Abstract:
The present invention discloses a method of strip casting an aluminum alloy from immiscible liquids that yields a highly uniform structure of fine second phase particles. The results of the present invention are achieved by using a known casting process to cast the alloy into a thin strip at high speeds. In the method of the present invention, the casting speed is preferably in the region of about 50-300 feet per minute (fpm) and the thickness of the strip preferably smaller than 0.08-0.25 inches. Under these conditions, favorable results are achieved when droplets of the immiscible liquid phase nucleate in the liquid ahead of the solidification front established in the casting process. The droplets of the immiscible phase are engulfed by the rapidly moving freeze front into the space between the Secondary Dendrite Arms (SDA).

Description:
RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. application Ser. No. 11/734,113, filed on Apr. 11, 2007. The entirety of this application is hereby incorporated herein by reference for the teachings therein. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates to the casting of metals and to a method of strip casting immiscible metals in particular. 
       BACKGROUND OF THE INVENTION 
       [0003]    Aluminum based alloys containing Sn, Pb and Cd are commonly used in bearings found in internal combustion engines. The bearing function in these alloys is performed by the soft second phase particle of the alloying element which melts in the event of lubricant failure and prevents contact between the aluminum in the alloy and the steel protected by the bearing. 
         [0004]    In the prior art, the soft second phase in these alloys separates during solidification and often appears in the form of non uniform distribution. In many cases the second phase forms at grain boundaries as a continuous layer, or the heavier component (Sn, Pb, Cd) settles to the bottom due to gravity segregation. Typically, heat treatment is required after cold rolling of the cast sheet to redistribute the soft phase. For Al—Sn alloys for example, this is done by an annealing treatment at 662° F. (350° C.) during which the soft phase melts and coagulates into a desired uniform distribution of unconnected particles. In a final processing step, the strip is bonded on a steel backing for use as bearings in engines. 
         [0005]    Twin roll casting of Aluminum based bearing alloys yields better distribution of the second phase particles compared to conventional ingot casting. A drawback of twin roll casting, however, is that the method is slow, yields low productivity and creates a distribution of the soft phase(s) that is not completely desirable. Suitable results are also produced using a powder metallurgy process; however this method is expensive. There is a need, therefore, for a method that results in higher productivity and yields a uniform distribution of fine particles of the soft phase in the aluminum matrix. 
       SUMMARY OF THE INVENTION 
       [0006]    The present invention discloses a method of strip casting an aluminum alloy from immiscible liquids that yields a highly uniform structure of fine second phase particles. The results of the present invention are achieved by using a known casting process to cast the alloy into a thin strip at high speeds. In the method of the present invention, the casting speed is preferably in the region of about 50-300 feet per minute (fpm) and the thickness of the strip preferably in the range of 0.08-0.25 inches. Under these conditions, favorable results are achieved when droplets of the immiscible liquid phase nucleate in the liquid ahead of the solidification front established in the casting process. The droplets of the immiscible phase are engulfed by the rapidly moving freeze front into the space between the Secondary Dendrite Arms (SDA). 
         [0000]    As the SDA are small under rapid solidification conditions, (in the range of 2-10 μm) the droplets of the immiscible phase are uniformly distributed in the cast strip and are very fine. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a flow-chart describing the method of the present invention; 
           [0008]      FIG. 2  is a schematic depicting an example of an apparatus that can perform the method of the present invention; 
           [0009]      FIG. 3  is a perspective view detailing apparatus that can be operated in accordance with the present invention; 
           [0010]      FIG. 4  is a cross-sectional view of the entry of molten metal to the apparatus illustrated in  FIGS. 2 and 3 ; and 
           [0011]      FIG. 5  is a photomicrograph of a transverse section of a strip produced in accordance with the present invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0012]    The accompanying drawings and the description which follows set forth this invention in its preferred embodiments. It is contemplated, however, that persons generally familiar with casting processes will be able to apply the novel characteristics of the structures and methods illustrated and described herein in other contexts by modification of certain details. Accordingly, the drawings and description are not to be taken as restrictive on the scope of this invention, but are to be understood as broad and general teachings. When referring to any numerical range of values, such ranges are understood to include each and every number and/or fraction between the stated range minimum and maximum. 
