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Causes and Prevention Methods of Common Defects for Casting Components

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Causes and Prevention Methods of Common Defects for Casting Components

Stomata (bubble, pupil, air pocket)
 
Feature
Pores are holes that are present on the surface or inside of a casting. They are round, elliptical or irregular. Sometimes a plurality of pores form an air mass, and the skin is generally pear-shaped. The shape of the pupil is irregular and the surface is rough. The air pocket is a concave surface of the casting and the surface is smooth. The visual inspection of the open hole reveals that the subcutaneous stomata can only be found after mechanical processing.
 
Cause of formation
 
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 paint is not good, its own exhaustibility is not good, and even its own volatilization or decomposition of gas.
4. There are holes and pits on the surface of the mold cavity. After the liquid metal is injected, the gas in the hole and the pit rapidly expands and compresses the liquid metal to form the pupil.
5. The surface of the mold cavity is rusted and not cleaned.
6. Raw materials (sand core) are improperly stored and are not preheated before use.
7, poor deoxidizer, or insufficient amount or improper operation.
Prevention method
 
1. The mold should be fully preheated. The particle size of the coating (graphite) should not be too fine and the gas permeability should be good.
2. Cast using a tilt casting method.
3. Raw materials should be stored in a ventilated and dry place, and should be preheated when used.
4. Select a deoxidizer (magnesium) with good deoxidation effect.
5. The pouring temperature should not be too high.
 
Shrinkage hole
 
Feature
 
The shrinkage hole is a surface roughened hole existing on the surface or inside of the casting, and the slight shrinkage hole is a plurality of small shrinkage holes which are dispersed, that is, the shrinkage, shrinkage or shrinkage of the crystal grains. It often occurs near the casting sprue, the root of the riser, the thick part, the thick and thin transition of the wall and the thick part with a large plane.
Cause of formation
 
1. The mold working temperature control does not meet the directional solidification requirements.
2, the paint selection is not appropriate, the thickness of the paint layer in different parts is not well controlled.
3. The position of the casting in the mold is not properly designed.
4. The design of the riser failed to achieve full complementation.
5. The pouring temperature is too low or too high.
 
Prevention method
 
1. Increase the temperature of the abrasive.
2. Adjust the thickness of the coating layer, the coating should be evenly sprayed, and the coating should not form part of the coating when it is applied.
3. Locally heat the mold or partially heat it with a heat insulating material.
4, the hot section is set with copper blocks, and the part is chilled.
5. Design the heat sink on the mold, or accelerate the cooling rate in the local area by water, etc., or spray water and spray outside the mold.
6. Use a detachable chiller block and place it in the cavity in turn to avoid insufficient cooling of the chill block itself during continuous production.
7. Design a pressurizing device on the mold riser.
8. The design of the gating system should be accurate and the appropriate pouring temperature should be selected.
 
Slag hole (solvent slag or metal oxide slag)
 
Feature
 
The slag hole is a bright hole or a dark hole on the casting. All or part of the hole is filled with slag, the shape is irregular, and the small-point flux slag is not easy to find. After the slag is removed, the hole is smooth, generally distributed in the pouring. In the lower part of the position, near the inner runner or at the corner of the casting, the oxide slag is mostly distributed in the form of a mesh on the surface of the casting near the inner runner, sometimes in the form of flakes, or irregular clouds with wrinkles, or forming a sheet-like interlayer. Or the inside of the casting is in the form of a floc. When breaking, it often breaks from the interlayer, and the oxide is among them, which is one of the roots of the crack formation of the casting.
 
Cause of formation
 
The slag hole is mainly caused by the alloy smelting process and the casting process (including the incorrect design of the casting system), the mold itself does not cause slag holes, and the metal mold is one of the effective methods to avoid slag holes.
 
Prevention method
 
1. The gating system is set up correctly or a cast fiber filter is used.
2. Adopt inclined pouring method.
3. Choose flux and strictly control quality.
 
