Method for enhancing obdurability of aluminum-copper alloy

Abstract

The invention discloses a method for enhancing the obdurability of an aluminum-copper alloy, and relates to the technical field of aluminum-copper alloy materials, and the method specifically comprises the following steps: S1, preparation of an aluminum-copper alloy melt: after an oxide layer on the surface of industrial pure aluminum is polished and removed, the industrial pure aluminum and an Al-50Cu alloy are put into a graphite crucible together, the graphite crucible is heated to 750 DEG C, and an Al-5Cu alloy melt is obtained after the industrial pure aluminum and the Al-50Cu alloy are completely molten; s2, primary refining and degassing; s3, intermediate alloy refining treatment is conducted, specifically, an Al-3Ti-4.35 La intermediate alloy is added into the refined melt, full stirring is conducted to enable the intermediate alloy to be evenly dispersed, heat preservation is conducted for 15 min at the temperature of 750 DEG C, and refined melt is obtained; s4, secondary refining; s5, casting molding; s6, customized heat treatment is conducted, specifically, an Al-5Cu as-cast sample is put into a muffle furnace to be subjected to two-stage solution treatment, and then water quenching is conducted immediately to the room temperature; and then the sample subjected to water quenching is placed in a constant-temperature drying box to be subjected to aging treatment, and the target Al-5Cu alloy is obtained. Through cooperation of the intermediate alloy and a specific process, comprehensive enhancement of the obdurability and the corrosion resistance of the aluminum-copper alloy is achieved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of aluminum-copper alloy materials, and particularly relates to a method for enhancing the strength and toughness of an aluminum-copper alloy through an intermediate alloy in cooperation with a specific refining process and customized heat treatment. BACKGROUND

[0002] Aluminum-copper alloy (such as Al-5Cu alloy) is a typical high-strength cast aluminum alloy, and is widely used in the fields of aerospace and automobile industry due to its high strength, good plasticity and toughness, and excellent process performance. The strength and toughness (the synergistic performance of strength and plasticity) is a core index for determining the service reliability, and mainly depends on the microstructure characteristics, including the grain size, the distribution morphology of the second phase (such as Al2Cu phase), and the precipitation state of the strengthening phase (such as θ' phase and θ'' phase).

[0003] At present, the strength and toughness of the aluminum-copper alloy is mainly improved by micro-alloying or single heat treatment, but there are the following limitations: (1) limited grain refinement effect: when the traditional single Ti or B element is used for refinement, the TiAl3 phase is easy to be coarse, resulting in uneven refinement, and the grain size is more than 200 μm. The coarse grains can significantly reduce the plasticity of the alloy, and easily cause stress concentration, leading to early fracture; (2) serious second phase segregation: Al2Cu phase as the main second phase is easy to segregate along the grain boundaries during solidification to form a continuous network structure, which can cut the matrix and cause the plasticity of the alloy to decrease sharply, and the strength and toughness are difficult to be improved simultaneously; (3) insufficient precision of strengthening phase regulation: in the traditional single-stage solid solution + aging treatment, the supersaturated solid solution is easy to directly precipitate coarse sheet-shaped θ phase (equilibrium phase), and the θ' phase (metastable strengthening phase) has low precipitation density and uneven distribution, which cannot effectively play a strengthening role, resulting in limited strength improvement; (4) poor process synergy: in the existing technology, the grain refinement and heat treatment process are independently designed, and the synergy is not formed, so it is difficult to realize the integrated strength and toughness improvement of “grain refinement-second phase regulation-strengthening phase optimization”.

[0004] Therefore, it is very important to develop a method for realizing the grain refinement, uniform distribution of the second phase and optimized precipitation of the strengthening phase through multi-process synergy, so as to enhance the strength and toughness of the aluminum-copper alloy. SUMMARY

[0005] In view of the insufficient strength and toughness of the aluminum-copper alloy, the present application provides a method for enhancing the strength and toughness of the aluminum-copper alloy through an integrated technical scheme of “intermediate alloy refinement + specific process synergy heat treatment”, and the core is to realize the grain refinement and second phase regulation by introducing the Al-Ti-La intermediate alloy, and to optimize the precipitation of the strengthening phase by combining the double-stage solid solution + aging treatment, so as to finally realize the significant improvement of the strength and toughness, which is especially suitable for the strength and toughness improvement of Al-5Cu alloy, and can be widely applied in the fields of aerospace, automobile industry and other fields with high requirements for the strength and toughness of the material.

[0006] To achieve the above object, the technical scheme adopted by the present application is as follows: The present application provides a method for enhancing the strength and toughness of aluminum-copper alloy, specifically comprising the following steps: S1: aluminum-copper alloy melt preparation: after the surface oxide layer of industrial pure aluminum is polished and removed, the aluminum-copper alloy is loaded into a graphite crucible together with Al-50Cu alloy, and is heated to 750 DEG C in a silicon-carbon rod resistance furnace, and after complete melting, an Al-5Cu alloy melt is obtained; S2: primary refining and degassing: 0.2wt.% of C2Cl6 is added to the Al-5Cu alloy melt for refining and degassing, until the white smoke is completely dispersed, the gas and impurities in the melt are removed, and a refined melt is obtained; S3: intermediate alloy refining treatment: Al-3Ti-4.35La intermediate alloy is added to the refined melt, and the intermediate alloy is uniformly dispersed by sufficient stirring, and the melt is refined and the second phase is controlled by the synergistic effect of Ti and La elements at 750 DEG C for 15 min, to obtain a refined melt; S4: secondary refining: 0.2wt.% of C2Cl6 is added to the refined melt for secondary refining, to further purify the melt and ensure that the intermediate alloy fully interacts with the matrix, to obtain a secondary refined melt; S5: pouring and forming: the secondary refined melt is taken out of the crucible, and after slagging, when the melt is cooled to 730 DEG C, it is poured into a steel mold preheated to 200 DEG C, and naturally cooled to room temperature, to obtain an Al-5Cu as-cast sample; S6: customized heat treatment: the Al-5Cu as-cast sample is placed in a muffle furnace for two-stage solid solution treatment, and then immediately water quenched; the water quenched sample is placed in a constant temperature drying box for aging treatment, to obtain the target Al-5Cu alloy.

[0007] Further, in the step S2, the stirring rate is 200 r / min, and the stirring time is 5 min; in the step S3, the stirring rate is 300 r / min, and the stirring time is 10 min.

[0008] Further, in the step S3, the addition amount of the Al-3Ti-4.35La intermediate alloy is 0.3wt.%.

[0009] Further, in the step S5, the specifications of the steel film are: inner diameter 120 mm, outer diameter 140 mm, and height 210 mm; the inner wall of the steel mold is polished.

[0010] Further, in the step S6, the two-stage solid solution treatment is specifically as follows: the Al-5Cu as-cast sample is placed in a muffle furnace, heated to 505 DEG C and kept for 2 hours, and then heated to 535 DEG C and kept for 8 hours.

[0011] Further, the rate of heating to 505 DEG C and heating from 505 DEG C to 535 DEG C is 7 DEG C / min.

[0012] Further, in the step S6, the water quenching is specifically: rapidly water quenching after solid solution, and keeping the supersaturated solid solution.

[0013] Further, in the step S6, the aging treatment is specifically: placing the water quenched sample in a constant temperature drying box, and continuously keeping at 170 DEG C for 8 hours, so that the theta' strengthening phase is uniformly precipitated.

[0014] Compared with the prior art, the beneficial effects of the present application are: The present application significantly enhances the strength and toughness of the aluminum copper alloy through the synergistic effect of "Al-3Ti-4.35La intermediate alloy + specific refining process + customized heat treatment", and the specific effects are as follows: (1) Grain refinement and synergistic improvement of strength and toughness: In the Al-3Ti-4.35La intermediate alloy, the TiAl3 particles formed by Ti element are heterogeneous nucleation cores, which can effectively refine the grain; La element as a rare earth element, forms a composition undercooling zone at the front of the solid-liquid interface, and inhibits the grain growth, and the two synergistically refines the aluminum copper alloy grain size from 206 μm to 112 μm (refining 45.6%), while increasing the proportion of high angle grain boundaries, reducing residual stress, and synchronously improving the strength and toughness. In the as-cast state, the tensile strength of the alloy is 199.5±5.1 MPa (increased by 39.4%), and the elongation is 16.4±0.7% (increased by 215.3%); after heat treatment, the tensile strength is 398.9±3.1 MPa (increased by 15.8%), and the elongation is 13.4±0.4% (increased by 123.3%).

