High-strength Al-Zn-Mg-Cu-Ag alloy

Abstract

This invention provides a high-strength Al-Zn-Mg-Cu-Ag alloy, comprising: 9.0-9.7% by weight zinc, 1.5-1.7% by weight magnesium, 1.2-1.6% by weight copper, 0.05-0.1% by weight zirconium, 0.05-0.1% by weight silver, and 86.8-88.2% by weight aluminum. The alloy of this invention has a strength of 720 MPa and a ductility of up to 10%.

Description

[Technical Field]

[0001] This invention relates to a high-strength Al-Zn-Mg-Cu-Ag alloy, and more particularly to the preparation composition and heat treatment process of a high-strength Al-Zn-Mg-Cu-Ag alloy. [Previous Technology]

[0002] 7000 series (Al-Zn-Mg) alloys with tensile strength greater than 500MPa are high-strength heat-treatable forged aluminum alloys. They mainly improve mechanical properties through precipitation strengthening, have excellent fatigue resistance and excellent specific strength, and are suitable for aerospace components that are often subjected to cyclic stress.

[0003] AA7056 alloy is a new type of high-strength 7000 series alloy. Traditional 7000 series alloys have a maximum Zn content of no more than 8.5 wt%, such as AA7055 with a maximum Zn content of 8.4 wt%. AA7056, however, has a Zn content between 8.5 and 9.7 wt%, and the Zn / Mg ratio is also increased to 3.5 to 6.0. The increased Zn content and Zn / Mg ratio help improve alloy strength and reduce quenching sensitivity, respectively, making it widely applicable in thick-material manufacturing and online quenching processes. Therefore, in addition to aerospace materials, AA7056 is increasingly being used in defense materials to improve vehicle mobility and environmental friendliness.

[0004] High-strength Al-Zn-Mg-Cu alloys are prone to the formation of a significant S-Al2CuMg phase after homogenization. This S-Al2CuMg phase, with a low corrosion potential (-0.88V), becomes a pitting corrosion site in corrosive environments, impairing the alloy's corrosion resistance. Furthermore, the melting point of the S-Al2CuMg phase is only about 490~501℃, which limits hot working (the extrusion speed of AA7075 alloy is only about 1m / min), far lower than the 15m / min of AA6063 alloy and the solution treatment temperature (supersaturated solution temperature). If the temperature is not properly controlled, overheating due to grain boundary melting can occur. In addition, this S-Al2CuMg phase can easily become the initiation site of cracks, impairing the alloy's toughness.

[0005] In summary, aluminum alloys still have defects. Therefore, the applicant of this case has painstakingly developed a high-strength Al-Zn-Mg-Cu-Ag alloy, which can increase the strength to 720MPa and the ductility to 10%. [Summary of the Invention]

[0006] In view of the shortcomings of the prior art, the main objective of the present invention is to provide a high-strength Al-Zn-Mg-Cu-Ag alloy. This is achieved by adjusting the Zn / Mg ratio of the high-zinc Al-Zn-Mg-Cu alloy to the Zn / Mg ratio of the precipitated strengthening phase η' (MgZn2), thereby optimizing the precipitation strengthening effect. Furthermore, the addition of trace amounts of silver causes the alloy to precipitate a dense strengthening phase, improving the alloy's mechanical strength. In addition, the S phase (Al2CuMg), which impairs the toughness of the high-zinc Al-Zn-Mg-Cu alloy, is eliminated through a three-stage strengthening solid solution treatment, enabling the alloy to achieve a strength of 720 MPa while maintaining a ductility of over 10%.

[0007] In order to achieve the above objective, a high-strength Al-Zn-Mg-Cu-Ag alloy according to the present invention comprises: 9.0 to 9.7% by weight of zinc, 1.5 to 1.7% by weight of magnesium, 1.2 to 1.6% by weight of copper, 0.05 to 0.1% by weight of zirconium, 0.05 to 0.1% by weight of silver, and 86.8 to 88.2% by weight of aluminum.

[0008] Preferably, the base material of the alloy can be an aluminum alloy casting or forging.

