the entire weld section and the unaffected parent material. Specimens in the as-welded, post-weld annealed and post-SPF have been analysed using standard microscopy techniques and Electron Back-Scattered Diffraction (EBSD). Welds in Aluminium Alloy (AA) 2004 with excellent room temperature properties have been created and shown to be capable of superplastic deformation achieving strain greater than 200%. Welds in AA5083, although producing excellent room temperature properties are unable to deform superplastically due to the difference in strengthening mechanisms employed by the different alloys. AA2004 contains Al3Zr which effectively pins the microstructure allowing grain boundary sliding to occur, AA5083 lacks this grain refinement element and so suffers from abnormal grain growth leading to early failure.