Abstract
This article mainly elaborates on the characteristics, locations, and mechanisms of shrinkage defects, including shrinkage cavities and porosity, in investment castings. It analyzes the primary causes of these defects and proposes preventive measures to effectively reduce them, lower costs, and improve efficiency.
1. Characteristics of Shrinkage Defects
Investment casting involves pouring molten metal into a high-temperature ceramic mold. During cooling and solidification, three types of shrinkage occur: liquid shrinkage, solidification shrinkage, and solid-state shrinkage. Shrinkage cavities and porosity primarily form in the last solidified areas of the casting.
| Type of Shrinkage Defect | Description |
|---|---|
| External Shrinkage | Rough, irregularly shaped holes exposed on the casting’s surface |
| Internal Shrinkage | Holes formed within the casting |
| Corner Shrinkage | Holes formed at concave corners of the casting |
2. Locations of Shrinkage Defects
Shrinkage defects typically occur in the last solidified and inadequately fed areas of the casting, often at hot spots and thick sections. The location also relates to factors such as gate placement, heat dissipation conditions, and the influence of the pouring system.
| Factor Influencing Shrinkage Location | Description |
|---|---|
| Gate Placement | Affects the flow and solidification sequence of molten metal |
| Heat Dissipation Conditions | Influenced by casting geometry, mold material, and environmental conditions |
| Pouring System | Impacts metal flow and solidification pattern |
3. Mechanisms of Shrinkage Defects
The volume of a casting decreases during three stages from pouring temperature to room temperature: liquid shrinkage, solidification shrinkage, and solid-state shrinkage.
| Stage of Shrinkage | Description | Formula |
|---|---|---|
| Liquid Shrinkage | Occurs when metal is in the liquid state | εV液 = αV液(t浇－t液) × 100% |
| Solidification Shrinkage | Occurs during the transition from liquid to solid | εV凝 depends on carbon content |
| Solid-State Shrinkage | Occurs after complete solidification | εV固 = αV固(t固－t室) × 100% |
4. Causes of Shrinkage Defects
| Cause | Description |
|---|---|
| Unreasonable Gating System Design | Prevents sequential solidification and adequate feeding |
| Unreasonable Casting Structure Design | Includes large variations in cross-sectional dimensions, isolated thick sections, and improperly sized fillets |
| High Liquid and Solidification Shrinkage Rates | Larger volumes of shrinkage defects |
| Improper Pouring Conditions | Faster pouring speeds increase the volume of shrinkage defects |
| Poor Cooling Capacity of the Mold | Slows cooling and solidification, increasing shrinkage |
| Poor Local Heat Dissipation of the Mold | Late solidification at concave corners, leading to corner shrinkage |
| Insufficient Pressure Head for Feeding | Reduces the flow speed of molten metal, affecting feeding effectiveness |
5. Prevention Measures
The basic principle for preventing shrinkage defects is to achieve sequential solidification and leave shrinkage cavities and porosity in the gating system.
| Prevention Measure | Description |
|---|---|
| Proper Gating System Design | Includes design of pouring cup, sprue, runner, and ingate to ensure sequential solidification and feeding |
| Improved Casting Structure | Analyze casting structure, adjust processing allowances, and use subsidies when necessary |
| Reduction of Metal Shrinkage Rates | Select metals with lower shrinkage rates and optimize pouring temperatures |
| Adequate Deoxidization | Reduce the influence of gas on solidification shrinkage |
| Suitable Pouring Conditions | Adopt practices such as high-temperature tapping and low-temperature pouring, with adjusted pouring speeds based on casting size and pouring method |
| Improved Local Heat Dissipation | Use techniques like cold irons to enhance cooling at critical areas |
| Increased Pressure Head for Feeding | Enhance feeding effectiveness by increasing the pressure head height |
6. Application Case
A critical investment casting component in a piece of equipment had a service life requirement of over 10,000 meters of travel but failed after 3,000-4,000 meters due to severe shrinkage defects. The addition of rare earth elements improved the fluidity and feeding ability of the molten metal, reducing shrinkage defects and enhancing the mechanical properties of the casting.
| Rare Earth Addition Method | Description |
|---|---|
| Pouring Ladle Injection | Achieved higher recovery rates and stable residual rare earth content |