Research On The Mechanism And Key Technology Of Brazing Of Complicated Shield Machine Carbide Scrapers

The shield cutters are widely used in the underground shield process under severe working conditions.The cutters often failed because of breakage or wevere wear.The maintance cost of the tool accounts for nearly 1/4 of the total construction cost.The scraper requires a large number of cemented carbide to be brazed at the steels at on one time.The area of the brazing surface is big and is divided into vertical surface,horizontal surface and inner mosaic structure,which affect the scraper quality.Therefore,the thesis focus on the selection of filler alloy,the optimization of brazing structure and the optimization of heating program.The thesis evaluates the brazing method and the induction heating technology of scraper.The quality of the scraper is evaluated by on-site service performance.Firstly,the microstructure and precipitation phase of BAg49ZnCuMnNi(Ag49Ni)joint brazed under different welding conditions are analyzed by using SEM,EDS and other methods.The microhardness of the precipitated phase and the shear strength of the joint were tested.The thesis evaluates the solidification process and strengthening mechanisam of Ag49 Ni filler alloy.The island-like ?-Cu solid solution(?-Cu(s.s))phase in the brazing joint has a dispersion strengthening effect due to high hardness,and the ?-Cu(s.s)layer at the interface has an interface strengthening effect.The distribution morphology of the ?-Cu(s.s)is affected by the welding parameters.The joint brazed at 770 °C for 30 s has the highest shear strength(345 MPA).If the welding temperature is too high or the holding time is too long,the number of island-like ?-Cu(s.s)phases decrease,and the thickness of the ?-Cu(s.s)layer increases.An ultrasonic-assisted brazing test platform was created.Ultrasonic vibration promotes the number of island-like ?-Cu(s.s)phases in the brazed joint.When the ultrasonic time is controlled within 0~15s,the mechanical properties of the brazed joint are improved.The microstructure and phase of the joints brazed with different sheet/mesh interlayer were analyzed by using SEM,EDS,XRD and other methods.The solidification nucleation mechanism and strengthening mechanism of joints brazed with different sheet/mesh interlayer were studied.The shear strength of the joints with SS304 mesh is highest(353 MPa).The mesh interlayer not only improves the mechanical properties of the brazed joint,but also improves the brazing processability.The average shear strength of the joints with Cu38Zn4Mn(CuZn)sheet interlayer is only 313 MPa.The joint performance is affected by the distribution of ?-Cu(s.s),?-Cu(s.s)phase and CuZn sheet interlayer in the brazing joint.The increase of welding temperature and the prolonged holding time make the dissolution of CuZn sheet interlayer.The chemical compositions of the filler alloy make the sub-eutectic–eutectic-hypereutectic transformation in the process.The mesh interlayer has a better effect on the performance of the cemented brazed joint than the sheet interlayer.Further,the Ansys software was used to establish the numerical calculation model of the induction heating of the scraper.The influence of the structure of the induction coil and process parameters on the heating power of the scraper was studied.The ABAQUS software was used to establish the calculation model of the scraper cooling temperature field-stress field.The distribution of internal temperature and residual stress in cooling process was studied.The results show that when the single-turn large coil has a coil inner ring size of 325 mm × 180 mm,a section height of 50 mm,a coil and scraper height difference of 12 mm,an alternating current of 15000 A,and a current frequency of 3500 Hz for heating,the overall heating power of the scraper is large,and the heating power of each part is relatively uniform.The preheating temperature is 300 °C,and the heating process adopts the 3-stage heating and heat preservation scheme of 400 °C-600 °C-800 °C.The simulation result is consistent with the heating law of the actual scraper brazing.The Mises stress in the scraper is the main factor of the fracture of the cemented carbide block.The residual stress has the maximum value at the junction of wc_c,wc_a and s_up.Finally,the actual brazing work of the scraper is carried out.The minimum shear strength of the brazed joint is 250 MPa,which meets the requirements of JB/T 11861-2014.The induction brazing process has little effect on the properties of YG15 C cemented carbide.The scrapers performed well under different geological conditions in the actual service,and meet the requirements for use.