Abstract:
An apparatus and method are disclosed in which ultrasonic vibration is used to assist the degassing of molten metals or metal alloys thereby reducing gas content in the molten metals or alloys. High-intensity ultrasonic vibration is applied to a radiator that creates cavitation bubbles, induces acoustic streaming in the melt, and breaks up purge gas (e.g., argon or nitrogen) which is intentionally introduced in a small amount into the melt in order to collect the cavitation bubbles and to make the cavitation bubbles survive in the melt. The molten metal or alloy in one version of the invention is an aluminum alloy. The ultrasonic vibrations create cavitation bubbles and break up the large purge gas bubbles into small bubbles and disperse the bubbles in the molten metal or alloy more uniformly, resulting in a fast and clean degassing.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
   This invention was made with Government support under Contract No. DE-AC05-00OR22725 awarded to UT-Battelle, LLC, by the U.S. Department of Energy. The Government has certain rights in this invention. 

   CROSS-REFERENCES TO RELATED APPLICATIONS 
   Not Applicable. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates to a method and apparatus in which high-intensity ultrasonic vibration is applied to a radiator that creates cavitation bubbles, breaks up purge gas (e.g., argon or nitrogen) which is intentionally introduced in a small amount into the melt in order to collect the cavitation bubbles and to make the cavitation bubbles survive in the melt, and induces acoustic streaming to disperse the bubbles uniformly in the melt, resulting in a fast and clean degassing of the molten metal or molten metal alloy. 
   2. Description of the Related Art 
   Porosity is one of the major defects in the casting of aluminum alloys, magnesium alloys, and steels. The formation of porosity is due to a significant solubility difference of gaseous elements in the liquid and solid phases, and an inadequate feeding of the solidification shrinkage of the alloy. For example, in aluminum alloys, hydrogen solubility in molten alloy is much higher than that in the solid. As a result, hydrogen porosity forms during the solidification of aluminum castings. The presence of porosity can be detrimental to the mechanical properties and corrosion resistance of the castings. Molten aluminum and aluminum alloys also contain solid non-metallic inclusions (e.g. aluminum oxides, aluminum carbides) and various reactive elements (e.g. alkali and alkaline earth metals). Non-metallic solid inclusions reduce metal cleanliness and the reactive elements and inclusions may create unwanted metal properties. 
   Therefore, before many molten metals and alloys are used for casting, unwanted components that may adversely affect porosity or other physical or chemical properties of the resulting cast product are removed. These unwanted components are normally removed from molten metals by introducing a purge gas below the surface of the molten metal. As the resulting purge gas bubbles rise through the mass of molten metal, the gas bubbles adsorb gases (e.g., hydrogen) dissolved in the metal and remove them from the melt. In addition, non-metallic solid particles are swept to the surface by a flotation effect created by the bubbles and can be skimmed off. If the gas is reactive with contained metallic impurities, the elements may be converted to compounds by chemical reaction and removed from the melt in the same way as the solid particles. This process is often referred to as “metal degassing”, i.e. reducing the gas content of a molten metal or metal alloy. 
   Several methods have been proposed for degassing molten metals and molten metal alloys. For example, U.S. Pat. No. 6,887,424 describes a process for inline degassing of a molten metal using a rotary device for generating bubbles of inert gas in the molten metal. U.S. Pat. Nos. 5,660,614 and 5,340,379 describe devices for injecting gas into molten aluminum. However, in these devices, degassing is a relatively slow process due to the large size of the bubbles that are produced. Other problems associated with these methods are the failure of moving graphite parts and the disturbance of the molten melt surface during rotary degassing, resulting in a significant oxide formation. Furthermore, the purging gas (usually argon) has to contain a few percent of chlorine in order make degassing efficient. Chlorine may be detrimental to the environment. 
   Molten metal degassing devices using ultrasonics have also been proposed. For example, Japanese patent application JP 2-173205 describes a dipping pipe of a degassing machine having an ultrasonic vibrator attached to the inner wall of the dipping pipe through which molten metal is drawn from a ladle into a vacuum tank. Japanese patent application JP 2-173204 describes a vacuum tank of a degassing machine having an ultrasonic vibrator attached to the bottom wall of the vacuum tank including dipping pipes for circulating molten metal. These methods utilize ultrasonics to assist vacuum degassing but vacuum degassing is seldom used in North America. 
   High-intensity ultrasonic vibration has been used to create cavitation bubbles for degassing molten metals. These methods can be used for degassing melts of small volume because hydrogen diffused to the cavitation bubble can diffuse back to the molten metal and a part of the cavitation bubbles can not survive for a longer time. 
