Aluminum-lithium alloy and slm forming method thereof

Abstract

The application discloses an aluminum-lithium alloy and an SLM forming method thereof, and the strength of an aluminum-lithium alloy test piece formed by laser selective melting is improved by adding a small amount of Ti element into the alloy and performing aging treatment at a low temperature, utilizing heterogeneous nucleation of a delta'-Al3Li phase to promote a secondary beta'-Al3Ti phase to be precipitated in a low-temperature aging process, thereby realizing synergistic strengthening of multiple precipitated phases. The Ti element in the alloy composition is combined with Al atoms, thereby inhibiting forming cracks in the laser selective melting process. In the growth process of fine grains, the feeding channel between the grains is short, the liquid phase feeding efficiency is high, and the generation of solidification cracks is effectively inhibited; and the fine equiaxed grains in the mushy zone in the solidification stage release the stress generated in the solidification process, the alloy cracking tendency is reduced, and the crack propagation resistance of the alloy is further improved. The application greatly reduces the cracking tendency of the alloy, and simultaneously improves the strength of the alloy through fine grain strengthening and precipitated strengthening of multiple secondary precipitated phases.

Description

Technical Field

[0001] This invention relates to the field of materials technology, specifically to an aluminum-lithium alloy with a strength higher than 510 MPa and its laser selective melting forming method. Background Technology

[0002] Aluminum-lithium alloys, due to their low density, high specific strength, and excellent corrosion resistance, have broad application prospects in the aerospace field. In recent years, the demand for integrated, complex, and precision manufacturing of high-strength aluminum-lithium alloy components has been continuously increasing. Traditional forming processes such as casting and extrusion are increasingly unable to directly produce high-strength aluminum-lithium alloy components. Selective Laser Melting (SLM) technology, due to its ability to achieve high-precision free-form forming of complex metal components, is attracting increasing attention in the manufacturing of high-strength and complex aluminum-lithium alloy components. However, most current aluminum-lithium alloy compositions are designed for traditional casting or welding processes, with wide solidification temperature ranges and large solidification shrinkage. Under the high cooling rate and high stress conditions of SLM forming, they often face problems such as severe cracking and low density of formed specimens. Therefore, there is an urgent need to design a new type of aluminum-lithium alloy that can suppress the tendency of alloy cracking and improve the formability of SLM.

[0003] Most current methods involve introducing microalloying elements into the alloy system, allowing the alloy to spontaneously form heterogeneous nucleation sites during solidification, thereby refining the grains and suppressing cracking.

[0004] In their doctoral dissertation, "The Influence of Zr Modification on the Microstructure and Mechanical Properties of Laser Selective Melting Al-Cu-Mg Alloys," Wang Y et al. disclosed a crack suppression method. This method involves adding approximately 1.5 wt.% Zr to aluminum alloy powder via pre-alloying, successfully refining the grain structure and suppressing cracks during laser selective melting. However, the significant addition of Zr significantly increases the alloy density. In invention publication CN117418147A, 0.1-1.2 wt.% Sc, 0.1-1.2 wt.% Zr, and 0.1-1.2 wt.% Y are introduced into the Al-Li-Cu alloy system to suppress cracking tendency and improve formability during SLM. However, the elements introduced into the alloy using this method are relatively expensive, significantly increasing production costs. In publication CN 116287913… In A's invention, one or more rare earth elements such as Sc, Ce, and Er are introduced into the Al-Li-Cu alloy system to improve its laser selective melting formability. The elements introduced into the alloy by this method are also relatively expensive, which will significantly increase the production cost. In addition, the relative atomic mass of the introduced elements is relatively large, which will significantly increase the alloy density, which is not conducive to the lightweighting of aluminum-lithium alloy structural parts.

[0005] In summary, the main method for suppressing cracking behavior and improving the formability of aluminum-lithium alloys during selective laser melting (SLM) is to introduce rare earth elements into the alloy through pre-alloying to refine the grains, suppress cracks, and improve formability. However, most of the introduced elements are either high-density or expensive, inevitably increasing the density of the aluminum-lithium alloy, hindering the lightweighting of aluminum-lithium alloy structural components, and increasing production costs. To address these problems, this invention introduces Ti, an inexpensive element with a relatively small atomic mass, into the aluminum-lithium alloy to suppress cracking and improve its SLM formability. Summary of the Invention

[0006] To overcome the shortcomings of high production costs and high alloy density in existing technologies, this invention proposes an aluminum-lithium alloy and its selective laser melting forming method.

[0007] The aluminum-lithium alloy proposed in this invention is composed of 1.4-2.1% Li, 2.5-3.3% Cu, 0.25-0.8% Mg, 0.1-0.6% Ag, 0.6-1.0% Ti and the balance Al; all percentages are by mass; the sum of the percentages of all components is 100%.

