Cold Spray (CS) (formerly a dynamic cold spray gas) is a layer deposition method. Solid powder (diameter 1 to 50 micrometers) is accelerated in supersonic gas jets up to 500-1000 m/s. During impact with the substrate, the particles undergo plastic deformation and adhere to the surface. To achieve a uniform thickness, the spray nozzle is scanned along the substrate. Metals, polymers, ceramics, composite materials and nanocrystalline powders can be stored by cold spraying. Particle kinetic energy, supplied by gas expansion, is converted to deformation of the plastic energy during binding. Unlike thermal spraying techniques, such as plasma spraying, arc spraying, spraying of flame, or high-speed oxygen fuel (HVOF), the powder does not melt during the spraying process.
Video Gas dynamic cold spray
Jenis
There are two types of CS. High pressure cold spray (HPCS) in which the working gas is nitrogen or helium at pressures above 1.5 MPa, flow rate of more than 2 m 3 /mnt, 18 kW warming power. It is used for spraying pure metal powder with size 5-50 Ã,Âμm. In low pressure cold spray (LPCS), the working gas is a compressed gas with a pressure of 0.5-1.0 MPa, a flow rate of 0.5-2 m 3 /minute and a 3- 5 kW. This is used to spray a mechanical mixture of metals and ceramic powders. The inclusion of ceramic components in the mixture provides high quality coatings with relatively low energy consumption.
Maps Gas dynamic cold spray
Basic principles
The most prevalent bonding theory in cold spraying is associated with "adiabatic shear instability" that occurs at the interface of the substrate of particles at or beyond a certain speed called critical velocity. When a spherical particle moving at a critical velocity affects the substrate, a strong pressure field spreads spherically into particles and substrate from the point of contact. As a result of this pressure field, a shear load is generated which accelerates the material laterally and causes local shear compression. Sliding loading under critical conditions causes an adiabatic shear instability in which thermal softening is predominantly locally above strain work and strain rate hardening, which leads to leaps breaking in strain and temperature as well as damage to flow stress. The phenomenon of adiabatic shear instability results in a thick material flow in the outflow direction with a temperature close to the melting temperature of the material. This drainage material is also a known phenomenon in explosive welding materials.
Key parameters in cold spray
There are several factors that may affect the quality of cold spray coating and deposition efficiency. The main influencing factors are:
- Type of gas, e.g. air, nitrogen, helium
- Gas pressure
- Gas temperature
- Particle size
- The ingredients of the ingredients of the feed, e.g. density, strength, melting temperature
- Nozzle type
- Substrate
- Kinetic deposition (transverse speed of gun, scan speed, number of movements...)
- Standby distance, ie the distance between the cold spray nozzle and the substrate.
Cold spray parameters are chosen according to the desired coating characteristics and economic considerations. This can be done by considering the correlation between process parameters and the properties of the final layer. There are also software packages available for this purpose.
Advantages and disadvantages
CS has many advantages that make the technology potentially very competitive. Being a cold process, the properties of early physical and chemical particles are maintained and substrate heating is minimal, resulting in a cold-worked micro structure in which no fusion and freezing takes place. Dynamic recrystallization with fine granules has been observed between the particles and the bonding region of the particles. In addition, this technology makes it possible to spray heat-sensitive materials and very different material combinations, because the mechanical adhesion mechanism is purely mechanical.
Other advantages are:
- High thermal and electrical conductivity of coatings;
- High density and coating hardness;
- High layer homogeneity;
- Low shrink;
- Possible to spray microparticles (5-10 m);
- The possibility to spray nanomaterials and amorphous materials;
- Short standby distance;
- Minimum surface preparation;
- Low energy consumption;
- Possibility of obtaining complex internal shapes and surfaces;
- High productivity due to high level of power feed;
- High deposition and efficiency levels;
- The possibility to collect and reuse 100% particles;
- No toxic waste;
- Without burning;
- Operational security increases due to the absence of high temperature gas emissions and radiation.
The jet obtained is a high-density particle beam due to the small size of the nozzle (10-15 mm 2 ) and the short stand-off distance (25 mm). This results in a high focus of jet and precise control over the deposition area. Finally, inducing compressive stress makes it possible to obtain a uniform and ultra-thick solid coating (20? M - 50 mm).
On the other hand, some difficulties can be found. For example, it is difficult to spray hard and fragile materials because, in this case, mechanical adhesion through plastic deformation may not be as effective as for ductile particles. Other issues may include:
- Almost nil ductility is in as-sprayed condition;
- The need for ductile substrates;
- Difficulty in processing pure ceramics and some alloys as work hardening alloys;
- the high cost of Helium;
- fouling and nozzle erosion.
Apps
Coating
The ability to store sensitive phase or temperature sensitive CS materials has positioned the technique to prepare upholstery is not possible with other thermal spray techniques. CS can generally be used to produce various metal, alloy, and metal-based composites, including materials that have very high melting temperatures (eg tantalum, niobium, superaloys). This process is also valuable for storing materials that are highly sensitive to the presence of oxygen and will be readily oxidized at high temperatures which are - results that impair the performance of these materials. Some examples of oxygen sensitive layers commonly produced with CS are aluminum, copper, titanium, and carbide composites (eg tungsten carbide), as well as coatings made of amorphous alloys.
Additional developments in CS are related to the deposition of ceramic materials in metals, especially titanium dioxide for photocatalytic effects, and the use of CS in the manufacture of additives.
Repair
Cold spray is now used to repair engine parts in minutes. Metal (nickel alloy) travel particles in a mixture of nitrogen and helium gas and gradually accumulate in the damaged part to create the desired surface. The robot controls the movement of the sprayer. The US Army uses technology to improve components on the Blackhawk helicopter. General Electric adapts technology to civilian applications.
Manufacturing
The manufacture of additives using cold spray technology can be used to develop parts and components quickly with a deposition rate as high as 45kg/h - much faster than other additive manufacturing methods. Unlike other additive-making methods such as selective laser fusion or the manufacture of electron beam additives, cold spray technology does not melt metals. This means that the metal is not affected by heat-related distortions, and its parts need not be made in an inert gas or a vacuum-enclosed environment, allowing for the creation of much larger structures. The largest and fastest 3D metal printer in the world has a 9 x 3 x 1.5m building envelope and uses a dynamic cold spray gas. This machine is operated by an advanced manufacturing company Titomic Limited. Manufacturing with cold spray technology provides advantages such as the ability to create shapes without shape or size limits, a more efficient buy-to-fly ratio when compared to machining, and be able to combine different metals to create hybrid metal parts - materials such as titanium alloys, copper, zinc, stainless steel, aluminum, nickel, even hastelloy and inconel can be sprayed together. Titomic has the exclusive right to commercialize a proprietary and patented CSIRO process for the application of cold-gas-cooled spraying of titanium or titanium alloy particles to the scaffold to produce the load structure.
References
Further reading
- The Cold Spray Action Team meets annually to discuss the latest cold spray technology, and to share data and experiences. Meeting presentations are available from 2011 onwards.
Source of the article : Wikipedia
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