How Are Titanium Alloy Impellers and Guide Wheels Cast?

Titanium-alloy impellers and guide wheels are cast under vacuum with protected melting and mold filling because molten titanium reacts readily with gases and many mold materials. The impeller example uses an investment graphite shell and centrifugal pouring; the guide wheel uses a treated graphite mold and vacuum centrifugal filling. Ti-6Al-4V Cast Titanium Alloy and Ti-5Al-2.5Sn Cast Titanium Alloy are among the cast materials discussed, with representative Ti-6Al-4V composition and properties listed below.

Composition and Properties of Ti-6Al-4V Cast Titanium Alloy

Note: The alloy commonly also contains small amounts of C, O, N, and H, with titanium as the balance.

Titanium alloy casting is difficult. Melting temperatures are generally above 1700°C, and titanium is highly reactive. Above 500°C it reacts with many materials, while molten titanium can react with almost all refractory materials and with oxygen, hydrogen, and nitrogen. Its fluidity is also poor. Mold materials, melting practice, and mold filling therefore require special controls. Molding materials need high refractoriness and thermal stability and must not react with the melt. Current mold systems include rammed graphite molds, investment-casting shells, ceramic molds, and zircon-sand molds.

Because of this reactivity, consumable-electrode ingot melting and titanium casting must take place under vacuum or inert-gas protection in a force-cooled crucible. Production commonly uses a vacuum skull-melting furnace.

Investment Graphite-Shell Titanium Impeller

This impeller uses an investment shell. The shell system uses phenolic resin as the binder and graphite powder and sand of different particle sizes as refractory filler and stucco. Shell strength reaches 5.7-6.8 MPa. Molds for impellers and fan rotors have an outer envelope of 350 mm × 350 mm × 350 mm. The mold is positioned at the center of the turntable in a vertical centrifugal casting machine, and the vacuum chamber is closed for evacuation and melting.

After melting, the skull furnace tilts automatically to pour. Mold speed during pouring is 200-400 r/min. The resulting castings can have a minimum wall thickness of 1.5-2 mm and a surface roughness of Ra 3.2-6.3 μm.

Graphite-Mold Titanium Guide Wheel

The guide wheel has a maximum diameter of 363.5 mm and 18 evenly distributed blades. The blade-tip wall is 4 mm thick, blade-thickness tolerance is less than 0.2 mm, and blade-arc error is less than 0.15 mm. Blade surfaces require Ra 6.3 μm, while machined guide-wheel surfaces have a 2-3 mm machining allowance.

A single annular core forms all blades, helping control blade dimensions and position. The blade core, upper and lower contour cores, and pouring-cup section are assembled inside an outer mold. Finished graphite molds and cores often contain moisture and require high-temperature treatment. Place the mold in a vacuum furnace, evacuate it, heat slowly to 950°C, hold for about 4 h, and cool with the furnace. Pour the treated mold as soon as possible to limit renewed gas absorption.

Pouring is performed in a vacuum consumable-electrode skull furnace with centrifugal filling under vacuum. Based on the mean radius, use a gravity factor G of 15 and a mold speed of 350 r/min. Cool the casting under vacuum after pouring.