An in-situ strengthening method to improve fracture toughness of aluminum alloys in additive manufacturing

Abstract

The application belongs to the technical field of aluminum alloy additive manufacturing, and particularly relates to an in-situ strengthening method for improving the fracture toughness of aluminum alloy in additive manufacturing, which comprises the following steps: (1) preparation before additive manufacturing; drying high-strength aluminum alloy powder and then putting the powder into a powder feeder; (2) setting of a remelting width of a forming path; the remelting width is controlled by a laser scanning interval method or a line energy density method; and (3) additive manufacturing and heat treatment; through the setting of the remelting width, partial grain remelting and re-nucleation and growth are realized, at the bottom of a molten pool, new crystal nuclei form fine equiaxed crystals with Al3(Zr, Er) as the core instead of epitaxial columnar crystals; through the alternate distribution of the columnar crystals and the equiaxed crystals, the crack propagation resistance is increased, and the fracture toughness of the material is significantly improved, while the high-strength characteristics are maintained.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of additive manufacturing of aluminum alloy, and particularly relates to an in-situ strengthening method for improving fracture toughness of aluminum alloy in additive manufacturing. BACKGROUND

[0002] Additive Manufacturing (AM), also known as 3D printing, is an advanced manufacturing technology that builds three-dimensional solid parts layer by layer. Unlike traditional subtractive manufacturing (such as cutting), additive manufacturing directly generates parts through digital models, with high design freedom, high material utilization, 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 manufacture three-dimensional solid parts by layering melting or sintering materials such as metal powder and ceramic powder. Laser additive manufacturing technology has high precision, high energy density, and good material adaptability, and is widely used in aerospace, medical devices, mold manufacturing, and other fields.

[0003] High-strength aluminum alloy is a type of aluminum alloy material with excellent mechanical properties (such as high strength, high toughness, and low density), which is commonly used as a key structural material 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 high strength and high plasticity, with excellent mechanical properties. However, the addition of Sc element results in high cost due to its rarity and difficulty in extraction, which limits its wider application. To reduce costs, domestic scholars propose 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 precipitates during aging, obtaining additive manufacturing high-strength temperature-resistant aluminum alloy with high strength and medium plasticity.

[0004] Fracture toughness is the ability of a material to resist crack propagation and is an important mechanical indicator for evaluating the material's resistance to fracture. It reflects the material's ability to resist brittle fracture in the presence of cracks or defects, and is of great concern in high-end manufacturing industries such as aerospace. Additive manufacturing 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 application. Therefore, it is urgent to develop new forming processes to obtain excellent fracture toughness. SUMMARY

[0005] The purpose of the present application is to provide an in-situ strengthening method for improving the fracture toughness of aluminum alloy in additive manufacturing, which controls the volume fraction of the remelted part of the cladding path in the additive manufacturing process, greatly improves the fracture toughness of high-strength aluminum alloy under the premise of ensuring the strength of the high-strength aluminum alloy, and enables the aluminum alloy to be used in actual production.

[0006] The present application 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 put it into the powder feeder; then, fix the shaped substrate in the shaped bin protected by inert gas, and use the atmosphere circulation system to ensure the low oxygen content in the shaped bin; the oxygen content of the shaped bin is less than 1000ppm.

[0010] The high-strength aluminum alloy powder uses 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 box, which removes the moisture in the high-strength aluminum alloy powder through drying operation and improves the uniformity of powder transmission in the additive manufacturing process.

[0012] The inert gas of the shaped bin uses argon or nitrogen; the inert gas reduces the oxidation reaction, which will cause the formation of an oxide layer on the surface of the material, affecting the interlayer bonding and the mechanical properties of the final part.

[0013] (2) Setting of the remelted width of the shaped path;

[0014] The remelted width is controlled by the laser scanning interval method or the line energy density method;

[0015] The remelted width w remelt = w - h, wherein w is the width of the single cladding path, and h is the laser scanning interval.

[0016] The laser scanning interval method refers to keeping the line energy density unchanged, adjusting the laser scanning interval h, and realizing the control of the remelted width w remelt .

[0017] The line energy density method refers to keeping the laser scanning interval unchanged, adjusting the line energy density, changing the cladding path width w, and realizing the control of the remelted width w remelt .

