U.S. patent application number 11/334813 was filed with the patent office on 2006-07-20 for aluminum alloy that is not sensitive to quenching, as well as method for the production of a semi-finished product therefrom.
This patent application is currently assigned to OTTO FUCHS KG. Invention is credited to Gernot Fischer, Matthias Hilpert, Gregor Terlinde.
Application Number | 20060157172 11/334813 |
Document ID | / |
Family ID | 35695568 |
Filed Date | 2006-07-20 |
United States Patent
Application |
20060157172 |
Kind Code |
A1 |
Fischer; Gernot ; et
al. |
July 20, 2006 |
Aluminum alloy that is not sensitive to quenching, as well as
method for the production of a semi-finished product therefrom
Abstract
An aluminum alloy that is not sensitive to quenching, for the
production of high-strength forged pieces that are low in inherent
tension, and high-strength extruded and rolled products, consisting
of: 7.0-10.5 wt. % zinc, 1.0-2.5 wt. % magnesium, 0.1-1.15 wt. %
copper, 0.06-0.25 wt. % zirconium, 0.02-0.15 wt. % titanium, at
most 0.5 wt. % manganese, at most 0.6 wt. % silver, at most 0.10
wt. % silicon, at most 0.10 wt. % iron, at most 0.04 wt. % chrome,
and at least one element selected from the group consisting of:
hafnium, scandium, strontium and/or vanadium with a summary content
of at most 1.0 wt. %. The alloy can also contain contaminants at
proportions of at most 0.05 wt. % per element and a total
proportion of at most 0.15 wt. %, wherein the remaining component
includes aluminum. The sum of the alloy elements zinc and magnesium
and copper is at least 9 wt. %. Furthermore, there can also be a
method for the production of a high-strength semi-finished product
low in inherent tension from this alloy.
Inventors: |
Fischer; Gernot;
(Meinerzhagen, DE) ; Terlinde; Gregor;
(Meinerzhagen, DE) ; Hilpert; Matthias;
(Meinerzhagen, DE) |
Correspondence
Address: |
WILLIAM COLLARD;COLLARD & ROE, P.C.
1077 NORTHERN BOULEVARD
ROSLYN
NY
11576
US
|
Assignee: |
OTTO FUCHS KG
|
Family ID: |
35695568 |
Appl. No.: |
11/334813 |
Filed: |
January 18, 2006 |
Current U.S.
Class: |
148/690 ;
148/417; 148/693 |
Current CPC
Class: |
C22F 1/053 20130101;
C22F 1/002 20130101; C22C 1/06 20130101; C22C 21/10 20130101; Y10T
428/12 20150115 |
Class at
Publication: |
148/690 ;
148/693; 148/417 |
International
Class: |
C22C 21/10 20060101
C22C021/10 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 19, 2005 |
DE |
10 2005 002 390.8 |
Claims
1. An aluminum alloy that is not sensitive to quenching, for the
production of high-strength forged pieces that are low in inherent
tension, and high-strength extruded and rolled products, consisting
of: 7.0-10.5 wt. % zinc; 1.0-2.5 wt. % magnesium; 0.1-1.15 wt. %
copper; 0.06-0.25 wt. % zirconium; 0.02-0.15 wt. % titanium; at
most 0.5 wt. % manganese; at most 0.6 wt. % silver; at most 0.10
wt. % silicon; at most 0.10 wt. % iron; at most 0.04 wt. % chrome;
at least one element selected from the group consisting of:
hafnium, scandium, strontium and vanadium with a summary content of
at most 1.0 wt. %; and a plurality of contaminants at proportions
of at most 0.05 wt. % per element with a total contaminant
proportion of at most 0.15 wt. %; wherein a remaining amount by wt
% is aluminum; and wherein a sum of the alloy elements zinc and
magnesium and copper is at least 9 wt. %.
2. The aluminum alloy according to claim 1, wherein an amount of
zinc and magnesium is in the form of a zinc:magnesium ratio that is
between 4.4 and 5.3.
3. The aluminum alloy according to claim 2, wherein the alloy
contains 1.6-1.8 wt. % magnesium and 0.8-1.1 wt. % copper.
4. The aluminum alloy according to claim 1, wherein the aluminum
alloy contains 0.8-1.1 wt. % copper and 0.3-0.5 wt. %
manganese.
5. The aluminum alloy according to claim 1, wherein said aluminum
alloy contains 0.8-1.1 wt. % copper and at most 0.03 wt. %
manganese.