[0015] (2) Uniform distribution of second phase, eliminating the performance short board: The La element in the intermediate alloy has strong interface adsorption, which can preferentially adsorb on the grain boundary growth front of Al2Cu phase, inhibit its continuous growth and segregation along the grain boundary, and make the Al2Cu phase change from the traditional network grain boundary distribution to the dispersed distribution in the grain and the grain boundary (size 2-5 μm (statistical results by Image J software)), effectively avoiding the stress concentration and plasticity decrease caused by the segregation of the second phase. At the same time, the uniformly distributed Al2Cu phase can further improve the strength of the alloy through "dispersion strengthening". Figure 5

[0016] ​(3) Precise control of the strengthening phase to maximize the strengthening effect: The two-stage solution treatment (505℃×2h+535℃×8h) achieved a full and uniform solid solution of Cu, laying the foundation for the precipitation of a high-density strengthening phase during subsequent aging; the aging process of 170℃×8h precisely controlled the type of precipitated phase, promoting the preferential precipitation of the thermodynamically metastable θ' phase (rather than the coarsened θ phase) from the supersaturated solid solution, and the θ' phase was uniformly distributed in the α-Al matrix (density reaching 1.2×10). 15 pcs / m 3 The alloy strength is significantly improved through the "dislocation hindering" mechanism.

[0017] (4) Simultaneous improvement in corrosion resistance, expanding service range: Refined grains (reducing grain boundary corrosion channels) and a uniformly distributed second phase (avoiding the formation of local microcells) synergistically reduce corrosion susceptibility. In 3.5% NaCl solution, the alloy corrosion rate decreased from 178.5 mg·m⁻¹. -2 ·h -1 Decreased to 119.0476 mg·m -2 ·h -1 (Reduced by 33.3%), the self-corrosion potential increased from -1.1949V to -1.1231V, and the self-corrosion current density increased from 56.23μA·cm. -2 Reduced to 25.95 μA·cm -2 The corrosion resistance is significantly improved.

[0018] In summary, this invention achieves comprehensive enhancement of the strength, toughness, and corrosion resistance of aluminum-copper alloys through the synergy of intermediate alloys and specific processes, making it suitable for industrial fields that require high-performance aluminum-copper alloys. Attached Figure Description

[0019] Figure 1 This is a detailed DSC thermal analysis curve of the Al-5Cu alloy; Figure 2 This is a detailed schematic diagram of the heat treatment process for Al-5Cu alloy; Figure 3 These are statistical charts of the alloy microstructure and grain size. (ac) represents the OM (Organic Microstructure) diagrams of the Al-5Cu, Al-5Cu+0.3wt.%Al-5Ti-1B, and Al-5Cu+0.3wt.%Al-3Ti-4.35La alloys, respectively; (df) represents the EBSD-IPF (Electronic Ecosystem Degradation and Interchange Function) diagrams of the Al-5Cu, Al-5Cu+0.3wt.%Al-5Ti-1B, and Al-5Cu+0.3wt.%Al-3Ti-4.35La alloys, respectively; and (gi) represents the statistical chart of the average grain size of the Al-5Cu, Al-5Cu+0.3wt.%Al-5Ti-1B, and Al-5Cu+0.3wt.%Al-3Ti-4.35La alloys, respectively. Figure 4 are the comparison of misorientation angle and kernel average misorientation maps of adjacent grain boundaries of Al-5Cu alloy with and without Al-Ti-La addition, wherein, (a-c) are the comparison of kernel average misorientation maps of Al-5Cu, Al-5Cu+0.3wt.%Al-5Ti-1B, Al-5Cu+0.3wt.%Al-3Ti-4.35La alloys in as-cast state, respectively; (d-f) are the statistics of high angle grain boundaries (HAGBs) of Al-5Cu, Al-5Cu+0.3wt.%Al-5Ti-1B, Al-5Cu+0.3wt.%Al-3Ti-4.35La alloys in as-cast state, respectively; Figure 5 are the scanning topography maps of samples after adding different intermediate alloys, wherein, (a-c) are the scanning topography maps of Al-5Cu, Al-5Cu+0.3wt.%Al-5Ti-1B, Al-5Cu+0.3wt.%Al-3Ti-4.35La alloys, respectively; (a1-c1) are the enlarged views of the red dashed curve parts in (a-c), respectively; (d, e) are the EDS area scan analysis of Al and Cu elements in (a1), respectively; (f) is the XRD pattern of Al-5Cu alloy; Figure 6 are the SEM topography maps of refined Al-5Cu alloy samples after heat treatment, wherein, (a-c) are the SEM topography maps of Al-5Cu, Al-5Cu+0.3wt.%Al-5Ti-1B, Al-5Cu+0.3wt.%Al-3Ti-4.35La alloys, respectively, (a1-c1) are the enlarged views of the dashed line parts in (a-c), respectively; in the figure, Point 1-Point 3 are the equilibrium phase CuAl2 precipitated after heat treatment; Figure 7 are the fracture topography maps of refined Al-5Cu alloy samples before and after heat treatment, wherein, (a-b) are the fracture topography maps of Al-5Cu, Al-5Cu+0.3wt.%Al-3Ti-4.35La before heat treatment (Cast state), respectively, (c-d) are the fracture topography maps of Al-5Cu, Al-5Cu+0.3wt.%Al-3Ti-4.35La after heat treatment (AHT state), respectively, in the figure, Clravage plane represents cleavage platform, Crack represents crack, Tearing ridge represents tearing ridge, Dimples represents dimples; Figure 8are fracture side section views of Al-5Cu alloy samples before and after heat treatment, wherein (a-b) are respectively fracture side section views of Al-5Cu, Al-5Cu+0.3wt.%Al-3Ti-4.35La before heat treatment (Cast state), (c-d) are respectively fracture side section views of Al-5Cu, Al-5Cu+0.3wt.%Al-3Ti-4.35La after heat treatment (AHT state); in the figure, yellow mark A represents intergranular fracture, and yellow mark B represents transgranular fracture; Figure 9 are TEM images of Al2Cu phase in Al-5Cu alloy with and without Al-Ti-La master alloy, wherein (a) is a TEM image of Al-5Cu alloy; (b) is an HRTEM image of the red dashed line part in (a); (c) is an enlarged view of the blue dashed line part in (b); (d) is a diffraction pattern of the Al2Cu phase in (a); (e) is a TEM image of Al-5Cu+0.3wt.%Al-Ti-La alloy; (f) is an HRTEM image of the red dashed line part in (e); (g) is an enlarged view of the blue dashed line part in (f); (h) is a diffraction pattern of the Al2Cu phase in (e); Figure 10 are TEM images of Ti2Al 20 La phase in Al-3Ti-4.35La master alloy refined Al-5Cu alloy, wherein (a) is a TEM image of Ti2Al 20 La phase; (b) is an enlarged view of the red dashed line part in (a); (c), (f) are respectively TEM-EDS point scanning results at Point 1, Point 2 in (b); (d) is a TEM-EDS area scanning result in (b); (e) is a diffraction pattern of the purple dashed line area in (b); Figure 11 are TEM and diffraction pattern images of θ' phase distribution and precipitated Al2Cu phase in Al-5Cu alloy with and without Al-Ti-La master alloy under AHT state, wherein (a) is θ' phase distribution of Al-5Cu alloy without Al-Ti-La master alloy under AHT state; (b) is TEM image of Al2Cu phase and Ti2Al 20 La phase in Al-5Cu alloy without Al-Ti-La master alloy under AHT state; (c), (d) are EDS area scanning results of Al, Cu elements in (b); (e) is a TEM image of θ' phase in Al-5Cu alloy with Al-Ti-La master alloy under AHT state; (f), (g) are enlarged views of the red dashed line part in (e); (h) is a TEM image of Al2Cu phase and Ti2Al 20TEM image of La phase; (i-l) are EDS area scan results of Al, Cu, Ti, La elements in (f), respectively; (a1, e1) are diffraction pattern images of θ' phase in (a, e), respectively; Figure 12 is a macroscopic model diagram of θ'-Al2Cu phase diffusion in the matrix; Figure 13 is a schematic diagram of a standard tensile specimen size; Figure 14 is a mechanical property diagram of the refined Al-5Cu alloy sample after heat treatment, wherein (a) is a tensile result curve diagram of the sample; (b) is a mechanical property columnar statistical diagram, wherein UTS (ultimate tensile strength), YS (yield strength), and elongation are shown; (c) is an alloy hardness statistical diagram; and (d) is a mechanical property comparison diagram of the present application and other 2-series aluminum alloys (such as 2219, 2319, Al-3.5Cu, Al-4.5Cu-5TiB2, etc.); Figure 15 is a corrosion morphology diagram of the Al-Ti-La alloy added and not added in the 3.5% NaCl solution after immersion for 24 hours, 96 hours, and 168 hours, respectively, in the AHT state, wherein (a, d, g) is Al-5Cu; (b, e, h) is Al-5Cu+0.3wt.% Al-5Ti-1B; and (c, f, i) is Al-5Cu+0.3wt.% Al-3Ti-4.35La; wherein Point 1-Point 12 correspond to SEM-EDS point analysis in (a-i), respectively; Figure 16 is a corrosion resistance performance of the Al-Ti-La alloy added and not added in the 3.5% NaCl solution in the AHT state, wherein (a) is a corrosion weight loss curve; and (b) is a corrosion rate change curve of the alloy under different periods; Figure 17 is an electrochemical performance test Tafel curve diagram of the Al-5Cu alloy refined sample after heat treatment, wherein (a) is a Tafel curve of the Al-5Cu, Al-5Cu+0.3wt.% Al-5Ti-1B, and Al-5Cu+0.3wt.% Al-3Ti-4.35La alloy in the AHT state; and (b) is an enlarged diagram of the pink dashed line part in (a). DETAILED DESCRIPTION

