Method for regulating low-melting phase content and improving mechanical properties of 2618 aluminum alloy

Abstract

The present application belongs to the technical field of aluminum alloy heat treatment, and particularly relates to a method for regulating the content of low-melting point phases in 2618 aluminum alloy and improving mechanical properties, the alloy composition by mass percentage being as follows: Cu: 1.9-2.7, Si: 0.1-0.25, Mg: 1.3-1.8, Mn: 0.02-0.04, Fe: 0.9-1.3, Ti: 0.04-0.1, Ni: 0.9-1.2, Zn < 0.10, single impurity element < 0.05, total impurity elements < 0.10, and the balance being Al. The preparation method comprises gradient homogenization annealing, low-temperature reverse extrusion, gradient solid solution treatment, straightening and aging treatment. The preparation method can make the low-melting point phases fully dissolve, the high-temperature strengthening phase Al9FeNi spheroidize and the size < 11 microns, and the final mechanical properties are as follows: tensile strength >= 460 MPa, yield strength >= 430 MPa and elongation >= 12%.

Description

Technical Field

[0001] This invention relates to a method for controlling the content of low-melting-point phases and improving the mechanical properties of 2618 aluminum alloy, belonging to the field of aluminum alloy heat treatment technology. Background Technology

[0002] 2618 aluminum alloy is a heat-treatable wrought aluminum alloy with good high-temperature strength and heat resistance. It is widely used in aerospace, automotive, and other fields, such as aircraft engine components and high-temperature environment parts in the automotive industry. This alloy grade mainly contains the following phases: Al2CuMg, Al2Cu, Al9FeNi, Mg2Si, and Al7Cu2Fe. Al2CuMg, Al2Cu, and Al9FeNi are all strengthening phases in the material, which can improve the alloy's strength and high-temperature performance through precipitation strengthening and dispersion strengthening. Al2CuMg and Al2Cu phases are low-melting-point phases and are prone to overheating during homogenization and solution treatment, affecting the final mechanical properties of the material.

[0003] During solidification, aluminum alloys often form dendritic structures, and alloying elements undergo severe segregation. Numerous non-equilibrium eutectic structures are frequently present at grain boundaries, significantly impacting the machinability and final mechanical properties of the alloy billet. To improve the final properties of the material and reduce internal structural defects, an optimal heat treatment method is needed: homogenization annealing to fully dissolve the internal low-melting-point phases into the matrix; solution treatment to form a supersaturated solid solution; and aging to precipitate fine strengthening phases. Patent CN110872674A proposes a two-stage homogenization treatment suitable for aluminum-copper alloys. The heat treatment method involves a first-stage homogenization temperature of 470-490℃ for 8-60 hours, and a second-stage homogenization temperature of 495-513℃ for 10-40 hours. The process involves water cooling, air cooling, or furnace cooling to room temperature to gradually dissolve the low-melting-point non-equilibrium eutectic phases, avoiding overheating. Patent CN110273116A proposes a process method to improve the solution treatment effect of 2618 aluminum alloy. The main content includes the use of new processes such as isothermal solution treatment, heated solution treatment, and cyclic cooling-heating solution treatment to gradually increase the solution temperature and solution treatment effect of 2618 aluminum alloy. This involves holding at 535℃ for 60–180 minutes, then heating to 547℃ for heated solution treatment, followed by cyclic cooling-heating solution treatment within the 547–535℃ range to ensure the alloy does not overheat. However, neither of these methods effectively dissolves the low-melting-point phase to improve the material's room temperature mechanical properties or reduce internal structural defects. Summary of the Invention

[0004] In order to better dissolve low-melting-point phases, improve the room-temperature mechanical properties of materials, and reduce internal structural defects, this invention provides a method for controlling the content of low-melting-point phases in 2618 aluminum alloy and improving its mechanical properties.