6. The aluminum alloy according to claim 1, wherein the aluminum
alloy contains 0.2-0.3 wt. % copper and 0.25-0.40 wt. % silver.
7. The aluminum alloy according to claim 1, wherein the aluminum
alloy contains 0.10-0.15 wt. % titanium.
8. The aluminum alloy according to claim 1, wherein the aluminum
alloy contains 0.001-0.03 wt. % boron.
9. The aluminum alloy according to claim 1, wherein the aluminum
alloy contains at most 0.30 wt. % scandium and at most 0.2 wt. %
vanadium, hafnium or cerium.
10. The aluminum alloy as in claim 1, wherein the iron and silicon
content is at most 0.08 wt. %, in each instance.
11. A method for the production of a high-strength semi-finished
product low in inherent tension, up to greater thickness values,
comprising the following steps: providing an aluminum alloy
consisting of: 7.0-10.5 wt. % zinc, 1.0-2.5 wt. % magnesium,
0.1-1.15 wt. % copper, 0.06-0.25 wt. % zirconium, 0.02-0.15 wt. %
titanium, at most 0.5 wt. % manganese, at most 0.6 wt. % silver, at
most 0.10 wt. % silicon, at most 0.10 wt. % iron, at most 0.04 wt.
% chrome, at least one element selected from the group consisting
of: hafnium, scandium, strontium and vanadium with a summary
content of at most 1.0 wt. %, a plurality of other contaminants at
proportions of at most 0.05 wt. % per element with a total
contaminant proportion of at most 0.15 wt. %, wherein the remaining
amount is aluminum, whereby the sum of the alloy elements zinc and
magnesium and copper is at least 9 wt. %. hot forming a plurality
of homogenized bars via forging, extrusion and/or rolling, in a
temperature range of 350-440.degree. C.; solution heat treating
said hot-formed semi-finished product at a temperature sufficiently
high to bring the alloy elements necessary for hardening into
solution uniformly distributed in the structure; quenching of the
solution heat treated semi-finished products in a quenching medium
comprising water, in a water/glycol mixture, or in a salt mixture
at a temperature between 100.degree. C. and 170.degree. C.; cold
forming the quenched semi-finished product to reduce a set of
inherent tensions that occurred during quenching in the quenching
medium; and artificial aging the quenched semi-finished product, in
at least one stage, wherein a heating rate, holding time, and
temperature is adjusted for optimization of the properties.
12. The method according to claim 11, wherein the step of cold
forming occurs by means of upsetting or stretching the
semi-finished product.
13. The method according to claim 11, wherein the cold forming rate
is 1-5%.
14. A method for the production of a high-strength semi-finished
product low in inherent tension, of medium thickness, from an
aluminum alloy, comprising the following steps: providing an
aluminum alloy consisting of: 7.0-10.5 wt. % zinc, 1.0-2.5 wt. %
magnesium, 0.1-1.15 wt. % copper, 0.06-0.25 wt. % zirconium,
0.02-0.15 wt. % titanium, at most 0.5 wt. % manganese, at most 0.6
wt. % silver, at most 0.10 wt. % silicon, at most 0.10 wt. % iron,
at most 0.04 wt. % chrome, at least one element selected from the
group consisting of: hafnium, scandium, strontium and/or vanadium
with a summary content of at most 1.0 wt. %, a plurality of other
contaminants at proportions of at most 0.05 wt. % per element with
a total contaminant proportion of at most 0.15 wt. %, wherein the
remaining amount is aluminum, whereby the sum of the alloy elements
zinc and magnesium and copper is at least 9 wt. %. hot forming of a
set homogenized bars by means of forging, extrusion and/or rolling,
in a temperature range of 350-440.degree. C.; solution heat
treating the hot-formed semi-finished product at a temperature that
is sufficiently high to bring the alloy elements necessary for
hardening into solution uniformly distributed in the structure;
quenching of the solution heat treated semi-finished products in
water, in a water/glycol mixture, or in a salt mixture at
temperatures between 100.degree. C. and 170.degree. C.; and
artificially aging the quenched semi-finished product, in at least
one stage, whereby the heating rates, holding times, and
temperatures are adjusted for optimization of the properties.
15. The method according to claim 14, wherein after the step of hot
forming, there is formed a semi-finished product having a greater
thickness, which is processed in cutting manner before the
subsequent heat treatment, in the way of pre-cutting, to reduce the
thickness of the semi-finished product by means of the cutting
processing, to such an extent that this pre-processed semi-finished
product has a medium thickness and the subsequent heat treatment is
carried out in accordance with the requirements corresponding to
semi-finished products having a medium thickness.
