Thermal Spray Process

 

Explosive spraying is a method in which oxygen and acetylene gas are mixed in a certain ratio in a specially designed combustion chamber and then detonated to heat and melt the powder and cause the particles to hit the surface of the part at high speed to form a coating.

 

 

Explosive spraying is a method in which oxygen and acetylene gas are mixed in a certain ratio in a specially designed combustion chamber and then detonated to heat and melt the powder and cause the particles to hit the surface of the part at high speed to form a coating. The biggest feature of explosive spraying is the high flight speed and high kinetic energy of particles, so the explosive spray coating has: First, the bonding strength between the coating and the matrix is high. Second, the coating is dense and has very low porosity. Third, the surface roughness of the coating is low after processing. Fourth, the surface temperature of the workpiece is low. In explosive spraying, when the acetylene content is 45%, the oxygen-acetylene mixture can produce a free combustion temperature of 3140°C, but it may exceed 4200°C under explosive conditions, so most powders can melt. The distance the powder is transported in the high-speed gun is much larger than that of the plasma gun, which is also the reason for the high particle speed. Explosive spraying can spray metal, cermet and ceramic materials. However, due to the high price of the equipment, loud noise, oxidizing atmosphere and other reasons, it is not widely used at home and abroad. The most successfully applied explosive spraying in the world is the patent obtained by the Linde Branch of the American Union Carbide Company in 1955. In our country, around 1985, the Aerospace Materials Research Institute of the Ministry of Aerospace Industry of China successfully developed explosive spraying equipment. In terms of Co/WC coating performance, the spraying performance is close to the level of Union Carbide in the United States.

 

 

The biggest characteristic of explosive spraying is that the thermal energy of a sudden explosion is used to heat and melt the spray material, and the high pressure generated by the explosion shock wave is used to spray the spray powder material at high speed onto the surface of the workpiece substrate to form a coating. Its main advantages are as follows.

(2) Thermal damage to the workpiece is small. Because explosive spraying is pulse type, the impact time of hot airflow and particles is short each time, and the nitrogen has a cooling effect on the workpiece. The temperature of the workpiece is lower than 200°C, so the thermal damage of the matrix is small, and deformation and phase change will not occur.
(3) The thickness of the coating is easy to control, the machining allowance is small, and maintenance operations are convenient.
(4) The roughness of the explosive spray coating is low, possibly less than 1.60 μm, and the roughness can reach 0.025 μm after grinding.
(5) During the spraying process, carbide and carbide-based powder materials will not produce carbon decomposition and decarburization, thus ensuring the consistency of the coating tissue composition and powder composition.
(6) Low oxygen consumption and low operating costs.

 

Plasma spraying is an advanced thermal spraying technology that uses the high temperature and high-speed airflow of plasma to spray coating materials onto the surface of the substrate to form a uniform, dense, and high-quality coating. This technology has ultra-high temperature characteristics, is suitable for spraying high melting point materials, and can achieve high density and high bonding strength of the coating.

 

 

(1) The base body is heated less and the parts are not deformed: Since the parts are not charged during spraying, the base metal does not melt, so the heat treatment properties of the base metal will not change, and some high-strength steel materials can be sprayed.
(2) Various types of materials and rich coating properties: Plasma spraying can use a variety of materials, including metals, ceramics, plastics, etc., so coatings with various properties can be obtained, such as wear-resistant coatings, heat-insulating coatings, and high-temperature resistant coatings. Oxide coating, insulating coating, etc.
(3) Stable process and high coating quality: The process parameters of plasma spraying can be quantitatively controlled, the process is stable, the coating reproducibility is good, and the bonding strength between the coating and the substrate is high.
The process parameters of plasma spraying include the selection of plasma gas, arc power, powder feeding amount, spraying distance and spraying angle, etc. Proper setting of these parameters is crucial to the quality of the coating. For example, the choice of plasma gas will affect the enthalpy and flow rate of the coating, the power of the arc will affect the temperature of the coating and the degree of melting of the particles, and the powder feeding amount and spraying distance will affect the deposition efficiency and uniformity of the coating. sex.
Plasma spraying technology is widely used in aerospace, automotive, electronics, medical and other fields. In the aerospace field, high-temperature alloys, ceramic coatings, etc. can be sprayed to improve the heat resistance of engines, turbines and other components; in the automotive field, anti-corrosion coatings, ceramic coatings, etc. can be sprayed to improve the corrosion resistance of automobile parts; In the electronics field, conductive coatings, insulating coatings, etc. can be sprayed to improve the performance and reliability of electronic components.