[0018] The remelt width refers to the width of a part of the solidified material being remelted due to the existence of heat conduction and heat affected zone between the interface of the newly deposited material layer and the previously solidified layer.

[0019] The width w of the single cladding track is measured by printing single-layer samples of single tracks with different process parameters; the specific measurement is realized by using a vernier caliper; the different process parameters are based on adjusting the laser power and scanning speed according to the linear energy density.

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

[0021] By setting the remelt width, part of the grains are remelted and nucleate and grow again, at the bottom of the molten pool, new crystal nuclei form fine equiaxed crystals with Al3(Zr, Er) as the core instead of epitaxial columnar crystals; thus, the crack propagation resistance is increased, and the fracture toughness of the material is significantly improved, while the high strength characteristics are maintained.

[0022] (3) additive manufacturing and subsequent processing thereof;

[0023] The model of the part is imported into the additive manufacturing equipment, and the required part is deposited in the forming bin according to the set process parameters, scanning strategy and remelt width; during the forming process, an inert gas protective atmosphere is ensured in the forming bin, and the oxygen content is always less than 1000 ppm.

[0024] After the forming is completed, the formed part is taken out after cooling to below 100 DEG C, and heat treatment is carried out by using an aging heat treatment process to obtain a formed part with high strength and high toughness characteristics.

[0025] The aging heat treatment process is: aging at 200-400 DEG C for 2-10 h, and air cooling to room temperature.

[0026] The beneficial effects of the present application are: by in-situ adjusting the process parameters (scanning pitch or linear energy density) during the forming process, the remelt width is accurately controlled, and the volume fraction of equiaxed crystals in the high-strength aluminum alloy component is significantly improved. The microstructure presents a unique feature of alternating distribution of equiaxed crystals and columnar crystals, effectively hinders the straight-line expansion of cracks, promotes the deflection and branching of the crack path, and thus reduces the stress concentration at the crack tip. Compared with the traditional forming technology, and under the heat treatment process (300 DEG C / 4h aging), the proportion of equiaxed crystals can be increased to a higher level, the fracture toughness is improved by 40.2%, and the stability of the tensile strength and elongation is maintained. This technical breakthrough not only solves the problem of insufficient fracture toughness of high-strength aluminum alloy in additive manufacturing, but also provides a lightweight material solution with high strength and high toughness characteristics for the aerospace field. BRIEF DESCRIPTION OF DRAWINGS

[0027] Figure 1 A schematic diagram of the relationship between the remelt width and the scanning pitch.

[0028] Figure 2 A schematic diagram of the relationship between the remelt width and the laser line energy density.

[0029] Figure 3 A schematic diagram of the part microstructure controlled by the scanning pitch to control the remelt width.

[0030] Figure 4 A schematic diagram of the part microstructure controlled by the laser line energy density to control the remelt width. DETAILED DESCRIPTION

[0031] Example 1

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

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

[0034] Step 1: Prepare the aluminum alloy powder used for laser selective melting, use 120℃, 2h drying parameters in a vacuum drying box to remove the moisture in the powder and improve the flowability of the powder, and after drying, take out the powder and pack it in a sealed bag for standby.

[0035] Step 2: Pack the powder into the forming bin, 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 Laser selective melting Al-Mg-Er-Zr alloy process parameters

[0037]

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

[0039] The SLM formed sample 1 using the process parameter No. 1 has a cladding channel width W of 0.12mm and a scanning pitch of 0.1mm, so the remelt region width is 0.02mm, which can ensure the stable formation of the sample without defects.

[0040] Six samples with a size of​ room temperature tensile samples, 3 of which are in the deposition direction (Z direction) and 3 of which are in the horizontal direction (X direction), and 4 samples with a size of 50x50x25mm 3 fracture toughness samples, 2 of which are in the horizontal direction (Z-X direction) and 2 of which are in the deposition direction (X-Z direction). All samples were tested after artificial aging at 300°C for 4h / AC, and the specific performance is shown in Table 2.

