Patent Application
20190368009
The present invention relates to high strength 7xxx aluminum alloy products and methods for making such products. The high strength 7xxx aluminum alloy product can be fabricated to plate, extrusion or forging products suitable for aerospace structural components, especially large commercial airplane wing structure applications requiring better strength, fatigue crack deviation resistance, anisotropic ductility, damage tolerance performance, and corrosion resistance performance.
High strength 7xxx (Al—Zn) aluminum alloy products are extensively used in aerospace structure application, in which the material strength, ductility, fracture toughness, fatigue resistance, and corrosion resistance are required simultaneously. Among these required material properties, the anisotropic ductility and fatigue crack branching resistance performances are very important but often not adequately addressed for aluminum alloy and product development.
The higher strength 7xxx aluminum alloys are being pursued assertively by airframe manufacturers and aluminum material manufacturers in order to aggressively reduce aircraft weight for fuel efficiency.
The fatigue crack branching or deviation, as shown in
In addition to the fatigue crack deviation, the anisotropic ductility of aluminum thick plate is another critical characteristic for aerospace application, especially for very popular monolithic part machining technology recently used in airframe manufacture industry. The anisotropic ductility refers to significant ductility changes when the tensile testing direction is away from the material metal flow or microstructural direction, commonly notated as rolling direction (L). The ductility is usually significantly lower when tensile direction is off the metal flow direction.
The chemical composition and processing have phenomenal influences on final production properties. In 7xxx aluminum alloys, zinc is the major alloying element for achieving high strength through age strengthening. In the more commonly used 7050 and 7075 aerospace aluminum alloys, zinc is in the range of 5.1 to 6.7 wt. %. Magnesium is added along with zinc to produce metastable and/or stable MgZn2 (η′ and/or η Phase) and its variant phases, which are the predominant precipitation hardening phases. Compositions with higher Zn and Mg usually result in higher strength. However, too high Zn and Mg content also negatively affect stress corrosion cracking (SCC) resistance and fracture toughness performance as well as anisotropic ductility and fatigue crack deviation resistance performances. In 7xxx aluminum alloys, copper is added in order to improve SCC resistance performance and strength. The Cu significantly increases the breakdown potentials, resulting in better corrosion resistance performance. The beneficial impact of Cu on corrosion resistance performance is also strongly affected by Zn level. In addition, concentrations of Cu that are too high also significantly increase the risk of high level of undesirable coarse Al2MgCu particles and macro-segregation from plate surface to center. During casting, large Al2CuMg particles can form during solidification. Such large particles normally can be dissolved during subsequent homogenization and solution heat treatment. If the Cu content is too high, however, this could promote extremely high levels of Al2CuMg particles, which cannot be dissolved during subsequent thermal treatments. Those undissolved Al2CuMg particles significantly reduce the strength and damage tolerance performance. Therefore, the optimized combination of Zn, Mg, and Cu is very critical for high strength, high damage tolerance, and corrosion resistance performance as well as excellent anisotropic ductility and fatigue crack branching resistance performances required by aerospace application.
The fatigue crack growth deviation and anisotropic ductility are also strongly affected by crystallographic texture, which is strongly affected by both chemistry and thermal mechanical processing.
In summary, the combination of the complicated age hardening behavior, as well as strict and comprehensive material performance necessitates a very fine, optimized, and probably very narrow chemistry range that needs to be discovered. Such new alloys are strongly needed for aerospace application, especially for large size commercial aircraft.
The high strength, better fatigue crack deviation performance, high anisotropic ductility 7xxx aluminum alloy products such as plates, forgings and extrusions, suitable for use in making aerospace structural components like large commercial airplane wing components, comprises, optionally consists of, 7.0 to 7.8 wt. % Zn, 1.1 to 2.2 wt. % Cu, and 1.1 to 2.1 wt. % Mg, one or more elements selected from the group consisting of up to 0.2% Zr, up to 0.2% Sc, up to 0.2% Hf, and the balance Al, and incidental impurities. The product provides high strength, high damage tolerance performance, better corrosion resistance performance as well as desirable fatigue crack deviation resistance performance, better anisotropic ductility suitable for aerospace application.
