EP1713603A2 - Method and apparatus for manufacturing strong thin-walled castings - Google Patents

Abstract

Method and apparatus for forming good quality strong webs having a minimum dimension of 3 mm or less between iron liners in aluminum engine blocks by utilizing pre-formed metallic inserts conjoined with the outer water jacket core, preferably having cooling liquid passages in such inserts bridging the outer water jacket and preferably shaped to fill the interliner gap and formed of the same alloy as the remainder of the block.

Description

TITLE

Method and Apparatus for Manufacturing Strong Thin- Walled Castings RELATED APPLICATION

Benefit is claimed of the prior filing date of provisional_application no. 60/531,278, filed

December 18, 2003 in accordance with 37 CFR §1.78(4) and 35 USC §119(e).

FIELD OF THE INVENTION

The invention broadly relates to the art of manufacturing thin-walled castings and is particularly useful for the manufacture of cast aluminum automotive engine blocks with very thin interliner walls.

BACKGROUND OF THE INVENTION

The description will be mainly in terms of its applicability to aluminum engine blocks where there is a constant push to increase the power to weight ratio, which results in the desire to utilize a minimum amount of light weight materials and small dimensions, yet with maintained strength and integrity, and thus effectiveness and reliability. In recent years for the manufacture of engine blocks, particularly for automotive applications, several processes are available; among which we can list (1) the sand package either low-pressure or gravity filled, wherein a sand mold comprising sand cores defining cavities of predetermined shapes, is filled with liquid aluminum alloy, which after solidification form the motor block, or (2) the semi-permanent low pressure molds or (3) gravity filled metallic molds with sand cores to form the interior features of the block. The design of the engine blocks has been changing over the time with a tendency to increase the power of engines. The dimensions of the motor blocks tend to be fixed by the dimensions of the car body. The blocks need to accommodate cylinders of larger volume, meaning larger diameter, within the same block volume. These designs pose a challenging problem to block manufacturers, because the cylinder liners (usually made of iron) are sought to be so close together that the aluminum wall formed as a web in the gap between said cylinder liners is becoming ever thinner (less than about 3 mm). With the gap between adjacent pairs of cylinder liners being so thin, the liquid aluminum volume filling such interliner cavity is relatively small and rapidly loses heat upon contacting said iron liners and consequently solidifies prematurely thus plugging the cavity and preventing liquid aluminum from filling the rest of said cavity. It has also been found that increasing the pressure in the aluminum alloy holding furnace does not solve the above-described problem, because the space through which the liquid aluminum must flow is too small. This uncontrolled solidification appears even when the iron liners are preheated to a temperature close to that of the liquid aluminum alloy. See U.S. Patent 5,421,397 issued June 6, 1995 to Robert K. Hembree et al, which discusses this same problem and teaches an elaborate system for not just preheating the liners, but actually using a casting plug 14 for forming the cylinder bore mold which has a computer temperature-controlled fluid pumped through it to delay the premature solidification at the thin interliner walls. But even this proposal, which is not likely to be cost effective, fails effectively to address the internal tension stresses that build up in the particularly vulnerable thin walls due to the simultaneous cooling of different materials in cohesive contact with one another (iron and aluminum). In other words, even if the aluminum is forced to fill the small interliner cavities another problem nevertheless arises, which is that the thin aluminum wall between the cylinder liners fails or develops cracks due to the thermal stresses generated by the rapid heat transfer from the relatively small amount of aluminum alloy between the liners and the relatively larger and colder mass of said liners. Thus, where the walls have been thinned by design considerations to be just a few millimeters, this cooling occurs while in contact with the liners which have different coefficients of expansion and contraction. This sets up stresses in the thin aluminum walls between the adjacent iron liners of the cylinder block, which thin walls crack on cooling, or later upon machining, or when the completed engine goes into service. This can cause oil leaks, compression loss, and other significant impairment of the engine. Even if the premature cooling does not plug the interliner cavity, the control of the cooling rate is nevertheless adversely and unpredictably affected and can result in undesirable differences in the crystalline structure of the cooled casting. There is also often the design need to provide for additional or more effective engine cooling passages in the interliner areas, because there is now less material to dissipate the heat from the cylinders; yet just maintaining even past cooling flow rates is becoming more difficult as the interliner gap becomes smaller. Cunent designs now seek aluminum wall interliner thicknesses of about 2 to 3 mm. Thus, the creation of a cooling passage reduces the aluminum wall thickness in such passages to about only 1 mm. Older techniques, such as drilling out passages in the interliner web do not work in such small dimensions (because the small diameter bits of such relatively long length would be costly, easy to break, and difficult to control without wandering). Also merely one round hole for forming the passages would be only about 1 mm in diameter and provide insufficient flow. To get sufficient flow, there needs to be several passages and/or the vertical height needs to be several times the horizontal width. Some cunent proposals for providing such cooling passages involve special core- making techniques, either using sand or other breakable materials like glass. These processes are not ideal, being typically of high cost and could be hazardous. Another cunent practice involves casting a solid section between the cylinder liners and then opening or machining a very thin gap in the interliner web, the top of which gap is later closed and sealed by welding (thus