16. The method as in claim 11, wherein the solution heat treatment
step is between 465 and 500 degrees Celsius.
17. The method as in claim 14, wherein the solution heat treatment
step is between 465 and 500 degrees Celsius.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application hereby claims priority from German
application Serial no. 102005002390.8 filed on Jan. 19, 2005 the
disclosure of which is hereby incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] The invention relates to an aluminum alloy that is not
sensitive to quenching, and which is used for the production of
high-strength forged pieces low in inherent tension, and
high-strength extruded and rolled products. Furthermore, the
invention relates to a method for the production of a semi-finished
product from such an aluminum alloy.
[0003] High-strength aluminum alloys are needed for the aeronautics
and space industry, in particular, bearing hull, wing, and chassis
parts which demonstrate high strength both under static stress and
under dynamic stress. The required strength properties can be
achieved, in the case of the aforementioned semi-finished products,
by using alloys from the 7000 group (7xxx alloys), in accordance
with the classification of aluminum alloys prepared by the Aluminum
Association (AA).
[0004] Die-forged pieces for parts that are subject to great stress
in the aeronautics and space industry, for example, parts made from
the alloys AA 7075, AA 7175, AA 7475 and, particularly preferably,
from the alloys AA 7049 and AA 7050, in America, and made from the
alloys AA 7010, AA 7049A, and AA 7050A in Europe.
[0005] A high-strength aluminum alloy of the aforementioned type is
known from WO 02/052053 A1, or U.S. Pat. No. 6,972,110 issued on
Dec. 6, 2005 to Chakrabarti et al., the disclosure of which is
hereby incorporated herein by reference. That reference discloses
an alloy having an increased zinc content as compared with earlier
alloys of the same type, coupled with a reduced copper and
magnesium content. The copper and magnesium content in the case of
this previously known alloy amounts to less than 3.5%, in total.
The copper content itself is indicated as being 1.2-2.2 wt.-%,
preferably 1.6-2.2 wt.-%. In addition to the elements zinc,
magnesium, and copper, this previously known alloy necessarily
contains one or more elements from the group zirconium, scandium,
and hafnium, with maximum proportions of 0.4 wt.-% zirconium, 0,4
wt.-% scandium, and 0.3 wt.-% hafnium.
[0006] The semi-finished products should be subjected to a special
heat treatment to produce the semi-finished products from one of
the aforementioned alloys. These products can be in the form of
forged pieces, wherein with this heat treatment, the extruded
profiles, or the rolled sheets are treated to have the desired
strength. This treatment includes quenching from solution heat
temperature, in most cases combined with subsequent cold forming at
medium thickness values of more than 50 mm. The cold forming serves
to reduce the tensions induced during quenching. The step of cold
forming can occur by means of cold upsetting or also by means of
stretching the semi-finished product, typically by 1-3%. The
semi-finished products produced should be as low in inherent
tension as possible, to minimize any undesirable drawing during
further processing. In addition, the semi-finished products and
also the finished parts produced from them should be low in
inherent tension, to give the designer the possibility of utilizing
the entire material potential. For this reason, the method steps to
be used for the production of parts for aeronautics and space
technology from the alloys AA 7050 as well as AA 7010, and also the
maximum thickness of the semi-finished products used for the
production of the parts, are standardized and/or prescribed. The
maximal permissible thickness is 200 mm and presupposes that after
quenching, the semi-finished product is necessarily subjected to a
cold forming step, for the reasons indicated above. With extruded
and rolled products, cold forming can be achieved in a fairly
simple manner, because of the geometry, which is generally simple,
via stretching in the longitudinal direction. With geometrically
complicated forged pieces, on the other hand, it is only possible
to achieve a uniformly high degree of upsetting with great effort
and expense, if it is even possible at all. In the course of
designing larger aircraft, larger and larger and, in particular,
thicker and thicker forged parts are constantly required.
SUMMARY OF THE INVENTION
[0007] The invention relates to a high-strength aluminum alloy that
is not sensitive to quenching, having the same or better strength
properties as the alloys AA 7010 and AA 7050 which, at the same
time, has lower inherent tensions due to quenching after cold
forming, and from which semi-finished products having a medium
thickness can be produced having great strength and fracture
resistance, without the need for a cold forming step to reduce
inherent tensions induced by quenching.
