METHOD OF MANUFACTURING A 7XXX-SERIES ALUMINIUM ALLOY PLATE PRODUCT HAVING IMPROVED FATIGUE FAILURE RESISTANCE
The invention relates to a method of manufacturing an 7xxx-series aluminium alloy plate product having improved fatigue failure resistance, the method comprising the steps of (a) casting an ingot made of an aluminium alloy of the 7xxx-series comprising (in wt. %): Zn 5 to 9, Mg 1 to 3, Cu 0 to 3, balance aluminium and incidental elements and impurities; (b) homogenizing and/or preheating the cast ingot; (c) hot rolling the ingot into a plate product by rolling the ingot with multiple rolling passes, characterized in that when the intermediate thickness of the plate is between 80 and 220 mm, at least a high reduction hot rolling pass is carried out with a thickness reduction of at least 25%, wherein the plate product has a final thickness of less than 75 mm. The invention is also related to an aluminium alloy plate product and an aerospace structural member produced by this method.
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The invention relates to a method of manufacturing a 7xxx-series aluminium alloy plate product having improved fatigue failure resistance. The plate product can be ideally applied in aerospace structural applications, such as wing skin panels and members, and other high strength end users.
BACKGROUND OF THE INVENTIONAl—Zn—Mg—(Cu) type alloys or AA7xxx-series alloys have been used for aircraft constructions for more than 50 years, and particularly for wing members, for example, inter alia, AA7055-series alloys have been used. These aluminium alloys possess a required balance of strength, fracture toughness and corrosion resistance, and are especially well suited for structural aerospace applications such as wing upper skin panels. This is disclosed for example in U.S. Pat. No. 5,221,377. This US patent discloses that in order to obtain these high mechanical characteristics, it is necessary to subject the alloys to a three-stage artificial ageing process. However, this US patent does not deal with the property of fatigue failure resistance of the AA7055 alloys.
It is known that high strength structural components, which excel in durability and damage tolerance as well as fatigue failure resistance, are highly desirable for aircraft manufacturers. Durability and damage tolerance can translate into long inspection intervals for an aircraft. An aircraft usually requires two types of inspections: initial inspection and periodic inspection during the operating life of the aircraft. Each type of inspection is very costly, because the aircraft must be taken out of service for the inspection to be performed. Inspections may require detailed visual inspection and extensive non-destructive testing of exterior and interior structures.
U.S. Pat. No. 7,097,719 discloses that the fatigue failure resistance of AA7055-series alloys can be improved by using an optimized alloy composition, later registered as the AA7255 alloy. However, to achieve the improved fatigue failure resistance it is necessary that the AA7255 alloys have much more stringent upper-limits for the Si- and Fe-levels than the AA7055 alloy. In particular, this US patent discloses that products made from the AA7255 alloy having lower Si and Fe levels than AA7055 (i.e. Si and Fe concentrations below 0.06 wt. %, preferably below 0.04 wt. %) exhibit better fatigue failure resistance. In particular, the US patent discloses in the Examples that alloys having less than 0.029 wt. % Si and less than 0.039 wt. % Fe (while maintaining Cu, Mg, Zn and Zr within the ranges of standard AA7055) achieved improvements in fatigue life with respect to standard AA7055 products when having higher Si- and Fe-levels. Accordingly, the fatigue life of an AA7255 aluminium alloy product with respect to a standard AA7055 product can be improved. Such an improvement delays the inspection intervals in an aircraft structure. However, keeping the content of impurities Si and Fe at such a very low level increases the costs for the aluminium alloy produced, as materials with a very high purity grade are to be sourced.
As fatigue performance, in particular fatigue failure resistance, is an important engineering parameter for aluminium alloy aerospace materials due to the cyclic stresses an aircraft experiences in service, there is a need for further improvement or further progress of fatigue failure resistance of AA7xxx-series alloys, including the AA7055-series alloys.
Thus, a need exists for Al—Zn—Mg—(Cu) type alloys having desirable strength, toughness and corrosion resistance properties as well as a high fatigue failure resistance. A need also exists for aircraft structural parts that exhibit a high fatigue failure resistance.