forming a cooling passage). This procedure requires expensive cutting/machining equipment and tools. OBJECTS AND SUMMARY OF THE INVENTION The present invention overcomes the above drawbacks of the cunent art by introducing a pre-formed web insert, with the required shape to fit in the narrowest space between the adjacent pairs of cylinder liners, at least along where the width is 3 mm or less. This insert is preferably made of aluminum of the same alloy or optionally of a different alloy or even of some other suitable material (such as bronze or copper) and is positioned in the desired place at the same time that the sand core defining the water jacket of the block is produced. This insert can have an hourglass shape similar to the pre-formed core discussed in U.S. Patent No. 6,298,899 and also similarly has the advantage of avoiding the "angle of inclination" discussed therein. Though similar in shape due to its positioning, the '899 core serves a very different purpose (involving a different concept). The applicants' pre-formed insert is significantly different with many more advantages. The '899 core is an impermanent sand core, while in contrast Applicants' pre-formed insert is a permanent solid structure that effectively remains in place as part of the casting after the water jacket core is removed. The Applicants' pre-formed insert can be a solid, or can additionally incorporate a thin- walled pipe, or preferably can alternatively have an integrally formed passage in it. The pipe or passage in such an insert is thus able to provide an effective stable cooling fluid passage through the ultra-nanow interliner gap. It is therefore an object of the invention to provide a method for manufacturing engine blocks of aluminum alloys, which method eliminates the problems of unfilled spaces, stresses, and/or uncontrolled cooling rates in the interliner webs between closely aligned pairs of cylinder block liners (or in any other casting, automotive or not, having the necessity of casting a strong thin-wall portion that otherwise would have flow, cooling, and/or differential stress problems), while preferably providing for an adequate cooling fluid passageway. It is another object of the invention to provide an engine block for automotive applications made of aluminum alloys wherein the web between adjacent cylinder liners is formed of a relatively stress-free pre-formed insert of a shape to fill the gap and a content suitable to function as part of the casting and to bond well chemically (including by some surface melting, possibly aided by an agent such zinc) and/or mechanically with the remainder of the casting and with the liners, preferably, being of the same aluminum alloy as the remainder of the casting. It is another object of the invention to provide a method for manufacturing an engine block of aluminum alloys wherein a cooling passage is formed in the nanow interliner gap of said block by providing a pre-formed insert with a cooling passage already formed therein, which is of a suitable material and shape so as to fit in said interliner gap, and which passage is placed so that it remains in flow communication with separate areas of the cooling- fluid jacket of said engine block. In one embodiment of the present invention, a method of casting a cast product is provided comprising pre-formed solid elements having at least one dimension thinner than about 3mm and separately casting thereafter the rest of the product with liquid metal with the pre-formed elements already in place thereby to form the final cast product. In another embodiment of the present invention, a method of manufacturing an engine block of aluminum alloy having cylinder liners comprising providing a mold for casting said engine block; inserting pre-formed solid elements each between a pair of said cylinder liners and filling said mold with molten aluminum alloy to form said motor block. In a further embodiment of the present invention, an engine block made of aluminum alloy is provided, comprising relatively stress-free pre-formed solid elements between said cylinder liners and the rest of said engine block being cast from said aluminum alloy forming an integral block after solidification of said casting. In further embodiments of the invention, the solid elements can incorporate a cooling passage pipe, or the pre-formed solid insert can have a cooling passage already formed therein (optionally with an in-place removable core). Brief description of the drawings Figure 1 is an isometric view of the core of a mold for forming the water jacket (i.e. cooling passages) of an aluminum engine block incorporating three pre-formed solid aluminum inserts (according to one prefened embodiment of the present invention), with each such insert having a bridging cooling passage core and each being incorporated by means of the bridging core with the jacket core at a respective interliner gap position, Figure 2 shows a top stylized view of an engine cylinder block with a cylindrical iron liner closely fitted into each of the cylinder bores and with a solid aluminum insert (having no passages therein) positioned in the interliner gap of each pair of liners in the finished block (thus showing the relationship of engine parts in which the invention can effectively be practiced). Figure 3 shows a stylized vertical section taken along section line A- A of Figure 2. Figure 4 shows a top stylized view similar to figure 2, but according to a different prefened embodiment of the present invention wherein the pre-formed inserts each incorporate a bridging cooling fluid flow passage (shown in dotted lines). Figure 5 shows a stylized vertical section taken along section line A-A of Figure 4.. Figure 6 is an isometric view a solid pre-formed insert showing two bridging cores, passing through and extending out from either end of the upper portion of the pre-formed insert, for forming interliner-gap-bridging cooling-fluid flow passages. Figure 7 is an isometric view showing two iron liners mounted on the core of a mold with a solid pre-formed insert (according to the prefened embodiment of figures 2 and 3) fitted in the interliner gap. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION The solution to the problems of filling a casting mold for an engine-block 8 having thin interliner walls 9 with a liquid aluminum alloy comprises incorporating a solid insert 10 made of a suitable material, for example the same aluminum alloy which will form the rest of the cast block. Other suitable materials for the pre-formed insert include, for example, bronze, copper and alloys and equivalents thereof. The pre-formed insert (designated generally by reference number 10) may be forged or extruded. The pre-formed insert 10 is placed between the cylinder liners 12 before introducing the liquid aluminum alloy into said mold. The insert 10 will be contained and locked in place by the liquid aluminum which solidifies forming the rest of the block 8. In a prefened embodiment, the perform insert (see inserts 10a or 10b) has grooves 14 to engage the liquid aluminum, thereby providing a better bond between the insert and the aluminum when the liquid aluminum cools and sets. Bonding may also occur from surface melting of the insert 10 during the casting pour. This may be aided by the addition of a bonding agent such as low melting zinc. In a prefened embodiment, because of the same or similar coefficients of expansion and cooling between the pre-formed 10 insert and the liquid aluminum alloy used in pouring into the block casting mold, one of the advantages of the present invention is that the pre- formed inserts 10 do not result in residual tension. In addition, even when the inserts 10 are not made of the same alloy, little or no thermal stresses are induced in the pre-formed inserts 10 due to shrinking of aluminum or due to expansion of the iron cylinder liners 12 (since the inserts 10 are already cooled to solid stress free form). Thus, cracking is minimized or eliminated. Further, the invention advantageously resists tensions or cracking caused by post-machining, procedures such as boring. In a further embodiment of the invention, a cooling-liquid passage 15 is preferably formed in or as part of a pre-formed insert 10; for example, by placing an insert 10b or 10c with a conduit for said cooling-fluid having the required shape to fit in the gap between the cylinder liners. The passage 15 can take the form of an embedded thin walled pipe 17 (made of steel or the like), see Figure 1 , or may be integrally form with the insert by use of a bridging core 16, see Figure 6. To facilitate the joinder of the insert cooling passage in the insert 10b to the remainder of the outer water jacket cooling passages (formed by the water jacket core 6), the removable bridging core 16, when used to form the passage 15 in the pre-formed insert 10b will remain in place until after the casting of the engine block is completed (and thereafter be removed with the removal of the jacket core 6). Known examples of such bridging cores include salt, carbon, or glass (see U.S. Patent No. 6,205,959). The present invention can be practiced, for example, with cunent processes and equipment, which can comprise a holding furnace for liquid aluminum alloy, a source of pressurized gas, normally nitrogen, which is injected into said holding furnace for pushing upwardly said liquid aluminum alloy through a suitable connecting conduit into a gate of a mold placed on top of said holding furnace. The liquid alloy is forced to enter all the mold cavities and after the mold is filled up, the flow of liquid is stopped by a suitable device, for example a slide valve or gate and said mold is then disconnected from the holding furnace and the process is repeated by a subsequent mold to be filled up. A mass of a heat-absorbing material also known as a chill (or a heat sink device) can be cunently placed under a proprietary process of the assignee of this application in contact with the liquid aluminum alloy in order to direct the solidification in the desired direction in order to produce good-quality castings. Such controlled cooling can be disrupted by the small interliner gaps that are now encountered in modern aluminum automotive engine blocks; but are overcome by use of the present invention. Less than 3 mm is a prefened condition for using this invention, but the method applies as well to thicker interliner dimensions when there is a need to provide interliner cross sections with reduced residual stresses due to the solidification process or for any of the other reasons discussed above. *** Those skilled in the art will recognize, or be able to ascertain without undue experimentation any of the numerous equivalents to the embodiments of the invention described herein. All such equivalents are considered to be within the scope of the instant invention and are encompassed by the claims that follow. Unless otherwise explained, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the invention belongs. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, suitable methods and materials are described herein. All publications, patent applications, patents, and other references mentioned herein are incorporated by reference in their entirety. In case of conflict, the present specification, including explanations of terms, will control. In addition, the materials, methods, and examples are illustrative only and not intended to be limiting. Although prefened embodiments of the present invention and modifications thereof have been described in detail herein, it is to be understood that this invention is not limited to those precise embodiments and modifications, and that other modifications and variations may be affected by one skilled in the art without departing from the spirit and scope of the invention as defined by the appended claims.

Claims

WHAT IS CLAIMED IS: 1. A method of casting a cast product comprising pre-formed solid elements having at least one dimension thinner than about 3 mm and casting the rest of the product with liquid metal to form the final cast product. 2. A method of manufacturing an engine block of aluminum alloys comprising cylinder liners comprising providing a mold for casting said engine block; inserting pre-formed solid elements between said cylinder liners and filling said mold with molten aluminum alloy to form said motor block. 3. An engine block made of aluminum alloy comprising pre-formed relatively stress free solid elements between said cylinder liners and the rest of said engine block being cast from said aluminum alloy forming an integral block after solidification of said casting. 4. A method according to claims 1 or 2 wherein the pre-formed element has at least one bridging passage therein. 5. A method according to claim 4, wherein the bridging passage is formed by a bridging core through each interliner gap between each pair of cast iron liners and extending between opposing halves of a water jacket core forming part of the engine mold.