Lost-wax Casting Of Micro Gear Titanium Alloy
Miniature gears are widely used in many precision machinery fields such as aerospace, electronic equipment, and medical devices, requiring small dimensions and high precision. Titanium alloys, with their advantages of low density, high strength, and good corrosion resistance, are ideal materials for manufacturing miniature gears.
Overview of Lost-Wafer Casting of Titanium Alloy for Miniature Gears
Miniature gears are widely used in many precision machinery fields such as aerospace, electronic equipment, and medical devices, requiring small dimensions and high precision. Titanium alloys, with their advantages of low density, high strength, and good corrosion resistance, are ideal materials for manufacturing miniature gears. Lost-wafer casting (investment casting) is a process method that can achieve the manufacturing of high-precision, complex-shaped parts, making it very suitable for the production of miniature gears.
Miniature Gear Titanium Alloy Lost-Wafer Casting Process
o. Mold Design and Manufacturing: Based on the design drawings of the miniature gear, a 3D model is created using computer-aided design (CAD) software, and then a high-precision mold is manufactured through CNC machining. The dimensional accuracy and surface quality of the mold directly affect the quality of the wax model. For miniature gears, the dimensional tolerance of the mold must be controlled within an extremely small range, such as ±0.01mm or even smaller.
o. Wax Material Selection and Treatment: A suitable wax material is selected. Generally, the wax material is required to have good fluidity, low shrinkage, and moderate strength. Common wax materials include paraffin-stearic acid wax materials.
o. Wax Molding: The wax material is heated and melted to a suitable temperature to remove impurities and air bubbles, ensuring the quality of the wax model.
o. Wax Injection Molding: The molten wax is injected into the mold, filling the mold cavity under specific pressure and temperature conditions. Injection pressure and temperature must be precisely controlled based on the wax's properties and the mold's structure; for example, injection pressure may be between 0.2 and 0.5 MPa, and temperature between 50 and 70°C. After the wax cools and solidifies, the mold is opened and the wax model is removed. The wax model is then trimmed, removing burrs, flash, and other excess material, and its dimensional accuracy and surface quality are checked.
o. Module Assembly: Individual wax models are assembled onto the sprue bar using welding or bonding to form a module. The module design must consider the flow of molten metal and venting to ensure smooth filling of the cavity during casting while simultaneously expelling gases. The size and shape of the sprue bar must be rationally designed based on the size and number of the micro-gears to ensure uniform molten metal supply and flow.
o. Coating: Immerse the mold assembly in the coating, ensuring a uniform layer covers the wax model surface. The coating typically consists of refractory materials (such as silica sol, zircon powder, etc.) and a binder, and its properties directly affect the quality of the shell. Care must be taken regarding the viscosity and thickness of the coating during application; multiple layers are generally required, each with potentially different compositions and particle sizes to create shell layers with varying properties.
o. Sanding: Immediately after coating, immerse the mold assembly in sand, allowing the sand to adhere to the coating surface. The particle size and material of the sand are selected based on the different shell layers, generally progressing from coarse to fine. Sanding increases the shell's strength and permeability.
o. Drying and Hardening: After each coating and sanding process, the mold assembly needs to be dried and hardened to solidify the binder in the coating, forming a robust shell. The drying and hardening time and conditions depend on the type of coating and environmental factors such as temperature and humidity, generally ranging from several hours to several days. o Dewaxing: The prepared mold shell is placed in a dewaxing device, where heating melts the wax model and allows it to flow out of the shell. Dewaxing methods include hot water dewaxing and steam dewaxing. The dewaxing temperature and time must be controlled according to the characteristics of the wax and the structure of the mold shell to ensure complete removal of the wax model without damaging the shell.
o Firing: The dewaxed mold shell is placed in a firing furnace for high-temperature firing to remove residual wax and moisture, improving the shell's strength and refractoriness. The firing temperature is generally between 800-1100℃, and the firing time depends on the size and thickness of the shell, typically 1-3 hours.
