Abstract:
Residual stress is reduced in light metal alloy articles, e.g. aluminum alloy articles, formed as castings against a sand casting mold body by incorporating a wax composition of suitable softening or melting temperature with the sand particles of the mold or core body. The hot cast metal heats adjoining surfaces of the mold body. As the cooling metal forms a solid shell, the surrounding sand particle and wax mixture are heated sufficiently to melt or soften the wax incorporated on or between sand particles. This softens portions of the rigid mold body that could otherwise restrain shrinking surfaces of the casting and produce unwanted stressed regions that are retained in the casting and must be removed by subsequent processing.

Description:
TECHNICAL FIELD 
     This invention pertains to the casting of molten metal against sand mold surfaces or sand core surfaces in making cast articles. More specifically, this invention pertains to making such sand particle casting bodies so as to minimize cracks, residual stresses, and the like in light metal alloy castings. 
     BACKGROUND OF THE INVENTION 
     The art of casting molten metal into sand molds to make useful articles has long been practiced. The casting art also includes casting molten metal into permanent molds in which sand cores are used to define internal surfaces of the casting. Today many ferrous and non-ferrous metal alloys are cast in green sand molds, resin bonded sand molds, or in other more permanent mold material structures using sand cores to define a portion of the surfaces of the cast articles. 
     Aluminum alloys are used in producing many cast articles, particularly in the automobile industry. Many engine components and other drive-train components are cast of various aluminum alloys in sand molds, and aluminum parts are produced by die casting or permanent mold casting in which sand cores are used. For example, there is a family of aluminum-based alloys variously containing, by weight, about five to twelve percent silicon, and smaller amounts of other alloying constituents such as copper, magnesium, and/or zinc. These alloys have good fluidity at pouring temperatures of, for example, about 700° C. for flowing into intricately shaped mold cavities in such casting practices. 
     Molding sand materials containing fine silica sand particles and small amounts of clay and water may serve as the mold or core material for casting aluminum alloys and other light metal alloys such as magnesium alloys. The pouring temperatures of these casting alloys are relatively low (as compared, e.g., to ferrous alloys or other higher melting point metal alloys) and special, high temperature resistant mold compositions are not normally required. Complex parts such as aluminum alloy engine cylinder blocks, engine head blocks and the like may be cast in sand molds with sand cores to good dimensional accuracy. But aluminum alloys have a high volumetric shrinkage upon solidification, and there is additional shrinkage as solidified cast metal experiences further cooling. The sand mold body is initially at ambient temperature and it has relatively low thermal conductivity. Those portions of the mold close to the mold cavity are heated by the sudden charge of hot metal. So mold surfaces and cores may expand in directions that press against surfaces of the solidifying cast metal. There are shapes in aluminum castings, such as those formed by surfaces in the cast body having intersecting faces at angles of about ninety degrees and lower, which may shrink extensively against acute angles (for example), adjacent sand mold surfaces and experience unwanted compressive or tensile stresses. This mold surface induced stress may cause cracks in affected surface regions of the cast light metal article. But more commonly, the cooled casting has regions of residual compressive or tensile stresses that may have to be relieved by a costly heat treatment. 
     There is a need for a method of making sand molds and sand cores that reduce such thermal shrinkage damage to cast light metal alloy parts. 
     SUMMARY OF THE INVENTION 
     In accordance with an embodiment of the invention, a mixture of sand particles and wax is used in making a core or a mold body (or a portion of a mold body) for casting aluminum alloys. In one embodiment, wax particles may be mixed and blended with sand particles in making the mold body. In another embodiment a solvent may be used to disperse the wax onto the surfaces of the grains of sand. The solvent may then be evaporated and the wax-coated grains of sand formed into the mold body or component. An entire mold may be formed of wax-containing sand. Alternatively, wax is used in making sections of the mold to lie near those cavity-defining surfaces where prior shape analysis or thermal analysis indicates that the mold (or core) may restrict shrinkage of the solidifying and cooling cast part and thereby damage the casting. 
     A wax material is selected that will melt (or soften appreciably) when a cast metal-heated mold section reaches a predetermined temperature (e.g., about 250° C.). The melted wax produces a softening of the heated region of the sand mold or core and that mold region provides less restraint to the enclosed hot cast body. Often the cast metal forms a solid shell by the time the wax melts and the shell helps to sustain the intended article shape as the molten interior solidifies and cools. Depending on the mold section structure, the melted wax may drain from the hot mold region and create porosity in the mold that reduces its restraint of the cast part. Whatever the mechanism, this softening of adjacent or nearby mold or core surfaces reduces the incidence of residual stress in the cast article. 
