Research On Brazeability Of Sintered NdFeB Rare Earth Permanent Magnet Materials

Sintered Nd Fe B is currently the most widely used rare earth permanent magnet material.In some electromagnetic-mechanical energy conversion equipment,the sintered permanent magnets are assembled into large and complex shaped components,which are subjected to high mechanical loads and thermal shocks in use.However,current glued assemblies have low joint strength and poor heat resistance,making it difficult to meet the requirements of use.Brazing is a joining technique that uses an alloy as the “glue” to form a metallurgical bond with the substrate,which is expected to meet the strength and heat resistance requirements of Nd Fe B splicing joints.In order to overcome this problem,this paper uses the physical vapour deposition(PVD)method to first prepare a protective layer of oxidation-resistant Cu on the surface of Nd Fe B under vacuum conditions,and braze under gas-protected conditions to make the Nd Fe B more easily wetted and joined.In order to reveal the weldability of brazing materials with different compositions and melting points on the surface coated Cu Nd Fe B,three types of brazing materials were used for wetting and welding tests at high,medium and low temperatures to characterize the brazing joint organization,strength and magnetic properties after brazing,etc.The main research contents and results are as follows:(1)Firstly,high temperature 72 Ag Cu alloy was selected as the brazing material,which was deposited onto the surface of the sintered Nd Fe B specimen by physical vapour deposition(PVD)method,and induction brazing was carried out under argon gas protection,and the microstructure and properties of the brazed joint were investigated with different induction heating times.The results show that the intergranular phase in Nd Fe B liquefies when induction heating melts 72 Ag Cu,and the melted 72 Ag Cu alloy first dissolves the Cu coating and then penetrates into the Nd Fe B and mixes with the intergranular phase to form a good metallurgical bond.The width of the brazing reaction layer increases with increasing heating time,and defects can occur when the brazing seam penetrates too thickly and spreads too much,so over-reaction should be avoided.The bending strength of the joint first increases with induction heating time,with the maximum bending strength(196.94 MPa)obtained at a heating time of 16 s,and then gradually decreases with increasing heating time.Induction heating reduces the coercivity of Nd Fe B by 16% and annealing after brazing restores its coercivity but with a slight loss of squareness.The performance of the brazed joint can be further improved by controlling the amount of brazing filler metal and induction heating parameters.(2)In order to reduce the thermal influence of brazing,a medium temperature brazing material with a melting point of about 520 °C was prepared by adding Zn,Cd,Sn and In elements to the Ag Cu alloy.The brazing material was ground into a fine powder and spread on the substrate surface to be brazed by induction heating for 10 s.The Ag-based medium-temperature brazing material also dissolves the Cu coating and reacts with the Nd Fe B surface by penetration,forming a brazing seam consisting of a central brazing material layer,a penetration reaction layer and a diffusion layer.As the heating time increased,the width of the brazing reaction layer widened,with a low reaction rate due to the lower brazing temperature.The maximum bending strength(70.9MPa)of the brazed joint bond strength was obtained at a heating time of 10 s,after which it gradually decreased with increasing heating time.Further control of the amount of brazing material is required to reduce the width of the brazing seam to avoid fractures occurring in the near surface layer of the Nd Fe B substrate due to mismatch in deformation during cooling,thus improving the bending strength.The coercivity is reduced by 2.94% after medium temperature brazing and annealing,with good squareness and little effect on the magnetic properties of the base material.(3)Finally,Sn Pb-SnAgCu soft solder was tried,and Nd Fe B was soldered using three methods: heating table heating,vacuum furnace heating and induction heating.The results showed that the Sn Pb solder was able to form a good wetting of the Cu coating on the surface of the substrate,and that it was not easy to completely dissolve the Cu coating;The SnAgCu alloy is slightly less wettable,but also forms a good wetting of the Cu plated layer and has a higher strength and melting point;with Sn Pb-SnAgCu composite brazing,micro-diffusion of the surface Cu layer occurs.The low temperature soft solder can be easily soldered with Nd Fe B with surface PVD coated Cu under the protection of flux,but the metallurgical reaction with Nd Fe B cannot occur after the Cu coating is melted through,which will lead to a significant decrease in strength.In summary,in this paper,high,medium and low temperature brazing materials were selected for the sintered Nd Fe B with PVD coated with Cu.It was found that the Cu coating on the surface can effectively prevent the Nd Fe B from being oxidised during the brazing process and can be wetted by various types of brazing materials.The silver based brazing material with medium to high temperature can form a metallurgical bond with the Nd Fe B substrate after the Cu coating has been dissolved off.The soft brazing material can be used to weld Nd Fe B coated with Cu.The bonding force depends on the bonding force between the Cu coating and the substrate,and it is necessary to avoid the surface layer of Cu being dissolved through.This paper provides some basis for the brazing connection of sintered Nd Fe B rare earth permanent magnet materials.