What is the role of boron and silicon added to self-fluxing alloys?

       (1) Boron and silicon can reduce the melting point of the alloy, and can lead to a wide temperature range between the solid phase and the liquid phase of the alloy. Both boron and silicon elements can be used with common matrix materials, such as nickel, cobalt, iron, etc., The eutectic with low melting point is formed at high temperature, which greatly reduces the melting point of the alloy. For example, the melting point of Ni-BA eutectic is 1095; the melting point of Fe-B eutectic is 1070 degrees; the melting point of Co-B eutectic is 1095 degrees; and the melting point of Fe-B eutectic is 1140 degrees. Silicon has less effect on the melting point of the alloy than boron.

    The melting point of the alloy is low, but it has a wide solid-liquid phase temperature range, which makes the alloy have excellent fluidity and wettability. Therefore, the manufacturability of the alloy is good, and the coating is beautifully formed.

    (2) Deoxidation reduction and slag formation of borosilicate. Boron and silicon are strong reducing agents. At various temperatures, the oxides they produce are more stable than those produced by elements such as nickel, cobalt, and iron. Therefore, boron and silicon elements have strong deoxidation and reduction effects on oxides of nickel, cobalt, iron and other elements. The action of boron, silicon and oxygen generates B2O3 and SiO2, respectively. The melting point of B2O3 is 580 degrees Celsius, and SiO2 is 1713 degrees Celsius. Although B2O3 has a low melting point, it has a high viscosity and is difficult to surface. However, when B2O3 and SiO2 are present at the same time, a low melting point borosilicate can be formed. For example, the melting point of 73% SiO2 and 27% B2O3 is 722 degrees Celsius. This borosilicate has low viscosity, low density and good fluidity, and is easy to float on the surface of the alloy, so that the welding layer alloy is protected from oxidation and the generation of pores.

    (3) Boron and silicon elements can improve the hardness of the alloy. Boron and silicon have dispersion strengthening and solid solution strengthening effects on the metallographic structure of the alloy. The second phase is dispersed in the alloy to increase its strength and hardness. This increase in strength and hardness is called dispersion strengthening. The main effect of boron is dispersion strengthening. Except for a very small amount of boron dissolved in nickel austenite, most of the boron is dispersed in the alloy in the form of intermetallic compounds such as Ni3B. When the alloy contains chromium, boron and chromium can form hard particles such as intermetallic compounds Cr2B and CrB. Boron also sometimes forms carbon boron compound hard particles with carbon in the alloy. They are all dispersed in the alloy. These hard particles are extremely hard. Experiments show that when the boron content is not high, the hardness of the coating increases significantly with the increase of boron. When the boron content exceeds 3.5%, its effect is not obvious. The role of silicon is mainly solid solution strengthening.

    In nickel-based self-fluxing alloys, the solubility of silicon in nickel at room temperature can reach 6%. Therefore, most of silicon can be solid-dissolved in nickel austenite, resulting in solid solution strengthening.

    In addition, part of the hard phase also forms a eutectic with the matrix phase. The amount of eutectic and the degree of dispersion distribution are related to the cooling rate of the alloy. The faster the solder layer solidifies, the less eutectic phase and the dispersion, on the contrary, the eutectic phase grains are large and aggregated. This eutectic phase increases the hardness and brittleness of the solder layer.

    The boron-silicon element also has a similar effect in iron-based and cobalt-based alloys. Generally, the content of boron in the alloy does not exceed 6%, and the content of silicon does not exceed 5%. If the content of boron and silicon is too high, more brittle compounds will appear, which will reduce the plasticity and toughness of the coating, increase the brittleness, and easily cause cracks.