FRICTION STIR PROCESSING OF 2xxx SERIES ALUMINIUM PM ALLOYS

Abstract

Friction stir welding (FSW) is a novel solid-state process known to facilitate the joining of materials that exhibit a poor response to conventional fusion welding technologies. Certain aluminium alloys in the 2xxx series are prime examples as their use in welded structures is desirable, but typically avoided in light of their acute sensitivity to solidification cracking. The desire to use these high strength alloys has historically resulted in less ideal joining methods such as brazing or riveting being implemented. To date, the majority of FSW research on these alloys has involved wrought products, leaving a clear void in the understanding of how those produced through aluminum powder metallurgy (APM) alloys respond. To address this shortfall, the response of two commercially relevant APM alloys denoted as PM2618 (Al-2.3Cu-1.6Mg-1Fe-1Ni-0.5Sn) and TC2000 (Al-1Mg-1.5Sn) to FSW was investigated in this study. The rotation speed and traverse rate of the tool were the principal process variables considered. In the PM2618 a variety of processing parameter combinations were found to produce defect-free welds when inspected through X-ray techniques coupled with metallographic inspection of polished cross sections. The stirred material was found to have a highly refined microstructure, showing an increase in hardness but without any apparent change to the nominal phase composition. Bend testing revealed significant improvements as a result of FSW. These included a near doubling of ductility, an average increase in yield strength in bending of 33%, and a 35% improvement in UBS. Bending fatigue behaviour was also investigated, with averaged gains of 27% measured relative to the as-sintered base material. Conversely, it was found that the TC2000 responded negatively to all processing parameter combinations used. While the stirred material in the microstructure exhibited a similar degree of refinement as seen in the PM2618, through X-ray examination it was found that specimen also contained tunnel defects and voids to varying extents.