Nozzle, vessel and spray forming

Abstract

 An improved nozzle which allows molten metals to be sprayed at low metal flow rates. This is useful for the Plasma Arc Melting process which used a hearth to prevent ceramic inclusions in the end product. The new nozzle design avoids metal freeze-off at the nozzle tip because a bore insert is positioned into a standard nozzle. The bore insert is a conduction heater, e.g., it is fabricated from boron nitride or the like. In use, molten metal is in contact with the insert so it maintains the insert at about the temperature of the molten metal; this then maintains the nozzle tip at a higher temperature than that of a standard nozzle. Freeze-off in the nozzle is non-occurring as long a molten metal is in the crucible.

Description

[0001] This invention relates to an apparatus for pouring molten metal from a crucible, and in particular to a nozzle for pouring molten metal at low flow rates. The apparatus permits spray forming of molten metals at lower flow rates than previously achieved using conventional apparatus.

Background of the Invention

[0002] A ceramic-free melting process for forming molten metal is the Plasma Arc Melting process. Further information regarding Plasma Arc Melting can be obtained from "Proceedings of the 1986 Vacuum Metallurgy Conference on Specialty Metals Melting and Processing, Pittsburgh, Pennsylvania" June 9-11, 1986 including Large Scale Plasma Melting and Remelting Tests by G. Sick and "Plasma Technology in Metallurgical Processing", including Ch. 7 Plasma Torches and Plasma Torch Furnaces, pp 77-87 (Iron & Steel Society, J. Feinman Edition 1987) all of which are hereby incorporated herein by reference. A decisive advantage in using plasma melting is the capability to melt with a high working pressure, typically atmospheric pressure, which can be varied over a wide range to prevent selective evaporation of alloying elements. Melting operations that must operate in a vacuum are more susceptible to composition variation in a desired alloy composition due to such selective evaporation of alloying elements. This is of particular interest for the melting, refining, casting and atomization of superalloys and titanium-base alloys. Superalloys are iron-base, colbalt-base, or nickel-base alloys which combine high-temperature mechanical properties, oxidation resistance, and creep resistance. Superalloys are useful for jet engine parts, turbo-superchargers, and extreme high-temperature applications.

[0003] Titanium-base alloys are considerably stronger than aluminum alloys and superior to most alloy steels in several aspects. Constituents of titanium-base alloys include vanadium, tin, copper, molybdenum and chromium, among other elements. Titanium alloys and superalloys are well suited for Plasma Arc Melting because they can be plasma melted without contamination, for example by water or oxygen, or change in composition, when a suitably designed plasma hearth process is utilized. The Plasma Arc Melting process excludes such contamination, such as humidity emitted from the inert plasma gas, because of the internal cooling circuit inside the plasma hearth. The water-cooling of the plasma torch makes it necessary that the plasma gas be sealed vacuum-tight against the water circuit.

[0004] Initially, in the Plasma Arc Melting process a high-voltage surge strikes an arc between the electrode and nozzle inside the plasma torch to start ionization of the plasma gas, e.g., argon; the nozzle inside the torch is a means to deliver the plasma gas. The torch operates briefly in the so-called non-transferable mode. A few seconds later, the potential is transferred to the melt via microprocessor control now operating in the transferred mode. The desired voltage gradient is adjusted by selecting the distance between the torch and the metal pieces in the vessel or crucible. The torch moving device starts circularly rotating with a pre-selected varying diameter. The diameter increases until a completed liquified melt surface is attained. The stirring action in the melt can be controlled via current, gas flow and torch-melt distance. The melt is then poured into a mold chamber by a bottom pouring crucible nozzle.

[0005] In such a process, a thin shell of the molten metal freezes against the hearth to provide the containment. Because the Plasma Arc Melting process is very energy intensive, melting rates are generally limited to about 10-15 lbs/minute, which is much slower than the rate used to spray molten metals.

