EP2629906A1

EP2629906A1 - Wettable injectors for degassing of molten metal

Info

Publication number: EP2629906A1
Application number: EP11776661.8A
Authority: EP
Inventors: J. Daniel Bryant; Norbert Babcsan
Original assignee: Alcoa Inc
Current assignee: Howmet Aerospace Inc
Priority date: 2010-10-18
Filing date: 2011-10-18
Publication date: 2013-08-28
Also published as: CA2815185A1; US20120090432A1; WO2012054478A1

Abstract

Various illustrative embodiments of an apparatus and method for reducing the dissolved hydrogen content of a molten metal alloy are provided. The disclosed embodiments can be utilized for the processing of molten metal alloys such as aluminum, and more particularly, for the removal of dissolved hydrogen from molten metal alloys such as aluminum. Gas permeable diffusers can be employed that are wettable by molten metal. When used as gas injectors, either in combination with ultrasonic oscillation or without, the gas permeable wettable diffusers can provide a high density of ultrafine inert gas bubbles that can be used to rapidly and efficiently reduce the level of dissolved hydrogen within the molten metal.

Description

WETTABLE INJECTORS FOR DEGASSING OF MOLTEN METAL

BACKGROUND

1. Related Applications

[0001] This application claims the benefit, and priority benefit, of U.S. Provisional Patent Application Serial No. 61/394,117, filed October 18, 2010, titled "Wettable Injectors for Degassing of Molten Metal," the disclosure of which is incorporated herein in its entirety.

2. Field of Invention

[0002] This invention relates generally to degassing of molten metal alloys, and more particularly, to an apparatus and method for reducing the dissolved hydrogen content of a molten metal alloy.

3. Description of the Related Art

[0003] In industrial applications, molten liquid metal alloys must often be degassed to remove dissolved hydrogen. In the absence of a degassing treatment, the monatomic hydrogen that has been absorbed by the molten alloy, from such sources as atmospheric moisture, will precipitate upon solidification as pores of diatomic hydrogen gas within the cast metal product. Such gas porosity represents a threat to the structural integrity of the product because gas porosity cannot be eliminated by secondary processing such as rolling, forging or extrusion. Because of this, the hydrogen content of molten metal alloys is closely monitored in commercial casting facilities, and means must be employed to reduce the level of dissolved hydrogen within the molten alloy prior to the casting operation. SUMMARY

[0004] Various illustrative embodiments of an apparatus and method for reducing the dissolved hydrogen content of a molten metal alloy are provided herein. The disclosed embodiments can be utilized for the processing of molten metal alloys such as aluminum, and more particularly, for the removal of dissolved hydrogen from molten metal alloys such as aluminum. Gas permeable diffusers can be employed that are wettable by molten metal. When used as gas injectors, either in combination with ultrasonic oscillation or without, the gas permeable wettable diffusers can provide a high density of ultrafine inert gas bubbles that can be used to rapidly and efficiently reduce the level of dissolved hydrogen within the molten metal.

[0005] In an illustrative embodiment, an apparatus for degassing a molten metal alloy is provided. The apparatus can include a container for holding the molten metal and a dispenser capable of dispensing purge gas. A diffuser can be provided that is in fluid communication with the molten metal. The diffuser can be wettable with respect to the molten metal and capable of receiving purge gas from the dispenser. The dispenser can also be capable of forming purge gas bubbles and emitting the purge gas bubbles into the molten metal. In another aspect, the diffuser can have a face with a plurality of pores formed thereon. The pores can be in fluid communication with the molten metal and capable of emitting the purge gas bubbles into the molten metal. In certain embodiments, the average diameter of the pores on the diffuser face is not greater than 200 microns. The molten metal can be aluminum. The molten metal can also comprise other metal alloys. The molten metal can contain dissolved hydrogen gas, and the purge gas bubbles can remove the dissolved hydrogen gas from the molten metal. In another aspect, the apparatus can also include an ultrasonic oscillator in direct mechanical communication with the diffuser. The ultrasonic oscillator can be disposed adjacent to the diffuser such that the diffuser lies within a sonicated field of the oscillator. The ultrasonic oscillator can also oscillate both below and above the cavitation power required for the molten metal.

