Abstract:
A casting method and mold design for optimization of material properties of a casting is disclosed, wherein the optimization is accomplished through control of a cooling rate of the casting to provide desired material properties throughout the casting.

Description:
FIELD OF THE INVENTION  
       [0001]     The invention relates to casting and more particularly to a casting method and mold design for optimization of material properties of a casting, wherein the optimization is accomplished through control of a cooling rate of the casting.  
       BACKGROUND OF THE INVENTION  
       [0002]     In a casting process, variations in material properties can occur in different regions within a casting. Amongst other reasons, the variations can occur when the different regions of the casting are permitted to cool, and thus solidify, at different rates. Castings having complicated geometries are particularly susceptible to these variations since the cooling rates in the various regions of the casting are geometry dependant. For example, in a region having a high ratio of local surface area to volume, the faster the region will tend to cool. In a region having a low ratio of local surface area to volume, the slower the region will tend to cool. This results in material properties which vary significantly from one region to another region within the casting.  
         [0003]     Such variations in material properties within a casting are often undesirable. The variations can cause problems with machinability or other processing. Issues with product performance can also arise.  
         [0004]     It would be desirable to develop a casting method and mold design for optimization of material properties of a casting.  
       SUMMARY OF THE INVENTION  
       [0005]     Concordant and congruous with the present invention, a casting method and mold design for optimization of material properties of a casting, has surprisingly been discovered.  
         [0006]     In one embodiment, the casting apparatus comprises a first mold pattern for forming a first mold cavity for receiving a molten material therein; and a second mold pattern positioned adjacent at least a portion of said first mold pattern, said second mold pattern forming a second mold cavity for receiving the molten material therein, wherein the molten material in the second mold cavity controls a cooling rate of the molten material in the portion of the first mold cavity.  
         [0007]     In another embodiment, a mold for casting comprises a mold having a first mold cavity and a second mold cavity formed therein and adapted to receive a molten material, the second mold cavity formed adjacent at least a portion of the first mold cavity, wherein the molten material received in the second mold cavity controls a cooling rate of the molten material received in the portion of the first mold cavity adjacent the second mold cavity.  
         [0008]     The invention also provides a method of controlling the cooling rate of molten material in a casting process that comprises the steps of providing a first mold pattern for forming a first mold cavity for receiving a molten material; providing a second mold pattern for forming a second mold cavity for receiving a molten material; positioning the second mold pattern adjacent at least a portion of the first mold pattern; and introducing molten material into the first mold cavity and the second mold cavity, wherein the molten material in the second mold cavity controls the cooling rate of the molten material in the portion of the first mold cavity.  
         [0009]     The apparatus and method of this invention are useful in a casting process. The apparatus and method are particularly useful in the casting of engine components such as engine blocks, cylinder heads, and complex transmission components, for example. 
     
    
     DESCRIPTION OF THE DRAWINGS  
       [0010]     The above, as well as other advantages of the present invention, will become readily apparent to those skilled in the art from the following detailed description of a preferred embodiment when considered in the light of the accompanying drawings in which:  
         [0011]      FIG. 1  is a top plan view of a casting apparatus known in the art;  
         [0012]      FIG. 2  is a graphical representation of temperature versus time data taken at three points along a crankshaft produced using the prior art casting apparatus of  FIG. 1 ;  
         [0013]      FIG. 3  is a top plan view of a crankshaft made with the prior art casting apparatus of  FIG. 1  and showing variations in Brinell hardness along a length of the crankshaft;  
         [0014]      FIG. 4  is a top plan view of an casting apparatus according to an embodiment of the invention;  
         [0015]      FIG. 5  is a perspective view of the casting apparatus of  FIG. 4 ;  
         [0016]      FIG. 6  is a graphical representation of temperature versus time taken at three points along a crankshaft produced using the casting apparatus of  FIG. 4 ;  
         [0017]      FIG. 7  is a top plan view of a crankshaft made with the casting apparatus of  FIG. 4  and showing the Brinell hardness along the length of the crankshaft; and  
         [0018]      FIG. 8  is a block diagram illustrating a method according to one embodiment of the invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0019]     The following detailed description and appended drawings describe and illustrate various exemplary embodiments of the invention. The description and drawings serve to enable one skilled in the art to make and use the invention, and are not intended to limit the scope of the invention in any manner. In respect of the methods disclosed, the steps presented are exemplary in nature, and thus, the order of the steps is not necessary or critical.  
