Abstract:
A method for casting articles from metal foam includes a molten metal bath and a foam forming means. The foam is drawn into a ladle, within a heated chamber, which transports a foam sample to a mould. The ladle deposits the foam sample into the mould and the mould is closed. Once cooled and hardened the formed article is removed. The system of the invention comprises a molten metal bath, a heated foam collecting chamber, a ladle for drawing a sample of the foam and for transporting the sample to a mould. The present invention provides an apparatus for carrying out.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention is directed to systems and methods for casting metal foam objects.  
         [0003]     2. Description of the Prior Art  
         [0004]     In the manufacture of products such as automobiles etc., there exists an increasing demand for components to be made from materials that have a high strength to weight ratio. In order to meet this demand, much emphasis has been placed on finding materials that are considerably low in weight yet high in strength for manufacturing such components. One such material that has been proposed is foamed metal.  
         [0005]     A metal foam is generally created by generating a gas in a molten metal bath so as to form a molten metal foam. The foam is then extracted and cooled. Metal foam offers various advantages as a replacement to standard metal such as meeting the above mentioned high strength to weight ratio, high shock or impact absorbing qualities, and sound absorbing qualities. The prior art teaches various methods for producing metal foam such as in U.S. Pat. Nos. 5,221,324 and 5,622,542. The known methods of generating the gas mentioned above include, among others: (1) the use of a gas supply, which blows or injects the gas into the molten metal; (2) the use of gas generating, or foaming agents, which release gas when heated; and, (3) the use of impellers to draw the desired gas into the molten metal bath. It is also know in the art to provide the molten metal with a number of additives to assist the foam in maintaining the integrity of the formed cells.  
         [0006]     Although the prior art provides various methods for producing metal foam slabs, which can be cut to desired dimensions, there is very little teaching of methods of forming the foam into three dimensional (3D) shapes of more complex geometries. U.S. Pat. No. 5,865,237 teaches one such method. In this reference, a metal powder and a gas evolving foaming agent are heated in a chamber to create a metal foam. While the foam is being generated, the molten mixture is forced into a mould cavity. The mixture is then allowed to continue to foam within the mould in order to ensure that the foam fills the entire volume of the cavity.  
         [0007]     The process taught by this prior art method includes various disadvantages. Firstly, the process must be carried out in a batch manner. That is, the production of a single piece involves each of the steps of charging the chamber with the required powders, melting the powders, forcing the material into the mould, finally, completing the foaming process, cooling the mould and extracting the finished article. For this reason, the process taught in U.S. Pat. No. 5,865,237 is very time consuming. Further, the step of forcing the foaming material into a mould cavity would require a force to be applied against the foam cells. This force would inevitably result in damage to some of the cells and, therefore, reduce some of the advantage of the foam material. In addition, the patent requires the use of a piston to force the foaming material into the mould. Since the piston of the &#39;237 patent, which is made of a metal, is maintained within the heated chamber at a temperature to maintain the molten metal in such state, it will be understood that the piston would have a tendency to seize due to damage caused by the heat. Further, the transfer of the foaming material must be done at a very specific time in the process in order to ensure that sufficient post-transfer foaming occurs. Finally, the method of forcing foaming material into the mould cavity taught by the &#39;237 patent does not allow of precise metering of such material. As such, the size and density of the final products would not be consistent.  
         [0008]     The present invention seeks to provide a metal foam casting system and process that mitigates at least some of the disadvantages of methods known in the alt.  
       SUMMARY OF THE INVENTION  
       [0009]     Thus, in one embodiment, the present invention provides a system for casting an article from a metal foam comprising: a molten metal bath; 
        a metal foam generator within the molten metal bath;     a chamber connected to the bath by a conduit;     a ladle in the chamber for receiving a sample of the foam;     a means for withdrawing the ladle from the chamber; and,     a mould having a mould cavity having a shape that is complementary to the article.     a means for withdrawing the ladle from the chamber; and,     a mould having a mould cavity having a shape that is complementary to the article.        
 
         [0017]     In another embodiment, the present invention provides a method of casting an article from a metal foam comprising: 
        providing a molten metal;     generating a foam from the molten metal;     drawing a sample of the foam;     transporting the sample to a mould;     cooling the mould; and,     withdrawing the formed article.       
 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]     These and other features of the preferred embodiments of the invention will become more apparent in the following detailed description in which reference is made to the appended drawings wherein:  
         [0025]     FIGS.  1  is a cross sectional view of a casting apparatus according to an embodiment of the invention in a foaming stage.  
         [0026]     FIGS.  2  is a cross sectional view of a casting apparatus after a foam sample is withdrawn.  
         [0027]      FIG. 3  is a cross sectional view of a mould in an open position with a foam transfer container.  
         [0028]      FIGS. 4 and 5  are cross sectional views of the mould in two stages of removal of the container.  