[0008] The specific process for preparing the aluminum-lithium alloy proposed in this invention is as follows:

[0009] Step 1, Prepare powdered raw materials:

[0010] The tip of an aluminum-lithium alloy rod is heated and melted using a rotating electrode method. The molten droplets at the tip of the rod are then ejected by high-speed rotation and solidified into spherical powder under the action of surface tension. The rotation speed is 15,000 to 25,000 r / min.

[0011] The obtained spherical powder was sieved to obtain particles with a diameter of 15–85 μm and a bulk density of 1.35–1.45 g / cm³. 3、 Powder raw materials.

[0012] The aluminum-lithium alloy rod is made by melting pure aluminum ingots, pure magnesium ingots, pure zinc ingots, pure silver ingots with a purity of 99.9% or higher, as well as Al-Cu master alloys, Al-Li master alloys, and Al-Ti master alloys, and pouring them into an ingot mold. After solidification, an aluminum-lithium alloy rod containing 1.4–2.1% Li, 2.5–3.3% Cu, 0.25–0.8% Mg, 0.1–0.6% Ag, 0.6–1.0% Ti, and the balance being Al is obtained; the percentages are by mass.

[0013] Step 2, drying of powder raw materials:

[0014] Drying is used to remove gases and moisture from the powdered raw materials.

[0015] Step 3, clean the substrate;

[0016] Step 4, draw the specimen model:

[0017] The 3D model of the specimen was drawn using Materialise Magics drawing software; the generated .stl file was then sliced ​​using conventional methods to obtain the specimen model that the 3D printer could read.

[0018] Step 5, Printing Preparation:

[0019] During the printing preparation, argon gas with a purity higher than 99.9% is continuously introduced into the forming chamber to expel oxygen from the forming chamber and keep the oxygen content below 100ppm throughout the forming process to prevent oxidation of the specimen during printing.

[0020] Once the oxygen content in the chamber drops below 100 ppm, preheating is activated to heat the aluminum alloy substrate to 150°C, thereby reducing thermal stress during the forming process and suppressing the tendency of the specimen to crack.

[0021] Step 6, print the specimen:

[0022] The specimen printing process involves multi-layer laser selective melting; each layer of laser selective melting includes powder spreading and scanning melting. The powder spreading in each layer is scanned and melted layer by layer using a laser to obtain a preform of the desired aluminum-lithium alloy specimen.

[0023] The specific process for printing the specimen is as follows:

[0024] Laser selective melting forming of the first layer: Turn on the 3D printer and lay the first layer of powder raw material on the surface of the aluminum alloy substrate. The thickness of the layer is 15-60μm. Start the laser to scan and melt the first layer of powder according to the three-dimensional model of the specimen, and complete the laser selective melting forming of the first layer of powder.

[0025] Laser selective melting to form the second layer: Repeat the process of laser selective melting to form the first layer. Specifically, a second layer of powder raw material with a thickness of 15-60 μm is laid on the surface of the first layer; the laser is activated to scan and melt the laid second layer of powder according to the three-dimensional model of the specimen, thus completing the laser selective melting to form the second layer of powder.

[0026] Continue repeating the process of spreading and melting the first layer of powder in the laser selective melting forming process, and complete the laser selective melting forming of each remaining layer until the specimen is prepared.

[0027] When printing the test specimen, the laser power is 120-240W, the scanning rate is 100-1000mm / s, the track spacing is 80-120μm, and the interlayer deflection angle is 67°.

[0028] Step 7, cut off the specimen:

[0029] After printing is completed and the aluminum alloy substrate has cooled to room temperature, the preform is removed and cut off from the substrate to obtain the semi-finished product of the required aluminum-lithium alloy specimen.

[0030] Step 8, Solution treatment:

[0031] The obtained aluminum-lithium alloy semi-finished product was subjected to solution treatment.

[0032] After solution treatment, the semi-finished aluminum-lithium alloy specimen was cooled to room temperature by water quenching, thus completing the solution treatment of the semi-finished aluminum-lithium alloy specimen.

[0033] During the solution treatment, the solution treatment temperature is 550–570℃ and the solution treatment time is 60–120 min.

[0034] Step 9, Time-sensitive processing:

[0035] The obtained solution-treated aluminum-lithium alloy specimen was subjected to aging treatment. After aging, it was cooled to room temperature by air cooling. The resulting aluminum-lithium alloy specimen was then obtained.

[0036] The aging treatment temperature is 160–180℃, and the aging treatment time is 24–48 hours.

[0037] The aluminum-lithium alloy proposed in this invention is suitable for SLM forming process, will not crack during forming, and has high strength.

