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What are the common defects in metal precision castings and how can we avoid them?

November 13, 2019

What are the common defects in metal precision castings and how can we avoid them?

1. Pinhole

Measures to prevent pinhole formation on stainless steel casting: Contaminated cast aluminum alloy materials, organic compounds and heavily oxidized materials are strictly prohibited. Control the smelting process and strengthen degassing and refining. Control the thickness of the metal coating, too thick and easy to produce pinholes. The mold temperature should not be too high, and the thick parts of the casting should be cooled, such as copper inlay or watering. Strictly control the water when using the sand mold, and use the dry core as much as possible.

2. Air hole

Measures to prevent venting: Improve the pouring and riser system to stabilize liquid flow and avoid air entrainment. The mold and core should be preheated, then coated, and must be baked before use. Mold and core design should consider adequate exhaust measures.

3. Oxide inclusions

Measures to prevent oxidation slag: strictly control the smelting process, rapid smelting, reduce oxidation, and completely remove slag. The aluminum-magnesium alloy must be melted under the covering agent. The furnace body and tools should be clean and free of oxides and should be preheated. The paint should be dry after use. The designed casting system must have stable flow, cushioning and slag removal capabilities. The inclined casting system is used to stabilize the liquid flow without secondary oxidation. The selected coating has strong adhesion, and the slag is formed in the casting during the casting process without peeling off.

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4. Cracking

Measures to prevent thermal cracking: Local overheating should be avoided in the actual gating system to reduce internal stress. The mold and core slope must ensure the above 5°. The inverted riser can be cored in solidification, and the core can replace the metal core when necessary. Control the thickness of the coating to ensure consistent cooling rate of the casting. Select the appropriate mold temperature based on the thickness of the casting. Refine the alloy structure and improve the thermal cracking ability. Improve the structure of the casting, eliminate sudden changes in sharp corners and wall thickness, and reduce the tendency of hot cracking.

5. Loose

Measures to prevent loose production: reasonable riser settings to ensure solidification and shrinkage. Appropriately reduce the working temperature of the metal mold. Control the thickness of the coating and thicken the thick wall. Adjust the cooling rate of each part of the metal mold to make the thick wall of the casting have greater cooling capacity. Properly reduce the metal casting temperature.

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