JP7282106B2 - Manufacturing method of 7xxx series aluminum alloy plate product with improved fatigue fracture resistance - Google Patents

Description

The present invention relates to a method for producing 7xxx series aluminum alloy plate products with improved fatigue fracture resistance. This plate product is ideally applicable to aerospace structural applications such as wing skin panels and members and other high strength end users.

Al-Zn-Mg-(Cu) type alloys or alloys of the AA7xxx series have been used in aircraft construction for over 50 years, especially in wing components, for example alloys of the AA7055 series, among others. there is These aluminum alloys provide the necessary balance of strength, fracture toughness, and corrosion resistance and are particularly suitable for structural aerospace applications such as wing upper skin panels. This is disclosed, for example, in US Pat. No. 5,221,377. This US patent discloses that the alloy must be subjected to a three-step artificial aging process to obtain these high mechanical properties. However, this US patent does not address the fatigue fracture resistance properties of the AA7055 alloy.

It is known that high strength structural components that are durable, damage tolerant, and fatigue fracture resistant are highly desirable for aircraft manufacturers. Durability and damage tolerance can lead to longer intervals between aircraft inspections. Aircraft typically require two types of inspections: initial inspections and periodic inspections during the life of the aircraft. Various inspections are very costly because the aircraft must be taken out of service to carry out the inspections. Inspection may require detailed visual inspection and extensive non-destructive inspection of external and internal structures.

US Pat. No. 7,097,719 discloses that the fatigue fracture resistance of AA7055 series alloys can be improved by using an optimized alloy composition later registered as AA7255 alloy. However, to improve fatigue fracture resistance, AA7255 alloy requires much tighter upper limits for Si and Fe levels than AA7055 alloy. In particular, according to this U.S. patent, articles made from AA7255 alloy having lower Si and Fe levels than AA7055 (i.e., Si and Fe concentrations less than 0.06 wt%, preferably less than 0.04 wt%) It is disclosed to exhibit superior fatigue fracture resistance. In particular, this U.S. patent states 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 ), disclose that improved fatigue life was achieved over standard AA7055 product at high Si and Fe levels. Therefore, the fatigue life of AA7255 aluminum alloy products relative to standard AA7055 products can be improved. Such improvements delay inspection intervals for aircraft structures. However, keeping the content of the impurities Si and Fe at such very low levels increases the cost of the aluminum alloys produced by procuring very high purity materials.

Due to the cyclic stresses that aircraft undergo during service, fatigue performance, especially fatigue fracture resistance, is an important engineering parameter for aluminum alloy aerospace materials, so the fatigue resistance of AA7xxx series alloys, including AA7055 series alloys. There is a need to further improve or advance the destructibility.

Therefore, there is a need for Al-Zn-Mg-(Cu) type alloys with desirable strength, toughness and corrosion resistance, as well as high resistance to fatigue fracture. There is also a need for aircraft structural components that exhibit high fatigue fracture resistance.

OBJECTS OF THE INVENTION It is an object of the present invention to provide a 7xxx series aluminum alloy plate product, particularly an AA7055 aluminum alloy plate product of similar dimensions and temper made by conventional methods, having increased fatigue fracture resistance compared to 7xxx series alloys. It is to provide a manufacturing method.

Another object of the present invention is to provide an aluminum alloy plate product having improved resistance to fatigue fracture over AA7055 plate product.

It is another object to provide aerospace structural members, such as upper wing skins, from improved fatigue resistant aluminum alloy plate products.

DETAILED DESCRIPTION OF THE INVENTION These and other objects and further advantages make a final thickness of less than 75 mm, preferably less than 50 mm ideally suited for use as an aerospace plate product with improved fatigue fracture resistance. or met or surpassed by the present invention to provide a method of manufacturing a final gauge aluminum alloy rolled plate product, the method comprising the following steps in that order:
(a) Casting an ingot of an aluminum alloy of the 7xxx series, the aluminum alloy comprising (in weight percent):
Zn 5-9,
Mg 1-3,
Cu 0-3,
Fe up to 0.20,
Si up to 0.15,
Zr up to 0.5, preferably 0.03 to 0.20,
casting an aluminum alloy containing residual aluminum and impurities;
(b) homogenizing and/or preheating the cast ingot;
(c) hot rolling the ingot into a plate product by rolling the ingot in a plurality of rolling passes, wherein the intermediate thickness of the plate is 80-220 mm, preferably 100-200 mm; where at least one high reduction hot rolling pass is performed with a thickness reduction of at least 25%;
(d) optionally solution treating and cooling the plate product to ambient temperature, preferably by quenching;
(e) optionally stretching the solution treated plate product;
(f) artificially aging the plate product, if necessary;

The term "comprising" in the context of aluminum alloys should be understood in the sense that the alloy may contain further alloying elements, as exemplified below.

