In-situ strengthening method for improving fracture toughness of aluminum alloy in additive manufacturing

Abstract

The invention belongs to the technical field of aluminum alloy additive manufacturing, and particularly relates to an in-situ strengthening method for improving fracture toughness of aluminum alloy in additive manufacturing, which comprises the following steps: (1) preparation before additive manufacturing; the high-strength aluminum alloy powder is dried and then put into a powder feeder; (2) setting the remelting width of a forming channel; the remelting width is controlled through a laser scanning spacing method or a linear energy density method; (3) carrying out additive manufacturing and heat treatment; according to the method, by setting the remelting width, remelting and re-nucleation growth of part of crystal grains are achieved, and at the bottom of a molten pool, new crystal nuclei form fine isometric crystals instead of epitaxial columnar crystals with Al3 (Zr, Er) as the core; and through alternate distribution of columnar crystals and isometric crystals, the crack propagation resistance is increased, the fracture toughness of the material is remarkably improved, and meanwhile the high-strength characteristic is kept.

Description

Technical Field

[0001] The present invention belongs to the technical field of additive manufacturing of aluminum alloys, and particularly relates to an in-situ strengthening method for improving the fracture toughness of aluminum alloys in additive manufacturing. Background Art

[0002] Additive Manufacturing (AM), also known as 3D printing, is an advanced manufacturing technology for fabricating three-dimensional solid parts by layer-by-layer material deposition. Different from traditional subtractive manufacturing (such as machining), additive manufacturing directly generates parts from digital models, with advantages such as high design freedom, high material utilization rate, and short manufacturing cycle. Among various metal additive manufacturing technologies, Laser Additive Manufacturing (LAM) is an advanced manufacturing technology that uses a high-energy laser beam as a heat source to fabricate three-dimensional solid parts by layer-by-layer melting or sintering of materials (such as metal powders, ceramic powders, etc.). Laser additive manufacturing technology has high precision, high energy density, and good material adaptability, and is widely used in fields such as aerospace, medical devices, and mold manufacturing.

[0003] High-strength aluminum alloys are a type of aluminum alloy material with excellent mechanical properties (such as high strength, high toughness, and low density), and are key structural materials commonly used in aerospace. In recent years, high-strength aluminum alloys represented by Al-Mg-Sc-Zr have received extensive attention and research in the field of additive manufacturing. The formed parts have the characteristics of high strength and high plasticity, and excellent mechanical properties. However, the added Sc element is costly due to its rarity and extraction difficulty, which limits its wider application. To reduce costs, domestic scholars have proposed using rare earth Er element to replace the expensive Sc element, obtaining Al3Er eutectic phase to refine grains, and precipitating Al3(Zr, Er) and Al3Er nano-precipitation phases during aging, to obtain additively manufactured high-strength and temperature-resistant aluminum alloys with high-strength and medium-plasticity mechanical properties.

[0004] Fracture Toughness is the ability of a material to resist crack propagation, and is an important mechanical index for evaluating the anti-fracture performance of a material. It reflects the ability of a material to resist brittle fracture in the presence of cracks or defects, and has received much attention in high-end manufacturing industries such as aerospace. Additively manufactured high-strength aluminum alloy Al-Mg-Er-Zr or Al-Mg-Sc-Zr alloy parts have high strength but poor fracture toughness, which greatly limits their practical applications. Therefore, it is urgent to develop new forming processes to obtain excellent fracture toughness. Summary of the Invention

[0005] The object of the present invention is to provide an in-situ strengthening method for improving the fracture toughness of aluminum alloy in additive manufacturing. By controlling the volume fraction of the remelted part of the cladding layer during the additive manufacturing process, on the premise of ensuring the strength of high-strength aluminum alloy, its fracture toughness is greatly improved, enabling the aluminum alloy to be applied to actual production.

