Microstructure And Properties Of Sintered NdFeB Permanent Magnet Joints Soldered With Zn-based Alloy

The sintered NdFeB permanent magnet is a kind of functional material with Nd₂Fe₁₄B as the hard magnetic phase and has been widely used in hybrid vehicles,wind power generation,electronic information,medical equipment and energy-saving home appliances due to its excellent magnetic properties,such as high residual magnetization,coercive force and maximum magnetic energy product.Because the sintered NdFeB permanent magnet is made by powder metallurgy,it is difficult to prepare large-size parts with complex shapes at one time.In order to meet the needs of modern manufacturing and fully utilized excellent magnetic properties,the sintered NdFeB permanent magnet should be connected with steel or other metals.At present,the connection methods used for sintered NdFeB permanent magnet are mechanical connection and adhesive connection.However,high-tech electromagnetic products continue to emerge.The requirements for high reliability,high strength and long life of the connection technology are getting higher and higher.The welding technology gradually attracted attention,such as laser spot welding and friction stir welding.The research work had a preliminary understanding of the weldability of the sintered NdFeB permanent magnet.However,there was lack of complete data support for the appropriate welding methods,properties of joints and the stability of the magnetic properties.In this paper,soldering process was adopted.Based on the solderability of sintered NdFeB permanent magnet and the basis of soldering selection,a kind of Zn-Sn-Bi-Nd solder suitable for connecting the sintered NdFeB permanent magnet was designed.Besides,the microstructure and properties of soldered joints between sintered NdFeB permanent magnet and steel were carried out.Finally,the magnetic properties of the sintered NdFeB permanent magnet should meet the required conditions.The solderability of sintered NdFeB permanent magnet were studied.Meanwhile,combined with the principle of soldering and alloying theory,the basic components of the solder were determined.As the heating temperature rising to 700°C,the grains of main phase Nd₂Fe₁₄B grew significantly,Nd-rich phase melted and agglomerated at the trident grain boundary,which reduced the demagnetization exchange coupling effect.The loss rates of B_r,H_cj and(BH)_max were 7.38%,18.77%and 19.38%,respectively.The melting point of zinc(419.58°C)is lower than the melting point of Nd-rich phase(655°C).Therefore,Zn-based solder had little effect on the magnetic properties of sintered NdFeB permanent magnet.Besides,it had strong interaction with steel.The Sn addition was beneficial to improve the plasticity of the Zn-based solder.When the Sn content was 6 wt.%,the spreading area of the solder on the sintered NdFeB permanent magnet and steel were 98.54 mm²and 216.23 mm²,respectively.The solder had poor wettability for the sintered NdFeB permanent magnet.Besides,the shear strength and elongation of the soldered joint were 45.3 MPa and 3.79%,respectively.The fracture was located at?-Sn phase of soldering seam.The effect of alloying elements(Bi and Nd)on the microstructure and properties of Zn-Sn solder and NdFeB/steel soldered joints was analyzed.The Bi addition generated Bi precipitated phase,refined?-Sn phase,and improved the wettability of the solder.The refined microstructure and the anodic barrier of Bi precipitates were the reasons for the corrosion susceptibility reducing by Bi addition.When the Bi content was 5 wt.%,the shear strength and elongation of soldered joint were 61.9 MPa and 2.56%,respectively.The position of fracture transfered from?-Sn phase to Bi precipitated phase,and the fracture type changed into a brittle-fracture pattern.The corrosion rate of the soldered joint was 0.055 mg?cm^-2?h^-1.Compared with the soldered joint without Bi addition,the shear strength of the soldered joint was increased by36.6%.Besides,the elongation and corrosion rate were decreased by 32.5%and 24.7%,respectively.The Nd addition generated Nd Sn₃ phase,which was dispersed in the solder,and refined?-Sn/Bi phase.The Nd addition could improve the wettability of the solder.Additionally,the improved compactness of the corrosion products with Nd addition prevented further corrosion of the solder.However,excessive Nd addition led to the formation of large-scale Nd Sn₃,resulting in deteriorating the corrosion resistance of the solder.When the Nd content was 0.15 wt.%,the shear strength and elongation of soldered joint were 85.6 MPa and5.47%,respectively.The position of fracture transfered from Bi precipitated phase to Nd Sn₃phase,and the fracture was characterized by ductile-brittle mixed fracture.The corrosion rate of joint was 0.031 mg?cm^-2?h^-1.Compared with the soldered joint without Nd addition,the shear strength and elongation of soldered joint were increased by 38.3%and 113.7%,respectively.Besides,the corrosion rate of soldered joint was decreased by 43.6%.The uniform formula design method was adopted.The spreading area of the solder,the shear strength and corrosion rate of soldered joint were used as indicators.In order to discuss the interaction of alloying elements on the properties and optimize the composition of Zn-based solder,the stepwise regression method was used to establish a quadratic regression model of the element and the objective function.The results showed that there were interactions between Sn,Bi,and Nd.Moderate Sn,more Bi and a small amount of Nd could improve wettability,shear strength and the corrosion resistance of the soldered joint.After comprehensively considering factors such as solder smelting and soldering seam forming,the composition of Zn-based solder was optimized as follows:6 wt.%Sn,5.7 wt.%Bi,0.13 wt.%Nd.The spreading area of solder was 152.36 mm².Besides,the shear strength and corrosion rate of the soldered joint were 110.7 MPa and 0.028 mg?cm^-2?h^-1,respectively.The effect of process parameters on the microstructure and properties of sintered NdFeB permanent magnet/steel soldered joint using Zn-Sn-Bi-Nd alloy was studied.The evolution of interfacial reaction of the soldered joint were revealed.Meanwhile,the kinetic theory was used to establish the diffusion kinetic equation of the Zn element in the sintered NdFeB permanent magnet.The soldered joint was divided into three areas:near the sintered NdFeB permanent magnet side(reaction layer I),including Nd Fe_5.5Zn(B)and?-FeZn₁₃ phase;soldering seam II,including Zn matrix,?-Sn,Bi precipitated phase and Nd Sn₃ phase;near the steel side(reaction layer III),including?-FeZn₁₃,?-FeZn₁₀ and?-Fe₃Zn₁₀ phases.When the holding time was 30s,as the soldering temperature rising from 410°C to 450°C,the thickness of the reaction layer I increased from 1.4?m to 2.1?m,and the thickness of the reaction layer III increasd from 12.3?m to 16.6?m.When the soldering temperature was 430°C,as the holding time extending from 20 s to 40 s,the thickness of the reaction layer I increased from 1.3?m to 5.6?m,and the thickness of the reaction layer III increased from 11.9?m to 16.1?m.When the soldered temperature was 430°C and the holding time was 30 s,the shear strength of the soldered joint was 110.7 MPa.In the case of low soldering temperature and short holding time,the interfacial reaction was insufficient,and weak links were generated at the NdFeB interface.The cracks were easy to initiate from Nd Fe_5.5Zn(B)phase.When the soldering temperature and holding time were moderate,the joint strength was determined by soldering seam.Further increasing soldering temperature and prolonging holding time,cracks initiated and propagated at the steel side due to the excessive growth of ?-FeZn₁₃ phase.