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To reduce the risks of defects caused by the impurities such as hydrogen, degassing is a critical process in aluminum casting. In most cases, this process is usually carried out by degassing machines. The baffle is an important component in the degassing machine. It is mainly applied to improve the degassing efficiency of molten aluminum and the durability of the equipment.
The baffle is usually installed around the rotating shaft or rotor of the rotary degasser. Through special structural design (porous, blade-shaped or other shapes), they make inert gases (nitrogen, argon or chlorine) be evenly dispersed into the molten aluminum, for the sake of increasing the contact area between bubbles and aluminum liquid, and improving the removal efficiency of impurities like hydrogen.
In high-temperature (700-800℃) molten aluminum, the baffle needs to withstand drastic thermal shock and chemical corrosion. The high-temperature stability and chemical inertness of silicon nitride can effectively isolate the melts from sensitive parts of the equipment (such as the driving device) to prevent erosion or oxidation.
By preventing molten aluminum from splashing and reducing destabilization of the liquid surface, silicon nitride baffle can decrease the probability of molten aluminum contacting with air, reduce the generation of oxide slag, and thus improve the quality of aluminum castings.
ADVANTAGES OF SILICON NITRIDE:
Silicon nitride has a melting point of up to 1900°C and can maintain high strength in molten aluminum (about 700°C) without deformation or softening.
The low thermal expansion coefficient (3.1-3.3×10⁻⁶/°C) and high thermal conductivity (40-50W/mK) can withstand frequent temperature changes during the melting process of aluminum and reduce the risk of cracking.
Silicon nitride has excellent corrosion resistance to molten aluminum, salt solvents and inert gase. It doesn’t react with most corrosive chemicals, which extends the service life.
With high hardness (HRA 92-94) and wear resistance, silicon nitride is suitable for the application of high-speed rotation, which can reduce wear caused by bubble impact or scours of molten aluminum.
COMPARISIONS OF DIFFERENT MATERIALS:
Material | Silicon Nitride | Silicon Carbide | Graphite |
Thermal Shock Resistance | Great | Excellent | Best |
Corrosion Resistance | Best | Good | Not good |
Costs | High | Lower | Lowest |
Life | 1 year or more | 3 months | 1 month |