[0006] The present invention adopts the following technical solutions:

[0007] An in-situ strengthening method for improving the fracture toughness of aluminum alloy in additive manufacturing, comprising the following steps:

[0008] (1) Preparation before additive manufacturing;

[0009] First, dry the high-strength aluminum alloy powder and then put it into the powder feeder; then, fix the formed substrate in a forming chamber protected by inert gas, and use an atmosphere circulation system to ensure a low oxygen content in the forming chamber; the oxygen content in the forming chamber is lower than 1000 ppm.

[0010] The high-strength aluminum alloy powder adopts Al-Mg-Er-Zr powder, specifically Al-7.00Mg-0.52Mn-1.22Er-1.43Zr-0.21Sc.

[0011] The drying is realized by a vacuum drying oven. Through the drying operation, the moisture in the high-strength aluminum alloy powder is removed, and the uniformity of the powder transmission during the additive manufacturing process is improved.

[0012] The inert gas in the forming chamber adopts argon or nitrogen; the oxidation reaction is reduced by the inert gas. Oxidation will cause an oxide layer to form on the material surface, affecting the interlayer bonding and the mechanical properties of the final part.

[0013] (2) Setting the remelting width of the forming track;

[0014] The remelting width is controlled by the laser scanning spacing method or the linear energy density method;

[0015] The remelting width w remelt = w - h, where w is the width of a single-pass cladding layer, and h is the laser scanning spacing.

[0016] The laser scanning spacing method means: keeping the linear energy density unchanged, adjusting the laser scanning spacing h to control the remelting width w remelt ;

[0017] The linear energy density method means: keeping the laser scanning spacing unchanged, adjusting the linear energy density, changing the cladding layer width w to control the remelting width w remelt ;

[0018] The remelting width refers to the width in the junction area between the newly deposited material layer and the previously solidified layer, where part of the solidified material is remelted due to heat conduction and the presence of the heat affected zone.

[0019] The width w of the single-pass cladding track is measured by printing single-pass single-layer specimens with different process parameters; specifically, the measurement is achieved using a vernier caliper; the different process parameters are based on the linear energy density to adjust the laser power and the scanning speed.

[0020] The linear energy density E line = P / V, where E line is the laser linear energy density (J / mm); P is the laser power (W); V is the scanning speed (mm / s).

[0021] By setting the remelting width, partial grain remelting and re-nucleation growth are realized. At the bottom of the molten pool, new crystal nuclei form fine equiaxed grains with Al3(Zr,Er) as the core, rather than epitaxial columnar grains; thereby increasing the crack propagation resistance and significantly enhancing the fracture toughness of the material while maintaining high strength characteristics.

[0022] (3) Additive manufacturing and subsequent processing;

[0023] Import the model of the part into the additive manufacturing equipment, and deposit the required part in the forming chamber according to the set process parameters, scanning strategy and remelting width; during the forming process, ensure that the forming chamber is in an inert gas protection atmosphere, and the oxygen content is always less than 1000 ppm.

[0024] After forming is completed, wait for the formed part to cool below 100 °C, take out the formed part, and perform heat treatment using the aging heat treatment process to obtain a formed part with high strength and high toughness characteristics.

[0025] Aging heat treatment process: Age at 200 - 400 °C for 2 - 10 h and air-cool to room temperature.

[0026] The beneficial effects of the present invention are as follows: By in-situ regulating the process parameters (scanning spacing or linear energy density) during the forming process, accurately controlling the width of the remelting zone, and significantly increasing the volume fraction of equiaxed grains in high-strength aluminum alloy components. Its microstructure presents a unique feature of alternating distribution of equiaxed grains and columnar grains, effectively hindering the straight propagation of cracks, causing the crack path to deflect and form branches, thereby reducing the stress concentration at the crack tip. Compared with traditional forming technologies, and under the heat treatment process (aging at 300 °C for 4 h), the proportion of equiaxed grains can be increased to a higher level, the fracture toughness is increased by 40.2%, and at the same time, the tensile strength and elongation are maintained stable. This technological breakthrough not only solves the problem of insufficient fracture toughness of additively manufactured high-strength aluminum alloys, but also provides a lightweight material solution with both high strength and high toughness characteristics for fields such as aerospace. Description of the Drawings

[0027] Figure 1 It is a schematic diagram of the relationship between the remelting width and the scanning spacing.