[0008] The invention further relates to a method for the production
of a semi-finished product having the desired properties from this
alloy.
[0009] A high-strength aluminum alloy that is not sensitive to
quenching according to an embodiment of the invention, comprises an
alloy consisting of: 7.0-10.5 wt. % zinc, 1.0-2.5 wt. % magnesium,
0.1-1.15 wt. % copper, 0.06-0.25 wt. % zirconium, 0.02-0.15 wt. %
titanium, at most 0.5 wt. % manganese, at most 0.6 wt. % silver, at
most 0.10 wt. % silicon, at most 0.10 wt. % iron, at most 0.04 wt.
% chrome, and at least one element selected from the group
consisting of: hafnium, scandium, strontium and/or vanadium with a
summary content of at most 1.0 wt. %. The alloy can also contain
contaminants at proportions of at most 0.05 wt. % per element and a
total proportion of at most 0.15 wt. %, wherein the remaining
component includes aluminum. In addition, the sum of the alloy
elements zinc and magnesium and copper is at least 9 wt. %.
[0010] The invention can also relate to a process for treating the
above alloy. That process can include a series of steps including
hot forming a plurality of homogenized bars via forging, extrusion
and/or rolling in the temperature range of 350-440.degree. C. Next
there can be a step of solution heat treating of a hot-formed
semi-finished product at temperatures that are sufficiently high to
bring the alloy elements necessary for hardening into solution
uniformly distributed in the structure, preferably at
465-500.degree. C. Next, there can be the step of quenching of the
solution heat treated semi-finished products in water, in a
water/glycol mixture, or in a salt mixture at temperatures between
100.degree. C. and 170.degree. C. Next there can be the step of
cold forming of the quenched semi-finished product to reduce the
inherent tensions that occurred during quenching in the quenching
medium. Next there can be the step of artificial aging of the
quenched semi-finished product, in at least one stage, whereby the
heating rates, holding times, and temperatures are adjusted for
optimization of the properties.
[0011] The terms used within the scope of these explanations with
regard to thickness are defined as follows: Semi-finished products
having a medium thickness have temper hardening thickness values of
50-180 mm. Semi-finished products having a greater thickness have a
temper hardening thickness of >180 mm.
[0012] Even semi-finished products having a thickness of more than
200 mm, particularly of 250 mm or more can be produced with the
alloy according to the invention that is not sensitive to
quenching, having the desired great static and dynamic strength
properties and, at the same time, good fracture resistance and good
stress crack corrosion behavior. Only at these greater thickness
values is a cold forming step carried out to reduce
quenching-induced inherent stresses, for practical reasons.
[0013] Furthermore, for medium thickness values, semi-finished
products produced from the alloy can be mildly cooled, for example
in a glycol/water mixture, without any noteworthy negative
influence on the very good material properties, after subsequent
warm settling. For this reason, the step of cold forming is not
necessary for medium thickness values, since the inherent stresses
induced with the mild cooling are non-critically low. Therefore it
is possible to produce semi-finished products in the medium
thickness range with this alloy, in a simple and inexpensive
manner, namely without a cold-forming step that would otherwise be
necessary.
[0014] The advantageous properties of the alloy as described above
can also be utilized to simplify the production process of a part
for the production of which a semi-finished product having a
greater starting thickness is required, and which part has a medium
thickness after being processed. Such a semi-finished product
having a greater thickness, for example a forged one, is
pre-processed by cutting, after the step of hot forming. The
pre-processing is designed so that the semi-finished product, which
will then be quenched within the course of hot forming, undergoes a
reduction in thickness. This reduction in thickness is necessary
for the production of the finished part, in any case, wherein the
pre-processed semi-finished product can be subjected to heat
treatment with mild quenching (glycol/water mixture), without
performing a cold forming step that is otherwise necessary for
greater thickness values.
[0015] Using an alloy according to an embodiment of the invention,
semi-finished products having a medium thickness can therefore be
quenched in mild manner, by means of glycol/water mixtures. With
semi-finished products having a greater thickness, such mild
quenching is not practical because of the minimum cooling speed
that is required. Accordingly, semi-finished products having a
greater thickness are quenched in water. As a result of this, these
semi-finished products are subsequently subjected to cold forming,
for example upsetting or stretching by 1-5%.
