Method for improving mechanical property of core of 7A04 thick-wall part

Abstract

The invention provides a method for improving the mechanical property of the core part of a 7A04 aluminum alloy thick-wall part, which comprises the following steps of: firstly, carrying out solution treatment on a forged free forge piece, and quickly transferring the free forge piece into a low-temperature water bath for water quenching to obtain a water-quenched 7A04 forge piece; the rapid transfer time is less than 10s, and the temperature of the low-temperature water bath is 5-20 DEG C; and carrying out liquid nitrogen treatment on the 7A04 forge piece obtained in the previous step to obtain the quenched 7A04 aluminum alloy forge piece. According to the method, the problem that the mechanical property of the core of the 7A04 series aluminum alloy thick-wall part is poor can be solved, the residual internal stress of the free forge piece is reduced, and follow-up machining can be normally carried out conveniently. According to the method, the quenching transfer time is shortened, supersaturated solid solution decomposition is reduced, the water temperature is reduced, the cooling speed is increased, the quenched free forge piece is subjected to liquid nitrogen treatment, residual stress in the forge piece is reduced, the mechanical property of the core of the thick-wall piece is improved, and therefore the problem that a large component cannot be thoroughly quenched is solved.

Description

Technical Field

[0001] The invention belongs to the technical field of preparation of 7A04 aluminum alloy thick-walled parts, and relates to a method for improving the mechanical properties of the core of the 7A04 thick-walled parts. Background Art

[0002] Since the beginning of its development, 7000 series aluminum alloy has been widely used in various industries due to its excellent performance. The research and development of 7000 series aluminum alloy has always been a hot topic in the metal field at home and abroad. Due to the application in the aerospace and automotive fields, large-scale structural parts require aluminum alloys to have not only high strength and high plasticity, but also good hardenability.

[0003] In actual engineering applications, because some workpieces are relatively large in size, the difference in cooling speed between the inside and outside of the workpiece during quenching will lead to differences in the internal and external structures and properties of the material. The industry usually uses the hardness distribution depth after quenching (the drop in hardness from the fast cooling end to the slow cooling end) to measure the quenching performance of the material, that is, the hardenability is characterized by the quenching depth. Hardenability is an important property of 7000 series aluminum alloys. Hardenability refers to the degree of difference between the performance of the alloy inside and the surface of the alloy after solution quenching. The greater the difference, the worse the hardenability of the alloy. This property has an important impact on the application of large-size aluminum alloy parts. Especially in the fields of aviation, shipbuilding, etc., if the hardenability of the aluminum alloy is not good, the material with a relatively thick cross-sectional area will be difficult to harden, which seriously affects the performance of the alloy. Therefore, there are high requirements for the hardenability of aluminum alloys in these fields. At present, the hardening depth of 7050 aluminum alloy used in the aerospace field is 120mm, the hardening depth of 7A04 aluminum alloy is only 50mm, and the hardening depth of 7085 aluminum alloy is the best, reaching 150mm. However, although the hardenability of 7050 and 7085 aluminum alloys is relatively good, the strength is below 600MPa and needs to be improved. Therefore, these aluminum alloys are far from meeting the dual requirements of modern industry for high strength and high hardenability of aluminum alloys, which is also the development direction of the next generation of 7000 series aluminum alloys.

[0004] Among them, 7A04 aluminum alloy is an Al-Zn-Mg-Cu series ultra-high strength aluminum alloy, also known as super-hard aluminum. It is the most commonly used super-hard aluminum. Although it has high strength and good heat treatment strengthening effect, its hardenability is relatively poor. For aluminum alloys, hardenability has become an important performance indicator of aluminum alloys due to industrial demand. With the increasing application of aluminum alloys, many aluminum alloy parts need to develop towards large-scale, which requires aluminum alloys to have higher hardness with a smaller difference. If the internal and external strength and structure are very different, residual stress will be formed, and even cracking will occur, which will seriously affect the performance of the material.

[0005] Therefore, how to find a more suitable way to improve the hardenability of aluminum alloys, especially for 7A04 aluminum alloy thick-walled parts, is an urgent problem to be solved, and it is also one of the focuses of many front-line researchers in the industry. Summary of the invention

[0006] In view of this, the technical problem to be solved by the present invention is to provide a method for improving the mechanical properties of the core of 7A04 thick-walled parts. The method provided by the present invention can solve the problem that large components cannot be hardened through, by reducing the quenching transfer time, reducing the decomposition of the supersaturated solid solution, by lowering the water temperature, increasing the cooling rate, and subjecting the quenched free forgings to liquid nitrogen treatment to reduce the residual stress inside the forgings, so that the mechanical properties of the core of the thick-walled parts are improved, and the process is simple, controllable, and highly executable on site, and is suitable for promotion and application of industrial-scale production.