[0020] In order to make the purpose and advantages of the present application more clear and explicit, the present application is further described in detail below in combination with embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application, and are not used to limit the present application.

[0021] The instruments, reagents, materials, etc. involved in the following examples, if not specifically stated, are conventional instruments, reagents, materials, etc. already existing in the prior art, which can be obtained through regular commercial channels. The experimental methods, detection methods, etc. involved in the following examples, if not specifically stated, are conventional experimental methods, detection methods, etc. already existing in the prior art.

[0022] In this embodiment, the experimental materials and instruments are as follows: Materials: industrial pure aluminum (99.7%), Al-50Cu alloy, Al powder (99.0wt.%,

[0023] Instruments: SG2-7.5-12 type silicon-carbon rod resistance furnace, KSL-1400X type muffle furnace, DGG-9146A type constant temperature drying oven.

[0024] Preparation Example 1 An Al-3Ti-4.35La intermediate alloy was prepared by in-situ reaction method of aluminum melt: first, Al powder (99.0wt.%, 80-100µm) and Ti powder (99.0wt.%, 45-65µm) were mixed in a molar ratio of Al:Ti=3:1, and mixed by a high-speed planetary ball mill at a speed of 160r / min for 3.5h. The uniformly mixed Al-Ti powder was pressed into a cylindrical preform block under the action of a universal test tensile-compression machine. The corresponding industrial pure aluminum was loaded into a graphite crucible and placed in a medium-frequency induction furnace, and the voltage of the medium-frequency induction furnace was gradually increased. The ring-shaped mold (size: inner diameter 45mm, outer diameter 70mm, height 70mm) was placed in a thermostatic drying oven at 200℃. When the melt temperature reached 1150℃, the furnace cover was half-opened to avoid oxidation and temperature runaway, and the dried Al-Ti preform block and La block were added to the bottom of the Al liquid and stirring was started. At a high temperature of 1150℃, the Al-Ti block and La block quickly participate in the reaction of the Al liquid, and after 3.5min of heat preservation, the melt temperature is lowered, and when it is lowered to 870℃, it is poured into a ring-shaped steel mold. The pouring process should be kept stable and fast to make it naturally cool to room temperature in the mold, and an Al-Ti-La intermediate alloy sample is obtained.

[0025] Example 1 The embodiment provides a method for enhancing the strength and toughness of an aluminum-copper alloy, which specifically comprises the following steps: S1: Preparation of aluminum-copper alloy melt: Selecting industrial pure aluminum (purity ≥ 99.7%) and Al-50Cu intermediate alloy (containing Cu 50wt.%) as raw materials; polishing the industrial pure aluminum to completely remove the surface oxide layer (thickness about 5-10 μm) to avoid the introduction of oxidized impurities into the melt; weighing the industrial pure aluminum and Al-50Cu alloy according to the composition ratio of Al-5Cu alloy (Cu content 5wt.%), loading into a graphite crucible (purity ≥ 99.9%, to avoid impurity pollution), placing in a silicon-carbon rod resistance furnace, heating at a rate of 10℃ / min to 750℃, and keeping for 30 min to completely melt the raw materials to form a uniform Al-5Cu alloy melt; S2: Primary refining and degassing: adding 0.2wt.% of C2Cl6 (based on the total mass of the melt) as a refining agent to the Al-5Cu alloy melt, with a stirring rate of 200r / min, and continuously stirring for 5 min to make C2Cl6 fully dispersed and react with H2, O2 and other gases in the melt (generating AlCl3, CO2 and other volatile products); keeping the furnace temperature at 750℃ during refining until the white smoke (volatile products) on the surface of the melt completely disperses, ensuring that the gas and impurity removal rate is ≥ 90%, and obtaining the refined melt; S3: Intermediate alloy refining treatment: adding 0.3wt.% of Al-3Ti-4.35La intermediate alloy, using a graphite stirring rod at a rate of 300r / min for 10 min to make the intermediate alloy completely dissolved and uniformly dispersed; after stirring, keeping at 750℃ for 15 min to refine the grains by forming TiAl3 heterogeneous nucleation core with Ti element, and to increase the composition undercooling degree by the enrichment of La element at the solid-liquid interface front to inhibit grain growth, obtaining the refined melt; S4: Secondary refining: adding 0.2wt.% of C2Cl6 again in the refined melt for secondary refining, repeating the stirring and holding operation of S2 (stirring rate 200r / min, holding at 750℃ until the white smoke disperses), further removing trace amounts of gas and undissolved impurities that may be introduced due to the addition of intermediate alloy, ensuring the purity of the melt, and obtaining the secondary refined melt; S5: Pouring and forming: taking out the secondary refined melt with the crucible, and using a graphite skimmer to remove the dross on the surface of the melt (removal rate ≥ 95%); when the melt naturally cools to 730℃, pouring it into a preheated steel mold (steel mold material is 45# steel, specifications: inner diameter 120mm x outer diameter 140mm x height 210mm, inner wall polished to reduce casting surface defects) at a uniform speed (pouring rate 50-60mL / s), and naturally cooling to room temperature to obtain the as-cast Al-5Cu sample; Using differential scanning calorimetry (DSC) to perform thermal analysis on the refined Al-5Cu alloy sample to determine the melting temperature of the low-melting eutectic phase of the alloy and to develop a heat treatment process scheme.Figure 1 To refine the DSC thermal analysis curves of the Al-5Cu alloy samples, from... Figure 1 As can be seen from the curve, the first peak occurs at 540.05℃, the second peak occurs at 567.76℃, and the dissolution peak of the α-Al matrix is ​​at 635.13℃. According to the empirical formula (1): T 固溶 =0.9~0.95T m (1) The first-stage solution temperature of the alloy should not exceed 513℃, and the second-stage solution temperature should not exceed 539.4℃. Therefore, the two-stage solution temperature is determined to be 505℃ + 535℃ to avoid overheating. To obtain better mechanical properties, a T6 aging treatment process (170℃ × 8h) is adopted, with the heat treatment regime as follows: Figure 2 As shown. The alloys before and after heat treatment are labeled Cast and AHT, respectively, for subsequent analysis and characterization studies.

