US3083424A - Method for producing coated die castings - Google Patents

Description

A ril 2, 1963 A. F. BAUER 3,083,424

METHOD FOR PRODUCING COATED DIE CASTINGS Filed May 7, 1959 2 Sheets-Sheet 1 IL VV INVENTOR Alfred F. Bauer ATTORN EY April 2, 1963 A. F. BAUER 3,083,424

METHOD FOR PRODUCING COATED DIE CASTINGS Filed May '7, 1959 2 Sheets-Sheet 2 Fig. 4.

IN V EN TOR. Alfred E Bauer BY @N i J Q ire Alfred Ferdinand Bauer, Toledo, (Phi-o, assignor to l latlonal Lead Company, New York, Nil, a corporation of New .t'ersey Filed May 7, 1959, filer. No. 311,611 ill Galina. (Cl. 22-293) This invention relates to a method for producing coated die castings where the coating constitutes a protective or other surface for at least a part of the die casting, and is particularly directed to the production of a die casting having a surface coating or layer which is strongly and permanently adhered to the body of the die cast material and is harder or more resistant to Wear or corrosion, or has other required or desired characteristics which the body of the die cast material lacks.

As used herein, and in the appended claims, the word coating means a relatively thick and substantial body of a surfacing material which is permanently mechanically locked to a die casting and distinguishes from a transitory or temporary coating such as would result from the use of a die lubricant or the like.

It has long een recognized that aluminum has numerous advantages over cast iron as a material for use in the pro duction of internal combustion engine blocks, not the least of such advantages being its low density, its high thermal conductivity, and its inertness or corrosion resistance with respect to cooling water. Aluminum is more expensive than is gray iron, so that its use in an engine block can be justified economically only if sufficient savings are achieved in processing. t present, sufiicient processing savings can be achieved only when an aluminum engine block is produced by a die casting technique. Before an aluminum internal combustion engine block can be produced commercially by die casting, it is necessary to provide, among other things, a wear-resistant cylinder bore surface which can be formed to a high degree of accuracy Without expensive machining operations. The method of the instant invention for applying a protective or other surface to a die casting is admirably suited for providing a wear-resistant cylinder bore in a die cast aluminum internal combustion engine block.

Methods that have previously been suggested for providing a wear-resistant cylinder bore surface have been found to be commercially unacceptable for one reason or another. For example, chrome plating can be used to provide the required wear-resistance, but is an unrealistically expensive and difiicult expedient. Similarly, spray-coating techniques can be used to provide a wearresistant bore surface, but spray coating techniques require special precautions to achieve satisfactory bonding of the sprayed metal to an aluminum bore, and produce coatin s which are initially uneven and rough, so that extensive machining is required. This, in turn, requires the spray-application of excessive amounts of a hard metal by comparison with the thin lining which remains after machining.

The instant invention is based upon the discovery of what has been denominated a transplant method which can be used to provide a hard, wear-resistant, cylinder bore surface in an aluminum die cast engine cylinder sleeve, as well as in other castings of aluminum and other metals. According to the method, such wear-resistant surface can be made with such a high degree of accuracy that only a simple honing operation is required after the die casting operation. The casting as it is formed in practicing the preferred embodiment of the method requires no draft in the interior surface of the cylinder, and the interior surface is as smooth as the core over which it is formed.