[0041] The sample was SLM formed using process parameter No. 2, with a cladding path width W of 0.12mm and a scanning pitch of 0.06mm, so the width of the remelted zone is 0.06mm. Under this parameter, the deposition layers of the sample all undergo some remelting process. Six samples were printed and formed using process parameter 2, with a size of 50x50x25mm room temperature tensile samples, 3 of which are in the deposition direction (Z direction) and 3 of which are in the horizontal direction (X direction), and 4 samples with a size of 50x50x25mm 3 fracture toughness samples, 2 of which are in the horizontal direction (Z-X direction) and 2 of which are in the deposition direction (X-Z direction). All samples were tested after artificial aging at 300°C for 4h / AC, and the specific performance is shown in Table 2.

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

[0043]

[0044]

[0045] From the relevant performance data in samples 1 and 2, it can be seen that by reducing the scanning pitch and thus increasing the width of the laser remelted zone, the fracture toughness can be significantly improved without changing the strength and elongation of the room temperature tensile performance, with an improvement ratio of 40.2%, and the improved fracture toughness is higher than the performance data of high-temperature deformed aluminum alloy 7075-T651 or Al-Mg-Sc-Zr reported in the current relevant literature. The improvement of fracture toughness is related to the formation of more strip-shaped equiaxed crystals in the scanning and remelting process. The formation of these equiaxed crystal regions causes more deflection during crack propagation, forms more crack branches, reduces the stress concentration at the crack tip, and thus significantly improves the fracture toughness.

[0046] Example 2

[0047] By changing the laser line energy density, the width of the single cladding path is controlled to obtain different remelted zone widths, and the relationship between the remelted width and the laser line energy density is shown in Figure 2 The specific implementation is as follows:

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

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

[0050] Table 3 Laser selective melting Al-Mg-Er-Zr alloy process parameters

[0051]

[0052] Step 3: Analyze the grain structure inside the samples of the two different process parameters using a scanning electron microscope equipped with an electron backscatter diffraction (EBSD) probe, and the results are shown in Figure 4

[0053] As can be seen from Figure 4 , when a higher laser line energy density is used, there are more remelted areas inside the sample due to the existence of a wider remelted width, and the bottom of the remelted area forms relatively small equiaxed crystals, while other parts of the sample under the action of a higher energy density, the grains are coarsened. This microstructure morphology combining coarse grains and fine grains is similar to the microstructure formed by process parameter 2 in Example 1, and it can be inferred that the wider remelted width obtained by using process parameter 2 helps to improve the fracture toughness of the final formed sample.

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

Claims

1. An in-situ strengthening method to improve fracture toughness of aluminum alloys in additive manufacturing, characterized in that, Comprising the following steps: (1) Preparation before additive manufacturing; Firstly, high-strength aluminum alloy powder is dried and put into a powder feeder; then, a shaped substrate is fixed in a shaping bin protected by inert gas; the high-strength aluminum alloy powder is Al-Mg-Er-Zr powder; (2) Setting of the remelting width of the forming path; The remelt width is controlled by a laser scanning pitch method or a line energy density method; the remelt width wherein, is a width of a single cladding pass, is a laser scanning pitch; The laser scanning interval method refers to keeping the linear energy density unchanged and adjusting the laser scanning interval Controlling the remelting width ; The line energy density method refers to keeping the laser scanning interval unchanged and adjusting the line energy density to control the remelting width ; a width of the single cladding pass is obtained by measuring single-pass single-layer test specimens printed with different process parameters; Different process parameters are laser power and scanning speed adjusted based on line energy density; the line energy density , is the laser line energy density; is the laser power; is the scanning speed; (3) Additive manufacturing and heat treatment thereof; The model of the part is imported into the additive manufacturing equipment, and the required part is deposited in the shaping bin according to the set process parameters, scanning strategy and remelting width; After the shaping is completed, the shaped part is cooled to below 100℃, and is subjected to aging heat treatment to obtain a shaped part with high strength and high toughness characteristics; the aging heat treatment refers to aging at 200-400℃ for 2-10h, and air cooling to room temperature.

2. The in-situ strengthening method of claim 1, wherein, The drying is achieved by using a vacuum drying box.

3. The in-situ strengthening method for improving fracture toughness of aluminum alloys in additive manufacturing according to claim 1, wherein, The inert gas of the shaping bin is argon or nitrogen; and an atmosphere circulation system is used to ensure low oxygen content in the shaping bin; the oxygen content of the shaping bin is less than 1000ppm.

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