[0038] Compared with the prior art, the beneficial effects achieved by the present invention are as follows:

[0039] 1. The alloy composition of this invention contains 0.6–1.0 wt.% Ti. During the rapid solidification process of laser selective melting, Ti atoms in the melt can combine with Al atoms to form a primary Al3Ti phase with an L12 structure. This phase precipitates before α-Al and has a low lattice mismatch with α-Al, serving as a heterogeneous nucleation core to promote the formation of equiaxed crystals, significantly refining grains and suppressing forming cracks during laser selective melting. Compared to elements such as Zr or Y, Ti has a lower relative atomic mass, and only a small amount of Ti needs to be introduced into the alloy system to obtain a high molar fraction of Al3X (X is Zr, Y, or Ti) primary phase during alloy solidification. This invention uses Thermo-Calc thermodynamic software to calculate the theoretical solidification path of the alloy with a Ti content of 0.8 wt.%, as shown below. Figure 3 As shown, during solidification, approximately 2 mol.% of the primary β′-Al3Ti phase precipitated first in the temperature range of 875–655 °C, while heavy elements such as Zr or Y require an addition of about 1.5 wt.% to obtain the same molar fraction of the primary β′-Al3X phase. Therefore, compared to heavy elements such as Zr or Y, the amount of Ti required to introduce the element is less, thus preventing an increase in the density of the aluminum-lithium alloy.

[0040] 2. Compared to introducing Sc into the alloy, introducing Ti can save production costs. Sc and Ti have similar relative atomic masses, and their effects on grain refinement and crack suppression are essentially equivalent at the same mass fraction. However, the current price of Sc is approximately 24,000 yuan / kg, while the price of Ti is only about 77 yuan / kg. Therefore, using Ti to replace Sc can significantly reduce production costs.

[0041] 3. Existing technologies often add large amounts of elements such as Sc and Zr to alloys, so the alloy strength mainly comes from the strengthening effect of Al3X. The strengthening potential of traditional strengthening phases such as Ti-Al2CuLi and δ′-Al3Li in aluminum-lithium alloys has not been fully explored. In this invention, by adding a small amount of Ti to the alloy and performing aging treatment at a lower temperature, the heterogeneous nucleation of the δ′-Al3Li phase promotes the precipitation of the secondary β′-Al3Ti phase during the low-temperature aging process. This results in the alloy simultaneously containing traditional strengthening phases such as Ti-Al2CuLi and δ′-Al3Li, as well as the secondary β′-Al3Ti phase, thereby achieving synergistic strengthening of multiple precipitated phases and improving the strength of laser selective melting-formed aluminum-lithium alloy specimens.

[0042] In summary, the high-strength aluminum-lithium alloy for SLM proposed in this invention comprises the following components by mass percentage: Li 1.4-2.1%, Cu 2.5-3.3%, Mg 0.25-0.8%, Ag 0.1-0.6%, Ti 0.6-1.0%, and the balance Al. Figure 1 and Figure 2 The image shows the particle size distribution and morphology of the powder used in this invention. It can be seen that the powder used in this invention has a diameter between 15 and 85 μm and good sphericity. Figure 3 As shown, the present invention predicts through thermodynamic analysis that during the SLM forming process, the alloy will first precipitate about 2 mol% of primary β′-Al3Ti particles in the melt. These particles can serve as heterogeneous nucleation nuclei to effectively promote the nucleation and growth of equiaxed crystals. Figure 4 The image shows the morphology of the deposited SLM high-strength aluminum-lithium alloy formed according to this invention under an optical microscope. It can be seen that within the forming process range of this invention, the specimen exhibits good formability, high density, and only a small number of porosity or poor fusion defects. Figure 5 and Figure 6 The image shows a scanning electron microscope (SEM) image and a backscattered electron diffraction (RSD) inverse pole figure of the specimen formed according to the present invention. It can be seen that the specimen formed according to the present invention has significantly refined grains and low texture strength. The microstructure consists of equiaxed crystals inside the molten pool and columnar crystals at the edge of the molten pool. The diameter of the equiaxed crystals is about 1.7 μm and the width of the columnar crystals is about 4 μm. During the growth of fine grains, the feeding channels between grains are short and the liquid phase feeding efficiency is high, which effectively suppresses the generation of solidification cracks. Moreover, the fine equiaxed crystals in the mushy region during the solidification stage can rotate and move under stress, thereby releasing the stress generated during solidification and reducing the tendency of the alloy to crack. At the same time, the fine equiaxed crystals formed after solidification can complicate the crack propagation path, further improving the alloy's resistance to crack propagation and increasing the alloy strength through grain refinement. Figure 7 This is a schematic diagram of the intracrystalline structure of the specimen formed according to the present invention under a transmission electron microscope. It can be seen that a large number of light-colored, blocky primary β′-Al3Ti phases have formed inside the grains, which is consistent with... Figure 3 The results are consistent with the thermodynamic calculations. Figure 8 This is a schematic diagram of the intracrystalline structure of the specimen formed according to the present invention after solution aging treatment under a transmission electron microscope. It can be seen that a large number of fine nano-precipitated phases have precipitated in the structure, including δ′-Al3Li phase, θ′-Al2Cu phase, χ-Al5Cu6Li2 phase and T1-Al2CuLi phase. In addition, due to the introduction of Ti element, a large number of nano-scale secondary β′-Al3Ti phases have also precipitated in the component, which plays a significant precipitation strengthening role. Figure 9The stress-strain curves of high-strength aluminum-lithium alloy specimens prepared by SLM forming and heat treatment are obtained from room temperature tensile tests. It can be seen that the SLM high-strength aluminum-lithium alloy formed by this invention, after solution aging treatment, achieves a yield strength of 448.3±2.2 MPa, a tensile strength of 516.5±2.7 MPa, and an elongation of 6.13±0.80%. Therefore, the SLM-specific high-strength aluminum-lithium alloy provided by this invention can promote the growth of equiaxed crystals by introducing primary Al3Ti particles as heterogeneous nucleation cores, greatly reducing the alloy's cracking tendency. Simultaneously, it improves the alloy's strength through fine-grain strengthening and precipitation strengthening effects of various secondary precipitates. Attached Figure Description