[0007] The present invention provides a method for improving the mechanical properties of the core of a 7A04 aluminum alloy thick-walled part, comprising the following steps:

[0008] 1) After the forged free forging is subjected to solid solution treatment, it is quickly transferred to a low-temperature water bath for water quenching to obtain a water-quenched 7A04 forging;

[0009] The time of the rapid transfer is less than 10s;

[0010] The temperature of the low-temperature water bath is 5 to 20°C;

[0011] 2) The 7A04 forging obtained in the above steps is treated with liquid nitrogen to obtain a quenched 7A04 aluminum alloy forging.

[0012] Preferably, the 7A04 aluminum alloy comprises, by mass percentage:

[0013]

[0014]

[0015] The balance is Al.

[0016] Preferably, the wall thickness of the 7A04 aluminum alloy thick-walled part is 100 to 180 mm;

[0017] The 7A04 aluminum alloy thick-walled parts include one or more of rings, forged plates, forged cakes, high-cylinder rings, flat rings and special-shaped parts.

[0018] Preferably, the forging method includes four-pier and three-draw;

[0019] The temperature of the solution treatment is 465-485°C;

[0020] The time of the solution treatment is 330 to 390 minutes.

[0021] Preferably, the low-temperature water bath comprises a low-temperature water bath with ice cubes added;

[0022] The cooling rate of the water quenching is 200-300°C / s;

[0023] The water quenching time is greater than or equal to 15 minutes.

[0024] Preferably, the liquid nitrogen treatment comprises placing liquid nitrogen in a container in which the 7A04 forging is placed.

[0025] Preferably, the liquid nitrogen treatment time is greater than or equal to 15 minutes;

[0026] The temperature of the 7A04 forging after the liquid nitrogen treatment is -196 to -160°C.

[0027] Preferably, the liquid nitrogen treatment further includes an artificial aging step.

[0028] Preferably, the artificial aging temperature is 160-170°C;

[0029] The artificial aging time is 570 to 630 minutes.

[0030] Preferably, the quenched 7A04 aluminum alloy forging is specifically a 7A04 aluminum alloy forging to be subsequently machined.

[0031] The present invention provides a method for improving the mechanical properties of the core of a 7A04 aluminum alloy thick-walled part, comprising the following steps: firstly, after the forged free forging is subjected to a solid solution treatment, it is quickly transferred to a low-temperature water bath for water quenching to obtain a water-quenched 7A04 forging; the time of the rapid transfer is less than 10s; the temperature of the low-temperature water bath is 5-20°C; and then the 7A04 forging obtained in the above steps is subjected to liquid nitrogen treatment to obtain a quenched 7A04 aluminum alloy forging. Compared with the prior art, the present invention believes that to solve the problem of how to improve the hardenability of aluminum alloys, it is necessary to first understand the factors that affect the hardenability of aluminum alloys. However, so far, there is no relatively complete and recognized theory in the academic community to answer this question. There are usually two main factors that affect the hardenability of aluminum alloys: first, the chemical composition of the aluminum alloy, which is an internal cause and also the fundamental reason. For 7000 series aluminum alloys, increasing the mass percentage ratio of Zn and Mg can improve the hardenability of the alloy, or reducing the Cu content in the 7000 series aluminum alloy can improve the hardenability of the alloy, and the micro-alloying element Zr can also improve the hardenability of the alloy. In addition to the above-mentioned internal factors, there are also some external factors that affect the hardenability of aluminum alloys, such as the cooling rate of alloy quenching, the size of the alloy, etc.

[0032] The purpose of quenching is to obtain a supersaturated solid solution. The degree of decomposition of the supersaturated solid solution directly affects the performance of the alloy after failure, so the alloy has the problem of quenching sensitivity. Quenching sensitivity reflects and describes the stability of the supersaturated solid solution and the difficulty of precipitation during quenching. The higher the quenching sensitivity, the easier it is for the supersaturated solid solution to decompose during quenching, and it is more difficult to cool the solid solution phase in a high temperature state to room temperature in a supercooled and supersaturated form. Aluminum alloys with high quenching sensitivity are more likely to precipitate out the equilibrium phase during slow cooling, which not only causes the supersaturated solid solubility of the alloy to decrease, but also because the precipitation of the equilibrium phase will absorb a large number of solute atoms, resulting in a decrease in the number of strengthening phases after aging, reducing the strength of the alloy.