[0026] S6: Customized heat treatment: A customized heat treatment process of "two-stage solution treatment + water quenching + aging" is performed, as detailed below: Two-stage solution treatment: The as-cast sample was placed in a muffle furnace and heated to 505°C at a rate of 7°C / min, and held for 2 hours (to fully dissolve the low-melting-point Al2Cu phase); then the temperature was increased to 535°C at a rate of 7°C / min and held for 8 hours (to promote the uniform dissolution of Cu elements into the Al matrix and form a supersaturated solid solution).

[0027] (2) Water quenching: After the solid solution treatment is completed, the sample is immediately immersed in 40℃ water for quenching. The cooling rate is ≥100℃ / s to suppress the precipitation of Cu element in the solid solution and retain the supersaturated state.

[0028] (3) Aging treatment: The water-quenched sample was placed in a constant temperature drying oven and kept at 170℃ for 8 hours to promote the precipitation of high-density, fine and uniform θ' strengthening phase (1.84nm (statistical results by Image J software)) in the supersaturated solid solution and to inhibit the coarsening of the θ phase (equilibrium phase).

[0029] Comparative Example 1 Except for step S3, which does not involve adding the Al-3Ti-4.35La master alloy, the remaining steps are the same as in Example 1.

[0030] Comparative Example 2 The difference from Example 1 is that the intermediate alloy added in step S3 is Al-5Ti-1B.

[0031] The compositions of the samples from Example 1 and Comparative Examples 1-2 are shown in Table 1: Table 1 Sample Composition

[0032] Experimental Example 1 1. Microstructure characterization After heat treatment, 10 mm x 10 mm x 10 mm samples were cut from the bottom of the sample, polished, and then etched with Keller reagent for 40 times of wiping on the sample surface. A Bruker D8 X-ray diffractometer (XRD) was used to analyze the phase of the sample and observe the change of the alpha-Al lattice constant during heat treatment, with a 2 theta scanning range of 10°-90°; Zeiss LSM800 optical microscope (OM) and Quanta FEG-450 scanning electron microscope (SEM) were used to observe and analyze the microstructure morphology, fracture morphology and corrosion morphology of the alloy, and FEI Talos F200X transmission electron microscope (TEM) produced by American FEI Company was used for in-depth observation of the microstructure morphology; the grain size, large-angle grain boundary and kernel average misorientation of the sample were analyzed by backscattered electron diffraction technology (EBSD) of Cess Gemini300 thermal field emission electron microscope. The results are as follows: (1) Microstructure characterization of Al-5Cu alloy refined by different intermediate alloys Figure 3 and Table 2 are the microstructure diagrams, EBSD-IPF diagrams, grain size distribution diagrams and grain size statistics table of Al-5Cu alloy before and after refinement. From Figure 3 (a-c) and Table 2, it can be seen that the microstructure of Al-5Cu alloy without intermediate alloy treatment has coarse grains, and the average grain size is 206±70µm. The macrostructure of the alloy treated by 0.3wt.% Al-3Ti-4.35La intermediate alloy has significantly smaller grains, and the average grain size is 112±51µm, as shown in Figure 3 (d-f). Compared with the untreated alloy, the average grain size of alpha-Al is reduced by 44.5%. It is worth noting that after adding Al-5Ti-1B intermediate alloy, the grain size is refined to 57±15µm, because the TiB2 particles contained in this intermediate alloy can act as heterogeneous nucleation core of alpha-Al matrix, increase the number of nucleation sites in the alpha-Al matrix, refine the grains and inhibit the formation of dendrites, thereby effectively refining the grains. Related studies have shown that Ti2Al 20 La phase in Al-3Ti-4.35La intermediate alloy does not have special coherent relationship with alpha-Al matrix, and it does not have obvious interaction with alpha-Al matrix, and the crystal face of Ti2Al 20 La phase has multiple matching relationships with the alpha-Al crystal face, among which (001) Al / / (001)Ti2Al20La , (111) Al / / (001) Ti2Al20La ,

[110] Al / /

[001] Ti2Al20La The mismatch degree δ of Ti2Al 20 La phase can also act as heterogeneous nucleation core of α-Al matrix, but due to the large size of Ti2Al 20 La phase, its fine-grain effect is not superior to that of adding Al-5Ti-1B master alloy in macroscopic performance. But in subsequent characterization analysis, we observed that it had a profound impact on the second phase Al2Cu, so that in the subsequent mechanical property and corrosion resistance tests, the sample added with Al-3Ti-4.35La master alloy showed more excellent results.

[0033] Table 2 Grain size statistics table

[0034] Figure 4 The statistical results and kernel average misorientation maps of high-angle grain boundaries (HAGBs) in the as-cast refined Al-5Cu alloy are shown in Figure 4 (d-f), with the addition of Al-3Ti-4.35La master alloy, the HAGBs in the alloy increased from 77.4% to 84.6%, which was attributed to the grain refinement and microstructure improvement of Al-3Ti-4.35La master alloy. High-angle grain boundaries have high energy at the interface, which can effectively hinder the movement of dislocations and play a key role in improving the mechanical properties of the alloy. Although the addition of Al-5Ti-1B master alloy can make the grain transform into equiaxed grains, and the proportion of high-angle grain boundaries is slightly higher than that of the sample added with Al-3Ti-4.35La master alloy, which is related to the better fine-grain effect shown in Figure 3 (b); but by observing Figure 4 (a), (b), in the KAM map of Al-5Cu alloy and Al-5Ti-1B master alloy, a larger green area is clearly visible, indicating that there is a larger stress concentration in the sample without adding Al-3Ti-4.35La master alloy, while Figure 4 (c) the green area is significantly reduced, the stress concentration is significantly reduced and significantly alleviated. The results show that the addition of Al-3Ti-4.35La master alloy not only helps to achieve grain refinement and ensure the proportion of favorable high-angle grain boundary distribution, but also can improve the release of residual stress, thereby effectively improving the mechanical properties of the alloy, and the final result is better than that of the sample added with Al-5Ti-1B master alloy.

[0035] Figure 5 The scanning morphology of Al-5Cu alloy samples under different refining processes is shown. The black and gray phase is the dendritic α-Al matrix phase, and the Figure 5 The bright white network phase in (a1) is analyzed by EDS, as shown in Figure 5 (d) and (e), the results show that the phase is mainly composed of Al and Cu elements, combined with Figure 5 the XRD pattern of (f), there is an obvious Al2Cu diffraction peak in the figure, so it is judged that the phase is Al2Cu phase. Since the sub-eutectic addition amount of Cu in this experiment is 5%, which is much lower than the eutectic point of Al-Cu alloy 33.2%, therefore α-Al becomes the main matrix phase of the alloy, and a large amount of Al atoms are consumed with the continuous growth of α-Al, and Cu atoms are continuously enriched in the front of the solidification interface. When the content of Cu atoms in the residual liquid reaches 33.2%, the eutectic reaction (formula 2) occurs, and finally Al2Cu phase is formed at the grain boundary.

[0036] (2) The Al2Cu phase in the Al-5Cu alloy without adding intermediate alloy is in the form of continuous mesh, and the grain boundary is thick, so its mechanical properties are poor. Figure 5 As shown in (b), although part of the Al2Cu phase is broken into short rod-shaped after adding Al-5Ti-1B intermediate alloy, but a relatively thick network structure is generated in the local, which produces a large stress concentration, which also confirms the Figure 4 the green area in (b) is distributed in a large area. However, with the addition of Al-3Ti-4.35La intermediate alloy, not only the grain is refined, but also the breaking effect of Al2Cu phase is significantly improved, which improves the morphology of Al2Cu phase, making Al2Cu phase discontinuous and finer, showing a near-spherical and small block structure in a dispersed distribution state, greatly reducing the stress concentration phenomenon of the alloy, thereby the performance of the alloy is obviously improved.

