We have successfully developed a high precision investment casting process for a large integral nozzle ring used in turbochargers. The nozzle ring is a critical component that converts high-temperature, high-pressure exhaust gas into rotational kinetic energy. Traditionally, welded structures were employed, but they suffered from multiple machining steps, significant welding deformation, and low overall strength. To overcome these limitations, we adopted high precision investment casting to produce the entire ring as a single monolithic part made of G-X40CrNiSi22.9 heat-resistant stainless steel. The overall dimensions of the casting are Ø656 × 112 mm, with a weight of 63 kg for the finished part and 150 kg for the poured system. This paper details our methodology, challenges encountered, and innovative solutions that ensured the success of this high precision investment casting project.
In high precision investment casting, every step from wax pattern fabrication to shell mold construction and pouring must be meticulously controlled. Our nozzle ring consists of three distinct segments: an outer ring, 24 evenly spaced nozzle vanes, and an inner ring. The outer ring has a near-rectangular cross-section revolved around the central axis; the vanes are extruded airfoil profiles; and the inner ring features an L-shaped cross-section with mounting lugs. The entire wax assembly must be built to tight dimensional tolerances, and the shell mold must withstand high centrifugal forces during casting. We will describe each stage of the process and highlight the key innovations that made this high precision investment casting feasible.
Wax Pattern Fabrication and Mold Challenges
For high precision investment casting, the wax patterns must be dimensionally stable and free of defects. We used medium-temperature wax for all patterns. The outer ring wax pattern was produced by gravity casting using a dedicated mold. The 24 nozzle vanes were individually injected in a wax injection machine (suitable for small patterns). However, the inner ring wax pattern mold had an outer dimension of 600 mm, while the distance between the tie bars of our standard wax injection machine was only 580 mm. This discrepancy made it impossible to mount the inner ring mold between the tie bars in the conventional manner.
To solve this problem, we devised a temporary fixture that allowed us to separate the clamping and injection functions. We placed a “dummy mold” (any mold smaller than the tie bar spacing) on the injection machine platen to satisfy the machine’s closing cycle. Then we extracted the injection nozzle and fixed it vertically above the inner ring mold using a custom support fixture, as illustrated conceptually below. By pressing the “close” button, the machine performed the clamping action on the dummy mold, while we manually aligned the injection nozzle onto the inner ring mold. The injection parameters were adjusted to ensure proper fill. This creative adaptation enabled us to produce the inner ring wax pattern without investing in a larger injection machine, thereby saving significant capital while maintaining the quality required for high precision investment casting.
| Pattern Component | Method | Mold Outer Dimension (mm) | Injection Machine Tie Bar Spacing (mm) | Solution |
|---|---|---|---|---|
| Outer ring | Gravity casting liquid wax | ~700 | N/A (gravity) | Standard mold |
| Nozzle vane (×24) | Injection molding | Small | 580 | Standard injection |
| Inner ring | Injection molding | 600 | 580 | Temporary fixture + dummy mold |
After producing all individual wax patterns, we assembled them into a complete wax cluster. The assembly sequence required precise positioning: the outer ring and inner ring were aligned concentrically, and the 24 vanes were inserted and welded at the correct angular spacing. A central sprue with a diameter of 80 mm was attached to the back of the inner ring flange, and a circumferential runner (60 mm wide, 10 mm high) provided feeding channels. The total wax assembly, including a 140 mm high riser, weighed approximately 63 kg (wax weight). This assembly was the master pattern for the subsequent shell mold.
Shell Mold Construction and Reinforcement
In high precision investment casting, the shell mold must possess sufficient strength and refractoriness to withstand the high pouring temperature (≥1580 °C) and the centrifugal forces during casting. Our standard shell mold building procedure uses a multiple-coat system: a primary coat of fused alumina powder with fused alumina sand, a transition coat of fused alumina powder with silica sand, and subsequent backup coats of silica powder with silica sand. For this large nozzle ring, we applied 10 layers total. However, due to the large diameter and complex geometry, we anticipated that the unsupported shell might crack or deform during the two-stage firing process (550 °C for 1 hour, then 850 °C for 2 hours) and especially during centrifugal pouring.
To reinforce the shell, we introduced a simple yet effective technique: after completing the transition coat, we wrapped the outer circumference of the shell with thin steel wire (less than 1 mm diameter) in a helical pattern. Then we continued with the backup coats, embedding the wire within the ceramic layers. This steel wire acts much like rebar in concrete, providing tensile strength and preventing crack propagation. The final shell thickness was uniform and the wire reinforcement significantly improved the shell’s structural integrity. The table below summarizes the shell building parameters.
| Layer | Description | Refractory Powder | Refractory Sand | Remarks |
|---|---|---|---|---|
| 1 (Prime) | Initial coat | Fused alumina | Fused alumina (fine) | – |
| 2 (Transition) | Second coat | Fused alumina | Silica sand | – |
| 3 | First backup | Silica powder | Silica sand | After this layer, apply steel wire wrap |
| 4–10 | Backup coats | Silica powder | Silica sand | Wire embedded |
The two-stage firing cycle ensured complete dewaxing and sintering of the shell. The low-temperature stage (550 °C) slowly removed the wax without cracking, and the high-temperature stage (850 °C) fully sintered the ceramic. The shell was removed from the furnace just before pouring, still hot (red-hot), to minimize thermal shock.
Melting and Centrifugal Casting
The nozzle ring alloy G-X40CrNiSi22.9 is a heat-resistant stainless steel with high chromium and nickel content. We melted the charge in a 150 kg medium-frequency induction furnace lined with magnesia (basic) refractory. Strict adherence to the charge formula ensured the target composition. The pouring temperature was controlled not lower than 1580 °C to guarantee fluidity for the thin trailing edges of the vanes.