[0006] In the plasma melting process, molten metal is usually contained in a water-cooled copper hearth as opposed to a ceramic crucible. But a ceramic crucible may also be used. A crucible, ceramic or otherwise, is formed from a heat-resistant material employed to hold another material, that in itself is at high temperature, or is subjected to high temperature. The crucibles are roughly cup or barrel-shaped and are made of materials such as clay, platinum, iron, and ceramic; platinum and ceramic crucibles are used in laboratories. Ceramic crucibles also have industrial applications. In the iron and steel industry, clay crucibles have been used, but at the present time, most manufacturers employ the ceramic crucible, which is especially designed to have high thermal expansions to match the base metal and to become glassy at temperatures low enough to prevent distortion of the underlying metal sheet. A crucible can act as the intermediate reservoir between the furnace and the mold. The crucible receives the molten metal produced elsewhere and conveys it from the point of melting to the point of casting or spray forming. In addition, the crucible can be used to hold a material being melted or burned, as in some processes for making steel, where the raw material is placed in the crucible and then sent into a hot furnace until the contents are melted by a method such as the Plasma Arc Melting process.

[0007] The number of applications for spray formed alloys has increased over the last decade. A suitable spray forming process is shown, for example, in U.S. Patent 3,909,921 incorporated by reference herein. Recently, the need for very high strength, high temperature alloys with improved fatigue and crack resistance have been identified for use in such circumstances as in the manufacture of jet engine disks, as well as for the aforementioned use of superalloys.

[0008] The presence of ceramic contaminants in advanced spray formed turbine disks, for instance, reduces low cycle fatigue resistance, hence reducing part life and increasing life cycle cost. Ceramic defects originate from the refracting melting systems used in both the original melt stock preparation and in the spray forming process. A spray forming process that provides a reduction in the size and frequency of ceramic inclusions in the sprayed deposit is, therefore, of major importance.

[0009] Generally, the conventional spray forming process is carried out at a flow rate of 50 to 100 lbs per minute, which is about five to ten times faster than the melting rate for the Plasma Arc Melting process. Hence, the flow rate of the spray forming process must be reduced in order for the Plasma Arc Melting process to be utilized with spray forming. Prior attempts to reduce the spray forming process flow rate to 10 to 15 lbs per minute by reducing the diameter of the pouring nozzle have failed because the metal tends to freeze (solidify) at the nozzle tip.

[0010] Traditionally, the standard pouring nozzle used in the metallurgic industry provides for a flow regulating bore in a section located at the bottom of the nozzle, furthest from the crucible, and having a melt plug located at the top of the nozzle, at the base of the crucible. The melt plug at the top of the nozzle is formed so that a melt superheat of approximately 70-80°C is achieved before pouring initiates.

[0011] The metal flow rate is controlled by the flow regulating bore diameter at the bottom of the conventional nozzle. At flow rates in excess of about 30 lbs per minute, the molten metal passes through the flow regulating diameter without difficulty because the volume of molten metal is sufficient to keep the bore of the nozzle at a melt superheat temperature, i.e., the volume of molten metal is sufficient to prevent "freeze-off" or solidification within the nozzle.

[0012] However, difficulty arises when the metal flow rate drops below about 30 lbs per minute. The decrease in volume of molten metal in the nozzle bore lowers the superheat temperature within the nozzle, which causes metal freeze-off to occur at the exit end of the nozzle. Thus, a significant problem of the prior art is that the nozzle assembly having its flow rate controlling diameter in the normal location at the base of the assembly cannot be used at low metal flow rates (such as those employed in Plasma Arc Melting) because of the drop in the superheat temperature in the nozzle, which results in metal freeze-off.

[0013] It would thus be desirable to provide a nozzle which can be used at lower molten metal flow rates, e.g., 10 to 15 lbs/minute, without suffering from the problems of previous nozzles. Such a nozzle is also desired because it can be used in the Plasma Arc Melting process.

Summary of the Invention

[0014] The present invention will be understood in summary with respect to the following stated objects.

[0015] A principal object of the present invention is to develop a clean molten metal spray process, that combines clean melting processes having low melting rates with controlled bottom pouring of a molten metal stream at low flow rates. As used herein, the term "low flow rate" means flow rates below about 30 lbs. per minute.

[0016] Another principal object of the present invention is to provide an improved nozzle for pouring molten metal from the bottom of a crucible at low flow rates.

