Metal treatment – Process of modifying or maintaining internal physical... – With casting or solidifying from melt
Reexamination Certificate
1999-12-10
2001-05-01
Wyszomierski, George (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
With casting or solidifying from melt
C148S698000, C148S902000
Reexamination Certificate
active
06224693
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method and apparatus for making aluminum alloys castings, wherein the heat-treatment processes of the prior art are simplified, by actually eliminating some traditional steps and equipment. The invention is applicable for example in the production of cylinder heads, motor blocks and the like, for automotive engines. The invention provides many advantages over the prior art heat-treatments, with an increased productivity of the casting plants, and lower capital and operation costs as well. The invention is particularly useful for producing aluminum alloys of the 3xx.x series of the classification of the Aluminum Association (AA), especially for T6 and T7 properties.
This invention is broadly applicable to the production of any aluminum alloy casting which in the past has derived meaningful benefit from quenching and artificial aging in an aging furnace. The invention eliminates the need for an aging furnace, while retaining the benefits thereof. This improvement has been styled herein as artificial self-aging (to distinguish from natural aging at ambient temperatures and from prior art artificial aging, which requires an aging furnace).
For a good background discussion and definitions of “heat treatable aluminum alloy castings”, “artificial aging” (see also “precipitation hardening”), “quenching”, “solution heat treatment”, “casting series 3xx.x”, and “T6 & T7 tempers” see the ASM Handbook series; particularly Volume 4 (1991), entitled “Heat Treating” (especially pages 841-879; see p.841 for “heat treatable”, p. 851 et seq. for “quenching” and p. 859 for “age hardening”) and Volume 2 (1990), entitled “Properties and Selection: Nonferrous Alloys and Special Purpose Materials”, (especially pages 15-41; see p.39 for “heat treatable”, p. 40 for “artificial aging”) both being tenth editions, and also Volume 15 (1988), entitled “Casting” ninth edition (especially pages 757-761, see pp. 760-1 for “quenching” and “aging”); all published by ASM International; the contents of which (including also the patents cited below) are incorporated herein by reference.
The present invention uses a selectively directed spray quench in a manner which. eliminates expensive equipment and reduces significantly the overall production time. The castings are preferably so quenched promptly after demolding in accordance with applicants' own recent patent to obtain the properties of a conventional “solution” heat treatment (such as the properties required by a T6 temper) but without the usual “solution” heat treatment in a furnace.
This invention is broadly applicable to the production of any aluminum alloy casting of the type having significant precipitation hardening with meaningful benefit from “solution” heat treating and aging.
BACKGROUND OF THE INVENTION
In the production of cast parts made of aluminum alloys it has always been thought in the past to be necessary for many such castings (especially with a T6 or T7 temper) to undergo an elaborate heat-treating process in order to impart to the cast parts the necessary mechanical properties (like hardness and tensile strength required for the demanding working uses of said parts).
It is known that the degree of hardness and other mechanical properties of the cast parts depend on the thermal history of the cast parts after having been cast in the mold. The Aluminum Association (AA) has classified the most used aluminum alloys and the several standard heat treatments used in the industry. Examples of such standard heat-treatments those denominated T6 and T7, which designate a standard set of mechanical properties developed by certain castings of primarily silicon-copper-aluminum alloys.
The automotive industry throughout the world demands very strict quality standards. Casting plants making aluminum motor parts must therefore be able to produce cast parts which consistently comply with the minimum levels of mechanical properties specified for each part. Since quality is a must, the casting plants follow those procedures and processes which are well tested and have proven reliable for many years. The production process currently followed in the industry comprises filling a mold with liquid aluminum alloy, cooling the cast part in the mold in order to obtain a solidified casting, extracting the casting from the mold, and allowing the cast part to cool-down naturally to ambient temperatures, and then subjecting batches of such cooled castings to the aforementioned “solution” heat-treating process. One way to reduce the heat load in the solution heat treatment: furnace, has been to remove the sand cores and riser portions of the castings after natural cooling and before the “solution” heat treatment. The heat treating of the prior art comprises heating the preferably trimmed castings in a furnace to temperatures above about 470° C. (typically in the range between 480° C. and 495° C.) for a certain period of time, usually in the range between at least 2 to 7 hours. This treatment is performed in order to bring back into solid solution the copper and/or other alloying elements that give the castings their hardness. It is known that, while the casting metal is in the molten state, the alloying elements are in solution in the aluminum substrate. During the cooling process, particularly if the cooling is carried out at a slow rate, there is a tendency for the different elements to become segregated. Therefore, traditionally the casting is re-heated in a “solution” heat treatment furnace for several hours, and only then is quenched, i.e. rapidly cooled down by a fluid quench from a temperature for example about 480° C. to around 85° C., so that the solid solution is preserved (before segregation can occur). Such post solution-treatment quench cooling may commonly be continued in a manner sufficient to bring the castings down to any of a number of different temperatures and at different rates according to the final properties of the alloy to be emphasized.
This quenching step produces a supersaturated solid solution that causes the alloy to harden naturally as time passes. Finally, in order to accelerate and improve this age hardening, the quenched castings are maintained at temperatures of about 200° C. in an “aging” furnace for about 2 or more hours. The time spent in the “Eaging” furnace at elevated artificial aging temperatures brings the alloy to at least a partial coherency in its structure giving the required hardness and strength properties.
U.S. pat. No. 5,788,784 to Koppenhoefer et al. discloses a process for heat-treating light metal castings that requires “a solution heat treatment furnace
2
, an adjoining quenching device
3
, as well as an aging furnace
4
”, all particularly for cylinder heads of piston engines. In the U.S. Pat. No. 5,788,784 process, after solidifying and removing the casting from the mold, said castings unconventionally are not naturally cooled, yet are still solution heat treated (claiming the advantage of using the residual heat of the casting present at the approximate 530° C temperature of such treatment). Thereafter, the castings are quenched with an air/water mixture down to 130° C. to 160° C., and then aged in a furnace at approximately 170° C. to 210° C. (thus taking advantage of some relatively minor residual heat carryover into the aging furnace), and are then finally cooled to room temperature after, for example, four hours of furnace aging. The castings are individually quenched with a mist-type fine mixture of air and water, which is “nozzle sprayed on all sides” of the casting.
Koppenhoefer asserts a number of advantages by reason of quenching the castings with an air-water spray, for example that a uniform and low-distortion cooling is achieved, that the adhering core sand is not wetted at the elevated quenching temperatures and can be collected clean and reused after regeneration, and that the residual heat of the casting remaining at 130° to 160° C. can be used to aid in the subsequent furnace aging step (by not cooling down the casting too much and lea
Garza-Ondarza Oscar
Mojica-Briseno Juan Francisco
Valtierra-Gallardo Salvador
Combs-Morillo Janelle
Frommer & Lawrence & Haug LLP
Safford A. Thomas
Tenedora Nemak S.A. de C.V.
Wyszomierski George
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