Abstract:
A method of transferring molten metal to a die casting mold is disclosed. The method includes providing a ladle with a dip well and a dispensing nozzle having a fluid metal filter formed therein as well as providing a receptacle fluidly between the ladle and the mold. Further the method includes delivering the molten metal from the ladle to the receptacle by positioning an exit face of the dispensing nozzle over the receptacle and rotating the ladle such that the exit face of the dispensing nozzle is repositioned proximal the bottom of the receptacle and conveying the molten metal that has been delivered to the receptacle into a mold cavity that is placed in fluid communication therewith.

Description:
BACKGROUND TO THE INVENTION 
       [0001]    This invention relates generally to an improved way to pour molten metal used in a casting operation, and more particularly to minimize the metal damage due to filling of shot sleeve of a horizontal high pressure die casting machine by using bottom filling of the shot sleeve and removal of inclusions present from the dip well. 
         [0002]    Low process cost, close dimensional tolerances (near-net-shape) and smooth surface finishes are all desirable attributes that make high pressure die casting (HPDC) a widely used process for the mass production of metal components. By way of example, manufacturers in the automobile industry use HPDC to produce near-net-shape aluminum alloy castings for engine, transmission and structural components. In a typical HPDC process, molten metal is introduced into shaped mold cavities through two metal transfer steps: a (first) low pressure tilt pour from a ladle to a filler tube (called a shot sleeve), and a (second) high pressure injection (such as upon movement of a piston in the tube) into the gating/casting cavity. 
         [0003]    The pouring of a molten material, such as metal, for example, into a casting mold is a significant process variable that influences the internal soundness, surface conditions, and mechanical properties, such as tensile strength, porosity, percent elongation and hardness, of a cast object. Many different designs for dipping/pouring ladles exist and are used in the foundry industry. The designs are normally chosen based upon the type of molten metal and casting mold used. Commonly used ladles make use of a slot, a lip and a baffle, or a dam at the top of the ladle to reduce inclusion of furnace metal oxides during metal filling, or the ladle may incorporate a stopper rod to control the flow of metal into and out of the ladle. 
         [0004]    Aluminum alloy castings are sensitive to molten metal delivery speed. Molten metals such as aluminum, for example, react with the air and create oxides, commonly known as dross, which upon mixing with the rest of the molten metal creates inclusions and highly porous regions in the cast object during solidification of the metal. When the delivery speed is too low, misruns and cold shuts may result; when it is too high, turbulent flow can entrap air or other gases that can in turn lead to oxide formations, as well as form surface molten aluminum that oxidizes when it comes in contact with ambient air. While many factors influence and account for undesirable properties in the cast object, two common sources of inclusions include formation of a dross layer on top of the molten metal, and the folding action of the molten metal caused by turbulent flow of the molten metal during pouring. Turbulent metal flow exposes the molten metal surface area to the air which creates the dross layer. Depending on the velocity of the molten metal, dictated by the pouring ladle and shot sleeve design and use, the molten metal may fold-over itself many times, thereby trapping oxygen and metal oxide layers therein and exposing additional surface area of the metal to the air. 
         [0005]    The concern over higher speed HPDC operations—while more efficient for large-scale production than their low-speed counterparts—is particularly acute considering that the high velocities are an inherent part of the higher delivery pressures. Both the entrapped (i.e., bi-film) and surface (i.e., top-layer) dross mix and subsequently solidify with the rest of the molten metal, which in turn leads to inclusions and highly porous regions that adversely impact structural and mechanical properties of the cast component. 
         [0006]    Research has shown that the entrained air (i.e., bi-film) variant of dross can arise if the velocity of the liquid metal is sufficiently high, and that such a velocity is believed to be between 0.45 m/s and 0.5 m/s for Al, Mg, Ti and Fe alloys. See, for example, Campbell,  Castings  (Elsevier Butterworth-Heinemann, 2003). Thus, it is desirable to keep metal delivery speeds under this critical velocity to significantly reduce the number of oxides being formed in the casting. Maintaining a low metal velocity below the critical velocity is not achievable in a standard tilt pour filling operation of a horizontal shot sleeve because of the required height in which it is poured. The typical free fall velocity of the aluminum alloy stream reaches over 2.5 m/s, five times higher than the recommended velocity. This metal damage is additive to the damage done during the high pressure injection phase. 
