Research On The Microstructure And Properties Of K417G Superalloy Joints Bonded By Wide Gap Powder Metallurgical Technology

The nickel-based K417G superalloy,a kind of typical precipitation-hardened alloy,is widely used in the hot-end components of aero engines and gas turbines due to its good structural stability and excellent mechanical properties.In recent years,with the development of industrial technology,higher requirements have been imposed on the performance of K417G superalloys.In order to improve the overall performance of K417G superalloys,the contents of the??-Ni₃(Al,Ti)matrix-strengthening phase are increased usually by increasing the contents of Al and Ti in the superalloys.However,with the increase of??content,the crack sensitivity of the superalloy increases greatly and the weldability of the superalloys declines seriously,which brings difficulties to the damage repair of the superalloys.Therefore,it has always been the goal of scientific researchers to design the gap filler metals with properties similar to those of K417G superalloys and to develop the technologies for repairing failed cracks in K417G superalloys.In order to achieve high-quality repair of wide gap cracks in a certain type of aero engine blades,this dissertation,based the mechanism of isothermal solidification,designed the filler metals composed of high-melting-point nickel-based alloy powders(composition similar to that of the base metal)and low-melting-point boron-containing nickel-based alloy powders,investigated the influence of filler metal composition and joining process parameters on the microstructures of the wide gap junction area,optimized the compositions of the filler metals and the joining process parameters,and analyzed the formation mechanism and the corresponding strengthening mechanism of the precipitates in the joint structure.In addition,the oxidation kinetics and oxidation mechanism of joint specimens of the repaired K417G superalloys were also studied.Firstly,based on the thermodynamic equilibrium phase diagram,this dissertation explored the influence of the low-melting-point powder contents(1 wt.%,3 wt.%,5 wt.%,7wt.%,10 wt.%,15 wt.%)on the filler metal solidification interval and the types and contents of the equilibrium phase.Theoretical calculation results showed that the equilibrium phase of the filler metal mainly includes?,??,and M₃B₂borides.As the contents of low-melting-point powders increased,M₃B₂borides gradually increased,and the solidification range of the filler metals expanded.Secondly,the above-mentioned 6 kinds of filler metal blocks were produced with the vacuum hot-pressing sintering technology.The results of the microstructural analysis of the blocks were basically consistent with those of the thermodynamic equilibrium phase diagram.With the increase in the contents of low-melting-point powders in the filler metals,the area fraction of M₃B₂borides in the blocks increased from 0.66%to 5.45%,and the morphology changed from fine granules to coarse bars.When the low-melting-point powders exceeded 7 wt.%,an abnormally coarse M₃B₂precipitated phases appeared in the front of the low-melting-point eutectic structure?/??.According to the calculation of the phase diagram and the results of microstructural analysis,4 kinds of filler metals(the contents of low-melting-point powders were respectively1 wt.%,3 wt.%,5 wt.%and 7 wt.%)were selected.By adjusting the joining process parameters,the effects of bonding temperature,holding time,and pressure on the microstructures and properties of the joint were further studied.The results showed that the joint,obtained by using 5 wt.%low-melting-point powders as filler metals and repairing the superalloy at 1200oC/30 min/20MPa,consists of 97.56%(area fraction,the same below)equiaxed/near-spherical?+??matrix and 2.44%M₃B₂particles dispersed in the matrix.The tensile strengths of the joint at room temperature and at 600oC were respectively 971±15MPa and 934±13 MPa,which reached the strength of the K417G alloy(950±21 MPa at room temperature,and 923±11 MPa at 600oC),achieving the high-quality joining of the joint to the wide gap.Based on the influence of the precipitated phases in the joint structure on the performance of the joint,the formation mechanism of M₃B₂borides in the joint structure was studied.In addition,based on the interface relationship between the precipitated phases and the matrix,the corresponding strengthening mechanism was analyzed.The results showed that the M₃B₂borides preferentially underwent primary nucleation at the?+??phase boundary,indicating that the?phase,divided by the strengthening phase??,acted as a high-energy nucleation point at this time.Considering the calculation results of the thermodynamic phase diagram,the content of??and?phase reached the highest at 1200°C,and the scale of the two phases reached the most uniform refinement,and more grain boundaries would provide more possibilities for the nucleation of M₃B₂borides.These factors all provided good thermodynamic and kinetic conditions for the uniform nucleation and growth of precipitated M₃B₂borides particles in the?+??matrix.After the sample is pre-deformed by 3%,the interface between the M₃B₂precipitated phases and the matrix phase in the joint structure changed from a semi-coherent phase interface to a coherent phase interface.This adaptive strengthening led to higher strength and better plasticity of the joint.In addition,the nano-dispersed carboborites M₂₃(C,B)₆,which was in-situ coherently precipitated from the matrix phase at the joint interface,also helped the improvement of the overall properties of the joint.Finally,the oxidation kinetics and oxidation mechanism of the joint,obtained by using 5wt.%low-melting-point powders as filler metals and repairing the superalloy at 1200oC/30min/20 MPa,were studied.The results showed that the oxidation resistance performance of the large gap junction area was better than that of the base metal,and the oxidation weight gain followed the parabolic law.Oxidized at 1000°C for 1440 min,the outer layer of the oxide film in the base metal was mainly Ni O and Ti O₂,the middle layer was dense Cr₂O₃,and the inner layer was Al₂O₃thin film layer.As the temperature further increased,Ni O and Cr₂O₃joined to each other,and they grew and generated internal stress,so that the oxide cracked or even peeled off.However,under the same conditions,the outer layer of the oxide film in the large gap junction area was mainly dense Cr₂O₃,and the inner layer was mainly Ti O₂and discrete Al₂O₃particles.The junction area of the wide gap has many grain boundaries due to the small grain size,and a dense Cr₂O₃oxide layer with oxidation resistance was formed at the initial oxidation stage,which effectively improved the oxidation resistance performance of the joints.