Oxidized slag typically appears on the upper surface of the casting, especially in areas where the mold is not properly ventilated, such as corners. These slag inclusions are usually gray or yellow in color and can be detected through X-ray inspection, mechanical processing, or during caustic washing, pickling, or anodization. The main causes include impure charge materials, excessive use of recycled scrap, poor system design, incomplete removal of slag from the alloy solution, and improper pouring techniques that lead to slag entrapment. To prevent this, it's important to sand-blast the charge and reduce the amount of recycled material. Improving the gating system’s ability to trap slag, using the right flux to remove impurities, ensuring smooth pouring while carefully monitoring for slag, and allowing the molten alloy to settle after refining are all effective measures.
Two-pore bubbles, often found in the walls of castings, are generally round or oval with a smooth, shiny surface, sometimes appearing oily or yellow. Surface bubbles can be identified through sandblasting, while internal ones are revealed by X-ray or mechanical inspection. On X-ray films, these pores appear black. Common causes include gas entrainment due to uneven alloy flow, contamination of molding sand with organic materials like coal dust or animal waste, poor ventilation in the mold or core, shrinkage holes near cold iron surfaces, and flawed pouring system design. To address these issues, it's essential to control the pouring speed to avoid gas entrapment, ensure that molding sand is free from organic contaminants, enhance the venting capacity of the mold and core, correctly handle cold iron placement, and optimize the pouring system design.
Shrinkage defects in aluminum castings commonly occur near the sprue, at thick sections, at the junctions of different wall thicknesses, or in thin walls with large flat areas. In the as-cast condition, the fracture appears gray, turning light yellow, grayish-yellow, or grayish-black after heat treatment. On X-rays, shrinkage shows up as filamentous patterns, and can also be detected through fluorescent low-magnification inspections. Causes include poor riser feeding, excessive charge, overheating around the sprue, high moisture content in the sand or improperly dried cores, coarse grain structure, and incorrect positioning of the casting in the mold. Additionally, high pouring temperatures and fast pouring speeds can contribute to this issue. Preventive measures include improving the riser design to ensure better metal feed, using clean and corrosion-free charge materials, placing risers and cold irons strategically in shrinkage-prone areas, controlling sand moisture and ensuring proper drying of cores, refining the grain structure, and adjusting the casting position in the mold. Lowering the pouring temperature and slowing down the pouring speed can also help minimize shrinkage defects.
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