Abstract:
A method of casting an article using a mold including a mold cavity, the method including the steps of providing a compound containing a halogen at at least a portion of a surface of the mold adjacent the mold cavity, pouring molten metal into the mold cavity and allowing the metal to cool and solidify, and removing the article from the mold.

Description:
CROSS-REFERENCE TO PRIORITY APPLICATIONS  
       [0001]     This application hereby claims the benefit of pending International Application No. PCT/GB2005/004712, filed Dec. 8, 2005, (and published Jun. 22, 2006, as WO 2006/064188 A1), which itself claims the benefit of pending United Kingdom Application No. 0427501.2, filed Dec. 16, 2004, (and published Jun. 21, 2006, as GB 2,421,207 A). These applications are hereby incorporated by reference in their entirety. International Application No. PCT/GB2005/004712 designates the United States and, accordingly, this U.S. non-provisional application is a continuation thereof.  
       FIELD OF THE INVENTION  
       [0002]     The present invention relates to a method of casting an article, particularly, but not exclusively to a method of sand-casting an aluminum alloy engine block.  
       BACKGROUND OF THE INVENTION  
       [0003]     Typically, an engine block is made by sand casting an aluminum alloy. In order to provide a suitable low friction, wear resistant, reciprocating surface, it is known to line the surface of each cylinder in the engine block in which a piston reciprocates, with a tubular cast iron liner. Such liners increase the weight of the engine and, as cast iron has a lower thermal conductivity than aluminum based alloys, the use of such liners is detrimental to the removal of heat from the engine cylinders.  
         [0004]     Thus, in order to remove the need for cast iron liners, various processes of coating the inner surface of each cylinder with a metal that is harder than aluminum to provide a low friction, wear resistant reciprocating surface have been developed, and these include electro-coating, plasma transfer wire arc coating, and laser alloying. For example, the plasma-spraying of a ferrous coating is described in U.S. Pat. No. 6,548,195, and use of laser alloying is described in U.S. Pat. No. 6,390,050 and European Publication No. EP 1041173 A1. Unfortunately, such processes only produce a satisfactory coating if applied to a relatively smooth substrate, and even after machining, the surface of a sand cast engine block includes significant porosity.  
         [0005]     This problem has been addressed in U.S. Pat. No. 5,931,213, which discloses a method of casting an engine block of aluminum in which cylindrical brass inserts are inserted into a sand mold, the brass inserts forming the cylinders in the engine block. As a result of the use of such high thermal conductivity inserts, the cooling rate of the aluminum alloy around the inserts is significantly higher than the cooling rate of the aluminum alloy elsewhere in the mold cavity. Consequently, the porosity in the resultant casting is significantly reduced in the volumes around the inserts, and, once the inserts are removed, machining of the interior of each cylinder produces a surface that is suitable for coating using the above-mentioned methods.  
         [0006]     This method has a number of disadvantages, however. Removal of the brass inserts can be difficult, particularly in a V-engine where the cylinders are inclined relative to the engine block, and adds an extra, relatively labor-intensive step in the manufacturing process. Moreover, the brass inserts are susceptible to mechanical damage, particularly after repeated exposure to molten aluminum alloy has caused the brass to anneal and soften, and must be replaced at regular intervals, which further increases the cost of the casting process.  
       SUMMARY OF THE INVENTION  
       [0007]     According to a first aspect of the invention, we provide a method of casting an article using a mold, including a mold cavity. The method includes the steps of providing a compound containing a halogen at at least a portion of a surface of the mold adjacent the mold cavity, pouring molten metal into the mold cavity, allowing the metal to cool and solidify, and removing the article from the mold.  
         [0008]     By virtue of the provision of such a compound, porosity formed by reaction at the surface of the casting adjacent the relevant portion of mold is reduced, and once machined, this surface provides a suitable substrate for electro-plating, plasma transfer wire arc coating etc. to produce a low friction, wear resistant surface.  
         [0009]     Preferably, the halogen is fluorine. The coating may include, for example, dipotassium fluorotitanate. Alternatively, the halogen may be chlorine, and may be, for example, a potassium chloride-magnesium chloride eutectic.  
         [0010]     Preferably, the halogen containing compound is provided in a coating applied to at least a portion of the mold surface which forms at least part of the mold cavity.  
         [0011]     Preferably, the method further includes the step of subjecting the article to isostatic pressure. In this case, preferably, the method further includes the step of heating the article to a temperature at which the entire article remains solid while applying the isostatic pressure.  
         [0012]     Subjecting the article to an isostatic pressure, particularly at high pressure, generally eliminates internal pores within the cast article, which improves the mechanical integrity of the article and reduces the risk of any such pores being exposed at the article surface during machining of the article.  
         [0013]     The metal is preferably predominantly aluminum. The metal may be an aluminum-silicon alloy, and is preferably a hypoeutectic aluminum-silicon alloy.  
         [0014]     Preferably, the coating is sprayed onto the mold surface.  
         [0015]     Preferably, the mold is made predominantly from sand.  
         [0016]     The article may be an engine block. In this case, preferably, the coated portions of the mold form cylinders in the engine block.  
         [0017]     According to a second aspect of the invention, we provide an article cast using the method of the first aspect of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0018]      FIG. 1  is a schematic illustration of the method of making an article according to the invention.  
     