[0016] The aforementioned properties of the semi-finished product
produced from this alloy, as mentioned above, are unexpected, since
contrary to the default values that result from the state of the
art, the copper content is clearly lower than was the case for
previously known high-strength aluminum alloys. According to a
preferred exemplary embodiment, the copper content is only 0.8-1.1
wt. %. At this value, the copper content is only about 50% of the
preferred copper content of the aluminum alloys known from WO
02/052053 A1 or U.S. Pat. No. 6,972,110. It is surprising that very
high strength values are achieved despite this. It is assumed that
these properties are based on the balanced composition of the alloy
components, which also includes the relative high zinc content
values and the magnesium content that is adapted to this. In the
balanced composition of the alloy elements, which are only allowed
in narrow limits, the sum of the elements magnesium, copper, and
zinc are at least 9 wt. %. It has been shown that the desired
strength properties can only be achieved if the elements magnesium,
copper, and zinc in total are more than 9 wt. %. This
characteristic of the alloy is a measure of the fact that the
products have the desired strength properties. This rule also
determines the heat treatability of the semi-finished products
produced with the alloy.
[0017] Particularly great static and dynamic strength properties
and particular non-sensitivity to quenching are obtained, along
with simultaneous great fracture resistance, if the copper content
is 0.8-1.1 wt. % and the magnesium content is 1.6-1.8 wt. %. This
corresponds to a zinc : magnesium ratio of 4.4-5.2. Thus, the
copper content clearly lies below the maximal solubility for copper
in the presence of the aforementioned magnesium content. This has
the result that the proportion of insoluble phases that contain
copper is very low, even taking into consideration the other alloy
elements and accompanying elements. This directly results in an
improvement of the dynamic properties and the fracture
resistance.
[0018] To further increase the strength of the alloy, it can be
advantageous to add silver. For economic reasons, the content will
be limited to 0.2-0.7 wt. %, particularly to 0.20-0.40 wt. %.
[0019] The manganese content of the alloy was limited to a maximum
0.5 wt. %. Manganese precipitates in the form of finely distributed
manganese aluminides, which can furthermore contain part of the
iron present in the alloy as a contaminant, in Al--Zn--Cu--Mg
alloys, during the homogenization of the extruded bars. These
manganese aluminides are helpful in controlling recrystallization
of the structure during heat treatment of the formed semi-finished
product. Experience has shown that the ability to through-harden an
Al--Zn--Cu--Mg alloy decreases with an increasing manganese
content. For this reason, the manganese content is limited.
[0020] The reduced effect of the manganese with regard to
controlling the structure is balanced out by means of adding
zirconium. According to a preferred exemplary embodiment, the
latter amounts to 0.14-0.20 wt. %. Zirconium also precipitates from
the structure during homogenization of the extruded bars, in the
form of zirconium aluminides. These aluminides are generally
configured to be more micro-dispersed than the manganese
aluminides. For this reason, they are particularly helpful with
regard to controlling recrystallization. The zirconium aluminides
that are formed are not made more coarse by the heat treatment that
is provided, and are stable in the selected temperature ranges, in
contrast to manganese aluminides. For this reason, zirconium is a
necessary component of the alloy.
[0021] The titanium contained in the alloy primarily serves for
making the grain fine during extrusion molding. A value of 0.03-0.1
wt. % titanium is preferred, particularly 0.03-0.06 wt. % titanium
added to the alloy.
[0022] The desired properties are achieved if the alloy components
are used in the proportions of the range indicated. Semi-finished
products having the required properties can no longer be produced
with an alloy in which one or more alloy components have a
proportion that lies outside the range indicated.
[0023] The semi-finished products are produced from this alloy with
the following steps:
[0024] Casting of bars of the alloy;
[0025] Homogenization or homogenizing of the cast bars at a
temperature that lies as close as possible below the starting melt
temperature of the alloy, for a heating and holding time that is
sufficient to achieve as uniform and as fine a distribution of the
alloy elements in the cast structure as possible, preferably at
460-490.degree. C.;
[0026] Hot forming of the homogenized bars by means of forging,
extrusion and/or rolling, in the temperature range of
350-440.degree. C.;
[0027] Solution heat treating of the hot-formed semi-finished
product at temperatures that are sufficiently high to bring the
alloy elements necessary for hardening into solution uniformly
distributed in the structure, preferably at 465-500.degree. C.;
Quenching of the solution heat treated semi-finished products in
water, at a temperature between room temperature and 100.degree.
