EVAPORABLE FOAM CASTING SYSTEM UTILIZING A HYPEREUTECTIC ALUMINUM SILICON ALLOY

Abstract: A method of casting utilizing an evaporable foam system with a hypereutectic aluminum silicon alloy. The alloy is composed by weight of 16 % to 19 % silicon, 0.4 % to 0.7 % magnesium, up to 1.4 % iron, up to 0.3 % manganese, up to 0.37 % copper and the balance aluminum. The molten alloy is introduced into a mold in contact with an evaporable foam pattern formed of polystyrene, or the like. The heat of the molten alloy will decompose and vaporize the pattern and the vapor will enter the interstices of the surrounding sand, while the molten alloy will fill the void caused by the vaporization of the pattern. The heat of crystallization caused by precipitation of the silicon on solidifaction of the alloy will temporarily slow the solidification rate of the alloy, thus increasing the time for elimination of pattern residue vapors from the molten alloy.

Description: EVAPORABLE FOAM CASTING SYSTEM UTILIZING A HYPEREUTECTIC ALUMINUM SILICON ALLOY Aluminum alloys, due to their light weight, have been used for casting engine blocks for internal combustion engines. Hypereutectic aluminum silicon alloys containing from 16% to 19% by weight of silicon are known to possess good wear resistant properties, achieved by the precipitated silicon crystals which constitute the primary phase. United States Patent 4,603,665 describes an improved hypereutectic aluminum silicon casting alloy having particular use in casting engine blocks for marine engines. The alloy of the aforementioned patent contains by weight from 16% to 19% silicon, up to 1.4% iron, 0.4% to 0.7% magnesium, up to 0.3% manganese up to 0.37% copper, and the balance aluminum. By minimizing the copper content in the alloy, the ternary aluminum-silicon-copper eutectic is avoided and the resulting alloy has a relatively narrow solidification temperature range. Evaporable foam casting is a known technique in which a pattern formed of an evaporable foam material is supported in a mold and surrounded by an unbonded particulate media, such as sand. When the molten metal contacts the pattern, the foam material vaporizes, with the vapor passing into the interstices of the sand, while the molten metal replaces the void formed by the vaporized foam material. In an evaporable foam casting process, it is desirable to slow the solidification rate of the molten metal to provide time for the elimination of vapors generated by the decomposition of the pattern from the molten alloy. If the molten metal solidifies too swiftly, vapor can be entrapped in the metal, resulting in porosity and a loss of mechanical properties. When dealing with aluminum alloys, increasing the pouring temperature of the molten metal to slow the solidification rate is not satisfactory. Not only does an increase in the pouring temperature increase the energy requirements, but hydrogen gas is soluble in aluminum alloys and the solubility of hydrogen increases rapidly with an increase in temperature If the temperature of the molten aluminum alloy goes above 1400°F, excessive quantities of hydrogen can be taken into solution, and on solidifying of the alloy, the hydrogen can show up as gas porosity, which will lower the mechanical properties of the alloy. Attempting to slow the solidification rate of aluminum alloys during evaporable foam casting by using an alloy with a relatively large solidification range has likewise not been satisfactory. A large solidification range can result in segregation during solidification in which the early solidified alloy may have a different composition from the later solidified alloy. The invention is directed to an evaporable foam casting system using a specific hypereutectic aluminum silicon alloy which, due to its composition, provides a slower solidification rate to provide high quality castings. In particular the invention provides a method of casting, comprising the steps of preparing a molten hypereutectic aluminum silicon alloy containing by weight from 16% to 19% silicon, casting said molten alloy into a mold into contact with an evaporable foam pattern surrounded by a finely divided media, the heat of said molten alloy acting to vaporize said pattern with the vapor passing into and being retained within said iδedia and said molten alloy filling the void resulting from the vaporization of said pattern, and precipitating the silicon in said alloy as said molten alloy cools to generate heat of crystallization to retard the solidification rate of said alloy and permit said vapor to escape from said molten alloy. The hypereutectic aluminum silicon alloy to be used in the casting method of the invention preferably contains by weight from 16% to 19% silicon, 0.4% to 0.7% magnesium, up to 1.4% iron, up to 0.3% magnesium, up to 0.37% copper and the balance aluminum. Due to the minimum copper content, the ternary aluminum-silicon-copper eutectic is avoided and the alloy has a relatively narrow solidification