[0037] (2) Microstructure of Al-5Cu alloy after heat treatment Figure 6 The scanning morphology of Al-5Cu alloy samples under different refining processes is shown. Al-5Cu alloy is subjected to double-stage solid solution treatment of 505℃×2h+535℃×8h, and then the Cu atoms in the alloy will be dissolved into the α-Al matrix to form a supersaturated solid solution, and then the T6 aging treatment process 170℃×8h will precipitate second phase particles from the matrix. Figure 6As shown in (a) and (a1), a large number of second-phase particles appear at the grain boundaries and are relatively large in size, while only a very small number of short rod-shaped or worm-shaped second-phase particles precipitate in the matrix. Since the energy of Cu atoms in the Al-5Cu alloy is higher than that at the grain boundaries, Cu atoms will spontaneously move towards the grain boundaries to reduce their own surface energy, resulting in the precipitation of Al2Cu phase particles near the grain boundaries. Figure 6 (c) and (c1) are SEM images of the Al-5Cu+Al-3Ti-4.35La alloy sample after heat treatment, and are consistent with... Figure 6 Compared to (a) and (b), more spherical or near-spherical second-phase Al2Cu particles precipitated in the matrix, and more small-sized spherical particles precipitated inside the grains, according to the bypass mechanism: (3) In formula (3), G is the shear modulus and b is the Burgers vector. That is, the strengthening mechanism is inversely proportional to the interparticle spacing of the second phase; the more particles in the second phase, the stronger the strengthening effect.

[0038] The addition of Al-3Ti-4.35La master alloy has an excellent grain refining effect, causing the second phase particles to precipitate in a spherical shape, suppressing the segregation of the second phase particles, and keeping them fine and dispersed in a uniform distribution at the grain boundaries and within the grains. As a result, the heat treatment strengthening effect of Al-5Cu+Al-3Ti-4.35La alloy is significantly improved.

[0039] Table 3 Figure 6 EDS point scan data at various points

[0040] (3) Refinement of Al-5Cu alloy and fracture analysis after heat treatment To investigate the effect of different intermediate alloy addition methods on the fracture type of Al-5Cu alloy, fracture surface analysis was performed, such as... Figure 7 As shown. Figure 7 (a) In the Al-5Cu alloy without the addition of Al-3Ti-4.35La refining agent, large-sized cleavage plateaus and tear ridges appeared, and some cracks were present. Since Cu elements are enriched at the grain boundaries to form a network of Al2Cu phase, this network of second phase is prone to cause crack initiation and propagation at the grain boundaries, resulting in this fracture exhibiting brittle fracture characteristics. Figure 7 (b) shows the fracture surface of the alloy after refinement with the Al-3Ti-4.35La master alloy. Numerous dimples were found, but no large cleavage plateaus were observed. Furthermore, tearing ridges were also observed on the alloy fracture surface. After heat treatment, the cleavage plateaus of the Al-5Cu alloy continued to grow, the cracks deepened, and it still exhibited brittle fracture characteristics, such as... Figure 7(c) shows. After heat treatment, the toughness of the alloy refined by Al-3Ti-4.35La master alloy becomes dense and deep, and the cleavage platform basically disappears, but a large number of tear ridges remain, as shown in Figure 7 (d) shows. The results show that after adding Al-3Ti-4.35La master alloy, the fracture mode of Al-5Cu alloy is changed, the larger size cleavage plane in the as-cast state is converted into smaller size toughness, and the toughness of the fracture is characterized by toughness, and after heat treatment, the toughness is obviously deepened. The fracture mode of Al-5Cu alloy is converted from brittle fracture to ductile fracture, and the plasticity and toughness of the alloy are significantly improved.

[0041] To further determine the transition of the fracture mode, the alloy is characterized by cross-section morphology of the fracture, as shown in Figure 8 As can be seen from Figure 8 (a), the fracture of the untreated Al-5Cu alloy cracks at the grain boundary, and the continuous network Al2Cu phase near the grain boundary is easy to become a crack source during tensile deformation, and some microcracks appear near it, resulting in poor plasticity of the material. As shown in Figure 8 (c), after heat treatment, Al2Cu phase still precipitates a lot at the grain boundary, which makes the grain boundary easy to produce microcracks, and the alloy still cracks along the grain boundary, showing brittle fracture characteristics, and has not been significantly improved. As shown in Figure 8 (b), after adding 0.3wt.% Al-3Ti-4.35La master alloy, due to the refinement of α-Al grains and the improvement of Al2Cu phase morphology, the degree of Al2Cu phase segregation at the grain boundary is reduced, and the stress concentration is reduced, which is consistent with Figure 4 (c) statistics, so the alloy cracks along the grain boundary and slows down when it cracks, and the elongation is significantly improved. As shown in Figure 8 (d), after the refined Al-5Cu alloy is heat treated, since most of the Al2Cu phase precipitates inside the grain, the stress concentration at the grain boundary is improved, so that the number of transgranular fractures at the fracture increases, and the number of intergranular fractures decreases, showing the characteristics of ductile fracture. In summary, the refinement of broken Al2Cu phase and α-Al matrix can suppress the cracking at the grain boundary, so that the elongation of the alloy is significantly improved, which is consistent with Figure 7 the results shown in

[0042] (4) TEM analysis of Al-5Cu alloy before and after refinement and heat treatment Figure 9 TEM, HRTEM and diffraction pattern diagram of Al2Cu phase in Al-5Cu alloy before and after refinement by Al-3Ti-4.35La master alloy. From Figure 9As can be seen in (a), the Al2Cu surface in the untreated alloy is smooth and appears as long rods at the grain boundaries. The coarse Al2Cu phase causes stress concentration when the alloy is subjected to external forces, thus leading to a decrease in the alloy's mechanical properties. From... Figure 9 (b) It was observed that the Al2Cu phase and the α-Al matrix exhibited a non-coherent interface relationship, due to the crystal structure of Al. The structure has a lattice constant a=b=c=0.4041nm, while the Al2Cu phase belongs to the I4 structure in the tetragonal crystal system, with lattice constants a=b=0.606nm and c=0.487nm. Figure 9 (c) is the selected area electron diffraction of the Al2Cu phase on the

[111] zone axis in (a). The high-index crystal plane (111) and the α-Al matrix exhibit (111)θ / / (111)α-Al. The mismatch degree of the matching relationship, with a mismatch degree greater than 6% with the α-Al matrix, indicates a high interfacial energy between the two phases. This makes it prone to crack initiation or becoming a crack propagation path in tensile tests, thus failing to realize the material's mechanical properties. After adding the Al-3Ti-4.35La master alloy, the Al2Cu phase is distributed in a near-spherical, uniform manner on the α-Al matrix, such as... Figure 9 As shown in (d), this distribution reduces the segregation of the brittle phase, decreases stress concentration, and thus improves mechanical properties. Figure 9 (f) is the selected area electron diffraction of the refined Al2Cu phase on the

[001] zone axis. After adding the master alloy, the phase relationship between the Al2Cu phase and the α-Al matrix was not changed. The two phases still maintained an incoherent interface relationship. However, with the addition of the Al-3Ti-4.35La master alloy, the increased grain boundary density and the number of nucleation sites in the α-Al matrix enhanced the mechanical properties of the alloy.