[0006] In another illustrative embodiment, a method for degassing a molten metal is provided. A molten metal can be provided with hydrogen gas dissolved therein. A purge gas can be introduced into a diffuser. The diffuser can be wettable with respect to the molten metal. Purge gas bubbles can be formed at the diffuser - molten metal interface. The dissolved hydrogen gas can be transferred from within the molten metal to the purge gas bubbles, such that the concentration of dissolved hydrogen gas in the molten metal is reduced.

[0007] In another illustrative embodiment, a method for degassing a molten metal containing dissolved hydrogen gas is provided. A face of a diffuser can be wetted with the molten metal. A purge gas can be flowed through a plurality of pores in the face. In certain embodiments, the pores can have a pore size in the range from approximately 2 - 200 microns. Purge gas bubbles can be produced at the pores in the diffuser. The purge gas bubbles can be emitted from the pores and into the molten metal. The dissolved hydrogen in the molten metal can be transferred to the purge gas bubbles, such that the concentration of the dissolved hydrogen gas in the molten metal is reduced. In another aspect, an ultrasonic oscillator can be provided that is in direct mechanical communication with the diffuser. The ultrasonic oscillator can also oscillate both below and above the cavitation power required for the molten metal.

[0008] It is to be understood that the subject matter herein is not limited to the exact details of construction, operation, exact materials, or illustrative embodiments shown and described, as modifications and equivalents will be apparent to one skilled in the art. Accordingly, the subject matter is therefore to be limited only by the scope of the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front perspective view of an apparatus for reducing the dissolved hydrogen content of a molten metal alloy according to certain illustrative embodiments set forth herein.

FIG. 2 is a front view of a contact angle (Θ) at the liquid-vapor and solid-liquid interface for a liquid droplet according to certain illustrative embodiments set forth herein.

FIG. 3A is a front perspective view of a purge gas bubble formed from a wettable injector according to certain illustrative embodiments set forth herein.

FIG. 3B is a front perspective view of a purge gas bubble formed from a non- wettable injector according to certain illustrative embodiments set forth herein.

FIG. 4 is a top view of a diffuser according to certain illustrative embodiments set forth herein.

FIG. 5 is a front perspective view of an apparatus for reducing the dissolved hydrogen content of a molten metal alloy having a sonotrode disposed adjacent the diffuser according to certain illustrative embodiments set forth herein.

FIG. 6 is a front perspective view of an apparatus for reducing the dissolved hydrogen content of a molten metal alloy having a sonotrode disposed in direct mechanical communication with the diffuser according to certain illustrative embodiments set forth herein.

FIG. 7 is a front perspective view of an apparatus for reducing the dissolved hydrogen content of a molten metal alloy having a sonotrode with a retaining cap disposed thereon according to certain illustrative embodiments set forth herein. DETAILED DESCRIPTION

[0009] Various illustrative embodiments of an apparatus and method for reducing the dissolved hydrogen content of a molten metal alloy are provided herein. In certain illustrative embodiments, the apparatus and method can employ gas permeable ceramic diffusers that are wettable by molten metal. The gas permeable diffusers can function as wettable injectors to provide ultrafine inert gas bubbles to the molten metal to reduce the level of dissolved hydrogen within the metal. In certain illustrative embodiments, the diffusers can be used in combination with ultrasonic oscillation to increase the dispersion of the gas bubbles in the metal. The dissolved hydrogen concentration can preferably be reduced to less than about 0.4 ml/100 gm at standard temperature and pressure.

[0010] In an illustrative embodiment, an apparatus 5 can include a container 10 for holding a molten metal alloy 20 and a dispenser 30 capable of dispensing purge gas into the molten metal alloy 20. The molten metal alloy 20 can comprise, for example, an aluminum alloy or other similar metal alloy. The purge gas can comprise argon gas or other similar inert gases. In certain embodiments, a small percentage of chlorine gas can also be included with the purge gas, as needed, to increase the effectiveness of the purge.

[0011] A diffuser 40 can be positioned adjacent to dispenser 30 and utilized to inject and disperse the purge gas into molten metal alloy 20. In certain illustrative embodiments, diffuser 40 can receive the purge gas from dispenser 30, form a plurality of purge gas bubbles 50, and then emit purge gas bubbles 50 into molten metal alloy 20. The dissolved hydrogen in molten metal alloy 20 will preferably diffuse through the interfaces of purge gas bubbles 50 as bubbles 50 pass through molten metal alloy 20.