[0009] Preferably, the forging or casting can be produced by melting it in a vacuum melting machine.

[0010] Preferably, the heat treatment process of the alloy may include a three-stage solution treatment, wherein the first stage solution treatment may be held at 470°C for 1 to 3 hours, the second stage solution treatment may be heated to 480°C and held for 1 to 3 hours, the third stage solution treatment may be heated to above 490°C and held for 1 to 3 hours, and then water quenched to room temperature after solution treatment.

[0011] Preferably, the heating rate between each stage of the three-stage solution treatment can be 10℃ / h to 20℃ / h.

[0012] Preferably, the heat treatment process may further include applying T76 artificial aging (100℃*6h+160℃*14h).

[0013] The above overview, the following detailed description, and the accompanying drawings are all intended to further illustrate the methods, means, and effects of the present invention in achieving its intended purpose. Other objects and advantages of the present invention will be explained in the following description and drawings. [Simplified Explanation of the Diagram]

[0033] The first figure is a flow chart of the heat treatment process of the high-strength Al-Zn-Mg-Cu-Ag alloy of the present invention.

Implementation Method

[0014] The following are specific examples illustrating the implementation of the present invention. Those skilled in the art can easily understand the advantages and effects of this invention from the content disclosed in this specification.

[0015] This invention provides a high-strength Al-Zn-Mg-Cu-Ag alloy, comprising: 9.0-9.7 wt% zinc, 1.5-1.7 wt% magnesium, 1.2-1.6 wt% copper, 0.05-0.1 wt% zirconium, 0.05-0.1 wt% silver, and 86.8-88.2 wt% aluminum. This invention achieves this by adjusting the Zn / Mg weight ratio of the alloy (approximately 5.4) and adding trace amounts of silver to form a dense precipitated strengthening phase.

[0016] In this embodiment, the base material of the alloy can be an aluminum alloy casting or forging, and the forging or casting can be made by melting it using a vacuum melting machine.

[0017] Please refer to Figure 1, which is a flow chart of the heat treatment process of the high-strength Al-Zn-Mg-Cu-Ag alloy of the present invention. The heat treatment process of the high-strength Al-Zn-Mg-Cu-Ag alloy of the present invention may include step S1: a three-stage solution treatment. The first stage solution treatment may be held at 470°C for 1 to 3 hours, the second stage solution treatment may be heated to 480°C and held for 1 to 3 hours, and the third stage solution treatment may be heated to above 490°C and held for 1 to 3 hours. In this embodiment, the heating rate between each stage of the three-stage solution treatment may be 10°C / h to 20°C / h, and finally, the alloy is water-quenched to room temperature.

[0018] Please continue to refer to Figure 1. The heat treatment process may further include step S2: applying T76 artificial aging (100℃*6h+160℃*14h).

[0019] As described above, in order to improve the strength of the aforementioned alloy and avoid overheating, the heat treatment process of the high-strength Al-Zn-Mg-Cu-Ag alloy of the present invention utilizes a three-stage strengthening solution treatment to eliminate the S phase (Al2CuMg) in the alloy and increase the supersaturation of the alloy. The solution treatment procedure is as follows: the first stage is held at 470°C for 1 to 3 hours; the second stage is heated to 480°C and held for 1 to 3 hours; the third stage is heated to above 490°C and held for 1 to 3 hours (the heating rate between each solution treatment stage is 10°C / hr to 20°C / hr); and finally, water quenching is performed in room temperature water.

[0020] Al-Zn-Mg-Cu is a heat-treatable alloy. Its strength is primarily increased by the precipitation of the η'(MgZn2) strengthening phase. When the Zn / Mg weight ratio of the alloy is close to that of η'(MgZn2) (approximately 5.4), a large amount of densely packed strengthening phase can precipitate, which is beneficial for improving the alloy's mechanical properties. Furthermore, the addition of trace amounts of silver can form nano-clusters composed of Zn-Mg-Ag-vacancies in the early stages of aging. These numerous vacancy-rich nano-clusters become precipitation sites for solid solution atoms during subsequent aging, thereby shortening the precipitate-free zone and forming a densely packed strengthening phase, thus improving the alloy's mechanical strength.