The invention is also especially useful in the production 35,683,424 Patented Apr. 2, 1963 of aluminum wheels for automotive vehicles. At the present time, work is being done to adapt a cast or forged aluminum wheel to this service because of the superior heat conductive qualities of aluminum alloys and because of their lighter weight. Since the aluminum is entirely unsuited for use as a braking surface, the present proposals call for the use of a liner made from cast iron or a similar material and bonded to the interior of the aluminum wheel, the liner furnishing the braking surface against which the force of the braking shoes is taken. The liners at the present time are quite thick and, because of their low heat-conductivity, operate at much higher temperatures at the braking surface than is desirable in spite of the ability of the surrounding aluminum to dissipate quite rapidly the heat absorbed at its interface with the liner. Since the physical properties of aluminum degenerate rapidly after its temperature reaches about 406 F.,. the interface must not be too drastically heated, and this restriction has limited the application of aluminum Wheels to vehicles in which the braking requirements are not too severe rather than for the most demanding service as was originally intended. It has been found that, by transplanting a liner layer to the interior of the Wheel, a thin and still satisfactory braking surface may be provided and one which requires Virtually no machining after it is formed. The coating formed on the casting by the method of the present invention may be given any suitable thickness as will become apparent hereinafter, but in general is much thinner than the liners previously proposed for known aluminum wheels. Because the braking surface material is thin, its temperature of operation will be much lower since the heat therefrom will be dissipated more rapidly into and from the aluminum body. Lowered operating temperatures mean lower thermal expansion of the wheels With a corresponding reduction in the liability of brake fading, and increased life for the brake linings.

It has also been found that the method of the invention has utility other than that in the automotive field. For

example, the interior of a die cast aluminum cooking utensil can be made resistant to the corrosive action of certain foods by transplanting a stainless steel surface thereon, or a die cast aluminum sole plate for an electric iron of either the steamor dry-type can be made stainproof by transplanting upon it a stainless steel surface. Aluminum bearings may have a bronze or other surface transplanted thereonto to provide better bearing properties. Outer applications for the method of the invention will be apparent to one skilled in the art from the following detailed discussion of the method.

it is, therefore, an object of the invention to provide an improved method for producing a die casting having a coated surface.

It is a further object of the invention to provide a method for producing such a die casting wherein the surface coating produced is extremely smooth and is of such accuracy that, at most, only a simple honing operation is required for finishing.

It is still another object of the invention to provide a method for producing a die cast cylinder sleeve for an internal combustion engine which is free fromtaper from end to end in its original state and requires, therefore, no additional machining beyond a simple honing operation.

It is still another object of the invention to provide a method for producing a die cast lightweight wheel for an automotive vehicle having a relatively thin wear-resistant coating exposed on its interior.

It is still another object of the invention to provide a method for applying a coating to a portion of a die assembly and then transplanting this coating onto an article produced in the die assem ly by a die casting technique. Other objects and advantages will be apparent from the following detailed description, and from the attached drawings, in which-- FIG. 1 is a sectional view of a die casting die assembly which can be used to produce a typical die casting by the method of the-invention; a

FIG. 2 is an enlarged fragmentary plan view of a metalized surface of a portion of the die assembly of FIG. 1 prior to the injection of the molten metal thereinto;

FIG. 3 is a partially sectioned perspective view of a casting produced in the die assembly of 1 1G. 1; and

FIG. 4 is an enlarged fragmentary photograph showing the cross sectional view of a typical casting produced by the method of the invention.

In the following specification, a specific example of an article produced by my new method, as well as a specific example of the method itself will. first be described. Thereafter, the specification will include a description of various modifications and changes that may be made to produce other and different articles 'of manufacture.

Referring now in more detail to the drawings, and in particular to FIG. 1, a die cast aluminum cylinder sleeve for an internal combustion engine can be produced in a die assembly which includes an ejector die 10, a cover die 12, a-separate core member 14, and a shot sleeve 16.

- The core member 14 has a body portion 18 which fits into a socket 20 provided therefor in the ejector die 10, there by accurately locating the core member concentric with a cylindrical cover die impression 22. Means for retaining the body portion of the core member within its mating sockethas not been shown since this may be in the form of any of several known mechanisms which are conventionally used by those skilled in the art. An annular die cavity 24 is thus formed between the core member 14 and a generally cylindrical surface of the impression 22.

