EP2546373A1

EP2546373A1 - Method of manufacturing an Al-Mg alloy sheet product

Info

Publication number: EP2546373A1
Application number: EP11173741A
Authority: EP
Inventors: Shangping Chen; Arjen Kamp
Original assignee: Aleris Aluminum Koblenz GmbH
Current assignee: Novelis Koblenz GmbH
Priority date: 2011-07-13
Filing date: 2011-07-13
Publication date: 2013-01-16
Also published as: WO2013007471A1

Abstract

The invention relates to a method of manufacturing an aluminium alloy sheet product, and comprising the steps of: (i) providing a feedstock for rolling and having a gauge in the range of 4 mm to 30 mm and being made from an aluminium alloy having at least 2.5% to 6% Mg and 0.05 to 1% Sc; (ii) cold rolling of the feedstock to a cold rolled sheet product having a gauge in the range of 0.8 mm to 6 mm; and (iii) annealing of the cold rolled sheet product at a temperature in the range of 225°C to 400°C.

Description

FIELD OF THE INVENTION

The invention relates to method of manufacturing Al-Mg-Sc sheet products. The Al-Mg-Sc sheet products obtained by this method are ideally for use in aerospace applications, and the like.

BACKGROUND TO THE INVENTION

As will be appreciated herein below, except as otherwise indicated, aluminium alloy designations and temper designations refer to the Aluminium Association designations in Aluminium Standards and Data and the Registration Records, as published by the Aluminium Association in 2011 and are well known to the person skilled in the art.
For any description of alloy compositions or preferred alloy compositions, all references to percentages are by weight percent unless otherwise indicated.
There are several Sc-containing aluminium alloys known in the art.
US Patent No. 5,624,632 discloses an aluminium alloy product, the composition is substantially zinc-free and lithium-free, and has 3% to 7% Mg, 0.05% to 0.2% Zr, 0.2% to 1.2% Mn, up to 0.15% Si, and 0.05% to 0.5% of a dispersoid-forming element selected from the group consisting of: Sc, Er, Y, Gd, Ho or Hf, balance aluminium and inevitable impurities. The aluminium alloy product is said to be especially suited for applications where damage tolerance is required.
US Patent No. 6,139,653 discloses aluminium alloy consisting of 4% to 8% Mg, 0.05% to 0.6% Sc, 0.1 to 0.5% Mn, 0.05% to 2% Cu or Zn, 0.05% to 0.20% Hf or Zr, and the balance aluminium and incidental impurities. It further discloses an aluminium alloy consisting of 4% to 8% Mg, 0.05% to 0.6% Sc, 0.05-2% Cu or Zn, 0.05-0.20% Hf or Zr, and the balance aluminium and incidental impurities.
US Patent No. 6,531,004 discloses a weldable, corrosion-resistant aluminium alloy having 5% to 6% Mg, 0.05% to 0.15% Zr, 0.05% to 0.12% Mn, 0.01% to 0.2% Ti, 0.05% to 0.5% by total of Sc and Tb, and at least 0.1% to 0.2% Cu or 0.1 % to 0.4% Zn, the balance being aluminium and unavoidable contaminants not exceeding 0.1 % Si. It is reported in this document that in the sensitised condition due to the low Mn-content an improved corrosion resistance is obtained.
There is a demand for aluminium-magnesium alloys containing Sc as purposive alloying element with an improved balance in strength and elongation.

DESCRIPTION OF THE INVENTION

It is an object of the invention to provide a method of manufacturing AlMgSc sheet material having an improved balance in strength and elongation.
This and other objects and further advantages are met or exceeded by the present invention providing a method of manufacturing an aluminium alloy sheet product, and comprising the steps of:

providing a feedstock for rolling and having a gauge in the range of 4 mm to 30 mm and being made from an aluminium alloy having at least 2.5% to 6% Mg and 0.05 to 1 % Sc;
cold rolling of the feedstock to a cold rolled sheet product having a gauge in the range of 0.8 mm to 6 mm; and
annealing of the cold rolled sheet product at a temperature in the range of 225°C to 400°C.