   There is still a need for an improved apparatus and method for the degassing of molten metals or molten metal alloys and to eliminate the use of chlorine in the purging gas. 
   SUMMARY OF THE INVENTION 
   The foregoing needs are met by the present invention which provides an apparatus and method in which ultrasonic vibration is used to assist in the degassing of molten metals or metal alloys thereby reducing gas content in the molten metals or alloys. A small amount of purge gas is used to provide stable bubbles for degassing and high-intensity ultrasonic vibration is used to create tiny cavitation bubbles, breakup the large purging gas bubbles into much smaller bubbles, which collect the cavitation bubbles along their way to escape at the surface of the melt, and to disperse the bubbles uniformly in the melt. 
   In one aspect, the invention provides an apparatus for degassing a liquid such as a molten metal or a molten metal alloy. The apparatus includes an ultrasonic transducer and a radiator coupled to the ultrasonic transducer. Activation of the ultrasonic transducer creates ultrasonic vibration in the radiator. The ultrasonic transducer may be a piezoelectric transducer that generates ultrasonic waves having a frequency of about 1000 Hz to about 2,000,000 Hz in response to electrical stimuli. 
   The radiator has a hollow shell defining a fluid flow path for transporting a purge gas. The shell includes at least one throughhole that extends from the fluid flow path to an outer surface of the shell. The through holes can be anywhere on the shell of the radiator exposed to the liquid. In one example form, the shell of the radiator has an outer cylindrical wall and an end wall that extends inward from the outer wall, and a plurality of throughholes extend from the fluid flow path through the end wall to the outer surface of the radiator. The radiator may be positioned in a wall of a container holding the liquid such that the outer surface of the radiator can contact the liquid in the container. Purge gas (e.g., argon or nitrogen) is transported through the radiator into the liquid (e.g., a molten metal or alloy such as an aluminum alloy) in the container (e.g., a refractory ladle, a crucible, a furnace, or in a reservoir in the trough transporting molten alloy). The ultrasonic vibrations of the radiator break up the large bubbles into small bubbles, as well as create cavitation bubbles and acoustic streaming in the melt. The acoustic streaming mixes the melt thoroughly, coalescing the cavitation bubbles with purge gas bubbles, and disperses the bubbles in the molten metal or alloy more uniformly, resulting in a fast and clean degassing. One of the purposes of using a small amount of purge gas is to make the ultrasonically induced cavitation bubbles survive in the melt such that the technology can be used for degassing of a large volume melt. For melt of small volume or of small liquid depth, no purge gas is needed since the cavitation bubbles can usually survive in the melt and escape from the melt surface. To induce cavitation bubbles, the intensity of the ultrasonic vibration should be high enough to create an instantaneous pressure on the order of a few MPa in the melt near the radiator. The cavitation threshold is about 1 MPa in molten aluminum. Cavitation is fully developed at 10 MPa, and pressure levels up to 100 MPa are possible. 
   Preferably, the radiator is positioned in the wall of the container such that the outer surface of the radiator is below liquid in the container. In this construction, the bubbles (cavitation bubbles or purge gas bubbles) rise through the mass of liquid, and the gas bubbles adsorb gases (e.g., hydrogen) dissolved in the liquid and remove them from the liquid. The container may include an inlet and an outlet for the liquid, and the outlet may be positioned above the inlet such that liquid having dissolved gas enters the inlet and degassed liquid exits the outlet at a higher level in the container. 
   A cooling jacket may be provided for surrounding the radiator for keeping the majority of the radiator at low temperatures by reducing heat transfer from the molten metal to the radiator. Also, in one form, the outer surface of the end wall of the radiator has a coating that resists bonding with the liquid in the container. Means for lowering air pressure above liquid in the container and/or means for regulating humidity above liquid in the container may be provided for enhanced degassing of the liquid. 
   In another aspect, the invention provides a method for degassing a liquid such as a molten metal or metal alloy. In the method, a radiator is coupled to an ultrasonic transducer. The radiator has a shell defining a fluid flow path for transporting a purge gas and the shell includes at least one throughhole that extends from the fluid flow path to an outer surface of the radiator. The liquid is contacted with the outer surface of the radiator, and the ultrasonic transducer is activated to create ultrasonic vibration in the radiator. Purge gas is then introduced into the liquid through the at least one throughhole in the radiator. Purge gas bubbles rise through the mass of liquid and are broken into smaller bubbles by the ultrasonic waves from the radiator. The purge gas bubbles adsorb cavitation bubbles and gases (e.g., hydrogen) dissolved in the liquid and remove them from the top of the liquid. 