[0005] To fully dissolve the low-melting-point phase, this invention is achieved through the following technical solution:

[0006] Step 1: Alloy composition control. Using Al ingots, Mg ingots, aluminum-iron master alloys, aluminum-titanium master alloys, aluminum-nickel master alloys, and aluminum-copper master alloys as raw materials, smelting is carried out to obtain 2618 aluminum alloy melt. The smelting temperature is 750℃, and the holding time is 15 minutes, resulting in a cast 2618 alloy billet. The alloy composition is controlled as follows (mass percentage): Cu: 1.9-2.7, Si: 0.1-0.25, Mg: 1.3-1.8, Mn: 0.02-0.04, Fe: 0.9-1.3, Ti: 0.04-0.1, Ni: 0.9-1.2, Zn < 0.10, individual impurity element < 0.05, total impurity element < 0.10, balance being Al.

[0007] Step 2: Gradient homogenization annealing treatment. The as-cast 2618 alloy billet is heated in a furnace to 470-495℃ at a rate of 5℃ / min and held for 10-30h. During this process, the Al2CuMg phase dissolves back. Then, the temperature is increased to 515-525℃ at a rate of 2℃ / min and held for 15-25h. During this process, 90% of the Al2Cu phase dissolves back. Then, the temperature is increased to 535-540℃ at a rate of 1℃ / min and held for 3-5h. During this process, the remaining Al2Cu phase further dissolves back, and the material does not overheat. Because most of the Al2Cu phase is dissolved back in the second homogenization stage, the overheating temperature of the Al2Cu phase is increased. Therefore, the material will not overheat during the high-temperature treatment in the third stage. Finally, the billet is cooled to room temperature with water mist. Water mist cooling can suppress grain coarsening and avoid the precipitation of a large amount of phase during cooling, preparing for subsequent high-temperature solid solution treatment.

[0008] Step 3: Cutting. Cut the sample after Step 2 to the appropriate length according to the extrusion requirements.

[0009] Step 4: Low-temperature reverse extrusion. After the treatment in step 3, the short ingots of uniform length are placed in a tunnel furnace and slowly heated for 24 hours. When the material temperature is between 380℃ and 400℃, it is held for more than 4 hours, and then reverse extrusion is carried out with an extrusion ratio of 6-30.

[0010] Step 5: Gradient solution treatment. The sample treated in Step 4 is heated in a vertical furnace to 510-530℃ at a rate of 5℃ / min and held for 1-5 hours. Then, the temperature is increased to 540-548℃ at a rate of 1℃ / min and held for 1-3 hours, followed by rapid water quenching. Samples of the solution-treated material are scanned to determine the approximate content of low-melting-point phases. Metallographic analysis is performed on the sample to observe overheating. Using a magnification of 200x as an example, metallographic observations are conducted at 5-20 different locations. If black pores are found, the material is considered overheated.

[0011] Step 6: Cold deformation. The sample after step 5 is straightened. The straightening deformation is controlled between 3-5%. The straightened sample is then cut off at both ends.

[0012] Step 7: Artificial aging. The samples treated in Step 6 are subjected to under-aging treatment in a horizontal furnace at an aging temperature of 200℃ for 3-20 hours, and then removed and air-cooled. After aging, spheroidized Al9FeNi phase with a size <11μm was observed in the 2618 aluminum alloy. The final mechanical properties are tensile strength ≥460MPa, yield strength ≥430MPa, and elongation ≥12%.

[0013] Beneficial effects

[0014] The three-stage gradient homogenization annealing treatment employed in this invention, compared to the two-stage homogenization treatment method described in patent CN110872674A, can better dissolve the low-melting-point Al2Cu phase and effectively eliminate the network structure, improving the microhardness and element diffusion uniformity of the alloy, thus ensuring improved alloy performance. The content of the low-melting-point phase limits the temperature during subsequent solution heat treatment. The homogenization treatment involved in this invention fully dissolves the Al2CuMg phase in the first stage, dissolves approximately 90% of the Al2Cu phase in the second stage, and further dissolves the remaining Al2Cu phase in the third stage. Furthermore, due to the preparatory work in the second stage, the melting point of the Al2Cu phase is increased, so the high temperature in the third stage will not cause overheating. Finally, after homogenization treatment, the Al2Cu phase content in the material is ≤3%.

