Abstract
This article delves into the characteristics, locations, and mechanisms of shrinkage defects such as shrinkages and porosity in investment casting. It analyzes the primary causes of these defects and presents comprehensive prevention strategies to effectively reduce them, thus lowering costs and enhancing productivity.
1. Introduction
Investment casting, also known as lost-wax casting, is a precise casting method widely used in industries due to its ability to produce complex geometries with high accuracy and surface finish. However, shrinkage defects, including shrinkages and porosity, remain a significant challenge in this process. These defects can compromise the mechanical properties and reliability of castings, leading to costly rework or scrap. This article aims to provide a comprehensive understanding of the causes of shrinkage defects in investment casting and propose effective prevention strategies.
2. Characteristics of Shrinkage Defects
2.1 Types of Shrinkage Defects
Shrinkage defects in investment casting can be categorized into the following types:
- Central Shrinkage: Appears as large, irregularly shaped holes with developed dendrite crystals at the final solidification point of investment casting.
- Exposed Shrinkage: Occurs on the external surface of the casting.
- Internal Shrinkage: Forms inside the casting.
- Corner Shrinkage: Develops at the concave corners of the casting.
2.2 Locations of Shrinkage Defects
Shrinkage defects typically occur in the last to solidify regions of investment casting, which are often the hot spots or thick sections. The location of shrinkages is influenced by factors such as the position of the ingate, heat dissipation conditions of different casting parts, the impact of the gating system on heat dissipation, and the relative positions of castings within the same mold cluster. Methods to predict and identify shrinkage defects include drawing isotherms or inscribed circles, dissecting castings, and utilizing casting process analysis software.
3. Mechanism of Shrinkage Defects
3.1 Stages of Contraction in investment Casting
During the cooling process from pouring temperature to room temperature, investment casting undergo three stages of contraction: liquid contraction, solidification contraction, and solid-state contraction. These stages are illustrated in Figure 4 for ZG270-500 steel.
Figure 4: Volumetric Contraction of ZG270-500
- Liquid Contraction: Occurs when the metal temperature drops from the pouring temperature to the liquidus temperature. The liquid contraction rate (εV液) can be calculated using the formula:varepsilonV液=αV液(t浇−t液)×100%where:
- εV液 is the liquid contraction rate,
- αV液 is the average volumetric contraction coefficient,
- t浇 is the pouring temperature,
- t液 is the liquidus temperature.
- Solidification Contraction: Takes place as the metal temperature decreases from the liquidus to the solidus temperature. The solidification contraction rate (εV凝) is related to the carbon content of the steel, as shown in Table 1.
| Carbon Content (ω(C), %) | εV凝 (%) |
|---|---|
| 0.10 | 2.0 |
| 0.35 | 3.0 |
| 0.45 | 4.3 |
| 0.70 | 5.3 |
| Table 1: Influence of Carbon Content on Solidification Contraction Rate |
- Solid-State Contraction: Occurs as the metal temperature further drops from the solidus temperature to room temperature. The solid-state volumetric and linear contraction rates can be expressed as:varepsilonV固=αV固(t固−t室)×100%varepsilon=α(t固−t室)×100%where:
- εV固 and ε are the solid-state volumetric and linear contraction rates, respectively,
- αV固 and α are the solid-state volumetric and linear contraction coefficients,
- t固 is the solidus temperature,
- t室 is the room temperature.
For ZG270-500 steel with a pouring temperature overheat of 100°C, the total volumetric contraction rate is calculated as:
varepsilonV总=εV液+εV凝+εV固=1.6%+3%+7.2%=11.8%
3.2 Mechanism of Shrinkage Formation
When molten metal is poured into the mold cavity, heat is primarily conducted through the mold shell, creating a temperature gradient (Figure 7). As the temperature decreases, the outer layer of metal first reaches the liquidus temperature and solidifies, forming a thin shell that gradually thickens inward until the entire cross-section of the casting solidifies.
Figure 8 illustrates the formation of central shrinkage in a cylindrical casting:
- Initial Pouring: Metal fills the mold cavity (Figure 8a).
- Initial Solidification: The outer layer solidifies due to heat dissipation through the mold shell (Figure 8b).
- Liquid Contraction and Solidification: The liquid core contracts, causing the liquid level to drop. As solidification progresses, the shell thickens, and a large void forms due to the combined liquid and solidification contractions (Figure 8c).
- Complete Solidification: The investment casting solidifies, leaving a nearly inverted conical central shrinkage (Figure 8d).
- Solid-State Contraction: The investment casting and shrinkage volume slightly decrease during further cooling (Figure 8e).