o Titanium Alloy Melting: Suitable titanium alloy raw materials are selected and batched according to alloy composition requirements. Vacuum induction melting and other methods are used to melt the titanium alloy raw materials. During the melting process, parameters such as melting temperature, time, and vacuum degree must be strictly controlled to ensure the uniformity and purity of the alloy composition. The melting temperature is generally between 1600 and 1800℃, and the vacuum degree needs to reach 10⁻³ - 10⁻⁴ Pa.
o. Pouring: The molten titanium alloy is poured into the mold shell under specific temperature and pressure conditions. Excessive pouring temperature will damage the mold shell and cause alloy element loss, while insufficient temperature will reduce the fluidity of the molten metal, affecting the filling quality. The pouring pressure and speed need to be precisely controlled according to the structure of the mold shell and the size of the micro gear to ensure that the molten metal can smoothly fill the cavity and form a complete micro gear.
o. Shell Removal: After pouring, once the casting has cooled to room temperature, the mold shell is removed using methods such as mechanical vibration or sandblasting. Care must be taken to avoid damaging the casting during the shell removal process.
o. Gate Cutting: The casting is cut off from the sprue bar, removing excess parts such as the sprue and riser. During cutting, the cut surface must be smooth and clean to avoid damaging the gear.
o. Heat Treatment: The cut micro-gears undergo heat treatment to improve their microstructure and properties. Common heat treatment processes include solution treatment and aging treatment. The temperature, time, and cooling rate of the heat treatment must be precisely controlled according to the composition and performance requirements of the titanium alloy.
o. Finishing: The micro-gears are finished using grinding, lapping, and other machining methods to improve their dimensional accuracy and surface quality. The precision requirements for finish machining are very high; dimensional tolerances may be controlled within ±0.005mm, and the surface roughness reaches Ra0.4 - Ra0.8μm.
o. Inspection: The finished micro-gears undergo comprehensive inspection, including dimensional accuracy, shape accuracy, surface quality, hardness, and metallographic structure. Inspection methods include coordinate measuring machine (CMM), microscopic observation, and hardness testing. Only products that pass inspection can proceed to the next process or be delivered for use.
Advantages of Lost-Wafer Casting of Titanium Alloy for Micro-Gears
High Precision
Lost-wafer casting can accurately replicate the shape and size of the wax model. For high-precision parts like micro-gears, it can achieve very high dimensional and shape accuracy, meeting the requirements for use in precision machinery.
Complex Shape Manufacturing
Lost-wax casting can produce miniature gears with complex tooth profiles and structures, shapes that are difficult to achieve using other machining methods.
High Material Utilization
Compared to traditional machining methods, lost-wax casting reduces material removal, increases material utilization, and lowers production costs.
Good Surface Quality
The cast miniature gears have a low surface roughness, reducing subsequent finishing work and improving production efficiency.
Challenges and Solutions in Lost-Wax Casting of Miniature Gears from Titanium Alloys
Difficulty in Titanium Alloy Melting and Casting
Titanium alloys are highly chemically reactive and easily react with elements such as oxygen and nitrogen in the air during melting and casting, affecting the alloy's properties. The solution is to use vacuum melting and casting equipment, strictly controlling the vacuum level during melting and casting to prevent titanium alloys from contacting air.
Compatibility Issues Between the Mold and Titanium Alloy
Titanium alloys can chemically react with the mold material at high temperatures, affecting the surface quality and performance of the casting. It is necessary to select suitable mold materials and coatings, optimize the mold manufacturing process, and improve the compatibility between the mold and the titanium alloy.
Deformation and cracking issues in micro gears
Due to their small size and complex structure, micro gears are prone to deformation and cracking during the casting process. This can be prevented by optimizing casting process parameters such as pouring temperature, pouring speed, and cooling rate, as well as by rationally designing the gating and riser system and mold structure, thereby reducing thermal stress and deformation in the castings.
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