Asa; first step in producing the die cast cylinder sleeve, 3. coating layer 26 is applied to a cylindrical impression portion 28 of the core 14. In contrast to the usual die casting techniques, the impression portion 28 of the'core does not need to be tapered wherepracticing the method of the invention in its preferred'embodiment. The coating layer 26 may conveniently be applied to the impression portion 28 of the core 14 by using a conventional metalizing gun to which a 420 stainless steel 1 wire is supplied. Within the gun, the wire is fed into an oXy-acetylene flame where it is melted, and molten alloy is carried from the gun'by a blast of compressed air and, onto the impression portion 28. It has been found to be preferable to roughen the impression surface '23 slightly, for example to-a smoothness of 20 to 30 RA LS? by light sandblasting before application of the coating layer 26. Such sandblasting promotes the proper adherence of the coating material to the core portion. The thickness of the coating 26'should be from about 0.015 inch to about 0.025 inch. The exterior of the coating 26 has an irregularly pitted and undercut surface exposed to the die cavity, as

can be seen readily in FIG. 2, which shows 'a portionof the exposed surface ofsuch a coating, produced as described, by metalizing, enlarged approximately eight times.

The application of the coating material by the conventional metalizing gun technique is particularly advantageous for the production of cylinder sleeves for internal combustion engines for the reason that at least portionsof' the coating material as applied are iron oxide, which oxide portions provide hard and wear-resistant parts in the exposed area of the liner.

Molten aluminum alloy of a selected type, under a pressure of from 4000 to 10,000 pounds per square inch,

, ings.

is then forced into the mold cavity 24 (FIG. 1) to completely fill the die cavity and to enter into all of the pits and undercuts on the err-posed surface of the coating 26. As the molten metal solidifies, it shrinks in and around the coating'z and is interlocked mechanically therewith forming a bond of tremendous strength. Within a few seconds after completion of injection of the aluminum, the two die halves are separated, and the casting 30 and the core member 14 are ejected as a unit.

' In the preferred, although not essential, practice of the present invention, the casting is permitted to cool while still in place on the core. in most instances, the core is made of a material having a lower coefficient of thermal expansion than the casting, for example, steel. If the two parts are cooled together, the aluminum casting is restrained by the core from shrinking to the full extent that it would shrink if it were not supported in its interior. The aluminum is, in effect, stretched by the cooling process beyond its elastic limit and takes a permanent set at a diameter determined by the diameter of the core. By pre-heating the core to a known temperature, and cooling the parts togetherto a known temperature, very accurate castings are obtained. The variation in diameter is within 2.001 inch in a 4 inch diameter sleeve.

The most convenient way of separating the core member 14 from the coated casting is by differential expansion of the parts. The aluminum casting with the strongly bonded coating may be heated quickly while the core remains cool and the expansion of the casting will allow the parts to be separated very easily. A significant advantage of this method is that the core may be formed without taper so that the cylinder produced requires no subsequent machining to produce a cylinder sleeve having a uniform diameter from top to bottom.

It has been found possibl by this method, to produce a cylinder sleeve having an interior surface coating, as die cast, smooth to 30 R.M.S. The completed aluminum die castingfil with the ferrous alloy coating on the interior surface thereof is shown in FIG. 3, with a portion broken away to show details of the construction. A cross-sectional view of such a casting, enlarged approximately fourteen times, constitutesFlG. 4- of the drawln FIG. 4, the die cast body of the sleeve is designated 3i and the coating is designated 25. The interlocking of the material of the coating and the body of the sleevecan readily be seen. I

it will be appreciated that the cylinder sleeve 38 is illustrative of articles which can be produced by the method of the invention, and that, in the sleeve, a coating is transplanted from a die portion to the cast sleeve. It will be understood that, in the cylinder sleeve, the transplanted coating serves a principal function of providing wearresis'tance and oil retention, but that coatings which provide other desired properties may be applied either to similar die castings or to die castings of othervaried shapes by this method, and that various changes can be made from the specific details set forth above. For example, coatings either substantially thicker or significantly thinner than the 0.015 inch to 0.025 inch range may be preferred for specific applications. Also, the coating may be applied to a die member, which can be either a fixed part of a die or a removable part, and various ways other than by metalizing. The die member can be dipped into a vessel containing a suspension of a desired coating material in a suitable carrier which is capable of causing the particles from which the coating is to be formed to adhere to the die sufficiently that they will not be washed away by metal entering the die during the casting operation. The carrier for the particles can be of such a nature that, upon its evaporation, the exposed surface of the coating will be rough and will contain pits and undercuts similar to asprayed coating. A similar type of a coating material can be applied to the die by, brushing or painting, or a machining operation might be used to provide the preferred rough surface on an originally smooth coating.

