Method for improving medium-temperature strength of additive manufacturing Al-Si alloy

Abstract

The invention discloses a method for improving the medium-temperature strength of an additive manufacturing Al-Si alloy, and belongs to the field of metal materials. According to the method, the Al-Si alloy serves as a base body, micron-sized AlB2 and Ti powder are added, laser irradiation energy in the additive manufacturing process is used for enabling the micron-sized AlB2 and Ti powder to react, and nanoscale TiB2 and D022-Al3Ti strengthening phases are synthesized in situ so that the medium-temperature strength of the material can be improved. By optimizing the content of the components, the adding proportion of the AlB2 powder and the adding proportion of the Ti powder are controlled to be 1.20-1.60 wt.% and 1.80-2.40 wt.% respectively, and the mass ratio of the AlB2 powder to the Ti powder is 2: 3. Through the synergistic effect of the two nanoscale second phases, the alloy can have high strength in a medium-temperature environment without post-heat treatment, and meanwhile, the high percentage elongation after fracture is kept. The yield strength of the alloy in the environment of 300 DEG C reaches 190 MPa, the tensile strength reaches 200 MPa, the ductility reaches 10%, and the strength upper limit of the selective laser melting near-eutectic Al-Si alloy in the working environment of 300 DEG C is broken through. The mechanical property of the aluminum alloy is comparable with that of Al-Ni, Al-Ce and other heat-resistant aluminum alloys prepared through selective laser melting, and the aluminum alloy is lower in cost and lighter in density.

Description

Technical Field

[0001] The present invention belongs to the field of metal materials, and particularly relates to a method for improving the medium-temperature strength of additively manufactured Al-Si alloys Background Art

[0002] In recent years, the rapid development of additive manufacturing technology has provided new paths and methods to better meet the increasingly stringent requirements of modern industry for material properties and component shapes. In the field of additive manufacturing of aluminum alloys, near-eutectic Al-Si alloys have become a research hotspot and the mainstream material for actual production due to their good formability, excellent room-temperature mechanical properties, and low cost, etc

[0003] Compared with the same material produced by traditional casting, the as-printed near-eutectic Al-Si alloy has a finer microstructure, such as a near-micron-scale Al-Si eutectic / α-Al cellular microstructure. Its microstructure endows the material with high yield and tensile strength at room temperature. However, the cellular structure in its as-printed material is a metastable structure generated by rapid solidification, which is extremely prone to fragmentation at higher temperatures (≥250°C), that is, the thermal stability of the microstructure is poor, and the heat resistance and medium-temperature strength of the material are poor. This results in the difficulty of this material to serve in medium- and high-temperature environments (≥250°C), greatly limiting its application range

[0004] The medium-temperature strength of Al-Si alloys can be improved by introducing thermally stable phases, such as Al3Ti, Al3Zr, TiC, TiCN, TiB2, etc. Among them, TiB2 particles have the characteristics of high melting point and good second-phase strengthening effect on the Al matrix, and are expected to solve the above problems. In the field of additive manufacturing, there are mainly two mainstream methods for preparing TiB2-reinforced near-eutectic Al-Si alloys. One is to mix TiB2 particles with Al alloy powder evenly by mechanical mixing and then print and form. This method has some problems: the strengthening effect of micron-scale TiB2 is limited, and nano-TiB2 has problems of high cost, high storage requirements, and difficulty in mixing. The other is to in-situ synthesize TiB2 particles by mixing salt reaction (KBF4 and K2TiF6) during alloy melting, and then directly prepare composite powder by atomization. This method has the phenomenon that high-melting-point TiB2 particles are easy to block the nozzle, and there are also problems of high furnace-opening cost and minimum order quantity, and it cannot fully utilize the advantages of small batch and high compositional design freedom of additive manufacturing

[0005] It can be seen that although the research on preparing TiB2-reinforced Al alloys by additive manufacturing technology has become mature in recent years, and the improvement of its room-temperature performance has been widely reported. However, problems such as its performance research in medium-temperature environments, cost control, compositional design, and the freedom of small-batch customization by customers still need to be solved Summary of the Invention

[0006] The present invention designs a novel method for improving the medium-temperature strength of additively manufactured Al-Si alloys. By utilizing the laser irradiation energy during the additive manufacturing process, micron-sized Ti and AlB2 powders undergo in-situ reactions during the material forming process to generate nanoscale TiB2 and D0 22 -Al3Ti strengthening phases. These two strengthening phases are fine and have good thermal stability, segregating at the cell walls and grain boundaries of the last solidified cellular structure. At medium temperatures, these particles play a role in pinning and stabilizing the metastable cellular structure and strengthening the grain boundaries, thereby significantly improving the medium-temperature strength of the additively manufactured Al-Si alloys.