[0043] Figure 1 This is a powder particle size distribution diagram of the high-strength aluminum-lithium alloy used for selective laser melting in this invention.

[0044] Figure 2 This is a scanning electron microscope image of the powder of the high-strength aluminum-lithium alloy used for selective laser melting in this invention.

[0045] Figure 3 The solidification path diagram of the aluminum-lithium alloy provided by this invention in the SLM rapid prototyping process was calculated using Thermo-Calc software;

[0046] Figure 4 This is a cross-sectional optical mirror image of a high-strength aluminum-lithium alloy specimen prepared under different laser selective melting forming parameters according to the present invention.

[0047] Figure 5 The cross-sectional microstructure of the high-strength aluminum-lithium alloy specimen obtained by the laser selective melting forming method of the present invention;

[0048] Figure 6 This is an EBSD scan of the cross-section of a high-strength aluminum-lithium alloy specimen obtained by the laser selective melting forming method of the present invention.

[0049] Figure 7 This is a microstructure diagram of the high-strength aluminum-lithium alloy specimen obtained by the laser selective melting forming method of the present invention.

[0050] Figure 8 This is a microstructure of the high-strength aluminum-lithium alloy specimen prepared by the laser selective melting method of the present invention after solution aging treatment.

[0051] Figure 9 The stress-strain curves of high-strength aluminum-lithium alloy specimens prepared by laser selective melting and heat treatment according to the present invention are obtained by room temperature tensile testing.

[0052] Figure 10The image shows the cross-sectional optical metallographic images of Ti-free aluminum-lithium alloy specimens prepared under different laser selective melting forming parameters according to the present invention.

[0053] Figure 11 This is an EBSD scan of the cross-section of a Ti-free aluminum-lithium alloy specimen prepared by the laser selective melting forming method of the present invention.

[0054] Figure 12 This is a flowchart of the present invention.

[0055] In the figure: 1 shows the solidification and precipitation process of the primary β′-Al3Ti phase in the melt; 2 shows the solidification process of α-Al in the melt after the precipitation of the primary β′-Al3Ti phase; 3 shows the solidification process of the intergranular eutectic phase in the final stage of solidification; 4 and 5 show the porosity defects in the modified SLM-formed specimen; 6 and 7 show the lack of fusion defects in the modified SLM-formed specimen; 8 shows the morphology of the columnar crystal region at the edge of the melt pool; 9 shows the morphology of the equiaxed crystal region at the center of the melt pool; 10 shows the blocky primary β′-Al3Ti particles in the deposited specimen after SLM forming modification; 11 shows the modified SLM-formed specimen after solution treatment and aging. 12 is the precipitated spherical secondary β′-Al3Ti strengthening phase; 13 is the precipitated δ′-Al3Li strengthening phase after solution aging of the SLM-molded modified specimen; 14 is the precipitated θ′-Al2Cu strengthening phase after solution aging of the SLM-molded modified specimen; 15 is the precipitated χ-Al5Cu6Li2 strengthening phase after solution aging of the SLM-molded modified specimen; 16 is the precipitated TI1-Al2CuLi strengthening phase after solution aging of the SLM-molded modified specimen; 17 is the precipitated pore defect in the unmodified SLM-molded specimen; 18 is the precipitated crack defect in the unmodified SLM-molded specimen. Detailed Implementation

[0056] This invention proposes a high-strength aluminum-lithium alloy for laser selective melting forming and its forming method. The technical solution of this invention will be described in detail through 12 embodiments.