         [0013]    Finally, for purposes of the description hereinafter, the terms “upper”, “lower”, “right”, “left”, “vertical”, “horizontal”, “top”, “bottom”, and derivatives thereof shall relate to the invention, as it is oriented in the drawing figures. 
         [0000]    The phrases “aluminum alloys”, are intended to mean alloys containing at least 50% by weight of the stated element and at least one modifier element. Suitable aluminum alloys include alloys of the Aluminum Association. 
         [0014]    The method of the present invention is depicted schematically in the flow chart of  FIG. 1 . As depicted therein, in step  100  a molten metal comprising aluminum and at least one immiscible phase is introduced into a suitable casting apparatus. In step  102 , the casting apparatus is operated at a casting speed greater than 50-300 fpm. In step  104 , the thickness of the cast strip is maintained at  0 .08-0.25 inch or smaller. 
         [0015]    The method of the present invention is suitable for use with known casting methods such as those disclosed in U.S. Pat. Nos. 5,515,908 and 6,672,368 for example. These methods produce thin strips at high speeds resulting in productivity in the range 600 to 2000 lb/hr per inch of width cast. 
         [0016]    An example of apparatus that can be employed in the practice of the present invention is illustrated in  FIGS. 2 ,  3  and  4  of the drawings. The apparatus depicted therein is in accordance with that disclosed in Commonly owned U.S. Pat. No. 5,515,908 and is presented as only one example of apparatus that can be used to achieve the results of the method of the present invention. 
         [0017]    The process will now be illustrated with respect to the apparatus depicted in  FIG. 2 , but is also applicable to the equipment depicted in  FIGS. 3 and 4 . As is depicted in  FIG. 2 , the apparatus includes a pair of endless belts  10  and  12  that act as casting molds carried by a pair of upper pulleys  14  and  16  and a pair of corresponding lower pulleys  18  and  20 . Each pulley is mounted for rotation about an axis  21 ,  22 ,  24 , and  26  respectively of  FIG. 2 . The pulleys are of a suitable heat resistant type, and either or both of the upper pulleys  14  and  16  is driven by a suitable motor means (not shown). The same is true for the lower pulleys  18  and  20 . Each of the belts  10  and  12  is an endless belt, and is preferably formed of a metal which has low reactivity or is non-reactive with the metal being cast. Quite a number of suitable metal alloys may be employed as well known by those skilled in the art. Good results have been achieved using steel and copper alloy belts. Other metallic belts can also be used such as aluminum. It should be noted that in this embodiment of the invention casting molds are implemented as casting belts  10  and  12 . However casting molds can comprise a single mold, one or more rolls or a set of blocks for example. 
         [0018]    The pulleys are positioned, as illustrated in  FIGS. 2 and 3 , one above the other with a molding gap therebetween. The gap is dimensioned to correspond to the desired thickness of the metal strip being cast. Thus, the thickness of the metal strip being cast is determined by the dimensions of the nip between belts  10  and  12  passing over pulleys  14  and  18  along a line passing through the axis of pulleys  14  and  18  which is perpendicular to the casting belts  10  and  12 . Molten metal to be cast is supplied to the molding zone through metal supply means  28  such as a tundish. The interior of tundish  28  corresponds in width to the width of the product to be cast, and can have a width up to the width of the narrower of the casting belts  10  and  12 . The tundish  28  includes a metal supply delivery casting tip  30  to deliver a horizontal stream of molten metal to the molding zone between the belts  10  and  12 . 
         [0019]    Thus, the tip  30 , as shown in  FIG. 4 , defines, along with the belts  10  and  12  immediately adjacent to tip  30 , a molding zone into which the horizontal stream of molten metal flows. Thus, the stream of molten metal flowing substantially horizontally from the tip fills the molding zone between the curvature of each belt  10  and  12  to the nip of the pulleys  14  and  18 . It begins to solidify and is substantially solidified by the point at which the cast strip reaches the nip of pulleys  14  and  18 . Supplying the horizontally flowing stream of molten metal to the molding zone where it is in contact with a curved section of the belts  10  and  12  passing about pulleys  14  and  18  serves to limit distortion and thereby maintain better thermal contact between the molten metal and each of the belts as well as improving the quality of the top and bottom surfaces of the cast strip. 