[0015] The gradient solution treatment method employed in this invention is more suitable for industrial production than the isothermal solution treatment, heating solution treatment, and cyclic cooling-heating solution treatment methods involved in patent CN110273116A. These methods place extremely high demands on the heating furnace and are complex to operate, failing to meet the requirements of rapid, convenient, and highly effective treatment needed in conventional industrial production. The gradient solution treatment method used in this invention can dissolve the incompletely dissolved phases and the phases precipitated after homogenization due to cooling into the matrix to form a supersaturated solid solution. It allows for more precise control of the nucleation and growth process of the precipitated phases, resulting in finer and more uniformly distributed precipitates, promoting grain refinement, thereby improving the toughness and plasticity of the alloy, and more effectively releasing residual stress generated during aluminum alloy processing, thus improving its dimensional stability.

[0016] The invention employs a three-stage homogenization + gradient solution heat treatment method. The former effectively dissolves low-melting-point phases, promotes full diffusion of alloying elements, eliminates compositional segregation, and improves microstructure uniformity. The latter facilitates industrial production, allowing excess phases to fully dissolve into the solid solution, ultimately improving the metal's plasticity, machinability, and room-temperature mechanical properties. Attached Figure Description

[0017] Figure 1 This is a scanned image of step (2) in Example 1;

[0018] Figure 2 EDS diagram of Example 1;

[0019] Figure 3 This is a scanned image of step (2) in Example 2;

[0020] Figure 4 EDS diagram of Example 2;

[0021] Figure 5 This is a scanned image of step (2) in Example 3;

[0022] Figure 6 EDS diagram of Example 3;

[0023] Figure 7 The scan image of step (1) of Comparative Example 1;

[0024] Figure 8 This is the EDS plot for Comparative Example 1;

[0025] Figure 9 The scan image is from step (1) of Comparative Example 2;

[0026] Figure 10 This is the EDS plot for Comparative Example 2;

[0027] Figure 11 The scanned image of step (1) in Comparative Example 3;

[0028] Figure 12 This is the EDS plot for Comparative Example 3;

[0029] Figure 13 The scan image is from step (2) of Comparative Example 4;

[0030] Figure 14 The EDS plot is shown in Comparative Example 4;

[0031] Figure 15 The image shown is the unburned metallographic image from Example 1.

[0032] Figure 16 The image shown is the unfired metallographic image from Example 2.

[0033] Figure 17 The image shown is the unfired metallographic image from Example 3.

[0034] Figure 18 The metallographic diagram of overburning in Comparative Example 1;

[0035] Figure 19 The metallographic diagram of overburnt metal in Comparative Example 2;

[0036] Figure 20 The metallographic diagram of overburnt metal in Comparative Example 3;

[0037] Figure 21 The metallographic diagram of overburnt metal in Comparative Example 4;

[0038] Figure 22 Metallographic image of Al9FeNi before homogenization treatment in Example 2;

[0039] Figure 23 Metallographic image of Al9FeNi after homogenization treatment in Example 2;

[0040] Figure 24 The image shows the metallographic diagram of the finished Al9FeNi product from Example 2. Detailed Implementation

[0041] The present invention will now be described in detail with reference to specific implementation examples. The present invention tests the tensile strength, yield strength, and elongation of each embodiment and comparative example. The test method for tensile strength is GB / T228.1-2021; the test method for yield strength is GB / T228.1-2021; and the test method for elongation is GB / T228.1-2021.

[0042] Example 1

[0043] This invention uses 2618 aluminum alloy billet, whose composition by mass percentage is as follows: Cu: 2.445, Si: 0.128, Mg: 1.62, Mn: 0.022, Fe: 1.135, Ti: 0.06, Ni: 1.077, Zn < 0.10, individual impurity element < 0.05, total impurity element < 0.10, and the balance is Al.

[0044] A method for controlling the content of low-melting-point phases and improving the mechanical properties of 2618 aluminum alloy includes the following steps:

[0045] (1) Alloy composition control: Al ingots, Mg ingots, aluminum-iron master alloys, aluminum-titanium master alloys, aluminum-nickel master alloys, and aluminum-copper master alloys were used as raw materials and smelted in a crucible resistance furnace to obtain 2618 aluminum alloy melt. The smelting temperature was 750℃ and the holding time was 15min.

