In modern hydropower generation, the turbine runner is a critical component that converts the pressure and potential energy of water into rotational mechanical kinetic energy, which is then transmitted through the main shaft to the generator. With increasing demands for dimensional accuracy and internal quality, integral casting has become the preferred manufacturing method for many runners. The runner structure typically consists of three parts: the crown, blades, and band. Integral casting involves pouring these three parts as a single piece, eliminating the need for subsequent welding and assembly. The advent of 3D printing casting technology has revolutionized this process by enabling the fabrication of complex sand cores that form the runner flow passage, followed by integral pouring. This approach reduces costs, shortens lead times, and improves casting surface quality while minimizing post-processing work. However, the runner experiences severe alternating loads, high-pressure water impact, and sediment erosion during operation, making casting defects such as shrinkage, porosity, and slag inclusion unacceptable. This article presents a comprehensive study on how to produce a high-quality integral casting runner using 3D printing sand cores, ensuring a defect-free casting that meets stringent quality standards.
Product Introduction
The target runner has an outer diameter of 2242 mm, a height of 1205 mm, a weight of 5701 kg, a maximum wall thickness of 170 mm, and a minimum wall thickness of 40 mm. The material is stainless steel GX4CrNi13-4. Its unique geometry presents a challenging casting scenario — the flow passages are narrow and curved, with multiple hot spots distributed along the blade-to-crown and blade-to-band junctions. The casting process must guarantee proper feeding, avoid turbulence, and ensure dimensional accuracy. The following table summarizes the key parameters of the runner:
| Parameter | Value |
|---|---|
| Material | GX4CrNi13-4 |
| Overall diameter | 2242 mm |
| Overall height | 1205 mm |
| Weight | 5701 kg |
| Maximum wall thickness | 170 mm |
| Minimum wall thickness | 40 mm |
| Number of blades | 13 (typical) |
Casting Process Design
Determination of Casting Orientation
Based on the structural characteristics and the need for effective feeding, the crown-up orientation was selected. In this orientation, the crown is positioned at the top, the band at the bottom, and the blades are arranged vertically. This allows the highest thermal gradient to be established from the top riser downward, promoting directional solidification. The schematic of this orientation is conceptually represented in the following 3D printing casting approach.
Simulation of Hot Spots and Feeder Design
Using MAGMA simulation software, the thermal distribution and potential shrinkage porosity were analyzed. The results indicated that the hot spots are concentrated at three primary regions: the junction between the band and blades, the junction between the crown and blades, and the main shaft area of the crown. These hot spots appear as spiral-shaped elongated zones due to the curved blade geometry. Traditional discrete small risers are inefficient and reduce yield. Instead, a combined feeding system was designed, consisting of a top riser for the crown and a series of side risers with pads for the band region.
The modulus calculation method was used for riser sizing. The modulus is defined as:
$$M = \frac{V}{A}$$
where \(V\) is the volume and \(A\) is the cooling surface area. The riser modulus must be larger than the casting modulus at the feeding zone:
$$M_{\text{riser}} = (1.2 \text{ to } 1.4) \times M_{\text{casting}}$$
For the crown region, a single top riser was placed on the main shaft end face. To ensure directional solidification from top to bottom, a tapered metal pad (feeding gradient of 8%–12%) was applied along the inner surface of the shaft, extending radially outward from the shaft root to the crown periphery. This pad creates a continuous feeding path without requiring additional risers on each blade junction, simplifying post-processing.
For the band region, the hot spots at the blade-band intersections form independent thermal nodes. A series of longitudinal risers with integrated pads were placed along the band outer circumference. Each pad is aligned with the corresponding blade and provides a feeding gradient of 10%–15% toward the riser. The following table summarizes the feeding parameters used:
| Region | Riser type | Modulus ratio (riser/casting) | Feeding gradient | Number of risers |
|---|---|---|---|---|
| Crown & shaft | Open top riser | 1.2 | 8%–12% (pad) | 1 |
| Band & blade junction | Side risers with pads | 1.3 | 10%–15% (pad) | 13 (one per blade) |
MAGMA simulation confirmed that the designed feeding system eliminates shrinkage porosity. The final porosity distribution after solidification showed no unacceptable defects. The simulation results are summarized in the following table:
| Region | Maximum porosity volume fraction (%) | Defect status |
|---|---|---|
| Crown | 0.5 | Acceptable (within standard) |
| Blade-crown junctions | 1.2 | Acceptable |
| Band | 0.8 | Acceptable |
| Blade-band junctions | 1.5 | Acceptable |
Gating System Design
The gating system must ensure smooth, rapid, and continuous filling of the mold cavity while minimizing turbulence and slag entrapment. An open, bottom-filled system was adopted. The molten metal flows from a single ladle through a central downsprue into a primary runner, then is divided into multiple branches. Each branch leads to a filter and then to an ingate located beneath each side riser pad. The filters reduce slag and control flow velocity. The gating system incorporates the following features:
- Single downsprue and open system to avoid backpressure.
