Causes and prevention methods of six common defects in castings

01 Slag holes (flux inclusions or metal oxide inclusions)

Features:

Slag holes are open or dark holes on castings. The holes are filled with slag in whole or in part. The shape is irregular. Small dots of flux inclusions are not easy to find. After the slag is removed, smooth holes appear. They are generally distributed in the lower part of the pouring position, near the inner runner or in the dead corner of the casting. Oxide inclusions are mostly distributed in a mesh on the surface of the casting near the inner runner. Sometimes they are in the form of thin sheets, or irregular clouds with wrinkles, or form sheet-like interlayers, or exist in the form of flocs inside the casting. When broken, they often break from the interlayer. The oxides are in it, which is one of the sources of cracks in the casting.

Causes of formation:

Slag holes are mainly caused by alloy smelting and pouring processes (including incorrect design of the pouring system). The mold itself will not cause slag holes, and metal molds are one of the effective ways to avoid slag holes.

Prevention and control methods:

1. Correctly set the pouring system or use a casting fiber filter.

2. Use an inclined pouring method.

3. Select flux and strictly control quality.

02 Cracks (hot cracks, cold cracks)

Features:

The appearance of the crack is a straight line or an irregular curve. The surface of the hot crack fracture is strongly oxidized and is dark gray or black, without metallic luster. The surface of the cold crack fracture is clean and has a metallic luster. Generally, the external cracks of the casting can be seen directly, while the internal cracks can only be seen with the help of other methods. Cracks are often related to defects such as shrinkage and slag inclusions. They often occur on the inside of the sharp corners of the casting, at the junction of thick and thin sections, and in the hot zone where the pouring head and the casting are connected.

Causes:

Metal mold casting is prone to crack defects, because the metal mold itself has no yield, the cooling speed is fast, it is easy to cause the internal stress of the casting to increase, the mold is opened too early or too late, the pouring angle is too small or too large, the coating layer is too thin, etc., which are easy to cause the casting to crack. When the mold cavity itself has cracks, it is also easy to cause cracks.

Prevention and control methods:

1. Attention should be paid to the processability of the casting structure, so that the parts with uneven wall thickness of the casting are evenly transitioned, and the appropriate fillet size is adopted.

2. Adjust the coating thickness to make the various parts of the casting reach the required cooling speed as much as possible to avoid forming too much internal stress.

3. Attention should be paid to the working temperature of the metal mold, adjust the mold slope, and timely core cracking, remove the casting and cool it slowly.

03 Porosity (bubbles, choking holes, air pockets)

Features:

Porosity is a hole existing on the surface or inside of the casting, which is round, oval or irregular in shape. Sometimes multiple pores form an air mass, which is generally pear-shaped under the skin. The shape of the choking hole is irregular and the surface is rough. The air pocket is a concave piece on the surface of the casting, and the surface is relatively smooth. The open hole can be found by appearance inspection, and the subcutaneous pores can only be found after mechanical processing.

Causes:

1. The mold preheating temperature is too low, and the liquid metal cools too quickly when passing through the pouring system.

2. The mold exhaust design is poor, and the gas cannot be discharged smoothly.

3. The coating is not good, the exhaust performance itself is poor, and even the gas is volatilized or decomposed.

4. There are holes and pits on the surface of the mold cavity. After the liquid metal is injected, the gas in the holes and pits expands and compresses the liquid metal rapidly, forming choking holes.

5. The surface of the mold cavity is rusted and not cleaned.

6. The raw materials (sand cores) are improperly stored and not preheated before use.

7. The deoxidizer is not good, or the dosage is insufficient or the operation is improper.

Prevention methods:

1. The mold should be fully preheated, the particle size of the coating (graphite) should not be too fine, and the air permeability should be good.

2. Use tilted pouring method for pouring.

3. Raw materials should be stored in a ventilated and dry place and preheated before use.

5. The pouring temperature should not be too high.

04 Shrinkage (shrinkage)

Features:

Shrinkage is a rough hole on the surface or inside of the casting. Slight shrinkage is many scattered small shrinkage holes, that is, shrinkage. Shrinkage or shrinkage has coarse grains. It often occurs near the pouring channel in the casting, the root of the riser, the thick part, the thick and thin transition of the wall, and the thick and thin part with a large plane.

Causes:

1. The working temperature control of the mold does not meet the requirements of directional solidification.

2. Improper coating selection, and poor control of the coating layer thickness in different parts.

3. Improper design of the position of the casting in the mold.

4. The design of the pouring riser fails to achieve the effect of fully compensating for shrinkage.

5. The pouring temperature is too low or too high.

Prevention and control methods:

1. Increase the temperature of the mold.

2. Adjust the thickness of the coating layer, spray the coating evenly, and avoid local coating accumulation when reapplying the coating after it falls off.

3. Heat the mold locally or use insulation materials to keep it warm locally.

4. Insert copper blocks in the hot spots to chill the local areas.

5. Design heat sinks on the mold, or accelerate the cooling speed of local areas by water, or spray water or mist outside the mold.

6. Use detachable chilling blocks and place them in the cavity in turn to avoid insufficient cooling of the chilling blocks themselves during continuous production.

7. Design a pressure device on the mold riser.

8. The pouring system should be designed accurately and the appropriate pouring temperature should be selected.

05 Cold shut (poor fusion)

Features:

Cold shut is a through seam or surface gap with rounded edges, separated by oxide scale in the middle, not completely fused together, and becomes "undercast" when the cold shut is serious. Cold shut often appears on the top wall of the casting, thin horizontal or vertical surfaces, thick and thin wall joints, or on thin auxiliary plates.

Causes:

1. Unreasonable design of metal mold exhaust.

2. Too low working temperature.

3. Poor coating quality (artificial, material).

Pub Time : 2024-12-08 19:38:13 >> News list