[0017] A further object of the present invention is to provide a bore insert at the upper portion of the nozzle which will preheat and maintain heat in the nozzle, particularly at the flow regulating bore of the nozzle, prior to and during metal flow into the nozzle; the insert acting as a conduction heater which minimizes the occurrence of metal freeze-off in the nozzle.

[0018] Another object of the present invention is to provide an improved spray forming process having molten metal pouring at low flow rates without freeze-off. Still another object of the present invention is to provide minimal friction to the flow of molten metal in a nozzle and to guide the molten stream into the atomization zone of a metal spray apparatus by increasing the inside diameter of the nozzle located below the bore insert.

[0019] In particular, the present invention provides a nozzle comprising: a member having length, a top portion, and a bottom portion and means defining an axial bore through the top and bottom portions, an insert comprising a conduction heater, and having a first axial bore which is generally concentric with the bore of the member, and means for retaining the insert in the bore of the member, so that the insert is positioned at the top portion of the member, and the first bore of the insert is in communication with the bore of the member.

[0020] In addition, the present invention also provides a vessel having such a nozzle; and, the present invention provides a method for melting and pouring metal or of spray forming employing such a nozzle.

Brief Description of the Drawings

[0021] Other objectives and features, and embodiments of the present invention are described in and will also be apparent from the following detailed description of the invention, taken in conjunction with the accompanying drawings, wherein:

FIG. 1 shows a cross-sectional view of a conventional pouring nozzle.

FIG. 2 shows a line graph showing molten metal flow rate versus conventional nozzle area or diameter.

FIG. 3 shows a cross-sectional view of the nozzle and insert of the present invention.

FIG. 3A shows a sectional view of the crucible without the nozzle.

FIG. 3B shows a sectional view of the crucible with the nozzle, but without the insert in the nozzle.

FIG. 4 shows the line graph of Fig. 2 with two additional data points showing flow rates for a nozzle having an insert of the present invention.

FIG. 5 shows a further embodiment of the present invention.

FIG. 6 shows an enlarged sectional view of the nozzle and insert of Fig. 5.

Detailed Description of the Invention

[0022] Referring to FIG. 1, the standard pouring means for a crucible 10 is a nozzle 11 assembly having a flow rate controlling bore diameter 12 at the bottom tip of the nozzle. The nozzle also has a melt plug cavity 13 at the base 15 of the crucible. Flow is then controlled by the flow regulating bore diameter at the bottom of the nozzle. FIG. 2 is a graph showing flow rate in pounds per minute, lbs./min., plotted on the ordinate versus diameter in millimeters, mm., or nozzle bore area in square millimeters, mm², plotted on the abscissa. Figure 2 shows that a larger nozzle bore area provides an increase in the flow rate. However, as nozzle bore area decreases producing lower flow rates of 30 lbs. per minute (shown as + sign data points), metal freeze-off occurs at the tip of the conventional nozzle shown in Fig. 1, because the temperature in the nozzle tip drops below the temperature of the molten metal inside the crucible due to the decrease in volume of the molten metal contained in the small nozzle area.

[0023] Spray deposition is a high flow rate process compared to conventional pouring techniques. Spray forming is typically carried out at a flow rate of 50 to 100 lbs./minute. Hence, early attempts to reduce the spray forming flow rate to 10-15 lbs./minute by reducing the pouring nozzle flow regulating bore diameter were not successful because the reduction in molten metal at the tip of the nozzle, causes the metal to cool to its freezing point.

[0024] Until recently spray forming at metal flow rates in the range of 10 to 15 lbs./minute has been non-existent. Hence, in order to achieve uniform and consistent spray forming coupled with low melting rate processes, metal flow rate must be lower. It is generally believed that crucible nozzle modification appears to be the only method of obtaining such low metal flow rate conditions.

[0025] The present invention attempts to combine the advantages of a spray forming process with the benefits of a ceramic-free melting process in order to avoid such disadvantages as ceramic inclusions in a final spray formed product such as a jet engine rotor disk. Note, however, that a ceramic crucible can be used with the nozzle and method of this invention.