C., or in a water/glycol mixture, or in a salt mixture at
temperatures between 100.degree. C. and 170.degree. C.; and
[0028] Artificial aging of the quenched semi-finished product, in
one stage or multiple stages, wherein the heating rates, holding
times, and temperatures are adjusted for optimization of the
properties.
[0029] There can be a method in which the artificial aging of the
quenched semi-finished product occurs in two stages. In the first
stage, the semi-finished product is heated to a temperature of more
than 100.degree. C. and held at this temperature for more than
eight hours, and in the second stage, it is heated to more than
130.degree. C. and heated for more than five hours. These two
stages can be performed directly following one another. The
semi-finished product treated with the first stage can also cool
off, and the second stage of artificial aging can be performed at a
later point in time, without having to accept any disadvantages
with regard to the desired properties of the semi-finished
product.
[0030] With greater thickness values, despite the non-sensitivity
of the alloy to quenching, it may be necessary to subject the
semi-finished product to a cold forming step after the step of
quenching, to reduce the inherent stresses that occurred during
quenching. It is practical if this occurs by means of upsetting or
stretching of the semi-finished product by typically 1-5%.
BRIEF DESCRIPTION OF THE DRAWINGS
[0031] Other objects and features of the present invention will
become apparent from the following detailed description considered
in connection with the accompanying drawings, which disclose one
embodiment of the present invention. It should be understood,
however, that the drawings are designed for the purpose of
illustration only and not as a definition of the limits of the
invention.
[0032] In the drawings wherein similar reference characters denote
similar elements throughout the several views:
[0033] FIG. 1 is a graph representing the strength behavior of
various AA 7xxx alloys as a function of the average cooling speed
during quenching from solution heat treatment temperature; and
[0034] FIG. 2 is a flow chart for a process for producing the
alloy.
DETAILED DESCRIPTION
[0035] The following are examples of different embodiments of the
invention.
EXAMPLES
[0036] To produce sample pieces to carry out the required strength
studies, two typical alloy compositions of the claimed aluminum
alloy were produced. The two alloys Z1, Z2 have the following
composition: TABLE-US-00001 Si Fe Cu Mn Mg Cr Zn Ti Zr Ti + Zr
Alloy Z1 0.05 0.05 0.95 0.39 1.70 0.002 8.35 0.035 0.12 0.155 Alloy
Z2 0.04 0.07 0.90 0.004 1.65 0.001 8.50 0.025 0.12 0.145
[0037] The alloys Z1, Z2 were cast to produce extrusion blocks
having a diameter of 370 mm, on an industrial scale. The extrusion
blocks were homogenized to balance out the micro-segregation
resulting from solidification. The blocks were homogenized in two
stages, in a temperature range of 465.degree. C.-485.degree. C.,
and cooled.
Example 1
[0038] After the casting skin of the blocks produced in this manner
had been lathed off, the homogenized blocks were pre-heated to
370.degree. C. and formed multiple times to produce free-form
forged pieces having a thickness of 250 mm and to a width of 500
mm.
[0039] Subsequently, the free-form forged pieces of alloy Z1 and Z2
were solution heat treated at 485.degree. C. for at least 4 hours,
quenched in water at room temperature, and subsequently
artificially aged between 100.degree. C. and 160.degree. C.,
wherein the artificial aging was carried out in two stages. In the
first stage, the semi-finished product was heated to more than
100.degree. C. and held at this temperature for more than eight
hours. The second stage, which was carried out immediately after
the first stage, took place at a temperature of more than
130.degree. C. for more than five hours.