The method according to the invention can be applied to a wide range of 7xxx series aluminum alloys, consisting of the following compositions, in weight percent:

Zn 5% to 9%, preferably 5.5% to 8.5%, more preferably 7% to 8.5%,
1% to 3% Mg,
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 residues that are aluminum and impurities. Typically, such impurities are present at <0.05% each and <0.15% total.

In further embodiments, the aluminum alloy comprises AA7010, AA7040, AA7140, AA7449, AA7050, AA7150, AA7055, AA7255, AA7081, AA7181, AA7085, AA7185, AA7090, AA7099, AA7199, and variations thereof. the chemical composition of have.

In certain embodiments, the aluminum alloy has a chemical composition within the AA7055 range.

As is clear as follows, the name and temple called aluminum alloy, unless it is specified as follows, is the ALUMINUM STANDARDS and Data and RE RE RE, published by Aluminium ASSOCIATION in 2016. The name of the Aluminium ASSOCIATION of GISTRATION RECORDS , which is well known to those skilled in the art.

In any discussion of alloy compositions or preferred alloy compositions, all references to percentages are by weight percent unless otherwise specified.

As used herein, the terms "≦" "up to" and "up to about" expressly include, but are not limited to, the possibility of zero weight percent of the particular alloying component to which it refers. not. For example, up to 0.4% Cr may include Cr-free alloys.

The method of the present invention preferably does not perform a cold rolling step when rolling the plate product to final gauge (thickness) and does not perform subsequent solution heat treatment, which adversely affects the balance of engineering properties in the final plate product. Avoid at least partial recrystallization during processing steps.

The final thickness of the rolled plate product is less than 75mm, preferably less than 50mm, preferably less than 45mm, more preferably less than 40mm and most preferably less than 35mm. In useful embodiments the final thickness of the plate product is greater than 10 mm, preferably greater than 12.5 mm, more preferably greater than 15 mm and most preferably greater than 19 mm.

This aluminum alloy is produced using casting techniques common in the art of casting products (e.g. DC casting, EMC casting, EMS casting) and preferably in the range of 300 mm or more (e.g. 400 mm, 500 mm or 600 mm). Depending on the thickness, it can be provided as a rolled ingot or slab. On a less preferred basis, slabs resulting from continuous casting, such as belt casters or roll casters, may also be used, which can be advantageous especially when producing thinner gauge end products. Crystal growth inhibitors such as those containing titanium and boron or titanium and carbon may also be used, as is well known in the art. After casting rolled alloy stock, the ingot is typically scalped to remove segregation zones near the casting surface of the ingot.
The rolled ingot is then homogenized and/or preheated.

The objectives of the homogenization heat treatment are known in the art to have the following objectives: (i) to dissolve as much as possible the coarse soluble phases formed during solidification, and (ii) to facilitate the dissolution step. to reduce the concentration gradient to Preheating also accomplishes some of these goals.

Generally, preheating refers to heating an ingot to a set temperature, soaking it at this temperature for a set time, and then starting hot rolling at about that temperature. Homogenization refers to a heating and cooling cycle applied to a rolled ingot whose final temperature after homogenization is ambient temperature.

A typical preheat treatment for the AA7xxx series alloys used in the method according to the invention would be a temperature of 400° C. to 460° C. with a soaking time ranging from 2 to 50 hours, more typically from 2 to 20 hours.

First, the soluble eutectic phases such as S, T, and M phases of the alloy stock are dissolved using normal industry practices. This is usually done by heating the stock to a temperature below 500° C., usually in the range of 450° C.-490° C., because the S-phase eutectic phase (Al ₂ MgCu phase) is about 489° C. in AA7xxx series alloys. °C and the melting point of the M phase (MgZn ₂ phase) is about 478 °C. As is known in the art, this can be achieved by homogenization in the temperature range described above and either cooled to the hot rolling temperature, or after homogenization the stock is subsequently treated before hot rolling. Cooled and reheated. The periodic homogenization process may be performed in two or more steps, if desired, and is typically performed in the temperature range of 430°C to 490°C for AA7xxx series alloys. For example, in a two-step process, there is a first step at 455° C.-465° C. and a second step at 470° C.-485° C. to optimize the melting process of the various phases depending on the exact alloy composition.

Immersion times at homogenization temperatures in accordance with industry practice generally range from 1 to 50 hours, depending on the alloy, as is well known to those skilled in the art. The heating rates that can be applied are those common in the art.