OBJECT OF THE INVENTIONIt is an object of the present invention to provide a method of manufacturing a 7xxx-series aluminium alloy plate product having a high fatigue failure resistance compared to 7xxx-series alloys and in particular AA7055 aluminium alloy plate products of similar dimensions and temper produced by conventional methods.
It is another object of the invention to provide an aluminium alloy plate product having improved fatigue failure resistance over AA7055 plate products.
It is another object to provide aerospace structural members, such as upper wing skins, from the improved fatigue resistant aluminum alloy plate product.
DESCRIPTION OF THE INVENTIONThese and other objects and further advantages are met or exceeded by the present invention providing a method of manufacturing an aluminium alloy rolled plate product having a final thickness or final gauge of less than 75 mm, preferably less than 50 mm, ideally suitable for use as an aerospace plate product, with improved fatigue failure resistance, the method comprising the steps, in that order, of:
- (a) casting an ingot of an aluminium alloy of the 7xxx-series, the aluminium alloy comprising (in wt. %):
- Zn 5 to 9,
- Mg 1 to 3,
- Cu 0 to 3,
- Fe up to 0.20,
- Si up to 0.15,
- Zr up to 0.5, preferably 0.03 to 0.20,
- balance aluminium and impurities;
- (b) homogenizing and/or preheating the cast ingot;
- (c) hot rolling the ingot into a plate product by rolling the ingot with multiple rolling passes, wherein, when at an intermediate thickness of the plate between 80 and 220 mm, preferably between 100 and 200 mm, at least one high reduction hot rolling pass is carried out with a thickness reduction of at least 25%;
- (d) optionally solution heat treating and cooling to ambient temperature, preferably by means of quenching, the plate product;
- (e) optionally stretching the solution heat treated plate product;
- (f) optionally artificially ageing of the plate product.
The term “comprising” in the context of the aluminium alloy is to be understood in the sense that the alloy may contain further alloying elements, as exemplified below.
The method according to this invention can be applied to a wide range of 7xxx-series aluminium alloys consisting of the following composition, in wt. %,
-
- Zn 5% to 9%, preferably 5.5% to 8.5%, more preferably 7% to 8.5%,
- Mg 1% to 3%,
- Cu 0% to 3%, preferably 0.3% to 3%,
- Si up to 0.15%, preferably up to 0.10%,
- Fe up to 0.20%, preferably up to 0.15%,
- one or more elements selected from the group consisting of:
- Zr up to 0.5%, preferably 0.03% to 0.20%,
- Ti up to 0.3%
- Cr up to 0.4%
- Sc up to 0.5%
- Hf up to 0.3%
- Mn up to 0.4%
- V up to 0.4%
- Ag up to 0.5%, and
- balance being aluminium and impurities. Typically, such impurities are present each <0.05%, total <0.15%.
In a further embodiment, the aluminium alloy has a chemical composition within the ranges of AA7010, AA7040, AA7140, AA7449, AA7050, AA7150, AA7055, AA7255, AA7081, AA7181, AA7085, AA7185, AA7090, AA7099, AA7199, and modifications thereof.
In a particular embodiment, the aluminium alloy has a chemical composition within the ranges of AA7055.
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 2016, 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.
The terms “≤” and “up to” and “up to about”, as employed herein, explicitly include, but are not limited to, the possibility of zero weight-percent of the particular alloying component to which it refers. For example, up to 0.4% Cr may include an alloy having no Cr.
In the method of the present invention, preferably no cold rolling step is carried out when rolling the plate product to final gauge (thickness) to avoid at least partial recrystallization during a subsequent solution heat treatment step resulting in adversely affecting the balance of engineering properties in the final plate product.
The final thickness of the rolled plate product is less than 75 mm, preferably 50 mm, preferably less than 45 mm, more preferably less than 40 mm, and most preferably less than 35 mm. In useful embodiments, the final thickness of the plate product is more than 10 mm, preferably more than 12.5 mm, more preferably more than 15 mm and most preferably more than 19 mm.