It has been surprisingly discovered that an aluminum alloy having an optimized chemistry range, associated with precise Zn, Mg and Cu contents along with deliberately controlled thermal mechanical processing, is capable of producing plate products with high strength, desirable fatigue crack deviation resistance performance, anisotropic ductility, damage tolerance, and corrosion properties never achieved before.
The inventive alloy has surprisingly desirable properties. The minimum tensile yield strength (TYS) along the rolling (LT) direction of the invention aluminum alloy is 65 ksi for 4 to 5 inch plate. Meanwhile, the minimum Kmax at crack deviation point for the inventive aluminum alloy is 31 ksi*in1/2 and preferably higher than 34 ksi*in1/2 for 4 to 5 inch plate. The minimum elongation is 1.4% for all tensile orientations including lowest orientations of ST-22.5 to ST-45.
In one embodiment, the high strength 7xxx thick plate aluminum alloy product offers a promising opportunity for significant fuel efficiency and cost reduction advantage for commercial airplanes, especially large size commercial aircraft. An example of such application of the present invention is the integral design wing box, which requires thick cross section 7xxx aluminum alloy products. Material strength is a key design factor for weight reduction. Also, important are ST tensile ductility, damage tolerance, corrosion resistance performance, such as exfoliation and stress corrosion resistance, and fatigue crack growth resistance.
The features and advantages of the present invention will become apparent from the following detailed description of a preferred embodiment thereof, taken in conjunction with the accompanying drawings, in which:
A high strength 7xxx aluminum alloy product is produced using a precise chemistry range. In one embodiment, the 7xxx aluminum alloy product is a thick plate high strength aluminum alloy product used in aerospace applications. The high strength 7xxx aluminum alloy product, with better fatigue crack deviation and anisotropic ductility, comprises, optionally consists of, optionally consist essentially of, 7.0 to 7.8 wt. % Zn, 1.1 to 2.2 wt. % Cu, and 1.1 to 2.1 wt. % Mg, one or more elements selected from the group consisting of up to 0.2% Zr, up to 0.2% Sc, up to 0.2% Hf, and the balance Al, and incidental impurities. The 7xxx aluminum alloy product provides high strength, high damage tolerance performance, and better corrosion resistance performance as well as desirable fatigue crack deviation resistance performance, and better anisotropic ductility suitable for aerospace application. Specifically, the tensile yield strength (TYS) along the rolling (LT) direction of the 7xxx aluminum alloy product is higher than 65 ksi but lower than 83ksi for 4 to 5 inch plate. The Kmax at crack deviation point, Kmax-dev for the 7xxx aluminum alloy product is higher than 31 ksi*in1/2 but lower than 61 ksi*in1/2 and preferably higher than 34 ksi*in1/2 but lower than 64 ksi*in1/2 for 4 to 5 inch plate. The minimum elongation is 1.4% but lower than 6% for all tensile orientations including lowest orientations of ST-22.5 to ST-45.
In one embodiment of the present invention, the high strength 7xxx aluminum alloy product consisting essential of: 7.3 to 7.6 wt. % Zn, 1.2 to 1.7 wt. % Cu, 1.2 to 1.7 wt. % Mg, one or more elements selected from the group consisting of up to 0.2 wt. % Zr, up to 0.2 wt. % Sc, and up to 0.2 wt. % Hf, ≤0.12 wt. % Si, ≤0.15 wt. % Fe, with the balance Al, and incidental impurities. The tensile yield strength (TYS) along rolling (LT) direction of the 7xxx aluminum alloy product is higher than 65ksi, but lower than 83 ksi, for 4 to 5 inch plate. The Kmax-dev at crack deviation point for the 7xxx aluminum alloy product is higher than 34 ksi*in1/2, but lower than 64 ksi*in1/2, for 4 to 5 inch plate. The minimum elongation is 1.4% for all orientations including lowest orientations of ST-22.5 to ST-45.
