Abstract: 2024 aluminum alloy was formed by selective laser melting technology, the effect of scanning interval on microstructure and mechanical properties at room temperature was studied. The results show that when the scanning interval is 0.12 mm, the microstructure of 2024 aluminum alloy is more fine, the hardness is up to 124 HB, and the tensile strength is 372 MPa.
Key words: selective laser melting; scanning interval; microstructure and properties
Because aluminum alloys have low viscosity, low density, and easy to form oxide films on the surface, the research progress on SLM forming of aluminum alloys is relatively slow. Most of them use Al-Si materials 1-3, which have relatively good laser absorption, while Al- Cu-based alloys are rarely reported. In 2011, K.Surekha and others studied the feasibility of forming 2XXX series aluminum alloy AA2219 powder, and found that the second phase particles (A12Cu) help improve the corrosion resistance of the alloy. Hu Zhang et al. 15A carried out research on Al-Cu-Mg alloy forming, and obtained high strength and dense aluminum alloy by adjusting laser energy density parameters. In general, 2XXX series aluminum alloys have large thermal conductivity and thermal expansion coefficient, and are prone to defects such as cracks during rapid solidification, which increases the difficulty of forming.29 This experiment mainly uses the self-developed HLJ-300 equipment to perform laser melting of selected 2024 aluminum alloy Research on forming process and microstructure properties.
The 2024 aluminum alloy spherical powder used in this experiment was produced by AVIC Maite, the composition is shown in Table 1, and the particle size distribution was 15-53 μm.
1.2 Test Steps
A metallographic specimen model of 10x mmx10 mmx10 mm was created, and a 2 mm thick block support was added to the bottom surface with a support interval of 2 mm. A tensile specimen was formed parallel to the deposition direction. The laser line scans the metal powder according to the preset path. After each layer is scanned, the forming cylinder is lowered by a layer thickness, and a new layer of powder is applied again until printing is completed. The metallographic specimens were polished with Keller reagent (1% HF, 1.5% HCI, 2.5% HNO, 95% H.O) for 5-7s.
The experimental process parameters are shown in Table 2.
2 Test Results And Discussion
The mechanical properties are analyzed according to the process parameters established in Table 2. The hardness and tensile strength of the aluminum alloy obtained by the final test are shown in Table 3.
It can be seen from Figure 3 that with the increase of the scanning interval, the hardness, tensile strength and tensile strength of the alloy first increase and then decrease, and the highest is at the scanning interval of 0.12mm, which are 124 HB and 372 MPa, respectively.
2.2 Metallographic Structure
When the scanning interval is 0.12 mm, two adjacent laser lines overlap and scan a part of the area. The molten liquid phase fills the surrounding voids and solidifies quickly. The structure is fine and uniform, and the molten pool overlaps well, as shown in Figure 46b). When the alloy has the highest hardness and tensile strength, when the scanning interval is 0.09 mm, as shown in Figure 4 (a), although the molten pool overlap rate is high, the powder absorbs too much energy, and the thermal cycle during scanning Causes local thermal stress accumulation, which is prone to defects such as deformation and hot cracking1, which makes the alloy’s hardness and tensile strength values the lowest, and much lower than the values of other groups; when the scan interval is 0.13 mm, as shown in Figure 4 (c ), The distance between adjacent laser lines is increased, the overlap rate of the molten pool is reduced, the surrounding voids are not filled well, and defects such as micropores and microcracks are generated, which reduces the hardness and tensile strength. Continued as shown in Figure 5. According to the fine grain strengthening theory, the relationship between grain size and strength can be described using the Hall-Petch formula:
(1) During the SLM forming of 2024 aluminum alloy, due to the high cooling rate, large undercooling, and fine grain structure, the number of grain boundaries and the AICu interface generated by the reaction increase the resistance of dislocation movement significantly. Improved strength of Al alloy. The phase diagram of the AI-Cu binary alloy is shown in Figure 6. In a metallographic sample with a scanning interval of 0.12 mm, through observation by a scanning electron microscope, it can be found that there are fine AICU phases generated inside the molten pool, as shown in Figure 7. As a matrix strengthening phase, the AlCu phase increases the strength and hardness of the alloy.
（1）2024 aluminum alloy is formed by SLM technology. When the scanning interval is 0.12 mm, the alloy has a fine microstructure, a hardness of 124 H1B3, a tensile strength of 372 MPa, and a high room temperature mechanical property;
（2） a molten pool Inside, there are fine lumps of Al and Cu phases. As a matrix strengthening phase, the hardness and tensile strength of the alloy are increased, and the room temperature mechanical properties of the alloy are significantly improved.
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