Study On Microstructure And Mechanical Properties Of Ni-based Boron-free Solder And Brazed Diamond By Adding Hf And Zr

Synthetic diamond is widely used in the manufacture of diamond tools due to its excellent mechanical and chemical properties.Compared with traditional methods of preparing diamond tools such as electroplating and sintering,brazing is more efficient and effective in preparing diamond tools.In order to achieve a high-strength bond between the diamond and the matrix,the brazing of the diamond requires an active filler to assist.Strong carbide elements such as Ti,Cr,W,etc.chemically react with the diamond surface to form carbides,which increases the connection strength between the diamond and the matrix,also known as the metallurgical bond strength.At present,the most widely used fillers are nickel-based fillers,copper-based fillers and silver-based fillers.Among them,nickel-based filler metals have become the first choice for preparing diamond tools due to their excellent mechanical properties,higher strength and better wettability.However,the melting point of nickel-based filler is relatively high,and brazing diamond at higher temperatures will cause thermal damage,thereby weakening the strength and brazing performance of the brazed joint;the hard and brittle phase formed during the brazing process will also cause The toughness of the joint deteriorates and cracks occur,which is not conducive to the improvement of the joint performance.In order to solve the above problems,this subject developed and prepared a new type of nickel-based boron-free solder,which was modified by adding two elements on the basis of optimizing the Cr element composition of the solder.The experimental study on the brazing of diamond with nickel-based boron-free composite brazing filler metal is also carried out.First,optimize the addition of Cr element in the new boron-free filler Ni-Cr-Si-Cu-Sn,and set the Cr composition as 5wt.%,15wt.%and 25wt.%,respectively.The microscopic morphology and element distribution of diamond were observed and analyzed by scanning electron microscope（SEM）and energy dispersive spectrometer（EDS）.The results showed that the increase of Cr content refined the grains of the brazing filler metal and played a role in element regulation in the brazing filler metal.When the Cr content is 15wt.%,the element distribution in the filler is relatively uniform.XRD shows that as the Cr content increases,the intermetallic compounds in the filler also increase.Because the intermetallic compound is a hard and brittle phase,the microhardness of the filler gradually increases,and the formation of a large number of intermetallic compounds affects the solid liquidus temperature of the filler,leading to an increase in the melting point of the filler.A comprehensive description of various characterizations,a Cr content of 15wt.%is the most suitable.Then,on the basis of optimizing the composition of the filler alloy,0wt.%～4wt.%Hf element is added to strengthen the phase,and the effect of different Hf element additions on the microstructure and properties of the filler alloy and brazed joints,as well as on the diamond surface carbides are studied.The results show that the addition of Hf element refines the grains of the filler,and as the Hf content increases,the grains become more refined,and at the same time the wettability of the filler also increases,but the wettability of the filler The better,the lower the exposure of diamond during brazing,which affects the friction and wear properties of the joint.Hf element can also form Ni-Hf compound with catalyst element Ni,which consumes Ni atoms in the brazing filler metal,thereby reducing the degree of graphitization of diamond.However,excessive Hf element will cause thermal damage to diamond.When the Hf content is 1wt.%,the solidus liquidus temperature of the filler is 937℃,and the diamond abrasive grains after brazing have good morphology and exposure.The interface between the diamond abrasive grains and the filler There are no obvious cracks and holes.At the same time,the addition of 1wt.%of Hf elements promotes the formation of Cr₇C₃at the interface,which improves the strength and hardness of the brazed joint while strengthening the metallurgical bond at the interface.The friction and wear experiment further shows that the friction and wear coefficient of the brazed joint with the addition of 1wt.%Hf is the lowest and the wear resistance is the best.Finally,change the type of element,add 0wt.%～2wt.%Zr element,and explore the effect of different Zr additions on the microstructure and properties of the brazing filler metal alloy and brazed diamond joints,as well as the effect of diamond surface carbides.The study found that the addition of Zr refines the grain structure of the solder alloy and reduces the melting point and melting point range of the solder alloy;The XRD pattern shows that the addition of Zr promotes the formation of intermetallic solid solution in the filler alloy,which has a solid solution strengthening effect on the filler alloy and improves the microhardness of the filler alloy.The SEM of the brazed joint shows that the addition of 0.5wt.%Zr effectively reduces the thermal damage of the diamond abrasive grains of the brazed joint,the diamond abrasive grains remain intact as a whole,and the brazing filler metal layer is relatively dense and uniform without obvious cracks,holes and other defects.The exposure of the brazed diamond is suitable,and the cutting edge is obviously exposed.It can be seen from the line scan at the interface between the brazing filler metal and the diamond abrasive grains that the uniform and dispersed distribution of Zr at the interface promotes the diffusion between elements and consumes the Ni element in the brazing filler metal alloy.Reduced erosion of the diamond surface by the brazing filler metal.Observing the carbide morphology of the diamond surface after etching,it is found that the addition of Zr promotes the formation of the carbide layer on the diamond surface,especially the formation of Cr₇C₃;in addition,the Zr also shorten the thermal expansion coefficient of the filler and diamond during high-temperature brazing.The difference reduces the generation of thermal residual internal stress at the interface,thereby reducing the probability of cracks in the thin carbide layer on the diamond surface.However,excessive Zr content will cause thermal damage and severe graphitization of the diamond.When the Zr content is 0.5wt.%,the degree of graphitization of the joint diamond is the lowest.The friction and wear experiment further shows that when 0.5wt.%Zr is added,the friction and wear coefficient is the lowest,most of the diamond cutting edges exist,and the wear resistance is the best.