[0040] Drawing samples were taken from the artificially aged
free-form forged pieces, on which the strength properties at room
temperature were determined in the sample positions "long" (L),
"long-transverse" (LT), and "short-transverse" (ST). The average
strength properties of the alloy Z1 and Z2 for a thickness of 250
mm with water quenching are shown in the following table:
TABLE-US-00002 Alloy Stress Direction R .sub.p02 (Mpa) Rm (Mpa) A
.sub.5(%) Z1 L 504 523 11.2 LT 502 533 5.2 ST 498 522 8.0 Z2 L 520
528 8.6 LT 508 530 4.0 ST 511 525 5.1
[0041] The results show that the R.sub.p02 and R.sub.m values are
almost identical for all three stress directions, and lie above 490
MPa for the stretching limit (R.sub.p02) and above 520 MPa for
tensile strength. The A.sub.5 values are highest for the L
direction, and reach at least 4% breaking elongation (A.sub.5) for
the two transverse directions. The fracture resistance K.sub.IC of
the sample positions L-T and T-L was determined using compact drawn
samples (W=50 mm) from the same free-form forged pieces, according
to ASTM-E 399. The K.sub.IC values are listed as follows:
TABLE-US-00003 Alloy Test Direction Position K.sub.IC (MPa m)
R.sub.p0.2 (MPa) Z1 L-T Edge 30.5 529 L-T Core 32.9 504 T-L Edge
23.1 516 T-L Core 20.4 502 Z2 L-T Edge 30.3 514 L-T Core 35.9 520
T-L Edge 23.6 514 T-L Core 21.8 508
[0042] The stress crack corrosion resistance was determined on
round samples for the LT and the ST position, according to ASTM G47
(alternating immersion test). The results are listed below for the
alloy Z1: TABLE-US-00004 Electrical Stress Direction Stress Mpa
Duration (Days) Conductivity LT 320 >30 34.7 ST 320 >30
34.7
[0043] For both test directions, lifetimes of more than 30 days are
obtained at stresses of 320 MPa. In typical specifications for
high-strength Al alloys, such as for AA 7050, for example, these
lifetimes are demanded at minimum stresses of 240 MPa. This means
that-the new alloy, despite clearly greater strength as compared
with the alloy AA 7050, at the same time has a stress crack
corrosion resistance that clearly lies above the minimum value for
AA 7050.
[0044] Analogously, forged pieces having the same parameters were
produced from the alloy Z1. In addition, the forged pieces were
cold-upset in the short transverse direction (ST) after solution
heat treatment and quenching, to reduce the inherent stresses
resulting from quenching. After the subsequent hardening, which was
performed in two stages, in accordance with the parameters
indicated above, the strength properties were determined at room
temperature, in the sample positions "long" (L), "long-transverse"
(LT), and "short-transverse" (ST). The results for the alloy Z1 are
listed in the following table: TABLE-US-00005 R .sub.p02 R.sub.m
Alloy Stress Direction (MPa) (MPa) A.sub.5 (%) Z1 L 504 523 11.2 LT
502 533 5.2 ST 498 522 8.0 Z1 + Cold Upsetting L 448 501 11.1 LT
468 516 6.7 ST 417 498 10.8
[0045] The results show that the R.sub.p02 and R.sub.m values for
all three stress directions are less, and that the lowest value was
found for the short-transverse direction (ST). The A.sub.5 values
are highest for the L direction, and reach-at least 6% breaking
elongation (A.sub.5) for the two-transverse directions. The
decrease in strength can be reduced by shortening the second
hardening stage. The fracture strength K.sub.IC in sample positions
L-T and T-L was determined according to ASTM-E 399, using compact
drawn samples (W=50 mm) from the same free-form forged pieces. The
K.sub.IC values are listed in the following table: TABLE-US-00006
Alloy Test Direction Position K.sub.IC (MPa m) R.sub.p0.2 (MPa) Z1
L-T Edge 30.5 529 L-T Core 32.9 504 T-L Edge 23.1 516 T-L Core 20.4
502 Z1 + Cold L-T Edge 38.9 485 Upsetting L-T Core 42.2 448 T-L
Edge 23.9 474 T-L Core 21.9 468
Example 2
[0046] In another series of experiments, free-form forged pieces
having a thickness of 150 mm and a width of 500 mm were produced
from alloy Z1 and, after solution heat treatment, were quenched in
water or a water/glycol mixture with approximately 20% and
approximately 40%, respectively, and warm settled as described
above. One forged piece was additionally cold upset after being
quenched in water. The influence of the various cooling media was
determined on drawn samples that were taken from the forged pieces
in the directions "long" (L), "long-transverse" (LT), and
"short-transverse" (ST). The average strength properties of the
alloy for a thickness of 150 mm for various cooling treatments are
shown as follows: TABLE-US-00007 Quenching R.sub.p0.2 R.sub.m
A.sub.5 Medium Stress Direction (MPa) (MPa) (%) Water(RT) L 551 573
10.3 LT 515 544 7.5 ST 505 549 8.0 Water (RT) + L 491 537 12.8 Cold
upsetting LT 465 520 8.7 ST 430 513 8.5 Water/Glycol L 545 566 12.5
(16-20%) LT 520 547 7.2 ST 512 548 8.3 Water/Glycol L 503 529 12.2
(38-40%) LT 493 525 5.0 ST 487 526 5.6
[0047] The results show that a reduction in the cooling speed by
adding glycol has hardly any influence on the strength properties
of the alloy. The ductility decreases only minimally with a
decreasing cooling speed, i.e. an increasing glycol content.
