I.Gas porosity (blowholes, choke holes and air pockets)
Pores are found on the surface or interior of the casting, and they are round, oval or irregular. Sometimes, several pores form a group of air holes, and the pores of the lower part are generally pear shaped. The shape of the choke hole is irregular, and the surface is rough, and the air pocket is a recess on the surface, and the surface is smoother. The hole can be found by visual inspection, and the subsurface porosity can be found only after machining.
- The mold preheating temperature is too low, and liquid metal cools too fast via the pouring system.
- The gas cannot be discharged smoothly due to poor exhaust design of the mold.
- The coating is not good, with poor exhaust performance, it even volatilizes or decomposes the gas.
- Holes and pits are found on the surface of the mold cavity. After the liquid metal is injected, the gas in the holes and pits expands rapidly, compressing the liquid metal and forming choke holes.
- The surface of mold cavity is rusted and not cleaned.
- The raw material (sand core) is not properly stored and preheated before use.
- The deoxidizer is poor, or the dosage is not enough or the operation is improper.
- The mold shall be fully preheated, the particle size of the coating (graphite) shall not be too fine, and the air permeability is good.
- The inclined pouring method is used.
- Raw materials shall be stored in a well ventilated and dry place and preheated before use.
- The deoxidizer (magnesium) with better deoxygenation effects shall be chosen.
- The pouring temperature should not be too high.
II. Shrinkage void (Shrinkage porosity)
The shrinkage void is a hole on the surface or inside of the casting, with a rough surface. When the shrinkage is small, there are many scattered small pores, namely shrinkage. The grain at the shrinkage or shrinkage point is coarse. Shrinkage often occur near the gate of the casting, the riser root, the thick parts, the junction of thick and thin walls and the thin wall with a large plane.
- The mold working temperature does not meet the requirements of directional solidification.
- Improper selection of coating and poor control of coating thickness at different parts.
- The location of the casting in the mold is not designed properly.
- The design of the gate and the riser fails to play the role of feeding.
- The pouring temperature is too low or too high.
- Improve the mold working temperature.
- Adjust the coating thickness and spray the coating evenly. The coating shall not be accumulated locally in the process of touch-up caused by peeling off.
- The mold is heated locally, or insulated locally with thermal insulation materials.
- The hot spot is inlaid with copper blocks for local quenching.
- The radiating fin is designed for the mold, or water is used to accelerate the cooling rate of the local area. Alternatively, water or mist is sprayed outside the mold.
- The demountable quench blocks are placed in the mold cavity in turn to avoid insufficient cooling of the quench blocks during continuous production.
- The mold riser is provided with a pressurization device.
- The gating system shall be designed accurately and the suitable pouring temperature shall be selected.
III. Slag hole (flux slag or metallic oxide slag inclusion)
The slag hole is an open hole or a hidden hole on the casting, and the whole hole or its local part is blocked by slag, with an irregular shape. The small punctate flux slag is difficult to find. The smooth holes are exposed after slag removal and generally distributed in the lower part of the pouring position, near the ingate or the dead corner of the casting. Oxide slag inclusions are mostly distributed on the surface of the casting near the ingate, sometimes in the form of flakes, irregular clouds with wrinkles, lamellar interlayers or flocculent in the interior of the castings. Castings often break from the interlayers in which oxide exists, which is one of the causes of cracks in castings.
The slag hole is mainly caused by alloy melting process and pouring process (including incorrect design of gating system). The mold itself will not cause slag hole, and the metal mold is one of the effective methods to avoid slag hole.
- The gating system is provided correctly or cast fiber filter is used.
- The inclined pouring method is used.
- Select proper flux and strictly control the quality.
IV. Crack (hot cracks and cold cracks)
The crack is straight lines or irregular curves. The surface of the hot crack fracture is strongly oxidized to be dark gray or black, without metallic luster. The surface of the cold crack fracture is clean with metallic luster. Generally, the external cracks of castings are visible, while the internal cracks can only be found with other methods. Cracks are often related to shrinkage porosity, slag inclusions and other defects. They are often found in the inner side of the casting sharp corner, the junction of thick and thin sections, and the hot spot area where the gate and the riser are connected with the casting.
Causes: Crack defects are likely to have in the process of metal mold casting. Since the metal mold itself has no deformability and has the fast cooling speed, the internal stress of the casting is likely to increase, the mold opening is too early or too late, the pouring angle is too small or too large, the coating layer is too thin, it is easy to cause cracks in the mold. If the mold cavity itself has cracks, cracks are likely to occur.
- Pay attention to the structural and technological properties of the castings, so that the parts with uneven wall thickness can be evenly transited, and the appropriate fillet size shall be determined.
- Adjust the thickness of the coating to make the parts of the casting reach the required cooling rate as far as possible to avoid the excessive internal stress.
- Observe the working temperature of the metal mold, adjust the inclination of the mold, pull out the core timely for cracking, and take out the casting for slow cooling.
V. Cold shut (poor fusion)
Cold shut is a through seam or surface crack with round edges, separated by oxide skin in the middle and not completely fused. The severe cold shut is called as insufficient pouring. Cold shut often appears on the top wall of the casting, on the thin horizontal or vertical plane, at the junction of the thin and thick walls or on the thin auxiliary plate.
- The exhaust design of metal mold is improper.
- The working temperature is too low.
- Poor coating quality (artificial material).
- The position of runner is improper.
- The pouring speed is too slow.
- The runner and exhaust system shall be designed correctly.
- For extensive thin-walled castings, the coating shall not be too thin, and the coating layer shall be thickened appropriately to facilitate the forming.
- The working temperature of the mold shall be increased appropriately.
- The inclined pouring method is used.
- The mechanical vibration metal mold is used for pouring.
VI. Sand hole
The sand hole is a relatively regular hole on the surface or inside of the casting, and its shape is consistent with the shape of sand particles. When the casting is just out of the mold, the sand particles embedded on the surface of the casting are visible and can be taken out. If multiple sand holes exist at the same time, the surface of the casting is orange peel shaped.
- Sand particles falling from the surface of the sand core are wrapped by liquid copper, causing holes on the surface of the casting.
- The surface strength of sand core is poor, burnt or not completely solidified.
- The size of the sand core does not match the external mold, and the sand core is crushed when the mold is assembled.
- The mold is dipped in graphite water contaminated by sand.
- The sand dropped from the friction between the ladle and the sand core at the runner is washed into the cavity with copper water.
- Fabricate sand cores strictly according to the process and check the quality.
- The size of sand core matches the external mold.
- The ink shall be cleaned up in time.
- Avoid friction between ladle and sand core.
- Blow off the sand in the mold cavity when placing the sand core.