Hot rolling of ingots is typically performed using multiple hot rolling passes in a hot rolling mill. The number of hot rolling passes is usually between 15 and 35, preferably between 20 and 29. When the hot rolled plate product reaches an intermediate thickness of 80 mm to 220 mm, preferably 100 mm to 200 mm, the process reduces the thickness by at least about 25%, preferably at least about 30%, most preferably at least about 35%. At least one high reduction hot rolling pass with thickness reduction is applied. In useful embodiments, the thickness reduction in this high reduction pass is less than 70%, preferably less than 60%, more preferably less than 50%. The "thickness reduction" of a rolling pass, also called reduction rate, is preferably the percentage by which the thickness of the plate is reduced in an individual rolling pass.

Such at least one high reduction hot rolling pass is not performed in conventional industrial hot rolling processes when producing 7xxx series plate products. Thus, a hot rolling pass between 80mm and 220mm, according to a non-limiting example of the present invention, can be described as follows (looking at plate intermediate thickness): 203mm-190mm-177mm-167mm-117mm- 102mm-92mm. A large reduction hot rolling pass from 167 mm to 117 mm corresponds to a thickness reduction of about 30%. For aluminum alloy plates produced by conventional hot rolling process, the reduction in thickness of each hot rolling pass is typically between 9% and 18% for intermediate thicknesses between 80mm and 220mm. . Therefore, a hot rolling pass of 80 mm to 220 mm, according to the example of the conventional method, can be described as follows (looking at the thickness in the middle of the plate): 203 mm - 188 mm - 166 mm - 144 mm - 124 mm - 104 mm - 92 mm . The method according to the invention therefore defines a hot rolling step in which at least one high reduction hot rolling pass is performed. This high reduction pass is defined by a reduction in thickness of at least about 25%, preferably at least about 30%, more preferably at least about 35%.

The hot rolling passes of the method of the present invention before and after the large reduction pass have reduction ratios comparable to those of the hot rolling passes of conventional hot rolling methods. Therefore, each hot rolling pass before and after the large reduction hot rolling pass can have a thickness reduction of between 8% and 18%. The reduction in thickness varies depending on the thickness of the plate, for example thick plates above 300 mm or thin plates below 30 mm, so that the thickness of the plate product in between 220 mm and 80 mm, preferably between 200 mm and 100 mm, most It is characteristic of the claimed method that the large reduction step is carried out, preferably when 200 mm to 120 mm has been reached. This thickness is chosen to ensure that high deformation/shear is consistent throughout the thickness of the plate product. For plate products thicker than 220 mm, it is more difficult to ensure consistent deformation across the plate. Generally, thicker plate products will deform less in the center (half thickness) of the plate product than in quarter thickness locations or subsurface regions.

Preferably, one large reduction hot rolling pass is performed. In an alternative embodiment, two high reduction hot rolling passes are performed. If one large reduction hot rolling pass is applied, this large reduction hot rolling pass is preferably one of the last seven or eight passes of the plurality of hot rolling passes.

Before starting the hot rolling process, the rolled ingot is at a temperature customary in the art, such as 390°C to 480°C, preferably 400°C to 460°C, more preferably 400°C to 430°C, For example, it is preheated to a temperature known to those skilled in the art of 410°C. It is therefore possible to maintain the inlet temperature of the hot rolling mill above 380°C, preferably above 390°C. The maximum temperature of the hot rolling pass is 450°C or less. This is because it has been observed that coarsening of the S phase can occur above this temperature and there is a risk of incipient melting.

When producing plate products with a final thickness of less than 50 mm, it has been found that the deformation rate during the hot rolling process also affects the properties of the final plate product. Therefore, the deformation rate during at least one high reduction pass in useful embodiments of the method is preferably lower than <1 s-1, preferably ≤0.8 s-1. This severe shear is believed to cause the fracture of constituent particles, such as Fe-rich intermetallic compounds.

The deformation rate during hot rolling for each rolling pass can be expressed by the following equation.

Deformation rate is the change in strain (deformation) of a material over time. This is sometimes called the "strain rate". This formula shows that not only the thickness of the entrance and the exit of the aluminum alloy plate, but also the rolling speed of the working rolls affects the deformation speed.

In conventional industrial-scale hot rolling practice, the deformation rate of each rolling pass is typically 2 s ⁻¹ or greater. As already outlined above, according to embodiments of the method according to the invention during this high reduction pass, the deformation rate is reduced to <1 s ⁻¹ , preferably to ≦0.8 s ⁻¹ . By using low deformation rates, it is possible to achieve higher shear within the plate material.

Additionally, the aluminum alloy plate product produced according to the present invention can be solution heat treated (SHT), preferably cooled by quenching, stretched, and heated to a final gauge step, if desired. It may be artificially aged after rolling. When solution heat treatment (SHT) is performed, similar to the homogenization heat treatment prior to hot rolling, the plate product is typically heated to a temperature in the range of 430°C to 490°C to remove all or all of the soluble zinc, magnesium and copper. Substantially all parts are placed in solution. After a set soak time at elevated temperature, the plate product must be rapidly cooled or quenched to complete the solution treatment procedure. Such quenching is preferably carried out by water quenching, for example by water immersion or water jet.

After cooling to ambient temperature, the plate product is further cold worked by stretching in the range of 0.5% to 8% of its original length to relieve residual stress therein and improve product flatness. You may let Preferably the stretch is in the range of 0.5% to 5%, more preferably 1% to 3%.

In a preferred embodiment, the plate product obtained according to the invention is artificially aged. All aging practices known in the art and those that may be subsequently developed can be applied to the AA7000 series alloy products obtained by the process according to the invention to develop the required strength and other engineering properties. good too.

In a particularly preferred embodiment, the plate product is artificially aged to T7 temper, preferably T79 or T77 temper. The artificial aging step can be performed in one step or multiple aging steps. Preferably, a two-step aging procedure is performed.

The desired structural shape, eg, integral wing spars, is then machined from these heat treated plate sections, more generally after artificial aging.

An advantage of the present invention is that aluminum alloy products exhibit improved fatigue fracture resistance without the need to maintain their iron and silicon content at very low levels. According to the prior art, both Fe and Si are generally considered detrimental to fatigue fracture resistance. However, aluminum alloy plate products produced by the method of the present invention are much more tolerant to the presence of Fe and Si, while still providing the necessary balance of properties such as high fatigue fracture resistance. In some embodiments, the alloy may contain more than 0.05% Fe, preferably more than 0.06% Fe. In one embodiment it may contain more than 0.05% Si, preferably more than 0.06% Si. In a more preferred embodiment, each of the Fe content and Si content is 0.07 wt% or more. In another embodiment, the Si content is between 0.06% and 0.10% and the Fe content is within 0.06% and 0.15%. Thus, for example, commercially available AA7055 aluminum alloy may be used in the claimed method.

In other embodiments, Fe and Si levels are kept at very low levels to achieve further improvements in properties. For example, the Fe content may be kept below 0.05%, preferably below 0.03%, and the Si content may be below 0.05%, preferably below 0.03%. .

AA7000 series alloy plate products made according to the present invention are used as aerospace structural components, among others, as fuselage frame members, upper and lower wing plates, thick plates for machined parts, thin sheets for stringers, It can be used as column members, rib members, floor beam members, and partition wall members.

In certain embodiments, aluminum alloy plate products are used as wing panels or members, and more particularly as upper wing panels or members.

Thus, plate products made according to the present invention have improved properties compared to plate products made according to conventional standard methods of this type of aluminum alloy that are otherwise worked to the same dimensions and same temper. ing.

Embodiments of the present invention are described herein by way of non-limiting example, and comparative examples representative of the state of the art are also provided.

1 is a graph of maximum net stress versus cycles to failure for plates prepared according to the method of the present invention and plates prepared by a conventional method; 1 is a graph showing the average logarithmic fatigue life of plates made by the method of the present invention and plates made by a conventional method, with a line connecting the data points corresponding to the averages;

The rolled ingot was a DC casting of aluminum alloy AA7055 having the composition shown in Table 1.
Table 1

The thickness of the rolled ingot was about 400 mm. Homogenization of the ingots was performed in a two-stage homogenization procedure at 465° C. (first stage) and 475° C. (second stage) followed by cooling to ambient temperature. After scalping, the ingot was preheated to 410°C for hot rolling. Hot rolling was performed in a hot rolling mill with a working roll radius of about 575 mm. Lots A and B were treated according to the invention, ie both lots underwent a large reduction pass during the hot rolling process. Lot A underwent a thickness reduction of about 30% (167 mm to 117 mm) and lot B underwent a thickness reduction of about 28% (165 mm to 118 mm) during the high reduction rolling pass. The rolling speed during this high reduction pass was about 25 m/min and the deformation speed was about 0.53 s ⁻¹ . Lots C, D, and E were processed according to conventional hot rolling (9% to 18% thickness reduction for each hot rolling pass from 220 mm to 80 mm thickness). The rolling speed during a standard hot rolling pass is about 105 m/min and the deformation speed is between 1.61 s ⁻¹ (188 mm inlet thickness) and 2.27 s ⁻¹ (123 mm inlet thickness). rice field. Plate A underwent 27 hot rolling passes and the high reduction pass was pass number 19. Plate B underwent 25 hot rolling passes and the high reduction pass was pass number 17.