   The method may be used with a container for the liquid. In this version of the method, the radiator is positioned in a wall of the container such that the outer surface of the radiator can contact liquid in the container. Preferably, the radiator is positioned in the wall of the container such that the outer surface of the radiator is below liquid in the container. As a result, the bubbles rise through the mass of liquid, and the gas bubbles adsorb gases (e.g., hydrogen) dissolved in the liquid and remove them from the liquid. 
   In one version of the method, a vacuum is created above the liquid in the container to enhance degassing. In another version of the method, humidity is regulated above the liquid in the container to enhance degassing. 
   In one specific application of the method, dissolved gases are removed from a molten metal or molten metal alloy in a container such as a refractory ladle, a refractory holding furnace, or any reservoir in the launder/trough of a molten metal transporting system. A radiator is coupled to an ultrasonic transducer. The radiator has a shell defining a fluid flow path for transporting a purge gas and the shell includes at least one throughhole that extends from the fluid flow path to an outer surface of the radiator. The outer surface of the radiator may have a coating that resists bonding with the molten metal or molten metal alloy in the container. Coatings that resist molten metal attack are suitable if they can be metallurgically bonded to the radiator. Example coatings include ceramic materials such as metal oxides, metal nitrides and metal carbides. The radiator is positioned in a wall of the container such that the outer surface of the radiator can contact the molten metal or molten metal alloy in the container. The ultrasonic transducer is activated to create ultrasonic vibration in the radiator, and a purge gas (e.g., argon or nitrogen) is introduced into the molten metal or molten metal alloy through the at least one throughhole. In one example application, the molten metal or molten metal alloy is selected from metals and metal alloys that are prone to porosity formation upon solidification from a melt. In another example application, the molten metal or molten metal alloy is selected from aluminum, aluminum alloys, magnesium, magnesium alloys, steels, or cast iron, and the dissolved gas is hydrogen in aluminum, aluminum alloys, magnesium, and magnesium alloys, and the dissolved gas is nitrogen or carbon containing gases in steels and cast irons. 
   These and other features, aspects, and advantages of the present invention will become better understood upon consideration of the following detailed description, drawings, and appended claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a cross-sectional view of a refractory ladle including an apparatus according to the invention. 
       FIG. 2  is a cross-sectional detailed view of an apparatus (part of the radiator and the cooling jacket) according to the invention shown in  FIG. 1 . 
       FIG. 3  is a top plan view of the apparatus according to the invention taken along line  3 - 3  of  FIG. 2 . 
       FIG. 4  shows an experimental apparatus used for the examples. 
       FIGS. 5   a  and  5   b  show micrographs of the porosity in solidified aluminum A356 specimens using alloy melts prepared at 740° C. under (a) a humidity of 60% in  FIG. 5   a , and (b) a humidity of 40% in  FIG. 5   b.    
       FIG. 6  is a graph showing the variation of densities for the solidified aluminum A356 specimens under different humidity levels. 
       FIGS. 7   a ,  7   b ,  7   c  show micrographs of the porosity in solidified aluminum A356 specimens after 0 minutes ( FIG. 7   a ), 1 minute ( FIG. 7   b ), and 4 minutes ( FIG. 7   c ) of ultrasonic vibration at 740° C. temperature and 60% humidity. 
       FIG. 8  is a graph showing the measured density of the solidified aluminum A356 specimens as a function of ultrasonic processing time in the melt of different initial hydrogen concentrations. 
       FIG. 9  is a graph showing the measured density of the solidified aluminum A356 specimens as a function of ultrasonic processing time in the alloy melt at different processing temperatures. 
       FIG. 10  is a graph showing the measured density of the solidified aluminum A356 specimens as a function of remnant pressure for vacuum degassing after 30 minutes of processing time. 
       FIG. 11  is a graph showing the measured density of the solidified aluminum A356 specimens as a function of processing time for vacuum degassing combined with and without ultrasonic degassing under a remnant pressure of 100 torr. 
       FIG. 12  is a graph showing the measured density of the solidified aluminum A356 specimens as a function of processing time for vacuum degassing combined with and without ultrasonic degassing under a remnant pressure of 1 torr. 
   

   Like reference numerals will be used to refer to like or similar parts from Figure to Figure in the following description of the drawings. 