[0007] A method for improving the medium-temperature strength of additively manufactured Al-Si alloys, characterized in that the preparation steps are as follows:

[0008] (1) Prepare spherical Al-Si alloy powders by gas atomization;

[0009] (2) Mix AlB2 and Ti powders with Al-Si powders evenly according to the designed ratio;

[0010] (3) Use a selective laser melting device to print and form the mixed powders, and adopt a laser remelting strategy;

[0011] (4) Post-treat the printed and formed samples.

[0012] Furthermore, for the method for improving the medium-temperature strength of additively manufactured Al-Si alloys, it is characterized in that the gas atomization powder preparation method in step (1) is: alloy melting is carried out according to the composition of Si (8.0 - 13.0 wt%), Fe (≤0.2 wt.%), Mn (≤0.01 wt.%), Ti (≤0.01 wt.%), Al (the balance), and powders are prepared by gas atomization process. After sieving, spherical alloy powders of 15 - 53 μm are obtained.

[0013] Furthermore, for the method for improving the medium-temperature strength of additively manufactured Al-Si alloys, it is characterized in that in step (2), AlB2 is irregularly shaped powder with a particle size distribution in the range of 1 - 20 μm, and Ti is spherical powder with a particle size distribution in the range of 1 - 20 μm; their addition amounts in the Al-Si alloy are 1.20 - 1.60 wt% and 1.80 - 2.40 wt.%, respectively, and the mass ratio is 2:3.

[0014] Further, the method for improving the medium-temperature strength of the additive manufacturing Al-Si alloy is characterized in that the specific parameters selected for the selective laser melting forming in step (3) are as follows: layer thickness of 25-35 μm, scanning spacing of 100-120 μm, substrate preheating temperature set at 200 °C, laser scanning strategy selected as stripe scanning with a 67° rotation between layers, laser power of 240-250 W, and scanning speed of 1400-1600 mm / s.

[0015] Further, the method for improving the medium-temperature strength of the additive manufacturing Al-Si alloy is characterized in that the laser remelting strategy in step (3) is as follows: after the first laser scan of each layer of material, powder spreading is not carried out immediately, but the laser is made to perform a second scan; this strategy can promote the decomposition of micron-scale modified powder and the synthesis of nano-scale strengthening phases. The scanning parameters and strategy of the remelting laser are the same as those of the initial laser.

[0016] Further, the method for improving the medium-temperature strength of the additive manufacturing Al-Si alloy is characterized in that the post-treatment in step (4) includes surface cleaning and grinding processes to obtain clean parts.

[0017] Principle of the present invention: Using the Al-Si alloy as the matrix, adding micron-scale AlB2 and Ti powders, and making them react by the laser irradiation energy during the additive manufacturing process to in-situ synthesize nano-scale TiB2 and D0 22 -Al3Ti strengthening phases to improve the medium-temperature strength of the material. By optimizing the component content, especially controlling the addition ratios of AlB2 and Ti powders at 1.20-1.60 wt.% and 1.80-2.4 wt.% respectively, and the mass ratio of the two is 2:3. Such a design forms approximately 2 wt.% TiB2 phase and approximately 2 wt.% D0 22 -Al3Ti phase in the matrix. This method is different from directly introducing nano-scale TiB2 particles, and has the advantages of controllable addition content and ratio, lower cost, being suitable for small batch and actual production, etc. In addition, an excessive amount of Ti element is introduced to generate additional D0 22 -Al3Ti particles interact with TiB2 and play a role in coordinating the strengthening effect. During the cooling and solidification process, due to the solidification sequence and characteristics, most of the in-situ generated second-phase particles are pushed to the grain boundaries and cellular structures that solidify last by the growing solid-liquid interface. These particles play a role in pinning and stabilizing the metastable cellular tissue walls and strengthening the grain boundaries in the medium-temperature environment.