         [0020]      FIG. 1  shows a casting apparatus  10  according to the prior art. The casting apparatus  10  is a casting pattern for a mold (not shown) as used in known casting processes such as investment casting, sand casting, permanent mold casting, and die casting, for example. The casting apparatus  10  includes a pair of mold patterns  12 , a gate  14 , and risers  16 .  
         [0021]     The mold patterns  12  include a flange  18 , a body portion  20 , and a stem  22 . The mold patterns  12  have a shape substantially similar to a desired cast object.  FIG. 1  shows the mold patterns  12  with a shape substantially similar to a crankshaft  24  as shown in  FIG. 3 . It is understood that the mold patterns  12  may have the shape of any desired cast object, such as engine blocks, cylinder heads, complex transmission components, and the like, for example.  
         [0022]     The gate  14  forms a conduit (not shown) in the mold that includes an inlet  26  to provide fluid communication with the mold patterns  12  and the risers  16 .  FIG. 1  shows the gate  14  located near the bottom of the casting apparatus  10 . It is understood that the gate  14  may be any size or shape and the gate  14  may be located in other areas of the casting apparatus  10 , as desired.  
         [0023]     The risers  16  are adapted to form reservoirs (not shown) that militate against the formation of cavities or voids in the desired cast object due to shrinkage of a molten material (not shown) during a cooling thereof.  FIG. 1  shows a pair of the risers  16  with one of the risers  16  associated with each of the mold patterns  12 . It is understood that any shape, size, location, and number of risers may be used, so long as an adequate amount of molten material is provided in the risers  16  to militate against the formation of cavities or voids in the cast object. It is further understood that the molten material may be any metal or non-metal, as desired.  
         [0024]     In use, the casting apparatus  10 , including the cavities formed by the mold patterns  12 , is filled with the molten material through the conduit formed by the inlet  26  of the gate  14 . In sand casting, for example, the cavities and the conduit are formed in a sand mold. The molten material flows through the casting apparatus  10 , filling the mold patterns  12  and the risers  16 .  
         [0025]     Once the mold cavity formed by the patterns  12  has been filled, the molten material is allowed to cool. Because metals are less dense in the liquid state than in the solid state, the volume occupied by the desired cast object will decrease as it cools. Thus, formation of cavities or voids is possible, generally at the last point to solidify. The risers  16  militate against the formation of cavities or voids in the desired cast object by providing additional molten material to the mold patterns  12 . Therefore, as the molten material solidifies and shrinks, any cavities or voids that form do so in the risers  16  and not in the desired cast object. Once the desired cast object has solidified and sufficiently cooled, the mold is opened or removed from the cast object. Hardened material from the cavities formed by the gate  14  and the risers  16  is attached to the desired cast object. The hardened material from the gate  14  and the risers  16  is removed from the desired cast object using methods known in the art, and then discarded or recycled.  
         [0026]      FIG. 2  shows a graph  27  of temperature versus time data for a casting produced from the casting apparatus of  FIG. 1 . The graph  27  includes a temperature axis  28  (the y-axis), a time axis  30  (the x-axis), a flange line  32 , a body portion line  34 , a stem line  36 , a shakeout line  38 , and a eutectoid line  40 .  
         [0027]     The flange line  32 , the body portion line  34 , and the stem line  36  represent a plot of temperature versus time measurements as measured by thermocouples (not shown) located at the flange  18 , the body portion  20 , and the stem  22 , respectively. The thermocouples measure the temperature of the molten material during solidification, cooling, and shakeout periods.  
