Methods for centrifugally casting highly reactive titanium metals

Abstract

Methods for centrifugally casting a highly reactive titanium metal involving providing a cold wall induction crucible having a plurality of induction coils and a removable bottom plate, using a power source to heat a titanium metal charge in the induction crucible to obtain a molten metal, preheating a secondary crucible and placing the preheated secondary crucible into a centrifugal casting machine, positioning the centrifugal casting machine having the secondary crucible beneath the induction crucible, withdrawing the bottom plate of the induction crucible and turning off the power source to the induction crucible to allow the molten metal to fall from the induction crucible into the secondary crucible, and accelerating the secondary crucible to centrifugally force the molten metal into a casting mold to produce a cast component.

Description

TECHNICAL FIELD[0001]Embodiments described herein generally relate to methods for centrifugally casting highly reactive metals. More particularly, embodiments herein generally describe methods for centrifugally casting highly reactive titanium alloys, and in particular, titanium aluminide alloys.BACKGROUND OF THE INVENTION[0002]Turbine engine designers are continuously looking for new materials with improved properties for reducing engine weight and obtaining higher engine operating temperatures. Titanium alloys (Ti alloys), and in particular, titanium aluminide based alloys (TiAl alloys), possess a promising combination of low-temperature mechanical properties, such as room temperature ductility and toughness, as well as high intermediate temperature strength and creep resistance. For these reasons, TiAl alloys have the potential to replace nickel-based superalloys, which are currently used to make numerous turbine engine components.[0003]Vacuum Arc Re-melting (VAR) is one techniqu...

AI Technical Summary

Problems solved by technology

Titanium electrodes used in the VAR process can be expensive because of the high cost of titanium billets / forgings, and the high cost of labor involved in creating an electrode from certified scrap or revert material.

Also, the requirement for a pre-alloyed electrode can make it difficult and expensive to produce non-standard alloys.

Furthermore, the need to use a water-cooled crucible can limit the degree of superheat achievable in the metal, which in turn can affect fluidity, leading to difficulty in filling thin-wall castings.

Moreover, the highest temperature exists where the arc strikes the metal, and high temperature gradients exist in the molten metal.

This can also affect the filling of molds and sets up poor temperature gradients in the solidifying casting.

For example, melting and casting from ceramic crucibles can introduce significant thermal stress on the crucible, which can result in the crucible cracking.

Such cracking can reduce crucible life and cause inclusions in the component being cast.

Moreover, the highly reactive TiAl alloys can break down the ceramic crucible and contaminate the titanium alloy with both oxygen and the refractory alloy from the oxide.

Similarly, if graphite crucibles are employed, the titanium aluminide can dissolve large quantities of carbon from the crucible into the titanium alloy, thereby resulting in contamination.

Such contamination can result in a loss of mechanical properties of the titanium alloy.

However, while cold crucible melting in copper crucibles can offer metallurgical advantages for the processing of the highly reactive alloys described previously, it can also have a number of technical and economic limitations including low superheat, yield losses due to skull formation and high power requirements.

In particular, the cold wall induction crucible suffers heat loss when the power to the crucible is terminated and the metal is allowed to slump against the water-cooled copper sides of the mold.

While the use of nozzles can provide many benefits over other common practices, the use of nozzles is not entirely without the potential for complications.

For example, cold hearth melting and bottom pouring of reactive metals like titanium can result in undesirable melt freeze-off in the nozzle.

In addition, many crucible / nozzle systems can struggle to provide the requisite control of liquid flow rate, minimize erosion of the nozzle, and minimize melt contamination.

However, while the magnetic induction field can both heat the metal and hold the molten metal suspended in space within the crucible, once the power source for the system is turned off, the metal can slip back into the water-cooled crucible and chill again before it can be poured.

This can result in incomplete filling of the mold.

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