[0018] Through the synergistic effect of two kinds of nanoparticles at medium temperature, which respectively play a role in pinning and stabilizing the metastable cellular tissue wall and strengthening the grain boundary, this alloy can have high strength in a medium-temperature environment without post-heat treatment, while still maintaining a high elongation after fracture. The yield strength of this alloy reaches 190 MPa at 300 °C, the tensile strength reaches 200 MPa, and the elongation reaches 10%, breaking through the strength limit of selective laser melting near-eutectic Al-Si alloy in a 300 °C working environment. Its mechanical properties are comparable to those of heat-resistant aluminum alloys such as Al-Ni and Al-Ce prepared by selective laser melting, and it has a lower cost and a lighter density. Description of the Drawings

[0019] Figure 1 is the typical technical roadmap of the present invention.

[0020] Figure 2 is the typical scanning electron microscope image of the mixed powder used in the present invention.

[0021] Figure 3 is the typical transmission electron microscope image of in-situ synthesized TiB2 and D0 22 -Al3Ti phases: (a)(d) Bright field images; (b)(e) High-resolution electron microscopy images; (c)(f) Electron diffraction patterns. Figure 4 is the TiB2 and D0 in the material of the present invention 22 -Al3Ti interact to form a core-shell structure compound with TiB2 as the core and D0 22 -Al3Ti as the shell: (a) Bright field image; (b) High-resolution electron microscopy image.

[0022] Figure 5 is the typical in-situ synthesized TiB2 and D0 22 -Al3Ti phases segregated at the cellular tissue wall and grain boundary: (a) Electron backscatter diffraction image and (b) Coaxial transmission Kikuchi diffraction: In-situ synthesized TiB2 and D0 22 -Al3Ti segregated at (a) grain boundaries and (b) cellular tissue walls. Figure 6 is the typical transmission electron microscope image of the phenomenon of TiB2 segregated at the cellular tissue wall and the TiB2 / Si binding interface: (a)(e) Bright field images; (b) and (c), (f) and (g) High-resolution electron microscopy images; (d)(h) Electron diffraction patterns.

[0023] Figure 7 is the typical mechanical property comparison diagram of the material of the present invention and AlSi12 alloy at 300 °C. Detailed Description of the Invention

[0024] The technical solution of the present invention will be further described below in conjunction with the accompanying drawings and specific embodiments. Figure 1 This is a typical technical roadmap of the present invention.

[0025] Example 1:

[0026] Prepare gas atomized AlSi12 alloy powder: Si (11.00 wt%), Fe (0.15 wt.%), Mn (≤0.01 wt.%), Ti (≤0.01 wt.%), Al (balance). After screening, powders with a particle size of 15 - 53 μm are retained, and the average particle size is about 31 μm. Irregular powders with a particle size of 1 - 10 μm are selected for AlB2, with an average particle size of about 5 μm, and spherical powders with a particle size of 1 - 10 μm are selected for Ti, with an average particle size of about 5 μm. The addition amount of AlB2 in the Al - Si alloy is 1.35 wt.%, and the addition amount of Ti in the Al - Si alloy is 2.025 wt.%.

[0027] Figure 2 The typical scanning electron microscope image of the mixed powder is shown. Subsequently, a selective laser melting equipment SLM125HL is used to print and form the material: the layer thickness is 30 μm, the scanning spacing is 110 μm, the substrate preheating temperature is set at 200 °C, the laser scanning strategy is selected as stripe scanning and rotated 67° between layers, the laser power and scanning speed are respectively set at 250 W and 1500 mm / s, and a laser remelting strategy is adopted. After density testing, its relative density > 99%.

[0028] After transmission electron microscope analysis ( Figure 3 ), nano - scale TiB2 and D0 22 -Al3Ti particles are in - situ synthesized during the material preparation process. As Figure 4 shown, the two second - phase particles can interact to form some core - shell structure compounds with TiB2 as the core and D0 22 -Al3Ti as the shell. As Figure 5 shown, through co - axial transmission Kikuchi diffraction analysis and electron backscatter diffraction analysis, the TiB2 and D0 22 -Al3Ti strengthening phases are segregated and distributed at the cell - like tissue walls and grain boundaries of the Al / Si eutectic. As Figure 6 shown, the TiB2 segregated at the cell - like tissue walls can form a certain orientation relationship with the Si phase here. At medium temperature, these particles can hinder the growth of these Si phases and the fragmentation of the cell - like tissue walls. At the same time, the second - phase particles segregated at the grain boundaries have the effect of strengthening the grain boundaries.

