A method for off-site alloying additive manufacturing of a high-strength, easily cracked aluminum alloy

Abstract

The application discloses an off-site alloying additive preparation method of a high-strength easy-to-crack aluminum alloy, which comprises the following steps: according to the element composition of the high-strength easy-to-crack aluminum alloy, element powders with corresponding proportions are configured and uniformly mixed; and selective laser melting is performed on the uniformly mixed high-strength easy-to-crack aluminum alloy element powder; wherein the particle size of a cracking element is controlled so that the cracking element is distributed in an enrichment state in a formed sample, and no thermal cracks are generated in the sample; homogenization treatment is performed on the printed sample, so that each element is uniformly diffused and an alloying process is realized; and aging treatment is performed on the sample after the homogenization treatment, so that the precipitation of a second phase is promoted to improve the strength of the second phase, and a high-strength easy-to-crack aluminum alloy without cracks is obtained. The method can complete the off-site alloying additive preparation of the high-strength easy-to-crack aluminum alloy without cracks, and promote the application of the high-strength easy-to-crack aluminum alloy.

Description

Technical Field

[0001] This invention belongs to the field of additive manufacturing technology for metallic materials, and particularly relates to a method for preparing easily cracked high-strength aluminum alloys by off-site alloying additive manufacturing. Background Technology

[0002] High-strength aluminum alloys that are prone to cracking are widely used in aerospace, automotive and other fields due to their advantages such as high specific strength, high specific stiffness, corrosion resistance and excellent thermal conductivity.

[0003] For decades, plastic forming, casting, and powder metallurgy have been the main processing methods for industrial production of easily cracked high-strength aluminum alloys. However, in practical applications, traditional processes have three main drawbacks: (1) significant resource waste and high costs during subtractive processing; (2) low design freedom for complex components, which usually require assembly after the separate production of several components; and (3) slow cooling rate, long solidification time, coarse grains, and uneven grain size distribution of large castings, resulting in low mechanical properties. Moreover, for castings, the final part can only be obtained after subsequent precision subtractive manufacturing using machine tools and other equipment. Therefore, this greatly limits the application of easily cracked high-strength aluminum alloys.

[0004] Selective laser melting is a processing method based on the discrete-stacking principle, which directly manufactures parts driven by the three-dimensional data of the parts. Its basic principle is to form the final part by selectively melting and stacking metal powder layer by layer. Its processing does not require molds and blanks of traditional machining, and has a high degree of design freedom and processing flexibility. Theoretically, it can form parts of any shape. Compared with traditional processing methods, selective laser melting technology has the following advantages: (1) Powder can be recycled and reused, and the material utilization rate is extremely high; (2) High degree of design freedom, which can directly prepare precision parts with complex structures, so the design of corresponding parts can be optimized and the total number of parts can be reduced; (3) High cooling rate and solidification rate, with the potential to form ultrafine grain structure.

[0005] However, these aluminum alloys are highly susceptible to hot cracking during selective laser melting (SLM). Similar to welding, the high temperature gradient and high cooling rate generated during SLM are insufficient for homogeneous nucleation to form equiaxed crystals. Therefore, in the absence of effective heterogeneous nucleating agents, easily cracked high-strength aluminum alloys typically form coarse columnar crystals with epitaxial growth. Furthermore, the wide solidification temperature range and high coefficient of thermal expansion of easily cracked high-strength aluminum alloys subject them to significant stress during solidification. Towards the end of solidification, the remaining liquid phase exists as a liquid film, which cannot effectively resist stress. Under stress, the liquid film ruptures, forming hot cracks, which significantly impacts the mechanical properties of easily cracked high-strength aluminum alloys. Overcoming hot cracking has become the most crucial issue in the field of SLM preparation of easily cracked high-strength aluminum alloys. Summary of the Invention

[0006] High-strength aluminum alloys prone to cracking are susceptible to hot cracking during selective laser melting due to their wide solidification temperature range and large coefficient of thermal expansion, which reduces their performance.

[0007] In order to prepare crack-free, easily cracked high-strength aluminum alloys, this invention proposes an off-site alloying additive manufacturing method for easily cracked high-strength aluminum alloys.

[0008] This invention is achieved through the following technical solution: This invention provides a method for off-site alloying additive manufacturing of easily crackable high-strength aluminum alloys, comprising: preparing powders of each element in appropriate proportions according to the elemental composition of the easily crackable high-strength aluminum alloy, and uniformly mixing them; the particle size of the crack-inducing element in the elemental powder must meet the requirement that the crack-inducing element exhibits an enriched distribution in the selective laser melting formed sample. Selective laser melting is performed on uniformly mixed high-strength aluminum alloy element powder that is prone to cracking. The crack-inducing elements are distributed in an enriched state in the sample, and no hot cracks are generated in the sample. The printed sample is subjected to homogenization and aging heat treatment to ensure uniform diffusion of each element and promote the precipitation of the second phase to improve its strength, thereby obtaining a high-strength, crack-free, easily cracked high-strength aluminum alloy.

