A high-elongation cast Al-Cu-Zn-Mg alloy with good thermal cracking resistance and a preparation method thereof

Abstract

This invention discloses a high-elongation cast Al-Cu-Zn-Mg alloy with good resistance to hot cracking. The alloy composition, by mass percentage, includes: Cu 3.5–4.5%, Mg 0.4–1.5%, Mn 0.2–0.5%, Ti 0.10–0.2%, Zn 2.5–3.5%, with the balance being Al and unavoidable impurity elements. Addressing the limitations of existing Al-Cu casting alloys, such as wide solidification temperature range, poor fluidity, and hot cracking, this invention provides a high-Zn-content Al-Cu-Zn-Mg alloy with good resistance to hot cracking. This alloy has a low melting point, a narrowed solidification temperature range, and excellent resistance to hot cracking and mechanical properties. It can be used for various high-strength cast aluminum alloy structural parts, effectively avoiding casting defects such as hot cracking, shrinkage cavities, and porosity, and significantly improving the overall mechanical properties of the alloy.

Description

A high elongation cast Al-Cu-Zn-Mg alloy with good resistance to hot cracking and its preparation method Technical Field

[0001] This invention belongs to the field of aluminum alloy development technology, specifically relating to a high elongation cast Al-Cu-Zn-Mg alloy with good resistance to hot cracking and its preparation method. Background Technology

[0002] Cast aluminum alloys offer a significant cost advantage over wrought aluminum alloys due to their shorter processing flow and near-net-shape forming. Al-Cu alloys are currently the most commonly used and widely used cast aluminum alloys, exhibiting high strength compared to other cast aluminum alloys such as Al-Si and Al-Mg, thanks to their ability to be strengthened through heat treatment. However, due to the limitations of the casting process, defects such as shrinkage cavities, porosity, and hot cracks in cast aluminum alloys can lead to a loss of mechanical properties. Therefore, improving the hot crack resistance of cast Al-Cu alloys while simultaneously enhancing their strength and toughness is crucial for expanding the applications of cast aluminum alloys.

[0003] The alloy composition of cast Al alloys is a major factor affecting their resistance to hot cracking. Due to the wide crystallization temperature range of Al-Cu alloys, a large amount of liquid and solid phases coexist during solidification, resulting in predominantly pasty solidification of the casting. This leads to poor fluidity and feeding ability of the alloy. The initial temperature of the liquid-solid two-phase coexistence region is the upper limit of the hot brittleness zone. As the temperature decreases to a certain point, the alloy strength begins to increase sharply; this temperature is called the lower limit of the hot brittleness zone. Within the hot brittleness zone, between the upper and lower limits, the difference in alloy strength easily leads to hot cracking. Simultaneously, a high Cu content in the alloy increases the number of coarse primary Al2Cu and Al2CuMg phases in the Al-Cu alloy. While this has a positive effect on improving alloy strength, it leads to a decrease in toughness and plasticity, which is extremely detrimental to controlling the alloy's hot cracking tendency, causing a significant increase in the tendency to crack. A large amount of brittle primary second phase reduces the fluidity of the alloy melt, resulting in poor casting performance and susceptibility to defects such as cracks.

[0004] Optimizing the composition of cast aluminum alloys by optimizing the main alloying elements and adding trace alloying elements can significantly improve the alloy's resistance to hot cracking and its mechanical properties. Studies have found that Al3Zr particles formed by adding Zr and Al to aluminum alloys nucleate before α-Al during solidification, becoming heterogeneous nucleation sites that refine the alloy microstructure and reduce hot cracking susceptibility. Since the refining effect of Zr is limited, the combined addition of Sc and Zr forms a Zr-rich shell encapsulating Sc Al3(Sc,Zr) particles, similar to Al3Sc particles. This core-shell structure of Al3(Sc,Zr) has a lower mismatch between the core and shell and the α-Al matrix, effectively refining the microstructure of the cast alloy and significantly improving its resistance to hot cracking and mechanical properties. However, the high price of metallic Sc ​​limits its industrial application. To overcome the drawbacks of grain refiners such as Al-Zr and Al-Sc, the addition of Nb to aluminum alloys can form nanoscale particles such as Al3Nb, NbB2, and NbC, with melting points of 1680℃, 3036℃, and 3490℃, respectively, thus preventing them from melting in molten aluminum. Al3Nb, NbB2, and NbC have similar lattice structures and constants to Al3Ti, TiB2, and TiC, respectively. Therefore, Al-Nb can significantly refine Al-Cu alloys, resulting in finely dispersed primary second-phase particles during solidification, without the appearance of the coarse and brittle AlCuSc heat-resistant phase caused by Sc. The Al3Nb nanoparticles also contribute to high grain size stability, preventing coarsening during subsequent homogenization, solution treatment, and other heat treatment processes.