[0029] Under the above - mentioned synergistic effect, this material has at 300 °C: yield strength 197 ± 11 Mpa, tensile strength 217 ± 10 Mpa, and elongation after fracture 10.5 ± 1.5%. The performance comparison with the original AlSi12 material is shown inFigure 7 as shown

[0030] Example 2:

[0031] Prepare gas atomized AlSi10 alloy powder: Si (10.40 wt%), Fe (0.12 wt.%), Mn (≤0.01 wt%), Ti (≤0.01 wt%), Al (the balance). After screening, powders with a particle size of 15 - 53 μm are retained, and the average particle size is about 29 μm. Irregular powders with a particle size of 1 - 10 μm are selected for AlB2, and the average particle size is about 5 μm. Spherical powders with a particle size of 1 - 10 μm are selected for Ti, and the average particle size is about 5 μm. The addition amount of AlB2 in the Al - Si alloy is 1.4 wt%, and the addition amount of Ti in the Al - Si alloy is 2.1 wt.%.

[0032] Subsequently, a selective laser melting equipment SLM125HL is used to print and form the material: the layer thickness is 35 μm, the scanning pitch is 100 μm, the substrate preheating temperature is set at 200 °C, the laser scanning strategy is selected as stripe scanning and the layer - to - layer rotation is 67°, the laser power and scanning speed are set at 260 W and 1600 mm / s respectively, and a laser remelting strategy is adopted. After density testing, its relative density > 99%.

[0033] This material has at 300 °C: yield strength 201 ± 10 Mpa, tensile strength 220 ± 16 Mpa, and elongation after fracture 10.1 ± 1.1%.

Claims

1. A method for improving the medium-temperature strength of an additive manufacturing Al-Si alloy, characterized in that The preparation steps are as follows: (1) Prepare spherical Al-Si alloy powder by gas atomization method; (2) Mix AlB2 and Ti powder with Al-Si powder evenly according to the designed ratio; (3) Use a selective laser melting equipment to print and form the mixed powder, and adopt a laser remelting strategy; (4) Post-process the printed and formed samples.

2. The method for improving the medium-temperature strength of the additive manufacturing Al-Si alloy according to claim 1, wherein The gas atomization powder preparation method described in step (1) is: alloy melting is carried out according to the composition of Si (8.0 - 13.0wt%), Fe (≤0.2wt.%), Mn (≤0.01wt.%), Ti (≤0.01wt.%), Al (the balance), and powder is prepared by gas atomization process. After screening, spherical alloy powder of 15 - 53μm is obtained.

3. The method for improving the medium-temperature strength of the additive manufacturing Al-Si alloy according to claim 1, characterized in that The AlB2 described in step (2) is irregular powder with a particle size distribution in the range of 1 - 20μm, and Ti is spherical powder with a particle size distribution in the range of 1 - 20μm; their addition amounts in the Al-Si alloy are 1.20 - 1.60wt% and 1.80 - 2.40wt.%, respectively, and the mass ratio is 2:

3.

4. The method for improving the medium-temperature strength of the additive manufacturing Al-Si alloy according to claim 1, characterized in that The specific parameters selected for the selective laser melting forming described in step (3) are: layer thickness 25 - 35μm, scanning spacing 100 - 120μm, substrate preheating temperature set at 200°C, laser scanning strategy selects stripe scanning and rotates 67° between layers, laser power 240 - 260W, scanning speed 1400 - 1600mm / s.

5. The method for improving the medium-temperature strength of an additive manufacturing Al-Si alloy according to claim 1, wherein The laser remelting strategy described in step (3) is: after the first scan of the laser for each layer of material, powder spreading is not carried out immediately, but the laser is made to perform a second scan; this strategy can promote the decomposition of micron-scale modified powder and the synthesis of nano-scale strengthening phases; the parameters and scanning strategy of the remelting laser are the same as those of the initial laser.

6. The method for improving the medium-temperature strength of the additive manufacturing Al-Si alloy according to claim 1, wherein The post-treatment described in step (4) includes surface cleaning and polishing processes to obtain clean parts.

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