The aluminium alloy can be provided as a rolling ingot or slab by casting techniques regular in the art for casting products, e.g. DC-casting, EMC-casting, EMS-casting, and preferably having a thickness in a range of 300 mm or more, for example 400 mm, 500 mm or 600 mm. On a less preferred basis slabs resulting from continuous casting, e.g. belt casters or roll casters, also may be used, which in particular may be advantageous when producing thinner gauge end products. Grain refiners such as those containing titanium and boron, or titanium and carbon, may also be used as is well-known in the art. After casting the rolling alloy stock, the ingot is commonly scalped to remove segregation zones near the cast surface of the ingot.
Next, the rolling ingot is homogenized and/or preheated.
It is known in the art that the purpose of a homogenisation heat treatment has the following objectives: (i) to dissolve as much as possible coarse soluble phases formed during solidification, and (ii) to reduce concentration gradients to facilitate the dissolution step. A preheat treatment achieves also some of these objectives.
Commonly a pre-heat refers to the heating of an ingot to a set temperature and soaking at this temperature for a set time followed by the start of the hot rolling at about that temperature. Homogenisation refers to a heating and cooling cycle applied to a rolling ingot in which the final temperature after homogenisation is ambient temperature.
A typical preheat treatment for AA7xxx-series alloys used in the method according to this invention would be a temperature of 400° C. to 460° C. with a soaking time in the range of 2 to 50 hours, more typically for 2 to 20 hours.
Firstly, the soluble eutectic phases such as the S-phase, T-phase, and M-phase in the alloy stock are dissolved using regular industry practice. This is typically carried out by heating the stock to a temperature of less than 500° C., typically in a range of 450° C. to 490° C., as S-phase eutectic phase (Al2MgCu-phase) have a melting temperature of about 489° C. in AA7xxx-series alloys and the M-phase (MgZn2-phase) has a melting point of about 478° C. As is known in the art this can be achieved by a homogenisation treatment in said temperature range and allowed to cool to the hot rolling temperature, or after homogenisation the stock is subsequently cooled and reheated before hot rolling. The regular homogenisation process can also be done in a two or more steps if desired, and which are typically carried out in a temperature range of 430° C. to 490° C. for AA7xxx-series alloys. For example in a two-step process, there is a first step between 455° C. and 465° C., and a second step between 470° C. and 485° C., to optimise the dissolving process of the various phases depending on the exact alloy composition.
The soaking time at the homogenisation temperature according to industry practice is alloy dependent as is well known to the skilled person, and is commonly in the range of 1 to 50 hours. The heat-up rates that can be applied are those which are regular in the art.
Hot rolling of the ingot is carried out with multiple hot rolling passes, usually in a hot rolling mill. The number of hot rolling passes is typically between 15 and 35, preferably between 20 and 29. When the hot rolled plate product has reached an intermediate thickness of between 80 mm and 220 mm, preferably between 100 mm and 200 mm, the method applies at least one high reduction hot rolling pass with a thickness reduction of at least about 25%, preferably of at least about 30% and most preferred of at least about 35%. In useful embodiments, the thickness reduction in this high reduction pass is less than 70%, preferably less than 60%, more preferred less than 50%. The “thickness reduction” of a rolling pass, also referred to as reduction ratio, is preferably the percentage by which the thickness of the plate is reduced in the individual rolling pass.
Such an at least one high reduction hot rolling pass is not carried out in conventional industrial hot rolling practices when producing 7xxx-series plate products. Therefore, the hot rolling passes between 80 mm and 220 mm according to a non-limitative example of the invention could be described as follows (looking at the plate intermediate thickness): 203 mm-190 mm-177 mm-167 mm-117 mm-102 mm-92 mm. The high reduction hot rolling pass from 167 mm to 117 mm corresponds to a thickness reduction of about 30%. For aluminium alloy plates produced by a conventional hot rolling process, the thickness reduction of each hot rolling pass is typically between 9% and 18% when at the intermediate thickness between 80 mm and 220 mm. Accordingly, the hot rolling passes between 80 mm and 220 mm according to an examples of the conventional method could be described as follows (looking at the plate intermediate thickness): 203 mm-188 mm-166 mm-144 mm-124 mm-104 mm-92 mm. Accordingly, the method according to the invention defines a hot rolling step wherein at least one high reduction hot rolling pass is carried out. This high reduction pass is defined by a thickness reduction of at least about 25%, preferably of at least about 30% and more preferred of at least about 35%.