Crack (hot crack, cold crack)
 
Feature
 
The appearance of the crack is a straight or irregular curve. The surface of the hot crack fracture is strongly oxidized to dark gray or black, no metallic luster, and the surface of the cold crack fracture is clean and has a metallic luster. Generally, the outer crack of the casting can be seen directly, and the internal crack can be seen by other methods. Cracks are often associated with defects such as shrinkage and slag inclusion, which occur mostly in the inner side of the sharp corner of the casting, at the junction of the thick section, and the hot joint where the riser is connected to the casting.
 
Cause of formation
 
Metal mold casting is prone to crack defects, because the mold itself has no retractability, the cooling speed is fast, and it is easy to cause the internal stress of the casting to increase. The opening type is too early or too late, the pouring angle is too small or too large, and the coating layer is too thin. It is easy to cause cracking of the casting, and it is easy to cause crack when the mold cavity itself has cracks.
 
Prevention method
 
1. Attention should be paid to the structural and structural properties of the castings, so that the parts with uneven wall thickness of the castings are evenly transitioned, and the appropriate rounded corners are used.
2, adjust the thickness of the coating, as far as possible to achieve the required cooling rate of each part of the casting, to avoid the formation of too much internal stress.
3, should pay attention to the working temperature of the metal mold, adjust the mold slope, and timely core cracking, take out the casting slowly.
 
Cold separation (poor fusion)
 
Feature
 
The cold partition is a kind of slit or surface with a rounded edge. The middle is separated by the scale, which is not completely integrated. When the cold partition is severe, it becomes “under-cast”. Cold insulation often occurs on the top wall of the casting, a thin horizontal or vertical surface, a thick wall joint or a thin support plate.
 
Cause of formation
 
1. Metal mold exhaust design is unreasonable.
2. The working temperature is too low.
3. The quality of the paint is not good (man-made, material).
4. The location of the sprue is improper.
5. The pouring speed is too slow.
 
Prevention method
 
1. Design the runner and exhaust system correctly.
2, large-area thin-walled castings, coatings should not be too thin, appropriate thickening of the coating layer is conducive to molding.
3. Appropriately increase the working temperature of the mold.
4, using the oblique casting method.
5. Casting with mechanical vibration metal mold.
 
Sand hole (sand hole)
Feature
 
A relatively regular hole is formed on the surface or inside of the casting, and its shape is consistent with the shape of the sand. When the mold is just released, sand grains embedded on the surface of the casting can be seen, and the sand can be extracted from the sand. When a plurality of sand eyes are simultaneously present, the surface of the casting is orange peel.
 
Cause of formation
 
The sand falling from the surface of the core is surrounded by the copper liquid and forms a hole with the surface of the casting.
1. The surface strength of the sand core is not good, burnt or not fully cured.
2. The size of the sand core does not match the outer mold, and the sand core is crushed when the mold is closed.
3. The mold smashed the graphite water contaminated with sand.
4. The sand that is rubbed off by the ladle and the sand core at the sprue is flushed into the cavity with the copper water.
 
Prevention method
 
1. The sand core is manufactured strictly according to the process and the quality is checked.
2. The size of the sand core matches the size of the outer mold.
3. The ink should be cleaned up in time.
4. Avoid rubbing the ladle with the sand core.
5. When blowing the sand core, blow the sand in the mold cavity.
Casting cracks are mainly divided into two categories, hot cracking and cold cracking!
 
Hot crack
 
The hot crack is the curved shape of the crack, the fracture is very irregular and the broken wire is connected, and the surface is wider, the narrower the inside, the hot cracking mechanism is: the molten steel begins to condense after injection into the cavity, when the crystal skeleton has formed and started After the wire is shrunk, since the internal molten steel is not solidified into solid state and the shrinkage is hindered, stress or plastic deformation occurs in the casting, and when they exceed the material strength limit at this high temperature, the casting will crack.
Pub Time : 2018-12-05 15:36:52 >> News list
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