The effect of cooling rate on casting is significant and can have a direct impact on the microstructure and properties of the cast material.
Higher cooling rates in aluminum alloy castings, such as A356 and A357 alloys, result in finer microstructures with smaller secondary dendrite arm spacing (SDAS) and refined eutectic particles. This finer microstructure leads to improved ductility and tensile properties in the cast material [3,4].
The cooling rate during the cooling phase of the casting process can be influenced by various factors. One factor is the post-heating and cooling process. It is important to reheat the casting evenly and then wrap it in a material that helps retain the heat and allows the part to cool as slowly as possible. Rapid cooling can lead to increased thermal gradients within the casting, which can result in uneven cooling and potential distortion or cracking [8].
During the cooling phase, different cooling phases can occur, including the vapor phase, boiling phase, and convection phase. The cooling rate can vary during these phases, and the control of these phases is crucial for achieving the desired properties in the cast material. The vapor phase, which occurs when the oil transforms into vapor due to the heat effect, leads to the fastest cooling due to the absorption of the latent heat of vaporization. However, excessive insulation caused by the formation of a vapor sheath around the part can reduce cooling-speed efficiency. The convection phase occurs when the temperature becomes lower and the vapor phase disappears, allowing the convection of the oil to finish the cooling to the equilibrium temperature [8].
It is important to note that the cooling of the part is never uniform due to different section thicknesses of the part itself. These cooling heterogeneities can lead to martensitic transformations at different times during the cooling phase, which can result in part expansion and distortions. The crossing of the Ms point (martensite start temperature) at different times can generate stress and potential distortions in the cast material [8].
In the case of welding, localized heating can cause restricted expansion, and the resulting stress depends on the thermal gradient between the heated zone (HZ) and the casting body. Pre-heating the casting before welding can help minimize the thermal gradient and reduce the tensile stress caused by welding. In cases where pre-heating is not possible, using low temperature welding processes and low melting point welding rods or wires can help minimize the stress and potential cracking [8].
In summary, the cooling rate during casting can have a significant effect on the microstructure, properties, and potential distortions or cracking in the cast material. Higher cooling rates in aluminum alloy castings can result in finer microstructures and improved ductility and tensile properties. Controlling the cooling phases and minimizing thermal gradients during cooling is important for achieving the desired properties and minimizing potential distortions or cracking. Pre-heating in welding can help reduce stress and potential cracking during the cooling phase.
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