1. Characteristics of oxide slag inclusion defects: oxide slag is mainly distributed on the upper surface of castings, at the corner of the mold which is not ventilated. Fractures are mostly gray-white or yellow, which can be found by X-ray fluoroscopy or mechanical processing, or by alkali washing, pickling or anodizing.
Reasons: 1. The burden is not clean and the amount of return charge is too much. 2. Poor design of pouring system. 3. The molten slag in the alloy liquid has not been cleaned up. 4. Improper pouring operation and slag entrainment. 5. Static time is not enough after refining and modification. Preventive measures: 1. The burden should be blown sand and the usage of return burden should be reduced properly. 2. Improving the design of gating system to improve its slag-retaining ability 3. Using appropriate flux to remove slag 4. When pouring, it should be stable and pay attention to slag-retaining 5. Alloy liquid should be stationary for a certain time before pouring after refining.
2. Porosity and defect characteristics: Porosity in the wall of three castings is generally round or elliptical, with smooth surface, usually bright oxide skin, sometimes oily yellow. Surface air holes and bubbles can be found by sandblasting, and internal air holes and bubbles can be found by X-ray fluoroscopy or mechanical processing. The air holes and bubbles are black on X-ray film.
Causes: 1. Casting alloy is not stable and involved in gas. 2. Organic impurities (such as coal cuttings, grass-roots horse manure, etc.) are mixed into the core sand. 3. Poor ventilation of casting and sand core. 4. There are shrinkage holes on the surface of chilled iron. 5. Poor gating system design. Prevention methods: 1. Correctly grasp the pouring speed to avoid involvement in gas. 2. The organic impurities should not be mixed into the core sand to reduce the gas emission of the moulding material. 3. Improve the exhaust capacity of (core) sand. 4. Correct selection and treatment of chilled iron. 5. Improving the design of gating system.
3. Shrinkage defect characteristics: Shrinkage defect of aluminium castings generally occurs in the thick part of the riser root around the inner runner, the thick and thin junction of the wall and the thin wall with a large plane. When cast, the fracture surface is gray, light yellow is gray, light yellow or gray black after heat treatment. Serious filiform shrinkage on X-ray negative can be found by X-ray and fluorescence low-power fracture.
Causes: 1. Riser feeding effect is poor. 2. Furnace charge contains too much gas. 3. Overheating around the inner runner 4. Excessive moisture in the sand mold and undrying of the sand core. 5. The grain size of the alloy is coarse. 6. The improper position of the casting in the mould. 7. The pouring temperature is too high and the pouring speed is too fast. Prevention methods: 1. Improving riser design by adding metal liquid from riser. 2. Furnace charge should be clean and corrosion-free. 3. A riser is set at the shrinkage and porosity of the casting, and chilled iron or chilled iron is combined with the riser. 4. Control the moisture content of moulding sand and the drying of sand core. 5. Take measures to refine grain size. 6. Improve the position of the casting in the mould to reduce the pouring temperature and speed.