[0057] The high-strength aluminum-lithium alloy for laser selective melting (SLM) proposed in this invention comprises, by mass percentage: 1.4–2.1% Li, 2.5–3.3% Cu, 0.25–0.8% Mg, 0.1–0.6% Ag, 0.6–1.0% Ti, and the balance Al. The composition of the powder used in the various embodiments of this invention is shown in Tables 1-1 and 1-2.

[0058] Table 1-1 Components of Examples 1 to 6

[0059]

[0060] Table 1-2 Components of Examples 7-12

[0061]

[0062] The specific process for preparing the high-strength aluminum-lithium alloy for laser selective melting forming proposed in this invention is as follows:

[0063] Step 1, Prepare powdered raw materials:

[0064] In preparing the raw materials, pure aluminum ingots, pure magnesium ingots, pure zinc ingots, pure silver ingots with a purity of 99.9% or higher, as well as Al-Cu master alloys, Al-Li master alloys, and Al-Ti master alloys, are melted and poured into an ingot mold. After solidification, an aluminum-lithium alloy rod containing 1.4–2.1% Li, 2.5–3.3% Cu, 0.25–0.8% Mg, 0.1–0.6% Ag, 0.6–1.0% Ti, and the balance being Al is obtained; the percentages are by mass.

[0065] The end of the bar is heated and melted using a conventional rotating electrode method, and the tiny droplets of molten material at the end of the bar are thrown out by high-speed rotation, solidifying into spherical powder under the action of surface tension; the rotation speed of the high-speed rotation is between 15,000 and 25,000 r / min.

[0066] The obtained spherical powder was sieved to obtain particles with a diameter of 15–85 μm and a bulk density of 1.35–1.45 g / cm³. 3、 Powder raw materials.

[0067] Step 2, drying of powder raw materials:

[0068] The powder raw material was placed in a vacuum environment at 120°C for 120 minutes to remove gas and moisture from the powder raw material.

[0069] Step 3, Clean the substrate:

[0070] The substrate material is 7075 aluminum alloy. The substrate was cleaned sequentially with acetone and then alcohol to remove oil and grease from its surface.

[0071] Step 4, draw the specimen model:

[0072] The 3D model of the specimen was drawn using Materialise Magics drawing software; the generated .stl file was then sliced ​​using conventional methods to obtain the specimen model that the 3D printer could read.

[0073] Step 5, Printing Preparation:

[0074] The powder is loaded into the powder chamber of the 3D printing device; the forming chamber is closed, and argon gas with a purity of over 99.9% is continuously introduced into the chamber to expel the oxygen in the forming chamber and keep the oxygen content below 100ppm throughout the forming process to prevent the specimen from oxidizing during printing.

[0075] Once the oxygen content in the chamber drops below 100 ppm, preheating is activated to heat the aluminum alloy substrate to 150°C, thereby reducing thermal stress during the forming process and suppressing the tendency of the specimen to crack.

[0076] Step 6, print the specimen:

[0077] The specimen printing process involves multi-layer selective laser melting; each layer of selective laser melting includes powder spreading and melting. The powder spread in each layer is scanned and melted layer by layer using a laser to obtain a preform of the desired aluminum-lithium alloy specimen.

[0078] The first layer is formed by selective laser melting; specifically, the 3D printer is turned on, and the first layer of powder raw material is laid on the surface of the aluminum alloy substrate with a thickness of 15-60μm; the laser is started to scan and melt the first layer of powder according to the three-dimensional model of the specimen, thus completing the selective laser melting of the first layer of powder.

[0079] The process of powder spreading and melting in the first layer of selective laser melting (SLM) is repeated to complete the second layer. Specifically, a second layer of powder raw material with a thickness of 15–60 μm is spread on the surface of the first layer; the laser is then activated to scan and melt the spread second layer of powder according to the three-dimensional model of the specimen, completing the 3D printing of the second layer of powder. The process of spreading and melting the first layer of powder in SLM is repeated again, and the remaining layers are processed layer by layer until the specimen is prepared, obtaining the preform of the desired aluminum-lithium alloy specimen.

[0080] When printing the test specimen, the laser power is 120-240W, the scanning rate is 100-1000mm / s, the powder layer thickness is 15-60μm, the track spacing is 80-120μm, and the interlayer deflection angle is 67°.

[0081] Table 2-1 Process parameters for each embodiment of step 6

[0082]

[0083] Table 2-2 Process parameters for six embodiments of step 2-2

[0084]

[0085] Step 7: Remove the specimen.

[0086] After the printing is completed and the aluminum alloy substrate has cooled to room temperature, the preform is removed and the specimen is cut off from the substrate using wire electrical discharge machining to obtain the semi-finished product of the required aluminum-lithium alloy specimen.

[0087] Step 8, solution treatment.