[0033] The decrease in quenching rate may cause the strength of aluminum alloy to decrease, which is particularly evident in the quenching of large components. It is mainly manifested in that in production practice, large components often have inconsistent cooling rates on the surface and the core, resulting in a large difference in hardness between the surface and the core of the workpiece during subsequent aging, affecting the overall performance. Therefore, the cooling rate must be ensured when the workpiece is quenched to avoid excessive decomposition of the supersaturated solid solution during the quenching process. In actual production, various quenching media with different cooling rates are often used to adjust the quenching rate. Quenching media can be divided into liquid (brine, molten salt, molten alkali, and special quenching oil, etc.), solid (fluidized bed, metal plate, etc.), gaseous (air, compressed air, liquid nitrogen, etc.). When the quenching medium cools faster, it is beneficial to quickly bring out the internal heat of the alloy and prevent the decomposition of the supersaturated solid solution, thereby obtaining higher strength. However, when the cooling rate is faster, deformation and cracking are prone to occur, and the workpiece is also prone to obtain higher residual stress. The ideal quenching cooling rate is to obtain higher and uniform strength and a larger quenching depth at the high temperature stage, and reduce cracking, eliminate deformation and residual stress.

[0034] Moreover, after the 7xxx series aluminum alloy is solid-solutionized, the alloying elements will melt into the aluminum matrix to form a supersaturated solid solution. Since the supersaturated solid solution is unstable and tends to decompose into a second phase, a higher quenching rate is required to retain it. During slow quenching, since the cooling rate of the alloy is less than the critical cooling rate, the supersaturated solid solution will decompose during the cooling to room temperature. The quenching rate allows the coarse quenched precipitate phase to have enough time to nucleate in the defects of the alloy (grain boundaries, phase boundaries, etc.), and absorb alloy solute atoms in the subsequent aging process to further coarsen. During the quenching and coarsening process, a large amount of alloying elements are consumed, so that a certain width of non-precipitated precipitation zone is formed around these coarse precipitates. The concentration of solute atoms is reduced, resulting in the small size of the second phase being difficult to precipitate. Slow quenching also leads to a decrease in the vacancy concentration of the alloy, which greatly reduces the nucleation points in the crystal during the subsequent aging treatment, making it more difficult for the fine second phase to precipitate. Therefore, the size and distribution of the precipitated phase in the crystal after air-cooled alloy aging are not as uniform as those of water-quenched. There are a large number of coarse second phases in the crystal, and the number of fine second phases is small and the density is also low. When quenching at room temperature with water, due to the fast cooling rate, the supersaturated solid solution is not easy to decompose and is retained at room temperature, and the atomic concentration of the alloy after quenching is high; at the same time, the high quenching rate also makes the alloy have a higher vacancy concentration, which makes the nucleation points in the matrix more during the subsequent aging treatment, so the precipitated phase of the water-quenched alloy after aging is uniform, fine, and dispersed.

[0035] After quenching, different regions in the alloy have different vacancy concentrations. Grain boundaries are vacancy traps. The diffusion of vacancies to grain boundaries during quenching results in a lower vacancy concentration near the grain boundaries; in areas far from the grain boundaries, since the vacancies have nowhere to diffuse, these areas have a higher vacancy concentration. A certain vacancy concentration gradient is formed near the grain boundaries and in areas far from the grain boundaries. During the aging stage, the quenching vacancy concentration in areas below the critical vacancy concentration is affected by the cooling rate of the quenching process. The lower the cooling rate, the more fully the vacancies diffuse, and the more difficult it is to precipitate fine second phase particles in the area near the grain boundaries. Therefore, the slowly quenched alloy has a wider grain boundary PFZ. In addition, the precipitation of coarse second phases at the grain boundaries during slow quenching leads to a decrease in the concentration of solute atoms near the grain boundaries, which also aggravates the widening of the PFZ. Therefore, the air-cooled sample has a wider grain boundary PFZ than the water-quenched sample.

[0036] Based on this, the present invention creatively designs a method for improving the mechanical properties of the core of 7A04 thick-walled parts. The present invention improves the mechanical properties of the core of 7A04 aluminum alloy thick-walled parts from external factors, by adding ice cubes to lower the water temperature, increasing the quenching cooling rate, and then treating it with liquid nitrogen to improve its mechanical properties while reducing the residual stress of the forging.