In accordance with the invention it has been found that with this method a sheet product can be provided having an improved ductility in combination with a high yield strength.
Part of the invention is that a feedstock for rolling is being provided and having a gauge in the range of 4 mm to 30 mm and being made from an aluminium alloy having at least 2.5% to 6% Mg and 0.05% to 1% Sc, and preferably also having at least Zr in a range of 0.03% to 0.4%.
Preferably the feedstock is prior to cold rolling not being subjected to a thermal or thermo-mechanical process at temperature in the range of 325°C to 450°C, for example as part of a pre-heat cycle or homogenisation cycle, so as to avoid the formation of coarse Al₃(Sc,Zr) precipitates that do not dissolve in any further processing steps and having a detrimental effect on the mechanical properties. Suitable feedstock can be obtained for example by continuous casting techniques, e.g. roll casting, belt casting or strip casting, having favourable high cooling rates and resulting in as-cast strip in a gauge range of 4 mm to 30 mm, e.g. about 10 mm or about 15 mm. This continuous cast strip can be coiled and stored for subsequent cold rolling in accordance with this invention.
It is important that as much as possible the Sc and, if present also the Zr, are in solid solution before the application of any rolling deformation in order to preserve a good deformability of the feedstock. If any Al₃(Sc,Zr) dispersoids have been formed in the feedstock before the rolling process, the size of the dispersoids should be as fine as possible, viz. these dispersoids should be smaller than 20 nm, and more preferably smaller than 15 nm. Thus preferably the feedstock is free from any Al₃(Sc,Zr) dispersoids having a size larger than 20 nm, and preferably is free from any such dispersoids larger than 15 nm.
The cold rolling can be carried out in one or more cold deformation steps depending on the starting gauge and the desired final gauge. Preferably each cold rolling step introduces a gauge reduction of maximum 35%.
The desired final gauge after cold rolling is in the range of about 0.8 mm to 6 mm, and preferably in the range of 0.8 mm to 5 mm.
In an embodiment of the method the cold rolling is carried out at ambient temperature.
In a preferred embodiment of the method according to this invention the cold rolling to final gauge is carried out using at least one cold rolling step at a sub-zero temperature. Preferably all cold rolling steps leading to the final gauge of the sheet product have been carried out at sub-zero temperature. It has been found that cold rolling at sub-zero temperatures lead an increased ductility compared to cold rolling at ambient temperature. Also the strength of the sheet product can be increased. Furthermore, it may lead to improved surface quality and to less edge cracking during rolling.
With sub-zero temperatures a processing temperature of the sheet product is meant of less than -25°C, and typically in a range of -200°C to -25°C, and more preferably in the range -200°C to -40°C.
Cooling to the rolling temperature can be done by placing or spraying cold media onto the sheet using liquid nitrogen or dry ice and which are preferred, but it should be understood that also other types of cooling liquids (dry ice/ethanol, dry ice/acetone, liquid nitrogen/solvent, and the like) or gases can be used. In an alternative approach a cold rolling process and apparatus can be used as disclosed in international patent application WO-02/087803-A1 published on 07 November 2002 and incorporated herein in its entirety by reference.
After cold rolling to final gauge the sheet products are annealed at a temperature in a range of about 225°C to 400°C. Preferably the annealing temperature is at least 250°C and more preferably at least 275°C. A preferred upper-limit for the annealing temperature is about 375°C, and more preferably 325°C. Annealing that these temperatures are to be carried out typically for several hours in the range of about 1.5 to 16 hours, for example for about 2 hours or about 8 hours. The annealing results in the formation of the fine, densely-distributed Al₃(Sc,Zr) precipitates in a nanostructured matrix. The formation of the fine, densely-distributed Al₃(Sc,Zr) precipitates having a size of less than 12 nm is believed to contribute to the observed enhancement in the ductility of the nanostructured alloy sheet as measured by an improvement in the uniform tensile elongation.
Following the annealing process the aluminium sheet product can be formed into a panel, for example a complex curved shape of a structural panel of an aircraft, e.g. fuselage panel or a wing stringer, using a roll forming operation, a stretch-forming operation or a creep-forming operation.
In an embodiment of the invention the aluminium alloy has a composition of, in wt.%,

