激 光 增 材 制 造 高 熵 合 金 颗 粒 增 强 铝 基 复 合 材 料 组 织 及性 能 研 究（特 邀）

Objective Laser additive manufacturing (LDM) is an advanced technology, which enables rapid manufacturing by layer deposition. Compared with traditional manufacturing methods, it does not require molds and has high material utilization and short preparation cycles. It is one of the most important technologies for high-performance complex components. Currently, the LDM technology has been applied to aluminum alloys, titanium alloys, nickel based high-temperature alloys, and their composites. Aluminum alloy has become a high-performance structural material with great potential for applications due to its advantages of low density and high specific strength. However, with the increasing demand for structural materials in the aerospace industry, the modulus and strength indicators of aluminum alloys are facing more stringent challenges. By adding strengthening phases to the aluminum matrix to prepare aluminum-based composites, the above-mentioned problems in aluminum alloys can be effectively improved. Particle reinforced aluminum-based composites have shown a wider range of application prospects due to their isotropy and superior processing properties. High entropy alloys (HEA) have excellent corrosion resistance, high hardness, excellent compressive strength, excellent wear resistance, high temperature performance, etc. HEA are reinforcement materials with great potential for future applications in composite materials. The AlCoCrFeNi HEA have a body-centered cubic (BCC) structure and a strength of up to 3531 MPa. After calculation, the interfacial energy of Al/AlCoCrFeNi is between -0.242 eV/Ų and -0.192 eV/Ų, indicating excellent interfacial performance. Many studies have shown that AlCoCrFeNi HEA particles reinforced aluminum-based composites can significantly improve the tensile strength and modulus of materials. At present, research on LDM of HEA particles reinforced aluminum matrix composites has just begun, and there is relatively little research on the effect of LDM process parameters on the microstructure and mechanical properties of HEA particles reinforced aluminum matrix composites. This article uses LDM to produce AlCoCrFeNi high entropy alloy particle reinforced AlMgScZr aluminum alloys. Through single pass multi-layer deposition experiments on the composite material, the influence of different process parameters on the formability of HEA particle reinforced aluminum-based composites is studied. By characterizing the microstructure of laser additive manufactured composite materials combining with mechanical testing experiments, the microstructure and mechanical properties of HEA particles reinforced aluminum-based composite materials produced by LDM are elucidated, laying a scientific foundation for the application of laser additive manufactured HEA/Al composite materials. Methods This work uses laser directed energy deposition (LDED) technology for sample preparation. The laser directed energy deposition equipment used is the laser manufacturing system, paired with the powder feeder. The powder feeding speed for the single-layer experiment is 1000 g/min, the laser power exploration range is 1500‒5000 W, and the scanning speed exploration range is 250 ‒ 750 mm/min. During the deposition process, argon is used as a protective atmosphere with a flow rate of about 20 L/min to ensure that the volume fraction of the oxygen is below 50×10^-6 and prevent the sample from being oxidized. Single pass process exploration is carried out on the Al-Cu alloy substrate after mechanical mixing of composites reinforced with 3% (mass fraction) HEA particles. Equal gradient laser power and scanning speed are set for cross experimental exploration. Based on single layer experiments, process parameters (scanning speed of 600 mm/min and laser power of 4000 W) with good formability are selected for multi-layer deposition experiments. The formed samples are shown in Fig. 1. Optical microscope (OM) and scanning electron microscope (SEM) are used to observe the microstructure of single and multi-layer samples, and energy dispersive spectroscopy (EDS) is used to analyze the elements in their tissues. Finer microstructure observation and elemental analysis are performed in transmission electron microscope (TEM). Hardness tests are conducted on different parts of samples using a micro Vickers hardness tester. Using an electronic universal testing machine for tensile testing, the tensile strain rate is 0.5 mm/min. Results and Discussions The morphologies of single-layer samples of 3% HEA reinforced aluminum-based composite materials formed under different laser process parameters are shown in Fig. 2. Based on the single pass sample forming situation and its corresponding laser process parameters, the single pass process of laser additive preparation of HEA particle reinforced aluminum matrix composites is divided into three typical regions according to the distribution of laser energy density: low