Design,Preparation And Integrated Research Of Structure And Function For Aluminum Matrix Composite With High Magnetic And Strength-Toughness

Traditional magnetic materials(such as Nd₂Fe₁₄B)are mainly iron-based magnetic materials,and although they have excellent comprehensive magnetic properties such as high remanence and coercivity,they have shortcomings such as high density,easy oxidation,high brittleness,and low toughness.With the rapid development of new energy vehicles,rail transit,and electronic information industries,new requirements have been put forward for magnetic materials,such as lightweight,miniaturization,high strength and toughness,to achieve the safety,energy saving and easy processing of magnetic bearings,motors,relays and other magnetic devices.Currently,widely used lightweight materials such as carbon fiber reinforced composites and aluminum steel etc.,although they possess high mechanical properties and lightweight characteristics,they do not have magnetic properties.Therefore,there is an urgent need for the development of lightweight,high strength and toughness magnetic materials.In this paper,2024Al and Nd₂Fe₁₄B particles were used as the matrix and the reinforcing phase respectively to prepare Nd₂Fe₁₄B_p/2024Al composite through high-energy ball milling,cold isostatic pressing,and microwave sintering.After optimizing the microwave sintering parameters,a certain amount of Co and Sm elements were added to the composite,and the effect of Co and Sm addition on the microstructure,mechanical/magnetic properties of the composite was determined.Finally,the optimized Nd₂Fe₁₄B_p/2024Al-7.5Co-1.5Sm was hot-rolled to prepare a new type of magnetic aluminum matrix composite with integrated structure and function.The microstructure,interface characteristics,and mechanical/magnetic properties of composite were tested and analyzed with modern analysis and testing methods,such as scanning electron microscopy（SEM）,transmission electron microscopy（TEM）,nanoindentation tester,X-ray diffraction,etc.The main research findings are as follows:（1）The effects of microwave sintering temperature and the amount of Nd₂Fe₁₄B added on the microstructure,mechanical and magnetic properties of Nd₂Fe₁₄B_p/2024Al composite were explored.There is no chemical reaction between 2024Al matrix and Nd₂Fe₁₄B particles during the sintering process,and the integrity of the two phases is still maintained.However,the addition of Nd₂Fe₁₄B particles reduces the melting point of 2024Al,which is more conducive to the advantages of microwave sintering at low temperature and fast sintering.When the sintering temperature is 490℃and the amount of Nd₂Fe₁₄B particles added is 15wt.%,the microstructure of the composite is optimized,with an average grain size of about 10μm,and the average particle size is 7μm.The surface morphology of the matrix is uniform,and the Nd₂Fe₁₄B particle is uniformly distributed.The interface between the matrix and particles is well bonded.The microhardness of the composite is 135.8HV,the yield strength is 190.5MPa,and the compressive strength is 248.9MPa,which is 17.9%,10.6%,and 6.6%higher than the pure 2024Al powder sintered sample,respectively.The magnetic parameters of the composite material are:B_r=0.17T,H_cj=3.51k Oe,（BH）_max=24.08k J/m³.The magnetic properties are mainly derived from Nd₂Fe₁₄B particles.When the particle size is in the range of 3-8μm and dispersed,it is beneficial to the magnetic performance.（2）The effects of Co addition on the microstructure and mechanical/magnetic properties of Nd₂Fe₁₄B_p/2024Al composites were studied.When the addition of Co is7.5wt.%,the grains are refined,and the dispersion of Co on the surface of Nd₂Fe₁₄B particles improves the interfacial bonding ability.The compactness,hardness,yield strength,compressive strength and maximum compression ratio of the composites are95.4%,152.1HV,210.0MPa,269.2MPa and 9.56%,respectively,which are 4.8%,12.0%,10.2%,8.2%and 14.8%higher than those of the Co-free sample.The magnetic properties reached the optimal values:B_r=0.20T,H_cj=4.60k Oe,（BH）_max=28.36k J/m³,which increased by 17.6%,31.1%and 17.8%respectively compared with the Co-free sample.The reason lies in three points:the increase of compactness;the stronger magnetic phase Nd₂（Fe,Co）₁₄B is formed after Co partially replaces Fe atoms in Nd₂Fe₁₄B alloy;Magnetic coupling occurs between Co soft magnetic and hard magnetic particles.