[0009] As a further explanation of the present invention, the easily cracked high-strength aluminum alloy is an aluminum alloy based on Al-Cu-Mg, Al-Mg-Si, or Al-Zn-Mg-Cu.

[0010] As a further explanation of the present invention, the elements in the easily cracked high-strength aluminum alloy include aluminum powder, copper powder, magnesium powder and manganese powder, wherein the cracking element copper powder is selected as spherical copper powder with a diameter of 75-106μm.

[0011] As a further explanation of the present invention, the aluminum powder is selected as spherical aluminum powder with a diameter of 15-53 μm, the magnesium powder is selected as spherical magnesium powder with a diameter of 15-53 μm, and the manganese powder is selected as irregular manganese powder with a diameter of 45 μm.

[0012] As a further explanation of the present invention, the mixing method of the elemental powder is selected as ball milling or vibration mixing.

[0013] As a further explanation of the present invention, the ball milling speed of the ball milling powder is 50~100 r / min, and the ball-to-material ratio is 2~4.

[0014] As a further explanation of the present invention, the vibration frequency of the vibratory mixing powder is 50-70Hz, and the ball-to-powder ratio is 0.5-2.

[0015] As a further explanation of the present invention, the laser power during the selective laser melting forming process is 100~500W; the scanning speed is 100~1400mm / s; the layer thickness is 0.03~0.06mm; and the scanning interval is 0.1~0.2mm.

[0016] As a further explanation of the present invention, the homogenization temperature in the homogenization process is lower than the solidus temperature of the easily cracked high-strength aluminum alloy.

[0017] Compared with the prior art, the present invention has the following beneficial technical effects: 1. Crack-prone aluminum alloys are prone to hot cracking during selective laser melting (SLM), affecting their performance. This invention suppresses hot cracking by controlling the distribution of crack-inducing elements during SLM. Specifically, the powder is changed from pre-alloyed powder to elemental powder, and the particle size of the crack-inducing elements is controlled to ensure their enrichment in the SLM sample, preventing hot cracking. After homogenization, the elements are evenly distributed, achieving alloying. Subsequent heat treatment improves its strength to meet application requirements.

[0018] 2. The present invention mainly controls the diffusion effect of crack-inducing elements in easily cracked high-strength aluminum alloys during the forming process, so that the crack-inducing elements in the printed sample are in an enriched state, suppressing the generation of hot cracks, and achieving alloying after homogenization treatment after printing. Attached Figure Description

[0019] Figure 1 This is the microstructure of the pre-alloyed powder after it has been formed and printed, as shown in Example 1 of this invention.

[0020] Figure 2 This refers to the microstructure of the printed state after forming in Embodiment 2 of the present invention.

[0021] Figure 3 This refers to the microstructure of the printed state after forming in Embodiment 3 of the present invention.

[0022] Figure 4 The image shows the microstructure and corresponding energy spectrum after homogenization treatment in Example 3 of this invention.

[0023] Figure 5 This is a comparison of the mechanical properties of the samples before and after heat treatment and the pre-alloyed powder printed samples in Example 3 of the present invention.

[0024] Figure 6 This is the microstructure after homogenization treatment in Example 4 of the present invention.

[0025] Figure 7 This is the aging hardening curve of the sample in Example 4 of the present invention during aging heat treatment at 185℃.

[0026] Figure 8 This is a schematic diagram of the process for the additive manufacturing method of off-site alloying of easily cracked high-strength aluminum alloys provided by the present invention. Detailed Implementation

[0027] To better understand the above-mentioned objectives, features, and advantages of the present invention, the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments. It should be noted that, unless otherwise specified, the embodiments of the present invention and the features thereof can be combined with each other.

[0028] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the description of the invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention.

[0029] This invention provides a method for additive manufacturing of easily cracked high-strength aluminum alloys through off-site alloying, comprising the following steps: Step 1: Prepare powders of each element in the appropriate proportion according to the elemental composition of the easily cracked high-strength aluminum alloy, and mix them evenly; the particle size of the crack-inducing element in the elemental powder must meet the requirement of being enriched in the formed sample after the selective laser melting forming process.