[0037] The improvement method provided by the present invention must reduce the quenching transfer time before quenching, reduce the decomposition of the supersaturated solid solution, reduce the water temperature, control the water temperature within a certain range, ensure the cooling rate, and control the wall thickness of the forging within a certain range to avoid the forging wall thickness being too thick, resulting in the situation of not being quenched through; at the same time, liquid nitrogen treatment is carried out after quenching, and residual stress is released during the extremely fast cooling process, which is conducive to subsequent machining. The method provided by the present invention can solve the problem of poor mechanical properties of the core of 7xxx series aluminum alloy thick-walled parts, can reduce the residual internal stress of free forgings, and facilitate the normal subsequent machining. The present invention can solve the problem of large-scale components not being quenched through, reduce the quenching transfer time, reduce the decomposition of supersaturated solid solution, reduce the water temperature, increase the cooling rate, and treat the quenched free forgings with liquid nitrogen to reduce the residual stress inside the forgings, so that the mechanical properties of the core of the thick-walled parts are improved, and the process is simple, controllable, high on-site executability, easy to implement, and suitable for the promotion and application of industrial-scale production.

[0038] Experimental results show that by adopting the method provided by the present invention, the mechanical properties of 7A04 thick-walled parts are significantly enhanced, the tensile strength and yield strength in the longitudinal, transverse and height directions are improved, the longitudinal elongation is significantly improved, and the transverse and height elongation are slightly decreased, but both are above 7%. Under the process conditions of liquid nitrogen treatment, machining does not produce large deformation, non-destructive testing is qualified, and physical and chemical testing is qualified. DETAILED DESCRIPTION

[0039] In order to further understand the present invention, preferred embodiments of the present invention are described below in conjunction with examples, but it should be understood that these descriptions are only for further illustrating the features and advantages of the present invention, rather than limiting the claims of the invention.

[0040] All raw materials of the present invention have no particular limitation on their sources, and can be purchased from the market or prepared according to conventional methods known to those skilled in the art.

[0041] There is no particular limitation on the purity of all raw materials in the present invention. The present invention preferably uses industrial pure materials or materials with conventional purity requirements in the field of 7A04 aluminum alloy preparation.

[0042] The brands and abbreviations of all raw materials of the present invention are conventional brands and abbreviations in the art. Each brand and abbreviation is clear and definite in the field of its related use. Those skilled in the art can purchase them from the market or prepare them by conventional methods according to the brands, abbreviations and corresponding uses.

[0043] The abbreviations of the processes used in the present invention are all conventional abbreviations in the field. The specific steps and conventional parameters of each abbreviation are clear and definite in its relevant field. Those skilled in the art can implement them in a conventional manner based on the abbreviations.

[0044] The present invention provides a method for improving the mechanical properties of the core of a 7A04 aluminum alloy thick-walled part, comprising the following steps:

[0045] 1) After the forged free forging is subjected to solid solution treatment, it is quickly transferred to a low-temperature water bath for water quenching to obtain a water-quenched 7A04 forging;

[0046] The time of the rapid transfer is less than 10s;

[0047] The temperature of the low-temperature water bath is 5 to 20°C;

[0048] 2) The 7A04 forging obtained in the above steps is treated with liquid nitrogen to obtain a quenched 7A04 aluminum alloy forging.

[0049] The invention firstly performs a solid solution treatment on the free forging after forging, and then quickly transfers it to a low-temperature water bath for water quenching to obtain a water-quenched 7A04 forging.

[0050] In the present invention, the time of the rapid transfer is less than 10 seconds, more preferably less than or equal to 8 seconds, more preferably less than or equal to 6 seconds, more preferably less than or equal to 5 seconds, and specifically can be 4 to 9 seconds, or 5 to 8 seconds.

[0051] In the present invention, the temperature of the low-temperature water bath is 5-20°C, or 7-18°C, or 10-15°C.

[0052] In the present invention, the 7A04 aluminum alloy preferably comprises, by mass percentage:

[0053]

[0054] The balance is Al.

[0055] In the present invention, the added amount of Si is preferably ≤0.5%, more preferably ≤0.45%, and more preferably ≤0.4%.

[0056] In the present invention, the amount of Fe added is preferably ≤0.5%, more preferably ≤0.45%, and more preferably ≤0.4%.