It is usually preferred, for economic reasons, to apply a coating, in practicing the method of the invention, to a die part which is separable from the remainder of the die. By operating in this manner, a number of identical die members can be coated as a preliminary operation, and then used as required so as not to slow down the machine cycle. If preferred, however, for any reason, the coating can be applied to a die or a die part which is mounted in a die casting machine.

The cylinder sleeve 3%, produced as described above, is a part produced according to the method of the invention wherein a smooth polished surface is desired. The method of the invention is equally effective to produce other parts where a surface that is other smooth, for example having a desired or required pattern, is preferred. in such case, it is necessary only to provide the desired or required pattern on the surface of the die part, so that the coating, when applied to the die part, follows the contour thereof, and, upon transplanting, irnparts such contour to the exposed surface of the casting.

A metalized or metal-sprayed coating, due to the manor of application, contains minute voids. For most applications, it is preferred that the coating be or" sufficient thickness and density that the voids do not extend through the coating layer. This is to prevent the passage of molten metal, dorin casting, through the voids to the surface which will be exposed. The density of the coating may be altered according to the technique employed in applying it to the die. If the coating is applied by spraying, the density thereof may be varied according to the distance maintained between the metalizing gun and the die; a technique known to the rnetalizing art. If the coating is applied by dipping or painting, its density may be varied according to the composition of the mixture.

Castings made by the method of the present invention may readily be given continuous coatings of varying depths and of varying properties. For example, a cylinder sleeve for an internal combustion engine is subject to unequal wear, unequal thermal stress, and unequal exposure to corrosive deposits from end to end. The area forming part of the combustion chamber is subject to the highest heat and to the highest wear, and must exhibit the greatest resistance to corrosive deposits resulting from combustion. immediately below this, in the area swept by the piston, the temperatures encountered are progressi ely lower and, below the line of upper reversal of the piston rings, the wear encountered is much less. At the lower end of the cylinder only limited wear takes place, and this is largely concentrated at the line of lower reversal of the piston rings at their bottom dead center position. Most advantageously, then, the coating that is formed in the casting should be chosen for heat and corrosion resistance at the top, for extreme wearresistance at the lines of piston ring reversal, and for oil retention and somewhat more moderate resistance to wear in the intermediate areas. A continuous coating composed of annular bands of different materials, and of different thicknesses can be deposited on the core 14 for the purposes outlined. Stainless steels, ceramic and cermet materials may be used for the portions of the sleeve meeting the most severe conditions, a molybdenum containing coating for those portions subject to the greatest wear, and much less expensive steel coatings for the balance of the sleeve.

The method or" this invention can best be practiced by the process of pressure die casting where the pressure exerted on the molten metal in the die cavity is effective to force it into mechanical interlocking relationship with the irregular surface of the coating. Such mechanical interlocking must produce, upon solidification of the die cast metal, a stronger bond between the coating and the metal than that existing between the coating and the wall of the die. Then, upon solidification of the metal, the

solidified casting and the mechanically interlocked coating are removed from the die. When, as is preferred, the surface of the coating that is exposed in the die cavity is pitted and undercut, the mechanical bond is sufliciently strong that, in spite of significant differences in thermal expansion properties between the two metals, the casting and coating do not separate even at elevated temperatures. For example, such die castings of aluminum with ferrous metal coatings do not separate even at the temperatures encountered in the combustion chamber of an internal combustion engine.

In its essential details, the method of the invention is for producing a die casting having a coated surface. Such method comprises applying a coating to a portion of a die constituting at least a part of a wall of a die cavity, which coating has an irregular surface exposed to the cavity, injecting molten metal into the die cavity and into mechanical interlocking relationship with the coating, the mechanical interlocking relationship producing upon solidification a stronger bond between the coating and the injected metal than the coating has to the die Wall, solidifying the metal in the die cavity, and removing the solidified casting and the coating from the die cavity.

