Thermal spray powder

       Thermal spraying powders Metals, alloys and composite powders commonly used in thermal spray hard-surface technology. Using thermal spraying (welding) methods such as oxygen-acetylene flame, plasma flame or oxygen-acetylene explosion flame, these unique powders are instantly heated, and in a molten or semi-melted state, they are sprayed and deposited in cold or cold or semi-melted state with high-speed heat flow. A surface strengthening layer is formed on the surface of the hot work piece to achieve the purpose of improving the wear resistance, corrosion resistance, heat resistance, oxidation resistance, or repairing the dimensions of the surface of the work piece. This is an economical, practical and efficient process technology that has been widely used. A brief history Thermal spraying technology first originated in 1910, when Dr. M V Schoop of Switzerland took the lead in spraying metal with an oxygen-acetylene flame spray gun. In the 1940s and 1950s, with the advent of new spraying equipment, spraying materials have gradually developed from metal wires to metal powders. Among them, the application of atomized pre-alloy powder and coated composite powder is more extensive and successful. Types Thermal spray powders are divided into self-fluxing alloys, self-bonding alloys, corrosion- and oxidation-resistant alloys, ceramics, cermets and composite powders. Each powder must consider the influence of its particle size composition, particle shape, fluidity and composition. Generally, the deposition rate of powder in the thermal spraying process should be between 80% and 95%. Self-fluxing alloy powder This powder has a very low melting point and can be melted or remelted in normal atmosphere and combined with the surface of the workpiece into a dense layer that is essentially non-porous. The self-fluxing alloy contains boron and silicon flux, which acts as a "getter" to prevent the oxidation of the nickel-chromium base alloy and form an oxidation-resistant boron silicate "glass". Boron is a strengthening agent for nickel, and it can lower the melting point of nickel-based alloys (which can be as low as 1040-1120°C). The borosilicate flux can significantly improve the fluidity and surface tension of the liquid alloy, and is conducive to the formation of a dense coating. The hardness of the coating ranges from HRC30 to 50, and its wear resistance and abrasion resistance are 20 times higher than that of steel with the same hardness. Self-fluxing alloys are divided into nickel-based, cobalt-based, iron-based and tungsten carbide-containing powders, almost all of which are prepared by atomization. It is often used for workpieces that require wear-resistant, corrosion-resistant thin coatings and high surface finish. Wear-resistant, corrosion-resistant, high-temperature alloy powder This is an alloy powder containing eutectic carbides in a cobalt-based matrix alloyed with chromium and tungsten. For example, various components of cobalt-chromium-tungsten alloy powder. It can meet the requirements of wear resistance, corrosion resistance and high temperature resistance at the same time. However, as the weight percentage of carbides decreases, its wear resistance and corrosion resistance decrease. This spray powder is expensive. Cheap alternatives with less or no cobalt have been developed. Oxidation-resistant alloy powder Oxidation-resistant alloys are not self-melting and are always used as sprayed. For example, high-quality stainless steel, high-chromium stainless steel, nickel-chromium alloy and high-purity spherical copper powder similar to 316 type. They are almost produced by the atomization method and can be used to prepare bright, dense, high-finish coatings or repair work surfaces. Composite powder With the development of spraying equipment, a new type of thermal spraying material with a wide variety and wide application range, namely composite powder, has been developed. It is often available as an exothermic white-sticky powder. Since then, various high melting point metals, alloys, oxides, carbides and other non-metallic materials can be thermal sprayed. Composite powders can combine various materials in different combinations, combining two or more different materials into multiphase heterogeneous particles. According to the powder structure, it can be divided into two categories: coated type and non-coated type. The former is a core material whose particle surface is completely and evenly covered with one or more, one or more layers of other materials; the latter is a coarser secondary powder composed of two or more fine material particles , It is mostly produced by high-energy ball mill mechanical alloying method. The composite powder is heterogeneous, but the composition and the distribution of various components between the particles are generally relatively uniform. Coated composite powder is an extremely good thermal spray material with broad development prospects. According to its material, performance and use, it can be roughly divided into the following types: (1) Self-bonding type. This kind of powder takes metal aluminum, titanium, zirconium, chromium and their alloys as the core, and is coated with nickel or cobalt on the outside to become self-bonding alloy powders such as nickel (cobalt) clad aluminum, titanium clad, and zirconium clad. The preparation methods of this composite powder include liquid deposition method, vapor deposition method and solid-phase thermal diffusion bonding method. Among them, the wet production of nickel or cobalt-coated composite powder can be directly prepared from the production process of nickel or cobalt. Take; The carbonyl method of vapor deposition is also quite successful in producing nickel-coated aluminum powder. (2) Wear-resistant type. For example, with WC, TiC, BN, Cr3C2, SiC, TiN, and diamond as the core, its surface is covered with metallic nickel, Cobalt, copper or alloy Ni-Cr-Al, Co-Cr-Al-Y, etc., can be used to make processing tools or wear-resistant coatings. (3) Sealable type. For example, an abradable, relatively soft powder (graphite, diatomaceous earth, calcium fluoride, etc.) is used as the core, and the surface is coated with metal. After processing or thermal spraying, an abradable coating or Components, achieve the purpose of lubrication and sealing. Such powders include nickel-coated graphite, copper-coated graphite, nickel-coated diatomaceous earth, nickel-copper-aluminum-coated calcium fluoride and so on. (4) Wear-resistant type. For example, titanium, zirconium, chromium, chromium oxide is used as the core to coat nickel, cobalt, copper, silver, gold and other composite powders for surface coating of wear-resistant parts. (5) Heat insulation and heat resistance type. For example, with particles such as zirconia, alumina and titania with good heat insulation or heat resistance as the core, coating the metal as a binder, thermal spraying can improve the adhesion of these oxides, minerals themselves and the workpiece matrix. It forms a good heat-insulating and heat-resistant coating to improve the heat resistance, wear resistance and erosion resistance of the surface of the workpiece. This coating has been widely used in aero engines and spacecraft.