         [0007]    Typical foundry ladles are referred to as tilt-pour ladles. These ladles are substantially cylindrical in shape with an external spout extending outwardly from the top thereof. The molten metal is typically transferred from the ladle to a casting mold through a pour basin. Turbulence of the molten metal also results when the molten metal is poured through the air and into the pour basin. One method of eliminating this turbulence is described in U.S. Pat. No. 8,522,857 for “Ladle for Molten Metal.” A ladle couples to the mold gating system and rotates to raise the metal above the junction. Two mold pieces are used to form the sprue and coupling orifice. This technology eliminates the need for a pour basin and the free falling metal stream. Its implementation to the filling of a horizontal shot sleeve is deterred by its one piece construction and lack of accessible parting lines. 
         [0008]    Porous ceramic foam materials have been used in metal melting furnaces and gravity pour gating systems. Filter efficiency in cleaning molten metal is described in U.S. Pat. No. 3,893,917 for “Molten Metal Filter”, U.S. Pat. No. 3,962,081 for “Ceramic Foam Filter”, and U.S. Pat. No. 4,056,506 for “Method of Preparing Molten Metal Filter”. The addition of filters in low pressure and gravity pour casting molds has been successfully implemented. Mold and core prints allow a filter to be seated in the metal flow path close to the casting cavity, reducing the metal velocity and capturing inclusions. However, there is no feature similar to the mold and core prints which allow a filter to be seated in the metal flow path in a horizontal shot sleeve. 
         [0009]    There is a continuing need for a production viable method of transferring molten metal from the ladle to a horizontal die casting shot sleeve which minimizes turbulence in the molten metal and militate against inclusions in a cast component. 
       SUMMARY OF THE INVENTION 
       [0010]    It is against the above background that embodiments of the present invention generally relate to methods to reduce the air entrainment and oxide film inclusions due to the gravity filling of a horizontal die casting shot sleeve. According to a first aspect of the present invention, a method of transferring molten metal to a die casting mold includes providing a ladle with a dip well and a dispensing nozzle formed therein as well as providing a receptacle fluidly between the ladle and the mold. The method also includes delivering the molten metal from the ladle to the receptacle by positioning an exit face of the dispensing nozzle over the receptacle and rotating the ladle such that the exit face of the dispensing nozzle is repositioned proximal the bottom of the receptacle. Additionally, the method includes conveying the molten metal that has been delivered to the receptacle into a mold cavity that is placed in fluid communication therewith. Further, the dispensing nozzle includes a fluid metal filter formed therein. 
         [0011]    According to another aspect of the present invention, a method of transferring molten metal to a die casting mold includes providing a ladle with a dip well and a dispensing nozzle receptor formed on opposite sides of the ladle. The method further includes affixing the dispensing nozzle to the dispensing nozzle receptor. Further, the method includes providing a horizontal shot sleeve fluidly between the ladle and the mold. The method additionally includes collecting the molten metal in the ladle and delivering the molten metal from the ladle to the horizontal shot sleeve by positioning an exit face of the dispensing nozzle over the horizontal shot sleeve and rotating the ladle such that the exit face of the dispensing nozzle is repositioned proximal the bottom of the horizontal shot sleeve. Further, the method includes conveying the molten metal that has been delivered to the horizontal shot sleeve into a mold cavity that is placed in fluid communication therewith. Additionally, the dispensing nozzle includes a fluid metal filter formed therein. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The following detailed description of the preferred embodiments of the present invention can be best understood when read in conjunction with the following drawings, where like structure is indicated with like reference numerals and in which: 
           [0013]      FIG. 1  is a simplified view of a gating system according to the prior art; 
           [0014]      FIG. 2  shows a representative bi-film produced by turbulence of the prior art; 
           [0015]      FIG. 3  shows a perspective view of a ladle comprising a screen according to an aspect of the present invention; 
           [0016]      FIG. 4  shows a perspective view of a ladle comprising a filter according to an aspect of the present invention; and 
           [0017]      FIGS. 5A and 5B  show sequential steps in delivering molten metal from the ladle of  FIGS. 3 and 4  to a shot sleeve according to an aspect of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0018]    Referring first to  FIG. 1 , in one form of HPDC, a network of fluidly connected channels may be used to convey the molten material to the mold cavities; such a network is commonly referred to as a gating (or charging) system  1 . In the figure, the notional component that corresponds to the depicted shot design being produced is a two-cavity automotive oil filter adapter  5 , although it will be appreciated by those skilled in the art that any other component compatible with HPDC manufacturing could also be shown without detracting from the nature of the present invention. Among other components, the gating system  1  may include the end of the shot sleeve biscuit  10 , a runner  20  and casting cavity gates  30 . 
