What problems can arise during the gravity casting of aluminum alloys?

One, Pores: Pores typically occur at the top of castings and are often found on the surface of protruding parts. Hidden pores within the casting interior require X-ray inspection and are discovered during the machining process of the casting.

The Causes: 1. Too fast casting speed, entraining air; 2. Poor mold exhaust gas; 3. Excessive molten aluminum flow; 4. High melting temperature; 5. Poor degassing of the alloy; 6. Excessive casting temperature; 7. Uncured sand cores, poor exhaust, or excessive gas generation.

Preventive Measures: 1. Pour metal liquid smoothly; 2. Add exhaust grooves or plugs in areas where gas is difficult to排出 from the metal mold, and clean them regularly; 3. Position the ladle as close to the sprue cup as possible during pouring; 4. Strictly control the aluminum liquid temperature to prevent overheating; 5. Properly degas the aluminum liquid; 6. Dry the cores, ensure the exhaust holes are clear, re-dry cores that have become damp, and hollow out the cores if they are particularly large; 7. Wait for the metal mold coating to dry before pouring.

Cracks: Most cracks appear in the internal angles of castings or at the transition between thick and thin sections. Cracks can be detected using dye penetrant inspection, air tightness tests, and X-ray examination in areas where the alloy liquid is introduced and where significant casting stresses occur. Cold cracks on aluminum castings can be discovered during the visual inspection after sand cores are cleaned. Causes: 1. Sharp angles on the casting with a significant difference in thickness; 2. Local overheating of the mold or excessively high pouring temperature; 3. Incorrect placement of cold iron; 4. Poor shrinkage compensation in the casting.

Preventive Measures: 1. Improve design by rounding off sharp angles on castings, ensuring even thickness transitions and rounded corners; 2. Select the gate and runner positions correctly, control the pouring temperature and coating thickness, properly place chilled iron, and enhance the feeding ability of the risers; 3. Apply asbestos thermal insulation coating on the mold riser sections.

Three: Segregation: Segregation is typically found in the central and upper portions of thicker castings, and can be identified through macroscopic analysis.

Reasons for the issue: 1. The aluminum melt composition was not properly mixed before casting; 2. The casting temperature was too high; 3. The metal mold temperature was excessively high, resulting in uneven coating and excessive thickness.

Preventive Measures: 1. Ensure thorough mixing of the alloy before pouring; 2. Reduce the pouring temperature and preheat the mold appropriately; 3. Design chilled iron, venting plugs, or employ air or water cooling in areas with slower cooling; 4. Add elements that hinder the alloy from segregating; 5. Thinner the casting wall thickness to accelerate solidification.