Aluminum Alloy Gravity Casting Common Defects and Preventive Measures

Release time:

2023-09-15 14:31

1. shrinkage: this defect often occurs in the thick part of the casting, or the thickness of the junction. Sometimes the surface of the casting is white, which is actually shrinkage.

Reasons: 1. Insufficient feeding of castings during crystallization; 2. The position of introducing alloy liquid is wrong; 3. The temperature of each part of the metal mold is inappropriate and does not conform to the principle of sequential solidification. 4, improper coating or coating falling off; 5, pouring temperature is too high; 6, pouring speed is too fast; 7, casting cooling is too slow; 8, casting burrs are too large.

Prevention measures: 1. Set up risers at thick parts of castings. The size and height of risers should be appropriate to achieve final solidification and improve the feeding effect of risers; 2. Distribute inner runners evenly along the circumference of castings, or open supplementary runners from the root of risers for supplementary pouring; 3. Adjust the temperature specification of each part of the metal mold to facilitate the sequential solidification of castings; 4. Different coating compositions and coating thicknesses shall be selected according to different requirements of the working part of the casting and the riser part, and the stripping shall be evenly supplemented. 5, appropriate to reduce the pouring temperature; 6, slow down the pouring speed; 7, in easy to produce shrinkage porosity parts, embedded with copper cold iron or vent plug, in order to accelerate the cooling.

2. cold insulation: This defect is generally produced on thin-walled castings with large horizontal surfaces and the final confluence of the alloy. After the casting is out of the mold, it can be found by visual inspection.

Causes: 1. The mold temperature is too low; 2. The temperature of aluminum liquid is too low; 3. Poor mold exhaust; 4. Poor gating system design, small number of internal gates and too small cross section; 5. The pouring speed is too slow or the pouring is interrupted. 6. The design wall thickness of the casting is too thin or lacks proper fillet.

Prevention measures: 1. Raise the mold temperature appropriately; 2. Raise the pouring temperature of aluminum liquid appropriately; 3, gas is not easy to discharge parts set up ventilation grooves or exhaust plugs, to maintain good exhaust; 4, appropriately increase the number of internal gates and internal gate section; 5, appropriately improve the pouring speed, avoid aluminum liquid pouring interruption; 6, according to the casting design process requirements to design a reasonable minimum wall thickness and casting fillet.

3. Porosity: Porosity is often produced in the upper part of the casting and often occurs on the surface of the protruding part of the casting. The hidden pores inside the casting must be found through X-ray and during the processing of the casting.

Causes: 1. The pouring speed is too fast and air is involved; 2, the mold exhaust gas is not good; 3, aluminum liquid flow too fast; 4, melting temperature is too high; 5, alloy degassing poor; 6, pouring temperature is too high; 7, sand core is not dry, poor exhaust or gas emission is too large.

Prevention methods: 1. Pour molten metal smoothly; 2, in the metal gas is not easy to exclude the parts of the additional exhaust slot or exhaust plug, and often clean; 3, pouring ladle as close as possible to the gate cup; 4, strictly control the temperature of aluminum liquid to prevent over-temperature; 5, aluminum liquid correctly degassing; 6, mud core should be dried, exhaust holes should be unblocked, mud core should be filled after moisture, the middle of the extra large mud core should be hollowed out; 7. After metal coating, it should wait for the coating to dry before pouring.

Cracks: most of the cracks appear at the internal angle of the casting and the part of the transition between thick and thin sections; The parts where alloy liquid is introduced into the casting and the parts with the greatest casting stress can be found by coloring inspection, air tightness test and X-ray inspection. Cold cracks on aluminum castings can be found by visual inspection after cleaning the sand core.

Causes: 1. There are sharp corners on the castings, and the thickness is quite different; 2. The mold is locally overheated or the pouring temperature is too high; 3. The cold iron is not placed correctly; 4. The casting is poorly supplemented;

Prevention methods: 1. Improve the design, remove the sharp corners of the casting, and try to make the casting wall thickness transition and round corners evenly; 2. Correctly select the gate and the position of the gate, control the pouring temperature and coating thickness, correctly place the cold iron, and increase the feeding ability of the riser; 3. Apply asbestos thermal insulation coating on the riser of the mold.

5. segregation: Segregation is generally distributed in the center and upper part of the thick part of the casting, which can be found during macro analysis.

Causes: 1. The aluminum liquid composition is not stirred evenly before pouring; 2. The pouring temperature is too high; 3. The metal type temperature is too high, the coating is uneven and too thick.

Prevention measures: 1. Stir the alloy liquid evenly as far as possible before pouring; 2. Properly reduce the pouring temperature and the preheating temperature of the metal mold; 3. Design cold iron, ventilation plug or use air cooling and water cooling at the parts with slow cooling. 4. Add elements that hinder the segregation of the alloy; 5. Thinning the casting wall thickness appropriately to accelerate solidification.

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