What types of plasma gases are commonly used in plasma spraying processes?
In the plasma spraying process, commonly used plasma gases mainly include the following types:
Hydrogen (H2): Hydrogen is a diatomic gas with high thermal conductivity and can significantly increase the temperature and thermal power of the plasma arc. Hydrogen is an indispensable gas when spraying refractory materials and ceramic materials. However, hydrogen is difficult to prepare and preserve, is costly, and poses certain safety risks, so its use is limited in some cases.
Nitrogen (N2): Nitrogen is also a diatomic gas with a high thermal enthalpy value. It absorbs a lot of heat during the ionization process and has a high energy utilization rate. Nitrogen is widely used in plasma spraying because of its convenient source and low price. However, nitrogen has certain oxidizing properties and is not suitable for spraying powders that are easily oxidized.
Argon (Ar): Argon is a monatomic gas with low arc voltage and low thermal enthalpy. However, it absorbs heat quickly during ionization and has a relatively small thermal conductivity. Argon has good arc starting and stabilizing properties. It is an inert gas with good protective properties and is suitable for spraying metals with strong chemical activity. Argon is one of the commonly used plasma gases, but its cost is relatively high.
Helium (He): Helium is a monatomic gas, also an inert gas, with extremely high enthalpy value. Helium absorbs a lot of heat during the ionization process, so it has a high enthalpy value at high temperatures. The physical and chemical properties of helium are very stable, but due to its extremely low content in the air and the very high cost of extraction, it is less used in industrial applications.
Water vapor and air: The heat transfer capacity of water vapor and air is also quite high, with wide sources, convenience and low cost. Therefore, people have begun to consider using water vapor and air as gas sources for plasma spraying, and have achieved certain results. For example, commercialized high-power water-stabilized plasma spraying equipment has been developed, but its structure is complex and the stability of the plasma jet during spraying needs to be improved.

 

Arc spraying is a thermal spraying technology that uses two continuously fed metal wires as consumable electrodes to generate arcs at their ends as a heat source. The molten wire is atomized by compressed air and sprayed onto the surface of the workpiece at high speed to form a coating. During this process, the metal wire is short-circuited and discharges at the moment of contact. Under the action of high-speed airflow and wire feeding mechanism, the metal at the end undergoes a series of processes such as melting, atomization, acceleration and deposition, and finally forms a coating.

 

 

(1) High energy utilization: Arc spraying converts electrical energy directly into thermal energy, and the thermal energy utilization rate is as high as 60%-70%, which is much higher than the 5%-15% of flame spraying.
(2) High cost-effectiveness: Since two metal wires are fed at the same time, the spraying efficiency is high and the cost is low.
(3) High coating bonding strength: All particles are sprayed by droplet atomization, so the coating bonding strength is high.
(4) Wide applicability: Corrosion-resistant material coatings can be sprayed according to different corrosive environments, and have universal applicability.

 

 

- Aerospace
- Car manufacturer
- Petrochemical
- Power capacitors and their electronic equipment
- Metal surface anti-corrosion, wear resistance, conductivity, insulation, decoration, etc.

 

- Staff must wear personal protective equipment, including safety helmets, protective glasses, protective masks, protective clothing, etc.
- Ensure that other equipment and items in the working area will not be affected by spraying, and take appropriate protective measures.
- When any potential safety hazards in arc spraying equipment are discovered, they should be reported immediately and measures should be taken to solve the problems in a timely manner.

Supersonic spraying involves mixing gaseous or liquid fuel with high-pressure oxygen and then burning it in a specific combustion chamber or nozzle. The resulting high-temperature, high-speed combustion flame is used for spraying. Since the speed of the combustion flame is several times the speed of sound, bright "Mach nodes" in the flame stream can be visually seen, so supersonic spraying is generally called supersonic flame spraying. By feeding the powder axially into the flame, the spray particles can be heated to a molten or semi-melted state and accelerated to a speed of up to 300-500m/s or even higher, thereby obtaining a dense, high-quality coating with high bonding strength. . The supersonic flame speed is very high, but the temperature is relatively low, about 3000°C. For WC-Co cemented carbide, it can effectively inhibit the decomposition of WC during the spraying process. The supersonic spray coating not only has high bonding strength, but also It is dense and has excellent wear resistance. Its wear resistance greatly exceeds the plasma spray coating, is equivalent to the explosive spray coating, and also exceeds the electroplated hard chromium layer and spray melt layer. It is extremely widely used.