The hot rolling passes of the method of this invention before and after the high reduction pass have a reduction ratio that is comparable with the reduction ratio of the hot rolling passes of the conventional hot rolling method. Accordingly, each hot rolling pass before and after the high reduction hot rolling pass could have a thickness reduction between 8% and 18%. Since the thickness reduction varies depending on the thickness of the plate, e.g. thick plates having more than 300 mm or thin plates having less than 30 mm, it is a feature of the claimed method that the high reduction step is carried out when the intermediate thickness of the plate product has reached between 220 mm and 80 mm, preferably 200 mm to 100 mm, most preferred between 200 mm and 120 mm. This thickness is chosen to ensure that the high deformation/shear is consistent throughout the entire plate product thickness. For plate products thicker than 220 mm it is more difficult to ensure a consistent deformation throughout the entire plate. Typically, in thicker plate products there would be less deformation in the center (half thickness) of the plate product than at the quarter thickness position or in the subsurface area.
Preferably, one high reduction hot rolling pass is carried out. In an alternative embodiment, two high reduction hot rolling passes are carried out. If one high reduction hot rolling pass is applied, this high reduction hot rolling pass is preferably one of the last seven or eight passes of the multiple hot rolling passes.
Before starting the hot rolling process, the rolling ingot is pre-heated to a temperature regular in the art and known to the skilled person, of e.g. 390° C. to 480° C., preferably 400° C. to 460° C., more preferred 400° C. to 430° C., such as 410° C. Accordingly, it is possible to maintain an entry temperature of the hot rolling mill of more than 380° C., preferably of more than 390° C. The maximum temperature for the hot rolling passes is not more than 450° C. because it has been observed that coarsening of the S-phase could occur above this temperature and there is a risk of incipient melting.
It has been found that, in the case of manufacturing a plate product having a final thickness of less than 50 mm, also a deformation rate during the hot rolling process has an influence on the final plate product properties. Therefore, the deformation rate during the at least one high reduction pass in a useful embodiment of the method is preferably lower than <1 s-1, preferably ≤0.8 s-1. This intense shearing is believed to cause a break-up of the constituent particles, e.g. Fe-rich intermetallics.
The deformation rate during hot rolling per rolling pass can be described by the following formula:
wherein
{dot over (ρ)} deformation rate (in s−1)
h0 entry thickness of the plate (in mm)
h1 exit thickness of the plate (in mm)
v1 rolling speed of the working rolls (in mm/s)
R radius of the working rolls (in mm).
The deformation rate is the change of strain (deformation) of a material with respect to time. It is sometimes also referred to as “strain rate”. The formula shows that not only the entry thickness and the exit thickness of the aluminium alloy plate, but also the rolling speed of the working rolls has an influence on the deformation rate.
For conventional industrial scale hot rolling practices, the deformation rate of each rolling pass is typically equal to or more than 2 s−1. As already outlined above, according to an embodiment of the method according to this invention during the high reduction pass the deformation rate is reduced to <1 s−1, preferably to ≤0.8 s−1. By using a low deformation rate, it is possible to achieve a more intense shearing within the plate material.
Furthermore, the aluminium alloy plate product manufactured by the present invention can be, if desired, solution heat treated (SHT), cooled, preferably by means of quenching, stretched and artificially aged after the hot rolling to final gauge step. If solution heat treating (SHT) is carried out, the plate product should be heated, similar as for the homogenization heat treatment prior to the hot rolling, to a temperature of typically in the range of 430° C. to 490° C., to bring all or substantially all portions of the soluble zinc, magnesium and copper into solution. After a set soaking time at the elevated temperature, the plate product should be rapidly cooled or quenched to complete the solution heat treating procedure. Such quenching is preferably carried out by water-quenching, e.g. via water immersion or water jets.
After cooling to ambient temperature, the plate products may further be cold worked by means of stretching in the range of 0.5% to 8% of its original length to relieve residual stresses therein and to improve the flatness of the product. Preferably the stretching is in the range of 0.5% to 5%, more preferably of 1% to 3%.