[0048] The fracture resistance K.sub.IC was determined in the
sample positions L-T and T-L, according to ASTM-E 399, using
compact drawn samples (W=50 mm) from the same free-form forged
pieces. The K.sub.IC values are contained in the following table:
TABLE-US-00008 Quenching Medium Test Direction K.sub.IC (MPa m)
R.sub.p.02 Water (RT) L-T 36.8 551 T-L 23.8 515 Water (RT) + Cold
L-T 39.1 491 Upsetting T-L 24.1 465 Water/glycol L-T 28.2 545
(16-20%) T-L 20.7 520 Water/glycol L-T 35.4 503 (38-40%) T-L 18.5
493
[0049] No clear dependence on the cooling speed is evident for the
L-T position, but for the T-L position, a trend towards slightly
lower values with decreasing cooling speed can be seen.
Example 3
[0050] To determine the strength properties, the alloy Z1 was also
cast in another example, analogous to the first example, and blocks
for extrusion were produced.
[0051] After the casting skin had been lathed off, the homogenized
blocks were pre-heated to over 370.degree. C. and pressed into
extrusion profiles having a rectangular cross-section, with a
thickness of 40 mm and a width of 100 mm.
[0052] Subsequently, the profiles were solution heat treated for at
least 4 hours at 485.degree. C., quenched in water at room
temperature, and subsequently artificially aged between 100.degree.
C. and 160.degree. C., in two stages (first stage: >100.degree.
C., >8 h; second stage: >130.degree. C., >5 h).
[0053] Drawn samples were taken from the artificially aged
extrusion profiles, on which the strength properties were
determined at room temperature, in the sample positions "long" (L),
"long-transverse" (LT), and "short-transverse" (ST). The average
strength properties of the alloy Z1 for an extruded rectangular
profile (40.times.100 mm) for water quenching with subsequent
stretching are listed in the following table: TABLE-US-00009
R.sub.p0.2 R.sub.m A.sub.5 Stress Direction (MPa) (MPa) (%) L 600
609 9.3 LT 554 567 7.1 ST 505 561 7.5
[0054] The results show that the R.sub.p02 and R.sub.m values are
highest in the L direction, at values of 600 MPa and 609 MPa,
respectively, and lowest in the ST direction, at values of 505 MPa
and 561 MPa, respectively. The A.sub.5 values are highest for the L
direction, and reach at least 7% breaking elongation (A.sub.5) for
the two transverse directions. The fracture resistance K.sub.IC in
the sample positions L-T and T-L was determined according to ASTM-E
399, using compact drawn samples (W=50 mm) from the same free-form
forged pieces. The average fracture mechanics properties of the
alloy Z1 and Z2 for a thickness of 250 mm and water quenching are
contained in the following table: TABLE-US-00010 R.sub.p0.2 Test
Direction K.sub.IC (MPa m) (MPa) L-T 50.9 50.9 T-L 30.7 30.7
[0055] FIG. 1 shows a diagram representing the strength behavior of
various AA 7xxx alloys as a function of the average cooling speed
during quenching from solution heat treatment temperature. It is
clearly evident in this representation that the loss in strength
when using the claimed aluminum alloy is significantly less, even
at low cooling speeds, than in the case of the comparison alloys AA
7075, AA 7010, and AA 7050.
[0056] The strength values of the products/semi-finished products
produced with the claimed alloy, determined within the scope of the
description of the invention, are significantly improved, in
particular with regard to stress crack corrosion resistance, as
compared with products of previously known alloys, which represents
a result that was not foreseeable in the form that occurred. The
results shown are also interesting in that the strength values
described can be particularly presented with artificial aging that
is carried out in only two stages.
[0057] FIG. 2 shows a flow chart for a process for producing the
alloy. For example, step 1 comprises providing the alloy which is
disclosed in the above examples. In step 2, the alloy is hot formed
as described above, and in step 3, the alloy is solution heat
treated as described above. In step 4, the alloy is quenched, while
in step 5, the alloy is optionally cold formed, while in step 6,
the alloy is artificially aged as described above.
[0058] Accordingly, while a few embodiments of the present
invention have been shown and described, it is to be understood
that many changes and modifications may be made thereunto without
departing from the spirit and scope of the invention as defined in
the appended claims.
* * * * *