Friction stir welding of PM2000 ODS alloy

Abstract

Friction stir welding has been used to join sheets of a ferritic, oxide dispersion strengthened alloy, PM2000. A stepped spiral probe, polycrystalline cubic boron nitride tool, with a shoulder diameter of 25 mm, was used to weld 4 mm thick plate in a butt joint configuration. The thermomechanically affected zone underwent dynamic recrystallisation during welding; the resultant microstructure consisted of equiaxed ferritic grains containing a dispersion of yttrium aluminium oxides. Microindentation measurements revealed a significant reduction in hardness within the weld zone, when compared to the parent material. The welding process induced an overall coarsening of the yttrium aluminium oxide particles and depletion in their number density. However, the precipitation of secondary phase particles, likely to be oxides, which took place during the welding process, is indicative that an element of mechanical alloying occurs during the welding process. Annealing the welds for 1 h at 1380°C produced a massive recrystallised grain structure in the weld zone and a uniform hardness across the parent and weld was achieved. Transmission electron microscopy showed that, subsequent to annealing, particles were coarser in the weld zone (33 nm mean diameter) than in the parent alloy (24 nm mean diameter). However, electron diffraction and energy dispersive X-ray spectroscopy confirm that the dominant oxide phase, YAlO₃ perovskite (YAP), was the same in both regions. Oxide particle size and number densities were not uniform throughout the weld. Focused ion beam prepared surfaces revealed particles within the size range of 50–600 nm diameter in material beneath the tool shoulder/workpiece contact area; the average size of dispersoids in this region was 130 nm.