Effects Of Sintering Methods And Co,Ni On Structure And Force/Magnetic Properties Of Nd₂Fe₁₄B_p/Al Composites And Mechanism Investigation

Traditional magnetic materials are mainly based on iron element.Taking Nd₂Fe₁₄B as an example,the density of Nd₂Fe₁₄B is larger than that of aluminum alloy.At the same time,Nd₂Fe₁₄B exhibits obvious brittleness,whose fracture toughness is similar to that of ceramics,and its processing performance is inferior.However,the magnetic devices used in modern rail transit demand the characteristics of light weight for high speed and energy saving,high strength and toughness for safe utilization,easy machining for the efficient processing of special-shaped devices and high thermal conductivity for high efficiency and safe operation in the electrical machinery and other energy conversion magnetic devices,etc.The research on the high magnetic and high strength and toughness aluminum matrix composite intends to solve the"weight and brittleness"and other major problems of available magnetic materials.The new target composite is a new type of material integrating structure and function together with high processability and electro-thermo conductivity.The Nd₂Fe₁₄B and pure aluminum are selected as reinforcement and matrix,two sintering processes as"ball-milling mixed powder-magnetic field cold isostatic pressure-microwave sintering（MWS）"and"ball-milling mixed powder-spark plasma sintering（SPS）"are designed to prepare the composites,meanwhile,the Co and Ni components as the additives.Through X-ray diffraction（XRD）,scanning electron microscopy（SEM）,transmission electron microscopy（TEM）,nano-indentation and others,the effects of MWS,SPS sintering methods and Co,Ni elements on microstructure,density,and mechanical/magnetic properties of Nd₂Fe₁₄B_p/Al composites(including remanence Br,coercive force H_c,and maximum magnetic energy product（BH）_max)were investigated.The main research conclusions are as follows:The effects of Co content on the structure and mechanical/magnetic properties of MWS,SPS-Nd₂Fe₁₄B_p/Al composites were studied.The results show that in the MWS process,partial Co atoms will enter into Nd₂Fe₁₄B and replace Fe atoms then form Nd₂（Fe,Co）₁₄B phase,which is an important factor for the magnetic enhancement.The optimized MWS-Nd2Fe14B_p/Al-5%Co composites has a stable structure,and the compactness,hardness,yield strength and compression strength are 94.5%,107.4HV,139.81MPa and 210.84MPa,with the highest mechanical properties in the experimental range.The corresponding magnetic properties are:Br=0.37T,H_c=0.65T,（BH）_max=45.12kJ/m³,which is the best within the experiment range.For the SPS samples with Co addition,the compactness of the samples is significantly higher than that of MWS due to the applied pressure in the process and the hardness and strength of the samples increase significantly,which is consistent with the compactness change rule and the distribution of particles is uniform.The maximum compactness,hardness,yield strength and compression strength of SPS-Nd2Fe14B_p/Al-2.5%Co samples with better mechanical properties are 99.1%,132.8HV,162.72MPa and 276.68MPa respectively in comparison to the MWS-5%Co sample,indicating that SPS is conducive to improving mechanical properties;The magnetic properties of SPS-Nd2Fe14B_p/Al-5%Co samples with better magnetic properties are:Br=0.31T,H_c=0.60T,（BH）_max=42.12kJ/m³,which is lower than that of MWS-5%Co sample,indicating that SPS is not conducive to improving the magnetic properties.The effects of Ni content on the structure and mechanical/magnetic properties of MWS,SPS-Nd₂Fe₁₄B_p/Al composites were studied.The results show that during the MWS process,after the addition of Ni,part of Ni and Al will react and form Al-Ni phases.The addition of Ni will not change the structure of Nd₂Fe₁₄B.The optimized MWS-Nd2Fe14B_P/Al-7.5%Ni samples have uniform structure,and the Ni-containing phases disperse in Al matrix,which plays a role of dispersion strengthening.The compactness,hardness,yield strength and compression strength are 95.7%,110.8HV,146.93MPa and 223.64MPa respectively,which are better than the MWS-5%Co sample,indicating that Ni is conducive to enhance the mechanical properties;The corresponding magnetic properties are:Br=0.29T,H_c=0.65T,（BH）_max=34.32 kJ/m³,which is lower than that of MWS-5%Co sample,indicating that Co is better than Ni in improving magnetic properties.For SPS samples with Ni addition,the compactness,hardness,yield strength and compression strength of optimized SPS-Nd2Fe14B_P/Al-5%Ni sample are 99.3%,145.1HV,171.16MPa and 278.84MPa respectively,with the highest mechanical properties within the experimental range;The corresponding magnetic properties are:Br=0.22T,H_c=0.57T,（BH）_max=30.45kJ/m³,which is the lowest magnetic properties in the experimental range.In comparison,Nd₂Fe₁₄B in MWS sample has a complete structure,which is beneficial to perform magnetic properties,but the lower compactness is unfavorable to the mechanical properties.Due to applied pressure,the SPS can improve the compactness and mechanical properties,and is unfavorable to the improvement of magnetic properties because it will destroy the structure of Nd₂Fe₁₄B.After adding Ni,the interface wettability and strength of Ni-Al and Nd₂Fe₁₄B-Al are improved,which is beneficial to the improvement of mechanical properties.After the addition of Co,Nd₂（Fe,Co）₁₄B strong magnetic phase will be formed,which is significantly helpful to the improvement of magnetic properties.The samples with the best and worst mechanical properties are SPS-5%Ni和MWS-5%Co,while the samples with the best and worst magnetic properties are MWS-5%Co and SPS-5%Ni respectively.Based on the aim of preparing high strength,toughness and magnetic properties aluminum matrix composite,SPS-5%Co is considered to be the sample with optimized comprehensive mechanical/magnetic properties.