     High melting point waxes are known that are suitable for use with sand particles in mold bodies for casting aluminum alloys. For example, polymeric reaction products of linear C 6 -C 12  dicarboxylic acids and a diamine of the formula, H 2 N(CH 2 ) n NH 2 , are commercially available as waxes with different melting point ranges. A wax with a specific melting range may be chosen by experience or pre-testing for use in casting a specific article shape of a specific alloy composition and pouring temperature. 
     In accordance with a practice of the invention, an analysis is made of the shape of an article to be cast using an aluminum alloy, magnesium alloy or other light metal alloy. Shape features of the article that may experience mechanical constraint due to shrinkage when cast in a sand mold are identified by observation or experience, and/or by structural and/or thermal analytical methods. This analysis may be performed, for example, using a suitable computer software program. Problems tend to arise in portions of a casting in which article surfaces merge, for example, at about ninety degrees or smaller angles. Some such shapes occur, for example, where the casting is shrinking around a relatively sharp edge or surface on the mold or core body. Such mold surfaces occur, for example, at the bottom of a cup-like structure where shrinkage of the casting occurs around a complementary cylindrical core. A cast shape having an I-section may likewise experience stress where the head and column of the “I” intersect and shrink against the complementary corner of the mold body. The method of this invention is practiced to minimize mold-caused compressive or tensile stresses on surfaces of the cast body. 
     Depending on the article to be cast, an entire sand mold (or core body) may be formed with wax coated on or filled between the sand particles. Or where it is convenient to prepare the mold in sections, selected mold sections may be made to include wax with a suitable melting point to minimize shrinkage restraint of the cast article, Critical surfaces of the mold or core are formulated with a wax and sand particle mixture in which the wax is selected to melt before the casting is damaged; for example, after a coextensive solid shell has formed around the remaining molten liquid. Melting of the wax alters the rigidity of a mold region in which it is contained in suitable quantity to reduce stress imparted to the hot, fragile casting. 
     Other objects and advantages of the invention will be apparent from the following descriptions of preferred embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side elevational view, partly in cross-section; illustrating a one-eighth section of an I-shaped aluminum alloy cast body and a sand particle mold. This view illustrates a momentary stage in the casting process in which molten aluminum alloy has been poured and a solidified shell has formed against the mold surface. This figure illustrates a selected region of the mold which is made with a wax and sand mixture to reduce compressive restraint at the intersection of the head and body portions of the I-structure. 
         FIG. 2  is a side elevational view illustrating a cross-section of a sand mold for casting an aluminum alloy cup-shape. This sectional view illustrates an embodiment in which a core is used with a hollow thin-wall sand particle outer structure and a cylindrical wax-sand particle inner structure. 
         FIG. 3  is a side view in cross-section of a cup of un-equal wall thickness as cast in the mold arrangement of  FIG. 2 . 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Compressive stresses may be applied to hot cast metal by adjacent sand mold surfaces as mold surfaces, heated by the hot cast metal, expand and cast metal surfaces cool and contract against the mold surfaces. This phenomenon arises, in part, from the incidence of thermal stresses generated in castings during solidification due to the difference in the coefficients of thermal expansion (CTE) of the hot solidified material and of the sand mold. When the hot metal is poured into the article-shaping cavity of an unheated sand mold, it loses (transfers) heat to the sand and as a result the adjacent mold material heats up and expands slightly. As the metal starts to solidify it contracts due to solidification shrinkage. Depending on the shape of the casting and mold cavity, the mismatch between the casting CTE and the sand mold CTE may cause mold/metal gap formation at certain locations and compressive engagement of the casting with the mold at other locations. In the former situation the cast metal shrinks away from the sand mold surface, while in the latter situation the casting shrinks against a mold surface. Shrinkage of cast material against a mold surface is constrained due to the resistance offered by the more rigid mold. Sometimes the shrinking metal encounters surfaces in a mold body intersection at more or less acute angles. This type of constraint, for example, may cause compressive or tensile stresses to develop in the casting. Usually such compressive or tensile stresses must be removed by an expensive heat treatment of the casting before the cast article is considered suitable for its intended use. 
