The present invention relates to a resistive melting furnace for the 5 production of superalloy components by means of investment casting process.

More specifically, the present invention refers to resistive melting furnaces, operating in high vacuum, dedicated to the production, by means of lost wax casting process or investment casting, of superalloy 10 components with grains having a directional structure (DS)/single crystal (SX), for aerospace, naval and industrial turbines.

An example of a known furnace is described in European patent application EP 0 559 251 A1 .

In the following reference will be made to shells made of ceramic 15 material, meaning internally shaped bodies made of refractory material having cavities which represent at a geometric-dimensional level the negative of the final production components, ie superalloy components for aerospace, naval and industrial turbines.

A resistive-type lost wax casting furnace according to the prior art is 20 shown in Figure 1 and consists of:

- a casting chamber 2, where the superalloy is cast by induction,

- a thermal chamber or hot chamber 3, placed below the casting chamber 2 and connected thereto through an upper valve 5, in which a ceramic shell 1 is placed and heated, said ceramic shell being internally

25 shaped and provided with cavities which represent at a geometric- dimensional level the negative of the final components of production, in which the molten alloy is poured,

- an extraction chamber or cold chamber 4, placed below the hot chamber 3 and connected thereto through a lower valve 6, where the

30 ceramic shell 1 is extracted from the hot chamber 3, with the solidification and cooling of the superalloy, and the generation of the desired grain structure.

In particular, the hot chamber 3 of this type of furnace is basically constituted by a hollow cylinder 7 made of graphite, called graphitic hot 35 chamber, which, externally heated by a graphite resistance 8, and placed inside an insulating pack 9 made of graphite, acts as an active element for radiative heating of the ceramic shell 1 , loaded inside and positioned on a cooling plate 10, generally made of copper, also called chill plate, cooled by a flow of water and moved by an electric piston 1 1 .

Through the movement of the piston 1 1 , the ceramic shell 1 is extracted from the thermal chamber 3 to the extraction chamber 4. A thermal deflector 12, also called ceramic baffle, is arranged at the interface between the thermal chamber 3 and the extraction chamber 4, with the function of thermal shield between the two chambers. The thermal deflector 12 is fundamental to ensure the thermal gradient necessary for the development of the desired grains structure in the superalloy component, as explained in detail in the following of the present description.

Referring to Figure 2, again referred to a resistive-type lost wax casting furnace according to the prior art, the resistance 8 is connected to a single electric power supply circuit and has two zones, an upper zone 81 and a lower zone 82 , having different cross-sections, ie different thicknesses: the lower zone 82, which is more affected by heat losses towards the cooling plate 10 and towards the cold chamber 4, has a smaller thickness, R82, than the thickness R81 of the upper zone 81 . In this way, with the same current passing through it, there is a greater radiated power in the lower zone 82, with a corresponding more intense heating of the low area of the hot chamber 7 made of graphite, to compensate for losses at the interface between the hot chamber 3 and the cold chamber 4.

Referring to Figure 3, always relative to a resistive lost wax casting furnace according to the prior art, the set cast temperature is read on the graphite hot chamber 7, at a depth of about 15 inches, corresponding to about half height of the same hot chamber 7, by means of a pyrometer and is retrocontrolled with the aid of a PID system.

The production process of components with the structure of the grains of the DS/SX type is mainly based on the imposition of a high spatial thermal gradient (ΔΤ/Δζ), having the module of the order of 10÷100 °C/cm, with specific values for each category of components, aligned with the direction of extraction from the hot chamber 3 and placed in corresponcence of the ceramic deflector 12, by:

- the generation and maintenance of a predetermined thermal field, as uniform as possible, inside the graphite hot chamber 7 and a predetermined cooling of the cooling plate 10 (cooling water temperature in the range 20÷24°C) and of the extraction chamber 4; - the use of a specific profile for the extraction of the shell from the thermal chamber 3 to the extraction chamber 4, according to a controlled descent program of the piston, with extraction speed in the range 0.1 ÷10mm/min, depending on the type and the geometry of each component in part.

It is known that the condition necessary to reduce the probability of growth of spurious grains in the melt during the process of solidification and generation of the grains is that of having a so-called "mushy zone", ie a spatial area within which the temperature passes from the liquidus temperature, TL, to the solidus temperature, TS, said area having the smallest possible width and an optimal orientation of the isotherms, that is, with an orientation of the isotherms that is fundamentally perpendicular to the extraction axis. These conditions are ensured inside the thermal deflector 12, or at the limit in its proximity, where the radial component of the thermal gradient is zero, or has very low values, thus maximizing the thermal gradient along the extraction axis.