[0043] Figure 10 Ti2Al in Al-5Cu alloy refined from Al-3Ti-4.35La master alloy 20 TEM image of the La phase. Figure 10 (a) Ti2Al in Al-5Cu alloy 20 La phase; Figure 10 (b) is an enlarged view of the red dotted line portion in (a). Figure 10 (c) and (f) are the TEM-EDS point scan spectra of Point1 and Point2 in (b), respectively. The results show that the Al, Ti and La atoms are approximately Al:Ti:La = 20:2:1. Figure 10 (e) represents the area within the pink dashed line in (b). Selected area electron diffraction along the zone axis further confirmed that the phase was Ti₂Al. 20 La particles have a size of approximately 200nm-300nm.Figure 10 (d) TEM-EDS surface scanning of (b), it can be observed that Ti2Al 20 La phase grows and enriches on the surface of Al2Cu phase, which plays a pinning effect on Al2Cu phase, and inhibits the growth of Al2Cu phase, which is due to Ti2Al 20 The interface between La phase and α-Al matrix has a low energy, and the rare earth phase Ti2Al 20 The concentration of dissolved rare earth element La around La is high, so part of the Al2Cu phase is dissolved in Ti2Al 20 La phase and α-Al matrix. At the same time, due to the existence of La-rich layer, it can not only ensure the existence of Ti2Al 20 La phase as a heterogeneous nucleation site of α-Al matrix, further inhibits the diffusion of Cu atoms at the phase boundary, which is conducive to hindering the tendency of Al2Cu phase to grow into a network, thereby breaking it into near-spherical shape and reducing the stress concentration of brittle phase.

[0044] Figure 11 TEM images of Al-5Cu alloy with and without Al-Ti-La master alloy in AHT state, as shown in Figure 11 (a), (e), a large number of needle-like θ' nanophase precipitates are observed along the

[001] θ' crystal band axis after aging treatment. After heat treatment, the density and average length of θ' nanophase precipitates in Al-5Cu alloy and Al-5Cu+0.3wt.%Al-3Ti-4.35La alloy are 2.73×10 -3 nm -2 and 21.6nm. Due to the addition of Al-3Ti-4.35La master alloy, the α-Al matrix is refined and the Ti2Al 20 La phase pinning effect on Al2Cu phase, although it does not change the morphology of θ' nanophase precipitates, but it makes the average length of θ' nanophase precipitates significantly reduced, and the density in bright field image significantly increased. At the same time, no dislocations are observed in both alloys, which is due to the rearrangement of dislocations caused by long-term aging treatment, resulting in the appearance of dislocation disappearance in the matrix. By observing Figure 11 (b), after aging treatment, the equilibrium phase Al2Cu also precipitates at the grain boundary, and its width is still 1μm, which is due to the increase of Cu atom content during aging, and the segregation of Cu atoms at the interface between the matrix and θ' nanophase precipitates, resulting in the appearance of equilibrium phase Al2Cu. This phase appears at the grain boundary, which is easy to produce micro-crack propagation source, leading to the decrease of mechanical properties of the alloy. After adding Al-3Ti-4.35La master alloy, Ti2Al 20The La phase grows on the surface and grain boundary edges of the equilibrium phase Al₂Cu, playing a certain pinning role and effectively inhibiting the further growth of the equilibrium phase Al₂Cu. Its widest point does not reach 500 nm. Meanwhile, Ti₂Al… 20 The La phase also hinders the diffusion of Cu atoms to the grain boundaries, significantly reducing stress concentration at the grain boundaries. Intragranular growth also effectively hinders the movement of some dislocations, thereby significantly improving mechanical properties.

[0045] The strengthening mechanism of Al-5Cu alloys with the introduction of 0.3 wt.% Al-3Ti-4.35La master alloy mainly involves second-phase strengthening, such as... Figure 12 The macroscopic model diagram of θ′-Al₂Cu phase diffusion within the matrix is ​​shown. Figure 12 It is known that during the two-stage solution treatment, Cu atoms diffuse, causing the Al2Cu phase to dissolve and distribute uniformly within the matrix, forming a large amount of dispersed phase. This results in a large number of uniformly dispersed supersaturated solid solutions (SSSS) in the α-Al matrix. With aging, a GP-like region structure first appears within the matrix. After the aging time reaches its peak, the GP-like region structure transforms into a GPⅠ region, resulting in a large number of uniformly distributed θ′ nano-precipitates within the matrix. After aging at 170℃ for 8 hours, a small amount of equilibrium phase Al2Cu precipitates, and Ti2Al is generated at the interface between Al2Cu and α-Al. 20 The La phase, a rare earth phase, is distributed in a small, near-spherical shape within the grain and at grain boundaries.

[0046] In summary, introducing 0.3wt.% Al-3Ti-4.35La master alloy is an effective means to promote the precipitation of the θ′ phase and maintain the dispersion of the Al2Cu phase. As a semi-coherent precipitate, the θ′ phase, according to the bypass mechanism (see formula (3), requires dislocations to bypass the θ′ phase to move, thereby increasing the deformation resistance, hindering dislocation movement and improving the yield strength and tensile strength of the alloy. At the same time, the uniformly distributed θ′ phase avoids the generation of stress concentration, making the deformation process more harmonious, delaying crack initiation, reducing the micro-cell effect during corrosion, delaying the activation of the electrochemical corrosion mechanism, and improving the plasticity and corrosion resistance of the Al-5Cu alloy.

[0047] 2. Mechanical property testing The microhardness of the samples was determined using a Wilson VH1102 microhardness tester. Ten points were tested on each sample, and the average value was taken as the hardness of the sample. Subsequently, the heat-treated samples were machined into 35×7×7mm shapes according to the GBT228.1-2021 standard. 3 "Ox bone" shaped bars, such as Figure 13The tensile test was carried out at room temperature by using a universal tensile testing machine (AG-10TA) with a crosshead speed of 0.2 mm / s. The mechanical properties of the samples were measured by connecting a 25 / 12.5 mm extensometer. The hardness test and tensile test were repeated at least three times for each sample to ensure the accuracy of the data, and the average value was taken as the result parameter.

[0048] Figure 14 Table 4 shows the mechanical properties and microhardness of the Al-5Cu alloy samples refined by adding different master alloys in the Cast state and the AHT state. The results show that compared with the Al-5Ti-1B master alloy, the addition of the Al-3Ti-4.35La master alloy can more effectively improve the mechanical properties of the Al-5Cu alloy. When 0.3wt.% Al-3Ti-4.35La master alloy is added, the ultimate tensile strength, yield strength, elongation and hardness of the alloy are increased from 344.4 MPa, 230.9 MPa, 6.0% and 108.4 HV to 398.9 MPa, 274.0 MPa, 13.4% and 128.6 HV, respectively, which are increased by 15.8%, 18.7%, 123.3% and 18.6%, respectively. This is because the morphology of the precipitated phase θ'-Al2Cu changes from short rod or vermicular to spherical or ellipsoidal, which reduces the cutting effect of the phase at the grain boundary and reduces the stress concentration, thereby greatly improving the mechanical properties. While the addition of 0.3wt.% Al-5Ti-1B master alloy only increases to 381.3 MPa, 258.8 MPa, 11.3% and 118.3 HV.

[0049] In addition, in the tensile sample diagram of the alloy, the Portevin-Le Chatelier (PLC) effect occurs. As shown in Fig. 4, the curve in (a) fluctuates. Related studies have shown that under certain deformation conditions, the plastic flow of metals may produce fluctuations, and the significant feature of this phenomenon is the occurrence of avalanche shear deformation bands with sawtooth-shaped drops. When the alloy exhibits the PLC effect, the dynamic interaction between mobile dislocations, neighboring dislocations and solute atoms intensifies, and the dislocations move violently and weaken instantaneously, resulting in a negative correlation between strain and stress, which manifests as unstable flow on the macroscopic curve. Figure 14

[0050] The Al-5Cu alloy of the present study is compared with other two-series aluminum alloys, as shown in Table 5. Figure 14 ​(d) can be clearly observed that the elongation of the alloy of the present study is increased by about 29.5% than other Al-Cu alloys. This presents the promoting effect of Al-3Ti-4.35La master alloy in improving the plasticity, in addition, the synergistic effect of strength-plasticity of Al-5Cu+Al-3Ti-4.35La alloy is superior to the Al-Cu alloys reported in previous studies, which shows the excellent performance of Al-5Cu+Al-3Ti-4.35La alloy.

[0051] In order to comprehensively evaluate the mechanical properties of the alloy, the mass index function Q (formula 4) of ultimate tensile strength and elongation is introduced to judge the comprehensive performance: Q = UTS + m x log (EL) (4) For two series aluminum alloys, m ≈ 150. After calculation, the Q value of Al-5Cu+Al-3Ti-4.35La alloy is about 567.9 MPa, which has the largest Q value and the best comprehensive performance.

