Thermal spraying and thermal spray welding technology

Thermal spraying technology and thermal spray welding technology are both technologies that use thermal energy (such as oxygen acetylene flame, arc, plasma flame, etc.) to melt coating materials with special properties and apply them to the workpiece to form a coating. It has the characteristic of being able to prepare relatively thick coatings (0.1-10mm), mainly used in manufacturing composite layer parts for repair.

1. Thermal spraying technology

① Principles and characteristics of thermal spraying technology

The process of using various heat sources to heat and melt or semi melt the coating material, and then using high-speed gas to disperse and refine the coating material and impact it onto the substrate surface at high speed, thus forming the coating.

The thermal spraying process mainly includes: melting of spraying materials; Atomization of spraying materials; Flight of sprayed materials; Impact and solidification of particles.

② Coating materials

Thermal spraying has certain requirements for coating materials, which must meet the following conditions: a wide liquid phase zone, which is not easy to decompose or evaporate at the spraying temperature; Good thermal stability; Good usability; Good wettability; Good solid fluidity (powder); The coefficient of thermal expansion is appropriate. Coating materials can be divided into wire and powder according to the shape of the spraying material.

③ The Bonding Mechanism of Thermal Spraying Coatings

·Mechanical bonding: Molten particles strike the surface of the substrate and spread into a flat liquid layer, which is embedded on the uneven surface and forms mechanical bonding.

·Metallurgical bonding: The diffusion and welding between the coating and the substrate surface is called metallurgical bonding.

·Physical bonding: When high-speed molten particles collide with the surface of the substrate, if the distance between the two sides of the interface is within the range of atomic lattice constant, the particles are bonded together by van der Waals force.

④ Formation process of coating

·The spraying material is heated to a molten state;

·The spraying material is atomized into tiny droplets and high-speed impacts the surface of the substrate. The greater the kinetic energy and impact deformation of the particles that impact the substrate, the better the bonding of the formed coating;

·The molten high-speed particles deform after impacting the surface of the substrate, and condense to form a coating.

Formation process of coating

The coating structure is composed of flat particles of varying sizes, unmelted spherical particles, inclusions, and pores. The reason for the existence of pores: low impact kinetic energy of unmelted particles; The shielding effect caused by different spraying angles; Solidification shrinkage and stress release effects. Appropriate pores can store lubricants, improve the insulation performance of coatings, reduce internal stress, and improve the thermal shock resistance of coatings. However, excessive pores will damage the corrosion resistance of coatings, increase the surface roughness of coatings, and thereby reduce the bonding strength, hardness, and wear resistance of coatings. Therefore, the number of pores should be strictly controlled during the preparation process of coatings.

2. Thermal spray welding technology

① The principle and characteristics of thermal spray welding technology

Thermal spray welding technology is a surface metallurgical strengthening method that uses a heat source to remelt or partially melt the coating material on the substrate surface, and condense it onto the substrate surface, forming a surface layer with metallurgical bonding with the substrate, also known as fusion bonding. Compared to other surface treatment processes, the microstructure obtained by thermal spray welding is dense, with few metallurgical defects and high bonding strength with the substrate. However, the selection range of materials used is narrow, the deformation of the substrate is much greater than that of thermal spray welding, and the composition of the thermal spray welding layer is somewhat different from the original composition.

② The classification of thermal spray welding technology mainly includes flame spray welding, plasma spray welding, etc.

Flame spray welding: First, powder is sprayed on the surface of the substrate, and then the coating is directly heated by flame to melt again on the surface of the substrate. The surface of the substrate is completely wetted, and there is mutual element diffusion at the interface, forming a solid metallurgical bond.

Flame spray welding features: simple equipment; Simple process; The bonding strength between the coating and the substrate is high; The coating has good resistance to erosion and wear.

Plasma spray welding: Using a plasma arc as a heat source to heat the substrate, forming a molten pool on its surface. At the same time, the spray welding powder material is fed into the plasma arc. The powder is preheated in the arc column, in a molten or semi molten state. After being sprayed into the molten pool by the flame, the gas and slag are fully melted and discharged. After the spray gun is removed, the alloy molten pool solidifies, ultimately forming a spray welding layer.

The characteristics of plasma spray welding: high production efficiency; Can spray weld refractory materials, low dilution rate, good process stability, easy to achieve automation, flat and smooth spray welding layer, uniform composition and structure, larger coating thickness, and precise control of the testing process.

③ The difference between thermal spray welding technology and thermal spray coating technology

·Surface temperature of the workpiece: When spraying, the surface temperature of the workpiece is less than 250 ℃; Spray welding should be>900 ℃.

·Bonding status: The spraying layer is mainly mechanically bonded; The spray welding layer is metallurgical bonding.

·Powder material: Self fluxing alloy powder is used for spray welding, and the spray powder is not restricted.

·Coating structure: The spray coating has pores, while the spray welding layer is uniform, dense, and without pores.

·Bearing capacity: The spray welding layer can withstand impact loads and high contact stresses.

·Dilution rate: The dilution rate of the spray welding layer is about 5%~10%, and the dilution rate of the spray coating is almost zero.