[0088] The obtained aluminum-lithium alloy semi-finished specimens were subjected to solution treatment. Specifically, the heat treatment furnace was heated to 550–570°C, and after the furnace temperature stabilized, the specimens were placed in the heat treatment furnace for solution treatment for 60–120 minutes. After solution treatment, the semi-finished aluminum-lithium alloy specimens were cooled to room temperature by water quenching. The solution-treated specimens were then obtained.

[0089] Table 3-1 Process parameters for each embodiment of step 8

[0090]

[0091] Table 3-2 Process parameters for each embodiment of step 8

[0092]

[0093] Step 9, Time-sensitive processing.

[0094] The semi-finished aluminum-lithium alloy specimens obtained after solution treatment were subjected to aging treatment. Specifically, the heat treatment furnace was heated to 170°C, and after the furnace temperature stabilized, the solution-treated specimens were placed in the heat treatment furnace for aging treatment for 36 hours. After aging, the specimens were cooled to room temperature by air cooling. Aluminum-lithium alloy specimens with a strength higher than 510 MPa were obtained.

[0095] Table 4-1 Process parameters for each embodiment in step 9

[0096]

[0097] Table 4-2 Process parameters for each embodiment in step 9

[0098]

[0099] This invention compares its technical solution with 12 comparative examples. The technical solutions and preparation processes of each comparative example, as well as the resulting technical effects, are described below:

[0100] Comparative Example 1

[0101] The components of the SLM-specific high-strength aluminum-lithium alloy in Example 1 were replaced with the following components by mass percentage: Li 1.6%, Cu 2.8%, Mg 0.28%, Ag 0.26%, and the balance Al. The laser power was replaced with 120W, the scanning rate was replaced with 100mm / s, and the remaining technical features, preparation process, and process parameters were the same as in Example 1. Ti-free Al-Li-Cu-Mg-Ag alloy specimens were prepared.

[0102] Comparative Examples 2-6

[0103] The laser scanning rate of SLM forming in Comparative Example 1 was replaced sequentially with 200 mm / s, 400 mm / s, 600 mm / s, 800 mm / s and 1000 mm / s. The other technical features were the same as those in Comparative Example 1. The schemes after replacing the above parameters were used as Comparative Examples 2 to 6 in sequence.

[0104] Comparative Examples 7–12

[0105] The laser power of SLM forming in Comparative Examples 1 to 6 was replaced with 240W, while the other technical features remained the same as those in Comparative Examples 1 to 6. The schemes after replacing the above parameters were used as Comparative Examples 7 to 12 in turn.

[0106] Performance testing

[0107] The particle size distribution of the SLM-specific high-strength aluminum-lithium alloy powder used in Examples 1-12 of this invention was tested, and the resulting particle size distribution test diagram is shown in Figure 1. Figure 1 It can be seen that the SLM-specific high-strength aluminum-lithium alloy powder used in this invention is mainly D. 10 =13.52μm, D 50 =31.38μm, D 90 =58.90μm.

[0108] The powder morphology of the SLM-specific high-strength aluminum-lithium alloy used in Examples 1-12 of this invention was observed using a scanning electron microscope (SEM), and the obtained SEM images are shown below. Figure 2 As shown. By Figure 2 It can be seen that the powder of the SLM-specific high-strength aluminum-lithium alloy used in Examples 1 to 12 of the present invention has a relatively uniform particle size and the powder morphology presents as a uniform spherical shape.

[0109] The alloy composition described in Example 4 of this invention was imported into Thermo-Calc thermodynamic calculation software to obtain the solidification path of the alloy under this composition during rapid solidification, as shown below. Figure 3 As shown. By Figure 3It can be seen that during the solidification process, the alloy described in this invention will first form a large number of primary Al3Ti phases in the melt. These primary Al3Ti phases can act as heterogeneous nucleation cores to promote the growth of α-Al equiaxed crystals, thereby inhibiting the cracking tendency of the alloy and improving its SLM formability.

[0110] The cross-sectional morphology of specimens prepared by the SLM high-strength aluminum-lithium alloy powder used in Examples 1-12 of this invention under different SLM forming parameters was observed using a Keyence VHX-2000 optical microscope. The obtained cross-sectional optical micrographs are shown below. Figure 4 As shown; where, Figure 4 Images (a-i) are, in order, cross-sectional optical mirror images of high-strength aluminum-lithium alloys prepared under different parameters in Examples 1-12. Figure 4 It can be seen that no cracks or defects appeared under different parameters of the SLM forming process; only extremely small pores and a small number of unmelted defects existed.

[0111] The specimen prepared using SLM high-strength aluminum-lithium alloy powder in Example 4 of this invention was observed under a ZEISS Gemini 500 field emission scanning electron microscope. The resulting microstructure images are shown below. Figure 5 As shown. By Figure 5 It can be seen that the SLM microstructure of the high-strength aluminum-lithium alloy provided by the present invention consists of equiaxed crystals inside the molten pool and columnar crystals at the edge of the molten pool. The grains are fine and there are only a few micron-sized pore defects inside the specimen.