range, less then 150°F, and preferably less than 100°F. When the molten alloy contacts the evaporable cam pattern in the mold, the heat of the alloy will decompose the foam material to vaporize the foam, the vapor passing into the interstices of the surrounding sand and the molten alloy filling the void created by vaporization of the foam material. Solidification of the alloy occurs in conjunction with the heat of crystallization of primary silicon. As the alloy contains a substantial quantity of silicon, the heat of crystallization slows the solidification rate temporarily, thus allowing additional time for the elimination of pattern residue vapors from the molten alloy. The decrease in solidification rate also permits casting of relatively thin sections or filling isolated areas of the pattern located relatively long distances from the ingate. These advantages are realized without increasing the initial pouring temperature of the molten alloy, nor through use of an alloy with a relatively large solidification range, which could cause segregation on solidifying. The cast alloy produced by the method of the invention has inherent soundness attributable to the relatively narrow solidification range, good corrosion resistance, and excellent wear resistance due to the precipitated silicon. In the drawings: Fig. 1 is a longitudinal section of a typical evaporable foam casting system that can be utilized; Fig. 2 is a section taken along line 2-2 of Fig. 1; and Figure 3 is a perspective view of the sprue. Fig. 1 illustrates a typical evaporative foam casting system which can be utilized. As illustrated, the casting system includes a mold 1 and a pattern assembly 2 is supported within the mold and surrounded by an unbonded particulate material 3, such as sand. The molten alloy is introduced into the mold through a funnel 4 which communicates with inlet assembly 5 of pattern assembly 2. Pattern assembly 2 includes a group of patterns ( corresponding in configuration to the part to be cast and which are formed of an evaporative foam material, such as expanded polystyrene. The polystyrene, polymethylmethacrylate, or alternative pattern material, may be coated with a synthetic resin or a pattern wax. The construction of the evaporable foam resin casting system is not critical and may take any desired form. Patterns 6 are supported from a central sprue 7 by a plurality of ingates 8 which can be formed of the same evaporable foam material as the patterns. As illustrated in Fig. 2, the sprue is generally rectangular in horizontal cross section having a central opening 9 and an open bottom. Two vertical rows of ingates 8 are associated with each side surface of sprue 7 and each row of ingates is connected to one of the patterns 6, so that, as illustrated, eight patterns are supported from the sprue 7. As shown in the drawings, ingates 8 are formed integrally with the respective pattern 6, and the inner flat end of each ingate is attached to the respective surface of sprue 7 through a layer of adhesive 10. The adhesive is a conventional type which will be vaporized by the heat of the molten alloy as it is introduced into the sprue and the vapor generated by vaporization of the adhesive will pass into the interstices of the sand. As described in the aforementioned patent application, ingates 8, alternately, can be integrally formed with sprue 7 and thus connected to the patterns 6 through use of a layer of adhesive, or the ingates can be separate pieces and connected through adhesives to both the patterns 6 and the sprue 7. As best illustrated in Fig. 3, the upper end of each side surface of sprue 7 is provided with an opening or recess 11 through which sand can flow into the interior chamber 9 of the sprue. In addition, opposite surfaces of the sprue are provided with openings 12 and 13, which also serve to admit sand to the internal chamber 9. Inlet assembly 5 includes a generally rectangular inlet member 14 formed of an evaporable foam material, such as polystyrene, and having a closed bottom, as shown in Fig. 3. The alloy to be used in the process of the invention is hypereutectic aluminum silicon alloy, such as that described in U.S. Patent 4,603,665. The preferred alloy contains, by weight 16% to 19% silicon, 0.4 to 0.7% magnesium, up to 1.4% iron, up to 0.3% manganese, up to 0.37% copper, and the balance aluminum. The magnesium acts to strengthen the alloy, while the iron and manganese tend to harden the alloy. The resulting alloy has increased machineability, with more stable mechanical properties at elevated temperatures. The copper content is maintained below 0.37% and preferably at a minimum. As the copper content is minimized, the aluminum-silicon-copper eutectic is correspondingly eliminated with the result that the alloy has a relatively narrow solidification range, below 150°F, and preferably less than 100°F. The alloy has a yield strength of 15,000 to 30,000 psi, an ultimate tensile strength in the range of 20,000 to 35,000 