[0005] Although the performance of cast Al-Cu alloys has improved, quality problems such as hot cracking, shrinkage cavities, porosity, and insufficient mechanical properties have not yet been effectively solved due to the poor fluidity and hot cracking resistance of Al-Cu alloys during the casting process. Summary of the Invention

[0006] The purpose of this invention is to overcome at least one deficiency of the prior art and provide a high elongation cast Al-Cu-Zn-Mg alloy with good resistance to hot cracking and its preparation method.

[0007] The technical solution adopted in this invention is:

[0008] In a first aspect, the present invention provides a high elongation cast Al-Cu-Zn-Mg alloy with good resistance to hot cracking, wherein the alloy comprises, by mass percentage: Cu 3.5-4.5%, Mg 0.4-1.5%, Mn 0.2-0.5%, Ti 0.10-0.2%, Zn 2.5-3.5%, with the balance being Al and unavoidable impurity elements.

[0009] In some instances, the alloy also contains 0.05 to 0.2% Zr by mass percentage.

[0010] In some instances, the alloy also contains a total of 0.05 to 0.5% Sc and Nb elements by mass percentage.

[0011] In some instances, the Sc content is 0.05–0.2% by mass percentage, and the Nb content is 0.10–0.30%.

[0012] Secondly, the method for preparing the Al-Cu-Zn-Mg alloy provided in the first aspect of the present invention includes the following steps:

[0013] 1) Weigh the raw materials of each element according to the proportion, melt them, remove the slag, and stir evenly;

[0014] 2) Refining under a protective gas, removing slag and filtering, and then casting to obtain alloy ingots;

[0015] 3) Take the alloy ingot prepared in step 2), perform homogenization heat treatment, and then perform cyclic intermittent aging treatment for 22-26 hours to obtain the high elongation cast Al-Cu-Zn-Mg alloy.

[0016] In some instances, the homogenization heat treatment is performed at a temperature of 450–500°C for a duration of 24–36 hours.

[0017] In some instances, the intermittent aging process involves aging at 110–170°C for 30–60 minutes, followed by water quenching at room temperature for 30–60 minutes.

[0018] In some instances, the melting temperature is 720°C to 760°C.

[0019] In some instances, the refining temperature is 680°C to 710°C.

[0020] In some instances, the casting temperature is 720°C to 740°C.

[0021] The beneficial effects of this invention are:

[0022] This invention addresses the shortcomings of existing Al-Cu casting alloys, such as wide solidification temperature range, poor fluidity, and hot cracking. It provides an Al-Cu-Zn-Mg alloy with high Zn content and good resistance to hot cracking. This alloy has a low melting point, a narrowed solidification temperature range, and excellent resistance to hot cracking and mechanical properties. It can be used for various high-strength cast aluminum alloy structural parts, effectively avoiding casting defects such as hot cracking, shrinkage cavities, and porosity, and significantly improving the overall mechanical properties of the alloy.

[0023] The main alloying component of the Al-Cu casting alloy of this invention has an increased Zn content of 2.5-3.5%. Increasing the Zn content further lowers the alloy's melting point, preventing severe oxidation during heating, reducing inclusions, and improving the alloy's fluidity. The addition of Zn to the Al-Cu-Mg alloy can form the η(MgZn2) strengthening phase through natural and artificial aging, thus increasing the alloy's strength.

[0024] In some embodiments of this invention, Zr, Nb, and Sc elements are added to aluminum alloys to refine the alloy microstructure and reduce hot cracking susceptibility. The combined addition of Sc and Zr forms Al3(Sc,Zr) particles with a Zr-rich shell encapsulating a Sc core, similar to Al3Sc particles. The core-shell structure of Al3(Sc,Zr) has a lower mismatch with the α-Al matrix, effectively refining the microstructure of the cast alloy and improving its hot cracking resistance and mechanical properties. Nb reduces the undercooling required for the alloy solidification process, forming Al3Nb nanoparticles that exhibit good coherence with the Al matrix. These Al3Nb nanoparticles maintain high grain size stability and do not coarsen during subsequent homogenization, solution treatment, and other heat treatment processes. Al-Nb significantly refines Al-Cu alloys, resulting in finer, more dispersed primary second phases during solidification. The refined eutectic microstructure reduces the alloy's hot cracking tendency and also improves the plasticity of the cast alloy. The design of Zr, Sc, and Nb elements significantly improves the elongation and hot cracking resistance of the cast alloy. Attached Figure Description

[0025] Figure 1 shows a typical stress-strain curve of the high elongation cast Al-Cu-Zn-Mg alloy of Example 1 of the present invention.