[0012] In certain illustrative embodiments, diffuser 40 can have a face 60 with a plurality of pores 70 formed thereon, each pore having a lip 75 formed at its interface with face 60. Diffuser 40 can be gas permeable, such that purge gas bubbles 50 can be emitted into molten metal alloy 20 through pores 70. Diffuser 40 is preferably in at least partial fluid communication with molten metal alloy 20, which means that face 60 of diffuser 40 can directly contact the fluid of molten metal alloy 20. Pores 70 are also preferably in fluid communication with molten metal alloy 20, which means that to some extent, the surface area near the lip 75 of any pore 70 also directly contacts molten metal alloy 20.

[0013] Various parts of apparatus 5, including but not limited to dispenser 30 and diffuser 40, can be constructed from a material that includes a wettable ceramic material such as titanium diboride (TiB₂) or silicon carbide (SiC) to produce and inject a fine dispersion of purge gas bubbles 50 in molten metal alloy 20. To the extent that purge gas bubbles 50 are emitted through pores 70, the material from which pores 70 are formed can also preferably be constructed from a wettable material. Purge gas bubbles 50 can provide increased degassing efficacy and can degas molten metal alloy 20 in shorter times than can be accomplished using conventional rotary nozzle methods, and can also remove dissolved hydrogen from greater volumes of molten metal alloy 20 than can be treated using non-wettable gas injectors.

[0014] Wettable generally means that the material 41 from which the diffuser 40 is constructed is capable of a contact angle of less than 90 degrees to a drop of the molten metal alloy 20. As illustrated in FIG. 2, the contact angle (Θ) is the angle at which the liquid- vapor interface (between alloy 20 and purge gas 51) meets the solid- liquid interface (between diffuser material 41 and alloy 20). The tendency of any drop of the molten metal 20 to spread out over a flat, solid surface increases as the contact angle decreases. Thus, the contact angle provides an inverse measure of wettability. A contact angle less than 90° (low contact angle) usually indicates that wetting of the surface is favorable, and the fluid drop will spread over a large area of the surface. A contact angle greater than 90° (high contact angle) generally means that wetting of the surface is unfavorable so the fluid will minimize contact with the surface and form a compact liquid droplet.

[0015] In certain illustrative embodiments, the effectiveness of apparatus 5 can be increased if apparatus 5 is used in combination with ultrasonic vibration. Diffuser 40 can be placed within the ultrasonic field of a sonotrode 90 (See FIG. 5), or alternatively, used as part of sonotrode 90 itself. In the latter configuration, an ultrasonic oscillator 80 (operating either below or above the cavitation power for molten metal 20) can be fitted with diffuser 40 at the end of sonotrode 90 (See FIG. 6). In certain illustrative embodiments, ultrasonic oscillator 80 can be used to provide ultrasonic energy to molten metal 20 in the vicinity of diffuser 40 and increase the dispersion of purge gas bubbles 50 in molten metal alloy 20. For example, oscillator 80 can operate above and/or below the cavitation power for molten metal alloy 20. Cavitation power refers to the amount of power needed to create cavities with molten metal alloy 20. Oscillator 80 can have sonotrode 90 connected thereto or disposed adjacent thereto. In certain illustrative embodiments, oscillator 80 and sonotrode 90 can be disposed adjacent to the diffuser (FIG. 5), or alternatively, oscillator 80 and sonotrode 90 can be disposed to directly contact the diffuser (FIG. 6). Oscillator 80 and sonotrode 90 can also surround all of, or part of, dispenser 30 to provide ultrasonic vibration to the diffuser 40 (FIG. 6). Sonotrode 90 can be exposed to ultrasonic vibration from oscillator 80, and then assist in transferring this vibratory energy to molten metal 20. Sonotrode 90 can be constructed from a wettable material such as TiB₂ or SiC or from a refractory metal.

[0016] In certain illustrative embodiments, a retaining cap 100 can be utilized to secure diffuser 40 in a position in direct mechanical communication with the sonotrode 90. (See FIG. 7). Retaining cap 100 can have an orifice 110 formed therein that allows purge gas bubbles 50 to exit dispenser 30, pass through diffuser 40 and orifice 110, and enter molten metal alloy 20. Retaining cap 100 can be securely affixed to sonotrode 90 such that diffuser 40 cannot be misaligned or substantially displaced due to ultrasonic vibration. Retaining cap 100 can be removable from sonotrode 90 such that diffuser 40 can be replaced, if desired.