The method of this invention can also be employed to transplant a coating from any other portion of the die cavity to the complementary portion of a pressure die casting. The examples given are for the purpose of illustration only and are not to be considered as limiting the scope of the followin claims.

What 1 claim is:

l. The method of producing a die casting having a taper-free cylindrical interior surface of a metal harder and of higher melting point than the body of the casting which comprises, initially forming a coating of a higher melting point harder metal on a smooth cylindrical core constituting a part of a wall of a die cavity by depositing a coating of such metal directly on said core, said coating having a rough and pitted surface exposed to said cavity, injecting molten metal of a melting point lower than said coating metal into said die cavity under pressure suilicient to force said molten metal into intimate mechanical'interlocking relationship with said coating, said mechanical interlocking relationship producing upon solidification a stronger bond between the coating and the iniected metal than the coating has to said smooth cylindrical core, solidifying the injected metal in the die cavity whereby the injected metal shrinks around said coated core, removing the solidified casting and coated core from the die cavity, cooling said casting and core together to a temperature sufiiciently low to cause a permanent deformation of said casting and coating, reheating at least said casting whereby ditlerential expansion of said core and casting causes radial separation thereof, and separating said core from said casting and interlocked coating. I

2. The method of producing a die casting having a cylindrical interior surface of a material harder and of higher melting point than the body of the casting which comprises, initially forming a coating of a higher melting point harder material on a smooth cylindrical core constituting a part of a wall of a die cavity by depositing a coating of such metal directly on said core, said coating having a rough and pitted surface exposed to said ca ity, injectin molten metal of a melting point lower than said coating material into said die cavity under pressure suficient to force said molten metal into intimate mechanical interlocking relationship with said coating, said mechanical interlocking relationship producing upon solidification a stronger bond between the coating and the injected metal than the coating has to said smooth cylindrical core, solidifying the injected metal in the die cavity whereby the injected metal shrinks around said coated core, removing the solidified casting and coated core from the die cavity, cooling said casting and core together to a temperature sutficiently low to cause a peraccuses m-anent deformation of said casting and coating, re-heating at least said casting whereby differential expansion ofsaid core and casting causes radial separation thereof, and separating said core from said casting and interlocked coating.

3. The method of producing a die casting having a cylindrical interior surf-ace of a material harder and of higher melting pointthan the body of the casting which comprises, initially depositing a coating of a higher Incling point harder material on a smooth cylindrical core constituting a part of a wall of asdie cavity, said coating having a rough and pitted surface exposed to said cavity, injecting molten metal of a melting point lower than said coating material into said die cavity under pressure sufiicient to force said molten metal into intimate mechanical interlocking relationship with said coating, said mechanical interlocking relationship producing upon solidification a stronger bond between the coating and the injected metal than the coating has to said smooth cylindrical core, solidifying the injected metal in the die cavity whereby the injected metal shrinks around said coated core, removing the solidified casting and coated core from the die cavity, and separating said core from said casting and interlocked coating.

4. The method of producing a pressure casting having an interior surface of a material, harder and of higher melting point than the body of the casting which comprises, initially depositing a coating of a higher melting point harder material on a smooth core constituting a part of a wall of a mold cavity, said coating having a rough-and pitted surface exposed to said'cavity, introducing under pressure molten metal of a melting point lower than said coating material into said mold cavity, establishing on said introduced metal in said cavity a pressure sufficient to force said introduced metal into intimate mechanical interlocking relationship with said coating, said mechanical interlocking relationship producing upon solidification a stronger bond between the coating and the introduced metal than said coating has to said core, solidifying said casting around said coated core, reheating at least said casting whereby differential expansion of said core and casting causes separation thereof,

and separating said core from said casting and interlocked coating.