     A method is provided for making a composite mold body of sand particles and a wax composition such that the incidence of residual compressive or tensile stresses in an aluminum alloy casting is reduced. A wax material is selected with a melting point such that heated regions of the mold body soften after the cast metal has formed a solid shell. The invention may be applied in the casting of other light metal alloys. 
     A wax composition is selected for mixture with sand particles in forming a mold body or core surface. Waxes are soft polymeric materials that may be mixed with sand particles or deposited on sand particles from a suitable removable solvent. In one embodiment a wax-like material is identified by, for example, a simple experiment or experience to soften sand particles in a mold body. The softening occurs due to the melting or softening of the sand and wax mixture due to heating from the cast metal. A wax is selected that softens regions of the mold body against which the cast metal may be expected to shrink. The melting point of the wax is selected so that the mold or core section may soften after a solid shell has formed on the cast metal and before the casting has hardened to the stage in which compressive or tensile stresses are frozen into the cast structure. In casting of aluminum alloy castings, for example, it is found that polymer waxes melting in the region of about 225° C. to about 275° C. are often suitable. As stated, the selection of a specific wax for the casting of a specific casting alloy into a predetermined article shape may be made by trying different waxes in different mold bodies while making a number of trial castings of the part to be made in large volume production. Alternatively, a casting simulation model or procedure can be used to determine the wax characteristics for the specific geometry of the cast article and the temperature of the cast metal. 
     Waxes are available that are formed of carbon, hydrogen, and oxygen-containing polymers and carbon, hydrogen, oxygen, and nitrogen-containing polymers. Typically these polymers are made of repeating monomer units in the polymer molecular chain. When the polymerization is stopped after the inclusion of, for example, about five to about ten monomer units the product has the characteristics of a wax. The molecular weight range of a particular mixture determines characteristics of the wax-like material. Each such wax may be used in the form of soft pliable particles having a melting range related generally to its molecular weight and monomer chain length. Or the wax may be dispersed in a solvent vehicle and deposited in the sand particles. One example of waxes suitable for selection and use in mold bodies of this invention are polymeric polyamide reaction products of linear C 6 -C 12  dicarboxylic acids and a diamine of the formula, H 2 N(CH 2 ) n NH 2 . Depending on the degree of polymerization waxes in this group of polymers may be prepared with individual melting points or ranges in a broad range of form about 200° C. to about 300° C. 
     A practice of the invention will be illustrated by reference to  FIG. 1 . As stated,  FIG. 1  illustrates a one-eighth section of a sand-particle mold  10  for casting of an I-shaped aluminum alloy cast body. An upper-quarter section of the sand mold  10  is illustrated in  FIG. 1 . Mold  10  has cavity defining surfaces, for example surface  13 , which are formed in sand mold  10  to confine the cast metal in the shape of the I-shaped body. A volume of molten aluminum  12  has been poured into the cavity of mold  10  through a mold gating and runner system, not shown in  FIG. 1 . Mold  10  is initially at about ambient temperature and the hot (e.g., about 700° C.) molten aluminum alloy melt is rapidly cooled and forms a solidified skin  14  on cavity defining surfaces (e.g., surface  13 ) of the mold body. 
     Mold cavity surfaces  17  and  18  define a portion of the I-shaped body where the head of the I-shaped body meets the vertical column of the body. This is a region of the cast body at which the casting may be expected to shrink against the substantially right angle edge formed by the intersection of mold cavity surfaces  17 ,  18 . Thus, a separate mold section  16  of mold has been prepared in which the sand particles are mixed with wax particles. Mold section  16  is assembled with the main portion of mold body  10  before the casting is poured. 
     As heat from the volume of cast molten metal  12  is conducted through solidified skin  14 , the surrounding regions of sand mold  10  and mold section  16  are heated. Mold section  16  contains a mixture of sand and wax in which the wax melts, for example at temperatures in the range of about 225° C. to about 275° C., to weaken mold section  16  and any other wax-containing sections of sand mold body  10 . The wax content of mold section  16  is suitable (for example, up to about fifty percent by weight of sand plus wax mixture) to weaken mold section  16  to minimize residual stress in the final solidified cast structure. 
     A computer simulation of coupled thermal-stress analysis was carried out for this one-eighth I-section part (as depicted in  FIG. 1 ) with (a) sand mold only (first simulation) and (b) sand mold embedded with wax mixture (second simulation) using a commercial casting software called ProCAST®. The ProCAST® database values of the material properties pertaining to aluminum-silicon alloy (like cast metal  12  in  FIG. 1 ) and a sand mold (like mold  10  in  FIG. 1  but without a wax containing mold section  16 ) were used in these simulations. 