   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention is carried out by “ultrasonic degassing”, which comprises vibrating a radiator at an ultrasonic frequency as a small amount of purge gas is introduced through the radiator into a molten material in order to facilitate the degassing of the molten material. The introduction of high-intensity ultrasonic vibration in the alloy melt generates an alternating pressure field within the alloy melt. This leads to the instantaneous variation in the local pressure from a minimum to a maximum at the ultrasonic vibration frequencies. The alternating pressure can be used to create tiny cavitation bubbles and acoustic streaming in the melt, and to break up bubbles produced by the purging gas. A small amount of purge gas (usually a fraction of what is used in the rotary degassing method) is introduced through the radiator to the melt in order to make the acoustically induced cavitation bubbles survive in the melt. In one embodiment, the purge gas is introduced through the radiator at a rate of 30 cubic feet per hour or less. The survival of the cavitation bubbles is achieved by collapsing the tiny cavitation bubbles into the purge gas bubbles. 
     FIG. 1  shows one example of an apparatus using ultrasonic vibration to speed up degassing of a melt. A refractory ladle  18  containing a molten metal or alloy melt  19  has an insulating outer wall  20  with a melt inlet  22  and a melt outlet  24 . Molten metal or alloy melt  19  is introduced into the ladle  18  at the melt inlet  22  at a location near the bottom of the ladle  18  and exits near the top of the ladle  18  at the melt outlet  24 . The melt outlet  24  has to be some distance below the melt surface  19   t  so that bubbles and surface oxide will not be sucked into the melt outlet  24 . An ultrasonic radiator  30  is positioned at the bottom of the outer wall  20  of the refractory ladle  18 . 
   Looking at  FIGS. 2 and 3 , the ultrasonic radiator  30  has a cylindrical outer wall  32  that defines a hollow cylindrical fluid flow path  34 . A purge gas is introduced into the fluid flow path  34  of the radiator  30  from a purge gas source (not shown). The choice of purge gas will depend on the melt being degassed. Noble gases are often suitable for degassing, and other gases such as nitrogen or chlorine are often used for certain applications. The cylindrical outer wall  32  of the ultrasonic radiator  30  terminates in an end wall  36  that substantially closes off the fluid flow path  34  of the radiator  30 . The cylindrical outer wall  32  and the end wall  36  form a hollow shell of the radiator  30 . Throughholes  38   a ,  38   b ,  38   c  and  38   d  extend through the end wall  36  of the hollow ultrasonic radiator  30  to the outer surface  36   t  which contacts with the melt  19 . Purging gas is introduced into the melt  19  from these throughholes  38   a ,  38   b ,  38   c  and  38   d  by way of the fluid flow path  34 . While four throughholes are shown in  FIG. 3 , the invention is not limited to any specific number of throughholes. Also, the location of the throughholes is not limited to the end wall  36 , e.g., the throughholes may be in the outer side wall  36 , or any location on the hollow shell of the radiator  30 . Preferably, the outer surface  36   t  of the end wall  36  of the radiator  30  is coated with a coating that prevents the top surface  36   t  of the radiator  30  from reacting or soldering with the molten metal melt  19 . Typically, the radiator  30  is formed from a metallic or ceramic material. 
   Still looking at  FIGS. 2 and 3 , a cooling jacket  40  surrounds the ultrasonic radiator  30 . The cooling jacket  40  is used to keep the ultrasonic radiator  30  at low temperatures. Coolant flows into the cooling jacket  40  in direction A of  FIG. 2  and then flows out of the cooling jacket  40  in direction B of  FIG. 2 . Air is one example coolant for use in the cooling jacket  40  to cool the radiator  30 . The cooling jacket  40  is protected using insulation materials  50  (shown in  FIG. 2 ). The ultrasonic radiator/cooling jacket assembly, is preferably placed at the bottom of the outer wall  20  of the refractory ladle  18  as shown in  FIG. 1 . 
   In accordance with the present invention, vibration at an ultrasonic frequency is operably applied to the radiator  30 . As is well known, ultrasound is a mechanical wave with a frequency at the top of or above the audible range that propagates by motion of particles within a medium. Preferably, the ultrasonic frequency is operably applied to the radiator  30  by an ultrasonic transducer that generates ultrasonic waves having a frequency of about 1000 Hz to about 2,000,000 Hz. Preferably, the frequency is in the range of 15 kHz to 25 kHz, and at an input power intensity in the range of 300 to 6000 watts, preferably in the range of 500 to 3000 watts. 