    
     DETAILED DESCRIPTION  
       [0019]     In accordance with an exemplary embodiment of the invention and as depicted in  FIG. 1 , the method is applied to sand casting an engine block  10  from a conventional hypoeutectic aluminum-silicon alloy, such as the 356 or 354 alloys detailed in Tables 1 and 2 (below):  
                                                   TABLE 1                           (356 Alloy)                Min.   Max.           Element   (weight fraction)   (weight fraction)   Ideal Content                    Cu   —   0.2%   N/A       Mg   0.4%   0.6%   &gt;0.5%       Si   6.5%   7.5%   &gt;7.00%       Fe   —   0.5%   N/A       Mn   —   0.3%   &gt;0.2%       Ni   —   0.1%   N/A       Zn   —   0.10%   N/A       Pb   —   0.10%   N/A       Sn   —   0.05%   N/A       Ti    0.05%   0.15%   &gt;0.10%       Sr   200 ppm   350 ppm   250 ppm       P   —    20 ppm   N/A       Na   —    20 ppm   N/A       Ca   —   100 ppm   N/A       Sb   —   200 ppm   N/A       Li   —    2 ppm   N/A       Al + additional   Balance   Balance   Balance       impurities                  
 
         [0020]    
       
         
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                   
               
               
                 (354 Alloy) 
               
             
          
           
               
                   
                 Min. 
                 Max. 
                   
               
               
                 Element 
                 (weight fraction) 
                 (weight fraction) 
                 Ideal Content 
               
               
                   
               
             
          
           
               
                 Cu 
                 1.5% 
                 2.0% 
                 1.80% 
               
               
                 Mg 
                 0.5% 
                 0.6% 
                 0.55% 
               
               
                 Si 
                 8.5% 
                 9.5% 
                 9.00% 
               
               
                 Fe 
                 — 
                 0.15% 
                 0.10% 
               
               
                 Mn 
                 — 
                 0.05% 
                 0.00 
               
               
                 Ni 
                 — 
                 100 ppm 
                 0.00 
               
               
                 Zn 
                 — 
                 0.05% 
                 0.00 
               
               
                 Pb 
                 — 
                 100 ppm 
                 0.00 
               
               
                 Sn 
                 — 
                 100 ppm 
                 0.00 
               
               
                 Ti 
                 0.11% 
                 0.14% 
                 0.12% 
               
               
                 Sr 
                 200 ppm 
                 250 ppm 
                 225 ppm 
               
               
                 P 
                 — 
                  20 ppm 
                 0.00 
               
               
                 Na 
                 — 
                 100 ppm 
                 0.00 
               
               
                 Ca 
                 — 
                 100 ppm 
                 0.00 
               
               
                 Sb 
                 — 
                  20 ppm 
                 0.00 
               
               
                 Li 
                 — 
                 100 ppm 
                 0.00 
               
               
                 Al + additional 
                 Balance 
                 Balance 
                 Balance 
               
               
                 impurities 
               
               
                   
               
             
          
         
       
     