[0046] (2) 2618 aluminum alloy billet was heated in a furnace to 470℃ at a heating rate of 5℃ / min and held for 30 hours, then heated to 515℃ at a heating rate of 2℃ / min and held for 20 hours, and then heated to 530℃ at a heating rate of 1℃ / min and held for 5 hours. Finally, the billet after the three-stage homogenization treatment was cooled to room temperature with water mist. Samples of the homogenized material were then scanned. Figure 1As can be seen, the Al2CuMg phase has been completely re-dissolved, and the Al2Cu phase content is <7%.

[0047] (3) Cut the processed billet into short pieces of equal length of 540mm. Place the short pieces into a tunnel furnace and slowly heat them for 24 hours. When the material temperature is between 380℃ and 400℃, keep them at that temperature for more than 4 hours and then perform reverse extrusion at a speed of 5.5mm / s and an extrusion ratio of 22.

[0048] (4) The treated semi-finished product was subjected to gradient solution treatment in a vertical T4 furnace. The specific method was as follows: the temperature was increased to 525℃ at a rate of 5℃ / min and held for 3 hours, then increased to 545℃ at a rate of 1℃ / min and held for 2 hours, followed by rapid water quenching. A sample scan of the solution-treated material revealed that the Al2Cu phase content was <5% ( Figure 2 Metallographic examination revealed no overheating. For example, at a magnification of 200x, metallographic observations at 5-20 different locations showed no black pores, confirming that the material was not overheated. Figure 15 ).

[0049] (5) The processed semi-finished product is cold-deformed to eliminate the internal stress generated by the round bar. The cold deformation amount is 5%.

[0050] (6) Cut off the head and tail of the semi-finished product after processing, leaving a finished product length of 2700mm, and then send the finished product into a horizontal furnace for under-aging treatment at 200℃ / 6h.

[0051] The finished product in this embodiment has a tensile strength of 453 MPa, a yield strength of 427 MPa, and an elongation of 10%.

[0052] Example 2

[0053] A method for controlling the content of low-melting-point phases and improving the mechanical properties of 2618 aluminum alloy includes the following steps:

[0054] (1) Alloy composition control: Al ingots, Mg ingots, aluminum-iron master alloys, aluminum-titanium master alloys, aluminum-nickel master alloys, and aluminum-copper master alloys were used as raw materials and smelted in a crucible resistance furnace to obtain 2618 aluminum alloy melt. The smelting temperature was 750℃ and the holding time was 15min.

[0055] (2) 2618 aluminum alloy billet was heated in a furnace to 480℃ at a heating rate of 5℃ / min and held for 20h, then heated to 520℃ at a heating rate of 2℃ / min and held for 15h, and then heated to 530℃ at a heating rate of 1℃ / min and held for 4h. Finally, the billet after the three-stage homogenization treatment was cooled to room temperature with water mist. Samples of the homogenized material were then scanned. Figure 3As can be seen, the Al2CuMg phase has been completely re-dissolved, and the Al2Cu phase content is ≤3%.

[0056] (3) Cut the processed billet into short pieces of equal length of 540mm. Place the short pieces into a tunnel furnace and slowly heat them for 24 hours. When the material temperature is between 380℃ and 400℃, keep them at that temperature for more than 4 hours and then perform reverse extrusion at a speed of 5.5mm / s and an extrusion ratio of 22.

[0057] (4) The treated semi-finished product was subjected to gradient solution treatment in a vertical T4 furnace. The specific method was as follows: the temperature was increased to 525℃ at a rate of 5℃ / min and held for 3 hours, then increased to 545℃ at a rate of 1℃ / min and held for 2 hours, followed by rapid water quenching. A sample scan of the solution-treated material revealed that the Al2Cu phase content was <1% ( Figure 4 Metallographic examination revealed no overheating. For example, at a magnification of 200x, metallographic observations at 5-20 different locations showed no black pores, confirming that the material was not overheated. Figure 16 ).