Mg: about 2.5 % to 6%, and preferably about 3.7% to 6%, and more preferably of about 3.7 to 4.7%,
Mn: 0 to about 1.4%, and preferably about 0.05% to 1.2%,
Sc: 0.05% to 1 %, and preferably 0.05% to 0.4%,
Ag: 0 to about 0.5%
Zn: 0 to about 2%
Cu: 0 to about 2%
Li: 0 to about 3%,

Fe: 0 to about 0.4%
Si: 0 to about 0.25%,

Typically inevitable impurities are present in a range of each up to 0.05% and in total up to 0.25%.
Iron can be present in a range of up to about 0.40% and preferably is kept to a maximum of about 0.25%. A typical preferred iron level would be in the range of up to 0.12%, for example about 0.03% or about 0.05%.
Silicon can be present in a range of up to about 0.25% and preferably is kept to a maximum of about 0.2%. A typical preferred Si level would be in the range of up to 0.12%, for example at a level of about 0.04%.
In an embodiment zinc can be present up to about 0.4%. Yet, in another embodiment Zn can be present as a strengthening element in a range of about 0.4% to 2%. A relatively high amount of Zn also has a positive effect of the corrosion resistance of the aluminium alloy. A more preferred upper-limit for the Zn-content is about 0.7%.
Cu can be present in the AlMgSc-alloy as strengthening element in a range up to about 2%. However, in applications of the alloy product where the corrosion resistance is a very critical engineering property, it is preferred to maintain the Cu at a low level of 0.25% or less, and preferably at a level of 0.1 % or less, and more preferably at a level of 0.04% or less.
Li can be present in the AlMgSc alloy in a range of up to about 3% to provide the product with a low density, high strength, good weldability, and a very good natural ageing response. If purposively added, the preferred Li level is in the range of 0.5 to 3%, and more preferably in a range of about 0.8 to 2%. In an alternative embodiment there is no purposive addition of Li and should be kept at impurity level of maximum 0.05%, and more preferably the aluminium alloy is lithium-free.
The AlMgSc alloy preferably has one or more elements selected from the group consisting of Zr 0.03% to 0.4%, Cr 0.03% to 0.4%, and Ti 0.005% to 0.3%. In the Al-Mg-Sc the preferred alloying element is Zr. A preferred range of the Zr addition is about 0.05% to 0.2%.
Optionally one or more elements selected from the group of (erbium, dysprosium, gadolinium, and hafnium) can be added whereby the total amount, if added, is in a range of 0.03% to 0.3%. The listed elements can be added to substitute in part the Sc in the AlMg alloy.
Ti may be added to the AlMgSc alloy as strengthening element or for improving the corrosion resistance or for grain refiner purposes.
In a particular preferred embodiment the aluminium alloy consisting of, in wt. %:

Mg: about 3.8% to 5.1 %, and preferably about 3.8% to 4.7%,
Mn: 0 to about 0.4%, and preferably 0 to about 0.25%,
Sc: 0.05% to 1 %, and preferably 0.05% to 0.4%,
Zn: 0 to about 0.4%
Cu: 0 to about 0.25%
Cr: 0 to about 0.12%
Zr: about 0.05 to 0.20%
Ti: 0 to about 0.20%
Fe: 0 to 0.4%, and preferably 0 to about 0.15%,
Si: 0 to 0.25%, and preferably 0 to about 0.10%,

In another particular preferred embodiment the aluminium alloy has a chemical composition within the ranges of AA5024.
The aluminium alloy product manufactured with this invention is especially suited for aerospace application, particularly fuselage skin, and for lower wing sections, upper wing sections, stringers, spars, pressure bulkheads of many airplanes.
The following example is provided to further illustrate the objectives and advantages of this invention. It is not intended to limit the scope of this invention in any manner, however.