         [0029]      FIG. 6  is a cross sectional view of the mould in a closed position.  
         [0030]      FIGS. 7A and 7B  are top and side cross sectional views, respectively, of a foam article prior to trimming.  
         [0031]      FIG. 8  is a cross sectional view of a trim press.  
         [0032]      FIGS. 9A and 9B  are top and side cross sectional views, respectively, of a foam article after trimming. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0033]     In  FIG. 1 , an embodiment of the system of the invention is illustrated. The system includes a heated chamber  10  that is connected to a molten metal bath  12  in which is contained a molten metal  14 . The bath  12  is maintained at a temperature sufficient to maintain the metal in a molten state using any known type of heating system. A gas inlet or injection port  16  is provided at the bottom of the bath  12 , through which is pumped a gas  13 . The gas  13  is bubbled through the molten metal  14  thereby causing the formation of a foam  18  from the molten metal  14 . The gas port  16  may include a porous nozzle which is permeable to the gas. Alternatively, the port may comprise any other known structure for allowing the gas to be bubbled through the molten metal. As is known in the art, the rising gas bubbles cause a foam  18  to form on the top surface of the molten metal  14 .  
         [0034]     In a preferred embodiment, the bath  12  is divided into two sections by means of a dividing wall  20 , thereby creating a foam forming section  22  of the bath. As shown, the gas port  16  is preferably positioned under the foam forming section  22  so as to cause the foam  18  to form in section  22 . It will be appreciated that the foam formation will preferentially occur in section  22  due to the generally vertical rise of the gas bubbles  13 . In order to ensure this, dividing wall  20  is partially submerged in the molten metal  14 .  
         [0035]     Dividing wall  20  includes a curved diverter  24 , which is one example of a means for directing the rising foam  18  towards the heated chamber  10 . Within the chamber  10 , a foam transfer container or ladle  26  is provided. The ladle  26  is connected to a reciprocating rod  28 , which causes the ladle to move laterally within the chamber  10 . It will be understood that any other means for moving the ladle  26  may be used. As shown in  FIG. 1 , the ladle  26  is first positioned proximal to a side wall of the bath  12 . Further, the ladle  26  is provided within the chamber  10  so that the opening  30  of the ladle  26  is generally at the same level as the side wall of the bath  12 . Such vertical positioning is provided to enable the foam  18  directed by the diverter  24  to enter into and fill the ladle  26 . It will be appreciated that any other orientation of the ladle  26  is possible while still enabling the filling of same with the foam  18 . For example, in another embodiment, the ladle can be positioned slightly above the bath  12  or a further diverter mechanism can be utilized to cause foam to fill the ladle  26 . In yet another embodiment, the foam  18  can be scooped or otherwise provided into the ladle. It will also be understood that the diverter  24  can be of any shape or orientation for serving the purpose mentioned above.  
         [0036]     Once the ladle  26  is filled with a sample  27  of foam, the rod  28  is withdrawn thereby withdrawing the ladle from the chamber  10 .  FIG. 2  illustrates the system of the invention with the ladle  26  withdrawn. As illustrated a side wall  32 , opposite the bath  12 , of the heated chamber  10  is provided with a closable opening  34  through which the ladle  26  can be passed. As will be understood, the opening  34  is preferably maintained in a closed position, as shown in  FIG. 1 , until the ladle  26  is to pass through, as shown in  FIG. 2 . In this manner, heat loss from the chamber  10  is minimized. However, it will be appreciated that the opening  34  may also be kept open and other means utilized to maintain the temperature within the chamber  10 . Further, as shown in  FIG. 2 , the opening  34  is dimensioned so as to minimize clearance of the ladle  26 . In this manner, as the ladle  26  is withdrawn, the upper end  31  of the opening serves to scrape off any foam that exceeds the height of the ladle opening  30 . In this manner, the volume of the foam sample  27  withdrawn by the ladle  27  is consistent from one extraction to another. Furthermore, by using such a scraping action and not compacting the foam into the ladle, the integrity of the cells forming the foam is maintained.  
         [0037]     As will also be understood, the purpose maintaining the chamber  10  in a heated state is to ensure that the foam  18  is not allowed to cool and solidify until the forming stage is complete (as will be described further below). In a preferred embodiment, the chamber  10  is maintained at a temperature of approximately 500-700° C.  
         [0038]      FIG. 2  also illustrates a pool  36  of molten metal and foam that spills from the bath  12 , which collects at the bottom of the chamber  10 . It will be appreciated that such spillage may be recycled back to the bath  12 .  