[0052] Table 4 Statistics of mechanical properties of different samples

[0053] 3. Corrosion resistance test The alloy prepared in Example 1 and Comparative Examples 1-2 in AHT state is processed into an alloy sample with a cuboid shape of 10 mm x 10 mm x 2 mm by wire cutting technology, and then the sample is placed in an acetone solution for ultrasonic cleaning for 20 min to remove surface stains. After cleaning the sample, it is polished and weighed with an electronic balance, and then placed in a constant temperature water bath containing a 3.5% NaCl solution, with the temperature set to 25±2℃. The sample is immersed for 24 hours, 96 hours and 168 hours, and the 3.5% NaCl solution is replaced every 24 hours. After each period of immersion, the sample is taken out and placed in an acetone solution for ultrasonic cleaning for 20 min to remove surface corrosion products, and then the sample is dried and stored in a sealed bag under vacuum for subsequent observation. Another group of polished samples are used to determine the Tafel curve by means of a CH Instruments three-electrode electrochemical workstation, with a Pt sheet as the auxiliary electrode, a saturated KCl calomel electrode as the reference electrode, and the potential scanning rate set to 0.001 v·s -1 , and the corrosion medium is still 3.5% NaCl solution.

[0054] (1) Immersion corrosion analysis of different Al-5Cu alloy samples after heat treatment Figure 15The evolution of corrosion morphology of Al-5Cu alloy with and without Al-3Ti-4.35La master alloy after different periods of immersion corrosion in 3.5% NaCl solution after heat treatment is shown. As can be seen from the figure, the deterioration of the three alloy samples is produced by pitting, and the corrosion morphology changes significantly with the extension of the immersion period. After 24 hours of immersion, the size of the pitting pit of the unrefined Al-5Cu alloy is larger than that of the other alloys, and the corrosion degree is more serious, as shown in Figure 15 (a). Continue to increase the immersion period, the alloy cracks along the pitting pit, the corrosion is intensified, the cracking size gradually increases, and the corrosion characteristics show three different significant changes: small size pitting pits are generated on the alloy surface, corrosion products gradually accumulate near the pitting pits and cracks are generated, and the accumulation of corrosion products is intensified and the structure is unstable. As can be seen from Figure 15 (d-f), after 96 hours of immersion, the accumulated corrosion products exhibit obvious crack propagation and structure fracture; when the immersion period is extended to 168 hours, the adhesion between the corrosion products and the α-Al matrix is sharply reduced, and then peeling occurs, as shown in Figure 15 (g-i). Relatively speaking, the sample with Al-3Ti-4.35La master alloy has a relatively complete surface morphology and fewer pitting pit nucleation points in the early stage of immersion corrosion, and with the extension of time, the corrosion products are in a dispersed distribution state and are distributed in a limited space. Although the corrosion product density increases and micro-cracks are generated after 168 hours of long-term immersion, the overall corrosion resistance is significantly improved compared with the other two alloys.

[0055] As can be seen from the SEM-EDS point analysis data table in Table 5, although different alloys show different progressive changes in corrosion morphology, the phase composition remains stable at different periods, indicating that the thermodynamic formation conditions remain unchanged during the overall immersion period. Al and O elements are mainly detected in the corrosion product regions at the edge, bottom and matrix of the pitting pit, indicating that an Al2O3 film is formed on the surface of the alloy. The passivation effect produced by the oxide film can effectively inhibit the increase of the corrosion rate. The consistency of the phase composition across the period proves that the corrosion evolution under the immersion conditions of 3.5% NaCl solution is mainly through the observation of the change of the microstructure rather than the change of the composition.

[0056] In the Al-5Cu alloy, the second phase particle Al2Cu has a higher potential than the α-Al matrix, thus forming a micro-battery with the α-Al matrix as the active anode, leading to preferential dissolution of the α-Al matrix at the anode. Although the Al2O3 film can provide protection, the presence of the Al2Cu phase makes the oxide film discontinuous, thus triggering local corrosion cracking, leading to local instability, which provides nucleation sites for pitting corrosion, thus activating the electrochemical corrosion mechanism through the coupling process of anodic dissolution and cathodic oxygen evolution, and the corrosion process is as follows: Al→Al 3+ +3e (5) O2+H2O+4e→4OH - (6) Provided is Al 3+ and OH - After that, the following reaction occurs under weak alkaline pH conditions: Al 3+ +3OH - →Al(OH)3 (7) Subsequently, Al(OH)3 is dehydrated to form stable Al2O3 and AlO(OH): 2Al(OH)3→Al2O3+H2O (8) Al(OH)3→AlO(OH)+H2O (9) In addition, Cl - not only destroys the surface oxide film of the matrix, but also further reacts with Al(OH)3 to form AlCl3: 3Cl - +Al(OH)3→AlCl3+3OH - (10) Therefore, the corrosion products and oxide films formed on the α-Al matrix cannot protect the matrix from contact with Cl - , thus allowing Cl - to penetrate into the matrix, promoting the decomposition of α-Al into Al 3+ . Further, the reaction of Al 3+ with hydroxide reduces the pH of the solution and increases the acidity, thus exacerbating corrosion, leading to the expansion of the pitting pit to the surrounding and the initiation of cracks. In the Al-5Cu+Al-3Ti-4.35La alloy, due to the presence of the Ti2Al 20 La phase, the spheroidization of the Al2Cu phase makes the continuity of the Al oxide film more complete, thus reducing the contact frequency of the α-Al matrix with Cl - , and further slowing down the corrosion rate of the other two alloys, inhibiting the rapid expansion of the pitting pit to the micro-crack, effectively improving the corrosion resistance of the alloy.

[0057] Table 5 corresponds to Figure 11 Statistical table of SEM-EDS point analysis data for Points 1-12 in the middle.

[0058] The corrosion resistance of Al-Ti-La alloys in the AHT state with and without Al-Ti-La alloys in 3.5% NaCl solution is as follows: Figure 16 . Figure 16 (a) shows the weight loss curves of the alloys after immersion in 3.5% NaCl solution. From the three curves, the weight loss patterns of the three alloy samples in the AHT state are similar, all exhibiting a direct proportional relationship. The weight loss increases significantly with prolonged immersion time. Compared to other periods, the weight loss of the alloys is smaller within the 24-48 hour period. The corrosion rate variation curves of the three alloys in the AHT state for each period are shown below. Figure 16 As shown in (b), the corrosion weight loss rate of the alloys increased rapidly in the initial stage of immersion. With continued immersion, the corrosion rate of each alloy slowed down, becoming less rapid. This is because Al oxides formed on the alloy surface during corrosion, covering a passivation layer and thus reducing the corrosion weight loss rate. After immersion for 168 hours, the Al-5Cu alloy lost 3 mg of weight, with a corrosion rate of 178.5714 mg·m⁻¹. -2 ·h -1 The Al-5Cu+Al-Ti-La alloy exhibited the smallest weight loss (only 2 mg) and the lowest corrosion rate (only 119.0476 mg·m⁻¹). -2 ·h -1 It exhibits good corrosion resistance during immersion, which is attributed to the addition of the Al-Ti-La master alloy, which allows the precipitated phases to be more evenly distributed in the matrix during heat treatment. Figure 6 The results show that the reduction in segregation reduces pitting corrosion, thereby decreasing the degree of corrosion of the alloy when immersed in 3.5% NaCl solution and improving the corrosion resistance of the alloy.

