WO2013152045A1

WO2013152045A1 - Structurally reinforced thin shells for directionally solidified vacuum investment casting

Info

Publication number: WO2013152045A1
Application number: PCT/US2013/035028
Authority: WO
Inventors: Charles M. Byrd; Thomas J. Todaro
Original assignee: Flc Flowcastings Gmbh
Priority date: 2012-04-02
Filing date: 2013-04-02
Publication date: 2013-10-10

Abstract

A shell mold for use in investment casting of a component from a casting material comprises a shell mold body which is produced using a direct digital manufacturing process and at least one structural support which is formed integrally with the shell mold body during the direct digital manufacturing process. The shell mold body comprises an outer surface and a cavity which conforms to the shape of the component, and the at least one structural support is located on the outer surface of the shell mold body.

Description

STRUCTURALLY REINFORCED THIN SHELLS FOR DIRECTIONALLY SOLIDIFIED VACUUM INVESTMENT CASTING

FIELD OF THE INVENTION

The present invention relates to the production of components by the vacuum investment casting process. More particularly, the invention relates to a reinforced ceramic shell mold which is used in the investment casting process. BACKGROUND OF THE INVENTION

Vacuum investment shell mold casting is a casting technique in which a casting material such as liquid metal is introduced into a ceramic shell mold and allowed to solidify while in an inert vacuum environment. Shell molds are usually produced in "shell lines" that are equipped with dip tanks and "sanders." The dip tanks contain a ceramic slurry which is used to coat a pattern, typically made of wax, of the component to be cast. Once the pattern is wetted with ceramic slurry, it is coated with an additional ceramic medium in the so-called "sanders." These devices contain ceramic meal which is allowed to cascade down in a waterfall or curtain. Either by manual or robotic means, the wetted wax patterns are evenly rotated in the sander to provide a uniform coating of material. With this dip-coat process, thickness and heat transfer are always inversely related. An arbitrarily thick layer of ceramic can be applied to the wax pattern, but in general the mold with have a more or less uniform shell thickness.

This method of making shell molds suffers from the following drawbacks:

- The successive steps of the dip-coat process can take days to complete.

- The dip-coat process requires a very controlled drying environment;

temperature and humidity must be carefully controlled.

- The dip-coat process has poor material utilization efficiency, that is, much of the ceramic slurry is wasted.

- The dip-coat process does not allow for precise shell thickness

management. Although the robotic process can produce a very

repeatable and uniform thickness across the entire part, it is unable to accurately reproduced variable thicknesses across uniform areas of the shell. For example, if the shell were a perfect cylinder the current process would not be able to vary the thickness in the radial or axial directions along the surface of the cylinder. - The dip-coat process is unable to effectively produce shells with features which deviate substantially from the wax pattern.

Shell molds serves several purposes in the investment casting process. They receive and contain the metal during the casting process, they define the form of the final component, they manage the thermo-mechanical stresses of the casting process, and they provide a thermal path from the alloy to the outside environment.

In providing this functionality, shell molds are typically required to have two contradictory attributes: mechanical strength (which is directly related to shell thickness) and thermal conductivity. In directionally solidified and single crystal vacuum investment casting in particular, it is desirable for technical and economic reasons to maximize the solidification rate of the casting material. However, the solidification rate is limited by the rate of heat transfer through the shell mold. Thus, it is desirable to increase the heat transfer rate through the shell mold in order to maximize the rate of heat transfer from the casting material to the hot and cold zones of the furnace.

While a very thin shell mold would solve the problem of low heat transfer rates through the shell mold, such an attribute would aggravate another problem, namely, the mechanical stability of the shell mold. If the shell mold is too thin, it will not be able to withstand the stresses of the casting process. In many instances, the shell mold may not even be able to support the mold assembly (gating, pour cup, etc).

The tradeoff between mechanical strength and heat transfer is rooted in the fact that, with conventional shell molds, the strength is inversely dependent on heat transfer through the shell. In other words, the thicker the shell, the better the mechanical strength but the worse the heat transfer. The opposite is also true. The thinner the shell, the better the heat transfer but the worse the mechanical strength.

Making the shell mold using a thin but strong ceramic material may appear to be a viable solution to this problem. However, this is not the case because a shell mold made of such a material, while exhibiting strength, could possibly overly constrain the casting during cooling, and this could produce both macroscopic and microscopic defects in the resulting metal part. Therefore, a ceramic shell mold is needed which exhibits both high thermal conductivity and mechanical strength but does not overly constrain the casting during cooling.