[0026] Figure 2 shows the CT scan results of the high elongation cast Al-Cu-Zn-Mg alloy casting prepared by the alloy in Example 1 of this invention. Detailed Implementation

[0027] The following disclosure provides many different implementations or examples for different ways of implementing the present invention.

[0028] The present invention will now be described in detail with reference to embodiments, comparative examples and experimental data. However, this is not intended to identify the key or decisive elements of the invention or to limit the scope of protection.

[0029] The alloy chemical composition by weight percentage in each embodiment is as follows: Cu 3.5–4.5%, Mg 0.4–1.5%, Mn 0.2–0.5%, Ti 0.10–0.2%, Zn 2.0–4.0%, Zr 0.05–0.2%. The raw materials are pure Al, Mg, and Zn ingots, and master alloys such as Al-50Cu, Al-20Mn, Al-5Ti, Al-5Zr, Al-3Sc, and Al-5Nb.

[0030] The composition of each embodiment and comparative example is shown in Table 1.

[0031] Table 1 Chemical composition (mass percentage) of the examples and comparative examples

[0032]

[0033] The preparation methods for each embodiment and comparative example are as follows:

[0034] 1) Raw material preparation: Weigh pure Al, Mg, and Zn ingots according to the proportions, and use Al-50Cu, Al-20Mn, Al-5Ti, Al-2Sc, Al-5Zr, and Al-10Nb master alloys as raw materials for smelting.

[0035] 2) Smelting and casting: Take the aluminum, magnesium, zinc ingots, aluminum master alloy and rare earth prepared in step 1), put them into the smelting furnace, heat and melt, remove slag and stir evenly, refine with protective gas, remove slag, let stand and filter, and cast to obtain alloy ingots.

[0036] 3) Solution aging: Take the alloy ingot prepared in step 2), perform homogenization annealing and solution heat treatment, and then perform cyclic intermittent aging treatment to obtain the high strength and toughness cast Al-Cu-Zn-Mg alloy.

[0037] Preferably, the melting temperature is 720℃~760℃; the refining temperature is 690℃~700℃; and the casting temperature is 720℃~740℃.

[0038] Preferably, the homogenization heat treatment temperature is 450–500°C; the homogenization heat treatment time is 24–36 h.

[0039] Preferably, the intermittent aging process involves aging at 110–170°C for 30–60 minutes, followed by water quenching at room temperature for 30–60 minutes to complete the intermittent aging; this intermittent aging process is repeated for a total of 22–26 hours.

[0040] Multi-component refining agents and degassing agents can be added during the smelting process. When the purity of the raw materials is high, these agents may not be necessary. The multi-component refining agents and degassing agents are those commonly used in this field, with a refining agent to smelting feed mass ratio of (1-3):100. The multi-component composite refining agent comprises: 20wt% NaCl, 20wt% KCl, 35wt% NaF, and 25wt% LiF; the degassing agent to smelting feed mass ratio is 1:100, and the degassing agent is hexachloroethane. The multi-component refining agents and degassing agents themselves have virtually no effect on the alloy's properties.

[0041] Comparison of properties of different Al-Cu-Zn-Mg alloys

[0042] Figure 1 shows a typical stress-strain curve of the high elongation cast Al-Cu-Zn-Mg alloy of Example 1 of the present invention. Adjusting the Zn content can form η(MgZn2) strengthening to improve the mechanical properties of the alloy. At the same time, the Zn content lowers the melting point of the alloy, improves the fluidity of the alloy, reduces the solidification temperature range of the liquid metal, and reduces the hot cracking tendency of the alloy.

[0043] Figure 2 shows the CT scan results of the high elongation cast Al-Cu-Zn-Mg alloy casting prepared by the alloy in Example 1 of this invention. The results show that the obtained casting has a complete shape, complete mold filling, and no casting defects such as incomplete filling, slag inclusions, or hot cracks, meeting the first-grade aluminum alloy casting standard.

