Improving Grain Structure and Dispersoid Distribution of Nanodiamond Reinforced AA6061 Matrix Composite Coatings Via High-Speed Additive Friction Stir Deposition

Nanodiamond (ND) is considered an excellent reinforcement agent. However, due to the intrinsic properties of these ultrafine diamond nanoparticles, numerous issues, such as agglomeration, hazardous chemical reactions, and weak interfacial bonding, prevent ND-reinforced aluminum matrix composites from maximizing their potential. In this study, a novel solid-state severe plastic deformation method, namely high-speed additive friction stir deposition (AFSD), was employed to improve the microstructure and dispersoid distribution of ND-reinforced aluminum matrix composites. AA6061 aluminum alloy and 0.5 wt% ND-reinforced AA6061 aluminum alloy matrix composite (ND/AA6061) were deposited on a soft AA1060 substrate plate using a stirring tool with V-grooves and a high rotational speed of up to 7000 rpm. The microstructure, precipitates, microhardness, and tribological properties of these two types of coatings were analyzed and compared to understand the influence of ND on the microstructure development and solute redistribution. The results show that the addition of 0.5 wt% ND not only significantly improved the grain refinement and grain size uniformity but also substantially promoted the dissolution and reprecipitation of the initial precipitates, resulting in a fine and uniform distribution of the final precipitates. As a result, the microhardness and wear rate of the ND/AA6061 coating obtained via AFSD were 50.1% higher and 34.6% lower than those of the A6061 coating, respectively, with the Orowan strengthening mechanism making the largest contribution to the yield strength. The results suggest that the high-speed AFSD method provides a novel strategy for fabricating ultrafine nanoparticle-reinforced aluminum matrix composites.