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Thermal spraying powders
Nov 07, 2017

Thermal spraying powders include metals, alloys and composite powders used for Thermal spraying of hard surface technology. These unique powders are instantaneously heated by means of thermal spraying such as oxygen-acetylene flame, plasma flame or oxy-acetylene explosion flame, and are spray-deposited with high-speed heat flux in a molten or semi-molten state on a cold or Hot workpiece surface, the formation of surface strengthening layer, in order to improve the workpiece surface wear resistance, corrosion resistance, heat resistance, oxidation or repair dimensions and other purposes. This is an economical, practical and efficient process technology, which has been widely used.


Brief history Thermal spraying technology originated in 1910, by Dr. M V Schoop, Switzerland, the first oxygen-acetylene flame spray gun for metal spraying. In the 1940s and 1950s, with the advent of new spraying equipment, the spraying material was gradually developed from metal wire to metal powder. Among them, the application of atomized pre-alloyed powder and coated composite powder was more extensive and successful.


Types of thermal spray powder is divided into self-fluxing alloy, self-bonding alloy, corrosion and oxidation-resistant alloys, ceramics, cermets and composite powders. Each powder must consider its particle size composition, particle shape, fluidity and compositional effects. Generally in the thermal spraying process powder deposition rate should be from 80% to 95%.


Self-fluxing alloy powder.

This powder has a very low melting point and can be melted or re-melted in the normal atmosphere to combine with the surface of the workpiece into a dense, essentially void-free layer. Self-fluxing alloys contain boron and silicon fluxes, acting as "getters" to prevent the nickel-chromium matrix alloy from oxidizing and forming an oxidation resistant boron silicate "glass." Boron is a nickel strengthening agent, but also reduce the melting point of nickel-based alloys (down to 1040 ~ 1120 ℃). Borosilicate flux can significantly improve the fluidity and surface tension of liquid alloys, is conducive to the formation of dense coatings, coating hardness range HRC30 ~ 50, its abrasion resistance, abrasion than the same hardness 20 times higher steel. Self-fluxing alloy is divided into nickel-based, cobalt-based, iron-based and tungsten carbide powder, almost all prepared by atomization. Commonly used in need of wear-resistant, corrosion-resistant coatings and high surface finish of the workpiece.


Wear-resistant, corrosion-resistant, heat-resistant alloy powder.

This is a kind of alloy powder containing eutectic carbide in a cobalt-based matrix alloyed with chromium and tungsten. For example, various components of cobalt-chromium-tungsten alloy powder. It can meet the wear-resistant, corrosion-resistant and high temperature requirements. However, as the weight fraction of carbides decreases, the wear resistance and corrosion resistance decrease. This spray powder is expensive, has been developed less cobalt or cobalt-free alternatives.


Oxidation resistant alloy powder.

Oxidation resistant alloy can not be self-fluxing, and is always used in sprayed state. For example, similar to the 316 type stainless steel, high chromium stainless steel, nickel-chromium alloy and high purity spherical copper powder. They are produced almost by nebulization and can be used to make bright, dense, high-gloss coatings or to repair work surfaces.


Composite powder

With the development of spray equipment, has developed a wide range of applications, a wide range of new thermal spray material, that is, composite powder. It is often available in exothermic white adhesive type powder available. Since then, a variety of high melting point metals, alloys, oxides, carbides and other non-metallic materials can be thermal sprayed. Composite powder can be a variety of materials for different combinations of two or more different materials combined into a heterogeneous heterogeneous particles, according to the powder structure can be divided into two types of coated and non-coated. The former is a core material of the particle surface completely and uniformly coated with one or more, one or more layers of other materials; the latter is composed of two or more small particles of coarse powder , And more use of high-energy ball milling mechanical alloying methods of production. The composite powder is heterogeneous, but the distribution of the components and the various components between the particles is generally relatively uniform.


Coated composite powder is a very good thermal spraying material, has broad prospects for development, according to its material, performance and use can be broadly divided into the following types:

(1) since the combination. The kind of powder is made of metal aluminum, titanium, zirconium, chromium and its alloy as the core, and is coated with nickel or cobalt as a self-adhesive alloy powder of nickel (cobalt) aluminum, titanium, zirconium and the like. This composite powder preparation methods are liquid deposition, vapor deposition and solid-state thermal diffusion bonding, etc., wherein the wet production of nickel or cobalt-coated composite powder can be directly from the nickel or cobalt production process Take; vapor deposition of carbonyl nickel powder production of aluminum is quite successful.

(2) wear-resistant type. Such as WC, TiC, BN, Cr3C2, SiC, TiN, diamond as the core, the surface coated with nickel, Cobalt, copper or alloy Ni-Cr-Al, Co-Cr-Al-Y, etc., can be used to make processing tools or wear-resistant coating.

(3) can be sealed type. Such as a wear-able, relatively soft powder (graphite, diatomaceous earth, calcium fluoride, etc.) as the core, the surface coating of metal, after treatment or thermal spraying and other processing to form an abradable coating or Parts, to achieve the purpose of lubrication and sealing. This powder is nickel-coated graphite (Figure a), copper coated graphite, nickel-coated diatomaceous earth, nickel-copper-aluminum-calcium fluoride and so on.

(4) wear-resistant type. Such as titanium, zirconium, chromium, chromium oxide as the core coated with nickel, cobalt, copper, silver, gold and other composite powder for wear-resistant parts of the surface coating.

(5) heat insulation type. If the core is made of zirconia, alumina (b) and titanium oxide with good thermal or heat resistance properties, the metal coated as a binder can be improved after thermal spraying, and the minerals themselves And the bonding of the workpiece substrate to form a good thermal insulation, heat-resistant coating to improve the surface of the workpiece thermal wear resistance erosion resistance. This coating has been widely used in aircraft engines and spacecraft.

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