laser energy density region, medium laser energy density region, and high laser energy density region. The results indicate that when the laser energy density is too low, the forming of the material is constrained, and the powder is difficult to completely melt, resulting in a smaller size of the melt pool, more defects in the samples, and even the inability to form composite materials. On the contrary, when the laser energy density is too high, the substrate readily melts under the intense heat, which adversely affects the overall formability. At the same time, excessive energy density can also cause the composite material powder to overheat, the melt pool to widen, and the formability to decrease. Within the appropriate energy density range (5‒7.8 J·mm^-1), the overall formability of the material is good, and the porosity is relatively low. Within this range, the selection of laser power and scanning speed is particularly crucial. Through the study of the medium energy density region, it is found that with a constant scanning speed, internal defects in the material first decrease and then increase with the increase of power, and the closer the parameters are to the boundary of the process window, the more significant the increase in defects is. This indicates that there exists an optimal combination of laser parameters within a certain range that can minimize defects within a single melt pool to the greatest extent possible. The mechanical property test results of Al-P and Al-C are shown in Fig. 9(a). It can be clearly seen that the addition of HEA particles significantly improves the microhardness, yield strength, and tensile strength of AlMgScZr alloy. The microhardness values of AlMgScZr alloy and HEA particle reinforced composite material produced by LDM are (93.8±1.2) HV and (130.3±5.2) HV, respectively. The addition of HEA particles increases the hardness of AlMgScZr alloy by about 38.9%. The tensile strength, yield strength, and fracture elongation of Al-P aluminum alloy are 320 MPa, 131 MPa, and 13.3%, respectively. The tensile strength, yield strength, and fracture elongation of composite material Al-C are 366 MPa, 230 MPa, and 6.8%, respectively. Compared with the matrix, the tensile strength of the composite material is increased by 14.7%, the yield strength is increased by 75.6%, and the fracture elongation is decreased by 48.8%. This indicates that after the addition of HEA particles, the high entropy precipitates and residual HEA particles inside the composite material can effectively improve the tensile strength and yield strength of the material, but the intrinsic brittleness of these precipitates also reduces the plasticity of the material. Conclusions LDM of HEA reinforced aluminum-based composites has good formability, without obvious defects such as pores and cracks. Through comparative experiments of single process parameters, it is found that the formability of HEA reinforced aluminum-based composites is optimal when the laser scanning speed is 600 mm/min and the power is 4000 W. The HEA particle reinforced aluminum matrix composite material deposited using these laser parameters partially retains the reinforcing particles in the microstructure, forming a stable and moderately thick interface layer at the interface between the particles and the matrix. The microstructure of AlMgScZr aluminum alloy matrix produced by LDM consists of matrix α-Al phase and diamond shaped Al₃(Zr, Sc) second phase with BCC structure. After adding HEA particles, the microstructure of HEA/Al composite materials produced by LDM consists of precipitated phases such as α-Al, Al₃(Zr,Sc) phase, (Al,Fe)₄Cr with BCC structure, as well as unmelted AlCoCrFeNi particles. The morphological distribution of AlCoCrFeNi reinforcement varies with forming height: a few intact particles persist at the top; in the mid-section the particles almost fully melt and re-precipitate as strip-shaped high-entropy phases; while at the bottom numerous smaller particles remain, and the melted part yields finer strip-shaped precipitates. Compared with the matrix aluminum alloy, HEA reinforced aluminum-based composites have significantly improved hardness and tensile strength, but reduced plasticity. Compared to those of the AlMgScZr aluminum alloy, the tensile strength of HEA reinforced aluminum-based composite material is increased by 14.7%, the yield strength is increased by 75.6%, the fracture elongation is decreased by 48.8%, and the hardness is increased by 38.9%. The fracture surfaces of both the matrix alloy and composite material samples exhibit lots of ductile dimples, manifested as ductile fracture. HEA particle reinforced aluminum-based composite materials have more pores inside, resulting in uneven distribution of stress and strain during the tensile process. Local stress concentration is more likely to occur during the tensile stage, leading to material fracture, and the plasticity is not as good as that of aluminum alloy samples without HEA particles.