Oxidized slag typically appears on the upper surface of the casting, especially at the corners where the mold lacks proper ventilation. These defects are usually gray or yellow in color and can be detected through X-ray inspection, mechanical processing, or during caustic washing, pickling, or anodizing. The presence of oxidized slag is often caused by unclean 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 inclusion. To prevent this, it's important to sand-blast the charge and reduce the amount of recycled material. Improving the gating system’s design to enhance its ability to trap slag is also crucial. Using the right flux to remove slag effectively, ensuring smooth pouring while carefully monitoring for slag, and allowing the molten alloy to settle for a period after refining can all help minimize the occurrence of oxidized slag.
Two-pore bubbles are commonly found in the walls of three castings, appearing as round or oval voids with smooth, shiny surfaces, sometimes with an oily yellow tint. Surface pores can be identified through sandblasting, while internal pores may be revealed via X-ray or mechanical testing, showing up as dark spots on the film. These defects occur due to gas entrapment in the alloy, contamination of the mold or core sand with organic impurities like coal dust or animal waste, poor venting of the mold or core, shrinkage holes on cold iron surfaces, or issues with the pouring system. To prevent these, it's essential to control the pouring speed to avoid gas entrainment, ensure that mold and core sands are free from organic contaminants, improve the venting of the molds and cores, properly handle and position cold iron, and optimize the pouring system design.
Shrinkage defects in aluminum castings usually occur near the runner, sprue riser, thick sections, junctions of different wall thicknesses, and areas with thin walls and large flat surfaces. In the as-cast condition, the fracture appears gray, turning light yellow, grayish-yellow, or grayish-black after heat treatment. X-ray and low-magnification fracture analysis often reveal filamentous shrinkage. Causes include poor riser feeding, excessive charge, overheating around the sprue, high moisture content in the sand, insufficient drying of the core, coarse grain structure, and incorrect positioning of the casting in the mold. High pouring temperature and fast pouring speed can also contribute. Prevention methods include improving the riser design and ensuring proper metal feed, using clean and corrosion-free charge materials, placing risers and cold irons strategically, controlling sand moisture and drying the core, refining the grain structure, and adjusting the casting position in the mold to lower the pouring temperature and speed.
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