[0112] The high-strength aluminum-lithium alloy specimen prepared by the SLM forming method in Example 4 of this invention was subjected to EBSD testing using an electron backscatter diffraction probe equipped with a Tescan Clara GMH electron microscope. The obtained EBSD scan images are shown below. Figure 6 As shown. By Figure 6 It can be seen that the overall texture strength of the specimen is low. The equiaxed crystal region has smaller grains and more random orientation distribution, while the columnar crystal region has slightly larger grains and exhibits a weaker

[001] texture, which is beneficial to reducing the anisotropy of the specimen's performance.

[0113] The specimen prepared using SLM high-strength aluminum-lithium alloy powder in Example 4 of this invention was observed using a Talos F200X field emission transmission electron microscope, and the intracrystalline microstructure images were obtained as follows: Figure 7 As shown. By Figure 7 It can be seen that the high-strength aluminum-lithium alloy prepared by SLM forming in this invention has a large number of blocky primary Al3Ti particles with a size of about 200nm in the crystal, which helps to refine the grain structure of the specimen, thereby suppressing cracks and improving mechanical properties. This experimental result is consistent with the thermodynamic prediction result of Thermo-Calc.

[0114] The high-strength aluminum-lithium alloy specimen prepared by the SLM forming method in Example 4 of this invention was subjected to solution aging treatment, and the microstructure of the specimen after solution aging treatment was observed using a Talos F200X field emission transmission electron microscope. The intragranular microstructure images are shown below. Figure 8 As shown. By Figure 8 It can be seen that after solution aging treatment, a large number of different types of fine nanoscale precipitates were precipitated in the alloy, including the δ′-Al3Li phase, θ′-Al2Cu phase, T1-Al2CuLi phase, and χ-Al5Cu6Li2 phase commonly found in aluminum-lithium alloys, as well as the secondary β′-Al3Ti phase introduced due to the addition of Ti element.

[0115] The SLM high-strength aluminum-lithium alloy specimens of Example 4 of this invention, after solution treatment and aging, were subjected to tensile tests three times at room temperature. The stress-strain curves obtained are shown below. Figure 9 As shown. Figure 9 As shown, the high-strength aluminum-lithium alloy for SLM provided by this invention, after solution treatment and aging, has a yield strength of 448.3±2.2MPa, a tensile strength of 516.5±2.7MPa, and an elongation of 6.13±0.80%.

[0116] The cross-sectional morphology of 12 groups of aluminum-lithium alloy specimens prepared under different alloy compositions according to Comparative Examples 1-12 of this invention was observed using a Keyence VHX-2000 optical microscope. The obtained cross-sectional optical micrographs are shown below. Figure 10 As shown; where, Figure 10 Images (a-i) are, in order, cross-sectional optical microscope images of high-strength aluminum-lithium alloys prepared under different parameters for Comparative Examples 1-12. Figure 10 It can be seen that within the process range, long cracks that almost penetrate the entire specimen are always present in the aluminum-lithium alloy specimens of Comparative Examples 1 to 12, and the crack rate of the specimens increases significantly with the increase of scanning rate.

[0117] The high-strength aluminum-lithium alloy specimen prepared by the SLM forming method in Comparative Example 4 of this invention was subjected to EBSD testing using an electron backscatter diffraction probe equipped with a Tescan Clara GMH electron microscope. The obtained EBSD scan images are shown below. Figure 11 As shown. By Figure 11 It can be seen that the deposited structure of the Ti-free specimen is composed of epitaxially grown coarse columnar crystals, and the specimen as a whole exhibits a high

[001] texture orientation, which will lead to severe performance anisotropy.

[0118] In summary, the high-strength aluminum-lithium alloy for SLM provided by this invention, through the introduction of Ti element modification, enables the formation of a large number of fine primary L12-type Al3Ti particles in the melt before α-Al solidification, which can act as heterogeneous nucleation cores, thereby effectively refining the grains and suppressing forming cracks. At the same time, after aging, a large number of finely dispersed secondary Al3Ti phases are formed in the alloy. These phases have a synergistic strengthening effect with the strengthening phases such as δ′-Al3Li phase, θ′-Al2Cu phase, T1-Al2CuLi phase, and χ-Al5Cu6Li2 phase in the aluminum-lithium alloy, effectively improving the mechanical properties of the alloy.