         [0028]     The shakeout line  38  graphically represents the time period when the crankshaft  24  is removed from the mold. The shakeout line  38  is shown at a time between one hundred (100) minutes and one hundred ten (110) minutes. It is understood that that shakeout line  38  can represent any desired time period based on the size or shape of the desired cast object, the temperature range of the material used to make the desired cast object, or other similar factors.  
         [0029]     The eutectoid line  40  graphically represents the temperature at which eutectoid transformation occurs. Eutectoid transformation occurs in a reaction wherein, upon cooling, one solid phase transforms isothermally and reversibly into two new solid phases that are intimately mixed.  FIG. 2  shows the eutectoid line  40  at a temperature of approximately 700 degrees Celsius. It is understood that the temperature at which eutectoid transformation occurs will vary based on the properties of the materials used to cast the desired cast object. Accordingly, the eutectoid line  40  may be at other temperatures, depending upon the material used. It is understood that some alloys, such as aluminum for example, do not go through a eutectoid transformation; however, a rate of solidification or rate of freezing may be controlled to affect the alloy&#39;s strength by altering a grain size, a dendrite arm spacing, or other similar characteristic of the alloy, as desired.  
         [0030]     The flange line  32 , the body portion line  34 , and the stem line  36  cross the shakeout line  38  at the same time, and the different parts of the crankshaft  24  represented continue to cool at different rates. The flange line  32 , the body portion line  34 , and the stem line  36  cross the eutectoid line  40  at different times. The flange line  32  crosses the eutectoid line  40  at a time between one hundred (100) minutes and one hundred ten (110) minutes, which is after the shakeout period. The body portion line  34  crosses the eutectoid line  40  at a time between sixty (60) minutes and seventy (70) minutes, which is before the shakeout period. The stem line  36  crosses the eutectoid line  40  at a time between forty (40) minutes and fifty (50) minutes, which is also before the shakeout period. It is understood that that the flange line  32 , the body portion line  34 , and the stem line  36  may cross the eutectoid line  40  at any time based on the size or shape of the desired cast object, the temperature range or properties of the material used to make the desired cast object, the casting process used, or other similar factors. Because the flange line  32 , the body portion line  34 , and the stem line  36  illustrate different cooling rates for different parts of the crankshaft  24  the material properties from one part of the crankshaft  24  to another vary.  
         [0031]      FIG. 3  shows a hardness distribution of the crankshaft  24  formed using the casting apparatus  10  known in the art. The Brinell hardness scale was used to show the non-uniform hardness distribution of the crankshaft  24 . The Brinell hardness scale characterizes the indentation hardness of materials through the scale of penetration of an indenter. The typical test uses a 10 mm diameter steel ball as the indenter with a 3,000 kgf (29 kN) force. For softer materials, a smaller force is used; for harder materials, a tungsten carbide ball is substituted for the steel ball. The hardness is calculated using the following equation: Brinell Harness Number (BHN)=P/[         *D*(D−√(D 2 −d 2 ))], where P is the force applied, D is the diameter of the indenter, and d is the diameter of the indentation. It is understood that there are a number of other hardness tests that may be used to determine the hardness distribution of the desired cast object.  
         [0032]     The crankshaft  24  includes a stem portion  42 , a lower body portion  44 , an upper body portion  46 , and a flange portion  48 . The stem portion  42  and the lower body portion  44  have BHNs between 141.0 and 220.03. The upper body portion  46  and flange portion  48  have BHNs between 273.0 and 326.0. The difference in hardness, represented by BHNs, is an effect of the stem portion  42  and the lower body portion  44  cooling at different rates than the upper body portion  46  and flange portion  48 . As described above, the difference in cooling rates from one region of a desired cast object to another region results in different rates of eutectoid transformation. Different rates of eutectoid transformation results in the desired cast object having different material properties from one region thereof to another.  