[0030] During selective laser melting, all elements melt and diffuse under the influence of concentration gradient and Marangoni convection. This regulates the particle size of crack-inducing elements, reduces their diffusion rate, and causes them to accumulate in the formed sample, thus inhibiting the formation of hot cracks.

[0031] Step 2: Selective laser melting is performed on the uniformly mixed easily crackable high-strength aluminum alloy element powder to prepare a printed sample. The crack-inducing elements in the easily crackable high-strength aluminum alloy are in an enriched state, and no hot cracks are generated in the sample.

[0032] Step 3: The printed sample is subjected to homogenization and aging heat treatment to ensure uniform diffusion of each element, thereby enabling the alloying process of the printed sample and promoting the precipitation of the second phase to improve its strength.

[0033] The homogenization diffusion process enables the uniform diffusion of various elements to achieve the alloying process. Then, the easily cracked high-strength aluminum alloy is subjected to aging heat treatment to promote the precipitation of the second phase and improve its strength to meet the application requirements.

[0034] High-strength aluminum alloys prone to cracking are susceptible to hot cracking during selective laser melting (SLM) preparation, affecting their performance. Therefore, this invention replaces pre-alloyed powder with elemental powder, controlling the particle size of crack-inducing elements to ensure their enrichment in the SLM sample, resulting in a crack-free printed sample. After homogenization, the elements are uniformly distributed and alloyed; subsequent heat treatment further enhances its strength to meet application requirements.

[0035] Furthermore, the easily cracked high-strength aluminum alloy is an aluminum alloy based on Al-Cu-Mg, Al-Mg-Si, or Al-Zn-Mg-Cu.

[0036] Furthermore, this invention requires controlling the distribution of crack-inducing elements in the formed sample to obtain a crack-free, easily cracked, high-strength aluminum alloy; therefore, the particle size of the crack-inducing elements is controlled. Preferably, the elements in the easily cracked, high-strength aluminum alloy include aluminum powder, copper powder, magnesium powder, and manganese powder, wherein the copper powder, the crack-inducing element, is selected as spherical copper powder with a particle size of 75-106 μm. More specifically, the aluminum powder is selected as spherical aluminum powder with a particle size of 15-53 μm, the magnesium powder is selected as spherical magnesium powder with a particle size of 15-53 μm, and the manganese powder is selected as irregular manganese powder with a particle size of 45 μm.

[0037] Furthermore, the mixing method for each element powder only needs to ensure uniform mixing; for example, ball milling, vibration mixing, or ultrasonic mixing can be selected. To obtain a crack-prone high-strength aluminum alloy, subsequent processes require a homogenization process to ensure the uniform distribution of each element. Therefore, to reduce homogenization time, the elements should be distributed as evenly as possible during the element powder mixing process, so that the elements are also evenly distributed in the printed sample.

[0038] Furthermore, in the selective laser melting (SLM) forming process, the sphericity of the powder directly affects the powder spreading quality, thus impacting the quality of the subsequently formed sample. To ensure powder flowability, energy input should be reduced during ball milling to minimize powder deformation; the ball milling speed for the powder mixing method is 50-100 r / min. The degree of powder mixing directly affects the subsequent homogenization temperature. To reduce homogenization time and obtain uniformly mixed elemental powder, the ball-to-powder ratio for the ball milling method is 2-4.

[0039] The degree of powder mixing directly affects the subsequent homogenization temperature. To reduce homogenization time and obtain uniformly mixed elemental powders, the vibration frequency of the vibration mixing method is 50-70 Hz. Powder mixing efficiency affects the overall sample processing efficiency. To improve powder mixing efficiency and ensure powder mixing quality, the ball-to-powder ratio of the vibration mixing method is 0.5-2.

[0040] Furthermore, for aluminum alloys, the extremely high laser reflectivity makes them difficult to form using selective laser melting (SLM), requiring higher laser power to melt the powder. However, excessively high laser power can cause the molten aluminum to vaporize, forming pores and affecting the final properties. Therefore, the laser power in the SLM forming process is 100-500W. In the SLM forming process, increasing the scanning speed reduces the stability of the molten pool, leading to molten splashing and a "spheroidizing" phenomenon; conversely, excessively low scanning speeds significantly increase the forming time. Therefore, the scanning speed in the SLM forming process is 100-1400mm / s. In the SLM forming process, layer thickness directly affects the powder spreading quality; a higher layer thickness results in better powder spreading quality, but excessively high layer thickness can negatively impact subsequent forming quality. Therefore, the layer thickness in the SLM forming process is 0.03-0.06mm. In the SLM forming process, the scanning interval directly affects the molten pool overlap rate. If the overlap ratio of the molten pool is too high, the energy input will be too large, and porosity defects will easily occur; if the overlap ratio is too low, some powder will not be completely melted. Therefore, the scanning interval in the selected area laser melting forming process is 0.1 ~ 0.2 mm.