[0119] Table 5-1 Performance parameters of aluminum-lithium alloy specimens obtained in each embodiment

[0120]

[0121] Table 5-2 Performance parameters of aluminum-lithium alloy specimens obtained in each embodiment

[0122]

Claims

1. A method for preparing aluminum-lithium alloy, characterized in that, The aluminum-lithium alloy consists of 1.4–2.1% Li, 2.5–3.3% Cu, 0.25–0.8% Mg, 0.1–0.6% Ag, 0.6–1.0% Ti, and the balance Al; all percentages are by mass; the sum of all component percentages is 100%; the specific process is as follows: Step 1, Prepare powdered raw materials: The tip of an aluminum-lithium alloy rod is heated and melted using a rotating electrode method. The molten droplets at the tip of the rod are then ejected by high-speed rotation and solidified into spherical powder under the action of surface tension. The rotation speed is 15,000 to 25,000 r / min. The obtained spherical powder was sieved to obtain particles with a diameter of 15-85 μm and a bulk density of 1.35-1.45 g / cm³. 3 Powder raw materials; The aluminum-lithium alloy rod is obtained by melting pure aluminum ingots, pure magnesium ingots, pure zinc ingots, pure silver ingots with a purity of 99.9% or higher, as well as Al-Cu master alloys, Al-Li master alloys, and Al-Ti master alloys, and pouring them into an ingot mold. After solidification, an aluminum-lithium alloy rod containing 1.4~2.1% Li, 2.5~3.3% Cu, 0.25~0.8% Mg, 0.1~0.6% Ag, 0.6~1.0% Ti, and the balance being Al; the percentages are by mass. Step 2, drying of powder raw materials: Drying is used to remove gas and moisture from the powdered raw materials. Step 3, clean the substrate; Step 4, draw the specimen model: The 3D model of the specimen was drawn using Materialise Magics drawing software; the generated .stl file was then sliced ​​using conventional methods to obtain the specimen model that the 3D printer could read. Step 5, Printing Preparation: Step 6, print the specimen: The specimen printing process includes multi-layer laser selective melting and forming; the laser selective melting and forming process of each layer includes powder spreading and melting; the powder spreading of each layer is scanned and melted layer by layer by a laser to obtain the preform of the desired aluminum-lithium alloy specimen; When printing the test specimen, the laser power is 120~240W, the scanning rate is 100~1000mm / s, the track spacing is 80~120μm, and the interlayer deflection angle is 67°. Step 7, cut off the specimen: After printing is completed and the aluminum alloy substrate has cooled to room temperature, the preform is removed and cut off from the substrate to obtain the semi-finished product of the required aluminum-lithium alloy specimen. Step 8, Solution treatment: The obtained aluminum-lithium alloy semi-finished product was subjected to solution treatment; After solution treatment, the semi-finished aluminum-lithium alloy specimen was cooled to room temperature by water quenching, thus completing the solution treatment of the semi-finished aluminum-lithium alloy specimen. Step 9, Time-sensitive processing: The semi-finished aluminum-lithium alloy specimen obtained after solution treatment is subjected to aging treatment; after aging, it is cooled to room temperature by air cooling; thus obtaining the aluminum-lithium alloy specimen.

2. The method for preparing aluminum-lithium alloy as described in claim 1, characterized in that, During the printing preparation described in step 5, argon gas with a purity higher than 99.9% is continuously introduced into the forming chamber to expel the oxygen in the forming chamber and keep the oxygen content below 100ppm throughout the forming process to prevent the specimen from oxidizing during printing. Once the oxygen content in the chamber drops below 100 ppm, preheating is activated to heat the aluminum alloy substrate to 150°C, thereby reducing thermal stress during the forming process and suppressing the tendency of the specimen to crack.

3. The method for preparing aluminum-lithium alloy as described in claim 1, characterized in that, In step 6, the specific process of printing the specimen is as follows: Laser selective melting forming of the first layer: Turn on the 3D printer and lay the first layer of powder raw material on the surface of the aluminum alloy substrate. The thickness of the layer is 15~60μm. Start the laser to scan and melt the first layer of powder according to the three-dimensional model of the specimen, and complete the laser selective melting forming of the first layer of powder. Laser selective melting to form the second layer: Repeat the process of laser selective melting to form the first layer; specifically, lay a second layer of powder raw material on the surface of the first layer, with a thickness of 15~60μm; start the laser to scan and melt the laid second layer of powder according to the three-dimensional model of the specimen, and complete the laser selective melting to form the second layer of powder. Continue repeating the process of selective laser melting and forming the first layer of powder and scanning melting, and then complete the selective laser melting and forming of the remaining layers one by one until the specimen is prepared.

4. The method for preparing aluminum-lithium alloy as described in claim 1, characterized in that, During the solution treatment described in step 8, the solution treatment temperature is 550~570°C and the solution treatment time is 60~120 min.

5. The method for preparing aluminum-lithium alloy as described in claim 1, characterized in that, The aging treatment in step 9 is performed at a temperature of 160~180°C for 24~48 hours.

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