By Harbin pei-ze material technology co., LTD | 23 February 2019 | 0 Comments

Metal and alloy materials for thermal spraying

An overview of the

  All the pure metals with independent performance can be used as thermal spraying materials, such as zinc, aluminum, lead, molybdenum, etc.In order to improve the performance of spraying, other elements are often added into the pure metal to form alloy materials.Among thermal spraying materials, metal materials have relatively low melting point, good toughness, high bonding strength, good thermal conductivity, and have certain corrosion resistance according to the different components. Therefore, it has become an important thermal spraying material and has been widely used.This chapter will focus on the basic properties and main applications of metal and alloy materials for thermal spraying.

   Metal and alloy materials for thermal spraying can be divided into powder and wire (bar) materials according to their shapes.According to the properties of materials can be divided into pure metal materials, alloys, self-melting alloys.

   In nickel base, iron base, cobalt base and copper base alloy add a certain amount of B, Si elements, forming low melting point eutectic alloy elements, this kind of alloy is called self-melting alloy, also known as self-flux alloy or eutectic alloy.In the melting process, it can deoxidize itself, make slag and moisten the surface of matrix.The addition of B and Si elements improves the performance of the coating.Their main functions in the alloy are as follows:

   (1) B and Si can reduce the melting point of the alloy and expand the temperature range between the solid phase and the liquid phase.

Both B and Si elements can be combined with common matrix materials such as Fe, Ni and Co to form eutectic with low melting point at high temperature, which greatly reduces the melting point of the alloy.Such as Ni - B (B4.2 %) eutectic melting point is 1045 ℃, Fe - B (B3.8 %) alloy eutectic melting point is 1161 ℃.Co - B alloy (B4.0 %) eutectic melting point is 1095 ℃.Si has less effect on melting point than B.

    (2) deoxidation reduction and slag formation of B and Si.

B, Si elements are strong reducing agents.At various temperatures, their oxides are more stable than those produced by Fe, Ni, Co and other elements.Therefore, elements B and Si have strong deoxidation and reduction effects on the oxides of Fe, Ni, Co and other elements.B and Si interact with oxygen to generate B2O3 and SiO2, respectively.B2O3 melting point is 580 ℃, SiO2 is 1713 ℃.Although B2O3 has a low melting point, it has a high viscosity and is difficult to surface.However, when B2O3 and SiO2 exist together, borosilicate with low melting point can be formed.For example, 73% of SiO2 and 27% B2O3 melting point is 722 ℃.This borosilicate viscosity is small, light specific gravity, good liquidity, easy to surface alloy, so that the coating alloy is protected from oxidation, prevent the generation of pores.

    (3) B, Si elements can improve the hardness of the alloy.

B and Si elements have diffusion strengthening and solid solution strengthening effects on the microstructure of the alloy.The dispersion of the second phase increases the strength and hardness of the alloy, which is called dispersion strengthening.

The main role of B is diffusion strengthening.Except a small amount of B is dissolved in ni-austenite, most of it is dispersed in the alloy in the form of intermetallic compounds such as Ni3B.When Cr is contained in the alloy, B and Cr can generate hard particles such as intermetallic compounds Cr2B and CrB.Boron sometimes forms hard carboron compound particles with the carbon in the alloy, which are dispersed in the alloy.These hard particles are extremely hard.The experiment shows that when the content of B is not high, the hardness of the coating increases significantly with the increase of the content of B.The effect was not significant when B content exceeded 3.5%.

   Si is mainly used for solid solution strengthening. In nickel-based self-melting alloys, the solubility of Si in nickel can reach 6% at room temperature.Therefore, most of the silicon can be solidly dissolved in nickel austenite, resulting in solid solution strengthening.

   In addition, some hard phases are eutectic with the matrix phase.The amount of eutectic and the degree of dispersion are related to the rate of cooling.The faster the coating solidifies, the less eutectic phase is dispersed, whereas the eutectic phase particles are larger and aggregate.This eutectic phase increases the hardness and brittleness of the coating.

    B, Si elements in iron - based, cobalt - based alloys have a similar role.The content of B and Si in general alloy does not exceed 6% and 5% respectively.Too high a content of B and Si will lead to more brittle compounds, which will reduce the plasticity and toughness of the coating, increase the brittleness, and easily cause cracks.

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