The physical-chemical mechanism that is at the origin of the directional growth process of the single grains (DS structure) or that determines the direction of the single crystal (SX structure) in the metal melt therefore imposes very restrictive conditions as regards the uniformity of the thermal field inside the hot chamber 3 and the thermal gradient (ΔΤ/Δζ) in the interface region between the hot chamber 3 and the cold chamber 4.

For this reason, there are stringent dimensional limitations on the various parts constituting the hot chamber 3, the cold chamber 4 and the interface between the two, including a well-defined geometric configuration of the resistance 8 which, as already mentioned previously, includes two zones, an upper zone 81 and a lower zone 82 of different thicknesses, respectively R81 and R82, and a minimum gap (in the order of a few mm) between the thermal deflector 12 and the cooling plate 10. Accordingly, a given configuration of the furnace assures the production of assemblies having a specific, well-defined diameter, and the need to produce assemblies having a diameter even slightly greater would automatically imply the need to purchase a new furnace.

This represents a limit, both in economic terms and in terms of size.

It is therefore an object of the present invention to overcome the drawbacks of the prior art by modifying the known melting fornace, so that they are able to house ceramic shells of different sizes, without having to purchase a new furnace.

It is therefore a first specific object of the present invention a resistive casting furnace for the production of superalloy components by means of investment casting process, said furnace comprising a melting chamber, a hot chamber or thermal chamber and a cold chamber or extraction chamber placed below said thermal chamber, a thermal interface zone, arranged between said thermal chamber and said extraction chamber, in which there is a thermal deflector, a cooling plate for housing a ceramic shell, and a piston for moving and supporting said cooling plate, said thermal chamber comprising a hollow cylinder externally surrounded by a cylindrical resistance and placed inside an insulating pack, said resistance being connected to an electric power supply circuit and having an upper and a lower zone, the thickness of said upper zone of the resistance being greater than the thickness of the lower zone of said resistance, wherein said resistance is configured in such a way that the thickness of said upper zone of the resistance is the maximum allowed by the limits of space of the upper zone (connected to the maintenance of the existing configuration of the electric power supply system) and the thickness of the lower zone of said resistance it is the minimum allowed by the requirements of mechanical strength of the lower zone.

In particular, according to the invention, said resistance is made of graphite.

Additionally it is second specific object of the present invention a reconfiguration method of a resistive melting furnace for the production of superalloy components by means of investment casting process, said furnace comprising a casting chamber, a hot chamber or thermal chamber and a cold chamber or extraction chamber placed below said thermal chamber, a thermal interface zone, placed between said thermal chamber and said extraction chamber, in which a thermal deflector is placed, a cooling plate for housing a ceramic shell, and a piston for moving and supporting said cooling plate, said thermal chamber comprising a hollow cylinder surrounded externally by a cylindrical resistance and placed inside an insulating pack, said resistance being connected to an electric power supply circuit and presenting an upper and lower zone, the thickness of said upper zone of the resistance being greater than the thickness of the lower zone of said resistance, said reconfiguration being functional to the treatment of ceramic shells larger than those of project,

in which the diametric measures of the components constituting said thermal chamber, the extraction chamber and said cooling plate are modified according to the diametric measures of said larger ceramic shells, and said resistance is also modified by reducing the thickness of the lower zone of said resistance as much as possible within the limits of mechanical strength of the lower zone and at the same time increasing the thickness of said upper zone of the resistance as much as possible within the limits of space of the upper zone.

The invention will now be described for illustrative but not limitative purposes, with particular reference to the drawings of the attached figures, in which:

- Figure 1 shows a front sectional view of a resistive melting furnace according to the prior art;

- Figure 2 shows a perspective view of a resistance of the graphitic chamber of the melting furnace of Figure 1 ;

- Figure 3 shows a front sectional view of a right half-portion of the melting furnace of Figure 1 , comprising a right half-portion of the hot chamber and a right half-division of the extraction chamber, with the piston at the top dead center;

- Figure 4 shows a front sectional view of the metal casing of the thermal chamber and of the extraction chamber, of the piston of a resistive melting furnace according to the prior art and of a ceramic shell;

- Figure 5 shows a front sectional view of the metal casing of the thermal chamber and of the extraction chamber, of the piston of a resistive melting furnace according to the invention and of a ceramic shell of larger diameter;

