I. Investment Casting: The Art of Precision Forming. Investment casting (investment casting) achieves precision forming through a composite structure of fusible molding material and refractory shell: 1. Mold Preparation: A wax-based or resin-based mold is made to resemble the part at a 1:1 scale. 2. Shell Construction: Refractory slurry (such as silica sol + corundum powder) is repeatedly applied to the surface of the mold to form 10-15 layers of shell with a thickness of 5-8mm. 3. Demolding and Firing: The molding material is removed by steam or hot oil, and the shell is strengthened by high-temperature firing at 900-1050℃. 4. Casting: Under vacuum or inert gas protection, molten titanium alloy at 1650-1750℃ is injected into the shell. This process can achieve a surface finish of Ra1.6μm, dimensional accuracy of CT4-CT5 grade, and material utilization rate is increased by more than 40% compared with traditional forging. It is particularly suitable for the production of complex structural parts such as turbine blades and artificial joints.
II. The "Material Forbidden Zone" Challenges in Titanium Alloy Casting: The chemical activity of titanium alloys in the molten state leads to three major technical challenges: 1. Interface Reaction: Interfacial diffusion occurs with common shell materials (such as SiO₂ and Al₂O₃), forming an α-case brittle layer. 2. Thermal Shock Damage: The instantaneous temperature difference of 200-300℃ during titanium molten casting causes shell cracking. 3. Gas Contamination: Adsorption of moisture and gas from the shell leads to porosity defects in the casting. Case Study: In the casting of a certain aero-engine blade, the traditional Al₂O₃ shell resulted in a 0.2mm thick α-case layer on the casting surface, requiring five additional machining processes for removal, resulting in a material loss rate as high as 35%.
III. Innovative Breakthroughs in Shell Materials Based on the characteristics of titanium alloys, researchers have developed three types of special shell material systems: 1. Carbonaceous Refractory Material System Representative Material: Artificial Graphite (Petroleum coke + pitch calcined at 2800℃) Advantages: • Refractoriness under vacuum > 2200℃ • Thermal expansion coefficient only 2.5×10⁻⁶/℃ • Strength increases with temperature (reaching 80MPa at 1000℃) Limitations: • Oxidation weight gain rate reaches 0.8mg/cm²·h (1000℃) • Thermal conductivity as high as 80W/(m·K), easily causing micro-cracks on the casting surface Improvement Solution: Using a Y₂O₃ coating on the graphite surface reduces the oxidation rate by 70%, increasing the casting qualification rate to 92%. 2. Oxide Ceramic System Material Gradient Design:
Key Parameters:
• Surface porosity < 8%
• High-temperature flexural strength > 15 MPa (1600℃)
• Thermal shock resistance (1100℃ water cooling) > 20 cycles
Application Effect: In the casting of an intermediate casing for a certain type of aero-engine, the use of this system improved the dimensional accuracy of the casting from ±0.3 mm to ±0.1 mm, and reduced the machining allowance by 60%. 3. Innovative Solution for Refractory Metal System: • Top Coating: Utilizes tungsten powder (particle size D50=5μm) + yttrium sol system, achieving a chemical stability level of 9 (0-10). • Back Layer Reinforcement: Molybdenum mesh reinforcement structure increases the shell's impact resistance by 3 times. Technical Breakthroughs: • After contacting TC4 titanium alloy at 1700℃ for 240 seconds, the interface reaction layer thickness is <15μm. • The casting surface roughness Ra is <0.8μm, achieving a mirror finish. Typical Application: Casting of artificial acetabular cups in the biomedical field, achieving "near-net-shape forming" without the need for subsequent polishing.
Titanium alloy investment casting, also known as the lost-wax casting method, involves creating a wax mold, covering it with refractory material, heating to remove the wax, pouring in molten titanium, and then cooling to obtain high-precision titanium alloy parts. This technology is particularly suitable for making complex-shaped components with high precision requirements, such as turbine blades for aero-engines and artificial joints, significantly reducing material waste and subsequent machining.
Core Process Flow:
Mold Preparation: Create a 1:1 investment mold of the part using wax or resin.
Shell Construction: Repeatedly apply refractory slurry (such as silica sol with corundum powder) to the investment mold to form a 10-15 layer shell, 5-8mm thick.
Demolding and Firing: Remove the wax mold using steam or hot oil, then fire the shell at a high temperature of 900-1050℃.
Pouring: Pour in molten titanium alloy at 1650-1750℃ under vacuum or inert gas protection.
Technical Advantages:
High Precision: Dimensional accuracy reaches CT4-CT5 level, surface finish Ra1.6μm.
High Material Utilization: Over 40% higher than traditional forging.
Suitable for Complex Parts: Capable of casting thin-walled, complex-structured parts.
Industry Applications
Aerospace: Aircraft engine components, aerospace structural parts.
Biomedical: Artificial joints, dental prostheses.
Other Fields: Ship propellers, missile structural parts, etc.
Technological Challenges and Breakthroughs
Challenges: High reactivity of molten titanium makes it prone to reacting with the mold shell to form a brittle layer, and it may also crack or develop porosity due to thermal shock.
Breakthroughs: New materials such as molybdenum mesh reinforced mold shells and oxide ceramic mold shells (such as zirconium oxide) have been developed, improving the quality and performance of castings.
Future Trends
Technology is developing towards lower cost, higher quality, and more environmentally friendly solutions. New materials and process optimization (such as digital twin simulation) are key areas, and future applications will be even broader.