[0058] (5) The processed semi-finished product is cold-deformed to eliminate the internal stress generated by the round bar. The cold deformation amount is 5%.

[0059] (6) Cut off the head and tail of the semi-finished product after processing, leaving a finished product length of 2700mm, and then send the finished product into a horizontal furnace for under-aging treatment at 200℃ / 6h.

[0060] The finished product in this embodiment has a tensile strength of 461 MPa, a yield strength of 438 MPa, and an elongation of 12%. Metallographic analysis of the finished product shows that the Al9FeNi phase size is <11 μm. Figure 24 ), Figure 22 The image shows the metallographic structure of Al9FeNi before homogenization treatment. Figure 23 The image shows the metallographic structure of Al9FeNi after homogenization treatment. As can be seen from the image, the particle size of Al9FeNi is significantly reduced after homogenization treatment.

[0061] Example 3

[0062] This invention uses 2618 aluminum alloy billet, whose composition by mass percentage is as follows: Cu: 2.445, Si: 0.128, Mg: 1.62, Mn: 0.022, Fe: 1.135, Ti: 0.06, Ni: 1.077, Zn < 0.10, individual impurity element < 0.05, total impurity element < 0.10, and the balance is Al.

[0063] A method for controlling the content of low-melting-point phases and improving the mechanical properties of 2618 aluminum alloy includes the following steps:

[0064] (1) Alloy composition control: Al ingots, Mg ingots, aluminum-iron master alloys, aluminum-titanium master alloys, aluminum-nickel master alloys, and aluminum-copper master alloys were used as raw materials and smelted in a crucible resistance furnace to obtain 2618 aluminum alloy melt. The smelting temperature was 750℃ and the holding time was 15min.

[0065] (2) 2618 aluminum alloy billet was heated in a furnace at a heating rate of 5℃ / min to 495℃ and held for 10 hours, then heated at a heating rate of 2℃ / min to 525℃ and held for 10 hours, and then heated at a heating rate of 1℃ / min to 530℃ and held for 3 hours. Finally, the billet after the three-stage homogenization treatment was cooled to room temperature with water mist. Samples of the homogenized material were then scanned. Figure 5 As can be seen, the Al2CuMg phase has been completely re-dissolved, and the Al2Cu phase content is <6%.

[0066] (3) Cut the processed billet into short pieces of equal length of 540mm. Place the short pieces into a tunnel furnace and slowly heat them for 24 hours. When the material temperature is between 380℃ and 400℃, keep them at that temperature for more than 4 hours and then perform reverse extrusion at a speed of 5.5mm / s and an extrusion ratio of 22.

[0067] (4) The treated semi-finished product was subjected to gradient solution treatment in a vertical T4 furnace. The specific method was as follows: the temperature was increased to 525℃ at a rate of 5℃ / min and held for 3 hours, then increased to 545℃ at a rate of 1℃ / min and held for 2 hours, followed by rapid water quenching. A sample scan of the solution-treated material revealed that the Al2Cu phase content was <4% ( Figure 6 Metallographic examination revealed no overheating in the material. For example, at a magnification of 200x, metallographic observations at 5-20 different locations showed no black pores, confirming that the material was not overheated. Figure 17 ).

[0068] (5) The processed semi-finished product is cold-deformed to eliminate the internal stress generated by the round bar. The cold deformation amount is 5%.

[0069] (6) Cut off the head and tail of the semi-finished product after processing, leaving a finished product length of 2700mm, and then send the finished product into a horizontal furnace for under-aging treatment at 200℃ / 6h.

[0070] The finished product in this embodiment has a tensile strength of 442 MPa, a yield strength of 411 MPa, and an elongation of 12%.

[0071] Comparative Example 1

[0072] Alloy composition control: Al ingots, Mg ingots, aluminum-iron master alloys, aluminum-titanium master alloys, aluminum-nickel master alloys, and aluminum-copper master alloys are used as raw materials. The mass percentage composition is as follows: Cu: 2.3, Si: 0.1, Mg: 1.55, Mn: 0.022, Fe: 1.11, Ti: 0.06, Ni: 1.12, Zn < 0.10, individual impurity element < 0.05, total impurity element < 0.10, balance being Al.