         [0033]      FIGS. 4 and 5  show a casting apparatus  60  according to an embodiment of the invention. The casting apparatus  60  is a casting pattern for a mold (not shown) that may be used in any known casting process such as investment casting, sand casting, permanent mold casting, and die casting, for example. The casting apparatus  60  includes a pair of first mold patterns  62 , a second mold pattern  64 , a gate  66 , and risers  68 . It is understood that more or fewer mold patterns  62 ,  64  can be used as desired.  
         [0034]     The first mold patterns  62  include a flange  70 , a body portion  72 , and a stem  74 . The first mold patterns  62  are adapted to form a mold cavity (not shown) in the mold which is in fluid communication with a mold cavity (not shown) formed by the gate  66 , and the risers  68 . The first mold patterns  62  have a shape substantially similar to a desired cast object. In the embodiment shown, the first mold patterns  62  have a shape substantially similar to a crankshaft  76 , as shown in  FIG. 7 . It is understood that the first mold patterns  62  may have the shape of any desired cast object such as engine blocks, cylinder heads, complex transmission components, and the like, for example.  
         [0035]     In the embodiment shown, the second mold pattern  64  includes an inner wall  78  and an outer wall  80 . The second mold pattern  64  is adapted to form a mold cavity (not shown) which is in fluid communication with a conduit (not shown) which is part of the cavity formed by the gate  66 , and the risers  68 . The inner wall  78  and the outer wall  80  of the second mold pattern  64  are configured to circumscribe the two-dimensional shape of the profile of the stem  74  of the first mold patterns  62 . The term circumscribe as used herein means that the inner wall  78  and the outer wall  80  of the second mold pattern  64  at least partially surround, in close proximity to but without actually contacting, the first mold pattern  62 . It is understood that the inner wall  78  of the second mold pattern  64  may be any shape or configuration desired such as the shape of a particular portion of the first mold patterns  62 , or any geometric shape, for example. It is also understood that the inner wall could circumscribe a particular portion of the first mold pattern  62  three-dimensionally. The outer wall  80  may be any shape or configuration, as desired. The second mold pattern  64  may also have a shape substantially similar to another desired cast object. It is further understood that the second mold pattern  64  may be positioned at any distance from the first mold pattern  62 , as desired. The exact configuration and position of the second mold pattern  64  will depend on the size, shape, and surface area of the first mold patterns  62 , the desired cooling rate of molten material filling the cavity formed by the first mold patterns  62 , and the like, for example. As shown, the second mold pattern  64  is positioned adjacent the stem  74  of the first mold patterns  62 . It is understood that the second mold pattern  64  may be positioned adjacent any portion of the first mold patterns  62  such as the flange  70 , the body portion  72 , or any combination of the portions of the first mold patterns  62 , as desired.  
         [0036]     The gate  66  forms the conduit that includes an inlet  82  to provide fluid communication with the first mold patterns  62 , the second mold pattern  64 , and the risers  68 . In the embodiment shown, the gate  66  is located near the bottom of the casting apparatus  60 . It is understood that the gate  66  may be any gate known in the art. The gate  66  may be any size or shape and the gate  66  may be located anywhere on the casting apparatus  60 , as desired.  
         [0037]     The risers  68  are adapted to form reservoirs (not shown) that militate against the formation of cavities or voids in the desired cast object due to shrinkage of a molten material (not shown) during a cooling thereof.  FIGS. 4 and 5  show a pair of risers  68  with one of the risers  68  associated with each of the first mold patterns  62 . It is understood that any shape, size, location, and number of the risers  68  may be used so long as an adequate amount of molten material is provided in the risers  68  to militate against the formation of cavities and voids in the desired cast object. It is further understood that the molten material may be any metal or non-metal, as desired.  
         [0038]     In use, the casting apparatus  60 , including the cavities formed by the first mold patterns  62  and the second mold pattern  64 , are filled with the molten material through the conduit formed by the inlet  82  of the gate  66 . The molten material flows through the casting apparatus  60  filling the mold cavities formed by the first mold patterns  62 , the second mold pattern  64 , and the risers  68 .  