[0028] Figure 2 It is a schematic diagram of the relationship between the remelting width and the laser line energy density.

[0029] Figure 3 It is a schematic diagram of the microstructure of the part where the scanning spacing controls the remelting width.

[0030] Figure 4 It is a schematic diagram of the microstructure of the part where the laser line energy density controls the remelting width. Detailed implementation manners

[0031] Example 1

[0032] Taking the laser selective melting of Al-7.00Mg-0.52Mn-1.22Er-1.43Zr-0.21Sc alloy as an example, by changing the cladding track spacing (scanning spacing) in the process parameters, different remelting widths are obtained. The relationship between the remelting width and the scanning spacing is as Figure 1 shown, and the schematic diagram of the formed microstructure is as Figure 3 shown.

[0033] Specifically, the method provided in this example includes the following steps:

[0034] Step 1: Prepare the aluminum alloy powder for laser selective melting. Use the drying parameters of 120°C for 2 h in a vacuum drying oven to remove the moisture in the powder and improve the fluidity of the powder. After drying, take out the powder and put it into a sealed bag for standby.

[0035] Step 2: Load the powder into the forming chamber, set the process parameters, and complete the printing process according to the predetermined program. The specific process parameters used are shown in Table 1.

[0036] Table 1 Process parameters for laser selective melting of Al-Mg-Er-Zr alloy

[0037]

[0038] Step 3: Cut the specimen from the forming substrate and perform artificial aging treatment at 300°C for 4 h / AC to obtain the final formed part.

[0039] Use the parameters of process parameter No. 1 to SLM form sample 1. Its cladding track width W is 0.12 mm, the scanning spacing is 0.1 mm, so the remelting zone width is 0.02 mm, which can ensure the defect-free and stable forming of the specimen.

[0040] Use process parameter No. 1 to print and form 6 specimens with dimensions of Room temperature tensile samples, with 3 in the deposition direction (Z direction) and 3 in the horizontal direction (X direction), and 4 samples printed and formed with dimensions of 50×50×25 mm 3 Fracture toughness samples, with 2 notches along the deposition direction (X-Z direction) and 2 notches along the horizontal direction (Z-X direction). All samples were tested for performance after artificial aging treatment at 300 °C for 4 h / AC. The specific performance is shown in Table 2

[0041] Samples were formed by SLM using the parameters of process parameter number 2. The width W of the cladding track is 0.12 mm, and the scanning spacing is 0.06 mm. Therefore, the width of the remelting zone is 0.06 mm. Under this parameter, the deposited layers of the specimens all experienced some remelting processes. Six samples with dimensions of Room temperature tensile specimens, with 3 in the horizontal direction (X direction) and 3 in the deposition direction (Z direction), and 4 samples printed and formed with dimensions of 50×50×25 mm 3 Fracture toughness specimens, with 2 notches along the horizontal direction (Z-X direction) and 2 notches along the deposition direction (X-Z direction). All specimens were tested for performance after artificial aging treatment at 300 °C for 4 h / AC. The specific performance is shown in Table 2

[0042] Table 2 Mechanical properties of selective laser melting Al-Mg-Er-Zr

[0043]

[0044]

[0045] From the relevant performance data of Samples 1 and 2, it can be seen that by reducing the scanning spacing and thus increasing the width of the laser remelting zone, the fracture toughness can be significantly improved with little change in the strength and elongation of the room temperature tensile performance. The improvement ratio of the fracture toughness reaches 40.2%. The improved fracture toughness is higher than the performance data of the high-temperature deformed aluminum alloy 7075-T651 or Al-Mg-Sc-Zr reported in the current relevant literature. The increase in fracture toughness is related to the formation of more banded equiaxed grains during the scanning remelting process. The formation of these equiaxed grain regions causes more deflections during the crack propagation process, forming more crack branches, reducing the stress concentration at the crack tip, and thus significantly improving the fracture toughness

[0046] Example 2

[0047] By changing the laser line energy density and controlling the width of a single cladding track, different widths of the remelting zone can be obtained. The relationship between the remelting width and the laser line energy density is as Figure 2 shown. The specific implementation method is as follows

[0048] Step 1: Prepare the aluminum alloy powder for selective laser melting. Dry the moisture in the powder in a vacuum drying oven to improve the fluidity of the powder. After drying, take out the powder and put it into a sealed bag for standby.

