Atlas of Casting Defects
Negative
Internal irregular cavities ranging from small dispersed or linear type cavities up to large cavities
Shrinkage cavities can range in size from very small (requiring magnification) to very large
Casting centerline between gates or in isolated heavy sections. Areas with sharp internal corners
Dendritic shrink, micro-shrink
Because this is a subsurface defect, if may not be discovered without x-ray examination, machining or sectioning through the casting. The edges of the void will appear
Molten alloys shrink as they solidify. As solidification progresses and the solid to liquid fraction increases, it becomes more difficult for liquid feed metal to reach the solidification front. Shrinkage occurs between dendritic arms. In larger defects, inadequate feed metal is provided to isolated hot spots in the casting.
| Area | Possible Cause | Potential Correction |
|---|---|---|
| Other (Mold design) | Inadequate or incorrect feeding |
Ensure adequate feeding of the area concerned to promote progressive solidification |
| Other (Mold design) | Incorrect solidification rate |
Modify the casting design to promote progressive solidification |
| Other (Mold design) | Vacuum in blind riser |
Ensure the v-notch in the riser prevents a vacuum from forming |
| Other (Mold design) | Blind riser too small |
Ensure the riser has adequate metal volume |
| Other (Mold design) | Low ferrostatic head pressure |
Increase the height of the mold. Use centrifugal force to increase head pressure |
| Other (Mold design) | Incorrect solidification rate |
Examine the molding technique. Modify the casting design to promote progressive solidification. |
| Foundry | Incorrect casting conditions |
Establish the correct casting conditions |
| Foundry | Incorrect solidification rate |
Use insulation on specific areas of the mold to increase thermal gradient |
| Foundry | Low metal temperature |
Increasing the metal temperature can improve the feeding distance |
| Foundry | Low mold temperature |
Increasing the mold temperature can improve feeding distance |
| Foundry | Vacuum created in the feeding system during solidification |
Ensure the metal in the head (pour cup) remains liquid longer than the casting. Insulate or use exothermic material on the pour cup and risers |
| Foundry | Dissolved gas level too high |
Use a heat makeup and melting practice that produce low dissolved gases |
| Other (Casting Processing) | Inadequate or incorrect feeding |
Use hot isostatic pressing (HIP) to close up the defects. Cap weld all surface connected shrinkage prior to HIP. |