| Ni-based superalloys with high Al+Ti content are precipitation strengthening alloy with superior high temperature, which are indispensable to hot section components of gas tuebine. γ’ will increase with the Al+Ti content in the alloy. Meanwhile, the alloy suffers from high susceptibility to hot cracking during welding and post-welding and post-weld heat treatment. The ability to achieve a crack-free weld and overlay, especially using the filler that matches, more closely matches base material for high γ’ superalloy including directionally solidified and single crystal superalloy has been the primary goal of manufacturing and repairing hot section components. In order to achieve the damage-free welding repair and coating method better matching the composition and structure of the base material, in this paper, a series of relevant research work has been carried out, including the electric-spark deposition and powder metallurgy remanufacturing. Based on the research of electric-spark deposition sandwich-like structure welding layer with post-weld heat treatment to heal welding cracks was unfeasible, we made a further study on the problem of powder metallurgy remanufacturing. The first one is how to ensure repairing strength; the second is the binder selection to fix powder to be molding; the last one is three-dimensional interface joint between powder body and the substrate alloy.In order to solve the first problem we pointed here, the high temperature superalloy IN738 with high Al+Ti content under vacuum sputtering powder was used as framework and different kind of activated powders were adopted for activation roast in order to lower the sintering temperature. The wettability of own-prepared Ni10-B alloy powder, commercial self-dissolve Nizs alloy powder, nano-sized Ni powder and IN738 alloy base were investigated The results show that:Ni10-B and Ni25 have good wettability under 1150 and 1200℃ which could be used as activation powder to be added in the framework.For the second point, we proposed a non-toxic and easy removed powder molding adhesive composition.8% addition of the binder was the best condition after the viscosity test and powder molding test. DSC-TGA was performed to study the post-mold degreasing process and the kinetic parameters were calculated. The results show that PMMA as the main binder with small amount of addictive binder can make the alloy powder plastic-like, which was kept the sample under 100℃ for 4h, and 200℃ for 4h, then put the sample in the vacuum oven maintaining 400℃ for 40min to ensure no binder left. The calcolation showed the binder removal have three steps (75-120℃,150-250℃ and 300-420℃). And the reaction activation energy were 69.6205kJ·mol-1,82.116kJ·mol-1 and 141.903KJ·mol-1, respectively. As for the evaporation and different composition of the binders, when decompositions of temperature increased every 10℃ at the lower temperature range, the reaction rate constant increased several times. When the temperature increased 50℃ in the high temperature range, the reaction rate constant would be increased by one order of magnitude.For the third, we studied the effects of the addition amount of Ni10-B, the roasting temperature, holding time on the sintered density of the sintering zone, and the base substrate. The results show that when 10% Ni10-B was added in the powder, a densed sintering zone and harmless, metallurgical bondings could form on the IN738 surface. The tensile test at room temperature and oxidation experiment under 900℃ results show that the tensile strength of the joints was more than 80% of that of the base substrate. The anti-oxidation of the bondings under 900℃ was inoxidizable, which is the same as the base substrate superalloy. The thermal fatigue test under 900℃ shows that:the crack length was similar to the base substrate afer 100 times cycle, but the thermal resistance of the interface was poor and the defects and Cr-B complex formation in the interface which cause the crack.At last, the optimized technology of powder metallurgy remanufacturing was applied to K403 turbine blades for practical test. The results show that with the powder metallurgy remanufacturing technology the damage part of the blades K403 could be remanufactured and the interface was metallurgical bonding. And the strength of the bonding could be 80% of the base substrate when the joint length 8mm.Further research about the feasibility of the powder metallurgy remanufacturing applied to low-carbon steel for Co-based coating was carried out. The results show that a thicker coating layer could be formed by this method. The metallurgical bonding was formed to connect the coating and stainless steel base. Fe, Co, Cr, Ni elements diffusion in the interface was the main force to form the metallurgical bonding. Compared with the plasma spraying method, the coating was thicker and fewer defects for this method, but the interface segregation occurred in hard phase. |
