Metallurgical bonding of coated inserts within metal castings

Details Metallurgical bonding of coated inserts within metal castings Metallurgical bonding of coated inserts within metal castings

1999-08-31

2002-11-26

Elve, M. Alexandra (Department: 1725)

Metal founding

Process

Shaping liquid metal against a forming surface

C164S310000

Reexamination Certificate

active

06484790

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Technical Field
This invention relates generally to methods for forming tenacious bonds between inserts and cast metal materials such as reinforcing inserts for castings of internal combustion engine components.
2. Description of the Prior Art
The substitution of light weight casting material, such as aluminum alloys, for cast iron is a known approach for weight reduction in a variety of applications such as internal combustion engine manufacture. For example, the use of aluminum alloys to form internal combustion components is well known for automotive engines or high performance racing or aircraft engines. Such substitutions, however, have often resulted in compromised performance and/or reliability.
A well known solution to some of the performance and reliability problems associated with the use of light weight casting material as a substitute for cast iron has been to provide high strength inserts at critical points where severe wear or high stress is known to occur.
One approach has been to substitute aluminum pistons with nickel-iron ring carriers for conventional cast iron designed as disclosed by E Mahle, “Alloy Iron Ring Carriers Reduce Cylinder Wear”, Auto Industries, Vol. 68, No. 19, May 1933, p578-82. However, wear of ring lands and rings in aluminum pistons causes reduction in engine output, blow-by of combustion gases, increased oil consumption, increased fuel consumption and piston slap (noise). Early attempts to correct these drawbacks involved placing gray iron inserts which had a coefficient of thermal expansion (CTE) of 0.0000067 in/in-° F. in aluminum pistons, which had a coefficient of thermal expansion of 0.0000134 in/in-° F. The differences in thermal expansion caused the carrier to loosen. The first successful pistons employed an aluminum-silicon alloy with a CTE of 0.000010 in/in-° F. with a Ni-resist carrier of about the same CTE. The Ni-Resist is an alloyed iron with 15% Nickel and 5% copper. When a Ni-Resist with even higher nickel and molybdenum content was used, an aluminum-copper alloy could be used for the piston, with improvements in thermal conductivity, machinability as well as thermal fatigue resistance.
However, when a ferrous alloy of low coefficient of thermal expansion is used, the tendency will be for the aluminum to grow against the restraining iron band during each heating cycle. This may minutely upset the aluminum so that when it cools, the fit will not be as close as it was originally. Subsequent heating cycles will exaggerate this condition until the ring band itself loosens on the piston.
Weight reduction by replacement of cast iron by light weight alloys incorporating inserts has not found general acceptance in heavy-duty diesel engines because of the severe performance and durability demands of the markets in which they have traditionally been used. One explanation for this is the difficulty of achieving an effective, durable metallurgical bond between the insert and the adjacent light weight cast material. For example, in “Engineering for Aluminum-Alloy Castings”, by T R Gauthier and H J Rowe of ALCOA, Mechanical Engineering, Vol. 70, No. 6, June 1948, p505-14, the authors discuss casting design from the standpoint of mechanical properties, section thickness, and the use of inserts. The authors assert that no metallurgical bond normally exists between inserts and aluminum. Projecting legs or dogs or at least knurling are said normally to be necessary to mechanically retain the insert in the casting, especially if a torque will be applied to the insert.
An exception to the general rule that light weight alloys with strengthening inserts are not generally used in diesel engines has been the use of aluminum pistons joined with cast iron ring carriers by an Al-Fin process disclosed in U.S. Pat. No. 2,396,730 to whitfield et al. Other techniques for pre-coating an insert prior to casting are disclosed in U.S. Pat. Nos. 2,849,790 to Zwicker et al.; U.S. Pat. No. 2,881,491 to Jominy et al.; U.S. Pat. No. 3,945,423 to Hannig, U.S. Pat. No. 4,997,024 to Cole et al. and U.S. Pat. No. 5,333,669 to Jorstad.
However, debond problems with the Ni-resist iron insert has given the Al-Fin process, and other pre-coat processes, a poor reputation. These pistons suffer from two problems. The first is that during casting the aluminum contracts more than the iron, which may place the interface in tension. The second problem with pistons of this type is the presence of brittle inter metallic compounds. It has been established that cracking in pistons occurs between gamma-Al
3
FeSi and a Fe
3
(Si
.9
Al
.1
) phase. The presence of these compounds is a function of casting temperature, cooling rate and bath composition and is not an inherent feature of Al-fin bonding.
J A. Lucas, “Aluminum Cylinder Blocks Cast in Permanent Molds”, Am. Mach., Vol., 66, No. 4, Jan. 1928, p173-174, describes a composite engine block including cast aluminum with several inserts. The liners were of cast iron with nickel added for wear resistance and to control thermal expansion. The liners were sand blasted and copper plated prior to being placed in the mold. The best casting alloy with regards to bonding to the inserts, soundness and proper shrinkage was experimentally found to be 99% aluminum 1% copper. Even small percentages of impurities were found to dramatically increase scrap rates.
J H Beile and C H Lund, “Current Status of Composite Casting as Bonding Technique”, Metals Eng. Quarterly, Vol. 6, No. 1, Feb. 66, p63-4, disclose a bonding technique for achieving metallurgical bonding requiring an absolutely clean surface on the inserts. Practical methods to prevent oxidation are to employ vacuum, inert atmospheres, or reducing atmospheres.
“Bonding Iron to Aluminum by Casting-On”, Light Metals, Vol. 21, No. 248, Nov. 1958, p355-6, describes the principles for producing metallurgical bonds between inserts and casting. This paper reports that the production of an intimate junction may be prevented by the presence of an oxide film on the outer surface of the aluminized coating on the insert.
Another approach that has been undertaken in an attempt to achieve acceptable metallurgical bonding between inserts and cast metal is disclosed in U.S. Pat. No. 5,429,173 to Wang et al. which discloses a casting process wherein the insert, such as a ferrous metal cylinder liner, is pre-coated with plural layers of alternating materials that are exothermically reactive to produce inter-metallic phases at the surface.
Yet another approach toward achieving an acceptably strong bond between cast aluminum alloy, such as would be suitable in forming an engine block, and inserts such as cast iron cylinder liners involves pre-coating the liner with a metal coating. Examples of such approaches are disclosed in U.S. Pat. No. 1,710,136 to Angel et al., U.S. Pat. No. 3,165,983 to Thomas and U.S. Pat. No. 5,005,469 to Ohta. While claims have been made that these methods produce metallurgical bonds, others have reported that such bonds are not formed on a consistent and reliable basis. See for example U.S. Pat. No. 5,280,820 to Voss et. al. This later patent discloses a method which attempts to overcome the deficiencies of the prior art by coating the insert with zinc, tin or cadmium or their alloys in a way to cause an outer oxidized surface to form, followed by the step of removing the oxidized surface, and casting molten metal, such as aluminum based material, around the insert to cause the coating to remelt and alloy with both the insert material and the cast material to form a metallurgical bond between the liner and cast material. While effective for the purposes disclosed, this later approach has the effect of allowing direct contact between the base material of the insert and the molten casting material. This direct contact can give rise to undesirable inter-metallic phases that negatively impact the quality of the bond.
While relevant, none of these prior art methods has been entirely successful in producing consistent, high strength bonds between inserts and

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