[0017] In certain illustrative embodiments, dispenser 30 can extend into the interior region of sonotrode 90 and deliver purge gas to the diffuser 40 (see FIG. 6). In certain illustrative embodiments, diffuser 40 can be mechanically or chemically bonded to sonotrode 90, or alternatively, sonotrode 90 can be fabricated entirely from wettable ceramics such as TiB2_, such that diffuser 40, including face 60 and pores 70, can all be subject to ultrasonic vibration.

[0018] Methods for reducing the dissolved hydrogen content of molten metal alloy 20 are also provided herein. In an illustrative embodiment, a purge gas can be introduced into diffuser 40. Diffuser 40 can be wettable with respect to molten metal 20. Purge gas bubbles 50 can be formed with diffuser 40. Purge gas bubbles 50 can be emitted from pores 70 of diffuser 40 and into molten metal 20 at a contact angle of less than 90° to molten metal 20. The dissolved hydrogen gas in molten metal 20 can diffuse into purge gas bubbles 50 such that all or substantially all of the dissolved hydrogen gas is removed from molten metal 20.

[0019] In an illustrative embodiment, face 60 of diffuser 40 can be wetted with molten metal 20. Purge gas can be flowed through a plurality of pores 70 in face 60. In an illustrative embodiment, pores 70 can have a pore size in the range from about 2 - 200 microns. Purge gas bubbles 50 can be produced at a number of pores 70, with each bubble 50 initiating as a hemi-spherical bubble with a diameter related to the diameter of the particular pore 70 from which it emerged. Purge gas bubbles 50 can be emitted from diffuser 40 and injected or dispersed into molten metal 20. The dissolved hydrogen gas can diffuse into purge gas bubbles 50 and can reduce the amount of dissolved hydrogen gas in molten metal 20. Also, diffuser 40 can be oscillated below and/or above the cavitation power of molten metal 20 to assist in dispersing purge gas bubbles 50. [0020] In certain illustrative embodiments, the diameter of any particular pore 70 will affect the diameter of the purge gas bubble 50 emerging from that particular pore 70 (see FIGS. 3A and 3B). If diffuser 40 is wettable by molten metal 20 such as, for example, liquid aluminum, the initial diameter of the hemispherical bubble 50 emerging from pore 70 will be set by the pore diameter. This pore diameter can therefore be selected by appropriate sizing of the particle size and volume fraction of the fugitive binders used in the fabrication of diffuser 40. For example, smaller pore diameters can result in an increased number of bubbles 50 each having a smaller size, but an overall increase in total surface area of bubbles 50 within molten metal alloy 20. Significantly, as the Stokes' velocity of bubbles 50 rising through a Newtonian fluid is reduced at smaller bubble diameters, the residence time of bubbles 50 within molten metal alloy 20 can likewise be increased. The combination of these two effects, an increased interfacial area between the purge gas and molten metal 20, and a decreased rise speed of bubbles 50 in their transit through molten metal 20, provide for greater opportunity for diffusion of hydrogen from molten metal 20 to the purge gas. An illustrative example of a wettable diffuser 40 is shown in FIG. 4, wherein diffuser 40 has been fabricated from a titanium diboride (TiB₂) material so as to have pores 70 on its face 60 that are gas permeable. TiB₂ can be sintered with fugitive binders (such as graphite) to yield connected porosity and also provide sufficient resilience to withstand the rigors of ultrasonic vibration. Other representative examples of wettable materials include SiC. Wettable diffuser 40 can provide a continuous stream of purge gas bubbles 50 into molten metal 20 when diffuser 40 is in fluid connection with a flow of inert purge gas.

[0021] The reduced diameter of bubbles 50 formed from diffuser 40 constructed of a wettable material according to the presently disclosed subject matter (see, e.g., FIG. 3A) can preferably increase the surface to volume ratio of the purge gas and can promote longer residence times for bubbles 50 within molten metal alloy 20. These two effects can enhance the kinetics of dissolved hydrogen diffusion into the inert gas. If a purge gas injector was constructed of non-wettable materials (such as some ceramics and transition metals), the diameter of the hemispherical bubble cap that forms would be determined not by the inner diameter of pore 70 from which bubble 50 emerges, but rather by the outer diameter of the pipe encompassing pore 70 (see, e.g., FIG. 3B). As a consequence of this phenomenon, the benefit of ever smaller physical pores 70 into any non-wetting purge gas dispenser is limited, as the effective size of the hemispherical gas bubbles 50 formed on the surface of such non- wetting purge gas dispenser would likely be more dependent on the diameter of the dispenser itself, rather than the diameter of pores 70 from which bubbles 50 emerge.