SUMMARY OF THE INVENTION

In accordance with the present invention, a shell mold is provided for use in the investment casting of a component from a casting material. The shell mold includes a shell mold body which is produced using a direct digital manufacturing process. The shell mold body comprises an outer surface and a cavity which conforms to the shape of the component, and the shell mold further includes at least one structural support which is formed integrally with the shell mold body during the direct digital manufacturing process. Moreover, the at least one structural support is located on the outer surface of the shell mold body.

In one embodiment of the invention, the shell mold body comprises a thickness of approximately 1 mm. Alternatively, the shell mold body comprises a thickness which is selected to provide a desired heat transfer rate from the casting material. In this embodiment, the thickness of the shell mold body may be sufficiently small to produce at least one area of structural weakness in the shell mold body, and the at least one structural support is positioned so as to reinforce this area.

In accordance with another aspect of the present invention, a method is provided for producing a shell mold for use in the investment casting of a component from a casting material. The method comprises determining a thickness of the shell mold body which is required to achieve a desired heat transfer rate from the casting material; identifying areas of potential structural weakness in the shell mold body based at least in part on the geometry of the shell mold body and the anticipated stresses on the shell mold body during the casting process; locating at least one structural support on the outer surface of the shell mold body in an area of potential structural weakness; and integrally forming the shell mold body and the at least one structural support using a direct digital manufacturing process.

Thus, it may be seen that the present invention decouples the thickness from the strength of the shell mold. This is accomplished by producing a shell mold having structural elements which are independent of the thickness of the shell. Also, since the structural supports are located on the exterior surface of the shell mold body, they will not interfere with the casting of the component. Furthermore, due to their discrete nature, the structural supports will provide the necessary strength while still allowing for sufficient movement of the shell mold during the casting/solidification process to minimize the likelihood of producing macroscopic and/or microscopic defects in the cast component.

The method of the present invention will allow a shell mold to be economically constructed with the following features:

- Integral cores that have, if needed, multiple levels of porosity.

- Shell molds with structural supports such as ribs, loops, spars, buttresses, struts, diagonals, braces, etc.

- Shell molds in which the structural supports are made with a ceramic

density that is different from that of the shell surface in contact with the cast material.

- Shell molds with structural supports that comprise "blind features", that is, features which cannot be produced from subtractive manufacturing processes such as the dip-coat process.

- Shell molds with mechanical properties that can be tailored to a part- specific casting process.

Using a direct digital manufacturing process, such as selective laser melting (SLM) or electron beam melting (EBM), shells with the aforementioned attributes can be easily and efficiently fabricated. Because vacuum investment casting is carried out with molds that contain several to dozens of parts, it would not be obvious that SLM or EBM could be used to produce suitable shells for investment casting. In fact, because current vacuum investment casting achieves economy of scale through the simultaneous casting of multiple parts, it would be counterintuitive to use a process, such as SLM/EBM, which is, in its current form, mostly only suited for producing single part molds. If instead the economy of scale is achieved through faster throughput on smaller casting units, the focus shifts towards trying to produce the thinnest suitable shell molds (those with the fastest processing rates due to enhanced heat transfer). The following table summarizes the difference between the weighting in shell design attributes from multiple part casting to single part casting: Table 1 :

These and other objects and advantages of the present invention will be made apparent from the following detailed description with reference to the accompanying drawings. In the drawings, the same reference numbers are used to denote similar components in the various embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a perspective representation of the shell mold according to an illustrative embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The shell mold of the present invention will now be described with reference to the illustrative embodiment shown in Figure 1. The shell mold of this embodiment, which is indicated generally by reference number 10, may be used in a conventional investment casting process to make any of a number of desired components. As shown in Figure 1 , for example, the shell mold 10 may be used to cast an airfoil component of a jet engine. Accordingly, the shell mold 10 includes a shell mold body 12 which comprises an outer surface 1 and a cavity 6 that conforms to the shape of the airfoil to be cast, and a number of structural supports 18 which are formed integrally with the shell mold body on the outer surface thereof. It should be noted, however, that the ceramic cores which may be required to define the internal features of the airfoil, such as the cooling channels, have been omitted for clarity.

The shell mold 10, including the shell mold body 12 and the structural supports 18, is ideally produced using a direct digital manufacturing technique, such as selective laser melting (SLM), electron beam melting (EBM),

steriolithography, or any other such suitable technique. An example of an SLM process which is suitable for making the shell mold 10 is disclosed in published European Patent Application No. EP 1772210 A2, which is hereby incorporated herein by reference. This patent application discloses a process for making ceramic casting cores for the casting of turbine engine components. The process involves focusing a laser beam on a selected region of a substrate and delivering a ceramic feed material to an area very close to the intersection of the laser beam and the substrate. The laser beam melts the feed material to form a melt pool, which then solidifies to form a clad track as the laser beam is directed along a path representing a cross section of the core being formed. The deposition apparatus is then moved upward and the process is repeated successively until the entire core is formed.