In a preferred embodiment, the plate products obtained by the present invention are artificially aged. All ageing practices known in the art and those which may be subsequently developed can be applied to the AA7000-series alloy products obtained by the method according to this invention to develop the required strength and other engineering properties.
In a particular preferred embodiment the plate product is artificially aged to a T7 temper, preferably to a T79 or T77 temper. The artificial ageing step can be carried out in one step or multiple-ageing steps. Preferably, a two-step ageing procedure is carried out.
A desired structural shape is then machined from these heat-treated plate sections, more often generally after artificial ageing, for example, an integral wing spar.
An advantage of the present invention is that the aluminium alloy product shows improved fatigue failure resistance without the need to maintain its iron and silicon contents at an extremely low level. According to the prior art, it is generally believed that Fe and Si are both harmful to the fatigue failure resistance. However, the aluminium alloy plate products manufactured by the method of the present invention are much more tolerant to the presence of Fe and Si while still delivering the required balance of properties including a high fatigue failure resistance. In an embodiment, the alloy may contain more than 0.05%, preferably more than 0.06%, Fe. In an embodiment it may contain more than 0.05%, preferably more than 0.06% Si. In a further preferred embodiment, each of the Fe- and Si-content is equal to or higher than 0.07 wt. %. In another embodiment the Si-content is between 0.06% and 0.10% and the Fe-content is within 0.06% to 0.15%. Accordingly, for example a commercially available AA7055 aluminium alloy can be used in the claimed method.
In other embodiments, the Fe and Si levels are kept at very low levels in order to achieve a further improvement in the properties. For example, the Fe content may be kept at less than 0.05%, preferably less than 0.03%, and the Si content may be less than 0.05%, preferably less than 0.03%.
The AA7000-series alloy plate product when manufactured according to this invention can be used as an aerospace structural component, amongst others as fuselage frame member, upper wing plate, lower wing plate, thick plate for machined parts, thin sheet for stringers, spar member, rib member, floor beam member, and bulkhead member.
In a particular embodiment the aluminium alloy plate product is used as a wing panel or member, more in particular as an upper wing panel or member.
Accordingly, the plate product manufactured according to the invention provides improved properties compared to a plate product manufactured according to conventional standard methods for this type of aluminium alloys having otherwise the same dimensions and processed to the same temper.
Embodiments of the invention will now be described by way of non-limiting examples, and comparative examples representative of the state of the art will also be given.
Rolling ingots have been DC-cast of the aluminum alloy AA7055, with a composition as given in Table 1
The rolling ingots had a thickness of about 400 mm. Homogenization of the ingots was carried out in a two-step homogenization procedure at 465° C. (first step) and 475° C. (second step) followed by cooling to ambient temperature. After scalping, the ingots were pre-heated to 410° C. for hot rolling. Hot rolling was carried out on a hot rolling mill having a work roll radius of about 575 mm. Lots A and B were processed in accordance with the invention, i.e. both lots receive a high reduction pass during the hot rolling process. During the high reduction rolling pass, lot A received a thickness reduction of about 30% (167 mm to 117 mm) and lot B received a thickness reduction of about 28% (165 mm to 118 mm). The rolling speed during this high reduction pass was about 25 m/min giving a deformation rate of about 0.53 s−1. Lots C, D, and E were processed according to a conventional hot rolling method (a thickness reduction between 9% to 18% for each hot rolling pass between 220 mm and 80 mm thickness). The rolling speed during the standard hot rolling passes was about 105 m/min giving a deformation rate of between 1.61 s−1 (entry thickness 188 mm) and 2.27 s−1 (entry thickness 123 mm). Plate A received 27 hot rolling passes, wherein the high reduction pass was pass number 19. Plate B received 25 hot rolling passes, wherein the high reduction pass was pass number 17.
The plates A, C, and E had a final thickness of 19 mm after the hot rolling process, and the plates B and D had a final thickness of 25.4 mm after the hot rolling process. After hot rolling, all the plates in final thickness were solution heat treated at a temperature of about 470° C., quenched and stretched for about 2%. An artificial ageing step was applied, bringing the plate products in a T7951 condition.
