Investment Casting of Titanium Alloys
Investment casting is the proven casting method of choice to ensure the highest possible quality of end product by using a shell system to help control the surface finish and detailing of the casting.
There are a wide range of titanium cast alloys to choose from however depending the application it important to consider the alloy choice carefully.
The main difficulty with casting titanium alloys is their reactivity with common elements in air like oxygen and nitrogen.
The investment casting process is the only metal casting process that can produce complicated shapes in high temperature alloys to a very high quality standard.
The shell system and the shell technology, including the control, maintenance, composition and operation, have the biggest effect on the quality of the casting. The shell system comprises a face coat system and a back-up system to produce the shell mould. Apart from the quality of the wax pattern, the composition of the slurry and the stucco’s used for the shell manufacture as well as the physical properties of the slurries, controls the surface finish of the casting and the definition of the features of the casting.
Titanium castings are successfully being implemented as cost-effective alternatives to forged and wrought products for high performance and increasingly cost-sensitive applications such as military and commercial air craft airframe structures. In some instances, these castings have been produced for half the cost of comparable forged and machined parts. For most of the last two decades, investment casting has been the preffered processing route for sophisticated titanium castings.
Table 1: Examples of cast titanium alloys
Table 2: Typical mechanical properties of cast titanium alloys
Figure 1: Various investment cast parts for applications in the low temperature section of a gas turbine engine
The most widely used titanium alloy nowadays is Ti-6Al-4V. This alloy possesses an excellent combination of strength, toughness and good corrosion resistance and finds application in aerospace, pressure vessels, aircraft compressor blades and discs, surgical implants etc. Aluminum stabilizes the hexagonal close-packed (hcp) α phase, and vanadium, being body-centered cubic (bcc), stabilizes the β phase. Because of high melting point and excessive reactivity of the melt with crucibles, melting and pouring of titanium alloys have to be performed under vacuum. Due to the high cost of titanium, the use of net-shape or near-net-shape technologies receive an increasing interest considering the large cost saving potential of this technology in manufacturing parts of complex shapes. Precision (investment) casting is by far the most fully developed net-shape technology compared to powder metallurgy, superplastic forming and precision forging. Production of precision castings of titanium alloys was considerably increased during last years due to significant cost savings compared with complicated process of machining. When Ti-6Al-4V is slowly cooled from the β region, α begins to form below the β transus temperature that is about 980°C. The kinetics of β→α transformation upon cooling strongly influences properties of this alloy. Contrary to wrought material, however, the possibilities to optimize the properties via the microstructural control are limited for cast parts to purely heat treatments. For many alloys mechanical properties of castings are inherently lower than those of wrought alloys. Nevertheless, heat treatment of titanium castings yields mechanical properties comparable, and often superior, to those of wrought products.