The manufacturing of complex, high-integrity components like impellers for turbomachinery presents significant challenges. Impellers are the rotating hearts of equipment such as centrifugal pumps, converting external mechanical energy into the kinetic and pressure energy of the fluid. Their sophisticated, streamlined blade profiles are extremely difficult and costly to achieve through machining alone. This is where the investment casting process proves its exceptional value. By enabling the production of near-net-shape, integral castings with intricate internal geometries, the investment casting process not only avoids the stress concentrations associated with machining but also allows for the direct realization of optimal hydrodynamic forms that enhance mechanical performance. However, for medium to large castings, particularly in demanding materials like duplex stainless steels, the risk of shrinkage porosity and gas defects remains a primary quality concern. This article details a first-person account of the systematic optimization of the investment casting process for a specific duplex stainless steel closed impeller, leveraging numerical simulation as a core tool for defect prediction and process refinement.
The subject component was a single-suction, six-channel closed impeller, a configuration where the blades are shrouded on both sides. The material was ASTM A890 3A duplex stainless steel, chosen for its excellent corrosion resistance and combination of strength and toughness. A preliminary analysis of the 3D model immediately identified the principal challenges inherent in the investment casting process for this geometry. The part featured significant variations in wall thickness, creating isolated thermal masses or “hot spots” that are prone to shrinkage during solidification. The key thick sections requiring careful feeding strategy were the central hub and the outer shroud, often called the “wear ring” or “口环” area. The basic dimensions critical for thermal and feeding calculations are summarized below.
| Feature | Dimension (mm) |
|---|---|
| Hub Diameter | 110 |
| Channel Width | 110 |
| Wear Ring (Shroud) Diameter | 495 |
| Overall Impeller Diameter | 611 |
| Blade Thickness Range | 8.5 – 10 |
| Total Casting Weight | ~122 kg |
The fundamental principle for preventing shrinkage in steel castings is to establish directional solidification, where the casting solidifies progressively from the areas farthest from the heat source (the feeder, or riser) towards the riser itself. This ensures a continuous feed path of liquid metal to compensate for volumetric shrinkage. The thermal gradient driving this is often assessed using the concept of modulus, $M$, defined as the volume-to-cooling-surface-area ratio:
$$ M = \frac{V}{A} $$
Areas with a higher modulus solidify more slowly and require proper feeding. Therefore, a successful investment casting process design must ensure the feeder has the largest modulus, followed by the casting’s thick sections. The initial process layout was conceived based on this principle. The casting was oriented with the wear ring facing upwards. A primary downsprue was attached to the spherical surface of the hub, terminating in a large cylindrical riser (feeder head) to feed the central mass. Additionally, three side gates were attached to the upper wear ring section. These served a triple purpose: to act as secondary feeders for the wear ring hot spots, to facilitate venting of gases and expulsion of slag, and to aid in wax removal during the dewaxing stage of the investment casting process.
To virtually validate and analyze this initial design before committing to costly tooling and trials, numerical simulation was employed using ProCAST software. The 3D model was meshed with due care for accuracy and computational efficiency. The key simulation parameters were defined to replicate foundry conditions as closely as possible.
| Parameter | Value |
|---|---|
| Pouring Temperature | 1620 °C |
| Initial Shell Temperature | 650 °C |
| Pouring Rate | 7 kg/s |
| Metal-Shell Heat Transfer Coefficient | 500 W/(m²·K) |
| Riser Top-Air Heat Transfer Coefficient | 100 W/(m²·K) |
| Shell-Air Heat Transfer Coefficient | 45 W/(m²·K) |
| Shrinkage Porosity Criterion (Critical Fraction) | 2.3% |
The simulation results were illuminating and pinpointed the shortcomings of the initial investment casting process scheme. The porosity prediction module clearly flagged potential shrinkage defects in two locations: the central hub and sections of the upper wear ring. Analyzing the solidification sequence and temperature fields revealed the root causes:
- Wear Ring Defects: The three side gates were insufficient in number and placement to cover the entire thermal mass of the wear ring. The feeding distance from each gate was limited, leaving isolated hot spots between the gates unsupplied with liquid feed metal, leading to micro-shrinkage.
- Hub Defects: A more complex issue was observed at the hub. The large surface area of the riser neck (the connection between the riser and the hub) caused it to lose heat rapidly to the environment. This effectively reduced its local modulus, causing it to solidify before the thicker hub section below it. Once this neck sealed, the hub became isolated from the liquid metal reservoir in the riser, resulting in a shrinkage cavity. This phenomenon was asymmetrical, more pronounced on the side of the hub opposite the side gates, as the gates provided some additional heat input delaying local solidification.
The solidification pattern at the hub showed an undesired “V”-type feeding mode from the riser, which is unstable and prone to defect formation. The governing heat transfer during this phase can be described by the transient heat conduction equation:
$$ \frac{\partial T}{\partial t} = \alpha \nabla^2 T $$
where $T$ is temperature, $t$ is time, and $\alpha$ is the thermal diffusivity. The premature freezing of the neck created an insulating barrier, altering the thermal gradient $\nabla T$ and disrupting directional solidification.
Based on these virtual findings, the investment casting process was systematically optimized. The modifications targeted both the feeding system geometry and the pouring method.