[0059] (2) Electrochemical corrosion analysis of Al-5Cu alloys refined by different intermediate alloys in the AHT state Figure 17 Table 6 shows the Tafel curves and statistical tables of the alloy's self-corrosion potential (Ecorr) and self-corrosion current density (Icorr) under the AHT state, refining the electrochemical performance test results of the Al-5Cu alloy samples. From... Figure 17As can be seen from (a) and Table 6, the self-corrosion potentials of the three alloys before and after adding the Al-3Ti-4.35La intermediate alloy are -1.1949 V, -1.1715 V and -1.1231 V, respectively. The corrosion potential of the Tafel curve moves in the positive coordinate direction. Since the corrosion potential is a thermodynamic parameter, the more positive the corrosion potential, the lower the corrosion tendency of the alloy, the more difficult the alloy is to corrode, and the better the corrosion resistance of the alloy. At the same time, in (a), the corrosion current on the side close to the anode increases sharply, and an obvious inflection point appears, indicating that pitting corrosion occurs in the alloy during the corrosion process, which is consistent with the corrosion morphology evolution shown in (b). Figure 17 Figure 15 As can be seen from Table 6, the self-corrosion current densities of the three alloys are 56.23 μA·cm -2 , 28.47 μA·cm -2 and 25.95 μA·cm -2 , respectively. The self-corrosion current is a kinetic parameter, and the smaller the self-corrosion current, the greater the impedance of the alloy during polarization, and the slower the corrosion rate. After adding the Al-3Ti-4.35La intermediate alloy, the self-corrosion current density of the alloy is the lowest, which also proves that the alloy has better corrosion resistance.

[0060] Table 6 Statistics of self-corrosion potential Ecorr and self-corrosion current density Icorr of the alloy

[0061] Comparative Example 3 Single-stage solid solution treatment: except that the two-stage solid solution in step S6 is replaced by single-stage solid solution (535℃×10h, the heating rate is 7℃ / min), the remaining steps are consistent with Example 1.

[0062] Test results: in the heat-treated state: tensile strength 352.3 MPa, elongation 9.8% (due to insufficient solid solution, the density of the strengthening phase is reduced, and the strength and toughness are lower than those of Example 1).

[0063] Mechanism analysis: In the present application, the Al-5Cu alloy exhibits excellent mechanical properties after adding the Al-3Ti-4.35La intermediate alloy. In order to understand its superior performance, it is particularly important to explore the potential mechanism of its enhancement. For this purpose, the following factors are summarized: (1) Fine grain strengthening The influence of average grain size on the mechanical properties of secondary aluminum alloys is usually reflected by the following Hall-Petch relationship (Formula 11): (11) In the formula, Δσ GB ​The figure represents yield strength, d represents average grain size, k represents Hall-Petch coefficient (k≈60 at room temperature), and σ0 is a constant. The Hall-Petch relationship shows that the finer the average grain size, the higher the yield strength. The addition of Al-3Ti-4.35La master alloy refines the as-cast Al-5Cu alloy (see Table 2 results), reduces casting defects, and the finer grains provide more grain boundaries, hindering dislocation movement and improving the alloy's resistance to deformation during tensile testing, thereby increasing the alloy's tensile strength.

[0064] (2) Second phase reinforcement For binary aluminum alloys, the precipitation of the second phase θ′-Al₂Cu plays a crucial role in improving mechanical properties. The θ′-Al₂Cu phase effectively hinders the movement of dislocations within the grains, thereby enhancing the alloy's yield strength and plasticity. This can be proven using the Orowan mechanism, as shown by the following formula: (12) (13) In the formula, λ is the interparticle spacing of the second phase; φAl2Cu is the volume fraction of the θ′ phase; θ′ is the average radius of the phase; M represents the Taylor factor, which is 3.1 in this invention; G is the shear modulus, which is approximately 27 GPa in Al-5Cu alloy; b corresponds to the Burgers vector, which has a value of approximately 0.286 nm.

[0065] according to Figure 11 Based on the statistical results of (a) and (e), the strengthening contribution of the θ′ phase can be calculated using equations (12) and (13). With the addition of the Al-3Ti-4.35La master alloy, the precipitation density of the θ′-Al2Cu phase increases and the average radius decreases. After calculation, it can be seen that the strengthening effect of the second phase in the Al-5Cu alloy is 198.3 MPa, while the strengthening effect of the second phase in the Al-5Cu+Al-Ti-La alloy is 333.2 MPa, which enhances the strengthening effect of the second phase.

[0066] (3) Solid solution strengthening A significant PLC effect was observed in the CAST state (see...). Figure 14 (a) indicates that during the casting process, Cu not only forms the Al₂Cu phase with Al atoms, but also dissolves a small portion into the matrix. After solution treatment and aging, the Cu atom solid solution induces lattice distortion in the matrix, inhibiting dislocation movement. This phenomenon can be demonstrated using the following formula: (14) In the formula, H is the solid solution strengthening coefficient, which is approximately 30 MPa / at.% in binary aluminum alloys.i represents the concentration of the i-th element. In the present application, the element concentration is calculated by EDS analysis (see Figure 6 ).

[0067] Through the comparison of examples and comparative examples, it can be seen that the present application can significantly improve the strength and toughness and corrosion resistance of aluminum-copper alloy through the synergistic effect of "Al-3Ti-4.35La intermediate alloy + specific refining process + customized heat treatment", and has outstanding technical advantages and industrial application value.

[0068] The above merely describes the preferred embodiments of the present application, and is not intended to limit the present application. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A method for enhancing the strength and toughness of aluminum-copper alloys, characterized in that: Specifically, it includes the following steps: S1: Preparation of aluminum-copper alloy melt: After removing the oxide layer on the surface of industrial pure aluminum by grinding, it is placed together with Al-50Cu alloy into a graphite crucible and heated to 750℃ in a silicon carbide rod resistance furnace. After complete melting, Al-5Cu alloy melt is obtained. S2: Initial refining and degassing: Add 0.2wt.% C2Cl6 to the Al-5Cu alloy melt for refining and degassing, stirring until white smoke is completely released to remove gas and impurities from the melt and obtain the refined melt. S3: Master alloy refining treatment: Add Al-3Ti-4.35La master alloy to the refined melt, stir thoroughly to make the master alloy evenly dispersed, and hold at 750℃ for 15 min to obtain a refined melt; S4: Secondary refining: 0.2 wt.% C2Cl6 is added to the refined melt for secondary refining to obtain a secondary refined melt; S5: Casting: The melt after secondary refining is taken out of the crucible, and after slag removal, when the melt temperature drops to 730°C, it is poured into a steel mold preheated to 200°C and allowed to cool naturally to room temperature to obtain an Al-5Cu as-cast sample. S6: Customized heat treatment: The Al-5Cu as-cast sample is placed in a muffle furnace for two-stage solution treatment, followed by immediate water quenching; the water-quenched sample is then placed in a constant temperature drying oven for aging treatment to obtain the target Al-5Cu alloy.

2. The method for enhancing the strength and toughness of aluminum-copper alloys according to claim 1, characterized in that: In step S2, the stirring rate is 200 r / min and the stirring time is 5 min; in step S3, the stirring rate is 300 r / min and the stirring time is 10 min.

3. The method for enhancing the strength and toughness of aluminum-copper alloys according to claim 1, characterized in that: In step S3, the amount of Al-3Ti-4.35La master alloy added is 0.3 wt.%.

4. The method for enhancing the strength and toughness of aluminum-copper alloys according to claim 1, characterized in that: In step S5, the specifications of the steel mold are: inner diameter 120mm, outer diameter 140mm, and height 210mm; the inner wall of the steel mold is polished.

5. The method for enhancing the strength and toughness of aluminum-copper alloys according to claim 1, characterized in that: In step S6, the two-stage solution treatment specifically involves placing the Al-5Cu as-cast sample into a muffle furnace, heating it to 505°C and holding it there for 2 hours, then continuing to heat it to 535°C and holding it there for 8 hours.

6. The method for enhancing the strength and toughness of aluminum-copper alloys according to claim 4, characterized in that: The heating rate to 505°C and the heating rate from 505°C to 535°C are both 7°C / min.

7. The method for enhancing the strength and toughness of aluminum-copper alloys according to claim 1, characterized in that: In step S6, water quenching specifically involves: rapid water quenching after solid solution treatment to retain the supersaturated solid solution.

8. The method for enhancing the strength and toughness of aluminum-copper alloys according to claim 1, characterized in that: In step S6, the aging treatment specifically involves placing the water-quenched sample in a constant temperature drying oven and maintaining it at 170°C for 8 hours.

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