Fatigue testing was performed according to DIN EN 6072 by using a single open hole test coupon having a net stress concentration factor Kt of 2.3. The test coupons were 150 mm long by 30 mm wide, by 3 mm thick with a single hole 10 mm in diameter. The hole was countersunk to a depth of 0.3 mm on each side. The test coupons were stressed axially with a stress ratio (min load/max load) of R=0.1. The test frequency was 25 Hz and the tests were performed in high humidity air (RH≥90%). The individual results of these tests are shown in Table 2 and
The invention is not limited to the embodiments described before, which may be varied widely within the scope of the invention as defined by the appending claims.
Claims
1. A method of manufacturing a 7xxx-series aluminium alloy plate product having improved fatigue failure resistance, the method comprising the following steps:
- (a) casting an ingot of an aluminium alloy of the 7xxx-series, the aluminium alloy comprising (in wt. %):
- Zn 5 to 9,
- Mg 1 to 3,
- Cu 0 to 3,
- Fe up to 0.20,
- Si up to 0.15,
- Zr up to 0.5,
- balance aluminium and impurities;
- (b) homogenizing and/or preheating the cast ingot;
- (c) hot rolling the ingot into a plate product by rolling the ingot with multi-pie rolling passes, characterized in that, when at an intermediate thickness of the plate between 80 and 220 mm, at least one high reduction hot rolling pass is carried out with a thickness reduction of at least 25%;
- wherein the plate product has a final thickness of less than 75 mm.
2. The method according to claim 1, wherein the method further comprises the steps of
- (d) solution heat treating the plate product;
- (e) cooling of the solution heat treated plate product;
- (f) optionally stretching the solution heat treated and cooled plate product, and
- (g) artificial ageing the solution heat treated and cooled plate product.
3. The method according to claim 1, wherein the method does not comprise a cold rolling step to final gauge.
4. The method according to claim 1, wherein the high reduction hot rolling pass is carried out with a thickness reduction of at least 30%.
5. The method according to claim 1, wherein a deformation rate during the high reduction hot rolling pass is <1 s−1.
6. The method according to claim 1, wherein the intermediate thickness of the plate before the high reduction hot rolling pass is be-tween 100 mm and 200 mm.
7. The method according to claim 1, wherein the Si-content and/or the Fe-content of the aluminium alloy is equal to or more than 0.05 wt. %.
8. The method according to claim 1, wherein the aluminium alloy has a composition consisting of
- Zn 5% to 9%,
- Mg 1% to 3%,
- Cu 0% to 3%,
- Si up to 0.15%,
- Fe up to 0.20%,
- one or more elements selected from the group consisting of: Zr up to 0.5%, Ti up to 0.3% Cr up to 0.4% Sc up to 0.5% Hf up to 0.3% Mn up to 0.4% V up to 0.4% Ag up to 0.5%,
- balance being aluminium and impurities.
9. The method according to claim 1, wherein the aluminium alloy has a composition in accordance with AA7055.
10. The method according to claim 1, wherein the final thickness of the plate product is less than 45 mm.
11. The method according to claim 1, wherein the final thickness of the plate product is more than 10 mm.
12. The method according to claim 1, wherein in the method step (c), a hot rolling mill entry temperature is more than 380° C.
13. The method according to claim 1, wherein the plate product is artificially aged to a T7 temper.
14. An aluminium alloy plate product manufactured according to claim 1 and having improved fatigue failure resistance.
15. An aerospace structural member manufactured from the aluminium alloy plate product obtained by the method according to claim 1.
16. Use of an aluminium alloy plate product manufactured according to claim 1, for the manufacture of an aircraft structural member.
Type: Application
Filed: Jun 5, 2019
Publication Date: Aug 12, 2021
Applicant: ALERIS ROLLED PRODUCTS GERMANY GMBH (Koblenz)
Inventors: Sabine Maria SPANGEL (Koblenz), Philippe MEYER (Koblenz), Achim BÜRGER (Koblenz), Matthias RÜBNER (Koblenz), Simon LACHNITT (Koblenz)
Application Number: 16/973,980