psi, and an elongation of 0% to 2.0%. Specific examples of the hypereutectic aluminum-silicon alloy to be used in the invention are as follows in weight percent: EXAMPLE I Silicon Iron Copper Manganese Magnesium Aluminum Solidification range EXAMPLE II Silicon Iron Copper Manganese Magnesium Aluminum Solidification range When the molten alloy at a temperature below 1400°F and generally at a temperature in the range of 1250°F to 1400°F is introduced into funnel 4, it will flow downwardly to the pattern assembly 2 and heat of the molten metal will vaporize the foam material of the inlet assembly 5, sprue 7, ingates 8, and the patterns 6, with the resulting vapors passing into and being captured in the interstices of the sand 3. On cooling from solution, the silicon in the alloy precipitates as relatively large crystals which generate substantial heat of crystallization. The heat of crystallization generated by precipitation of the silicon crystals slows the solidification rate, by nonexternal means, while within the physical/thermodynamic constraints of nature. This allows additional time for the escape of vapors from the molten alloy, thereby minimizing gas porosity in the solidified alloy. The choice of silicon is ideal for this purpose because silicon has the highest heat of fusion of any element in the periodic table. As the solidification rate is slowed, the method of the invention permits relatively thin or complicated sections to be cast and also permits isolated areas of the pattern, located a relatively long distance from the ingate, to be cast without defects. With the invention, the solidification rate is slowed, not by increasing the initial pouring temperature of the alloy, but through the heat of crystallization generated by the precipitation of the silicon crystals. As the hypereutectic aluminum silicon alloy has a relatively low solidification range, less than 150°F, segregation on solidification is correspondingly minimized.

Claims: CLAIMS 1. A method of casting, comprising the steps of preparing a molten hypereutectic aluminum silicon alloy containing by weight from 16% to 19% silicon, casting said molten alloy into a mold into contact with an evaporable foam pattern surrounded by a finely divided media, the heat of said molten alloy acting to vaporize said pattern with the vapor passing into and being retained within said media and said molten alloy filling the void resulting from the vaporization of said pattern, and precipitating the silicon in said alloy as said molten alloy cools to generate heat of crystallization to retard the solidification rate of said alloy and permit said vapor to escape from said molten alloy. 2. A method of casting contoured components for an internal combustion engine, comprising the steps of preparing a molten hypereutectic aluminum silicon alloy containing by weight from 16% to 19% silicon, forming an evaporable foam pattern having a shape substantially identical to a component of an internal combustion engine, supporting said evaporable foam pattern in a mold, connecting said pattern through a sprue with the exterior of the mold, filling the mold with a generally inert finely divided media to surround said pattern, introducing said alloy through said sprue to said pattern with the heat of said molten alloy acting to vaporize said pattern with the vapor passing into and being contained within said media and said molten alloy filling the void created by vaporization of said pattern, and cooling the molten alloy to precipitate silicon from said alloy as relatively large crystals and generate heat of crystallization to retard the cooling rate of said alloy and permit said vapor to escape from said molten alloy. 3. The method of claim 1, and including the step of maintaining said molten alloy at a temperature below 1400° F . 4. The method of claim 1, wherein said alloy has a solidification range less than 150°F. 5. The method of claim 2, and including the step of forming the pattern from expanded polystyrene. 6. A method of casting, comprising the steps of preparing a molten hypereutectic aluminum silicon alloy containing by weight from 16% to 19% silicon, casting said molten alloy into a mold into contact with an evaporable foam pattern surrounded by a finely divided material, vaporizing said pattern by the heat of said molten alloy with the vapor passing into and being retained within said material and said molten alloy filling the void resulting from the vaporization of said pattern, and generating heat internally of said molten alloy in said mold by precipitating silicon as silicon crystals from said alloy to thereby retard the cooling rate of said molten alloy and permit said vapor to escape from said molten alloy.

Inventors: CLEARY TERRANCE M, DONAHUE RAYMOND J, HESTERBERG WILLIAM G, CLEARY, TERRANCE, M., DONAHUE, RAYMOND, J., HESTERBERG, WILLIAM, G.
Applicants: BRUNSWICK CORP, BRUNSWICK CORPORATION
Application year: 1988
Main classification: B22D21
IPC codes: B22C9, B22D21, B22D27