[0057] In the present invention, the added amount of Cu is preferably 1.2% to 2.0%, more preferably 1.3% to 1.8%, more preferably 1.4% to 1.7%, more preferably 1.5% to 1.6%.

[0058] In the present invention, the added amount of Mn is preferably 0.2% to 0.6%, more preferably 0.25% to 0.55%, more preferably 0.3% to 0.5%, more preferably 0.35% to 0.45%.

[0059] In the present invention, the added amount of Mg is preferably 1.8% to 2.8%, more preferably 2.0% to 2.6%, and more preferably 2.2% to 2.4%.

[0060] In the present invention, the added amount of Cr is preferably 0.1% to 0.25%, more preferably 0.13% to 0.22%, and more preferably 0.16% to 0.19%.

[0061] In the present invention, the added amount of Zn is preferably 5.0% to 7.0%, more preferably 5.4% to 6.6%, and more preferably 5.8% to 6.2%.

[0062] In the present invention, the added amount of Ti is preferably ≤0.1%, more preferably ≤0.09%, and more preferably ≤0.08%.

[0063] In the present invention, the wall thickness of the 7A04 aluminum alloy thick-walled part is preferably greater than 100 mm, more preferably 100-180 mm, more preferably 110-160 mm, and more preferably 120-150 mm. Specifically, the 7A04 aluminum alloy thick-walled part in the present invention can be Φ1882*Φ1666*898 mm, and its wall thickness is 108 mm.

[0064] In the present invention, the 7A04 aluminum alloy thick-walled parts preferably include one or more of rings, forged plates, forged cakes, tall tube rings, flat rings and special-shaped parts, and more preferably rings, forged plates, forged cakes, tall tube rings, flat rings or special-shaped parts.

[0065] In the present invention, the forging method preferably includes four-pier and three-draw.

[0066] In the present invention, the temperature of the solution treatment is preferably 465-485°C, more preferably 467-483°C, and even more preferably 469-481°C.

[0067] In the present invention, the solution treatment time is preferably 330 to 390 minutes, more preferably 340 to 380 minutes, and more preferably 350 to 370 minutes. Specifically, the solution treatment time is determined according to the wall thickness of the product.

[0068] In the present invention, the low-temperature water bath preferably includes a low-temperature water bath with ice cubes added. In engineering, the temperature is high in summer, and adding ice cubes is a way to lower the water temperature.

[0069] In the present invention, the cooling rate of the water quenching is preferably 200-300°C / s, more preferably 220-280°C / s, and more preferably 240-260°C / s.

[0070] In the present invention, the water quenching time is preferably greater than or equal to 15 min, more preferably greater than or equal to 16 min, and more preferably greater than or equal to 17 min.

[0071] In the present invention, the temperature of the 7A04 forging after water quenching is preferably cooled to room temperature.

[0072] In the present invention, the 7A04 forging obtained in the above steps is finally treated with liquid nitrogen to obtain a quenched 7A04 aluminum alloy forging.

[0073] In the present invention, the liquid nitrogen treatment method preferably includes placing liquid nitrogen in a container where the 7A04 forging is placed.

[0074] In the present invention, the liquid nitrogen treatment time is preferably greater than or equal to 15 min, more preferably greater than or equal to 16 min, and more preferably greater than or equal to 17 min.

[0075] In the present invention, the temperature of the 7A04 forging after liquid nitrogen treatment is preferably -196 to -160° C., more preferably -190 to -165° C., more preferably -185 to -170° C., more preferably -180 to -175° C. Liquid nitrogen treatment is cryogenic treatment, and the temperature is minus 196° C.

[0076] In the present invention, the liquid nitrogen treatment preferably further includes an artificial aging step.

[0077] In the present invention, the temperature of artificial aging is preferably 160-170°C, more preferably 162-168°C, and more preferably 164-166°C.

[0078] In the present invention, the artificial aging time is preferably 570 to 630 min, more preferably 580 to 620 min, and more preferably 590 to 600 min.

[0079] In the present invention, the quenched 7A04 aluminum alloy forging is preferably a 7A04 aluminum alloy forging to be subsequently machined.

[0080] The present invention is to complete and refine the overall technical solution, and better improve the hardenability and uniformity of 7A04 aluminum alloy thick-walled parts. The method for improving the mechanical properties of the core of 7A04 aluminum alloy thick-walled parts may specifically include the following contents:

[0081] The present invention adds ice cubes to lower the water temperature, increases the quenching cooling rate, and then undergoes liquid nitrogen treatment to improve the mechanical properties and reduce the residual stress of the forging.