[0044] The alloys obtained in the examples and comparative examples were subjected to performance tests. Three-dimensional CT was used to detect casting defects. The strength and elongation were tested according to GB / T 228.1-2010 "Metallic materials – Tensile testing – Part 1: Tests at room temperature". The performance test results of each example and comparative example are shown in Table 2.

[0045] Table 2 Performance test results of each embodiment and comparative example

[0046]

[0047] As shown in Table 2, Examples 1 and 2, as well as Example 9, demonstrate the effect of Zn content on the mechanical properties and hot crack resistance of the alloy. A suitable Zn content lowers the alloy's melting point, preventing severe oxidation during high-temperature melting, thus reducing oxide inclusions and casting defects. Zn improves the alloy's fluidity and narrows the solidification temperature range of the liquid metal, thereby reducing the alloy's tendency to form hot cracks. Zn can also form the η(MgZn2) strengthening phase, increasing the alloy's strength.

[0048] Examples 3 to 8 demonstrate the improvement of the plasticity and hot crack resistance of cast alloys by trace elements Zr, Sc, and Nb. In aluminum alloys, Zr reacts with Al to form Al3Zr particles, which can act as heterogeneous nucleation sites during Al matrix solidification, refining the alloy microstructure and reducing hot crack susceptibility. The combined addition of Sc and Zr forms Al3(Sc,Zr) particles with a Zr-rich shell encapsulating a Sc core, similar to Al3Sc particles. The core-shell structure of Al3(Sc,Zr) has a lower mismatch with the α-Al matrix, effectively refining the microstructure of the cast alloy and improving its hot crack resistance and mechanical properties. Nb reduces the supercooling required for alloy solidification, forming Al3Nb nanoparticles that exhibit good coherence with the Al matrix. These Al3Nb nanoparticles maintain high grain size stability and do not coarsen during subsequent homogenization, solution treatment, and other heat treatment processes. Al-Nb significantly refines Al-Cu alloys, resulting in finer and more dispersed primary second phases during solidification. This refinement of the eutectic structure reduces the alloy's susceptibility to hot cracking and also improves the plasticity of the casting alloy. The design of Zr, Sc, and Nb elements significantly enhances the elongation and hot cracking resistance of the casting alloy.

[0049] In summary, compared with the comparative alloy without Zr, Sc, and Nb, the addition of Zr, Sc, and Nb elements in the alloy of this invention can improve the mechanical properties of the Al-Cu-Zn-Mg casting alloy, while significantly reducing the hot cracking tendency factor of the alloy.

[0050] The above is a further detailed description of the present invention and should not be considered as a limitation on the specific implementation of the present invention. For those skilled in the art, simple deductions or substitutions without departing from the concept of the present invention are all within the protection scope of the present invention.

Claims

1. A high-elongation cast Al-Cu-Zn-Mg alloy with good resistance to hot cracking, characterized in that, The alloy comprises, by mass percentage: Cu 3.5–4.5%, Mg 0.4–1.5%, Mn 0.2–0.5%, Ti 0.10–0.2%, Zn 2.89–3.5%, Zr 0.05–0.2%, Sc 0.05–0.15%, Nb The alloy contains 0.10-0.30% Al and unavoidable impurity elements. The preparation method includes the following steps: 1) Weigh the raw materials of each element according to the proportion and melt them. After removing the slag, stir them evenly; 2) Refine them under a protective gas, remove the slag and filter them, and then cast them to obtain an alloy ingot; 3) Take the alloy ingot prepared in step 2) and perform homogenization heat treatment, and then perform cyclic intermittent aging treatment for 22-26 hours. The intermittent aging treatment step is as follows: first age at 110-170℃ for 30-60 minutes, and then quench at room temperature for 30-60 minutes to obtain the high elongation cast Al-Cu-Zn-Mg alloy.

2. The Al-Cu-Zn-Mg alloy according to claim 1, characterized in that, The homogenization heat treatment temperature is 450–500℃, and the homogenization heat treatment time is 24–36 h.

3. The Al-Cu-Zn-Mg alloy according to claim 1, characterized in that, The melting temperature is 720℃~760℃.

4. The Al-Cu-Zn-Mg alloy according to claim 1, characterized in that, The refining temperature is 680℃~710℃.

5. The Al-Cu-Zn-Mg alloy according to claim 1, characterized in that, The casting temperature is 720℃~740℃.

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