In the process of casting, the three dimensional flexible welding platform of cast iron is unavoidable to meet the defects of shrinkage, bubble, segregation and so on, which causes the low yield of the castings, and the production of the re furnace is faced with a large amount of human and electrical energy consumption. How to reduce casting defects is a problem that foundry people have always been concerned about.
In order to reduce the defects of casting, our company has arranged ten major criteria for reducing casting defects, hoping to help the colleagues in foundry industry.
1. Good castings start from high quality melting
Once casting is started, the first step is to prepare, check and handle the melting process. If required, the acceptable minimum standard can be adopted. A better option, however, is to prepare and adopt near zero defect melting schemes.
2, avoid the turbulent inclusion on the free surface
This requires that the front free surface (meniscus) flow rate be too high. For most metals, the maximum flow rate is controlled at 0.5m/s. For closed gating systems or thin-walled components, the maximum flow rate will be increased appropriately. This requirement also means that the height of the metal liquid can not exceed the critical value of the “drop” height.
3, to avoid laminar inclusions on the surface of metal liquid
This requires that the front end of any metal stream should not stop flowing in the mold filling process. The metal meniscus at the early filling stage must remain in motion and free from the influence of thickening of the surface shell, which will become part of the casting. To achieve this effect, the front end of the liquid metal can be designed to expand continuously. In practice, only bottom injection “uphill” can achieve continuous rising process. (for example, gravity casting starts upward from the bottom of the runner).
4. Avoid gas (producing bubbles)
Avoid bubbles coming from the casting system and enter the cavity. It can be achieved by the following ways:
The rational design of the ladder gate cup;
The reasonable design of the straight runner is fast and full.
The rational use of “dams”;
Avoid “well type” or other open gating systems.
Use small cross section runner or use the ceramic filter near the runner at the runner.
Use the degassing device;
There is no interruption in the pouring process.
5, avoid the hole in the sand core
Avoid bubbles formed in sand core or sand mold into the cavity metal. The sand core must ensure very low gas content, or adopt appropriate exhaust to prevent the formation of the sand core. It can not be repaired with clay core or mold unless it is completely dry.
6, avoid shrinkage
Due to the influence of convection and unstable pressure gradient, the castings with large cross-section can not be fed upward. Therefore, it is necessary to follow all the feeding laws to ensure a good feeding design. The computer simulation technology is used to verify the actual samples. Control the flare level at the junction of sand mold and sand core; control the thickness of casting coating (if any); control the temperature of alloy and mold.
7. Avoid convection
The damage of convection is related to the time of solidification. Both thin-walled and thick walled castings are free from convective hazards. For medium thickness castings, the convection hazard is reduced by casting structure or process.
Avoid upward shrinkage;
Flipping after pouring
8, reduction of segregation
Prevention of segregation and control within the standard range or customer permitted component overrun area. If possible, try to avoid channel segregation
9. Reduction of residual stress
Do not use water (cold water or hot water) medium for quenching after light alloy solid solution treatment. If the casting stress does not seem to be large, polymer quencher or forced air quenching can be used.
10. A given datum point
All castings must be provided with positioning datum points for dimensional inspection and processing.