In one embodiment, the high strength 7xxx aluminum alloy product includes ≤0.12 wt. % Si, preferably ≤0.05 wt. % Si. In one embodiment, the high strength 7xxx aluminum alloy product includes ≤0.15 wt. % Fe, preferably ≤0.08 wt. % Fe. In one embodiment, the high strength 7xxx aluminum alloy product includes Zr in the range from 0.05 to 0.15 wt %. In one embodiment, the high strength 7xxx aluminum alloy product includes ≤0.04 wt. % Cr, preferably no Cr is added to the alloy other than that provided as an incidental impurity.
In one embodiment of the present invention, the high strength 7xxx aluminum alloy product is a 3-10 inches thick plate, extrusion, or forging product. Another embodiment of the present invention, the high strength 7xxx aluminum alloy product is a 4-8 inches thick plate, extrusion, or forging product. In one embodiment of the present invention, the high strength 7xxx aluminum alloy product is a 3-10 inches thick plate product, or 4-8 inches thick plate product.
It is understood that the ranges identified above for the 7xxx aluminum alloy product includes the upper or lower limits for the element selected and every numerical range and fraction provided within the range may be considered an upper or lower limit. For example, it is also understood that within the range of 7.0 to 7.8 wt. % Zn, the upper or lower limit for Zn may be selected from 7.0, 7.1, 7.2, 7.3, 7.4, 7.5, 7.6, 7.7, and 7.8 wt. %. In one embodiment, the 7xxx aluminum alloy product includes 7.46-7.8 wt % Zn. In another embodiment, the 7xxx aluminum alloy product includes 7.3-7.6 wt. % Zn. For example, it is also understood that within the ranges of Zn provided herein, the 7xxx aluminum alloy product includes 1.1 to 2.2 wt. % Cu, the upper or lower limit for Cu may be selected from 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.1, and 2.2 wt. %. In one embodiment, the 7xxx aluminum alloy product includes 1.2 to 1.7 wt. % Cu. In another embodiment, the 7xxx aluminum alloy product includes 1.2-1.55 wt. % Cu. For example, it is also understood that within the ranges of Zn and Cu provided herein, the 7xxx aluminum alloy product includes 1.1 to 2.1 wt. % Mg, the upper or lower limit for Mg may be selected from 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0 and 2.1 wt. %. In one embodiment, the 7xxx aluminum alloy product includes 1.2-1.7 wt. % Mg. In another embodiment, the 7xxx aluminum alloy product includes 1.1-1.44 wt. % Mg. In another embodiment, the 7xxx aluminum alloy product includes 1.55-2.1 wt. % Mg. For example, it is also understood that within the ranges of Zn, Cu, and Mg provided herein, the 7xxx aluminum alloy product includes one or more elements selected from the group consisting of up to 0.2 wt. % Zr, up to 0.2 wt. % Sc, and up to 0.2 wt. % Hf, the upper or lower limit for Zr, Sc and/or Hf may be selected from 0.00, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.10, 0.11, 0.12, 0.13, 0.14, 0.15, 0.16, 0.17, 0.18, 0.19, and 0.20 wt. %. In one embodiment, the 7xxx aluminum alloy product includes up to 0.2 wt. % Zr, 0 wt. % Sc, and 0 wt. % Hf. In another embodiment, the 7xxx aluminum alloy product includes up to 0.11 wt. % Zr, 0 wt. % Sc, and 0 wt. % Hf. In another embodiment, the 7xxx aluminum alloy product includes 0.05-0.15 wt. % Zr, 0 wt. % Sc, and 0 wt. % Hf. For example, it is also understood that within the ranges of Zn, Cu, and Mg provided herein, the 7xxx aluminum alloy product includes ≤0.12 wt. % Si, the upper or lower limit for Si may be selected from 0.00, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.10, 0.11, and 0.12 wt. %. In one embodiment, the 7xxx aluminum alloy product includes ≤0.05 wt. % Si. For example, it is also understood that within the ranges of Zn, Cu, and Mg provided herein, the 7xxx aluminum alloy product includes ≤0.15 wt. % Fe, the upper or lower limit for Fe may be selected from 0.00, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.10, 0.11, 0.12, 0.13, 0.14, and 0.15 wt. %. In one embodiment, the 7xxx aluminum alloy product includes ≤0.0.8 wt. % Fe. For example, it is also understood that within the ranges of Zn, Cu, and Mg provided herein, the 7xxx aluminum alloy product includes ≤0.04 wt. % Cr, the upper or lower limit for Cr may be selected from 0.00, 0.01, 0.02, 0.03, and 0.04 wt. %.