Optimization 1: Redesign of the Gating and Feeding System
Two critical changes were made:
- Increase in Side Gates: The number of side gates was doubled from three to six. This ensured that every junction between the wear ring and a blade was directly topped by a feeding gate, eliminating the previous isolated hot spots and establishing a more uniform thermal and feeding environment around the circumference.
- Enlargement of the Primary Riser: To address the premature freezing of the riser neck, the modulus of the riser base was increased. This was achieved by specifying a riser with a larger base diameter. The riser top was also redesigned into a hexagonal shape for easier handling and wax pattern assembly within the investment casting process workflow. The new design aimed to satisfy the fundamental requirement for safe feeding:
$$ \frac{M_{\text{riser}}}{M_{\text{casting}}} > 1.2 $$
where $M_{\text{riser}}$ is the modulus of the riser neck and $M_{\text{casting}}$ is the modulus of the hub hot spot.
Simulation of this revised model showed dramatic improvement. The wear ring was now completely free of predicted shrinkage, as each hot spot was effectively fed by a dedicated gate. However, a minor defect signal persisted in the central hub region, although significantly reduced. Analysis confirmed that while the larger riser helped, the fundamental “V”-type solidification pattern from the bottom of the hub upwards remained, indicating that the hub’s bottom surface was acting as a heat sink, cooling too slowly relative to the riser neck.
Optimization 2: Modification of the Pouring Method
The initial pouring setup involved introducing the molten metal down the center of the pouring cup and sprue. Simulation of the filling process revealed that this central stream continuously impinged on the bottom of the hub, creating a localized “re-heating” effect. This kept the hub bottom at an artificially high temperature for an extended period, disrupting the desired thermal gradient and contributing to the residual shrinkage risk. The governing energy transfer during filling can be related to the kinetic and thermal energy of the stream:
$$ E_{\text{thermal}} \propto \dot{m} C_p (T_{\text{pour}} – T_{\text{liquidus}}) $$
where $\dot{m}$ is the mass flow rate and $C_p$ is the specific heat. The concentrated impingement localized this energy input.
The solution was to change from a central downpour to a tangential, side-pouring technique. In this optimized investment casting process, the metal is directed to flow along the wall of the pouring cup before entering the sprue. This simple change yielded two major benefits:
- Elimination of Local Re-heating: It prevented the direct, high-velocity stream from hitting the hub bottom, allowing for a more uniform and controllable temperature distribution in the critical lower section of the mold cavity.
- Improved Mould Integrity: The tangential entry significantly reduced the turbulence and erosive force of the incoming metal, lowering the risk of mold erosion or inclusion generation.
The final simulation, incorporating both the optimized feeding system and the side-pouring method, confirmed the success of the investment casting process optimization. The predicted shrinkage porosity in the hub was completely eliminated. The solidification sequence now showed a more favorable progressive solidification from the blade tips and hub bottom towards the riser and side gates. The final temperature field and solid fraction isosurves confirmed a sound feeding pattern.
| Stage | Key Modification | Simulation-Predicted Outcome | Primary Defect Addressed |
|---|---|---|---|
| Initial Design | 3 side gates, central riser, center pouring. | Shrinkage in wear ring and hub. | N/A (Baseline) |
| Optimization 1 | Increase side gates to 6, enlarge riser base diameter. | Wear ring defects eliminated; hub defect reduced. | Wear Ring Shrinkage |
| Optimization 2 | Change from center pouring to tangential side pouring. | Hub shrinkage defect completely eliminated. | Hub Shrinkage & Local Re-heating |
The optimized investment casting process was translated into production. The wax patterns were assembled into trees according to the new design, followed by ceramic shell building, dewaxing, firing, and casting. The duplex stainless steel was melted and poured using the side-pouring technique. After shakeout and standard cleaning processes, the cast impellers were subjected to rigorous non-destructive testing and subsequent machining. The final machined components confirmed the simulation predictions: both the wear ring and the hub areas were free from shrinkage defects, meeting all dimensional and quality specifications. This successful outcome validated the virtual optimization loop and demonstrated a significant improvement in yield and reliability for this complex component.
In conclusion, this case study underscores the power of integrating numerical simulation into the investment casting process development cycle. The systematic approach—from initial thermal analysis to feeding system redesign and finally to filling method optimization—allowed for the virtual identification and correction of defects before any metal was poured. The key learnings can be formalized into guiding principles for similar closed impeller castings:
- Comprehensive Feeding: Ensure all major thermal masses are directly fed by adequately sized risers or gates. The feeding distance $L_f$ for a section can be estimated and must be respected:
$$ L_f \propto \sqrt{M} $$
where a larger modulus $M$ allows for a longer feeding distance. - Riser Efficacy: Design risers not just for volume but for thermal dominance. The riser neck must remain liquid longer than the casting section it feeds, a condition dependent on its modulus and heat transfer environment.
- Controlled Filling: The pouring method is an integral part of the investment casting process. Minimizing turbulent impingement and localized reheating is crucial for establishing a favorable initial temperature gradient for directional solidification.
By adhering to these principles and leveraging simulation tools, the investment casting process can be robustly optimized to produce high-integrity, complex geometry components like duplex stainless steel impellers with high yield and consistent quality.