Did you know that cast titanium is so unstable that it is immune to both seawater and chemical corrosion? The properties of titanium that prevent chemical corrosion, however, are the very properties that work to confound common casting techniques.
In its pure form, titanium is so unstable that it reacts spontaneously with oxygen to form a protective layer of titanium oxide. Similar reactions occur with many of the alloyed forms of titanium, including the “bread-and-butter” of the aerospace industry: Grade 5 Ti6Al4V.
When it is heated, the titanium reaction with oxygen is so violent that it causes great challenges in the casting process. During this process, molten titanium reacts with even the smallest amount of trace oxygen present in most refractory compounds. Refractories are the materials typically used to produce investment casting molds. The extreme reactivity of titanium, combined with the ubiquitous nature of oxygen in the environment, presents a challenge for manufacturers to cast titanium effectively. We will discuss below three common techniques for producing cast titanium components: rammed graphite, metal injection molding, and investment casting.
Rammed Graphite for Cast Titanium
One method to manage titanium’s reactive properties is to produce casting molds with graphite, a chemically inert material. This particular process of minimizing mold reaction with titanium is known as rammed graphite casting. Graphite “sand” is first mixed with water and pitch syrup – amongst other binding agents – and is then tamped – or rammed – against a pattern to create a mold. The graphite mold is then subsequently dried and baked to burn off the binding materials, leaving behind a pure graphite shell. This mold shell is then placed into a vacuum environment where molten titanium is then poured into the shell. After which it solidifies and is allowed to cool. The graphite shell is then broken away, leaving the final cast titanium product.
Due to the relatively coarse texture of the graphite sand, the rammed graphite process tends to produce titanium castings with rougher surface finishes. While the rammed graphite process is relatively cost-effective for large bulky castings, it produces castings with low detail and precision. The tradeoff between precision tolerances and cost make rammed graphite an economically viable option for many industries where precise dimensional control is not required.
Metal Injection Molding for Cast Titanium
At the other end of the spectrum from the rammed graphite method is Metal Injection Molding, or MIM. Finely ground titanium powder is blended with appropriate alloying powders and mixed with a thermoplastic binder at an elevated temperature. The binder adds plasticity to the mixture, allowing it to be injected into a precision machined mold.
This injection process produces a high precision shape with tight tolerances and fine features. The molded part is then exposed to solvents and catalysts to remove the binding thermoplastics. After the binder is removed, the part is fragile, porous, and up to 40% empty space. The part is then sintered to remove this empty space by heating to almost its melting point and applying pressure. The sintering process removes the pore volume, causes the part to shrink substantially, and solidifies the molded part into a solid titanium alloy with its final shape.
The MIM process offers excellent dimensional controls and very detailed features. These benefits combined with an excellent surface finish make MIM an ideal process for small net-shape castings where precision and presentation are paramount. The multi-step MIM process involves high cost, however, and requires a significant initial investment in order to fabricate the injection molds.
Titanium Investment Casting
We have identified two extremes related to the processes to produce cast titanium: rammed graphite, with relatively low cost and precision; and Metal Injection Molding with higher precision but higher costs.
Many cast titanium applications, however, require a balance between these two extreme options. Investment casting – also known as the lost wax method – achieves an ideal balance between precision and cost. The investment casting process begins with the production of an exact wax model – or pattern – of the final product. This pattern is typically produced via either injection molding or by 3D printing. The pattern is then coated in a ceramic material to create a “shell” – or mold. The mold is placed into a furnace where the wax is melted and drained from the shell, leaving behind an empty cavity in the shape of the final casting.
The shell is then placed into a vacuum chamber, where Oxygen concentration is virtually zero. Molten titanium is then poured into the ceramic mold, which fills the shell cavity vacated. The titanium is allowed to solidify and cool, then the ceramic mold is destroyed and removed from the component casting, revealing the final cast titanium part.
While it is a less costly process relative to MIM, investment casting can produce titanium castings with reasonably tight tolerances for intricate and complex details and features. Investment casting can also produce relatively smooth surface finish.
Investment casting is a very cost-effective process to produce near-net and net shape cast titanium components. Due to its relatively high precision, it can be used to limit unnecessary machining operations and material waste. This makes investment casting an ideal process that FS Precision has practiced and improved upon for more than 50 years! Our NADCAP compliant near-net titanium investment castings represent our company’s commitment to high-quality and cost-effective titanium alloy components.
If you would like to learn more, watch our investment casting video. Or if you’d like to explore how we can assist you with your needs for cast titanium, please click on our Get A Quote button at the lower left, and send us your project details. We will send you an estimate of cost and lead time to assist you in your program development process.
Producing cast titanium components is very challenging because of its extremely unstable nature in its elemental form. In the presence of oxygen, elemental titanium will spontaneously react to form the more stable titanium oxide compounds. This reaction becomes particularly violent at elevated temperatures.
In spite of these challenges, the benefits of titanium for critical aerospace, subsea, and other applications has led to the development of a variety of methods for forming complex shapes in titanium alloys.
- Metal injection molding (MIM) is a good method for small shapes with intricate detail, high precision, and smooth surface finish such as jewelry. MIM involves a costly multi-step process with relatively high initial investment in tooling
- Rammed graphite casting is relatively low cost compared to MIM, as it does not require costly initial tooling investment. Rammed graphite, however, does not produce fine detail, high precision, or smooth surface finish.
- Investment cast titanium achieves an ideal middle ground for most aerospace, and subsea applications. Investment casting produces reasonably fine detail and tolerances at a reasonable cost.
FS Precision is one of the largest titanium casting suppliers in the world, and is an emerging global leader for structural investment castings for fixed wing, rotary wing, space launch systems. Our AS9100D certified investment casting foundries are located in the USA and Taiwan.
Learn more About FS Precision AS9100D titanium aerospace castings.