Introduction

Additive manufacturing (AM) designates a layer-by-layer manufacturing technology of a component with complex geometry (Ref 1, 2). Based on 3D model in CAD files (e.g., stl), objects with complex geometry or shape can be produced. Different from the traditional subtractive manufacturing such as milling and machinery, additive manufacturing can effectively avoid the unnecessary waste in terms of time and cost. In such AM processes, the high-energy sources like laser or electron beam can be used to melt down the powder bed selectively, whereas the resulting solidification can achieve the fabrication. It includes the selective laser melting (SLM) (Ref 2, 3), selective laser sintering (SLS) (Ref 4) and electron beam melting (EBM) (Ref 5). However, for such high-energy-based methods, the disadvantages like residual stress and unwanted phase transformations due to high processing temperature cannot be avoided.

As a recently emerged technology, the cold-spraying (CS) technique (Ref 6, 7) can fabricate the sample through the solid-state deposition of feedback powders without melting and solidification. In this process, powders are accelerated to high velocity under the effect of supersonic flow (Ref 8). The successful bonding occurs through the intensive plastic deformation of solid-state particles upon high-velocity impact at a temperature below the melting point (Ref 9,36, 39). No attention was drawn on the influence of peening effect on adhesion strength of a cold-sprayed coating. Many results based on TEM observation (Ref 52, 53) indicated that the existence of the interfacial defects such as microvoid and oxide film debris could inhibit the direct contact between the fresh metals of single particle and substrate and the formation of diffusion zone. Generally, an oxide-free Ni-Al interface is indispensable for the formation of diffusion zone during heat treatment, which allows the direct metal-to-metal contact for atomic diffusion. Accounting for the above findings of the bonding features of single-particle and full coating deposition, it may prudently suggest that the strengthened peening effect caused by the successive impact of particle is the determining factor for the formation of metallurgical bonding during full coating deposition.

During the high-velocity impact, the oxide films originally existing on the particle and substrate surfaces are disrupted, which results in the partial exposure of fresh metals of particle and substrate. With the continuous impact of subsequent particles, the oxide film debris is further broken up. Meanwhile, further deformation of deposited metal provides an opportunity for the fresh metal contact. Ultimately, the metallurgical bonding was generated under the consecutive high contact pressure due to the peening effect of subsequent particle impacts. As shown in Fig. 3, the metallurgical bonding could not be formed directly between Ni splat and Al substrate due to insufficient kinetic energy of single Ni particles. However, for the full coating deposition (Fig. 7), massive impacts of subsequent particles with higher velocity at higher propelling gas pressure can provide more kinetic energy to promote the formation of metallurgical bonding. Such strengthened peening effect can significantly increase the area of metallurgical bonding between coating and substrate and result in a stronger metallurgical bonding inside the coating. Such promoted plastic deformation of Ni particle can also explain the decreasing coating porosity at higher propelling gas pressure (see Fig. 6). The strengthened peening effect significantly improves the coating bonding strength (Fig. 10). Thus, it is able to convince that this point of view about the strengthened peening effect at higher propelling gas pressure can provide a new direction to improve the coating properties by cold spray additive manufacturing. For example, it is possible to improve significantly the bonding strength and densification rate of a cold-sprayed sample by using situ shot peening, which may break through the bottleneck of the development of cold spray equipment.

Conclusions

In this paper, cold-sprayed Ni coating and individual splats were deposited on Al substrate with different propelling gas pressures. Heat treatment results show that metallurgical bonding generated only at the interface of coating deposition instead of the one of single splat even at a high spray parameter (3.7 MPa, 600 °C). Meanwhile, the results of Ni-Al diffusion zone indicated a more intensive metallurgical bonding increased at higher propelling gas pressure. It can be believed that the strengthened peening effect by the successive impacts of the particle with higher kinetic energy is beneficial for the formation of metallurgical bonding. Therefore, the increased bonding strength of cold-sprayed Ni coating is the result of the combined effect of an increase in peening effect at a higher propelling gas pressure. It is worth mentioning that the strengthened peening effect may help improve the bonding strength and decrease the porosity of cold-sprayed coating.