5. The method of producing a die casting haying a cylindrical interior surface of a metalharder and of higher melting point than the body of the casting which comprises, initially depositing a coating of a higher melting point harder metal on a smooth cylindrical core con-stituting a part of a wall of a die cavity, said coating having a rough and pitted surface exposed to said cavity, injecting molten metal of a melting point lower than said coating metal into. said die cavity under pressure sufiicient to force said molten metal into intimate mechanical interlocking relationship with said coating, said mechanical interlocking relationship'producing upon solidification a stronger bond between the coating and the injected metal than the coating has to said smooth cylindrical core, solidifying the iniected metal in the die cavity whereby the injected metal shrinks around said coated core, establishing a lower temperature in said core than in said castin whereby differential expansion of said core and casting causes radial separation thereof, and separating said core from said casting and interlocked coating. 6. The method of producing a die casting having a coated inner cylindrical surface which comprises, initially depositing a coating on a cylindrical core portion of a die constituting at least a part of a wall of a die cavity, said coating having a rough and pitted surface exposed to said cavity, injecting under pressure molten metal having a higher coefiicient of thermal expansion than said core into said die cavity and into mechanical interlocking relationship with said coating, said mechanical interlocking relationship producing upon solidification a stronger bond between the coating and the injected metal than the coat ing has to said core portion, solidifying the injected metal in the die cavity, removing the solidified casting and the coated core portion from the die cavity, cooling the core and casting together to a temperature sufiiciently low to cause permanent deformation of said casting and coating and separating the solidified casting and interlocked coating from the core portion.

7. The method of producing a die casting having a surface of a metal of a higher melting point than the body of the casting which comprises, initially depositing a coating of a higher melting metal on a portion of a die constituting at least a part of'a wall of a die cavity, said coating having a rough and pitted surface exposed to said cavity, injecting under pressure molten metal of a melting point lower than said coating metal into said die cavity, and into mechanical interlocking relationship with said coating, said mechanical interlocking relationship producing upon solidification a stronger bond between the coating and the iniected metal than the coating has to said die wall, solidifying the metal in the die cavity, and removing the solidified casting and the interlocked coating from the die cavity.

8. An article of manufacture produced by the method of claim 7. a

9. The method of producing a casting having a surface of a metal of a higher melting point than the body of the casting which comprises, initially depositing a coating of a higher melting metal on a portion of a mold constituting at least a part of a wall of a mold cavity, said coating having a rough and pitted surface exposed to said cavity, introducing under pressure molten metal of a melting point lower than said coating metal into said mold cavity, forcing said molten metal into mechanical interlocking relationship with the exposed surface of said coating, said mechanical interlocking relationship producing upon solidification a stronger bond between the coating and the introduced metal than the coating has to said mold wall, solidifying the metal in the mold cavity, and removing the solidified casting and the interlocked coating from the mold cavity.

it). The method of producing a casting having a coated surface which comprises, initially depositing a coating on a portion of a die constituting atleast a part of a wall of a die cavity, said coating having an irregular surface exposed to said cavity, introducing under pressure molten metal into said die cavity, forcing said molten metal into mechanical interlocking relationship with said coating, said'mechanical interlocking relationship producing upon solidification of said introduced metala stronger bond between the coating and the metal than the coating has to said die wall, solidifying the metal in the die cavity, and

, removing the solidified casting and the interlocked coating from the die cavity.

References 'Citcd in the file of this patent UNITED STATES PATENTS Jepon Sept. 24, 1957 OTHER narnanncas Practical Considerationsin Die Casting Design, New Jersey Zinc. 00., copyright 1948, printed by Marhridge Printing Co., New York 14, N.Y.; p. 152 relied on.

Die Casting, by H. H. Doehlencopyright 1951, first edition, McGraw-Hill Book Co., New York; page relied on.

Claims (1)

1. THE METHOD OF PRODUCING A DIE CASTING HAVING A TAPER-FREE CYLINDRICAL INTERIOR SURFACE OF A METAL HARDER AND OF HIGHER MELTING POINT THAN THE BODY OF THE CASTING

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