Bonding mechanism and microstructural evolution in mechanically-alloyed nanodiamond-reinforced Al6061 composite particle deposits in cold spray

Abstract

This study aimed at synthesizing a novel Al6061-ND metal matrix composite (MMC) particle feedstock via high-energy mechanical alloying (HE-MA) method for potential lightweighting applications. A material/milling design-of-experiment (DOE) was carried out to optimize the design parameters, including composite batch design, milling time, and ball diameter. It was found that the 5-mm diameter ball with 10:1 ball-to-powder ratio (BPR) could achieve the desired 5–100 μm particle size distribution (PSD) target with up to an 80 % yield within 2–4 h milling. Specifically, the addition of 1 wt% nanodiamond (ND), as opposed to 5 wt% or 10 wt%, in the MMC composite merited in its ability to produce superior PSD of the desired range. Despite milling being conducted in a controlled environment, oxide content was traced in the milled product indicating the need for a material-specific heat-treatment strategy incorporation. X-ray diffraction (XRD) analysis of both particles and deposits containing Al6061-1wt.%ND MMC revealed that (1) the synthesized Al6061 matrix contained nano crystallites in the order of 10–20 nm range; and (2) are linked to the pinning effect of the embedded ND in the matrix, respectively. High-resolution transmission electron microscopy (HRTEM) confirmed the formation of atomic-scale metallurgical bonding at the deposit-substrate interface, with conclusive evidence of the presence of ND in the Al matrix. Overall, the Al6061-1wt.%ND MMC, with optimized HE-MA synthesis method, was found to be a suitable candidate as feedstock for cold spray additive manufacturing of lightweight components.