         [0020]    The casting apparatus shown in  FIGS. 2 ,  3  and  4  includes a pair of cooling means  32  and  34  positioned opposite that portion of the endless belt in contact with the metal being cast in the molding gap between belts  10  and  12 . The cooling means  32  and  34  thus serve to cool the belts  10  and  12  just after they pass over pulleys  16  and  20 , respectively, and before they come into contact with the molten metal. As illustrated in  FIGS. 2 and 3 , the coolers  32  and  34  are positioned as shown on the return run of belts  10  and  12 , respectively. The cooling means  32  and  34  can be conventional cooling means such as fluid cooling tips positioned to spray a cooling fluid directly on the inside and/or outside of belts  10  and  12  to cool the belts through their thicknesses. 
         [0021]    Thus molten metal flows horizontally from the tundish through the casting tip  30  into the casting or molding zone defined between the belts  10  and  12  where the belts  10  and  12  are heated by heat transfer from the cast strip to the belts  10  and  12 . The cast metal strip remains between and is conveyed by the casting belts  10  and  12  until each of them is turned past the centerline of pulleys  16  and  20 . Thereafter, in the return loop, the cooling means  32  and  34  cool the belts  10  and  12 , respectively, and remove therefrom substantially all of the heat transferred to the belts in the molding zone. The supply of molten metal from the tundish through the casting tip  30  is shown in greater detail in  FIG. 4  of the drawings. As is shown in that figure, the casting tip  30  is formed of an upper wall  40  and a lower wall  42  defining a central opening  44  therebetween whose width may extend substantially over the width of the belts  10  and  12 . 
         [0022]    The distal ends of the walls  40  and  42  of the casting tip  30  are in substantial proximity to the surface of the casting belts  10  and  12 , respectively, and define with the belts  10  and  12  a casting cavity or molding zone  46  into which the molten metal flows through the central opening  44 . As the molten metal in the casting cavity  46  flows between the belts  10  and  12 , it transfers its heat to the belts  10  and  12 , simultaneously cooling the molten metal to form a solid strip  50  maintained between casting belts  10  and  12 . Sufficient setback (defined as the distance between first contact  47  of the molten metal  46  and the nip  48  defined as the closet approach of the entry pulleys  14  and  18 ) is provided to allow substantially complete solidification prior to the nip  48 . 
         [0023]    To produce the results yielded by the method of the present invention utilizing the apparatus described in  FIGS. 2-4 , a molten aluminum based alloy comprising a phase that is immiscible in the liquid state is introduced via tundish  28  of  FIG. 3  through casting tip  30  into the casting zone defined between belts  10  and  12 . Preferably, the dimensions of the nip between belts  10  and  12  passing over pulleys  14  and  18  should be in the range of  0 . 08  to  0 . 25  inches , and the casting speed is  50  - 300  fpm. Under these conditions, droplets of the immiscible liquid phase nucleate ahead of the solidification front and are engulfed by the rapidly moving freeze front into the space between the SDA spaces. Thus, the resulting cast strip contains a uniform distribution of the droplets of the immiscible phase. 
         [0024]    Turning now to  FIG. 5  a photomicrograph of a section of a Al-6Sn strip  400  produced in accordance with the present invention is shown. The strip shows a highly uniform distribution of fine Sn particles  401  which are 3 μm or smaller. This result is several times smaller than particles that would result from material made from an ingot or by roll casting which are typically 40-400 μm in size. Moreover, the strip produced by the present invention requires no heat treatment for re-distribution of the soft phase and is ideal for providing the required lubricating properties for use in bearings for example. If so desired the strip can be used in as-cast form without being subject to additional fabrication such as rolling for example. 
         [0025]    Having described the presently preferred embodiments, it is to be understood that the invention may be otherwise embodied within the scope of the appended claims.