[0041] Furthermore, to avoid defects caused by remelting of some microstructures during the homogenization process, the homogenization temperature must be lower than the solidus line in easily cracked high-strength aluminum alloys.

[0042] All easily cracked high-strength aluminum alloys are heat-treatable, meaning their strength can be improved by promoting the precipitation of the second phase through appropriate aging treatment. To maximize their strength, the aging process must be selected based on the type of easily cracked high-strength aluminum alloy.

[0043] The following describes a preferred embodiment in detail.

[0044] Table 1 shows the elemental composition of the high-strength aluminum alloys used in Examples 1-4 of this invention.

[0045] Example 1

[0046] The easily cracked high-strength aluminum alloy used in this embodiment is 2024 aluminum alloy. Step 1, Selective Laser Melting: Selective laser melting is performed on 2024 aluminum alloy pre-alloyed powder. The laser power is 230W, the scanning speed is 500mm / s, the scanning interval is 0.17mm, and the layer thickness is 30μm.

[0047] Microstructure observation was performed on the samples formed using 2024 pre-alloyed powder. The horizontal microstructure is as follows: Figure 1 As shown in (a and b), where, Figure 1 (a) is a secondary electron image. Figure 1(b) is a backscattered electron image. The sample was formed using pre-alloyed powder, so Cu diffused completely and uniformly during the forming process, resulting in a large number of hot cracks that were distributed in a network. Example 2

[0048] The easily cracked high-strength aluminum alloy used in this embodiment is 2024 aluminum alloy. Step 1, Element Powder Mixing: Prepare the corresponding element powders according to the elemental composition of 2024 aluminum alloy. The aluminum powder is selected as spherical aluminum powder with a diameter of 15-53μm, the copper powder is selected as spherical copper powder with a diameter of 15-53μm, the magnesium powder is selected as spherical magnesium powder with a diameter of 15-53μm, and the manganese powder is selected as irregular manganese powder with a diameter of 45μm. Mix the prepared powders evenly by vibration mixing method, with a ball-to-powder ratio of 0.5, a vibration frequency of 60Hz, and a vibration time of 2h.

[0049] Step 2, Selective Laser Melting: Selective laser melting is performed on 2024 aluminum alloy element powder. The laser power is 190W, the scanning speed is 500mm / s, the scanning interval is 0.17mm, and the layer thickness is 30μm.

[0050] Microscopic observation of the above-mentioned printed sample revealed the following microstructure: Figure 2 As shown in (a and b), where Figure 2 (a) is a horizontal mirror image. Figure 2 (b) is a horizontal BSE image, where the gray area is the aluminum matrix and the white area is the Cu-rich region. Due to the Cu particle size of 15-53 μm, incomplete diffusion occurred during the forming process, resulting in hot cracks, but the hot crack content decreased. Example 3

[0051] The easily cracked high-strength aluminum alloy used in this embodiment is 2024 aluminum alloy. Step 1, Element Powder Mixing: Prepare the corresponding element powders according to the elemental composition of 2024 aluminum alloy. The aluminum powder is selected as spherical aluminum powder with a diameter of 15-53μm, the copper powder is selected as spherical copper powder with a diameter of 75-106μm, the magnesium powder is selected as spherical magnesium powder with a diameter of 15-53μm, and the manganese powder is selected as irregular manganese powder with a diameter of 45μm. Mix the prepared powders evenly by ball milling. The ball-to-powder ratio is 4:1, the rotation speed is 100rpm, and the ball milling time is 4h.

[0052] Step 2, Selective Laser Melting: Selective laser melting is performed on 2024 aluminum alloy element powder. The laser power is 190W, the scanning speed is 500mm / s, the scanning interval is 0.17mm, and the layer thickness is 30μm.

[0053] Step 3, Homogenization and Heat Treatment: The printed sample is subjected to homogenization and heat treatment. The homogenization process is 510℃@14 days, and the heat treatment is 510℃@2h-water quench-185℃@8h.

