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System and filter for filtering hard alpha inclusions from reactive metal alloys

System and filter for filtering hard alpha inclusions from reactive metal alloys description/claims

This invention was made with government support under Contract No. F33657-97-C-0030 awarded by Department of the Air Force. The government may have certain rights in this invention.

The present invention relates generally to manufacturing components from reactive metal alloys and, more particularly, to filtering hard alpha inclusions from reactive metal alloys during casting of components from the same.

In many industries today, such as the biomedical and aerospace industries, components experience severe service conditions and, thus, are often made from titanium alloys or superalloys. For example, turbine-powered aircraft contain critical rotating components in the engine, such as the fan, compressor and the turbine sections, that are made from titanium alloys and/or superalloys. Generally, such alloys are manufactured by secondary remelting processes, such as plasma arc cold hearth melting (PAM), electron beam cold hearth melting (EBM), vacuum arc remelting (VAR), and electroslag remelting (ESR). During the manufacturing of the components, quality control takes a significant role because failure of such components can lead to catastrophic loss of the complex system as well as other losses.

In quality control, one of the most important quality issues for titanium alloys and superalloys is melt-related inclusions. In this regard, inclusions can consist of unusually coarse segregated phases formed in the melt, or as exogenous materials having origins outside the deliberate alloy constituents. In the case of exogenous materials for investment castings, one type of inclusion is mold shell fragments. Mold shell fragments are inadvertently released from the ceramic shell mold during casting as a result of high thermal stresses and erosion of the mold by the molten metal. The ceramic mold innermost layer (that faces the molten metal) typically contains rare earth metal oxide(s), such as erbia, that are utilized because of their high melting point and chemical compatibility with the reactive titanium melt. Upon release, the mold shell fragments may be incorporated into the body of the casting itself, and thereby become inclusion defects.

Another type of exogenous inclusion, peculiar to titanium and other reactive metal alloys with solvus temperatures that rise with interstitial oxygen, nitrogen or carbon content, is “hard alpha”, also known as Type I inclusions. Hard alpha inclusions originate within such alloys during process operations, such as welding, flame cutting, grinding, cutting and even furnace air leaks, that expose the molten alloy to elements in air, particularly oxygen, nitrogen, and carbon. When such exposure occurs, the alloy takes the elements into solution where the elements simultaneously stabilize and embrittle the alpha phase of the alloy. By stabilizing and embrittling the alpha phase, a defect is created within the alloy that is very similar in most other respects to the base alloy. Particulate debris from such operations can inadvertently migrate to the casting furnace and enter the ceramic shell mold as the casting pour takes place. Because hard alpha inclusions have a melting point exceeding that of the clean alloy, hard alpha inclusions can survive exposure to the melt, enter the mold and become a brittle inclusion. Additionally, hard alpha inclusions can enter the primary metal supply stream and unknowingly become part of the melt stock.

As stated, hard alpha inclusions originate during certain process operations within titanium and other reactive metal alloys with solvus temperatures displaying a positive slope as oxygen, nitrogen or carbon are added. Such process operations are integral to other process streams at the foundry where the components are manufactured and, as such, the process operations cannot be totally eliminated or isolated from the casting activity. As a result, detailed contamination control plans are typically implemented to prevent the generation and introduction of hard alpha debris into the foundry. Such contamination control plans, however, have a number of drawbacks. While contamination control plans aid in preventing the generation and introduction of hard alpha debris, such control plans generally do not remove hard alpha debris that actually do form or become introduced from operations external to the foundry. Also, contamination control plans typically add cost to the manufacture of the components, and add time required in the production schedule of the components. Additionally, such contamination control plans are generally difficult to enforce among manufacturers, and cannot be easily validated. In this regard, because of the limitations of contamination control plans and the associated risk of component failure, the design of components made from such alloys still typically accounts for the presence of hard alpha inclusions.

In light of the foregoing background, the present invention provides a system and filter for filtering hard alpha inclusions from reactive metal alloys. In contrast to conventional contamination control plans, the system and filter of the present invention remove hard alpha debris that forms or becomes introduced into the foundry. As such, the design of components made from relevant alloys manufactured according to the present invention need not account for the presence of hard alpha inclusions. Also, the system and filter add relatively little cost to the manufacture of components, when compared to the cost to develop and implement a conventional detailed contamination control plan. Utilizing the system and filter of the present invention does not add any appreciable time to the production schedule of components. Further, because the system and filter of the present invention remove hard alpha inclusions as opposed to attempting to control or limit the formation of such contaminants, no need exists to enforce or validate implementation of the filter as required by conventional contamination control plans.

According to one embodiment, the present invention provides a system for filtering hard alpha inclusions from a reactive metal alloy, such as titanium. The system includes a vessel that is capable of holding the reactive metal alloy in a molten form, and can pour the molten reactive metal alloy. The system also includes a receptacle for receiving the molten reactive metal alloy poured from the vessel. And to prevent at least some hard alpha inclusions from entering the receptacle, the system includes a filter disposed between the vessel and the receptacle through which the molten reactive metal alloy passes before being received by the receptacle.

The filter includes a frame, and a porous surface that is disposed within the frame such that the frame extends peripherally about the porous surface. The porous surface defines a plurality of openings that are sized to permit the reactive metal alloy in molten form to pass therethrough. The filter is comprised of a material having a melting point that exceeds a melting point of the reactive metal alloy and is at least partially insoluble in the molten reactive metal alloy. The material of the filter can have a solubility less than a predetermined percent by weight in the molten reactive metal alloy, such as less than twenty-five percent by weight. Also, the material of the filter can have a melting point greater than a melting point of the reactive metal alloy by at least a predetermined amount, such as at least 500 degrees Celsius. For example, the filter can comprise an alloy including at least one of niobium, molybdenum, tantalum, rhenium and tungsten.

Further, to limit solidifying of the molten reactive metal alloy on the filter as the molten reactive metal alloy passes through the filter, the system can include a heating element in thermal contact with the filter. More specifically, the heating element can preheat the filter to thereby limit the solidifying of the molten reactive metal alloy within the openings defined by the porous surface of the filter. And in another embodiment, the system further includes a chamber defining an internal cavity within which the vessel, receptacle and filter are disposed. In this embodiment, the internal cavity is isolated from an external environment. Also, the heating element is capable of preheating the filter by passing current through the filter.

The system and filter of the present invention, therefore, filter hard alpha inclusions from reactive metal alloys with solvus temperatures displaying a positive slope. In contrast to conventional contamination control plans, the system and filter of the present invention add relatively little cost to the manufacture of components, and do not add any appreciable time to the production schedule of components. Further, in contrast to conventional contamination control plans, the system and filter of the present invention filter out hard alpha inclusions as opposed to attempting to control or limit the formation of such contaminants. And as such, no need exists to enforce or validate implementation of the filter.

Having thus described the invention in general terms, reference will now be made to the accompanying drawings, which are not necessarily drawn to scale, and wherein:

FIG. 1 is a block diagram of a system for filtering hard alpha inclusions from a reactive metal alloy, according to one embodiment of the present invention;

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