The term “consisting essentially of” shall be construed to mean that no intentional additional of other elements are added beyond those recited in order to provide the novel and basic features of the present invention. It is understood that due to impurities and/or leaching from contact with manufacturing equipment, trace quantities of such elements may, nevertheless, find their way into the final alloy product. It is to be understood, however, that the scope of this invention should not/cannot be avoided through the mere addition of any such element(s) in quantities that would not otherwise impact on the combination of properties desired and attained herein. Such novel and basic characteristics of the present invention include high strength, better fatigue crack deviation performance, and high anisotropic ductility over other aluminum alloys. Such tensile yield strength (TYS) along the rolling direction (LT) is higher than 65 ksi for 4 to 5 inch plate. The Kmax-dev at crack deviation is higher than 34 ksi*in1/2 for 4 to 5 inch plate. The minimum elongation is 1.4% for all orientations including lowest orientation of ST-22.5 to ST-45.
The term “incidental impurities” can include relatively small amounts, less than 0.1 wt. %, or less than 0.05 wt. %, or less than 0.01 wt. % of other elements. Incidental impurities can be present in significant amounts and add desirable or other charactersitics on their own without departing from the scope of the invention so long as the alloy retains the desirable characteristics set forth herein, namely high strength, better fatigue crack deviation performance, and high anisotropic ductility over other aluminum alloys. In one embodiment of the present invention, the incidental impurities are present up to 1 wt. %, preferably ≤0.5 wt. % or ≤0.1 wt. %, of the total weight of the 7xxx aluminum alloy product.
The high strength 7xxx aluminum alloy product may be used to produce plates, extrusions, and forging products. In one embodiment, the high strength 7xxx aluminum alloy product is used to produce a wrought product that is a rolled plate including any of the chemistries provided in the above-mentioned embodiments. The rolled thick plate may be manufactured using known process conditions such as homogenization, hot-rolling, solution heat treatments and ageing treatments.
In one embodiment, ingots of the high strength 7xxx aluminum alloy product may be cast, homogenized, hot rolled, solution heat treated, cold water quenched, optionally stretched, and aged to desired temper. In one embodiment, the high strength 7xxx aluminum alloy is a plate subjected to a final T7651 and T7451 tempers in the thickness range from 2 inch to 10 inch. The ingots may be homogenized at temperatures from 454 to 491° C. (849 to 916° F.). The hot rolling start temperature may be from 385 to 450° C. (725 to 842° F.). The exit temperature may be in a similar range as the start temperature, for example, the exit temperature is within 100° F., more preferably 80° F., of the start temperature. The plates may be solution heat treated at temperature range from 454 to 491° C. (849 to 916° F.). The plates are cold water quenched to room temperature, for example, between 50-95° F., and may be stretched at about 1.5 to 3%. The quenched plate may be subjecting to any known aging practices known by those of skill in the art including, but not limited to, two-step aging practices that produce a final T7651 or T7451 temper. When using a T7651 temper, the first stage temperature may be in the range of 100 to 140° C. (212 to 284° F.) for 4 to 24 hours and the second stage temperature may be in the range of 135 to 200° C. (275 to 392° F.) for 5 to 20 hours. In one embodiment, the second stage temperature is (135 to 152° C.) (275-305° F.) for 5 to 20 hours.
Although the following examples demonstrate various embodiments of the present invention, one of skill in the art should understand how additional high strength 7xxx aluminum alloy products can be fabricated in accordance with the present invention. The examples should not be construed to limit the scope of protection provided for the present invention.
Twenty three (23) industrial scale ingots were cast by commercial DC (Direct Chill) casting process and processed to different thickness plates. Table 1 gives the chemical compositions of 23 ingots.