The final thickness of plates A, C and E after the hot rolling process was 19 mm and the final thickness of plates B and D was 25.4 mm after the hot rolling process. After hot rolling, all plates of final thickness were solution heat treated at a temperature of about 470°C, quenched and stretched about 2%. An artificial aging step was applied to bring the plate product to the T7951 condition.

Fatigue tests were performed according to DIN EN 6072 using single open-hole test coupons with a net stress concentration factor Kt of 2.3. The test coupon was a single hole 150 mm long x 30 mm wide x 3 mm thick and 10 mm in diameter. The hole was countersunk to a depth of 0.3 mm on both sides. The test coupons were axially stressed with a stress ratio (minimum load/maximum load) of R=0.1. The test frequency was 25 Hz and the test was performed in humid air (RH≧90%). The individual results of these tests are shown in Table 2 and Figures 1 and 2. The lines in FIG. 2 are interpolations between the calculated log-mean data points.

Table 2

In FIG. 1, it can be seen that by using the method of the present invention, it is possible to significantly improve the fatigue life and thus the fatigue fracture resistance as compared to AA7055 alloy plates prepared by conventional methods. It is shown. For example, when a net sectional stress of 175 MPa is applied, plate A has a life of 470,421 cycles, corresponding to a 3.2 times improvement in life compared to AA7055 alloys, alloys C and E, which have a life of 142,655 cycles. are doing. Therefore, for an alloy prepared by the method of the present invention and having a final thickness of 19 mm, the life of 200000 cycles (see log mean curve in FIG. , corresponding to a maximum net stress of about 210 MPa for the present invention. This represents an improvement of 20% or more, which aircraft manufacturers can take advantage of to increase aircraft design stresses, thereby saving weight while maintaining the same aircraft inspection interval.

FIG. 2 compares the logarithmic average of lots A and B produced according to the process of the present invention with the logarithmic average of lots C, D, and E produced according to conventional processes for the same alloys as in FIG. , with lines showing the interpolation between the calculated log-mean data points. From this figure it is clear that the method of the present invention leads to improved fatigue life over the conventional method by using the same alloy composition.

The invention is not limited to the embodiments described above, but may vary widely within the scope of the invention defined by the appended claims.

Claims (12)

A method for manufacturing a 7xxx series aluminum alloy plate product with improved fatigue fracture resistance, the method comprising the steps of:
(a) casting an ingot of a 7xxx series aluminum alloy, said aluminum alloy comprising (in weight percent):
Zn 5-9,
Mg 1-3,
Cu 0-3,
Fe up to 0.20,
Si up to 0.15,
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%,
casting an aluminum alloy ingot consisting of aluminum and a residue that is an impurity;
(b) homogenizing and/or preheating the cast ingot;
(c) hot rolling said ingot into a plate product by rolling said ingot in a plurality of rolling passes, wherein said plate has an intermediate thickness of 80-220 mm under at least one high reduction; the hot rolling pass is characterized by being performed with a thickness reduction of at least 25%;
wherein said plate product has a final thickness of less than 75 mm and greater than 10 mm;
the deformation rate during said high reduction hot rolling pass is <1 s ⁻¹ ,
The manufacturing method. 2. The method of claim 1, further comprising the steps of:
(d) solution treatment of the plate product;
(e) cooling the solution treated plate product;
(f) optionally stretching the solution treated and cooled plate product;
(g) artificial aging of the solution heat treated and cooled plate product. 2. The method of claim 1, wherein the method does not include a cold rolling step to final thickness . A method according to any preceding claim, wherein the high reduction hot rolling pass is performed with a thickness reduction of at least 30%. A method according to any one of claims 1 to 4, wherein the plate has an intermediate thickness of between 100 mm and 200 mm before the high reduction hot rolling pass. The method according to any one of claims 1 to 5, wherein said Si content and/or said Fe content in said aluminum alloy is 0.05 wt% or more. A method according to any preceding claim, wherein the aluminum alloy has a composition according to AA7055. A method according to any preceding claim, wherein the plate product has a final thickness of less than 45 mm. A method according to any preceding claim, wherein the final thickness of the plate product is greater than 12.5mm. The method according to any one of the preceding claims, wherein in step (c) the inlet temperature of the hot rolling mill is above 380°C. The method of any one of claims 1-10, wherein the plate product is artificially aged to a T7 temper. Use of the aluminum alloy plate product produced according to any one of claims 1-11 for the production of aircraft structural members.

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