Gravity casting alone would have resulted in incomplete filling of the airfoil sections because the vanes have a very thin trailing edge (about 1.5 mm). Therefore, we adopted centrifugal casting. The shell mold was placed on a rotating table, and the molten metal was poured into the central riser while the mold rotated at 2 revolutions per second (2 rps). The centrifugal force drove the metal radially outward, ensuring that all 24 vanes filled completely and the outer ring was sound. The rotational speed was chosen based on the following analysis.
The centrifugal pressure generated at a radius \( r \) is given by:
$$ p = \frac{1}{2} \rho \omega^2 r^2 $$
where \( \rho \) is the density of liquid steel (~7.0 g/cm³ or 7000 kg/m³), \( \omega \) is the angular velocity in rad/s, and \( r \) is the radius. With \( \omega = 2\pi \times 2 \text{ rps} = 4\pi \text{ rad/s} \approx 12.57 \text{ rad/s} \), and the maximum radius of the outer ring \( r_{\text{max}} = 0.328 \text{ m} \), the centrifugal pressure at the outer periphery is:
$$ p_{\text{max}} = \frac{1}{2} \times 7000 \times (12.57)^2 \times (0.328)^2 \approx 0.5 \times 7000 \times 158.0 \times 0.1076 \approx 59500 \text{ Pa} \ (0.595 \text{ bar}) $$
This pressure, though modest, was sufficient to overcome capillary resistance in the thin vanes and ensure complete filling. The table below lists the casting parameters.
| Parameter | Value |
|---|---|
| Melting furnace capacity | 150 kg (medium frequency) |
| Lining material | Magnesia (basic) |
| Pouring temperature | ≥1580 °C |
| Rotational speed | 2 rps (≈12.57 rad/s) |
| Pouring method | Centrifugal via central sprue |
| Total poured weight | 150 kg |
| Finished casting weight | 63 kg |
| Alloy grade | G-X40CrNiSi22.9 |
After solidification and cooling, the ceramic shell was removed, and the riser was cut off by turning on a lathe. The final casting was subjected to chemical analysis, mechanical testing, dimensional inspection, and surface examination. The chemical composition met the specification, as shown in the typical analysis below.
| Element | C | Si | Mn | Cr | Ni | P max | S max |
|---|---|---|---|---|---|---|---|
| Specification | 0.30–0.50 | 0.30–1.50 | ≤1.00 | 22.0–24.0 | 8.0–10.0 | 0.035 | 0.030 |
| Actual | 0.38 | 1.02 | 0.65 | 23.1 | 9.2 | 0.020 | 0.015 |
Results and Quality Verification
Every casting produced by this high precision investment casting process was inspected to ensure it met the stringent requirements of turbocharger operation. The mechanical properties were evaluated using separately cast test bars (poured with the same batch). Typical values are given below.
| Property | Value |
|---|---|
| Yield strength \( R_{p0.2} \) | ≥ 450 MPa |
| Tensile strength \( R_m \) | ≥ 650 MPa |
| Elongation \( A \) | ≥ 10% |
| Hardness | 200–260 HB |
Dimensional inspection of the nozzle ring showed that all vane profiles were within tolerance, the outer and inner ring concentricity was within 0.3 mm, and the angular spacing of the 24 vanes was uniform. No porosity or shrinkage defects were detected by X-ray inspection. The surface finish was superior to that of welded counterparts, requiring minimal machining. The integral construction eliminated weld joints, thus improving fatigue strength and resistance to thermal cycling.
Discussion and Lessons Learned
This project demonstrated several key lessons for high precision investment casting of large annular components:
- Adaptive tooling: When standard wax injection machines cannot accommodate large molds, a temporary fixture with a dummy mold can be a cost-effective solution. This approach allowed us to produce the inner ring pattern without purchasing new equipment, a major advantage for low-volume production.
- Shell reinforcement: Wrapping the shell with thin steel wire is a simple and reliable method to enhance strength, especially for large-diameter shells subjected to centrifugal forces. The wire must be fine enough to be fully embedded in the ceramic layers without causing cracking during firing.
- Centrifugal casting: For thin-walled airfoil sections, centrifugal casting ensures complete filling. The rotational speed of 2 rps proved adequate; higher speeds might risk mold rupture or excessive segregation.
- Process reliability: By combining these innovations, we achieved a high yield rate for the initial production run. The overall success of this high precision investment casting confirms that even oversized wax patterns can be handled with creativity and rigorous process control.
In high precision investment casting, every detail matters. From the wax injection parameters to the firing schedule, from the shell coating viscosity to the pouring temperature, all variables must be optimized. Our experience with the large nozzle ring provides a template for other large, circular castings that exceed conventional equipment limits. The methodology is particularly valuable for yearly low-volume requirements where dedicated large injection machines are not justified.
Conclusion
We have successfully implemented high precision investment casting for a large integral nozzle ring with an outer diameter of 656 mm and a height of 112 mm, composed of G-X40CrNiSi22.9 heat-resistant steel. By temporarily adapting the wax injection machine with a dummy mold and a vertical injection fixture, we overcame the size limitation of the inner ring pattern. Shell mold strength was enhanced by embedding fine steel wire in the backup coats. Centrifugal casting at 2 rps ensured complete fill of the thin vane sections. The final castings met all dimensional, chemical, and mechanical specifications, outperforming traditional welded designs. This case study demonstrates that high precision investment casting can be extended to large components without prohibitive capital investment, as long as creative process modifications are applied. The lessons learned will guide future developments in high precision investment casting for turbocharger and similar applications.