Alloys 1 to 7 are invention 7xxx aluminum alloy products. Alloy 8 to 23 are not invention 7xxx aluminum alloy products either due to higher Zn than 7.8 wt. % or lower Zn than 7.0 wt. %. In addition, Alloy 8 is not an invention 7xxx aluminum alloy product due to higher Mg than 2.1 wt. % and higher Cr than 0.04 wt. %.
Ingots were homogenized, hot rolled, solution heat treated, quenched, stretched and aged to final T7651 temper plates in the thickness range from 4 inch to 6 inch. The ingots were homogenized at a temperature from 465 to 485° C. (869 to 905° F.). The hot rolling start temperature is from 400 to 440° C. (752 to 824° F.). The exit rolling temperature is in the similar range as start temperature, i.e. with 80° F. The rolling reduction of each pass was deliberately controlled to achieve target temperature during hot rolling process.
The plates were solution heat treated at temperature range from 470 to 485° C. (878 to 905° F.), cold water quenched to room temperature and stretched at about 1.5 to 3%. A two-step aging practice was used to produce final T7651 temper. The first stage temperature is in the range of 110 to 130° C. (230 to 266° F.) for 4 to 12 hours and the second stage temperature is in the range of 145 to 160° C. (293 to 320° F.) for 8 to 20 hours.
Five invention alloy plates (ID #1, 2, 5, 6, 7) were selected for strength and fracture toughness aging response study. The 0.2% offset yield strength (TYS) along transverse direction (LT) was measured at quarter thickness (T/4) and mid thickness (T/2) under ASTM B557 specification. The plane strain fracture toughness (K1c) in T-L orientations at quarter thickness (T/4) was measured under ASTM E399 using CT specimens. Tables 2 give the results of tensile and fracture toughness as function of second stage aging time under 305° F. temperature. As expected, the strength decreases and fracture toughness increases as aging time increases. More importantly, it also shows that the chemistry strongly affect the strength and fracture toughness aging response. This can be more clearly demonstrated in
Based on the aging responses of both strength and fracture toughness, the desirable aging time for the optimized combination of strength and fracture toughness was selected as T7651 temper product aging practice. After production aging, the comprehensive material characterization including fatigue crack growth deviation and anisotropic ductility was conducted for the final T7651 temper plate.
Table 3 gives the tensile and fracture toughness of production aged 7xxx aluminum alloy plates. Both tensile and fracture toughness testing samples were cut from T/4 (quarter thickness) location. The different chemistry alloys have different properties.
The fatigue crack growth deviation was evaluated based on ASTM E647. The coupons orientation is L-S, which has the highest chance to have crack deviation during crack propagation.
The determination of external crack deviation was based on “anything that would normally invalidate the E647 FCG test (up to the point of crack deviation) would invalidate the Kmax-dev test (e.g. crack growth out of plane by more than 20° or crack deviation after the remaining ligament criterion is exceeded). After the deviation branching point determined, the crack length was measured and calculated by three point weighted average method based on fracture sample. The equation for weighted average length is a=(front+back+2*center)/4.
The fatigue cycles, crack length and Kmax-dev at the crack deviation point were given in Table 4 for invention and non-invention alloy lots.
The anisotropic tensile properties, especially anisotropic tensile ductility, can be significantly different for different testing directions. Such orthotropic material behavior is very important for high strength thick plate aerospace application.
In general, the elongation is relatively high at ST direction and then very low at ST-22.5, ST-35, and ST-45, and then quickly becomes very high for ST-67.5 and L directions. The lowest elongation at the worse direction for all patent alloy lot is 1.4% and the average elongation of all lots and all directions is 6.2%.
Stress corrosion resistance is critical for aerospace application. The standard stress corrosion cracking resistance testing was performed in accordance with the requirements of ASTM G47 which is alternate immersion in a 3.5% NaCl solution under constant deflection. Three specimens were tested per sample. All specimens survived 20 days testing without failing under 26ksi stress level in ST direction as shown in Table 6.
Although the present invention has been disclosed in terms of a preferred embodiment, it will be understood that numerous additional modifications and variations could be made thereto without departing from the scope of the invention as defined by the following claims:
