Cold gas dynamic spray or simply cold spray (CS) is a process in which solid powders are accelerated in a de Laval nozzle toward a substrate. If the impact velocity exceeds a threshold value, particles endure plastic deformation and adhere to the surface. Different materials such as metals, ceramics, composites and polymers can be deposited using CS, creating a wealth of interesting opportunities towards harvesting particular properties. CS is a novel and promising technology to obtain surface coating, offering several technological advantages over thermal spray since it utilizes kinetic rather than thermal energy for deposition. As a result, tensile residual stresses, oxidation and undesired chemical reactions can be avoided. Development of new material systems with enhanced properties covering a wide range of required functionalities of surfaces and interfaces, from internal combustion engines to biotechnology, brought forth new opportunities to the cold spraying with a rich variety of material combinations. As applications multiply, the total number of studies on this subject is expanding rapidly and it is worth summarizing the current state of knowledge. This review covers different material systems that have been studied up to now with an emphasis on potential innovative applications. This includes metallic, ceramic and metal matrix composite (MMC) coatings and their applications. Polymer (both as substrate and coating) and metal embedment in the polymer are also covered. CS has emerged as a promising process to deposit nanostructured materials without significantly altering their microstructure whereas many traditional consolidation processes do. Relevant material systems containing nanostructured powders are also considered. A critical discussion on the future of this technology is provided at the final part of the paper focusing on the microstructural bonding mechanisms for those relatively less explored material systems. These include MMCs involving more than one constituent, ceramics, polymers and nanostructured powders. Future investigations are suggested especially to quantitatively link the process parameters and the behaviour of the material systems of interest during impact.

Additive manufacturing (AM) is a new paradigm for the design and production of high-performance components for aerospace, medical, energy, and automotive applications. This review will exclusively cover directed energy deposition (DED)-AM, with a focus on the deposition of powder-feed based metal and alloy systems. This paper provides a comprehensive review on the classification of DED systems, process variables, process physics, modelling efforts, common defects, mechanical properties of DED parts, and quality control methods. To provide a practical framework to print different materials using DED, a process map using the linear heat input and powder feed rate as variables is constructed. Based on the process map, three different areas that are not optimized for DED are identified. These areas correspond to the formation of a lack of fusion, keyholing, and mixed mode porosity in the printed parts. In the final part of the paper, emerging applications of DED from repairing damaged parts to bulk combinatorial alloys design are discussed. This paper concludes with recommendations for future research in order to transform the technology from “form” to “function,” which can provide significant potential benefits to different industries.

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Cold spray is a high deposition rate coating process, in which solid powder particles are accelerated to velocities ranging between 300 and 1200 m s − 1 through a de Laval nozzle, impact and adhere to a substrate or previously deposited coating. The powder feedstock remains well below its melting temperature during deposition. Therefore, cold spray is categorised among solid state deposition methods, and offer significant advantages as compared to other traditional thermal spray techniques.Particles/substrate interaction during high velocity impact and the resultant bonding is fundamental, and determines physical and mechanical properties of the coating/substrate system. Numerous experimental and computational efforts have been carried out to provide a better understanding of the bonding mechanism in cold spray. [1][2][3][4][5][6][7][8][9][10][11][12] These efforts raised the consciousness that adhesion only occurs when the powder particles exceed a critical impact velocity. The knowledge of critical velocity is a key point to determine the optimised spraying parameters, and reduce the manufacturing cost by increasing the deposition efficiency. Experimental investigations show that the critical velocity depends on the spray material properties, 13,14 particle temperature, size distribution, surface oxidation [15][16][17][18] as well as substrate nature and properties. 19,20 The exact bonding mechanism is still a matter of debate. However, adiabatic shear instability is known as the predominant bonding precursor in cold spray. Adiabatic shear instability occurs at the particle substrate interface during impact. A strong pressure field is introduced as a result of particle impact on the substrate. Consequently, a shear load is generated, which accelerates the material laterally. This causes localised shear straining, which, under critical conditions, leads to adiabatic shear instability. Experimental results show that the minimal particle impact velocity needed to produce shear localisation at the particles/substrate interface, correlates quite well with the critical velocity for particle deposition by the cold spray process in a number of metallic materials. The initiation of adiabatic shear instability is usually described by thermal softening in competition with strain and strain rate hardening. During high strain rate deformation, strengthening occurs as a result of dislocation generation and movement. On the other hand, heat generated by plastic work softens the material. At a certain point, softening dominates the material response and results in the flow stress breakdown and onset of adiabatic shear band formation. 21 This phenomenon often leads to material jet formation in the powder particle or both particle and substrate (shown in Fig. 1), that is a precursor of adhesion in cold spray.The success of the cold spray process mainly depends on proper acceleration of the powders to an impact velocity window, set by the critical and erosion velocities. 3,22

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