Synthesis and characterisation of advanced ball-milled Al-Al2O3 nanocom- posites for selective laser melting

Selective laser melting (SLM) offers significant potential for the manufacture of the advanced complex-shaped aluminium matrix composites (AMCs) used in the aerospace and automotive domains. Previous studies have indicated that advanced composite powders suitable for SLM include spherical powders with homogeneous reinforcement distribution, a particle size of less than 100 μm and good flowability (Carr index less than 15%); however, the production of such composite powders continues to be a challenge. Due to the intensive impacts of grinding balls, the highenergy ball-milling (HEBM) process has been employed to refine Al particles and disperse the nano Al2O3 reinforcements in the Al matrix to improve their mechanical properties. Notwithstanding, the specific characteristics of ball-milled powders for SLM and the effect of milling and pause duration on the fabrication of composite powders have not previously been investigated. The aim of this study was to synthesise Al-4 vol.% Al2O3 nano-composite powders using HEBM with two different types of milling and pause combinations. The characteristics of the powders subjected to up to 20 hours of milling were investigated. The short milling (10 minutes) and long pause (15 minutes) combination provided a higher yield (66%) and narrower particle size distribution range than long milling (15 minutes) and a short pause (5 minutes). The nano Al2O3 reinforcements were observed to be dispersed uniformly after 20 hours of milling, and the measured Carr index of 13.2% indicated that the ball-milled powder offered good flowability. Vickers micro-hardness tests indicated that HEBM significantly improved the mechanical properties of the ball-milled powders.