[0082] Step 1: After four piers and three draws, the free forgings are obtained, and then solution treated at 470°C. Ice cubes are added to the quenching furnace to reduce the water temperature to 15°C to increase the quenching rate. The quenching transfer time is controlled at 7S to reduce the decomposition of the supersaturated solid solution.

[0083] Step 2: Based on the first step, liquid nitrogen is added to the free forgings to fully reduce the residual stress of the forgings based on the principle of thermal expansion and cooling rate, so as to facilitate subsequent machining.

[0084] The above content of the present invention provides a method for improving the mechanical properties of the core of 7A04 thick-walled parts. The method designed by the present invention for improving the mechanical properties of the core of 7A04 thick-walled parts improves the mechanical properties of the core of 7A04 aluminum alloy thick-walled parts from external factors, by reducing the quenching transfer time, reducing the decomposition of the supersaturated solid solution, adding ice cubes to reduce the water temperature, increasing the quenching cooling rate, and then treating it with liquid nitrogen to improve its mechanical properties while reducing the residual stress of the forging. Through the method of the present invention, the mechanical properties of thick-walled 7A04 forgings can be improved, solving the problem that the mechanical properties of thick-walled 7A04 forgings do not meet the requirements.

[0085] The improvement method provided by the present invention must reduce the water temperature before quenching, control the water temperature within a certain range, ensure the cooling rate, control the quenching transfer time within a certain range, reduce the decomposition of the supersaturated solid solution, and control the wall thickness of the forging within a certain range to avoid the forging wall thickness being too thick, resulting in the situation of not being quenched through; at the same time, liquid nitrogen treatment is carried out after quenching, and residual stress is released during the extremely fast cooling process, which is conducive to subsequent machining. The method provided by the present invention can solve the problem of poor mechanical properties of the core of 7xxx series aluminum alloy thick-walled parts, can reduce the residual internal stress of free forgings, and facilitate the normal subsequent machining. The present invention can solve the problem of large-scale components not being quenched through, reduce the quenching transfer time, reduce the decomposition of supersaturated solid solution, reduce the water temperature, increase the cooling rate, and treat the quenched free forgings with liquid nitrogen to reduce the residual stress inside the forgings, so that the mechanical properties of the core of the thick-walled parts are improved, and the process is simple, controllable, and highly executable on site. It is easy to implement and suitable for the promotion and application of industrial-scale production.

[0086] Experimental results show that by adopting the method provided by the present invention, the mechanical properties of 7A04 thick-walled parts are significantly enhanced, the tensile strength and yield strength in the longitudinal, transverse and height directions are improved, the longitudinal elongation is significantly improved, and the transverse and height elongation are slightly decreased, but both are above 7%. Under the process conditions of liquid nitrogen treatment, machining does not produce large deformation, non-destructive testing is qualified, and physical and chemical testing is qualified.

[0087] In order to further illustrate the present invention, a method for improving the mechanical properties of the core of a 7A04 thick-walled component provided by the present invention is described in detail below in combination with embodiments. However, it should be understood that these embodiments are implemented on the premise of the technical solution of the present invention, and detailed implementation methods and specific operating processes are given only to further illustrate the features and advantages of the present invention, rather than to limit the claims of the present invention. The protection scope of the present invention is not limited to the following embodiments.

[0088] Example 1

[0089] For chemical composition control, please refer to the composition control range of 7A04 aluminum alloy given in the specification.

[0090] The 7A04 aluminum alloy free forgings after four-pier and three-draw were solutionized at a solution temperature of 470°C and a solution time of 360 min.

[0091] Add ice cubes to the quenching water tank and adjust the water temperature to 15°C.

[0092] After the solution treatment is completed, the forging is transferred to the quenching pool for 7S.

[0093] After the forgings are transferred to the quenching pool, the residence time in the quenching pool is 17 minutes.

[0094] After the water quenching is completed, the forging is first transferred to a container, and then liquid nitrogen is poured into the container, and the forging is cooled to -170°C.

[0095] The forgings are then transferred to an aging furnace for artificial aging at a temperature of 165°C and an aging time of 600 min.

[0096] After artificial aging, mechanical properties testing is carried out.

[0097] Example 2

[0098] For chemical composition control, please refer to the composition control range of 7A04 aluminum alloy given in the specification.

[0099] The 7A04 aluminum alloy free forgings after four-pier and three-draw were solutionized at a solution temperature of 470°C and a solution time of 360 min.