4. Causes of Shrinkage Defects
4.1 Unreasonable Gating System Design
An improperly designed gating system can hinder sequential solidification or adequate feeding during the cooling process. Sequential solidification involves controlling the metal’s solidification process to ensure that it solidifies in a clear sequence towards the riser, with later-solidifying metal feeding earlier-solidifying regions. If the gating system is not designed correctly, shrinkages will form in the last to solidify areas of investment casting (Figure 9).
Figure 9: Effect of Riser on Shrinkage
4.2 Unreasonable Casting Structure Design
- Large Variations in Sectional Dimensions: Rapid solidification of thin sections compared to adjacent thick sections disrupts sequential solidification.
- Isolated Thick Sections: These sections may not receive adequate feeding.
- Improper Fillet Radii: Too small a radius results in rapid transitions between thick and thin sections, impeding feeding. Too large a radius creates new hot spots, also hindering feeding.
- Lack of Redesign for Casting-Substitute Components: Components designed for forging or welding without considering casting process characteristics may not allow for adequate feeding.
4.3 High Liquid and Solidification Contraction Rates
- High Liquid Contraction Rate: Increases the volume of shrinkages formed. The liquid contraction rate (εV液) is proportional to the average volumetric contraction coefficient (αV液) and the superheat (t浇 – t液).
- High Solidification Contraction Rate: Also increases shrinkage volume. Inadequate deoxidation during melting can lead to gas evolution during solidification, reducing the solidification volume contraction but expanding the porosity region.
4.4 Improper Pouring Conditions
Faster pouring speeds increase the liquid contraction and reduce the time available for later-poured metal to feed earlier-solidified regions, resulting in larger shrinkages.
4.5 Poor Cooling Capacity of the Mold Shell
Weaker cooling capacity slows down the cooling and solidification of metal within the mold cavity, reducing the amount of feeding provided by later-poured metal and increasing shrinkage volume.
4.6 Poor Local Heat Dissipation Conditions
Poor heat dissipation at concave corners, for example, delays solidification in these areas, leading to corner shrinkages if feeding is inadequate.
4.7 Insufficient Pressure Head for Feeding
Low pressure heads slow down metal flow, reducing feeding effectiveness and causing shrinkages in unfed regions.
5. Prevention Strategies
5.1 Proper Design of Gating System
A well-designed gating system facilitates sequential solidification and adequate feeding. Key components of an investment casting gating system include:
- Sprue Cup: Holds the molten metal and establishes pressure for filling and feeding, typically positioned 70-100 mm above the wax pattern.
- Straight Sprue: Acts as a support during shell making and often doubles as a riser. Its cross-sectional area should be 1.4 times that of the ingate.
- Cross Sprue: Distributes metal, skims slag, and can assist in feeding. If used for feeding, its cross-sectional area should be 1.1-1.3 times that of the straight sprue; otherwise, 0.7-1.0 times.
- Ingate: Connects the straight or cross sprue to investment casting. Its design, including location, number, shape, length, and cross-sectional size, significantly impacts filling, solidification, feeding, and investment casting defects.
Table 2: Design Parameters for Ingate
| Parameter | Considerations |
|---|---|
| Location | Avoid impacting thin sections or cores; place at hot spots for sequential solidification. |
| Number | Typically one, but more for complex shapes or multiple hot spots. |
| Shape | Rectangular, circular, square, or扇形 based on the filling area. |
| Length | 8-12 mm for easily removable ingates, 10-15 mm for gas-cut ingates. |
| Cross-Sectional Size | Determined by the hot spot diameter (D内 = KD节, K = 0.7-0.9) or equivalent hot spot method. |
5.2 Improvement of Casting Structure
Modify the investment casting structure to facilitate sequential solidification and adequate feeding:
- Analyze the Casting Structure: Identify hot spots and define feeding zones.
- Uniform Wall Thickness: Adjust processing allowances to ensure uniform wall thickness and add fillets at transitions to improve feeding.
- Subsidiary Riser Technique: Add thickened sections (subsidiaries) near the riser to enhance feeding channels and riser effectiveness. However, excessive use increases machining allowances and may defeat the purpose of investment casting.
5.3 Reduction of Metal Contraction Rates
- Select Low-Contraction Alloys: Choose metals with low liquid and solidification contraction rates.
- Optimize Pouring Temperature: Higher pouring temperatures increase liquid contraction but improve feeding if the gating system remains effective. Lower temperatures risk premature solidification of the gating system, reducing feeding effectiveness.