[0054] Microscopic observation of the above-mentioned printed sample revealed the following horizontal microstructure: Figure 3 As shown in (a and b), where, Figure 3 (a) and Figure 3 (b) All images are backscattered electron images; the gray areas represent the aluminum matrix, and the white areas represent the Cu-rich regions. Compared to Example 2, the Cu particle size increased from 15-53 μm to 75-106 μm, and its specific surface area decreased by 80%-58%, reducing the diffusion rate of Cu during the forming process. Therefore, Cu was distributed in an enriched state in the sample, suppressing the generation of hot cracks. Microstructure observation of the homogenized elemental powder 2024 revealed the following microstructure: Figure 4 As shown in (a), Figure 4 (b and c) are the corresponding energy spectra, showing that all elements have diffused uniformly. Mechanical properties of the heat-treated elemental powder 2024 were then tested. Figure 5 The mechanical properties of 2024 elemental powder in printed state, solution state, aged state and 2024 pre-alloyed powder printed samples are compared. Compared with 2024 pre-alloyed powder printed samples, the high-strength aluminum alloy obtained by this method can maintain high plasticity while also having high strength. Example 4

[0055] The easily cracked high-strength aluminum alloy used in this embodiment is 2024 aluminum alloy. Step 1, Element Powder Mixing: Prepare the corresponding element powders according to the elemental composition of 2024 aluminum alloy. The aluminum powder is selected as spherical aluminum powder with a diameter of 15-53μm, the copper powder is selected as spherical copper powder with a diameter of 75-106μm, the magnesium powder is selected as spherical magnesium powder with a diameter of 15-53μm, and the manganese powder is selected as irregular manganese powder with a diameter of 45μm. Mix the prepared powders evenly by ball milling. The ball-to-powder ratio is 4:1, the rotation speed is 100rpm, and the ball milling time is 4h.

[0056] Step 2, Selective Laser Melting: Selective laser melting is performed on 2024 aluminum alloy element powder. The laser power is 190W, the scanning speed is 500mm / s, the scanning interval is 0.17mm, and the layer thickness is 30μm.

[0057] Step 3, Homogenization and Heat Treatment: The printed sample is subjected to homogenization and heat treatment. The homogenization process is 510℃@14 days, and the heat treatment is 510℃@2h-water quench-185℃@8h.

[0058] Microscopic observation was performed on the homogenized elemental powder 2024, and its microstructure is as follows: Figure 6 As shown, where Figure 6 (a) is a horizontal backscattered electron image. Figure 6 (b) is a vertical backscattered electron image, indicating that Cu, Mg, and Mn elements have diffused uniformly during the homogenization process. The homogenized elemental powder 2024 was then heat-treated. Figure 7 The image shows the aging hardening curve of the sample at an aging temperature of 185℃. The curve indicates that elemental powder 2024 reaches its peak hardness of 80.1 HV after an aging time of 8 hours. 0.1 .

[0059] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for additive manufacturing of easily cracked high-strength aluminum alloys by off-site alloying, characterized in that, include: Prepare powders of each element in appropriate proportions according to the elemental composition of easily cracked high-strength aluminum alloy, and mix them evenly. The crack-prone high-strength aluminum alloy is an Al-Cu-Mg based aluminum alloy. The elements in the crack-prone high-strength aluminum alloy include aluminum powder, copper powder, magnesium powder, and manganese powder. Specifically, the crack-inducing copper powder is selected as spherical copper powder with a particle size of 75-106 μm; the aluminum powder is selected as spherical aluminum powder with a particle size of 15-53 μm; the magnesium powder is selected as spherical magnesium powder with a particle size of 15-53 μm; and the manganese powder is selected as irregular manganese powder with a particle size of 45 μm. The particle size of the crack-inducing elements must meet the requirement of exhibiting an enriched distribution in the selective laser melting (SLM) formed sample. Selective laser melting was used to prepare printed samples from uniformly mixed, easily cracked, high-strength aluminum alloy element powder. The laser power during the selective laser melting process was 100-500 W; the scanning speed was 100-1400 mm / s; the layer thickness was 0.03-0.06 mm; and the scanning interval was 0.1-0.2 mm. The printed sample is subjected to homogenization and aging heat treatment to ensure uniform diffusion of all elements and obtain a crack-free, high-strength aluminum alloy; the homogenization temperature in the homogenization treatment is lower than the solidus temperature of the crack-free high-strength aluminum alloy.

2. The method for off-site alloying additive manufacturing of easily cracked high-strength aluminum alloy according to claim 1, characterized in that, The mixing method for elemental powders can be either ball milling or vibration mixing.

3. The method for preparing easily cracked high-strength aluminum alloy by off-site alloying additive manufacturing according to claim 2, characterized in that, The ball milling speed for the powder mixing is 50-100 r / min, and the ball-to-powder ratio is 2-4.

4. The method for preparing easily cracked high-strength aluminum alloy by off-site alloying additive manufacturing according to claim 2, characterized in that, The vibration frequency of the vibratory mixing powder is 50-70 Hz, and the ball-to-powder ratio is 0.5-2.

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