[0100] Add ice cubes to the quenching water tank and adjust the water temperature to 18°C.

[0101] After the solution treatment is completed, the forging is transferred to the quenching pool for 8S.

[0102] After the forgings are transferred to the quenching pool, the residence time in the quenching pool is 17 minutes.

[0103] After water quenching, the forgings are first transferred to a container, and then liquid nitrogen is poured into the container to cool the forgings to -160°C.

[0104] The forgings are then transferred to an aging furnace for artificial aging at a temperature of 165°C and an aging time of 600 min.

[0105] After artificial aging, mechanical properties testing is carried out.

[0106] Comparative Example 1

[0107] For chemical composition control, please refer to the composition control range of 7A04 aluminum alloy given in the specification.

[0108] The 7A04 aluminum alloy free forgings after four-pier and three-draw were solutionized at a solution temperature of 470°C and a solution time of 360 min.

[0109] Add ice cubes to the quenching water tank and adjust the water temperature to 25°C.

[0110] After the solution treatment is completed, the forging is transferred to the quenching pool for 7S.

[0111] After the forgings are transferred to the quenching pool, the residence time in the quenching pool is 17 minutes.

[0112] After the water quenching is completed, the forging is first transferred to a container, and then liquid nitrogen is poured into the container, and the forging is cooled to -170°C.

[0113] The forgings are then transferred to an aging furnace for artificial aging at a temperature of 165°C and an aging time of 600 min.

[0114] After artificial aging, mechanical properties testing is carried out.

[0115] Comparative Example 2

[0116] For chemical composition control, please refer to the composition control range of 7A04 aluminum alloy given in the specification.

[0117] The 7A04 aluminum alloy free forgings after four-pier and three-draw were solutionized at a solution temperature of 470°C and a solution time of 360 min.

[0118] Add ice cubes to the quenching water tank and adjust the water temperature to 15°C.

[0119] After the solution treatment is completed, the forging is transferred to the quenching pool for 12 seconds.

[0120] After the forgings are transferred to the quenching pool, the residence time in the quenching pool is 17 minutes.

[0121] After the water quenching is completed, the forging is first transferred to a container, and then liquid nitrogen is poured into the container, and the forging is cooled to -170°C.

[0122] The forgings are then transferred to an aging furnace for artificial aging at a temperature of 165°C and an aging time of 600 min.

[0123] After artificial aging, mechanical properties testing is carried out.

[0124] Comparative Example 3

[0125] For chemical composition control, please refer to the composition control range of 7A04 aluminum alloy given in the specification.

[0126] The 7A04 aluminum alloy free forgings after four-pier and three-draw were solutionized at a solution temperature of 470°C and a solution time of 360 min.

[0127] Add ice cubes to the quenching water tank and adjust the water temperature to 15°C.

[0128] After the solution treatment is completed, the forging is transferred to the quenching pool for 7S.

[0129] After the forgings are transferred to the quenching pool, the residence time in the quenching pool is 17 minutes.

[0130] The forgings are then transferred to an aging furnace for artificial aging at a temperature of 165°C and an aging time of 600 min.

[0131] After artificial aging, mechanical properties testing is carried out.

[0132] See Table 1, which shows the performance data of 7A04 aluminum alloy forgings prepared in the embodiments of the present invention and the comparative examples.

[0133] Table 1

[0134] Tensile strength MPa Yield strength MPa Elongation MPa Require ≥510 ≥420 ≥5 Example 1 553 444 13 Example 2 548 439 14.5 Comparative Example 1 505 410 10.5 Comparative Example 2 495 397 11.2 Comparative Example 3 482 375 12.3

[0135] The above data of the present invention are analyzed: the purpose of quenching is to obtain a supersaturated solid solution. The degree of decomposition of the supersaturated solid solution directly affects the performance of the alloy after failure, so the alloy has the problem of quenching sensitivity. Quenching sensitivity reflects and describes the stability of the supersaturated solid solution and the difficulty of precipitation during quenching. The higher the quenching sensitivity, the easier it is for the supersaturated solid solution to decompose during quenching, and it is more difficult to cool the solid solution phase in a high temperature state to room temperature in a supercooled and supersaturated form. Aluminum alloys with high quenching sensitivity are more likely to precipitate out the equilibrium phase during slow cooling, which not only causes the supersaturated solid solubility of the alloy to decrease, but also because the precipitation of the equilibrium phase will absorb a large number of solute atoms, resulting in a reduction in the number of strengthening phases after aging, reducing the strength of the alloy.