[0021] By holding the temperature at 480°C, the low-melting-point S phase (Al2CuMg) is dissolved back into the matrix, and the solution temperature, which was originally limited by the melting point of the S phase, can be increased to above 490°C. Increasing the solution temperature can increase the supersaturation of the alloy, and more strengthening phases can be precipitated during subsequent aging, thereby increasing the strength of the alloy. Eliminating the S phase can also improve the toughness of the alloy.

[0022] Generally speaking, the strength of Al-Zn-Mg-Cu alloy is difficult to exceed 650MPa. However, through the alloy composition and three-stage solid solution treatment of the present invention, the strength of Al-Zn-Mg-Cu alloy can be increased to 720MPa. Furthermore, due to the complete elimination of the S phase, the ductility of the alloy can also be improved. Therefore, the high-strength alloy of the present invention can be used not only in the aerospace field but also as a ballistic material for the defense industry, and has more applications than the general low-strength Al-Zn-Mg-Cu alloy.

[0023] The high-strength Al-Zn-Mg-Cu-Ag alloy of the present invention, analyzed by differential scanning calorimetry (DSC), showed a significant endothermic peak (4.86 J / g) in the as-cast state for the T-Al2Mg3Zn3+η'-MgZn2 phase re-dissolution. After homogenization treatment (470℃*24h), the S-Al2CuMg phase (0.88 J / g) precipitated. A significant endothermic peak (0.88 J / g) for the S-Al2CuMg phase dissolution was observed at 495℃ on the heating curve from 490℃ to 550℃ (heating rate 10℃ / min). The exothermic heat released after the Al-Zn-Mg-Cu-Ag alloy under single-stage solution treatment was 3.12, 2.34, and 1.41 (J / g), respectively. After a three-stage enhanced solution treatment, the precipitation amount of the Al-Zn-Mg-Cu-Ag alloy increased again to 3.35, 2.56, and 1.66 (J / g), respectively. This shows that the three-stage treatment can indeed dissolve a larger number of solid atoms in the quenched alloy compared to a one-stage treatment, thus suggesting that the three-stage treatment can achieve higher mechanical strength. Furthermore, at 495℃, significant heat absorption (0.72 J / g) of S-Al2CuMg phase re-dissolution was observed in the one-stage solution-treated alloy. The one-stage solution treatment only eliminated a portion of the S-Al2CuMg phase in the Al-Zn-Mg-Cu-Ag alloy. However, after the three-stage enhanced solution treatment, the residual S-Al2CuMg phase was completely eliminated, clearly demonstrating that the three-stage solution treatment is indeed more effective than the one-stage treatment in eliminating residual S-Al2CuMg phase in the alloy.

[0024] The high-strength Al-Zn-Mg-Cu-Ag alloy of the present invention was tested for tensile properties. The tensile curves of the Al-Zn-Mg-Cu-Ag alloy after different solution treatments and aging were observed. When the alloy was aged, precipitation strengthening effect was obviously produced. The strength of the alloy with three-stage solution treatment (723MPa) was higher than that of the alloy with one-stage solution treatment (708MPa). This can also be seen from the differential scanning calorimetry (DSC) analysis, which shows that the three-stage solution treatment increased the precipitation amount and precipitation kinetics of the strengthening phase η'-MgZn2 phase.

[0025] The tensile curves show that the ductility of the three-stage solution-treated alloy (10.6%) is improved compared to the one-stage solution-treated alloy (9.2%) due to the increase in strength. Furthermore, the tensile test specimens after artificial aging of the alloy were observed. By observing the microstructure of the tensile direction (TD) plane around the fracture surface, it was found that many microcrack initiation points and propagation cracks were observed on the residual S-Al2CuMg phase (37wt% Al, 48wt% Cu, and 16wt% Mg as determined by EDS analysis) in the one-stage solution-treated alloy. These propagation cracks are highly correlated with the S-Al2CuMg phase. In contrast, the residual amount of S-Al2CuMg phase in the three-stage solution-treated alloy is significantly reduced compared to the one-stage solution-treated alloy, which will result in a reduction in the number of tensile crack initiation points, thereby improving the ductility of the alloy.