EXAMPLE

To test the tensile properties of the aluminium alloys formed in accordance with the invention, a number of rolled sheet samples were prepared, and subjected to testing.
An AlMgSc-alloy of composition 3% Mg, 0.6% Mn, 0.3% Sc, 0.15% Zr, 0.15% Fe, 0.05% Ti, 0.05% Si had been produced by strip casting and having a gauge of 10 mm.
Three different processing routes have been applied:

the "previous" route or conventional route, which includes preheating to cast strip at 325°C for 5 hours, and then hot rolling from 10 mm to 6 mm, followed by cold rolling at room temperature from 6 mm to 3 mm;
cold rolling of the cast strip at room temperature ("RT Rolling") from 10 mm to 3 mm;
submerging of the cast strip into liquid nitrogen ("Cryorolling") for about 10 minutes and cold rolling in steps from 10 mm to 3mm, and whereby prior to each step the sheet is being submerged again into the liquid nitrogen;

After the cold rolling to 3 mm final gauge all three sheet products had been annealed for 2 hours at 320°C. The two sheets having been processed in accordance with the present invention had not been homogenised or pre-heated prior to the cold rolling process applied.
Tensile properties were determined using small Euro norm tensile samples according to EN485-2 and the results are listed in Table 1.
From the results of Table 1 it can be seen from the comparison of "previous" and "RT Rolling" in accordance with the invention, that where the sheet in accordance with the invention has not been subjected to a high-temperature heat treatment prior to room temperature cold rolling, that an increase in strength and a significant increase in elongation is being obtained. The feedstock used in the method in accordance with this invention were free from any Al₃(Sc,Zr) dispersoids having a size of more than 20 nm.

The increase in elongation can be further enhanced by cold rolling in accordance with the invention at sub-zero temperatures.

Table 1. Tensile properties of the sheet materials.

			Tensile properties
	Rolling practice	Annealing	Rp [MPa]	Rm [MPa]	Ag [%]	A [%]
Previous	HR+CR	2h@320°C	365	429	5.1	6.3
Invention	RT Rolling	2h@320°C	384	428	7.2	9.0
Invention	Cryorolling	2h@320°C	382	429	7.5	10.6

This example is merely to illustrate to effect of the processing on the properties. Further advantages in properties can be achieved by optimising the alloy composition and the processing route.
While various embodiments of the technology described herein have been described in detail, it is apparent that modifications and adaptations of those embodiments will occur to those skilled in the art. However, it is to be expressly understood that such modifications and adaptations are within the spirit and scope of the presently disclosed technology.

Claims

Method of manufacturing an aluminium alloy sheet product, and comprising the steps of:
- providing a feedstock for rolling and having a gauge in the range of 4 mm to 30 mm and being made from an aluminium alloy having at least 2.5% to 6% Mg and 0.05 to 1 % Sc;

- cold rolling of the feedstock to a cold rolled sheet product having a gauge in the range of 0.8 mm to 6 mm; and

- annealing of the cold rolled sheet product at a temperature in the range of 225°C to 400°C.
Method according to claim 1, wherein the cold rolling is carried out with one or more cold reduction passes of each maximum 35% reduction.
Method according to claim 1 or 2, wherein the cold rolling is carried out at ambient temperature.
Method according to claim 1 or 2, wherein the cold rolling is carried out at a sub-zero temperature in the range of -200°C and -25°C.
Method according to any one of claims 1 to 4, wherein the annealing temperature is in the range of 250°C to 325°C.
Method according to any of one claims 1 to 5, wherein the annealed sheet product is being formed into a panel.
Method according to claim 1, wherein the annealed sheet product is being formed to a panel using a forming process selected from the group of roll forming, stretch-forming, and creep-forming.
Method according to any one of claims 1 to 7, wherein the aluminium alloy has a composition having,
Mg 2.5 % to 6%, and preferably 3.7% to 6%,

Mn 0 to 1.4%, and preferably 0.05% to 1.2%,

Sc 0.05% to 1 %, and preferably 0.05% to 0.4%,

Ag 0 to 0.5%

Zn 0 to 2%

Cu 0 to 2%

Li 0 to 3%,
optionally at least one or more elements selected from the group consisting of (Zr 0.03% to 0.4%, Cr 0.03% to 0.4%, and Ti 0.005% to 0.4%),
optionally one or more elements selected from the group of (Er, Dy, Gd, and Hf) in a total amount of 0.01 % to 0.3%,
Fe 0 to 0.4%

Si 0 to 0.25%,
inevitable impurities and balance aluminium.
Method according to any one of claims 1 to 7, wherein the aluminium alloy has a composition within the ranges of AA5024.
Method according to any one of claims 1 to 9, wherein the aluminium alloy sheet product is in the form of fuselage skin, a panel for a lower wing section, a panel for an upper wing section, a stringer, or a spar.