[0026] In particular, to achieve the required cleanliness, the Plasma Arc Melting process is utilized. The Plasma Arc Melting process utilizes continuous feed of molten metal into a series of skull lined hearths to avoid introduction of ceramics. Multiple hearths provide sufficient residence time for inclusion flotation, or sinking and entrapment in the skull. Bottom pouring from the final hearth is then utilized to regulate the liquid metal feed into the atomization zone of a spray forming apparatus. The inability to maintain a stream of molten metal at these low melting rates due to metal freeze-off occurring at the standard pouring nozzle tip (See FIG. 1) is costly in terms of both inefficient use of equipment and material loss. Consistent initiation of metal flow from the hearth, and continuation of flow are essential for successful processing by metal atomization.

[0027] Further, the present invention provides a new nozzle design which avoids metal freeze-off at the nozzle tip yet permits pouring at low metal flow rates. Shown in Figure 3 is crucible 20 (See also FIG. 3A for sectional view of crucible without nozzle) having a nozzle 21 (See also FIG. 3B for sectional view of nozzle without the bore insert) at the base 25 of crucible 20 for pouring molten metal. Nozzle orifice 21A is generally circular and is shown in cross section. Nozzle orifice 21A has steps 22 and 23 at the junction 24 of the crucible base 25. Steps 22 and 23 on the interior of nozzle 21 to provide support for insert 27A (See also FIG. 6). Portion 21D of nozzle 21 has a first diameter 21B. Steps 22 and 23 have a second and third diameter larger than the diameter of portion 21D. Insert 27A is preferably made a material resistant to reaction with the molten metal, for example, boron nitride. Insert 27A has a flow controlling insert bore 27. Insert 27A controls the melt flow rate, and insert bore 27 acts as the flow controlling section in the nozzle.

[0028] The ratio of length (L) to the diameter (D) of insert bore 27 is preferably maintained at approximately 2.5 to 3.0, more preferably about 2.7, for maximum flow rate at a particular diameter. Insert 27A acts as a conduction heater, and is preferably designed so that the flow orifice 26 is in the shape of a truncated cone, i.e., it has a smaller diameter at its upper portion (where it contacts insert bore 27) and it becomes wider to where the insert is in contact with nozzle wall 21C, wall 21C contacts crucible 20. Insert bore 27 has generally parallel sides and preferably has a length of about 8.1 to 8.6 mm. and a diameter of about 3.0 to 3.5 mm for low molten metal flow rates. Insert bore 27, at its upper portion is in communication with orifice 29; orifice 29 can have a slightly larger diameter than that of insert bore 27. Orifice 29 is in communication with insert bore 27, and flow orifice 26 preferably varies in diameter from that of insert bore 27 to that of portion 21D or discharge orifice 28. Discharge orifice 28 is the opening at the bottom terminus of portion 21D. Nozzle orifice 21A, accordingly runs from discharge orifice 28 to portion 21D to flow orifice 26 (of insert 27A) to insert bore 27 (of insert 27A) to orifice 29. Flow orifice 26, portion 21D and orifice 28 are configured to have a diameter large enough to minimize, or preferably, prevent contact with the molten metal stream exiting bore 27. For example, portion 21D (from discharge orifice 28 to flow orifice 26) can be about 40 mm. in length and about 7.0 mm in diameter. Portion 21D is also suitably designed to have a diameter 21B corresponding to the wider section of flow orifice 26.

[0029] Low molten metal flow rates can be attributed to the novel design of the bottom pour nozzle of the present invention which has a decrease in the nozzle area. The low flow rates achieved in nozzles of this invention are shown in FIG. 4. Low flow rates of about 12 and 20 lbs. per minute were produced without freeze-off in the nozzle. As shown in Figure 4, flow rates of 12 to 20 lbs. per minute are significantly lower than the lowest flow rates that could be produced in standard nozzles without freeze-off of the molten metal stream in the nozzle.