     The results from the first simulation indicated that (not shown in the Figures) the maximum residual stress encountered by the casting was at the intersection of mold cavity surfaces  17  and  18  and that residual stress was approximately 90 MPa after the entire liquid metal had solidified (i.e., past the casting stage of skin formation  14  as depicted in  FIG. 1 ). The second simulation used the same conditions as the first except that the CTE for the composite mold part  16  was changed from 10 −5 /° C. to −10 −5 /° C. at a temperature higher than 200° C. to simulate the softening effect due to the presence of wax at the same location of intersection between mold surfaces  17  and  18  in the casting  14  region. The second simulation indicated the maximum residual stress to be 60 MPa. These results confirm the benefit of the use of a wax-continuing mold section  16  in the “I” casting embodiment. The computer simulation estimated reduction in residual stress in the corner section was about 30%. 
     Another embodiment of the invention will be illustrated by reference to  FIGS. 2 and 3 .  FIG. 2  is an elevational view, in cross-section, of sand mold body  20  for casting a round cylindrical cup structure  30  as illustrated in cross-section in  FIG. 3 . Cup structure  30  is representative of cast articles that have a cup portion with vertical wall segments  32 ,  34  of varying thickness and a base portion  36  of still a different dimension. Wall segments  32 ,  34  form substantially right angle intersections at arc segments  38 ,  40  with base portion  36 . The right angle between wall segments  32 ,  34  and base  36  means that there is a likelihood of residual stress being present in an article produced by casting a molten aluminum alloy (or an alloy of another light metal) in a sand mold. Other acute angle intersections between walls of cast articles present like situations for the retention of stress in a cast light metal article. 
     In this embodiment, sand mold  20  ( FIG. 2 ) may be formed of sand particles bonded with water moistened clay particles. Sand mold  20  defines mold cavity  22  for the casting of cup  30  ( FIG. 3 ). Sand mold  20  has a round cylindrical surface  21  defining mold cavity  22 . Mold surface  21  also defines the exterior walls of cup  30 , and a round flat surface  23  defining the exterior bottom surface of cup  30 . Sand mold  20  would likely also have a gating and runner system, not shown, for pouring molten aluminum alloy to fill cavity  22  by molten metal flow into the bottom of cavity  22  and then upwardly into the vertical walls of the cavity. 
     Supported on bottom mold surface  23  with aluminum alloy chaplets or the like (not shown) is a thin wall, cup shaped, sand particle core  24  for defining the interior surfaces of arcuate wall portions  32 ,  34  and the base portion  36  of cup  30 . Inserted within thin wall, sand particle core  24  is a second core body  26  that is cylindrical and composed of a mixture of sand particles and wax. (Note: alternatively, instead of a second core body the core itself may be formed of a mixture of sand and wax, with the wax dispersed in selected regions within the core). The cylindrical and bottom walls of sand particle core  24  are thin (for example a couple of millimeters thick) to maintain structural integrity of cavity  22  for the accurate shaping of cup  30  as solidified metal skin forms on the surfaces of mold  20  and core  24 . But the thin walled core  24  in not strong enough to cause residual stress in regions  38 ,  40  of cast cup  30 . Moreover, the wax composition and content of core  26  is such that the wax softens or melts as solidification of cup  30  continues. Suitable softening of wax and sand particle core  26  contributes to the residual stress-free casting of cup  30 . 
     Mixtures of wax and sand-containing casting molds and cores are, thus, used to reduce the formation of residual stress in aluminum alloy castings and other light metal alloy castings. The shape of a potential casting and mold arrangement is evaluated to pre-determine the location of potential residual stress caused by shrinkage of the solidifying and cooling casting against a rigid mold or core surface. Such mold body surfaces are suitably weakened by helpful placement of a softenable mold structure. The mold structure is made softenable by use of a suitable wax. The composition of the wax is selected to melt or soften at a mold body temperature when the fragile casting is shrinking against the casting-heated mold body surface. 
     In one embodiment, wax particles may be mixed with sand particles to form a softenable mold body member. In another embodiment, sand particles may be coated using a solution of the wax with subsequent solvent removal as necessary. 
     The practice of the invention has been illustrated with examples of some specific embodiments. But the illustrations are not intended to be limiting of the scope of the invention. A worker skilled in the arts of metal casting and mold construction will recognize that other embodiments of the invention will readily be adaptable for other cast article shapes and other casting situations.