   The ultrasonic radiator/cooling jacket assembly shown in  FIGS. 1-3  can be installed in a refractory ladle that connects a melting furnace and a casting machine. Purge gas bubbles (e.g., argon) introduced through throughholes  38   a ,  38   b ,  38   c  and  38   d  in the end wall  36  of the radiator  30  break up from large bubbles into small bubbles by ultrasonic vibrations. These small bubbles released at the throughholes  38   a ,  38   b ,  38   c  and  38   d  of the end wall  36  of the radiator  30  will then travel upwards in the melt  19 , collecting the tiny cavitation bubbles induced by ultrasonic vibration as they travel upwards. Hydrogen (or other dissolved gases) in the melt  19  will then diffuse to the cavitation bubbles and the purge gas bubbles and escape at the top surface  19   t  of the melt  19 . Since molten metal with dissolved gases is introduced at the bottom of the ladle  18  at melt inlet  22  and degassed molten metal is discharged at the top of the ladle  18  at melt outlet  24 , the ultrasonic radiator  30  can be used for continuous degassing of molten metal for industrial applications. 
   Thus, the invention involves: (1) a radiator design that allows purge gas to be delivered through the radiator  30  to the melt  19  to break up the large purge gas bubbles into small bubbles, as well as to create a large number of tiny cavitation bubbles and acoustic streaming in the melt; (2) the use of a small amount of purge gas to create small bubbles (under the assistant of ultrasonic vibration) that collect those tiny cavitation bubbles and the dissolved hydrogen (i.e. making the cavitation bubble survive in the melt); (3) a cooling jacket  40  that keeps the major part of the radiator  30  at low temperatures, ensuring that the radiator  30  works and vibrates for extended time; (4) the placement of the radiator  30  at the bottom of the melt  19  thereby allowing the small bubbles released from the radiator  30  to travel upward throughout the melt  19 ; and (5) the design of the ladle  19  with melt inlet  22  and the radiator  30  near the bottom of the ladle  18 , and the melt outlet  24  at locations close to the top of the ladle  18 . 
   Alternative configurations of the invention are also possible. For example, multiple ultrasonic radiators  30  can be used in the ladle  18  for faster degassing, and the radiator  30  shown in  FIGS. 1-3  can be placed on the side or top of the outer wall  20  of the ladle  18 . The through holes  38   a ,  38   b ,  38   c , and  38   d  can also be placed at the side wall of the radiator exposing to the melt if the end wall of the radiator is not flush with the cooling jacket. Also, the shape of the ladle  18  can be varied to make bubble distribution more uniform. In addition, ultrasonic vibration alone (without purging gas) can also be used for the degassing for shallow melt or melt of a small volume (a few kilograms). 
   In another version of the invention, degassing is improved by lowering the air pressure above the melt  19  by way of a means for lowering air pressure above the melt. A suitable air tight cover is provided over the ladle  18 , and vacuum can be used at the top of the ladle  18  below the cover for fast degassing (gas removal), and ultrasonic vibrations can be used to assist the vacuum degassing. By vacuum, we mean a pressure below 760 torr. Preferably, the vacuum is below 200 torr, and most preferably, the vacuum is below 25 torr. The vacuum can be achieved and maintained by exhausting air above the ladle  18  with a vacuum pump. 
   In another version of the invention, degassing is improved by lowering the relative humidity above the melt  19  by way of a means for regulating humidity above the melt  19 . For example, a suitable air tight cover is provided over the ladle  18 , and air of a lower dew point can be obtained by compressing the atmosphere above the melt with a compressor and passing the compressed air through a dehumidifier. The dew point of the obtained air is thereby lowered. The portion above the surface of the melt  19  in the ladle  18  is rendered to be an atmosphere of air having a lower dew point than that of the surrounding atmosphere and is maintained at a lower humidity. A preferred relative humidity above the above the melt is 30% to 80%, and most preferably 40% to 60%. 
   The invention has many advantages, including without limitation: (1) fast degassing because small bubbles are generated creating large bubble/melt interfacial surface areas; (2) a significant reduction of the consumption of purge gas (argon or other gas) that is used for degassing; (3) less dross formation because the melt surface  19   t  is tranquil; (4) a clean process, that is, less emission, less dross formation, and preferably no use of chlorine; and (5) no moving parts compared with rotary degassing. 
   EXAMPLES 
   The following Examples have been presented in order to further illustrate the invention and are not intended to limit the invention in any way. For a small volume melt, no purge gas is needed because the cavitation bubbles can escape from the melt surface. All the following data were obtained without using any purge gas. 