[0030] A further embodiment of the present invention can be seen completely assembled in FIG. 5. Nozzle assembly 31 is located in the base 25 of the melting crucible 30. The upper portion of the nozzle 31 contains insert 37A, having a dome 39 which extends above inside base 25 of melt crucible 30 and into an admission cavity 32 drilled into the bottom face of a solid charge 34 in crucible 30. Insert 37A has a flow controlling insert bore 33 having the same diameter and length to diameter ratio requirements explained above for the nozzle in Figure 3. When charge 34 is melted, molten metal completely envelopes dome 39 so that the walls of flow regulating bore 33 approach the temperature of molten metal charge 34. Hence, freeze-off in the bore of insert 37A will be minimized as long as molten metal remains in the crucible 30. A small nickel or superalloy plug can be placed in insert bore 33 prior to melting of charge 34, so that pouring is not initiated until the plug is melted by the molten metal charge in the crucible 30. In this way the molten metal charge can be additionally superheated up to about 200°C, preferably charges are superheated to about 80° to 140°C. The remaining bore sections in insert 37A and nozzle 31 are configured to have a diameter large enough to minimize, or preferably, prevent contact with the molten metal stream exiting flow controlling bore 33.

[0031] The following non-limiting example is given by way of illustration only and is not to be considered a limitation of this invention, many apparent variations of which are possible without departing from the spirit of scope thereof.

Example 1: Inadequacy of Prior Art at Low Flow Rates

[0032] Spray forming with nickel based superalloys was performed with a series of standard pouring nozzles (FIG. 1) having a flow regulating diameter at the bottom of the nozzle and a melt plug cavity at the top of the nozzle using flow rates in excess of and below 30 lb/min. The flow regulating bore diameters in the nozzles ranged from about 3.0 mm to 7 mm. A metal charge was melted by induction in a ceramic crucible, with melt superheats of 70-80°C achieved before pouring was initiated. Spray forming occurred successfully at flow rates in excess of 30 lbs per minute for nozzles having bore diameters of 4 mm. or greater. Nozzles having 3 and 3.5 mm. bore diameters for flow rates below 30 lbs per minute yielded only one satisfactory atomization run out of eight runs. A satisfactory run occurred when the melt crucible was completely emptied. The seven unsatisfactory runs, i.e., incomplete emptying of the crucible, were the result of freeze-off at the nozzle tip. Slight variations to the standard nozzle design did not prevent freeze-off at the nozzle tip.

Example 2: Superiority of the Present Invention

[0033] 

[0034] Fifteen spray forming runs with nickel based superalloys were performed with the novel nozzle of the present invention, using flow rates below 30 lbs/min, particularly at 10 to 20 lbs/min. The bore of the insert located at the top of the nozzle had a length of about 9.0 mm, an insert bore diameter of 3.0 mm or 3.5 mm and acted as the flow regulator at the base of the crucible. The remainder of the nozzle below the insert (portion 21D, FIG. 3) was about 42 mm in length. Melt superheats as low as 40°C were achieved before pouring was initiated. Of the fifteen runs, twelve satisfactory runs (complete emptying) were obtained. This represents an 80% success rate achieved by the nozzle of the present invention at low flow rates. The three unsuccessful runs (where the melt crucible did not completely empty) can be attributed to human error in performing the runs, and not to the design of the nozzle. Thus, overall, metal freeze-off did not occur at the nozzle tip of the present invention when lower melt temperatures and lower flow rates were used. Accordingly, the nozzle of the present invention is surprisingly superior for low molten metal flow rates, as compared to the prior art nozzle.

Claims

1. A nozzle comprising:
   a member having length, a top portion, and a bottom portion and means defining an axial bore through the top and bottom portions,
   an insert comprising a conduction heater, and having a first axial bore which is generally concentric with the bore of the member, and
   means for retaining the insert in the bore of the member, so that the insert is positioned at the top portion of the member, and the first bore of the insert is in communication with the bore of the member.

2. A vessel for pouring a stream of molten metal comprising:
   a body comprised of a bottom of a heat resistant material, and sidewalls of heat resistant material,
   the sidewalls rising from the bottom and defining means defining an opening for accepting a stream of molten metal, the bottom having a means defining an orifice,
   a nozzle positioned in the orifice for the discharge of molten metal from the vessel, the nozzle comprising:
   a member having length, a top portion, and a bottom portion and means defining an axial bore through the top and bottom portions,
   an insert comprising a conduction heater, and having a first axial bore which is generally concentric with the bore of the member, and,
   means for retaining the insert in the bore of the member, so that the insert is positioned at the top portion of the member, and the first bore of the insert is in communication with the bore of the member.