         [0019]    Referring next to  FIG. 2 , multiple forms of defects in an aluminum alloy are shown. Upon heating into liquid (i.e., molten) form  100 , various streams of aluminum (for example, first stream  110  and second stream  120 , as well as droplets  130 ) interact in varied ways. When processed in an oxygen-containing environment, oxide films  140  may form on the outer surface of the liquid aluminum, including the first stream  110 , second stream  120  and droplets  130 . A bi-film  170  forms when the two oxide films  140  from respective first stream  110  and second stream  120  meet. Bi-films also form when turbulence-induced droplets land on the metal stream, as shown at  150 . While bi-films  150 ,  170  are an inherent part of almost every casting process, they are generally not detrimental to casting mechanical properties unless the oxide film  140  is entrained in the bulk of the alloy, as shown at location  160  due to the folding action when two separate streams, first stream  110  and second stream  120 , meet at large angles (typically more than  135  degrees, where the splashing action of one stream collapses onto another stream to form a cavity therebetween). Such a formation can have significant impacts on overall material integrity and subsequent casting scrap rates. Likewise, entrained gas  180  may form from the pouring action of liquid metal, creating additional entrained oxides. As mentioned above, when liquid metal is poured or forced into a mold or shot sleeve in a conventional manner, it is possible to trap large gas bubbles. 
         [0020]    Referring next to  FIGS. 3 and 4 , a ladle  200  includes a main body  202 , hollow interior  204 , and an opening  206  for receiving molten metal  100 . The opening  206  has a size that accommodates a dipping operation (such as into a crucible, dip well or related device) while permitting the ladle  200  to hold a sufficient quantity of the molten metal  100  in the hollow interior  204  during transport. For example, the opening  206  may be a substantially open top used for filling the hollow interior  204  with the molten metal  100 . As a non-limiting example, the main body  202  may be in the form of a partial cylinder with capped ends. Other shapes for the main body  202  may also be used, as desired. 
         [0021]    The main body  202  has a dispensing nozzle  208  formed therein. In one form, the dispensing nozzle  208  may be integral or non-reversibly attached with the sidewall  210  of the main body  202 . In other forms, the sidewall  210  comprises a dispensing nozzle receptor  212  to which the dispensing nozzle  208  may be reversibly attached such as with a threaded connection. The dispensing nozzle  208  ranges from a minimum length of approximately 100 mm to a maximum length of approximately 350 mm. A funnel panel (not shown) may form part of the sidewall  210  of the portion of the main body  202  that is adjacent the dispensing nozzle  208  and may be used to help direct the molten metal  100  toward the dispensing nozzle  208  when the ladle  200  is rotated to orient the dispensing nozzle  208  downward. An orientation of the sidewall  210  may be such that it is angled downwardly when the main body  202  is rotated is rotated to orient the dispensing nozzle  208  downward. 
         [0022]    The dispensing nozzle  208  further has a fluid metal filter  220  formed therein. The fluid metal filter  220  captures inclusions such as deleterious oxides transferred from the dip well bath allowing inclusion free molten metal  100  to pass through. In addition, the fluid metal filter  220  reduces the metal velocity exiting the dispensing nozzle  208 , reducing the turbulence and oxide generation of the metal stream as it fills the shot sleeve. 
