| As the pillars of a great power,Shield machine is an essential equipment for urban rail transit construction.Shield cutter is the "tooth" of shield tunneling construction,which is composed of cemented carbide and high toughness steel by brazing.Due to poor brazing performance of large-scale cemented carbide,the reliability of brazed joint is affected.In service process,there are premature cracking,edge chipping and tooth dropping of shield cutter,which reduce working life of shield cutter and greatly affect construction efficiency.According to statistics,maintenance time of shield cutting tools accounts for 3%～20%of whole construction period,and cost of replacing shield cutting tools accounts for about 25%of total construction cost.Therefore,by the means of brazing seam morphology,microstructure and structure design,to study reliability of shield cutter cemented carbide joint,and explore solutions to improve brazing performance of cemented carbide and strength of brazing seam,is of great important engineering significance to improve performance of domestic cutter,prolong working life of shield machine and reduce tunneling cost of shield project.Firstly,concept of brazing seam design is introduced.Surface of cemented carbide is textured by laser to enhance wettability,remove free carbon,oxide,and other impurities.It is found that textured pit can change morphology of brazing seam,increase spreading area of liquid solder on cemented carbide,and reduce wetting angle.Take cylindrical pit as example,based on Wenzel,Cassie and Cassie-Baxter wetting model,the relationship between distribution density of pits,filling state of solder and wetting angle is deduced.The calculation of specific surface area increment of cemented carbide is realized by establishing textured pit model.Combine with stress analysis of solder pit phase,the effect of laser texturing on improvement of interface strength between cemented carbide and brazing seam is summarized.The results of shear test show that fracture is located in brazing seam instead of cemented carbide/brazing seam interface,which shows that laser texturing effect is significant.Secondly,Co-Ni coating on cemented carbide is firstly deposited by electro deposition technology,to design structure of brazing seam.The effects of plating time on morphology,thickness,composition and plating speed of coating are recorded and analyzed.Based on observation of brazing seam microstructure,it is found that the content of strengthening phase α-Cu(s.s)increased in brazing seam.According to Daken-Gurry interaction strength theory,the strengthening mechanism of coating elements on brazing seam strength and interface joints strength are revealed and analyzed.The results of shear test show that thickness of coating affects brazed joint reliability.When coating time is more than 20 minutes,coating induces internal stress,and tends to peel away from cemented carbide.Finally,Cupronickel(CuZnNi),Kovar(FeNiCo)and 304 stainless steel are used as interlayer materials,to carry out structure design of brazing seam.About plant-type interlayer,the effects of thickness and brazing temperature on reliability of brazed joint are studied.The strengthening mechanism of interlayer elements on strength of brazing seam,and strength of interface between base metal and brazing seam is analyzed.The effect of plant-type interlayer on release thermal stress is analyzed.The synergistic mechanism of pore phase of brazing seam and stainless steel mesh to release thermal stress is revealed.Test results show that shear strength of joint can be significantly improved by using Kovar and 304 stainless steel mesh as interlayer,and the shear fracture is located in brazing seam,while the improvement by using Cupronickel as interlayer is not obvious,and the shear fracture is located at interface between brazing seam and interlayer. |
