Influence of base metal microstructure on microstructural development in aluminium based alloy friction stir welds

The formation of 'onion rings' during friction stir welding of many aluminium based alloys has previously been related to the process parameters ( forward and rotational tool motion). The 'onion rings' have also been observed to contain fine and coarse grain bands and particle rich and particle poor bands. However, the influence of variations in the base metal microstructure on the presence of the 'onion rings' has not been investigated and is the subject of this paper. 2xxx and 5xxx sheet materials in various tempers prepared by both direct chill and continuous casting have been characterised through thickness using optical and electron microscopy. Stereological studies of the intermetallic particle distributions in the base metal and the welds were carried out. The relationship between the banding of constituent particles ( e. g. Al-x( Fe, Mn)(y)Si-z and MgxSi in AA5xxx and the Al2CuMg or Al2Cu in AA2xxx) along the rolling direction and the formation of the 'onion rings' structure and the banded grain structure was investigated. Within the range of welding parameters investigated, it was found that base materials with clear banding of the intermetallic particles showed a defined 'onion rings' structure with a banded grain and particle structure, whereas similar area fractions of intermetallic particles randomly distributed in the base metal did not give clear 'onion rings' on friction stir welding. The intermetallic particle distribution had a greater effect on the 'onion ring' formation than variations in the processing parameters.