         [0023]    In one form, the fluid metal filter  220  is a screen  222 . In various embodiments the screen  222  is disposed proximal the exit face  226  of the dispensing nozzle  208 . For example, the screen  222  may be placed at 70%, 80%, or 90% along the length of the dispensing nozzle  208  so as to be closer to the exit face  226  than the dispensing nozzle receptor  212 . The length of the dispensing nozzle  208  being represented by the axis spanning from the attachment to the dispensing nozzle receptor  212  to the exit face  226 . In further embodiments the screen  222  is disposed distal the exit face  226  of the dispensing nozzle  208 . For example, the screen  222  may be placed at 10%, 20%, 30%, or 40% along the length of the dispensing nozzle  208  so as to be closer to the dispensing nozzle receptor  212  than the exit face  226 . In yet further embodiments the screen  222  is disposed on or at the exit face  226  of the dispensing nozzle  208 . In still yet further embodiments the screen  222  is disposed on or at the face of the dispensing nozzle  208  opposite the exit face  226  and near the dispensing nozzle receptor  212  of the dispensing nozzle  208 . 
         [0024]    The screen  222  is configured to capture inclusions such as deleterious oxides transferred from the dip well bath while allowing inclusion free molten metal  100  to pass through. In various embodiments the screen  222  comprises fiberglass. In further embodiments the screen  222  may comprise, for example, steel wire mesh, fiber ceramic cloth, or tinplate. 
         [0025]    The mesh size of the screen  222  determines the minimum particle size of inclusions such as deleterious oxides transferred from the dip well bath which are captured. In various embodiments the screen comprises an approximately 16 to 20 mesh with approximately 1.1 to 0.9 mm width opening and an approximately 51 to 46% open area A non-limiting exemplary screen includes a 20 mesh screen with a width opening of 0.9 mm and an open area of approximately 46%. If the mesh size is too small flow of the inclusion free molten metal  100  is unnecessary constricted while a mesh which is too large allows deleterious inclusions to pass through 
         [0026]    In another form, the fluid metal filter  220  is a porous ceramic filter  224 . In various embodiments the porous ceramic filter  224  is disposed proximal the exit face  226  of the dispensing nozzle  208 . For example, the porous ceramic filter  224  may be placed at 60%, 70%, 80%, or 90% along the length of the dispensing nozzle  208  so as to be closer to the exit face  226  than the dispensing nozzle receptor  212 . In further embodiments the porous ceramic filter  224  is disposed distal the exit face  226  of the dispensing nozzle  208 . For example, the porous ceramic filter  224  may be placed at 10%, 20%, 30%, or 40% along the length of the dispensing nozzle  208  so as to be closer to the dispensing nozzle receptor  212  than the exit face  226 . In yet further embodiments the porous ceramic filter  224  is disposed at the exit face  226  of the dispensing nozzle  208 . In still yet further embodiments the porous ceramic filter  224  is disposed on or at the face of the dispensing nozzle  208  opposite the exit face  226  and near the dispensing nozzle receptor  212  of the dispensing nozzle  208 . 
         [0027]    The thickness of the porous ceramic filter  224  is represented by the dimension of the porous ceramic filter  224  extending along the length of the dispensing nozzle  208 . In embodiments, the porous ceramic filter  224  has a thickness of approximately 22 mm. In further embodiments, the porous ceramic filter  224  has a thickness of approximately 12 mm. Additionally, one skilled in the art would appreciate that additional filter thickness are possible such as a porous ceramic filter  224  representing 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, or 100% of the length of the dispensing nozzle  208 . 
         [0028]    The porous ceramic filter  224  is configured to capture inclusions such as deleterious oxides transferred from the dip well bath while allowing inclusion free molten metal  100  to pass through. Exemplary, non-limiting, ceramics for the porous ceramic filter  224  include mullite, alumina silicate and kyanate In further embodiments the porous ceramic filter  224  may comprise, for example, phosphate bonded alumina. 
         [0029]    The pore size of the porous ceramic filter  224  determines the minimum particle size of inclusions such as deleterious oxides transferred from the dip well bath which are captured. Non-limiting exemplary pore sizes include 10 pores per inch and 15 pores per inch porous ceramic filters. If the pore size is too small flow of the inclusion free molten metal  100  is unnecessary constricted while a pore size which is too large allows deleterious inclusions to pass through. The pore size selection of the porous ceramic filter  224  may be made to allow for a 6 pound/sec flow rate of molten metal  100  into the horizontal shot sleeve with no oxide films larger than 1×1 mm. 