         [0021]     It should be appreciated, however, that the method may equally be applied to the making of any cast article using any metal or metal alloy, such as an aluminum-magnesium alloy, or magnesium based alloys.  
         [0022]     A mold  12  is formed from first  12   a  and second  12   b  mold parts made from zircon or silicon sand using conventional sand-casting techniques. The cylindrical portions of an upper mold part  12   a,  which form cylindrical cavities, i.e., the cylinders in the engine block, are then coated with a coating  14  containing a halogen, such as fluorine. In this example, the halogen is fluorine, and the coating material contains dipotassium fluorotitanate powder (K 2 TiF 6 ). Other halogen containing compounds, such as a mixed potassium chloride—magnesium chloride eutectic, potassium borofluoride, or aluminum chloride may alternatively be used.  
         [0023]     The K 2 TiF 6  powder is mixed with a solvent, a filler material such as zircon powder, and a gelling agent. The coating material, for example, may comprise 60 weight percent powder (the powder comprising 25 weight percent zircon flour and 75 weight percent K 2 TiF 6  dry milled powder) and 40 weight percent IPA solvent.  
         [0024]     The coating is typically sprayed onto the mold surface, but may also be painted onto the mold. Alternatively, dry K 2 TiF 6  powder may be rubbed onto the mold surface, or even added directly to the sand used to form the mold at levels of approximately 0.5 weight percent K 2 TiF 6 .  
         [0025]     The mold parts  12   a  and  12   b  are clamped together to form a mold cavity, and molten aluminum-silicon alloy is pored into the mold cavity. The mold is then allowed to cool until the alloy has solidified. The as-cast engine block  10  is then removed from the mold  12 .  
         [0026]     Usually during sand-casting of an alloy such as a hypoeutectic aluminum-silicon (Al—Si) alloy, hydrogen ions migrate from the mold  12  into the liquid metal adjacent the mold surface. As the molten metal solidifies, dissolved hydrogen is ejected from the solidification front into the remaining liquid metal, which results in the formation of a plurality of sub-surface elongate micropores that extend into the casting, generally perpendicular to the surface of the casting, up to a depth of 3 to 4 mm. The inclusion of strontium in the alloy is believed to enhance this process. Conventionally, during machining of the cast article, less than 3 mm of material is removed from the article surface, and therefore such machining exposes these pores at the article surface, and renders the surface unsuitable for coating using plasma transfer wire arc coating, electro-plating and similar processes.  
         [0027]     Where the K 2 TiF 6  coating is present, however, the coating reduces transfer of hydrogen ions into the molten alloy and therefore significantly reduces the surface and sub-surface microporosity of the casting. It is believed that this occurs because the fluorine in the coating reacts with the hydrogen ions before they can dissolve in the molten alloy. As a result, the surface of the casting adjacent the coating, in this example the interior surfaces of the cylinders in the engine block, may be machined and coated as described above without surface microporosity having a deleterious effect on the integrity of the coating.  
         [0028]     Even where a K 2 TiF 6  coating is used, such a casting will, however, include internal pores, formed not as a result of reaction with the mold as described above, but as a result of the relatively low cooling rate associated with sand casting. The internal pores may be exposed at the article surface during machining of the casting, and therefore it is desirable to eliminate these pores in addition to the surface microporosity.  
         [0029]     The as-cast engine block is therefore subjected to hot isostatic pressing using a conventional, commercially available Al HIPPING process, the Bodycote Densal® II process, for example. In such a process, the engine block is placed in a container of fluid, heated to a temperature close to the melting temperature of the alloy but at which the alloy remains solid (e.g., 40° C. below the solidus temperature of the alloy), and the fluid pressurized (e.g., to 1,000 atmospheres of pressure). The engine block is typically retained in the pressurized, heated fluid for forty five minutes to one hour.  
         [0030]     The combination of fluid pressure and elevated temperature causes the internal pores to cave in and the material formerly surrounding each pore to diffusion bond. Thus, the internal pores are substantially eliminated and the mechanical integrity of the casting improved. The casting may thus be machined with substantially reduced risk of exposing internal pores at the casting surface.  
         [0031]     It should be appreciated that HIPPING cannot be used to remedy surface microporosity, since the pressurized fluid would fill the surface pores and provide internal support preventing the pore from caving in. Moreover, HIPPING is ineffective in eliminating sub-surface microporosity, since the layer of aluminum alloy separating each pore from the article surface is relatively thin, and is easily ruptured under the pressure of the pressurized fluid, thus exposing the pore interior to the pressurized fluid and preventing consolidation of the material around the pore. In order to address the problem of surface/sub-surface porosity and internal porosity, it is therefore necessary to use a mold coated with or containing a halogen containing compound in addition to HIPPING.  
         [0032]     After HIPPING, the casting is then machined to the required dimensions and surface roughness. In this example, the interior surfaces of the cylinders are machined in preparation for the application of a low friction, wear resistant coating using plasma transfer wire arc coating, electro-plating, or similar processes.  
         [0033]     When used in this specification and claims, the terms “comprises” and “comprising” and variations thereof mean that the specified features, steps or integers are included. The terms are not to be interpreted to exclude the presence of other features, steps or components.  
         [0034]     The features disclosed in the foregoing description, or the following claims, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate, may, separately, or in any combination of such features, be utilized for realizing the invention in diverse forms thereof.