Technology Briefing 693/1999
P L Threadgill and A J Leonard
It is self evident that the static performance of friction stir welds will be dependent on the weld microstructure, which in turn will be dependent on the processing parameters used. Recent experience has indicated that the development of microstructures in friction stir welds is very dependent on the material being welded. Although a considerable volume of work has now appeared describing various aspects of microstructures in aluminium based alloys, these are not always typical of what is found in other materials.
A classification system for microstructures was proposed based on knowledge gained from aluminium alloys, but subsequent experience with other alloys has shown that this scheme cannot be generally applied to other materials. A revised version, believed to be applicable to all materials is therefore proposed. This discriminates only between the unaffected parent material, the material which has been affected only by heat and not deformed (the heat affected zone (HAZ)), and material which has been affected by heat and deformation (the thermomechanically affected zone (TMAZ)).
The report summarises what is known on microstructures of friction stir welds in aluminium and several other materials, including a number of titanium alloys, steels, copper, magnesium alloys and zinc. This is believed to be the first time that some of these microstructures have been described.
The objectives of this report are to:
- Introduce a revised microstructural classification scheme for friction stir welds
- Review and summarise the development of microstructures in friction stir welds in various metals and alloys
- Compare the microstructures of various metallic materials after friction stir welding.