[0136] A decrease in quenching rate may cause a decrease in the strength of aluminum alloys, which is particularly evident in the quenching of large components. This is mainly manifested in that in production practice, large components often have inconsistent cooling rates on the surface and core, resulting in a large difference in hardness between the surface and core of the workpiece during subsequent aging, affecting the overall performance. Therefore, the cooling rate must be ensured when quenching the workpiece to avoid excessive decomposition of the supersaturated solid solution during the quenching process.

[0137] During the solution quenching process, rapid cooling leads to uneven temperature distribution, forming large quenching residual stress inside the material. The residual stress not only affects the performance of alloy structural parts, but also causes dimensional instability during the machining of parts. Under the premise of ensuring the mechanical properties of the structural parts, the water temperature is generally controlled to reduce the residual stress of the product and avoid quenching cracking. For large and complex structural parts, a small amount of cold pressing plastic deformation can redistribute the residual stress inside the component, which is an effective way to reduce quenching residual stress.

[0138] The above is a detailed introduction to a method for improving the mechanical properties of the core of a 7A04 thick-walled part provided by the present invention. In this article, specific examples are used to illustrate the principles and implementation methods of the present invention. The description of the above embodiments is only used to help understand the method and its core ideas of the present invention, including the best mode, and also enables any technician in the field to practice the present invention, including the manufacture and use of any device or system, and the implementation of any combined method. It should be pointed out that for ordinary technicians in this technical field, without departing from the principles of the present invention, the present invention can also be improved and modified in a number of ways, and these improvements and modifications also fall within the scope of protection of the claims of the present invention. The scope of patent protection of the present invention is defined by the claims and may include other embodiments that can be thought of by those skilled in the art. If these other embodiments have structural elements that are not different from the literal expression of the claims, or if they include equivalent structural elements that are not substantially different from the literal expression of the claims, then these other embodiments should also be included in the scope of the claims.

Claims

1. A method for improving the mechanical properties of the core of a 7A04 aluminum alloy thick-walled part, characterized in that: The following steps are involved: 1) After the forged free forging is subjected to solid solution treatment, it is quickly transferred to a low-temperature water bath for water quenching to obtain a water-quenched 7A04 forging; The time of the rapid transfer is less than 10s; The temperature of the low temperature water bath is 5 to 20°C; 2) The 7A04 forging obtained in the above steps is treated with liquid nitrogen to obtain a quenched 7A04 aluminum alloy forging.

2. The preparation method according to claim 1, characterized in that: The 7A04 aluminum alloy, measured by mass percentage, comprises: Si: ≤0.5%; Fe: ≤0.5%; Cu: 1.2%~2.0%; Mn: 0.2%~0.6%; Mg: 1.8%~2.8%; Cr：0.1％～0.25％； Zn: 5.0%~7.0%; Ti: ≤0.1%; The balance is Al.

3. The preparation method according to claim 1, characterized in that: The wall thickness of the 7A04 aluminum alloy thick-walled part is 100 to 180 mm; The 7A04 aluminum alloy thick-walled parts include one or more of rings, forged plates, forged cakes, high-cylinder rings, flat rings and special-shaped parts.

4. The preparation method according to claim 1, characterized in that: The forging method includes four piers and three draws; The temperature of the solution treatment is 465-485°C; The time of the solution treatment is 330 to 390 minutes.

5. The preparation method according to claim 1, characterized in that: The low-temperature water bath comprises a low-temperature water bath with ice cubes added; The cooling rate of the water quenching is 200-300°C / s; The water quenching time is greater than or equal to 15 minutes.

6. The preparation method according to claim 1, characterized in that: The liquid nitrogen treatment method includes placing liquid nitrogen in a container where the 7A04 forging is placed.

7. The preparation method according to claim 1, characterized in that: The liquid nitrogen treatment time is greater than or equal to 15 minutes; The temperature of the 7A04 forging after the liquid nitrogen treatment is -196 to -160°C.

8. The preparation method according to claim 1, characterized in that: The liquid nitrogen treatment further includes an artificial aging step.

9. The preparation method according to claim 8, characterized in that: The temperature of artificial aging is 160-170°C; The artificial aging time is 570 to 630 minutes.

10. The preparation method according to claim 1, characterized in that: The quenched 7A04 aluminum alloy forging is specifically a 7A04 aluminum alloy forging to be subsequently machined.

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