[0026] The high-strength Al-Zn-Mg-Cu-Ag alloy of this invention underwent elastic energy testing. Shot tests were conducted using 7.62mm × 51mm NATO M80 steel plates after different solution treatments and aging. This test used only 100mm × 100mm × 2mm alloy plates, and the kinetic energy consumed by the alloy was calculated by comparing the initial velocity before penetration and the final velocity after penetration. The one-stage and three-stage solution-treated specimens of the novel Al-Zn-Mg-Cu-Ag alloy consumed 254J and 261J of bullet impact kinetic energy, respectively, approximately 8% of the initial bullet velocity, demonstrating that the three-stage solution treatment indeed improves the alloy's resistance to bullet impact compared to the one-stage solution treatment.

[0027] In summary, the present invention is a high-strength Al-Zn-Mg-Cu-Ag alloy, which has the following characteristics:

[0028] 1. This new high-strength Al-Zn-Mg-Cu-Ag alloy can improve the strength of the existing 7000 series high-strength (Al-Zn-Mg-Cu) aluminum alloy, while also improving the alloy ductility. Its performance is better than that of the existing 7000 series high-strength (Al-Zn-Mg-Cu) alloy.

[0029] 2. The composition of this new high-strength Al-Zn-Mg-Cu-Ag was developed to improve the strength and ductility of the traditional 7000 series high-strength (Al-Zn-Mg-Cu), but the three-stage solution treatment can also be widely applied to other high-strength aluminum alloy castings or forgings.

[0030] 3. The new three-stage strengthening solution treatment method can improve the ductility of the alloy while increasing its strength. Compared with the old one-stage solution treatment, it has a higher tolerance for engineering design safety factor.

[0031] 4. This invention adjusts the Zn / Mg ratio of the high-zinc Al-Zn-Mg-Cu alloy to the Zn / Mg ratio of the precipitated strengthening phase η' (MgZn2), thereby optimizing its precipitation strengthening effect. A trace amount of silver is added to induce dense precipitation of the strengthening phase, improving the alloy's mechanical strength. Furthermore, the S phase (Al2CuMg), which impairs the toughness of the high-zinc Al-Zn-Mg-Cu alloy, is eliminated through a three-stage strengthening solid solution treatment, resulting in an alloy strength of 720 MPa while maintaining a ductility of over 10%.

[0032] The above embodiments are merely illustrative of the features and effects of this invention and are not intended to limit the scope of the substantive technical content of this invention. Any person skilled in the art may modify and change the above embodiments without departing from the spirit and scope of the invention. Therefore, the scope of protection of this invention should be as set forth in the following patent application claims.

Claims

1. A high-strength Al-Zn-Mg-Cu-Ag alloy, comprising 9.0-9.7% by weight zinc, 1.5-1.7% by weight magnesium, 1.2-1.6% by weight copper, 0.05-0.1% by weight zirconium, 0.05-0.1% by weight silver, and 86.8-88.2% by weight aluminum, wherein the heat treatment process of the alloy includes a three-stage solution treatment: the first stage solution treatment is held at 470°C for 1-3 hours; the second stage solution treatment is heated to 480°C and held for 1-3 hours; and the third stage solution treatment is heated to above 490°C and held for 1-3 hours, followed by water quenching to room temperature.

2. The high-strength Al-Zn-Mg-Cu-Ag alloy as described in claim 1, wherein the base material of the alloy is an aluminum alloy casting or forging.

3. The high-strength Al-Zn-Mg-Cu-Ag alloy as described in claim 2, wherein the forging or casting is produced by melting it using a vacuum melting machine.

4. The high-strength Al-Zn-Mg-Cu-Ag alloy as described in claim 1, wherein the heating rate between each stage of the three-stage solution treatment is 10℃ / h to 20℃ / h.

5. The high-strength Al-Zn-Mg-Cu-Ag alloy as described in claim 1, wherein the heat treatment process further includes a T76 artificial aging heat treatment condition of 100℃*6h+160℃*14h.

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