5.4 Adequate Deoxidation During Melting
Reduce gas evolution during solidification by thoroughly deoxidizing the metal, thereby minimizing porosity and aiding in shrinkage reduction.
5.5 Selection of Appropriate Pouring Conditions
Adopt pouring strategies such as high-temperature tapping and low-temperature pouring, using techniques like “fast initial, slow subsequent, and final top-up” for large mold clusters to promote sequential solidification and effective feeding.
5.6 Improvement of Local Heat Dissipation Conditions
Enhance local cooling by arranging wax patterns with appropriate spacing in multi-piece clusters or placing hot spots at the cluster edges for singles. Use chills (preferably brass or stainless steel) during shell making to improve cooling at concave corners and reduce corner shrinkages.
5.7 Increase in Pressure Head for Feeding
Higher pressure heads enhance metal flow and feeding effectiveness. For large castings, conventional risers may be insufficient; instead, use insulated risers with exothermic covers to delay upper solidification and prevent secondary shrinkages. In extreme cases, consider centrifugal or pressure casting.
6. Conclusion
6.1 Fundamental Principle for Reducing Shrinkage Defects
The key to minimizing or eliminating shrinkage defects is to achieve sequential solidification in investment casting.
6.2 Considerations for Sequential Solidification
- Casting Structure: Ensure reasonable design.
- Gating System Design: Ensure it is correctly designed.
- Compensatory Measures: Adjust processing allowances or use the subsidiary riser technique to create a temperature gradient favoring sequential solidification.
- Supporting Measures: Optimize mold shell temperature and pouring conditions.
6.3 Potential Drawbacks of Sequential Solidification
Large temperature gradients can introduce residual stresses, leading to investment casting distortion or cracks.
6.4 Interconnected Prevention Methods
Strategies for reducing or eliminating shrinkages and porosity are interconnected and require thorough analysis, multiple solution proposals, and iterative validation through production practice to achieve desired results.
7. Application Case Study
A critical investment casting component (16 kg) in a piece of equipment was required to last over 10,000 meters of usage but failed after only 3,000-4,000 meters due to severe shrinkages and porosity on the wear surface, reducing wear resistance and leading to premature failure (Figure 13).
7.1 Material and Chemical Composition
The modified material, ZG20CrMnMoRe, included rare earth elements to improve metal fluidity, feeding ability, and mechanical properties (Table 3).
| Material | Chemical Composition (wt.%) |
|---|---|
| ZG20CrMnMoRe | C: 0.15-0.25, Si: 0.40-0.75, Mn: 0.90-1.20, Cr: 1.10-1.40, Mo: 0.20-0.30, P ≤ 0.04, S ≤ 0.04, Re: 0.15 |
| Table 3: Chemical Composition of ZG20CrMnMoRe |
7.2 Addition of Rare Earth Elements
- Addition Method: Pouring ladle injection was found optimal, offering high recovery and stable residual rare earth content.
- Addition Amount: Calculated based on deoxidation, desulfurization, burn-off, and residual requirements. An addition of 0.15% rare earth alloy via pouring ladle injection was found best, with a recovery rate of 50-80% and a residual content of 0.08-0.12%.
7.3 Casting and Testing
Castings of both ZG20CrMnMo and ZG20CrMnMoRe were poured and tested. The ZG20CrMnMoRe castings exhibited improved density, reduced porosity, and enhanced mechanical properties (Tables 6-9).
| Casting Type | Casting Count | Defective Count (Porosity, etc.) | Microstructure Density |
|---|---|---|---|
| With Rare Earth | 15 | 0 | Slight porosity visible after nitric acid etching |
| Without Rare Earth | 10 | 2 | Severe shrinkages and porosity visible after nitric acid etching |
| Table 6: Casting Results |
| Material | Mechanical Property | Value |
|---|---|---|
| ZG20CrMnMo | Bending Strength | 187 MPa |
| Impact Toughness (True Carburized) | 1.2 J·cm^-2 | |
| Impact Toughness (False Carburized) | 2.2 J·cm^-2 | |
| ZG20CrMnMoRe | Bending Strength | 198 MPa |
| Impact Toughness (True Carburized) | 1.8 J·cm^-2 | |
| Impact Toughness (False Carburized) | 6.4 J·cm^-2 | |
| Table 8: Mechanical Properties |
| Material | Wear Test Result (After 40,000 Revolutions) |
|---|---|
| ZG20CrMnMo | Weight Loss: 3.7872 g, Dimension Reduction: 0.8810 mm |
| ZG20CrMnMoRe | Weight Loss: 3.2678 g, Dimension Reduction: 0.7170 mm |
| Table 9: Wear Test Results |