Examples of 3D printing preparation methods for domestic aluminum-based nanocomposites
With the increasing demand for high-performance lightweight alloy materials, especially in the fields of aerospace, automotive, medical, etc., the design and preparation of novel metal matrix composite materials is attracting more and more researchers' attention. . Conventional aluminum-silicon alloys have received extensive attention due to their significant advantages in terms of specific strength, wear resistance, and thermal expansion coefficient, but their performance is no longer sufficient for existing needs.
The aluminum-silicon composite material obtained by particle reinforcement can significantly improve the mechanical properties of the traditional aluminum-silicon alloy. It has been extensively studied and applied in practical engineering. The commonly used reinforcements include Al2O3, TiC, TiB, SiC, etc. . Metal materials used in laser additive manufacturing include stainless steel, tool steel, titanium alloy, nickel-based superalloy, Co-Cr-Mo alloy, aluminum alloy, etc., but the research on laser additive manufacturing of metal matrix composites is relatively less. In this issue, 3D Science Valley and Guyou came to experience the results of the 3D printing preparation method of aluminum-based nanocomposites at Nanjing University of Aeronautics and Astronautics.
At present, the particle reinforced aluminum matrix composites manufactured by laser additive are mainly faced with such problems during the forming process:
- Since aluminum has a high laser reflectivity to the laser, it is generally difficult for the low-power laser to completely melt the aluminum alloy. The addition of the reinforcing particles can increase the absorption rate of the laser to a certain extent, but the reinforcing particles are excessively added. Resulting in a decrease in material elongation properties;
- Studies have shown that reducing the particle size of the reinforcement to nanoscale can effectively improve the mechanical properties of the metal matrix composite, such as increasing the strength and reducing cracks, but when the size of the reinforcing particles is reduced to the nanometer scale, the particles will be strongly The van der Waals force and the great surface tension are closely agglomerated, which is not conducive to enhancing the uniform dispersion of the particles in the matrix. In the laser additive manufacturing process, the unique Marongoni flow in the formed molten pool can be To uniformly disperse the second phase, but the Marangoni flow is intimately connected to the temperature field of the molten pool;
- Since the reinforcing particles usually added are ceramic phases, the wettability between the ceramic phase and the matrix phase is poor, and the difference in thermal expansion coefficient between them tends to be large, which results in a liquid phase formed during the forming process. It is not possible to spread evenly, and at the same time, a large shrinkage stress is generated in the subsequent solidification process to cause cracks.
In order to solve the above-mentioned technical problems, Nanjing University of Aeronautics and Astronautics provides an aluminum-based nanocomposite based on SLM forming, which is used in the field of laser additive technology to effectively solve the process performance of aluminum-based nanocomposites in the laser additive process. The mechanical properties are not matched, the particle distribution is uneven, and the wettability between the ceramic phase and the substrate phase is poor, so that the obtained product has good interface bonding and excellent mechanical properties.
Nanjing University of Aeronautics and Astronautics is processing aluminum-based nanocomposites in a high-purity argon atmosphere, maintaining a positive pressure of 0.9-1.2 atm during the forming process. Processing parameters and powder properties are the two most important factors affecting the final shape of the laser during processing. From the viewpoint of powder composition, the addition of rare earth elements and ceramic particles will inevitably enhance the absorption rate of the aluminum alloy powder to the laser, thereby ensuring that the molten pool has a sufficient amount of liquid phase under the laser power. On the one hand, the addition of the ceramic phase, its particle size, density and mass fraction will affect the laser absorption rate. On the other hand, the laser forming process parameters also significantly affect the thermodynamic properties of the molten pool during the forming of the aluminum-based nanocomposites and the subsequent microstructure and properties.
In view of these factors, the Nanjing University of Aeronautics and Astronautics program has the following advantages:
- the proportion of fine
The powder component comprises an aluminum-silicon alloy powder, a rare earth phase and a ceramic phase, wherein the rare earth phase is any one of La, Nd, Sm or Y, and the selected rare earth elements are in accordance with their thermal properties (melting point, coefficient of thermal expansion and surface tension). The principle of being between the matrix phase and the reinforcing phase is selected to ensure good wetting property between the ceramic reinforcing phase and the matrix during laser processing and to avoid cracking during solidification due to excessive difference in thermal properties. The content is controlled at 0.3-0.8wt%, avoiding excessive addition to cause performance deterioration; ceramic particles are selected for carbide, aiming to generate in-situ reaction during forming, improve interface structure, select nanometer size in size, and use small size The surface interface effect is effective to improve the toughness of the material. In addition, the addition of the ceramic phase can effectively improve the absorption rate of the powder to the laser and improve the processing performance of the powder, but the added content should be controlled at 4-6 wt% to ensure that the material is not damaged. The enhancement phase is too high and causes a decrease in ductility.
- Gradient interfacial layer aluminum-based nanocomposites form a gradient interface layer with a certain thickness between the reinforcing phase and the matrix phase, and the gradient changes from the matrix phase to the reinforcing phase Al and rare earth elements. During the loading process, it is often easier to strengthen the particles. The stress concentration causes cracking, but the existence of such a gradient interface layer effectively alleviates the occurrence of stress concentration, thereby strengthening and toughening the material; at the same time, the reinforcing particles become more fine due to the addition of rare earth elements. The sleekness also reduces the chance of stress concentration inside the material during loading.
- The uniform powder is coated with the ceramic reinforcing phase and the rare earth phase by high-energy ball milling. With the secondary ball milling effect, the powder satisfying the SLM forming process can be effectively obtained, that is, it has good fluidity, sphericity and uniformity. The composition of the composition, the narrow particle size distribution, the powder preparation method is simple and easy to operate.
- Controlling the effective body energy density Controls the good forming quality by optimizing the effective body energy density in SLM forming. The effect of effective body energy density is reflected in the stability of the molten pool, temperature field, flow field and accompanying laser in laser processing. The influence of the microstructure on the SLM process is reflected in the comprehensive analysis of the physical properties of the powder and the processing parameters. The molten pool formed by the manufacturing process of Nanjing University of Aeronautics and Astronautics has good stability, and the surface of the formed part has a smooth and corrugated melting path, and almost no spheroidization effect is obtained and a nearly full dense structure is obtained. Microstructural analysis showed that the reinforced particles obtained a uniform dispersion distribution, and the matrix grains were fine and grew in a cell structure.
(Editor)
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