         [0039]     Once the mold cavities have been filled, the casting apparatus  60  and the desired cast object are allowed to cool. Because metals are less dense in the liquid state than in the solid state, the volume occupied by the desired cast object will decrease as it cools. Thus, formation of cavities or voids is possible, generally at the last point to solidify. The risers  68  militate against the formation of cavities or voids in the desired cast object by providing additional molten material to the cavity formed by the first mold patterns  62 . Therefore, as the molten material solidifies and shrinks any cavities or voids that form, do so in the risers  68  and not in the desired cast object. A cooling rate of the portion of the cast object adjacent the mold cavity formed by the second mold pattern  64  is controlled by the heat radiating from the molten material in the mold cavity formed by the second mold pattern  64 . If the portion of the cast object formed in the cavity formed by the first mold patterns  62  adjacent to the cavity formed by the second mold pattern  64  would normally cool faster than the other portions of the cast object, the cooling rate of the cast object formed in the cavity formed by the first mold patterns  62  is decreased by the radiating heat. It is understood that a plurality of mold patterns may be used to control the cooling rate of the first mold pattern  64  without departing from the scope of the invention.  
         [0040]     Once the desired cast object has solidified and sufficiently cooled the mold is opened and the desired cast object is removed. Hardened material from the cavities or conduits formed by the gate  66 , the risers  68 , and the second mold pattern  64  is attached to the desired cast object. The hardened material is removed from the desired cast object using methods known in the art, and then discarded or recycled.  
         [0041]      FIG. 6  shows a graph  83  of temperature versus time data for a casting produced from the casting apparatus  60 . The graph  83  includes a temperature axis  84  (the y-axis), a time axis  86  (the x-axis), a flange line  88 , a body portion line  90 , a stem line  92 , a shakeout line  94 , and a eutectoid line  96 .  
         [0042]     The flange line  88 , the body portion line  90 , and the stem line  92  represent a plot of temperature versus time measurements as measured by thermocouples (not shown) located at the flange  70 , the body portion  72 , and the stem  74 , respectively. The thermocouples measure the temperature of the molten material during solidification, cooling, and shakeout periods.  
         [0043]     The shakeout line  94  graphically represents the time period when the crankshaft  76  is removed from the mold. For purposes of illustration, the shakeout line  94  is shown at a time between one hundred (100) minutes and one hundred ten (110) minutes. It is understood that that shakeout line  94  may represent any desired time period based on the size or shape of the desired cast object, the temperature range of the material used to make the desired cast object, or other similar factors.  
         [0044]     The eutectoid line  96  graphically represents the temperature at which eutectoid transformation occurs. Eutectoid transformation occurs in a reaction wherein, upon cooling, one solid phase transforms isothermally and reversibly into two new solid phases that are intimately mixed.  FIG. 6  shows the eutectoid line  96  at a temperature of approximately 700 degrees Celsius. It is understood that the temperature at which eutectoid transformation occurs will vary based on the properties of the materials used to cast the desired cast object. Accordingly, the eutectoid line  96  may be at other temperatures depending upon the material used. It is understood that some alloys, such as aluminum for example, do not go through a eutectoid transformation; however, a rate of solidification or rate of freezing may be controlled to affect the alloy&#39;s strength by altering a grain size, a dendrite arm spacing, or other similar characteristic of the alloy, as desired.  
         [0045]     The flange line  88 , the body portion line  90 , and the stem line  92  have substantially similar cooling rates and cross the shakeout line  94  at substantially the same time. Because the flange line  88 , the body portion line  90 , and the stem line  92  have substantially similar cooling rates, the flange line  88 , the body portion line  90 , and the stem line  92  cross the eutectoid line  96  at the substantially same rate and time. In the embodiment shown, the flange line  88 , the body portion line  90 , and the stem line  92  all cross the eutectoid line  96  at the same rate, at a time between one hundred five (105) minutes and one hundred ten (115) minutes, which is after the shakeout period. As a result of the substantially similar cooling rates and the substantially similar rates of eutectoid transformation, the material properties from one portion to another within the casting will be substantially the same. It is understood that that the flange line  88 , the body portion line  90 , and the stem line  92  may cross the eutectoid line  96  at other times based on the size or shape of the desired cast object, the temperature or properties of the molten material used to make the desired cast object, the casting process used, or other similar factors.  