- Figure 6 shows a diagram illustrating the temperature variation as a function of the position in the furnace (expressed in inches and shown with reference to Figure 3), in the case of the initial configuration (STD Resistance on SB), of an intermediate configuration, wherein the diametric measures of the components constituting the hot chamber, the cold chamber and the cooling plate have been modified, but the thicknesses of the two zones of the graphite resistance have not been modified (STD Resistance on SBB) and in the case of the final configuration, in which both the diametric measures of the components constituting the hot chamber, the cold chamber and the cooling plate have been modified, as well as the thicknesses of the two zones of the graphite resistance (Modified resistance on SSB);

- Figure 7 shows a diagram illustrating the variation of the width of the mushy zone according to the position of the piston (expressed in inches and shown with reference to Figure 3), in the case of the initial configuration (STD Resistance on SB), of an intermediate configuration, in which the diametric measures of the components constituting the hot chamber, the cold chamber and the cooling plate have been modified, but the thicknesses of the two areas of the graphite resistance have not been changed (STD resistance on SBB) and in the case of the final configuration, in which both the diametric measures of the components constituting the hot chamber, the cold chamber and the cooling plate, and the thicknesses of the two zones of the graphite resistance (Modified resistance on SSB) have been modified; and

- Figure 8 shows a diagram illustrating the variation of the center position of the mushy zone according to the position of the piston (expressed in inches and shown with reference to Figure 3), in the case of the initial configuration (STD Resistance on SB), of an intermediate configuration, in which the diametric measures of the components constituting the hot chamber, the cold chamber and the cooling plate have been modified, but the thicknesses of the two zones of the graphite resistance have not been modified (STD resistance on SBB) and in the case of the final configuration, in which both the diametric measures of the components constituting the hot chamber, the cold chamber and the cooling plate, and the thicknesses of the two zones of the graphite resistance have been modified (Modified resistance on SSB).

According to the present invention, a resistive melting furnace for the production of superalloy components by means of investment casting process, realized according to the prior art, of the type shown in Figure 1 , can be reconfigured to accommodate assemblies, ie ceramic shells 1 , having a larger diameter, preserving the structure of the melting chamber 2 unaltered, and modifying only the thermal chamber 3 of the furnace and the extraction chamber 4, basically by means of an increase in the diametric measures of all the cylindrical components.

The objective of this diametral increase in the useful working volume inside the thermal chamber (3) of the furnace consists of: - the increase of production capacity and in reduction of production costs by means of castings of assemblies with a greater number of components, and therefore the increase of the alloy efficiency, the reduction of consumption (energy, raw materials and ancillary materials) and of working times connected to alloy cutting, to the preparation of wax assemblies, to the development of ceramic shells and to the melting process;

- the possibility of producing new superalloy components having larger dimensions.

This resizing of the resistive melting furnace for the production of superalloy components by means of investment casting process was carried out with the objective of maintaining furnace thermal performances equivalent to the initial configuration, already optimized to get as close as possible to an ideal thermal profile. The latter provides:

- a uniform temperature inside the thermal chamber 3, equal to the set cast temperature;

- a high thermal gradient module at the interface between the thermal chamber 3 and the extraction chamber 4.

The ideal thermal profile can not be achieved in practice, since the thermal field in the thermal chamber 3 suffers from the upper and lower edge effects, where the temperature decreases due to losses towards the two cold chambers, ie melting chamber 2 and extraction chamber 4.

In particular, the conversion strategy aims at maintaining on the final configuration of the resistive melting furnace for the production of superalloy components by means of investment casting process according to the present invention a thermal field equivalent to that of the furnace in the initial configuration, to use the same/equivalent cast/extraction programs already set up for the components in production on the initial configuration and which consequently can be produced on the new configuration.

As already mentioned above, the physical-chemical mechanism that is at the origin of the directional growth process of single grains (DS structure), or that determines the direction of the single crystal (SX structure) in the metal melt, imposes very restrictive conditions as regards the uniformity of the thermal field inside the hot chamber and the thermal gradient (ΔΤ/Δζ) in the interface region between the hot chamber 3 and the cold chamber 4. For this reason there are stringent dimensional limitations on the various parts constituting the hot chamber 3, the cold chamber 4 and the interface between the two, including a well-defined geometrical configuration of the resistance 8, which envisages, as already mentioned previously, an upper zone 81 and a lower zone 82, R81 and R82, respectively, having different thicknesses, and a minimum gap (in the order of a few mm) between the thermal deflector 12 and the cooling plate 10. Accordingly, a given configuration of the furnace assures the production of assemblies having a specific, well-defined diameter, and the need to produce assemblies having a diameter even slightly greater would automatically imply the purchase of a new furnace.