[0022] The embodiments of a degassing apparatus and method provided herein utilize minimal volumes of inert gas, thus reducing gas cost. Also, enrichment with chlorine gas can be decreased or avoided, which reduces environmental concerns and saves on maintenance in the exhaust systems. Also, removal of hydrogen using decreased flows of purge gas can reduce dross formation during processing, which directly reduces metal loss and indirectly reduces dross reclamation costs. Also, rapid degassing may allow for effective in-trough degassing, thus reducing the need for draining and/or flushing of large rotary head treatment boxes during alloy changes and the associated losses in productivity.

[0023] It is to be understood that the subject matter herein is not limited to the exact details of construction, operation, exact materials, or illustrative embodiments shown and described, as modifications and equivalents will be apparent to one skilled in the art. Accordingly, the subject matter is therefore to be limited only by the scope of the appended claims.

Claims

CLAIMS WHAT IS CLAIMED IS:

1. An apparatus for degassing a molten metal, the apparatus comprising:

a container for holding the molten metal;

a dispenser capable of dispensing purge gas; and

a diffuser in fluid communication with the molten metal from the container, the diffuser being wettable by the molten metal and capable of receiving the purge gas from the dispenser, forming purge gas bubbles from the purge gas, and emitting the purge gas bubbles into the molten metal.

2. The apparatus of claim 1, wherein the diffuser has a face with a plurality of pores formed thereon, the pores being in fluid communication with the molten metal and capable of forming purge gas bubbles from the purge gas and emitting the purge gas bubbles into the molten metal.

3. The apparatus of claim 2, wherein the average diameter of the pores is not greater than 200 microns.

4. The apparatus of claim 1 , wherein the molten metal is aluminum.

5. The apparatus of claim 1 , wherein the molten metal contains dissolved hydrogen gas and the purge gas bubbles are capable of removing the dissolved hydrogen gas from the molten metal.

6. The apparatus of claim 1, further comprising an ultrasonic oscillator disposed adjacent to the diffuser such that the diffuser lies within a sonicated field of the oscillator.

7. The apparatus of claim 6, wherein the ultrasonic oscillator is in direct mechanical communication with the diffuser.

8. The apparatus of claim 6, wherein the ultrasonic oscillator is operable below or above the cavitation power required for the molten metal.

9. The apparatus of claim 6, wherein the diffuser oscillates below the cavitation power required for the molten metal.

10. The apparatus of claim 6, wherein the diffuser oscillates above the cavitation power required for the molten metal.

11. The apparatus of claim 1 , wherein the composition of the diffuser includes a wettable material comprising titanium diboride.

12. The apparatus of claim 1 , wherein the composition of the diffuser includes a wettable material comprising silicon carbide.

13. A method of degassing a molten metal, the method comprising:

providing a molten metal alloy with hydrogen gas dissolved therein;

introducing a purge gas into a diffuser, the diffuser being in fluid communication with the molten metal at a diffuser-molten metal interface and wettable with respect to the molten metal;

forming purge gas bubbles at the diffuser-molten metal interface;

injecting the purge gas bubbles from the diffuser into the molten metal;

transferring the dissolved hydrogen gas from the molten metal to the purge gas bubbles; and reducing the concentration of dissolved hydrogen gas in the molten metal.

14. A method of degassing a molten metal containing dissolved hydrogen gas, the method comprising:

contacting a face of a diffuser with the molten metal;

wetting the face of the diffuser with the molten metal;

flowing a purge gas through a plurality of pores in the face, the pores having a pore size in the range from 2 - 200 microns;

producing purge gas bubbles at the pores;

emitting the purge gas bubbles from the pores and into the molten metal; transferring the dissolved hydrogen gas from the molten metal to the purge gas bubbles; and reducing the concentration of the dissolved hydrogen gas in the molten metal.

15. The method of claim 14, further comprising oscillating the diffuser below or above the cavitation power of the molten metal.

16. The method of claim 14, further comprising disposing the diffuser within the sonicated field of an oscillator.

17. The method of claim 14, further comprising attaching the diffuser to the sonotrode of an oscillator.