To what extent does friction-stir welding deteriorate the properties of powder metallurgy Al?

Abstract

Powder metallurgy (PM) fabricated HITEMAL is a near- and sub-micrometer aluminum (Al) material that is stabilized by a small fraction of a nano alumina (Al2O3) secondary phase. This phase forms in situ from surface Al2O3 films present on as-atomized Al powders during compaction. HITEMAL showed unique mechanical properties and creep performance at high homologous temperatures owing to its ultra-fine Al grain structure effectively stabilized by Al2O3. However, conventional fusion welding is generally not applicable to PM Al. This study reports on friction stir welding (FSW) that was applied to join bars extruded from Al powders of different particle size. As PM Al performance crucially depends on the Al2O3 phase, we focused on changes to this phase that were, induced by FSW. Thermal stability, mechanical properties, and deformation behavior of the weld materials were studied versus changes in the morphology and distribution of the Al2O3 phase during FSW. The study confirmed that FSW can be successfully used to join PM Al, although some detrimental transformation of the Al2O3 phase occurred upon FSW. This transformed Al2O3 phase did not provide sufficient stabilization under severe plastic deformation induced by a rotating tool action. The transformation involved dynamic recrystallization and minor Al grain growth in weld zones. This led to a decrease in the mechanical strength of the weld compared with that of the respective base materials. While this negative effect was pronounced for HITEMAL with the finest Al grain size, it was negligible in the coarse-grained material produced from the coarsest Al powder.