Tailoring strength and ductility in additive manufacturing or repair is key to successful applications. Therefore, cold spraying must be tuned for maximum amounts of well-bonded internal interfaces as well as sufficient softening of the highly workhardened deposit. Zinc (Zn) with its low melting temperature is an ideal model system to study phenomena associated with high strain rate deformation and local temperature distributions, both, in single impacts and thicker deposits. Bonding and recrystallization can be facilitated by covering selected wide parameter regimes in cold spraying. Despite the low temperatures, Zn single splats already show recrystallization at internal interfaces, the respective amounts then scaling with increasing process gas temperatures. At higher process temperatures, deposits are almost fully recrystallized. The recrystallization seems to improve bonding at internal and at deposit-substrate interfaces. Under optimum conditions, an ultimate deposit cohesive strength of up to 135 MPa and an elongation to failure of 18.4% are reached, comparable to that of laser-manufactured or bulk Zn parts. This demonstrates a welltuned interplay between high amounts of bonded interfaces and softening by recrystallization that allows for deriving bulk-like performance of cold sprayed material without additional posttreatments. Correlations between microstructures, mechanical properties, and fracture mechanisms supply information about prerequisites needed for reaching high ductility as obtained in damage and failure modes of deposits and bulk materials in global and local approaches.

High-entropy alloys are of great interest due to their unique phase structure. They are constructed with five or more principal alloying elements in equimolar or near-equimolar ratios and thus derive their performance from multiple elements rather than one. In this work, solid-state cold spraying is used for the first time to produce a FeCoNiCrMn high-entropy alloy coating. As a low-temperature process, cold spraying completely retained the high-entropy phase structure in the coating without any phase transformation. Examination shows that the grains underwent significant refinement due to dynamic recrystallization and that the coatings are much harder than the feedstock powder because of increased dislocation density and grain boundaries.

Using cold spraying (CS), a surface layer with a modified microstructure and enhanced mechanical properties was formed on a 3.2 mm thick friction stir welded (FSW) AA2024-T3 joint. The combined effect of “shot peening effect (SPE)” and “heat flow effect (HFE)” during CS were used to enhance joint mechanical properties. The microstructure evolution of the FSW AA2024-T3 joints in the surface layer following CS coatings and their effect on mechanical properties were systematically characterized with electron back-scattered diffraction, transmission electron microscopy, differential scanning calorimetry and mechanical tests. Based on these experiments, a grain refinement, finer and more S phases, and improved amount of Guinier-Preston-Bagaryatsky (GPB) zones produced by CS treatments are proposed. The deposition of aluminium coating on the joint, lead to hardness recovery in the stir zone and the development of two low hardness zones as the density of GPB increased. The tensile properties of FSW AA2024-T3 joints improved with the application of the aluminium coatings. Experiments and analysis of the enhanced mechanical properties mechanism indicate that SPE with a high plastic deformation and HFE with an intensive heat flow are necessary for the production of refined grains and increased numbers of GPB zones.

Cold spray (CS) is characterized as a solid-state process of high deposition efficiency for metallic coatings as well as additive manufacturing of metals. However, due to high velocity impact and extensive deformation of particles during CS, the as-received coatings or deposits may present anisotropic characteristics which could influence the performance of deposits. Hence this study aims to investigate the anisotropic behaviors of CS copper deposits in a systematic way. The microstructure and micromechanical properties of the deposits both in the cross-section (v-face) and in the parallel plane to the surface (p-face) were characterized. Tensile tests were performed at various loading angles with respect to the nozzle moving direction in the p-face. It is revealed that there exist strong microstructural and mechanical anisotropies in CS deposits. Different interparticle interaction results in more severe particle impact deformation in v-face than p-face, with larger elastic modulus and microhardness values. The tensile tests show an unexpected anisotropy in both ultimate tensile strength and elongation, with the highest performance occurring at the angle of 20°. The in-plane tensile anisotropy could be attributed to the parallel multiple passes. Therefore, a novel weave-spraying method was proposed, which can greatly reduce the tensile anisotropy of CS deposits.

In this paper, a preliminary analysis on the residual stress evolution of cold-sprayed coatings taking copper coating as an example was performed. An integrated frame of calculation was proposed based on the simulation results obtained from the developed thermo-mechanically coupled Eulerian model. In a single Cu splat, the residual stress appears to be large tensile stress near the surrounding area of the interface and be small tensile stress at the bottom. In single pass Cu coating, the residual stress is variable and there exists not only compressive stress but also tensile stress. When the impacting velocity changes, the resultant residual stresses also vary in both cases. The present simulated results were related to the reported experiments by others, while there is much difference.

A previous study demonstrated through experiments that surface oxide films of metallic particles have significant influence on the deposition efficiency, microstructure, and properties of cold sprayed coatings. To clearly reveal the underlying mechanism, this study focuses further on the effect of particle oxidation on deposition behavior through the designed use of oxidized copper particles having an average size of about 81 µm. The oxidized Cu powder was deposited by cold spraying to enable analysis of single impacts and coating deposition. In addition, the rebounded copper particles were collected for analysis of their morphology in comparison to the adhered particles. Results show that the deposition efficiency of the oxidized Cu particles is much lower than that of the annealed Cu powder. The thick oxide film prohibits the bonding of particles to an extreme extent. The clearance of interface oxide inclusions by plastic deformation of particles was also observed.

In the present investigation, Al5056 alloy-based metal matrix composites having SiC particles of different sizes were prepared by cold spraying. The average particle sizes of the used SiC feedstock varied from 66.94 µm to 1.64 µm. The calculated results by the FLUENT software show that the velocity of SiC particle decreases with the increase of the particle size, while the kinetic energy is greatly increased with increasing the particle size. The addition of SiC particles effectively reduces the porosity of the composite coatings. Furthermore, the porosity of the composite coatings increases with the decrease of SiC particle size, while the content of SiC in the coatings decreases with the decrease of SiC size. In addition, two effects of SiC particles determine the coating properties, i.e., the advantageous peening effect and the disadvantageous effect that reduces the effective contact between the coating and substrate.