Stock material or miscellaneous articles – All metal or with adjacent metals – Composite; i.e. – plural – adjacent – spatially distinct metal...
Reexamination Certificate
2002-04-24
2004-08-17
Koehler, Robert R. (Department: 1775)
Stock material or miscellaneous articles
All metal or with adjacent metals
Composite; i.e., plural, adjacent, spatially distinct metal...
C148S516000, C148S527000, C148S529000, C148S535000, C148S536000, C428S577000, C428S588000, C428S594000, C428S636000, C428S637000, C428S638000, C428S926000, C428S940000
Reexamination Certificate
active
06777106
ABSTRACT:
CLAIM FOR PRIORITY
This application claims priority under 35 U.S.C. §119 to French Patent Application No. 0105500, filed Apr. 24, 2001 which is incorporated herein by reference in its entirety.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to metal blocks suitable for use in machining applications, for example for manufacturing large size toolings or molds, or structural components for high capacity aircraft.
2. Description of Related Art
Metal blocks used for machining large size parts are generally rolled metal plates or forged blocks. When manufacturing such thick metal plates, and especially thick plates in aluminum alloys, static mechanical properties in the centre of such plates are usually lower than those same properties in the centre of thinner metal plates or sheets. More specifically, the tensile strength (R
m
) the yield strength (R
p0.2
) and the ultimate elongation (A) tend to decrease (often to unsatisfactory levels) when the thickness of the metal sheet or plate is increased by virtue of a given manufacturing process. As an example, the European standard EN 485-2 as of November 1994 specifies for rolled plates in EN AW-6061 aluminum alloy in the metallurgical condition T651, the following minimum values: R
m
min=290 MPa for plates with a thickness between 12.5 mm and 100 mm and a R
m
min=265 MPa for plates with a thickness between 150 mm and 175 mm. For ultimate elongation properties, the reduction is even more significant: the guaranteed minimum value is 8% for plates with a thickness between 12.5 mm and 40 mm, and 4% for plates with a thickness between 150 mm and 175 mm. According to the EN 485-1 standard, for plates with a thickness of more than 40 mm, the longitudinal axis of the specimen should be located at a distance from one of the rolling surfaces equal to a quarter of the thickness, and for plates with a thickness of less than 40 mm, to half the thickness.
This drop in the static mechanical properties is more significant or marked when the mechanical properties are analyzed at various levels below the rolled surface. For example in the case of a metal plate with a thickness of 200 mm, by taking out a specimen at 25 mm, 50 mm and 100 mm from below the surface, it can be seen that the properties drop off accordingly. This phenomenon is well known to one skilled in the art and its causes are multiple. Work-hardening of the metal plate during the rolling process may increase its R
m
and R
p0.2
values, but is limited by the design of the hot rolling mill. In order to obtain a metal plate with a final thickness of 100 mm by a rolling process which is to include a reduction of the thickness by half, it is generally necessary to start from a rolling ingot with a thickness of at least 200 mm. In order to obtain in the same way a metal plate with a final thickness of 400 mm, it is necessary to start with a rolling ingot with a thickness of at least 800 mm. However, currently no rolling mill for rolling such a thick ingot or plate is available. Thick plates or inglos may be work-hardened by forging, but for this, very powerful forging presses must be available, which only exist in rare locations, and such methods are very expensive.
In the case of thick metal plates of quench hardenable alloys, the quench rate influences the static mechanical properties. The local quench rate for a given volume of the metal plate is determined by the thermal conductivity of the material and therefore depends on the thickness of the metal plate, or, more precisely, on the distance of the particular volume element from the surface in contact with the quenching medium.
In the case of quenched metal plates, the quenching process induces residual stresses, which may lead to deformation of the metal plate, notably when the plate is machined. These stresses are therefore undesirable and should be minimized, for example, by stretching the quenched metal plate. Stretching machines available in most factories often do not accept metal plates with thicknesses of more than 100-200 millimeters, and their power is also often limited. Relief of internal stresses in metal plates may also be obtained by compressing such metal plates under a forging press. In this case, the thickness of the metal plate may be larger, but then the maximum compression stress that the forging press is able to provide becomes a factor of limitation.
The drop in the local static mechanical properties versus thickness is generally undesirable. That is, when machined parts are manufactured from thick metal plates, it is the local mechanical properties near the new surface generated by the machining process that determine the properties of the machined part. For example, when an injection mold for plastic parts is manufactured by machining a thick steel or aluminum alloy block, the designer of the mold must take into account the gradient of the static mechanical properties based on the thickness of the metal block, as opposed to the overall mechanical properties of the block. Namely, because the block will be shaped and machined, it is the static property values below the outer surface that become important and any decrease in the properties at certain depths below the surface must be taken into account.
Another drawback of prior art products relates to the machining operation itself. As an example, when thick metal plates in aluminum alloys are machined to a great depth, it is observed that the machinability of the metal is reduced upon penetrating into the inside of the plate, as the metal is softer deep down as opposed to metal that is close to the plate's original outer surface. Also, polishing of machined surfaces and chemical or electrolytic graining are of a poorer quality for deeply machined surfaces than surfaces obtained from an area near the original outer surface of the thick plate. This is because precipitate formation in the centre of thick aluminum plates and precipitate formation close to the surface are not necessarily the same.
To overcome these drawbacks, one skilled in the art has hitherto favored a metallurgical approach by working either on the composition of the alloy used, or on the manufacturing route. This is notably the case for aluminum alloys. For example, in U.S. Pat. No. 6,077,363 (incorporated herein by reference), residual stresses of a AlCuMg alloy metal sheet are minimized by selecting an optimized chemical composition, notably with regards to manganese, iron and silicon, and by selecting a manufacturing process comprising several thermomechanical processing steps.
U.S. Pat. No. 5,277,719 (Aluminum Company of America) (incorporated herein by reference) presents a method for manufacturing a thick low porosity plate in an aluminum alloy from the 7xxx series, by employing a first preforging step with a reduction ratio of at least 30%, which precedes the hot rolling. Patent application EP 723 033 A1 (Hoogovens Aluminum Walzprodukte) (incorporated herein by reference) describes a method for manufacturing a thick metal plate in an aluminum alloy, by conducting one or several forging steps after a first hot rolling step. The goal of these methods is mainly to improve fatigue strength. However the method described in EP 723 033 also leads to a slight reduction in the drop of the tensile strength for metal plates with a thickness of more than 8 inches (about 205 mm).
Patent application EP 989 195 (Alusuisse Technology & Management AG) (incorporated herein by reference) provides a method for reducing residual stresses in AlCuMg alloy sheets, aimed at obtaining homogeneous precipitation of submicron phases of Al
3
Zr in the thickness of the metal sheet. These metal sheets may be obtained by hot rolling a rolling ingot, or they can be directly manufactured from cast plates, without any rolling.
These different means provided by the state of the art induce constraints in terms of: (i) the selection of the alloys, (ii) the metallurgical conditions, and (iii) the manufacturing method for the metal plate and of its thickness. Moreover, they ar
Catteau Frédéric
Godard David
Connolly Bove & Lodge & Hutz LLP
Koehler Robert R.
Pechiney Rhenalu
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