         [0039]      FIG. 3  illustrates the ladle completely withdrawn from the chamber  10  and positioned two halves of a mould  39 . The mould  39  includes a first half  38  and a second half  40 . Each half of the mould  39  is provided with a cavity,  42  and  44  respectively, which together form a mould cavity that conforms to the outer shape of the article to be formed. Each of the cavities  42  and  44  are also provided with a partial notch  45 . When the mould halves are brought together, the notches  45  combine to form an overflow space around the article. This will be described further below.  
         [0040]     Once the ladle is positioned between the mould halves as in  FIG. 3 , the contents of the ladle must then be poured into one of the cavities in the mould halves. Preferably, the mould halves are provided below and above the ladle so that the ladle need simply be emptied into the lower mould cavity.  
         [0041]     FIGS.  3  to  5  illustrate a preferred embodiment of the ladle  26  which is designed to facilitate the emptying of the foam sample  27  into the mould cavity. In this embodiment, the ladle is formed in four sections, two pieces forming the base,  46  and  48 , and two pieces forming the sides,  50  and  52 . When the pieces  46 ,  48 ,  50  and  52  are connected together, they form the complete ladle as shown in  FIG. 3 . When the foam sample  27  is to be emptied into the mould cavity, the ladle  26  is dismantled by disconnecting the various pieces. In one embodiment, as shown in  FIGS. 4 and 5 , the dismantling process begins with the base pieces  46  and  48  first being separated, by sliding them away from each other, followed by separation of the side pieces  50  and  52  in a similar manner. The initial removal of the base pieces ensures that the foam sample  27  is maintained in the desired size for pouring into the mould cavity.  
         [0042]      FIG. 6  illustrates the mould  39  in a closed position, enclosing the foam sample  27  within the mould cavity. As shown, when the mould  39  is in the closed position, the notches  45 , described above, combine to form an overflow space  54  into which flows any excess foam that exceeds the volume capacity of the mould cavity. After the mould is cooled, the formed metal foam article can be removed.  
         [0043]     In the preferred embodiment, the mould  39  is formed of sand as is commonly known in the art. Sand offers various advantages when forming moulds, including low material and mould manufacturing cost and also very low heat transmission. With regard to the latter aspect, as a poor heat transmitter, sand would allow the foam within the mould to remain at its near molten state temperature. However, it will be understood that the sand mould can be replaced with a steel mould as well. Such steel moulds would require heating as is known in the art to prevent premature cooling and hardening of the foam. Methods for using steel moulds are taught, for example, in U.S. Pat. No. 5,865,237.  
         [0044]     It will be understood that during the transfer of the foam sample  27  from the heated chamber into the closed mould, the foam should be maintained at a molten temperature in order to keep the foam in a formable molten state. In a preferred embodiment, cooling of the molten foam is prevented by rapidly transferring the foam sample to the mould and completing the casting process. Such rapid transfer avoids the need for any external heat requirements. Moreover, since the mould is preferably made of sand held together with moisture, any external heat would lead to lead to evaporation of the moisture and collapse of the mould. In another embodiment, the region where the ladle is moved may be heated in a manner similar to the chamber  10  so as to prevent the foam sample from cooling. In such case, it will be understood that, in the event that a sand mould is used, the mould itself would not be heated for the reasons mentioned above. Further, where metal moulds are used, it will be appreciated that the entire region of passage of the ladle and the mould itself can be heated to the desired temperature. In such case, the mould can be cooled after closure to enable hardening of the cast foam.  
         [0045]      FIGS. 7A  and B illustrate a top and side view, respectively, of a foam article  58  removed from the mould of  FIG. 6 . As shown, the article  58  includes a ring  60  comprising the excess foam that was contained in the overflow space  54 .  
         [0046]      FIG. 8  shows a trim or nipping press  62  for removing the ring  60  on the formed article. The press  62  includes a base  64  for supporting the article  58 , having a first blade  66 . The article is positioned on the base  64  so that the ring  60  rests on the first blade  66 . The press also includes a pressing portion  68 , having a second blade  70 , which cooperates with first blade  66  to form a nip. The base  64  and pressing portion  68  are both provided with a cavity between their respective blades to accommodate the article  58 . In operation, the article  58  is positioned on the base  64  as indicated previously. The pressing portion  68  is then moved towards the base  64  so that the two blades are brought into contact. In this manner, the ring  60 , is nipped or trimmed off the article  58  by the cutting action of the cooperating blades  66  and  70 .  
         [0047]      FIGS. 9A  and B illustrate the article after the trimming process.  
         [0048]     In the above description, the foaming process has been described as using a gas supply port in the molten metal bath. However, it will be appreciated that any other foaming process may be used. For example, as taught in U.S. Pat. No. 5,865,237 and other references, a metal foam may be generated using foaming agents in a molten metal instead of a gas supply means. Further, the molten metal may be supplied with various additives that are know to stabilize the foam formed there from. In another embodiment, an impeller may be provided in the bath  12 , which draws air into the molten metal. In other embodiments, the gas port  16  of the invention may also comprise a rotating impeller or a vibrating nozzle.  
