Study On Repairing Of Thermal Cracks Of K417G By Powder Metallurgy

Superalloy components used in engine commonly suffer from abrasion, impact, hightemperature gas and thermal fatigue. So, hot cracks will form in the components and thenmake the components fail. The application of advanced technology to repair the defective andeven damaged parts is helpful to prolong their service life, and will make considerableeconomic benefits. Based on the principle of powder metallurgy (PM), a novel kind of repairtechnique of crack is developed. In the region repaired by PM method, the microstructureexhibits more uniform, no linear eutectic of low melting point forms, and bulk borides orsegregations are lack. The interface of the repaired regions and the matrix exhibitsmetallurgical bonding. In addition, the repair process is simple and suitable to repair inbatches. Therefore, the systemic study on PM repair of K417G Ni-based superalloycomponent used in engine has great practical significance.In this study, micro-liquid phase sintering as a PM technique was used to repair crackwith wide gap. With the increase of sintering temperature and holding time, the relativedensity of repaired joint grows and the porosity decreases. After repairing by sintering at1220℃for4h, the relative density of repaired joint is up to the highest value of92.5%. Theresults of XRD, OM, SEM, TEM analysis indicates that not only K417G nickel-basedsuperalloy but also the repaired joints contain γ-phase, mushroom-shaped γ+γ’ eutectic anddispersed small block of MC carbides. In the repaired joints, a small amount of boride phaseis found additionally. The matrix and the repaired regions both have high hardness values.Under the same sintering conditions, the hardness of the matrix is slightly higher than therepaired regions. For the sample sintered at1220℃for4h, the microhardness is high andmore uniform, with a maximum hardness of527HV. Meanwhile, the hardness of the crackrepair area is close to that of the base material. Moreover, the tensile strength and elongationof the repaired joint are lower than those of K417G nickel-based superalloy, and the brittlefracture feature dominates the fracture surface.The antioxidation property of the joint repaired at1220℃for4h and the furthercomparision with K417G nickel-based superalloy were studied. With regard to the isothermaloxidation of the repaired joint, the weight augment by oxidation exhibits linear approximationwith the oxidizing time under700℃and800℃. However, a parabolic approximationisshown under900℃. During the oxidation process of repaired joint, both the outer and inneroxidation occurred. So, the existence of the oxide films can be clearly divided into inner andouter oxidation in the surface of sample. The outer oxide layer mainly consists of NiO, Al2O3 and Cr2O3, with a small amount of TiO2and NiCr2O4spinel compound. Differently, the inneroxide layer mainly consists of Al2O3and a minor amount of Cr2O3. As the temperatureincreases, there is significant variation in the thickness of the outer oxide layer, and the inneroxide layer. Compared with the matrix, the repaired joints have a near oxidation resistance,which is demonstrated by the similar oxidation kinetics law, oxidation weight gain andstructure of oxide layer.The corrosion resistance and thermal fatigue behavior of the repaired joints by sinteringat1220℃for4h were also investigated.During the hot corrosion at800℃and850℃, theweight gain basically changes with time in parabolic approximation law. However, the weightgain does not follow the parabolic law during the hot corrosion under900℃. The presence ofNaCl accelerates the hot corrosion. Moreover, the thermal fatigue test shows that the thermalfatigue cracks initiates in the binding region between the matrix alloy and the repaired joint.The crack initiation is mainly caused by the generation, aggregation and connectivity ofstress-induced oxidative holes. Grain boundaries, carbides and eutectic structure can facilitatethe crack initiation and propagation. The thermal fatigue crack is found to progress mainlyalong the dendritic crystals. The minor crack progression is along the grain boundaries. It isfound that the grain boundaries and the eutectic and carbides distributed on dendritic crystals,easily crack or separate from the matrix under thermal stress. Subsequently the crackpropagation channels can form.