3. A vessel as set forth in claim 2 wherein the body comprises a water cooled copper hearth.

4. A vessel as set forth in claim 2 wherein the body comprises a ceramic crucible.

5. A vessel or nozzle as set forth in claim 1 or 2 wherein the
bore of the member comprises a first portion having a second length and a first diameter, a second portion having a second length and a second diameter, and a third portion having a third length and a third diameter, wherein the second diameter is greater than the first diameter, the third diameter is greater than the second diameter and the second and third diameters define steps and the means for retaining the insert is positioned above the first diameter so that the first diameter communicates with the first bore of the insert.

6. A nozzle as claimed in claim 5 wherein the first bore of the insert has a first portion and a second portion, the second portion having a length and a diameter, the diameter of the second portion being less than the first diameter of the bore of the member, and the first portion being positioned below the second portion and in communication with both the second portion and the bore of the member, the first portion having a varying diameter, the varying diameter varying from comprising the first diameter of the bore of the member where the first portion communicates with the second portion.

7. A vessel as set forth in claim 5 wherein the first bore of the insert has a first portion and second portion, the second portion having a length and a diameter, the diameter of the second portion being less than the first diameter of the bore of the member, and the first portion being positioned below the second portion and in communication with both the second portion and the bore of the member, the first portion having a varying diameter, the varying diameter varying from comprising the first diameter of the bore of the member where the first portion communicates with the bore of the member to comprising the diameter of the second portion of the insert where the first portion communicates with the second portion.

8. A nozzle as set forth in claim 6 or a vessel as set forth in claim 7 wherein the
first portion of the first bore of the insert has a shape comprising that of a truncated cone.

9. A nozzle as set forth in claim 8 wherein the ratio of the length of the second portion of the first bore of the insert to the diameter of the first bore of the insert comprises 2.5 to 3.0.

10. A vessel as set forth in claim 8 wherein the ratio of the length of the second portion of the first bore of the insert to the diameter of the first bore of the insert comprises 2.5 to 2.9.

11. A nozzle as set forth in claim 9 or a vessel as set forth in claim 10 wherein the ratio comprises 2.7.

12. A nozzle or a vessel as set forth in claim 11 wherein the second portion of the first bore of the insert has a diameter comprising about 3.0mm to about 3.5mm.

13. A nozzle or a vessel as set forth in claim 1, 9 or 12 wherein the insert is comprised of boron nitride.

14. A nozzle as set forth in claim 12 wherein the first portion of the bore of the member has a length comprising about 40 mm.

15. A nozzle as set forth in claim 1 wherein the member is comprised of zirconia.

16. A method for melting metal and pouring the molten metal at a select suitable temperature and low flow rate comprising:
   applying a heat source to solid metal in a suitable vessel to form molten metal,
   and pouring said molten metal at said select suitable temperature and low flow rate by a nozzle comprising:
   a member having length, a top portion, and a bottom portion and means defining an axial bore through the top and bottom portions,
   an insert comprising a conduction heater, and having a first axial bore which is generally concentric with the bore of the member, and
   means for retaining the insert in the bore of the member, so that the insert is positioned at the top portion of the member, and the first bore of the insert is in communication with the bore of the member.

17. The method of claim 16 wherein the heat source comprises a plasma torch.

18. The method of claim 17 wherein the vessel has a bottom and the nozzle is positioned thereat.

19. The method of claim 17 further comprising transferring the molten metal from the vessel to a second vessel having a bottom with the nozzle positioned thereat.

20. A method for spray forming comprising pouring molten metal at a select suitable flow rate and a suitable temperature through a nozzle comprising:
   a member having length, a top portion, and a bottom portion and means defining a generally axial bore through the top and bottom portions,
   an insert comprising a conduction heater, and having a first axial bore which is generally concentric with the bore of the member, and
   means for retaining the insert in the bore of the member, so that the insert is positioned at the top portion of the member, and the first bore of the insert is in communication with the bore of the member.

21. The method of claim 20 wherein the flow rate is less than 30 lbs/min.

22. The method of claim 20 wherein the temperature is a superheat of about 40°C to 200°C.

23. The method of claim 20 wherein the temperature is a superheat of about 80° to 140°C.

Drawing