Progress on Cold Spray Additive Manufacturing of Aluminum-based Intermetallic Compounds

Aluminum-based intermetallic compounds are recognized for their excellent high-temperature mechanical properties, high resistance to oxidation and corrosion at elevated temperatures, and low density. These characteristics make them suitable for a broad range of applications, including protective coatings and structural components in aerospace and other industries. High-energy-beam additive manufacturing technologies, such as laser and electron beam melting processes, are effective for the rapid fabrication of complex metal structures. However, depositing aluminum-based intermetallic compounds remains challenging due to their complex phase structures and inherent brittleness, which often lead to flaws and defects, particularly cracks. Cold spraying, a process characterized by low processing temperatures and high deposition rates, shows significant potential for the low-heat-input fabrication of aluminum-based intermetallic compounds. This paper summarizes and analyzes recent advancements in the preparation of aluminum-based intermetallic compounds using cold spraying. Effects of powder design and fabrication routes on deposition behavior and deposit properties are discussed. These routes include intermetallic compound powder deposition, mixed elemental metal powder cold spraying followed by heat treatment, and cold spraying of mechanically ball-milled pseudo-alloy powders with subsequent heat treatment. First, the deposition behavior of intermetallic compound powders is reviewed and discussed. In the cold spraying process, successful deposition and bonding of spraying particles rely on plastic deformation induced by particle impact. However, due to the intrinsic brittleness of intermetallic compounds at room temperature, it is difficult to deposit particles directly using intermetallic compound powders as feedstock. In practice, when relatively soft materials are used, only a single layer of intermetallic particles can be mechanically embedded into the substrate layer. Subsequent spraying does not contribute to building up of the deposit because achieving successful bonding between intermetallic particles is highly challenging. Although high gas temperatures, even up to 1 000 ℃, are used to soften intermetallic particles, depositing a thick, high-quality deposit remains elusive. Thus, preparing intermetallic deposits using intermetallic feedstock powders continues to be a challenge. To address this issue, an alternative strategy involving the formation of intermetallic compounds during or after deposition has been extensively investigated. Following this strategy, mechanically mixed powders containing aluminum (Al) and other elemental powders, such as iron (Fe), nickel (Ni), or titanium (Ti), are used as feedstock materials. Due to the excellent plastic deformability of elemental metal powders, deposits containing mixed elemental metal particles can be easily deposited by cold spraying at relatively low gas temperatures and pressures. Post-spray heat treatment or annealing is then performed to activate interdiffusion between the Al and Fe / Ni / Ti phases, facilitating the formation of intermetallic compounds. However, the higher deposition efficiency of Al powder compared to Fe / Ni / Ti powders often results in cold-sprayed composite deposits with a higher Al content than the feedstock powder, complicating precise control of the chemical composition. This challenge is particularly pronounced when the feedstock powder contains more than three elemental metal powders. Additionally, during heat treatment, the long diffusion paths required for intermetallic compound formation frequently lead to the creation of numerous Kirkendall pores, and achieving a single intermetallic phase proves difficult. To overcome these challenges, a method involving the cold spraying of mechanically milled pseudo-alloy powders, followed by heat treatment, is proposed. By controlling the intensity and duration of high-energy ball milling, pseudo-alloy powders with alternating submicron lamellae of various metals can be prepared from mechanically mixed metal powders. The composition of the pseudo-alloy powder can be precisely controlled by adjusting the proportions of the raw materials. Moreover, the pseudo-alloy powder retains the plastic deformation capability of the original elemental metals, ensuring efficient deposition during cold spraying. The fine microstructure of the alternating submicron metal lamellae significantly shortens diffusion paths during heat treatment, effectively mitigating the formation of Kirkendall pores in the deposit. Finally, the effects of post-treatments such as friction stir processing (FSP) and hot isostatic pressing (HIP) on the microstructure and properties of the deposit are summarized. FSP treatment greatly refines the microstructure of deposits sprayed with mechanically mixed elemental metal powders, resulting in structures featuring alternating submicron metal lamellae and partially formed intermetallic compounds. This refinement significantly shortens diffusion paths between phases and prevents the formation of Kirkendall pores. However, it is challenging to process parts with complex shapes using this method. In contrast, HIP applies isostatic pressure during treatment, closing Kirkendall pores and making it suitable for parts with complex geometries. Overall, cold spraying of mechanically mixed elemental metal powders followed by HIP treatment, as well as cold spraying of mechanically milled powders combined with subsequent heat treatment, have been shown to produce aluminum-based intermetallic compounds with low porosity and high hardness. By comparing and analyzing the advantages and limitations of different technological routes, this study aims to provide guidance for the cold-spraying additive manufacturing of aluminum-based intermetallic compounds.