An example of a steriolithography process which is suitable for producing the shell mold 10 is disclosed in U.S. Patent Application Publication No. US 2010/0003619 A1 , which is hereby incorporated herein by reference. This patent application discloses a technique for making ceramic shell molds which can be used for casting airfoils for turbine engines. The particular process described in this patent application is referred to as large area maskless photopolymerization (LAMP). In this process, an optical imaging system comprising, e.g., an ultraviolet light source is used to scan the surface of a photo-curable ceramic material in order to solidify a layer representing a cross section of the shell. The solidified layer is then lowered beneath the surface of the material and the process is repeated successively until the entire shell is produced.

In accordance with the present invention, the shell mold body 12 is formed with a minimum thickness T which will result in a desired heat transfer rate from the cast material to the furnace surroundings. In order to compensate for any reduction in mechanical stability due to the decreased thickness of the shell mold body 12, one or more structural supports 18 is formed integrally with the shell mold body in those areas which are determined to require strengthening.

However, in order to not interfere with the shape of the component being cast, the structural supports 18 are positioned on the outer surface 14 of the shell mold body 12.

Given the geometry of the airfoil or other component to be cast, the desired heat transfer or solidification rate of the cast material, the thermal conductivity of the shell mold material and other process parameters known to persons of ordinary skill in the art, a desired thickness T for the shell mold body 12 can be determined using, e.g., a suitable heat transfer simulation computer program. The inventors have determined that an optimal heat transfer or solidification rate can be achieved for many airfoil designs when the thickness T is in the range of 1 mm.

Once the desired minimum thickness T has been determined, any areas of potential weakness in the shell mold body 12 can be identified using, e.g., a suitable finite element analysis computer program based on such factors as the geometry of the shell mold body, the mechanical properties of the shell mold material, and the anticipated stresses on the shell mold body during the phases of the casting process, including the pour and cooling phases. Then, once the areas of potential weakness are identified, the number, shapes and locations of the structural supports 18 which are required to eliminate these areas can be designed using, e.g., the same finite element analysis computer program. The final shape of the shell mold 10, including the shell mold body 12 and the supports 18, can then be input into a suitable direct digital manufacturing device in order to produce a shell mold with the desired thickness and mechanical strength.

Furthermore, the structural supports 18 should be designed so as not to overly impede the radiant heat transfer from the surface of the shell mold body 12. Accordingly, the cross section of the structural support 18 at its intersection with the shell mold body 12 should be minimized. This may be achieved, for example, by designing the supports 18 to be thin, but longer and taller. Such a configuration will minimize the amount of shading which each support 18 provides with respect to the surface of the shell mold body 12.

As an example, for the shell mold 10 shown in Figure 1 , an area of potential weakness, which is identified generally by reference number 20, may be found to exist along the midspan of the suction side 22 of the shell mold body 12. As a result, it may be determined that the shell mold 10 is required to be provided with two generally rectangular supports 18 which are configured and located as shown in order to eliminate this area of potential weakness.

It should be recognized that, while the present invention has been described in relation to the preferred embodiments thereof, those skilled in the art may develop a wide variation of structural and operational details without departing from the principles of the invention. Therefore, the appended claims are to be construed to cover all equivalents falling within the true scope and spirit of the invention.

Claims

What is Claimed Is:

1. A shell mold for use in investment casting of a component from a casting material, the shell mold comprising:

a shell mold body which is produced using a direct digital manufacturing process, the shell mold body comprising an outer surface and a cavity which conforms to the shape of the component; and

at least one structural support which is formed integrally with the shell mold body during the direct digital manufacturing process;

wherein the at least one structural support is located on the outer surface of the shell mold body.

2. The shell mold of claim 1 , wherein the shell mold body comprises a thickness of approximately 1 mm.

3. The shell mold of claim 1 , wherein the shell mold body comprises a thickness which is selected to provide a desired heat transfer rate from the casting material.

4. The shell mold of claim 2, wherein the thickness of the shell mold body is sufficiently small to produce at least one area of structural weakness in the shell mold body and the at least one structural support is positioned so as to reinforce this area.

5. A method for producing a shell mold for use in investment casting of a component from a casting material, the shell mold comprising a shell mold body having an outer surface and a cavity which conforms to the shape of the component, the method comprising:

determining a thickness of the shell mold body which is required to achieve a desired heat transfer rate from the casting material;

identifying areas of potential structural weakness in the shell mold body based at least in part on the geometry of the shell mold body and the anticipated stresses on the shell mold body during the casting process;

locating at least one structural support on the outer surface of the shell mold body in an area of potential structural weakness; and

integrally forming the shell mold body and the at least one structural support using a direct digital manufacturing process.