Advantages and disadvantages of cold spray technology

       As mentioned earlier, cold spraying is a spraying technique based entirely on aerodynamic principles. The particles are heated at a low temperature and remain solid, and the solid particles are deposited on the workpiece surface through plastic rheology caused by "adiabatic shear instability" or through mechanical processes such as severe plastic deformation under conditions of extremely high stress, strain and strain rate. . Therefore, during the spraying process, the particles are not prone to oxidation, burning loss, phase change, and organizational change.



The residual stress of the coating is mainly compressive stress, and the spraying efficiency is high, and thick coatings and even bulk materials can be prepared. Based on the above characteristics, this technology has obvious advantages in the preparation and development of new material coatings such as nano, amorphous materials and other thermally sensitive material coatings, and has low energy consumption, recyclable material resources, and no environmental pollution. It is a green spray technique. Compared with the thermal spraying method, it is concluded that cold spraying has the following advantages.



1) The spraying rate is high, up to 3kg/h; the deposition efficiency is high, up to 80%.



2) The chemical composition and microstructure of the coating can be consistent with the raw materials, basically there is no oxidation, alloy composition burning, grain growth and other phenomena, can spray heat sensitive materials and active metals and polymer materials, suitable for in amorphous and nanocrystalline



Preparation of coatings.



3) Composite coatings can be prepared by mechanical mixing of powders with different physical and chemical properties.



4) Small thermal influence on the substrate, extremely slow grain growth rate (possibly maintaining nanostructure), close to forged structure (higher hardness than traditional coatings), stable phase structure and chemical composition, basically no need for shielding, The spraying loss is small, and the spray width can be adjusted to <3mm.



5) The appearance of the coating is consistent with the surface appearance of the substrate, which can reach a high level of surface roughness, and the spraying distance is extremely short.



6) The residual stress of the coating is low, and all are compressive stress, which reduces the limit on the thickness of the coating.



7) The bonding strength of cold spray coating is high, which can reach more than 100MPa, which can fully meet the requirements of strong load and long life in aviation, aerospace and other fields.



8) The coating is dense, with few pores, and the density can reach 98%, which can prepare coatings with high thermal conductivity and high electrical conductivity. The conductivity of cold sprayed pure copper coating is 90%, and the conductivity of flame sprayed coating and HVOF sprayed coating is <50%.



9) The oxide content of the coating is low, the oxide content (mass fraction) of cold spraying is only 0.2%, and the oxide content (mass fraction) of powder flame spraying and HVOF spraying is 1.1.% and 0.5%, respectively.



10) Cold spraying is basically non-polluting to the environment, and the sprayed powder can be recycled and reused.



11) Simple and safe operation, no heat radiation.



Of course, it also has the following shortcomings:



1) Helium is sometimes required to produce high-quality coatings. Helium must also be used when spraying iron-based, steel-based, nickel-based or superalloys, resulting in increased costs.



2) The efficient deposition of particles and the preparation of stable high-quality coatings are largely dependent on the properties of the particles and substrate materials.



The advantages exhibited by cold spray far outweigh its disadvantages, and the advent of advanced helium gas circulation devices and powder injection methods downstream of the spray gun at low pressures indicate that the industry is committed to improving these shortcomings according to application needs.