         [0046]      FIG. 7  shows a hardness distribution of the crankshaft  76  formed using the casting apparatus  60 . The Brinell hardness scale was used to show the non-uniform hardness distribution of the crankshaft  76 . It is understood that there are a number of other hardness tests that may be used to determine the hardness distribution of the desired cast object.  
         [0047]     The crankshaft  76  shown includes a stem portion  98 , a lower body portion  100 , an upper body portion  102 , and a flange portion  104 . In the embodiment shown, the stem portion  98 , the lower body portion  100 , the upper body portion  102 , and the flange portion  104  all have BHNs between 273.0 and 326.0. This data is used for illustrative purposes to show the consistent material properties and is not intended to limit the scope of the invention. The substantially uniform hardness is a result of the stem portion  98 , the lower body portion  100 , the upper body portion  102 , and the flange portion  104  cooling at the substantially same rate. As described above, the substantial similarity of the cooling rates from one region of the desired cast object to another results in similar times of eutectoid transformation. Similar rates of eutectoid transformation cause the desired cast object to have substantially similar material properties from one portion of the desired cast object to another.  
         [0048]      FIG. 8  schematically illustrates a method  110  of controlling the cooling rate of molten material in a first mold cavity according to one embodiment of the invention. The method  110  may be used with any known casting process, such as investment casting, sand casting, permanent mold casting, and die casting, for example.  
         [0049]     In a first step  112 , a first mold cavity is formed which is adapted to receive a molten material. The first mold cavity may be any size or shape as desired to produce a desired cast object. The first mold cavity may be the size or shape of a crankshaft, an engine block, a cylinder head, a complex transmission component, and the like, for example.  
         [0050]     In a second step  114 , a second mold cavity is formed which is adapted to receive a molten material. The second mold cavity may be configured to circumscribe the two-dimensional profile of a portion of the first mold cavity. It is understood that second mold cavity may be any shape or configuration desired such as the shape of a particular portion of the first mold cavity, or any geometric shape, for example. It is also understood that the second mold cavity could circumscribe a particular portion of the first mold cavity three-dimensionally. The second mold cavity may be any shape or configuration, as desired. The second mold cavity may also have a shape substantially similar to another desired cast object. It is further understood that the second mold cavity may be positioned at any distance from the first mold cavity, as desired. The exact configuration and position of the second mold cavity will depend on the size, shape, and surface area of the first mold cavity, as well as the desired cooling rate of the molten material in the first mold cavity.  
         [0051]     In a third step  116 , the second mold cavity is positioned adjacent a portion of the first mold cavity. It is understood that the second mold cavity may be placed adjacent one side, two sides, or completely surrounding the portion of the first mold cavity.  
         [0052]     In a fourth step  118 , a molten material is introduced into the first mold cavity and the second mold cavity, wherein the heat radiating from the second mold cavity controls the cooling rate of the molten material inside the portion of the first mold cavity. The cooling rate of the portion of the molten material in the first mold cavity adjacent to the second mold cavity may be controlled so that the portion adjacent to the second mold cavity cools at a substantially similar rate as the remaining portions of the molten material in the first mold cavity. Alternatively, the cooling rate of the portion of the molten material in the first mold cavity adjacent to the second mold cavity may be controlled so that the portion adjacent to the second mold cavity cools at a slower rate than the rest of the molten material in the first mold cavity.  
         [0053]     From the foregoing description, one ordinarily skilled in the art can easily ascertain the essential characteristics of this invention and, without departing from the spirit and scope thereof, can make various changes and modifications to the invention to adapt it to various usages and conditions.