         [0030]    The porosity of the porous ceramic filter  224 , in combination with pore size, determines the difficulty in passing the molten metal  100  through the porous ceramic filter  224 . Porosity, also known as void fraction, is a measure of the void or “empty” spaces in a material, and is a fraction of the volume of voids over the total volume, between 0 and 1, or as a percentage between 0 and 100%. In general, with thickness and pore size being equal, the lower the porosity, the more resistance the molten metal  100  experiences passing through the porous ceramic filter  224 . In this example, if the porosity is too low flow of the molten metal  100  is unnecessary constricted, while a porosity which is too large allows deleterious inclusions to pass through. The porous ceramic filter  224  captures inclusions such as deleterious oxides transferred from the dip well bath by obstructing their path and causing them to become captured in the cellular structure of the porous ceramic filter  224 . By varying the thickness of the porous ceramic filter  224 , one can enhance the ability for the inclusions to depth load and capture inclusions along the length of the porous ceramic filter  224  instead of merely face loading and blocking all inclusions from entering the porous ceramic filter  224  at all. In combination with the pore size, the porosity selection may be made to allow for a 6 pound/sec flow rate of molten metal  100  into the horizontal shot sleeve with no oxide films larger than 1×1 mm. 
         [0031]    In operation, the fluid metal filter  220  and ladle configuration of  FIGS. 3 and 4  is augmented by having the exit face  226  of the dispensing nozzle  208  extend to be proximal to the bottom of a receptacle  300  such as a shot sleeve, runner or related fluid-conveying receptacle. For clarity, the receptacle  300  is referred to as the shot sleeve  300  throughout this disclosure but other types of receptacles are equally envisioned. The exit face  226  of the dispensing nozzle  208  is extended proximal to the bottom of the shot sleeve  300  by rotating the ladle  200  about an axis extending transversely across the dispensing nozzle  208  with the exit face  226  of the dispensing nozzle  208  disposed over a fill opening of the shot sleeve  300 . The rotation of the ladle  200  is illustrated in  FIGS. 5A and 5B . 
         [0032]    By placing the exit face  226  of the dispensing nozzle  208  proximal the bottom of the shot sleeve  300 , the delivery of the molten metal  100  from the dispensing nozzle  208  to the shot sleeve  300  takes place with a minimal unimpeded drop as a way to reduce the turbulent effects of a conventional vertical delivery. Such an arrangement promotes low velocity molten metal  100  delivery. Thus, using the present approach, the molten metal  100  may be contact poured at the lowest point of the shot sleeve  300  and then have a greatly reduced amount of turbulence in the molten metal from ladle  200  in entering the confined environment of the shot sleeve  300 . Specifically, extending the exit face  226  of the dispensing nozzle  208  toward the bottom of the shot sleeve  300  allows a bottom fill system; significantly, the recommended metal fill velocity is kept very low in the present system (preferably below 0.5 m/s for most aluminum-based alloys). 
         [0033]    The ladle  200  is compatible with many existing dip well furnace and ladler equipment. For example, robotic manipulation of the ladle  200  is achievable in the same manner as present systems. Significantly, the pouring efficiency of a conventional tilt ladle pour process is preserved while minimizing the formation of turbulence of the molten metal  100  during introduction into the shot sleeve  300 , as well as removal of inclusions transferred from the dip well bath. Importantly, the method of the present invention also reduces initial metal stream surface area and oxide film formation. 
         [0034]    It is noted that terms like “preferably,” “commonly,” and “typically” are not utilized herein to limit the scope of the claimed invention or to imply that certain features are critical, essential, or even important to the structure or function of the claimed invention. Rather, these terms are merely intended to highlight alternative or additional features that may or may not be utilized in a particular embodiment of the present invention. Moreover, the term “substantially” is utilized herein to represent the inherent degree of uncertainty that may be attributed to any quantitative comparison, value, measurement, or other representation. As such, it may represent the degree by which a quantitative representation may vary from a stated reference without resulting in a change in the basic function of the subject matter at issue. 
         [0035]    Having described the invention in detail and by reference to specific embodiments thereof, it will be apparent that modifications and variations are possible without departing from the scope of the invention defined in the appended claims. More specifically, although some aspects of the present invention are identified herein as preferred or particularly advantageous, it is contemplated that the present invention is not necessarily limited to these preferred aspects of the invention.