[0073] (1) Using 2618 aluminum alloy billet, the billet was heated to 465℃ in a furnace at a heating rate of 5℃ / min and held for 24h. The treated billet was then cooled to room temperature by water mist. Sampling and scanning of the treated material revealed residual Al2CuMg and Al2Cu phases. Figure 7 ).

[0074] (2) Cut the processed billet into short pieces of equal length of 540mm. Place the short pieces into a tunnel furnace and slowly heat them for 24 hours. When the material temperature is between 380℃ and 400℃, keep them warm for more than 4 hours and then perform reverse extrusion at a speed of 5.5mm / s and an extrusion ratio of 22.

[0075] (3) The semi-finished product after treatment was solution treated in a vertical T4 furnace. The solution treatment process was to hold at 535℃ for 3 hours, followed by rapid water quenching. Sampling and scanning of the solution-treated material revealed that the Al2Cu phase content was >15% ( Figure 8 Metallographic examination revealed that the material showed signs of overheating. For example, at a magnification of 200x, observing the metallographic patterns at 5-20 different locations revealed numerous black pores, indicating that the material was overheated. Figure 18 ).

[0076] (4) The processed semi-finished product is cold-deformed to eliminate the internal stress generated by the round bar. The cold deformation amount is 5%.

[0077] (5) Cut off the head and tail of the semi-finished product after processing, leaving a finished product length of 2700mm, and then send the finished product into a horizontal furnace for under-aging treatment at 200℃ / 6h.

[0078] The finished product of this comparative example has a tensile strength of 437 MPa, a yield strength of 392 MPa, and an elongation of 9%.

[0079] Comparative Example 2

[0080] Alloy composition control: Al ingots, Mg ingots, aluminum-iron master alloys, aluminum-titanium master alloys, aluminum-nickel master alloys, and aluminum-copper master alloys are used as raw materials. The mass percentage composition is as follows: Cu: 2.3, Si: 0.1, Mg: 1.55, Mn: 0.022, Fe: 1.11, Ti: 0.06, Ni: 1.12, Zn < 0.10, individual impurity element < 0.05, total impurity element < 0.10, balance being Al.

[0081] (1) Using 2618 aluminum alloy billet, the billet was heated to 465℃ and held for 10h in a heating furnace at a heating rate of 5℃ / min, then heated to 490℃ and held for 12h at a heating rate of 2℃ / min, and finally the homogenized billet was cooled to room temperature by water mist. Sampling and scanning of the homogenized material revealed that there was almost no residue of the Al2CuMg phase, and the residual amount of the Al2Cu phase was >13% ( Figure 9 ).

[0082] (2) Cut the processed billet to obtain short pieces of equal length of 540mm. Put the short pieces into the tunnel furnace and slowly heat them for 24 hours. When the material temperature is between 380℃ and 400℃, keep them at that temperature for more than 4 hours and then perform reverse extrusion at a speed of 5.5mm / s and an extrusion ratio of 22.

[0083] (3) The semi-finished product after treatment was solution treated in a vertical T4 furnace. The solution treatment process was to hold at 535℃ for 3 hours, followed by rapid water quenching. Sampling and scanning of the solution-treated material revealed that the Al2Cu phase content was <11% ( Figure 10 Metallographic examination revealed that the material showed signs of overheating. For example, at a magnification of 200x, observing the metallographic patterns at 5-20 different locations revealed numerous black pores, indicating that the material was overheated. Figure 19 ).

[0084] (4) The processed semi-finished product is cold-deformed to eliminate the internal stress generated by the round bar. The cold deformation amount is 5%.

[0085] (5) Cut off the head and tail of the semi-finished product after processing, leaving a finished product length of 2700mm, and then send the finished product into a horizontal furnace for under-aging treatment at 200℃ / 6h.

[0086] The finished product of this comparative example has a tensile strength of 440 MPa, a yield strength of 408 MPa, and an elongation of 9.5%.

