Metal treatment – Process of modifying or maintaining internal physical... – With casting or solidifying from melt
Reexamination Certificate
2001-03-01
2003-11-18
Wyszomierski, George (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
With casting or solidifying from melt
C148S554000
Reexamination Certificate
active
06648993
ABSTRACT:
BACKGROUND
1. Field of the Invention
The present invention relates to castings made from alloys having large differentials between their liquidus and solidus temperatures.
2. Background
Cast products are typically not used in applications that can result in major catastrophe, especially where service failure cannot be predicted. For example, because of their low fatigue properties, castings are typically not used for making structural aircraft components. Similarly, castings are typically not used for making commercial hand tools, high speed tools and bearing steels because of poor mechanical and fracture toughness problems.
One reason why castings are not used in these applications is casting porosity. Casting porosity can result from a number of different phenomena including liberation of gas during solidification from the molten state, which is commonly referred to as “gas porosity.” Casting porosity can also be due to shrinkage of the liquid metal during solidification without sufficient flow of liquid metal into the solidifying region, which is commonly referred to as “interdendritic” or “shrink porosity.”
Casting porosity can be an especially significant problem in alloys having large differentials between their liquidus and solidus temperatures, e.g. differentials on the order of 100° C. or more. By “liquidus temperature” is meant the temperature at the alloy becomes 100% liquid upon heating. “Solidus temperature” is that temperature at which the alloy becomes 100% solid when cooled. Such “high freezing range” alloys inherently take longer to cool from 100% molten to 100% solid. This, in turn, allows increased casting porosity to occur, since casting porosity occurs only during solidification—i.e., while the alloy is in a semi-solid state between its liquidus and solidus temperatures. Moreover, because cooling time is directly related to casting size, shrink porosity can become especially pronounced when castings made from these alloys are larger in size, e.g. castings whose minimum thickness dimension is 1 inch or more.
Accordingly, it is an object of the present invention to provide new technology for making alloy castings with reduced casting porosity.
In addition, it is a further object of the present invention to provide such reduced porosity alloy castings even when made from alloys having large differentials between liquidus and solidus temperatures.
A still further object of the present invention is to provide such improved low porosity castings when made from such large differential alloys, even when the casting has a minimum thickness dimension of 1 inch or more.
SUMMARY OF THE INVENTION
These and other objects are accomplish by the present invention which is based on the discovery that casting porosity can be largely reduced, and essentially eliminated in some instances, by subjecting the casting to hot isostatic pressing (“HIP”).
Accordingly, the present invention provides a new process for reducing casting porosity in a casting made from an alloy having a solidus/liquidus temperature differential of at least 50° C. comprising subjecting the casting to hot isostatic pressing.
In addition, the present invention provides a new casting made from an alloy having a solidus/liquidus temperature differential of at least 50° C., the casting having a minimum thickness dimension of 1 inch and further having a casting porosity of 50% or less of the porosity of an otherwise identical casting not having been subjected to hot isostatic pressing.
DETAILED DESCRIPTION
In accordance with the present invention, the casting porosity of a casting made from an alloy having a large differential between its liquidus and solidus temperatures (hereinafter “high freezing range alloy”) is reduced and/or essentially eliminated by subjecting the casting to hot isostatic pressing.
Castings
The present invention is applicable to any type of casting including bulk castings and near net shape castings. In this context, a “bulk casting” is a mass of solid alloy whose size and shape are dictated by convenience in terms of manufacture, storage and use. Bulk castings are sold commercially in a variety of different forms including rods, bars, strips and the like. Transforming these bulk products into discrete, shaped products in final form usually requires some type of substantial shaping operation for imparting a significant change in shape to the casting. This significant change in shape may occur by some type of cutting operation for removing part of the casting and may also include a mechanical deformation step such as bending or forging for imparting a curved or other non-uniform, non-rectilinear or non-orthogonal shape to the casting. In some instances, the casting may be worked, before or after final solution anneal, to affect its crystal structure throughout its bulk.
A “near net shape” casting, on the other hand, is a casting whose shape when taken out of the mold is the same as, or approximately the same as, the shape of the ultimate product to be made. Only minor shaping, in addition to removing the sprues, gates, runners and hot tops and deburring the casting surfaces, is required to achieve final shape. Such minor shaping may include some type of cutting operation (e.g. drilling, sawing, milling, etc.) to impart holes or other fine shape changes to the casting body. Wrought processing, as further described below, is not involved. Where the ultimate product is small, a single near net shape casting may be composed of multiple near net shape sections which are separated from one another to form the ultimate products.
Skilled metallurgists readily understand the difference between “bulk” and “near net shape” castings.
The present invention is primarily directed to making improved castings (both bulk and near net shape) which are unwrought. In this connection, it is well understood in metallurgy that the crystal structure and hence properties of many alloys can be significantly affected by subjecting the alloy to substantial, uniform mechanical working (deformation without cutting), typically on the order of 40% or more in terms of area reduction. Accordingly, most alloys of this type are available commercially either in wrought (worked) form or in cast (unwrought) form. See, for example, Kirk Othmer,
Concise Encyclopedia of Chemical Technology
, Copper Alloys, pp 318-322, 3d. Ed., © 1985. See, also, the APPLICATION DATA SHEET, Standard Designation for Wrought and Cast Copper and Copper Alloys, Revision 1999, published by the Copper Development Association. The present invention is primarily applicable to unwrought castings—i.e., castings which have not been subjected to mechanical deformation carried out to effect a noticeable change in the crystal structure and properties of the alloy forming the casting.
The present invention can also be used to enhance the properties of a previously wrought processed casting—i.e., a casting which has already been subjected to wrought processing. Wrought processing inherently reduces or eliminates casting porosity while improving microstructure, and so the beneficial effect achieved by the present invention—enhancement of properties due to reduction in casting porosity—is not as great in this embodiment. Nonetheless, hot isostatic pressing of a previously wrought processed casting still containing residual casting porosity will further reduce this porosity, thereby improving its properties at least somewhat.
Although the present invention is applicable to castings of any size, it is particularly useful when practiced on “large” castings, i.e. castings whose minimum thickness dimension (including minimum wall thickness dimension in the case of hollow and other similar products) is at least 1 inch. Castings whose minimum thickness dimension is at least about 3 inches, and especially at least about 4 or 6 inches, are of particular interest. The rate at which heat can be extracted from a mass of metal in a mold depends, among other things, on the ratio of its volume to its surface area. Since “larger” castings generally have greater volume/surface area
Brush Wellman Inc.
Calfee Halter & Griswold LLP
Wyszomierski George
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