- Multiple ingates (one per blade riser pad) to achieve uniform filling.
- Ceramic filter at each ingate to prevent reoxidation slag from entering the cavity.
- High-refractory ceramic tubes for runner channels to reduce sand erosion.
The pouring time was calculated based on the Chvorinov rule and optimized using simulation. The final pouring parameters are listed below:
| Parameter | Value |
|---|---|
| Pouring temperature | 1580°C |
| Pouring time | 55 s |
| Number of ingates | 13 |
| Filter type | Ceramic foam (10 ppi) |
| Downsprue diameter | 80 mm |
| Cross-section ratio (runner:ingates) | 1 : 1.2 |
Mold and Core Assembly Using 3D Printing Casting Technology
The complex internal flow passages formed by the blades demand extremely high dimensional accuracy. Traditional hand-molded cores cannot achieve the required ±1 mm tolerance. Therefore, a hybrid approach was employed: the external mold was made from CNC-machined wood patterns and resin-bonded sand, while the internal cores (blade flow passage core and shaft core) were produced via 3D printing casting technology. The 3D printed sand cores are printed in one piece (or several segments that are subsequently assembled) with a dimensional accuracy of ±1 mm. This minimizes post-machining and ensures near-net shape.
The process sequence for mold and core assembly is as follows:
- External mold preparation: Wood patterns for the band outer contour and crown upper surface are machined using a 5-axis CNC. The pattern is mounted on a molding box, and resin-bonded silica sand is compacted around it. After curing, the pattern is removed, forming the outer cavity.
- 3D printing of internal cores: The flow passage core (which defines the blade surfaces and the inner cavity of the runner) is printed using a binder jetting process with furan resin as binder and silica sand. The shaft core is printed separately. All cores are post-cured and inspected.
- Core setting: The printed flow passage core is placed into the bottom mold (band cavity). The shaft core is inserted into the top mold (crown cavity). The cores are supported by chaplets and vents.
- Closing: The top mold (with shaft core) is lowered onto the bottom mold (with flow passage core). The joint is sealed with a ceramic paste to prevent metal penetration.
- Gating and riser assembly: Ceramic tubes for runners and ingates are installed, and the riser sleeves (exothermic or insulating) are placed on the mold surface above the pads.
The above figure illustrates a typical sand casting part produced using 3D printing casting technology, showing the intricate core geometries achievable.
The complete assembly is then transported to the pouring pit. After melting and refining the GX4CrNi13-4 alloy in an electric arc furnace, the steel is tapped at 1600°C and poured into the mold. After solidification and cooling, the mold is knocked out, and the casting is subjected to heat treatment (austenitizing, quenching, and tempering) to achieve the required mechanical properties.
Production Verification and Quality Inspection
Several actual runner castings were produced using the above process. The following aspects were evaluated:
- Dimensional accuracy: The flow passage dimensions (including blade profile, band inner diameter, and crown outer diameter) were measured using a coordinate measuring machine (CMM). All dimensions were within the specified tolerance of ±1.5 mm. The 3D printed cores contributed significantly to achieving this precision.
- Surface quality: Visual inspection showed no burn-on sand, veining, or metal penetration. The surface roughness was less than 6.3 μm Ra on flow surfaces.
- Internal quality: Ultrasonic testing (UT) and radiographic testing (RT) were performed as per ASTM A609. No shrinkage cavities, gas pores, or inclusions larger than 2 mm were found. The defect levels met the highest quality class specified for hydro turbine runners.
- Mechanical properties: Tensile test specimens taken from the casting (at the crown and band regions) showed the following:
| Property | Measured | Specification |
|---|---|---|
| Tensile strength | 800 MPa | ≥ 760 MPa |
| Yield strength (0.2% offset) | 620 MPa | ≥ 550 MPa |
| Elongation | 18% | ≥ 15% |
| Impact toughness (KV, 20°C) | 65 J | ≥ 45 J |
| Hardness (HB) | 280 | 260–310 |
The results demonstrate that the 3D printing casting process, combined with optimized feeding and gating, consistently produces runner castings that meet or exceed all technical requirements.
Conclusion
In summary, the integral casting of a large hydraulic turbine runner using 3D printing casting technology has been successfully developed and validated. The key innovations and outcomes are:
- An optimized feeding system with a single top riser and multiple side risers, combined with tapered metal pads, eliminates shrinkage porosity even in complex spiral hot spots.
- Open, bottom-filled gating with filters ensures smooth filling and minimal slag entrapment.
- 3D printing casting technology enables the production of highly accurate sand cores (±1 mm tolerance) for the runner flow passages, reducing the need for extensive machining and improving surface quality.
- The process is cost-effective and shortens the manufacturing cycle compared to traditional fabrication methods.
- Actual production runs confirm that castings exhibit excellent dimensional accuracy, internal soundness, and mechanical properties in compliance with international standards.
The adoption of 3D printing casting technology for integral runner production represents a significant advancement in hydro turbine manufacturing, offering improved quality, reduced lead times, and enhanced reliability for critical power generation components.