 

 

In addition, the supersonic spray gun needs to be cleaned frequently during the spraying process to prevent excessive deposition of metal powder. During the working process of the spray gun, the voltage at the two poles reaches tens of thousands of volts. Excessive dust can easily produce sparks, affecting construction and causing accidents. The main methods for cleaning the spray gun are a high-pressure air gun and a dry rag.
The application of supersonic spraying (HVOF) technology With the development and improvement of thermal spraying technology in my country, the quality requirements for sprayed coatings are becoming higher and higher. The high-speed gas method developed in the United States and other countries in recent years is a new process for preparing high-quality coatings. Because the supersonic flame spraying method has many advantages.

Flame spray welding is a commonly used metal surface treatment technology. It uses an oxyacetylene flame to heat alloy powder until it melts or reaches a high plasticity state, and then sprays it onto the surface of the workpiece to form a protective or enhanced coating. This process can be used for surface strengthening or repair of various carbon steel and low alloy steel parts. However, when using it, attention needs to be paid to the characteristics of the matrix material to avoid excessive differences in linear expansion coefficients or the matrix containing substances that are easy to react with oxygen. elements causing difficulty in spray welding.

 

 

Wide scope of application: suitable for surface treatment of a variety of metal materials.
Relatively low cost: Flame spray welding is less expensive than other surface treatment technologies.
Easy to operate: The process flow is relatively simple and easy to master.
Improve wear resistance and corrosion resistance: The spray welding layer can significantly improve the wear resistance and corrosion resistance of the workpiece.

 

 

Flame spray welding is widely used in the following fields:
Mechanical parts repair: Used to repair worn or damaged mechanical parts surfaces.
Surface strengthening: Improve the wear resistance and fatigue resistance of parts.
Anti-corrosion treatment: used to improve the corrosion resistance of parts and extend their service life.

 

When performing flame spray welding, you need to pay attention to the following points:
Workpiece surface treatment: The surface needs to be strictly purified and roughened before spraying to ensure the adhesion of the coating.
Spraying specifications: Strictly control the spraying specifications to avoid overheating of the workpiece. Especially when the coating is thick, intermittent spraying is required to cool down.
Spraying angle: Pay attention to the angle of the spray gun when spraying to ensure even distribution and good adhesion of the coating.
Remelting temperature control: The control of remelting temperature is very critical. Too high or too low temperature may affect the quality of the coating and the performance of the workpiece.

 

 

Laser cladding is a new type of surface modification technology. It adds cladding materials to the surface of the substrate and uses high-energy-density laser beams to melt them together with the thin layer on the surface of the substrate to form a metallurgically bonded additive melt. Cladding. This process can significantly improve the wear resistance, corrosion resistance, heat resistance, oxidation resistance or electrical properties of the base material surface, thereby achieving the purpose of surface modification or repair, meeting the specific performance requirements of the material surface while saving a large amount of material costs. 

 

 

Low dilution rate: The laser cladding layer is metallurgically bonded to the substrate, and the dilution rate is low, usually only 5% to 8%, which means that a thinner coating can be used to achieve the required performance requirements.
High hardness and wear resistance: The cladding layer has a fine and dense structure, a higher hardness and better wear resistance.
Small heat-affected zone: Due to the high heating temperature of the laser beam, the heat-affected zone is small and the workpiece deforms less.
Stable quality of the cladding layer: The quality of the laser cladding layer is stable and easy to realize automated production.
Wide range of material selection: The laser beam power density is high, the heating temperature is high, and the selection range of cladding materials is wider.

 

Laser cladding processes can be roughly divided into two categories according to the supply method of cladding materials:
Prepositioned laser cladding: First, the cladding material is pre-placed on the cladding part of the substrate surface, and then the laser beam is used to scan and melt the cladding material and the substrate surface.
Synchronous powder-feeding laser cladding: powder or wire cladding materials are synchronously fed into the molten pool through the nozzle during the cladding process.

 

Surface modification: such as surface modification of gas turbine blades, rolls, gears, etc.
Surface repair: such as surface repair of rotors, molds, etc.
Laser additive manufacturing: Through layer-by-layer laser cladding, parts with a three-dimensional structure are obtained.

 

The development trends of laser cladding technology are mainly concentrated in the following aspects:
Basic theoretical research: In-depth understanding of the physical and chemical mechanisms in the laser cladding process.
Design and development of cladding materials: Develop new cladding materials to meet higher performance requirements.
Improvement and development of laser cladding equipment: improve the performance and stability of laser cladding equipment.
Establishment of theoretical model: Establish an accurate theoretical model of laser cladding to guide actual production.
Laser cladding rapid prototyping technology: develop fast and efficient laser cladding prototyping technology.
Automation of cladding process control: Realize automated control of the laser cladding process to improve production efficiency and product quality.