   A. Experimental Apparatus and Methods 
   A study of ultrasonic degassing in an aluminum A356 alloy melt was performed. Aluminum alloy A356 typically contains 6.5-7.5% silicon, 0.2% max. iron, 0.2% max. copper, 0.1% max. manganese, 0.2-0.4% magnesium, 0.10 max. zinc, and 0.2% max. titanium. Typical applications of aluminum alloy A356 are airframe castings, machine parts, truck chassis parts, aircraft and missile components, and structural parts requiring high strength. 
   An experimental apparatus was constructed including an ultrasonic device which comprised: (i) an ultrasonic processor including a power supply, an air cooled transducer, a booster, a horn, and a radiator; and (ii) a heating unit including a furnace and a temperature controller.  FIG. 4  shows the experimental apparatus which included an ultrasonic generator  1 , a furnace controller  2 , a temperature indicator  3  for the alloy melt, a pneumatically operated device  4 , an air inlet line  5 , a transducer  6 , a booster  7 , a horn  8 , a radiator  9 , an electric furnace  10 , and a graphite crucible  11  inside the furnace  10 . Samples were solidified in a vacuum unit set at 28 inches Hg (pressure: 50 torr). 
   Ultrasonic degassing was carried out in the aluminum A356 alloy melt under three conditions: (1) Humidity: the humidity was varied from 40% to 60%; (2) Temperature of the melt: four melt temperatures, 620° C., 660° C., 700° C. and 740° C. were tested; and (3) Volume/size of the melt or the size of crucible: the weight of the melt was 0.2 kg, 0.6 kg and 2 kg, respectively. 
   B. Results 
     FIGS. 5   a ,  5   b  and 6 show the effect of humidity on the solidified aluminum A356 alloy specimens.  FIGS. 5   a  and  5   b  show micrographs of the porosity in solidified aluminum A356 specimens using alloy melts prepared at 740° C. under (a) a humidity of 60% in  FIG. 5   a , and (b) a humidity of 40% in  FIG. 5   b .  FIG. 6  is a graph showing the variation of densities for the solidified aluminum A356 specimens under different humidity levels (40%, 50% and 60%). 
     FIGS. 7   a ,  7   b ,  7   c  and  8  show that degassing can be achieved in a few minutes using ultrasonic vibration.  FIGS. 7   a ,  7   b ,  7   c  show micrographs of the porosity in solidified aluminum A356 specimens after 0 minutes ( FIG. 7   a ), 1 minute ( FIG. 7   b ), and 4 minutes ( FIG. 7   c ) of ultrasonic vibration at 740° C. temperature and 60% humidity.  FIG. 8  is a graph showing the measured density of the solidified aluminum A356 specimens as a function of ultrasonic processing time in the melt of different initial hydrogen concentrations. 
     FIG. 9  shows that ultrasonic degassing is more efficient at temperatures higher than 700° C.  FIG. 9  is a graph showing the measured density of the solidified aluminum A356 specimens as a function of ultrasonic processing time in the alloy melt at different processing temperatures. 
     FIG. 10  shows the effect of vacuum degassing.  FIG. 10  is a graph showing the measured density of the solidified aluminum A356 specimens as a function of remnant pressure for vacuum degassing after 30 minutes of processing time. 
     FIG. 11  shows the effect of vacuum degassing under 100 torr with the assistance of ultrasonic vibrations.  FIG. 11  is a graph showing the measured density of the solidified aluminum A356 specimens as a function of processing time for vacuum degassing combined with and without ultrasonic degassing under a remnant pressure of 100 Torr. 
     FIG. 12  shows the effect of vacuum degassing under 1 torr with the assistance of ultrasonic vibrations.  FIG. 12  is a graph showing the measured density of the solidified aluminum A356 specimens as a function of processing time for vacuum degassing combined with and without ultrasonic degassing under a remnant pressure of 1 Torr. 
   Thus, the invention provides a method and apparatus for removing impurities such as hydrogen from molten aluminum alloys or magnesium alloys. Among other things, the aluminum industry, metalcasting industry, and automotive industry can use this technology for fast and clean degassing. 
   Although the present invention has been described in considerable detail with reference to certain embodiments, one skilled in the art will appreciate that the present invention can be practiced by other than the described embodiments, which have been presented for purposes of illustration and not of limitation. Therefore, the scope of the appended claims should not be limited to the description of the embodiments contained herein.