[0087] Comparative Example 3

[0088] Alloy composition control: Al ingots, Mg ingots, aluminum-iron master alloys, aluminum-titanium master alloys, aluminum-nickel master alloys, and aluminum-copper master alloys are used as raw materials. The mass percentage composition is as follows: Cu: 2.3, Si: 0.1, Mg: 1.55, Mn: 0.022, Fe: 1.11, Ti: 0.06, Ni: 1.12, Zn < 0.10, individual impurity element < 0.05, total impurity element < 0.10, balance being Al.

[0089] (1) Using 2618 aluminum alloy billet, the billet was heated to 465℃ and held for 10h at a heating rate of 5℃ / min in a heating furnace, then heated to 490℃ and held for 12h at a heating rate of 2℃ / min, and finally the homogenized billet was cooled to room temperature by water mist. Sampling and scanning of the homogenized material revealed that there was almost no residue of the Al2CuMg phase, and the residual amount of the Al2Cu phase was >13% ( Figure 11 ).

[0090] (2) Cut the processed billet into short pieces of equal length of 540mm. Place the short pieces into a tunnel furnace and slowly heat them for 24 hours. When the material temperature is between 380℃ and 400℃, keep them warm for more than 4 hours and then perform reverse extrusion at a speed of 5.5mm / s and an extrusion ratio of 22.

[0091] (3) The treated semi-finished product was subjected to gradient solution treatment in a vertical T4 furnace. The specific method was to heat it to 500℃ at a heating rate of 5℃ / min and hold it for 3 hours, then heat it to 535℃ at a heating rate of 1℃ / min and hold it for 2 hours, followed by rapid water quenching. Sampling and scanning of the solution-treated material revealed that the Al2Cu phase content was <9% ( Figure 12 Metallographic examination revealed that the material showed signs of overheating. For example, at a magnification of 200x, observing the metallographic patterns at 5-20 different locations revealed numerous black pores, indicating that the material was overheated. Figure 20 ).

[0092] (4) The processed semi-finished product is cold-deformed to eliminate the internal stress generated by the round bar. The cold deformation amount is 5%.

[0093] (5) Cut off the head and tail of the semi-finished product after processing, leaving a finished product length of 2700mm, and then send the finished product into a horizontal furnace for under-aging treatment at 200℃ / 6h.

[0094] The finished product of this comparative example has a tensile strength of 446 MPa, a yield strength of 419 MPa, and an elongation of 11%.

[0095] Comparative Example 4

[0096] This invention uses 2618 aluminum alloy billet, whose composition by mass percentage is as follows: Cu: 2.445, Si: 0.128, Mg: 1.62, Mn: 0.022, Fe: 1.135, Ti: 0.06, Ni: 1.077, Zn < 0.10, individual impurity element < 0.05, total impurity element < 0.10, and the balance is Al.

[0097] A method for controlling the content of low-melting-point phases and improving the mechanical properties of 2618 aluminum alloy includes the following steps:

[0098] (1) Alloy composition control: Al ingots, Mg ingots, aluminum-iron master alloys, aluminum-titanium master alloys, aluminum-nickel master alloys, and aluminum-copper master alloys were used as raw materials and smelted in a crucible resistance furnace to obtain 2618 aluminum alloy melt. The smelting temperature was 750℃ and the holding time was 15min.

[0099] (2) Using 2618 aluminum alloy billet, heat it to 480℃ in a heating furnace at a heating rate of 5℃ / min and hold for 30h, then heat it to 500℃ at a heating rate of 3℃ / min and hold for 20h. Finally, cool the homogenized billet to room temperature with water mist. Samples of the homogenized material are then scanned. Figure 13 As can be seen, the Al2CuMg phase has been completely re-dissolved, and the Al2Cu phase content is <5%.

[0100] (3) Cut the processed billet into short pieces of equal length of 540mm. Place the short pieces into a tunnel furnace and slowly heat them for 24 hours. When the material temperature is between 380℃ and 400℃, keep them at that temperature for more than 4 hours and then perform reverse extrusion at a speed of 5.5mm / s and an extrusion ratio of 22.

