Thick alznmgcu alloy products with improved properties

Details Thick alznmgcu alloy products with improved properties Thick alznmgcu alloy products with improved properties

2000-01-19

2001-11-13

Wyszomierski, George (Department: 1742)

Metal treatment

Stock

Age or precipitation hardened or strengthened

C420S532000, C420S541000

Reexamination Certificate

active

06315842

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to products made from an aluminum alloy of the AlZnMgCu type (the 7000 series according to the Aluminum Association designation) with thicknesses greater than 60 mm. These products can be hot-rolled plates or sheets, forged blocks or extruded products. In cases where the product does not have a parallelepipedic shape, the term thickness refers to the smallest dimension of the product at the time of quenching (for example, the thickness of the thinnest wall for a section).
DESCRIPTION OF RELATED ART
Thick rolled, forged or extruded products made of aluminum alloys from the 7000 series are used to mass produce—by cutting, surfacing or machining—high strength pieces for the aeronautics industry, for example wing elements such as wing spars or fish plates, and fuselage elements such as frames, or mechanical engineering pieces like machine-tool components or molds for plastics.
These pieces must have a set of properties that are frequently antithetical to one another, requiring difficult compromises in the precise definition of the chemical composition and in the transformation range of the products used.
In effect, in the heat treated state, the products must simultaneously have:
high mechanical strength in order to limit the weight of metal used,
sufficient toughness to reduce the crack propagation rate,
good fatigue resistance due to their use in structures subject to vibrations or stresses which are not constant over time,
sufficient stress corrosion resistance.
Moreover, the alloy must be able to be cast and transformed under proper conditions so as to obtain acceptable metallurgical quality. The transformation which follows the casting of the plate or billet usually comprises a homogenization, a hot transformation by rolling, forging or extrusion, a natural aging, a quenching (for example by immersion in or spraying with a quenching liquid), a possible de-stressing by cold traction or compression, a natural aging and an artificial aging.
The cooling during the quenching can be more or less rapid. What is meant here by the quench rate is the average cooling speed (in ° C./s) of the product from 450° to 280° C. at quarter thickness. A product is said to be quench sensitive if its static mechanical properties, such as its yield strength, decrease when the quench rate decreases, which naturally has a greater chance of occurring in thick products.
In order to obtain high mechanical strength, as well as good toughness, a fibrous structure is generally sought, which is obtained by avoiding too great a recrystallization of the alloy. For this purpose, one or more elements called “antirecrystallants” such as Zr, Ti, Cr, Mn, V Hf, or Sc are added to the composition. Thus, the compositions registered with the Aluminum Association for the alloys 7010 and 7050 comprise an addition of Zr at contents from 0.10 to 0.16%, and from 0.08 to 0.15%, respectively.
This is clearly illustrated by the recent article by DORWARD et al., “Grain Structure and Quench-Rate Effects on Strength and Toughness of AA7050 AlZnMgCuZr Alloy Plate”, Metallurgical and Materials Transactions A, Vol. 26A, pp. 2481-2484, which indicates, for example for 7050, a Zr+Ti content of 0.14%, and shows the effect, for 14-mm thick plates produced in the laboratory and not de-stressed, of extreme variations in the recrystallization rate between 15 to 80% on the yield strength of plates in the T6 temper. It also shows the effect on the quench sensitivity of 7050 of a quench rate of less than 20° C./s, which corresponds to the quench rate of products with thicknesses greater than about 50 mm.
However, these laboratory experiences are different from industrial practice, since the final thickness of 14 mm is obtained by a tepid-rolling which results in a relatively refined microstructure that is quite different from the microstructures that normally characterize thick plates obtained by hot rolling.
According to the DORWARD article, the effect of the recrystallization rate on L-T toughness diminishes with the quench rate. By way of example,
FIG. 6
in the article by DORWARD et al. shows that for a quench rate of 8° C./s (which corresponds to a half-thickness of about 100 mm, characteristic of a heavy plate for the application considered), the L-T toughness is the same for a recrystallization rate of 15% or 50%, and is reduced by about 10% when the recrystallization rate goes up to 90%.
The addition of antirecrystallant elements, which would make it possible to limit the recrystallization, has the distinct disadvantage of reducing the ability of the product to harden after quenching and annealing, especially when it is thicker, since it hardens less at the core than on the surface, resulting in significant differences in the mechanical properties.
Thus, the article by M. CONSERVA and P. FIORINI, “Interpretation of Quench Sensitivity in AlZuMgCu alloys”, Metallurgical Transactions, Vol. 4, March, 1973, pp. 857-862, mentions a loss of structural hardening capacity, measured in terms of the density of GP zones, for thin sheets of Al—Zn5.5—Mg2.5—Cu1.6 alloy with an addition of either 0.23% Cr or 0.22% Zr relative to the same alloy without these additions.
This article teaches What zirconium is more effective than chromium in limiting the loss of the hardening power of the alloy during annealing. But even in the presence of zirconium, when the quench rate is 4° C./s, that is the quench rate at the core of a product approximately 200 mm thick immersed in cold water, the loss of hardening power is considerable and the zirconium no longer makes it possible to limit the quench sensitivity. The article also shows that, for the composition tested, even in the absence of chromium or zirconium, a loss of hardening power is observed for a quench rate of the order of 4° C./s.
In order to reduce quench sensitivity, Russian metallurgists have proposed the alloy V93, or 1930 according to the Russian standard GOST 11069, which does not include any antirecrystallant elements, but which has a very different composition from that of the alloys 7010 and 7050, including in particular a high iron content (between 0.20 and 0.45%) which is unfavorable to toughness and fatigue resistance.
The article by H. A. HOLL, “Investigations into the possibility of reducing quench sensitivity in high-strength AlZnMgCu alloys”, Journal of the Institute of Metals, July 1969, pp. 200-205, makes the same observation as to the harmful effect of the elements Zr, Mn, Cr and V, that is the antirecrystallants, but also of Fe and Si at commercial purity levels, on the hardenability of thin sheets. This means that in order to reduce the quench sensitivity of these alloys, it is necessary to use compositions with low Fe and Si contents, which increases production costs with respect to alloys of commercial purity. However, the teaching of this article, which relates to thin sheets, cannot be transferred to heavy plates, due to the microstructural differences which result from the different production processes.
Finally, the Applicant performed a measurement of the yield strength R
0.2
in the L and TL directions on sheets of different thicknesses made from treated alloy 7050 in the T7451 temper intended for the aeronautics industry and observed a loss of about 0.5 MPa per mm of additional thickness.
FIGS. 1 and 2
show the statistical distribution of these values for the L direction and the TL direction, respectively. These results match those in the above-mentioned article by DORWARD et al., which shows, in the T6 temper, a loss on the order of 40 MPa between quench rates of 25° C./s and 8° C./s, which approximately corresponds to the cooling speeds in cold water at the core of plates with respective thicknesses of 60 and 150 mm. Thus, the prior art does not indicate, for thick products made from alloys of the 7000 type, any means which make it possible to simultaneously control recrystallization using zirconium to obtain high strength and toughness, and to limit the quench sensitivity so as to obtain homogeneous mechanical propert

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