Defect characteristics and detection methods
Fine wrinkles on the surface of casting or dark gray flakes peeling off from the surface of casting; dark lines or knife edge lines in the interior of casting, which can be distributed in all parts of casting. Found after casting out of box or sand cleaning, sandblasting or X-ray
Causes of formation
- The alloy surface is strongly oxidized and the oxide skin is thickened. When the pouring speed is too slow, it floats on the casting surface or gets involved in the casting
- The molding sand has too much water content or the core is not dried or the sand mold is tamped too tightly and the air permeability is poor
- Unreasonable design of pouring system makes alloy liquid oxidize violently
Prevention methods and remedial measures
- Melt the alloy strictly according to the melting process of the alloy to prevent strong oxidation of the alloy liquid
- Pour the mold in a steady and even liquid flow, reduce the pouring height as much as possible, shorten the pouring time, and reduce the oxidation of the alloy liquid
- Properly extend the baking time of the sand mold. It is forbidden to install the exothermic core in the mold and put the assembled sand mold on the hot base plate to prevent local condensation of water. The assembled sand mold shall be poured within 4h, and can be unpacked after 4H. If there is no damage after inspection, it can be used only after re baking, otherwise it will be scrapped
- The surface of sand mold cavity can be baked with blowtorch. However, it must be noted that excessive baking or uneven baking is not allowed, so as to prevent the surface quality of the mold from deterioration or burning off the protective additives in the sand, resulting in burning black
- In order to prevent secondary oxidation, a uniform and thin layer of sulfur powder can be sprinkled on the surface of the mold cavity when casting the complex shape of high magnesium aluminum alloy