What Is The Investment Casting Technology Of Titanium Alloys?

Titanium alloys feature high chemical activity, poor machinability, and difficulty in forming complex structural components via traditional casting methods. Investment casting technology of titanium alloys has overcome this challenge, becoming the mainstream forming method for complex and precision titanium alloy components, and promoting the extensive application of titanium alloys in various high-end fields.

 

Image source: JC Casting《What is Titanium Investment Casting》

 

It is a process to fabricate a wax pattern with the same shape as the target component and then coat it with refractory materials to form a mold shell. The next step is melting and removing the wax pattern through high-temperature roasting, pouring molten titanium alloy into the mold shell in a specific environment. Then it can obtain precision castings after cooling, solidification, shell breaking, and subsequent treatments.

 

Wax Pattern Preparation

• First, numerical simulation software is used to optimize the preparation process. The wax pattern and mold models are designed according to the 3D model of the casting, and process parameters are set to predict and avoid potential defects.

 

• Then, molten wax is injected into the metal mold. The initial wax pattern is taken out after cooling. Wax patterns with surface defects are repaired, polished, cleaned, and dried as needed to ensure smooth surfaces and precise dimensions. In recent years, additive manufacturing technology has been applied to wax pattern preparation, which not only greatly shortens the mold development cycle but also enables the rapid forming of wax patterns with complex structures.

Mold Shell Preparation

• During preparation, the qualified wax pattern with a gating system is repeatedly dipped into zirconia or alumina-based refractory slurries, followed by sanding and drying to form a mold shell with 4–10 layers. High-end castings require three layers of special mold shells to enhance structural strength.

 

• After drying, the mold shell is roasted at a high temperature of 870–980℃ to completely remove wax and water, and sinter the mold shell into a ceramic structure. This process improves the strength, air permeability, and dimensional stability of the mold shell. The mold shell materials must be strictly matched to the characteristics of titanium alloys to avoid casting defects such as inclusions and blowholes caused by high-temperature chemical reactions.

Melting and Pouring

• Titanium alloys has extremely high chemical activity at high temperatures, so melting and pouring must be carried out in a vacuum environment with a limited pressure or under argon protection. This prevents reactions between the alloy melt and air, crucibles, or mold shells to ensure the purity and mechanical properties of the castings.

 

• Melting adopts vacuum induction or plasma arc melting technology. Titanium alloy ingots are heated to achieve complete melting and homogenization, removing impurities and gases.

 

• Pouring requires precise control of temperature, speed, and mold filling pressure. Numerical simulation is used to optimize the gating system, predict and avoid mold filling defects, ensure stable and complete filling of the mold cavity with the alloy melt, and prevent issues such as misruns, shrinkage cavities, and cracks.

 

• After pouring, the casting must be cooled for no less than 2 hours. Controlled cooling rate reduces internal stress and avoids deformation and cracking.

Post-treatment

• Shell Breaking and Component Extraction: The mold shell is broken via high-pressure water jetting or vibration. The casting is taken out, and the gating system is cut off.

 

• Surface Treatment: The residual shell and oxide scale are removed via primary sandblasting with 30–80 mesh sand. The casting is pickled in a mixed solution of nitric acid and hydrofluoric acid to remove a surface layer with a thickness of ≤5 mm. Secondary multi-stage sandblasting is performed with 80–250 mesh sand, followed by polishing to achieve the designed surface roughness. High-end castings undergo additional surface activation treatment to improve coating compatibility.

 

• Performance Improvement: Heat treatment, hot isostatic pressing, and other processes are applied according to the application requirements of the casting. It can eliminate internal stress, refine grains, and enhance strength, toughness, and corrosion resistance. Some castings are subject to non-destructive testing to detect internal hidden defects, ensuring operational reliability.

 

• It offers high forming precision and excellent surface quality. The dimensional tolerance of castings can be controlled within ±0.1 mm, and the surface roughness reaches Ra1.6–6.3 μm. This greatly reduces the amount of subsequent machining, even enabling near-net shape forming, thus lowering manufacturing costs and material waste.

 

• It is very suitable for forming difficult-to-machine structural components such as complex curved surfaces, thin-walled parts, and small cavities. It avoids the negative impacts of processes like welding and brazing on the strength and integrity of castings.

 

Ruihang Group mainly produces the raw materials for your precision manufacturing. For more details,please reach us to the email:Sam.Rui@bjrh-titanium.com