As a researcher in advanced manufacturing, I have extensively studied the application of lost wax investment casting for producing high-performance titanium alloy components. Titanium alloys are renowned for their low density, high specific strength, excellent corrosion resistance, low thermal conductivity, and small linear expansion coefficient, making them ideal for aerospace, marine, and chemical industries. However, the high production costs often limit their widespread use. Lost wax investment casting, as a near-net-shape forming technique, minimizes material waste and reduces manufacturing expenses, particularly for complex parts like impellers with intricate curved blades. In this article, I will detail the process design, implementation, and results of using lost wax investment casting to fabricate a ZTC4 titanium alloy impeller, emphasizing key aspects such as wax pattern design, gating system optimization, and quality control.
The lost wax investment casting process begins with the creation of a precise wax pattern. For the impeller with complex曲面 blades, we employed 3D printing to fabricate the wax model, accounting for linear shrinkage that varies across the structure. The shrinkage ratio in lost wax investment casting typically ranges from 1.0% to 2.0%, but due to the impeller’s geometry, we implemented a segmented approach. Specifically, the shrinkage scale was set at 1.0% near the blade roots and gradually increased to 2.0% towards the distal ends. This ensures dimensional accuracy after solidification. The linear shrinkage of titanium alloy, generally between 0.5% and 0.8%, was compensated by applying reverse deformation corrections. We assigned a correction of 0.8% at the proximal regions, decreasing to 0.5% distally, with thicker sections adjusted to 1.0% to counteract non-uniform cooling and deformation. The relationship for shrinkage compensation can be expressed as: $$ \Delta S = S_0 \times (1 + k_s) $$ where $$ \Delta S $$ is the adjusted dimension, $$ S_0 $$ is the original size, and $$ k_s $$ is the shrinkage factor, which varies segmentally. This meticulous design in lost wax investment casting guarantees that the final shell dimensions align with the intended specifications, crucial for achieving the desired水流线 profile in the blades.
To further illustrate the wax pattern design parameters, consider the following table summarizing the segmented shrinkage and correction factors applied in the lost wax investment casting process:
| Region | Shrinkage Scale (%) | Reverse Deformation Correction (%) |
|---|---|---|
| Blade Proximal | 1.0 | 0.8 |
| Blade Mid-section | 1.5 | 0.65 |
| Blade Distal | 2.0 | 0.5 |
| Thick Sections | 1.8 | 1.0 |
The gating and riser system in lost wax investment casting plays a pivotal role in ensuring defect-free castings. For this impeller, we adopted a top-pouring approach with a annular gating system to facilitate stable filling and directional solidification. By dispersing five inner gates, we enhanced the upward flow of molten alloy, reducing turbulence and minimizing defects like cold shuts, porosity, and shrinkage. The design promotes the accumulation of impurities and gases in the risers, located at the blade tips and central hub. Additionally, vent holes were strategically placed from proximal to distal regions to improve mold venting. The fluid dynamics during filling can be modeled using the Bernoulli equation: $$ P + \frac{1}{2} \rho v^2 + \rho g h = \text{constant} $$ where $$ P $$ is pressure, $$ \rho $$ is density, $$ v $$ is velocity, and $$ h $$ is height. This equation helps optimize the gating design in lost wax investment casting to maintain laminar flow and prevent oxidation. The table below outlines the key gating system parameters used in this lost wax investment casting process:
| Parameter | Value |
|---|---|
| Pouring Position | Vertical Top-Pour |
| Number of Inner Gates | 5 |
| Riser Type | Annular and Open |
| Vent Holes | Distributed Along Blades |
Shell building is a critical phase in lost wax investment casting, especially for reactive metals like titanium. We selected yttria-stabilized zirconia as the face coat material due to its high chemical inertness, paired with a zirconium acetate binder. The backup layers consisted of alumino-silicate materials and silica sol, reinforced with mullite sand. After coating, the shell was dried and dewaxed using hot water, followed by firing in a reducing atmosphere at 900–950°C for 2–2.5 hours to carbonize the binder and enhance strength. Post-firing, the shell underwent vacuum degassing at 950–1000°C with a vacuum level below $$ 5 \times 10^{-2} \, \text{Pa} $$, held for 2–2.5 hours to remove residual gases. This rigorous shell preparation in lost wax investment casting ensures minimal reaction with the molten titanium, preserving the alloy’s integrity. The thermal treatment can be described by the Arrhenius equation for diffusion: $$ D = D_0 \exp\left(-\frac{Q}{RT}\right) $$ where $$ D $$ is the diffusion coefficient, $$ D_0 $$ is a constant, $$ Q $$ is activation energy, $$ R $$ is the gas constant, and $$ T $$ is temperature. This governs the removal of volatiles during shell processing.
