In the copper-aluminum casting process, several different casting stresses are encountered. They are thermal stress, phase change stress, and shrinkage stress. For castings, the causes of these three stresses are different, so the applicable control method is different. It also varies.
The first concern is the shrinkage stress in the copper-aluminum casting. It is caused by the tensile stress caused by the mold and the core during the shrinkage of the casting. Under normal circumstances, the stress is temporary, once the casting is opened. The box will disappear automatically.
However, be aware that if the open time of the copper-aluminum casting is not properly grasped, the shrinkage stress will not disappear according to the convention, but it will leave hot cracks on the surface of the casting, especially in the process of cast aluminum alloy, due to shrinkage. Hot cracking caused by stress is more common.
It can be seen that the residual stress in the casting of copper-aluminum casting reduces the mechanical properties of the alloy and affects the machining accuracy of the casting, which can usually be eliminated by annealing. According to past experience, as long as the alloy has good thermal conductivity and no phase change in the cooling process, and the structural design of the casting is reasonable, the residual stress of the casting is generally small.
Aluminum alloy easily absorbs gas. This is a major feature of copper and aluminum castings, and the higher the temperature of the aluminum alloy melt, the more hydrogen it absorbs; in addition, it also inhales when it is poured into the mold. The liquid metal entering the mold descends with the temperature, the solubility of the gas decreases, excess gas is precipitated, and a part of the unoccupied gas remains in the casting to form pores.
However, if the copper-aluminum casting has a large pressure generated by bubble heating, the surface of the pores is smooth and there is a bright layer around the pores; if the pressure generated by the bubbles is small, the inner surface of the pores is more wrinkled and becomes a major defect of the casting.
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