Micro Turbine Titanium Alloy Lost Casting
Micro-turbines have wide applications in aerospace, energy, and other fields. Their complex shapes and high precision requirements make manufacturing processes quite challenging. Titanium alloys, due to their low density, high strength, and good corrosion resistance, are ideal materials for manufacturing micro-turbines.
Overview of Lost-Wafer Casting of Titanium Alloy for Micro-Turbines
Micro-turbines have wide applications in aerospace, energy, and other fields. Their complex shapes and high precision requirements make manufacturing processes quite challenging. Titanium alloys, due to their low density, high strength, and good corrosion resistance, are ideal materials for manufacturing micro-turbines. Lost-wafer casting is a precision casting process, well-suited for manufacturing parts with complex shapes and high precision requirements, especially micro-turbines.
Process Steps of Lost-Wafer Casting of Titanium Alloy for Micro-Turbines
o. Mold Design: First, based on the micro-turbine design drawings, a 3D model is created using computer-aided design (CAD) software to determine the precise dimensions and shape of the wax model. Considering factors such as shrinkage during the casting process, the model dimensions are appropriately modified.
o. Mold Manufacturing: Using CNC machining or other methods, the designed model is transformed into an actual mold. Mold materials are typically chosen from aluminum alloys to ensure sufficient strength and precision.
o. Wax Injection: The wax is heated to an appropriate temperature to achieve good fluidity. Then, the wax material is injected into the mold using an injection molding machine. It is held under pressure and temperature for a period of time to ensure the wax fully fills the mold cavity. After the wax cools and solidifies, the mold is opened and the wax model is removed.
o Wax Model Finishing: The removed wax model is inspected and finished, removing excess flash, burrs, etc., to ensure the dimensional accuracy and surface quality of the wax model meet the requirements.
o Selecting a Gating System: Based on the shape, size, and weight of the micro turbine, a suitable gating system is selected, including sprues, runners, and ingates. The gating system design must ensure that the molten metal fills the cavity smoothly and evenly, avoiding defects such as turbulence and air entrapment.
o Welding Wax Models: Multiple wax models are connected to the gating system by welding to form a complete module. During welding, it is essential to ensure a firm connection and good seal between the wax model and the gating system to prevent leakage during subsequent shell-making processes.
o. Applying the Top Coating: The mold assembly is immersed in a specially formulated top coating, ensuring a uniform coating layer on the wax model surface. The top coating typically consists of refractory materials (such as zircon powder), binders (such as water glass or silica sol), and additives. Its fine particle size ensures the surface quality of the casting. After coating, a layer of fine sand is sprinkled on the mold assembly surface to increase the coating thickness and strength.
o. Drying and Hardening: The mold assembly with the top coating is placed in a drying chamber for drying and hardening under specific temperature and humidity conditions. The drying and hardening time depends on the type of coating and environmental conditions, generally ranging from several hours to tens of hours.
o. Applying the Back Coating: After the top coating has dried and hardened, the back coating is applied sequentially. The back coating contains coarser-grained refractory materials, primarily serving to increase the strength and rigidity of the shell. The method for applying the back coating is similar to that for the top coating. After each layer of coating, sanding and drying/hardening are required. Generally, multiple layers of back coating are needed until the mold shell reaches a sufficient thickness.
o Steam Dewaxing: The prepared mold shell is placed in a dewaxing kettle, and high-pressure steam is introduced, causing the wax model to melt and flow out of the mold shell. The advantages of steam dewaxing are its fast speed, high efficiency, and minimal damage to the mold shell.
o Hot Water Dewaxing: The mold shell can also be placed in hot water, causing the wax model to melt and float on the surface, thus achieving dewaxing. Hot water dewaxing equipment is simple and inexpensive, but the dewaxing time is longer and it is prone to problems such as mold shell cracking.
o Heating Stage: The dewaxed mold shell is placed in a firing furnace and heated slowly at a controlled rate. This allows the moisture and residual wax in the mold shell to fully evaporate, while simultaneously causing a chemical reaction in the binder, improving the strength and refractoriness of the mold shell. The heating rate should not be too rapid to avoid cracking of the mold shell due to excessive thermal stress.
o Holding stage: After the furnace temperature reaches the predetermined firing temperature, maintain it for a period of time to allow the mold shell to fully sinter. The firing temperature and holding time depend on the material of the mold shell and the requirements of the casting. Generally, the firing temperature is between 800-1200℃, and the holding time is 1-3 hours.
o Cooling stage: After firing, slowly reduce the furnace temperature to allow the mold shell to cool gradually. The cooling rate should not be too rapid to avoid cracking of the mold shell.
o Titanium alloy melting: Place the titanium alloy raw material into melting equipment such as a vacuum induction furnace and melt it under vacuum or inert gas protection. During the melting process, strictly control the temperature, time, and alloy composition to ensure the quality of the titanium alloy.
o Casting: After the titanium alloy has been melted to a suitable temperature and fluidity, pour it into the preheated mold shell. During pouring, pay attention to the pouring speed and method to avoid molten metal impacting the mold shell, which could damage the shell or cause defects in the casting.
o Sand Removal: After the casting has cooled and solidified, remove the mold shell and sand core using methods such as vibration sand removal or shot blasting to expose the casting surface.
o Gating Cutting: Use cutting equipment to separate the casting from the gating system.
o Heat Treatment: According to the performance requirements of the titanium alloy, perform appropriate heat treatment on the casting, such as solution treatment and aging treatment, to improve the strength, hardness, and toughness of the casting.
o Machining: Perform necessary machining on the casting, such as turning, milling, and grinding, to achieve the dimensional accuracy and surface roughness required by the design.
o Inspection: Conduct a comprehensive inspection of the casting, including dimensional measurement, visual inspection, metallographic analysis, and non-destructive testing, to ensure that the casting quality meets standards and usage requirements.
Advantages of Lost-Wafer Casting of Micro-Turbine Titanium Alloys
High Precision
Lost-wafer casting can accurately replicate the shape and size of the wax model, enabling the manufacture of complex, high-precision micro-turbines that meet stringent requirements for dimensional and shape accuracy.
Good Surface Quality
Due to the fine particle size of the coating on the mold shell, the surface roughness of the casting is low, resulting in better surface quality and reducing the workload of subsequent machining.
High Material Utilization
Lost-wafer casting allows for precise design of the gating system and wax model based on the shape and size of the part, reducing metal waste and improving material utilization.
Suitable for Mass Production
By manufacturing molds and modules, mass production of micro-turbines can be achieved, improving production efficiency and reducing production costs.
Challenges of Lost-Wafer Casting of Micro-Turbine Titanium Alloys
Difficulty in Melting and Casting Titanium Alloys
Titanium alloys have high chemical reactivity and easily react with elements such as oxygen and nitrogen in the air during melting and casting, forming impurities such as oxides and nitrides, which affect the performance of the castings. Therefore, melting and casting must be carried out under vacuum or inert gas protection, placing high demands on equipment and processes.
Difficulty in shell quality control
The quality of the shell directly affects the quality of the casting. The shell's strength, permeability, and thermal stability require strict control. During shell making, factors such as coating formulation, coating process, and drying and hardening conditions all influence the shell's quality, necessitating precise process control.
Casting defect control
Porosity, shrinkage cavities, and cracks are prone to occur during the lost-wax casting of micro-turbine titanium alloys, requiring effective control measures. For example, optimizing the gating system design, controlling melting and pouring temperatures, and improving the shell's permeability can reduce casting defects.
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