[0049] Step 2: Load the powder into the forming chamber, set the process parameters, and complete the printing process of two specimens respectively using Process Parameters 1 and 2 in Table 3 according to the predetermined program.

[0050] Table 3 Process Parameters of Selective Laser Melting of Al-Mg-Er-Zr Alloy

[0051]

[0052] Step 3: Use a scanning electron microscope equipped with an electron backscatter diffraction (EBSD) probe to analyze the grain microstructure inside the two groups of specimens with different process parameters. The results are as Figure 4 shown.

[0053] From Figure 4 it can be seen that when a higher laser line energy density is used, due to the existence of a wider remelting width, there are more remelting regions inside the specimen. Relatively fine equiaxed grains will form at the bottom of the remelting region, while at other parts, the grains will coarsen under the action of a higher energy density. The microstructure morphology of the combination of coarse grains and fine grains is similar to the microstructure formed by Process Parameter 2 in Example 1. It can be inferred that the wider remelting zone width obtained by using Process Parameter 2 helps to improve the fracture toughness of the finally formed specimen.

[0054] The above is only the preferred embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any modification and replacement based on the technical solutions and inventive concepts provided by the present invention should be covered within the protection scope of the present invention.

Claims

1. An in-situ strengthening method for improving the fracture toughness of aluminum alloys in additive manufacturing, characterized in that, It includes the following steps: (1) Preparation before additive manufacturing; First, dry the high-strength aluminum alloy powder and then put it into the powder feeder; then, fix the formed substrate in the forming chamber protected by inert gas; (2) Setting the remelting width of the forming track; The remelting width is controlled by the laser scanning spacing method or the linear energy density method; the remelting width w remelt = w - h, where w is the width of a single-pass cladding bead and h is the laser scanning spacing; The laser scanning spacing method means that while keeping the linear energy density constant, the laser scanning spacing h is adjusted to control the remelting width w remelt ; The linear energy density method refers to keeping the laser scanning pitch unchanged and adjusting the linear energy density to control the remelting width w remelt ; The width w of the single-pass cladding track is obtained by measuring single-pass single-layer specimens with different process parameters; The different process parameters adjust the laser power and the scanning speed based on the linear energy density; the linear energy density E line = P / V, E line is the laser linear energy density; P is the laser power; V is the scanning speed; (3) Additive manufacturing and its heat treatment; Import the model of the part into the additive manufacturing equipment, and deposit the required part in the forming chamber according to the set process parameters, scanning strategy and remelting width; After forming, wait for the formed part to cool below 100°C, and obtain the formed part with high strength and high toughness characteristics through aging heat treatment.

2. An in-situ strengthening method for improving the fracture toughness of aluminum alloy in additive manufacturing according to claim 1, characterized in that The high-strength aluminum alloy powder is Al-Mg-Er-Zr powder.

3. An in-situ strengthening method for improving the fracture toughness of aluminum alloy in additive manufacturing according to claim 1, characterized in that The drying is realized by a vacuum drying oven.

4. An in-situ strengthening method for improving the fracture toughness of aluminum alloy in additive manufacturing according to claim 1, characterized in that, The inert gas in the forming chamber is argon or nitrogen; and an atmosphere circulation system is used to ensure a low oxygen content in the forming chamber; the oxygen content in the forming chamber is lower than 1000 ppm.

5. An in-situ strengthening method for improving the fracture toughness of aluminum alloy in additive manufacturing according to claim 1, characterized in that, The aging heat treatment means aging at 200-400°C for 2-10 h and air-cooling to room temperature.

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