（3）On the basis of optimizing the amount of Co addition,the effect of Sm addition on the microstructure and mechanical/magnetic properties of Nd₂Fe₁₄B_p/2024Al-7.5Co composite was studied,and the optimal amount of Sm addition was obtained to be1.5wt%.The average particle size of Nd₂Fe₁₄B phase is about 5μm,evenly distributed in the matrix;For the 2024Al matrix,Sm particles can reduce the surface tension of the composite material,and the Al₃Sm phase formed as a crystal nucleus distributed at the grain boundaries can promote nucleation,inhibit grain growth,and refine grains;In addition,the addition of Sm can play a role as solid solution strengthening.The compcatness,hardness,yield strength,and compressive strength are 97.1%,185.3HV,230.4MPa,and 284.9MPa,respectively,and the maximum compression ratio is 12.62%,which increased by 1.7%,21.9%,9.7%,5.9%,and 32.0%compared to the sample without Sm,respectively.The strength and toughness of the composite material were further improved.For the magnetic properties,the B_r is 0.23T,the H_cj is 4.65k Oe,and the（BH）_max is 29.10k J/m³,which are increased by 15.0%,10.9%,and 2.6%compared to the Sm-free sample,respectively.It is due to the increase of compactness after adding Sm,the uniform distribution of Nb-rich phase,grain boundary pinning and grain refinement,the formation of hard magnetic phase Sm₂O₅ and the strong interaction between soft and hard magnetic phases.（4）The effect of different rolling deformation amounts on the microstructure,mechanical properties,and magnetic properties of Nd₂Fe₁₄B_p/2024Al-7.5Co-1.5Sm composite at a rolling temperature of 450℃was studied,and the optimal hot rolling deformation amount was found to be 45%.The morphology is optimal with Nd₂Fe₁₄B particles distributed more evenly and finely on the matrix.The compcatness is 98.8%,which is 1.8%higher than the sintered sample.The strengthening mechanism of hot rolling on composites mainly includes:1)Mechanical strengthening.During the hot rolling process,the particle phase after hot rolling is more evenly dispersed on the matrix,which enhances the load transfer;2)Structural strengthening.The microstructure of the composites was optimized during the hot rolling,which improved the mechanical properties;3)Interface strengthening.Hot rolling improves the interfacial bonding strength between the particle phase and the matrix and promotes interfacial strengthening.The elastic modulus and nanohardness values of the matrix are 142.9 GPa and 0.93 GPa,respectively.The elastic modulus and nanohardness values at the interface are 177.9 GPa and 1.25 GPa,respectively,which are significantly improved compared to the sintered sample;The microhardness,yield strength,compressive strength and maximum compression ratio of the composite were 208.5HV,252.2MPa,300.5MPa and 15.56%respectively,which increased by 12.5%,9.5%,5.6%and 23.3%respectively compared with the sintered samples.For the magnetic properties,the B_r is 0.26T,H_cj is 4.70k Oe,and the（BH）_max is 30.32k J/m³,increased by13.0%,1.2%,and 4.5%compared to the sintered sample,respectively.The reason is that hot rolling improves the lattice distortion state and the number of dislocations increases significantly.In summary,the microstructure,mechanical and magnetic properties of the composites were gradually improved by composition design,parameter optimization,addition of Co and Sm elements and hot rolling.Finally,the goal of“preparing an aluminum matrix composite with high strength,toughness and high magnetism”was achieved.The prepared Nd₂Fe₁₄B_p/2024Al-7.5Co-1.5Sm composite has good mechanical properties and certain magnetic properties after microwave sintering at490°C for 30 min and hot rolling with 45%deformation.