By way of example, in order to produce assemblies having a diameter greater than 3.5" (inches), a reconfiguration of an already existing furnace, in which the maximum diameter of the assemblies was 5.5", equal to the diameter of the thermal plate 10, has been carried out, with lower costs compared to the purchase of a new furnace. Referring to the comparative figures in Figures 4 and 5, the modification of the furnace, ie the conversion from the initial configuration, also called Small Bore (SB), to the final configuration, which can be defined SuperSmall Bore (SSB), consisted mainly of a 3.5" oversizing on diametric measures, in particular:

- of the diameter D, of the components constituting the hot chamber

3 (with i=1 ÷n, where n is the number of cylindrical components of the hot chamber);

- of the diameter d _e of the cold chamber 4 and of the cooling plate d„.

On the other hand, the diameter of the piston 1 1 remained unchanged.

The resizing of the graphitic resistance, on the other hand, can not be limited to an increase of 3.5" on the diametric measures, but must take into account more complex variables, which entail a solution that is not obvious to an expert in the sector, as will be explained below.

In fact, in order to preserve the characteristics of thermal stability, efficiency and reliability of the furnace, the oversizing of the hot chamber 3 and of the cold chamber 4 has been carried out while maintaining the relative distance between the various components constituting the thermal chamber 3 and the extraction chamber 4 , by appropriately modifying, according to the new dimensions, the position of the rotation axis of the opening/closing system of the lower valve 6 of the thermal chamber 3 and of the movement mechanisms of the same system.

The diametrical increase described above determined, with respect to the initial configuration, Small Bore (SB), an increase in radiative thermal losses through the larger surface open to the interface between the hot chamber 3 and the cold chamber 4, generating:

- an increase in the power absorbed by the resistance 8, to bring the furnace to the cast temperature, due to the greater volume of the hot chamber 3 and the greater radiative and conductive thermal losses towards the cold chamber 4;

- a variation of the thermal field inside the thermal chamber 3, ie an overall decrease of the temperature and of the thermal gradient module (ΔΤ/Δζ) at the interface between the thermal chamber 3 and the extraction chamber 4, as shown in Figures 6 and 7.

To ensure, in the final configuration SuperSmall Bore (SSB), a thermal field and a thermal gradient equivalent to the initial configuration, Small Bore (SB), the thicknesses R81 and R82 of the two zones of the resistance 8 have been appropriately modified. The obvious solution for a technician of the sector would be the further reduction of the thickness R82 of the lower area of the graphite resistance, to compensate for the greater thermal losses towards the cold chamber. Instead, to define the optimal geometrical configuration of the graphite resistance to be used on the final configuration, numerical simulations of the thermal exchanges were carried out inside the hot chamber 3 and the cold chamber 4, using a finite element model experimentally validated. Through these simulations it was surprisingly discovered that the higher the R81 /R82 ratio, the more the thermal profile in the final configuration approaches that in the initial configuration and the ideal configuration. Therefore, in addition to the diameter increase of 3.5", the main modification used on the graphite resistance in the final configuration, SuperSmall Bore, consisted in reducing the thickness R82 of the lower zone 82, within the technical limit connected to the mechanical strength of the element itself; and in the increase of the thickness R81 of the upper zone 81 , within the overall limits of space.

In correspondence to this modification of the graphitic resistance 8, the thermal field inside the furnace hot chamber in the final configuration becomes more uniform, ie the temperature in the whole volume of the hot chamber 3 is closer to the set cast temperature and improves with respect to the initial configuration, approaching the ideal profile as shown in Figure 6. Simultaneously, as shown in Figure 7, the thermal gradient module (ΔΤ/Δζ) at the interface between the thermal chamber 3 and the extraction chamber 4, for different positions of the piston, during the phase of extraction of the assembly from the hot chamber 3, improves with respect to the initial configuration. At the same time, as shown in Figure 8, the center of the mushy zone moves towards the deflector (as shown in Figure 3, the center of the deflector is located at a height of about 22 inches), even improving with respect to the initial configuration.

The present invention has been described by way of illustration, but not of limitation, according to its preferred embodiments, but it is to be understood that variations and/or modifications may be made by those skilled in the art without thereby departing from the relative scope of protection, as defined in the attached claims.