Melting and pouring were conducted in a vacuum consumable electrode arc skull furnace to handle titanium’s high reactivity. We used secondary ingots as electrodes and maintained precise parameters to achieve a sound casting. The melting process involved an initial arc strike current of 1000–1500 A, preheating at 3000–5000 A for 60–80 seconds, and a main melting current of 28–36 kA with a voltage of 34–41 V. The vacuum was stabilized at 3.0–5.0 Pa, and the crucible cooling water temperature was kept below 45°C to prevent thermal shock. The total melting time ranged from 45 to 60 minutes, ensuring thorough homogenization. The energy input during melting can be approximated by: $$ E = I \times V \times t $$ where $$ E $$ is energy, $$ I $$ is current, $$ V $$ is voltage, and $$ t $$ is time. This controlled environment in lost wax investment casting minimizes contamination and defects. The table below summarizes the melting parameters employed:
| Parameter | Value |
|---|---|
| Arc Strike Current (A) | 1000–1500 |
| Preheat Current (A) | 3000–5000 |
| Preheat Time (s) | 60–80 |
| Melting Current (kA) | 28–36 |
| Melting Voltage (V) | 34–41 |
| Vacuum Level (Pa) | 3.0–5.0 |
| Melting Time (min) | 45–60 |
| Crucible Temperature (°C) | ≤45 |
After casting, the impeller underwent post-processing and rigorous quality assessments. We performed hot isostatic pressing (HIP) at 920±10°C under 100–140 MPa for 2–2.5 hours to heal internal voids, followed by furnace cooling. Surface inspection via visual and dye penetrant testing revealed no cracks, cold shuts, or flow marks, with a surface roughness of 3.2 μm, exceeding the required 6.3 μm. Dimensional checks confirmed compliance with CT9 level tolerances per GB/T6414-1999; for instance, the designed size of (290±3.2) mm measured 292.8 mm post-casting. Internal quality was evaluated using X-ray radiography, showing no defects such as porosity, shrinkage, or inclusions, meeting GJB2896A-2007 C级 standards. Chemical composition analysis from samples taken at the blade distal, proximal, and hub regions aligned with ZTC4 specifications, as detailed in the table below. The homogeneity can be expressed using the mixing index: $$ M = 1 – \frac{\sigma}{\bar{c}} $$ where $$ \sigma $$ is the standard deviation of composition and $$ \bar{c} $$ is the mean concentration. This underscores the effectiveness of lost wax investment casting in producing consistent alloys.
| Sample Location | Al (%) | V (%) | Fe (%) | Si (%) | C (%) | N (%) | H (%) | O (%) | Ti (Balance) |
|---|---|---|---|---|---|---|---|---|---|
| Blade Distal | 5.98 | 4.08 | 0.17 | 0.08 | 0.069 | 0.022 | 0.0078 | 0.11 | Remaining |
| Blade Proximal | 6.01 | 4.10 | 0.19 | 0.09 | 0.070 | 0.020 | 0.0080 | 0.12 | Remaining |
| Center Hub | 5.99 | 4.09 | 0.18 | 0.08 | 0.071 | 0.021 | 0.0079 | 0.13 | Remaining |
| Standard Range | 5.5–6.8 | 3.5–4.5 | ≤0.30 | ≤0.15 | ≤0.10 | ≤0.05 | ≤0.015 | ≤0.20 | Remaining |
In conclusion, the lost wax investment casting process demonstrated exceptional capability in manufacturing complex titanium alloy impellers. By integrating segmented wax pattern scaling and reverse deformation corrections, we achieved precise dimensional control. The top-pouring gating system with dispersed inner gates ensured stable filling and high internal quality. Shell construction with advanced refractory materials and vacuum melting parameters further contributed to the success. This approach not only reduced costs and production cycles compared to traditional machining but also yielded components with superior surface finish, mechanical properties, and metallurgical integrity. The lost wax investment casting method proves to be a robust solution for aerospace and marine applications, highlighting its potential for future advancements in near-net-shape manufacturing of high-performance alloys.