[0101] (4) The semi-finished product after treatment was subjected to gradient solution treatment in a vertical T4 furnace. The specific method was as follows: the extruded rod was subjected to isothermal solution treatment at 535℃ for 90 min; then the 2618 aluminum alloy that had undergone isothermal solution treatment was heated to 547℃ for 10 min, then cooled to 535℃ for 10 min, then heated to 547℃ for 10 min, then cooled to 535℃ for 10 min, then heated to 547℃ for 10 min, and finally quenched in water. The material after solution treatment was sampled and scanned, and it was found that the Al2Cu phase content was <3% ( Figure 14 Metallographic examination revealed overheating. For example, at 200x magnification, observing the metallographic patterns at 5-20 different locations and finding black pores indicates overheating of the material. Figure 21 ).

[0102] (5) The processed semi-finished product is cold-deformed to eliminate the internal stress generated by the round bar. The cold deformation amount is 5%.

[0103] (6) Cut off the head and tail of the semi-finished product after processing, leaving a finished product length of 2700mm, and then send the finished product into a horizontal furnace for under-aging treatment at 200℃ / 6h.

[0104] The tensile strength of the finished product in this embodiment is 443 MPa, the yield strength is 421 MPa, and the elongation is 10%. The room temperature mechanical properties of the embodiment and the comparative example are shown in Table 1.

[0105] Table 1. Room temperature mechanical properties of extruded bars after T6.

[0106]

[0107] It is understood that the present invention has been described through some embodiments, and those skilled in the art will recognize that various changes or equivalent substitutions can be made to these features and embodiments without departing from the spirit and scope of the invention. Furthermore, under the teachings of the present invention, these features and embodiments can be modified to adapt to specific situations and materials without departing from the spirit and scope of the invention. Therefore, the present invention is not limited to the specific embodiments disclosed herein, and all embodiments falling within the scope of the claims of this application are within the protection scope of the present invention.

Claims

1. A method for controlling the content of low-melting-point phases and improving the mechanical properties of 2618 aluminum alloy, characterized in that, Includes the following steps: (1) Alloy composition control: The mass percentage of alloy composition is controlled as follows: Cu: 1.9-2.7, Si: 0.1-0.25, Mg: 1.3-1.8, Mn: 0.02-0.04, Fe: 0.9-1.3, Ti: 0.04-0.1, Ni: 0.9-1.2, Zn < 0.10, individual impurity element < 0.05, total impurity element < 0.10, and the balance is Al; (2) Alloy gradient homogenization annealing treatment: Cooling after three-stage gradient homogenization annealing treatment; The steps of three-stage gradient homogenization annealing treatment are: heating to 470-495℃ at a rate of 5℃ / min and holding for 10-30h, heating to 515-525℃ at a rate of 2℃ / min and holding for 15-25h, heating to 535-540℃ at a rate of 1℃ / min and holding for 3-5h. (3) Extrusion: The alloy is heated to 380℃-400℃ and held for more than 4 hours, and then reverse extrusion is carried out with an extrusion ratio of 6-30. (4) Gradient solution treatment: The sample is subjected to two-stage solution treatment and quenched in water; two-stage gradient solution treatment: the temperature is raised to 510-530℃ at a rate of 5℃ / min and held for 1-5h, and then raised to 540-548℃ at a rate of 1℃ / min and held for 1-3h. (5) Cold deformation: The deformation amount is 3-5%; (6) Artificial aging: hold at 200℃ for 3-20h; after underaging treatment, air cool to obtain 2618 aluminum alloy with low melting point phase content and improved mechanical properties.

2. The method according to claim 1, characterized in that, In step (1), the alloy uses Al ingots, Mg ingots, aluminum-iron master alloys, aluminum-titanium master alloys, aluminum-nickel master alloys, and aluminum-copper master alloys as raw materials, and is smelted in a crucible resistance furnace to obtain 2618 aluminum alloy melt. The smelting temperature is 750℃ and the holding time is 15min.

3. The method according to any one of claims 1-2, characterized in that, After aging, the Al9FeNi phase in the 2618 aluminum alloy becomes spheroidized with a size of <11μm, and its mechanical properties are tensile strength ≥460MPa, yield strength ≥430MPa, and elongation ≥12%.

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