         [0049]     It will be understood by persons skilled in the art that the present invention can be used to form articles from metal foams of varying densities. The density of the foam (which is a function of the size and wall thickness of the cells forming the foam) will depend on a variety of factors such as the speed of gas addition, the amount and type of foaming additives added to the molten metal.  
         [0050]     Another embodiment of the mould of the invention is shown in  FIGS. 10 and 11 , wherein elements common to previous figures are shown with common reference numerals. In this embodiment, a mould is shown generally at  74 . The mould  74  includes a die region  76 , which conforms to the shape of the final object to be formed and a funnel region or riser  80 . The funnel region  80  serves to guide the foam  27  from the ladle  26 . As described above, the ladle  26  is preferably in four sections as described above, namely, side walls  50  and  52  and a base formed of pieces  46  and  48 . In  FIG. 10 , the base pieces  46  and  48  are shown in the separated position, which allows the foam  27  to drop into the mould. In this process, the interior wall  82  of the funnel region  80  of the mould  74  directs the foam into the die region  76 . The mould  74  may be formed with steel, ceramic, graphite, sand or other materials.  
         [0051]     In one embodiment, a plunger (not shown) may be used to force the foam  27  into the die region  76 . It will be understood that such plunger will conform to the dimensions of the funnel region  80 . The plunger can be made of refractory materials. Alternatively, the mould  74  can be vibrated to force the foam  27  into the die region  76 . In other embodiments, the foam can be forced into the die region using a vacuum, by applying air pressure, or by spinning the mould. Various other means will be apparent to persons skilled in the art.  
         [0052]      FIG. 11  illustrates the mould  74  after the foam has been delivered into the die region  76 . The ladle is not shown in this figure.  
         [0053]     As can be appreciated, the mould of this embodiment does not need to be closed to form the final product. However, the mould is separable so as to enable removal of the formed product. It will also be appreciated that the proportion of the funnel region has been exaggerated in order to depict the features of the mould and that the actual proportions and dimensions will be dependent on the final product being formed and will be easily determined by persons skilled in the art.  
         [0054]      FIG. 12  illustrates an article  84  formed in the mould of  FIGS. 10 and 11 . As can be seen, a portion  86  of the formed article  84  may conform to the lower portion of the funnel region  80  of the mould  74 . As above, this portion  86  of the article can be easily trimmed by cutting or other means resulting in the final desired article  87 , the outer shape of which corresponds to the shape of the die region or cavity  76 . Again, the proportion of the trimmed portion  86  has been exaggerated in order to illustrate this embodiment.  
         [0055]     Referring again to  FIG. 10 , the die region  76  of the mould  74  is provided with a vent or well  88  for collecting any liquid metal that was not foamed.  FIG. 11  illustrated the well  88  wherein liquid, non-foam metal is collected.  FIG. 12  illustrates a solid metal piece  90 , which corresponds to the molten metal that collects in the well  88 . As will be understood, such extraneous piece  90  is easily cut off from the desired finished article.  
         [0056]      FIG. 13  illustrates a mould  92  in the open position, after forming the desired article. The mould comprises a metal casing  94  the inside of which is filled with sand  96 . The interior portion of the mould includes a die cavity  98  formed into the sand. The upper end of the mould is provided with a funnel or guide as described above but not shown in  FIG. 13 . The lower end of the mould includes a well  100  as described above. The well as shown is of a “dove tail” shape.  
         [0057]      FIG. 14  illustrates an article  102  formed in the mould of  FIG. 13  prior to finishing. As can be seen, the article  102  includes a first end  104  that corresponds to the shape of a portion of the funnel (described above). A second end  106  corresponds to the liquid metal that entered the well  100 .  
         [0058]      FIG. 15  illustrates the article  102  of  FIG. 14  after trimming of the first and second ends.  
         [0059]     The present invention provides a casting process that does not require the foam generation step from being halted as with the prior art. As such, the invention allows for a continuous process for generating foam, portions of which can be withdrawn and cast in a mould. It will be appreciated that in another embodiment, the system can be provided with multiple ladles each drawing samples from the same chamber but at sequential times. Such ladles would then deposit the respective samples to different moulds. In this manner, the invention provides for a continuous process for casting metal foam articles.  
         [0060]     In the preferred embodiment, the metal is aluminum. However, it will be appreciated that any other metal may be utilized in the invention.  
         [0061]     Although the invention has been described with reference to certain specific embodiments, various modifications thereof will be apparent to those skilled in the art without departing from the spirit and scope of the invention as outlined in the claims appended hereto. Although the invention has been described with reference to certain specific embodiments, various modifications thereof will be apparent to those skilled in the art without departing from the spirit and scope of the invention as outlined in the claims appended hereto.