GH738 alloy showcases commendable high-temperature performance, rendering it a prevalent choice for the production of crucial aeroengine components, including sealing rings, turbine discs, and fasteners. Nevertheless, GH738 sealing rings are susceptible to accelerated wear in strenuous service environments, leading to surface deterioration and eventual failure of the sealing mechanism. To improve the wear resistance of the surface of the GH738 sealing ring and prolong its service life, the GH4169 alloy, which has a slightly higher hardness than the GH738 alloy, is selected for repair. However, because of the rapid melting and coagulation during the laser deposition repair process, the sedimentary microstructure is typically metastable. Heat treatment is necessary to optimize the microstructure and improve the performance. Therefore, the study of the microstructural properties resulting from heat treatment is a research priority for the laser deposition repair of superalloys. Consequently, the GH738 alloy undergoes laser deposition repair using GH4169 alloy powder, followed by analyses of the resulting microstructural properties, microhardness distribution, and friction and wear behavior of both the deposited and subsequently heat-treated specimens. This study aims to establish a theoretical foundation for the repair of GH738.
The laser deposition repair test performed in this study employs a synchronous powder-feeding laser additive manufacturing system. The process parameters used are the laser power of 1200 W, scanning speed of 7 mm/s, powder feeding rate of 6 g/min, spot diameter of 3 mm, and lap rate of 40%. Subsequently, aging heat treatment tests are performed on the repaired specimens in a vacuum tube high-temperature sintering furnace under vacuum conditions. The microstructure of the specimen is observed using an optical microscope and scanning electron microscope (SEM) with energy dispersive X-ray spectroscopy (EDS), which is followed by a regional chemical composition analysis. The microhardness of the deposited specimens is tested by a digital microscopic Vickers hardness tester with the test load force of 3 N. A rotary microcomputer-controlled universal friction and wear testing machine is used to test the friction and wear performance at room temperature with constant load of 15 N, speed of 200 r/min, and wear time of 15 min. An electronic balance is used to measure the mass before and after specimen wear to calculate the amount of frictional wear. Friction and wear morphologies are examined using a digital microscope.
The lowest microstructures within the sedimentary repair zone exhibit columnar dendrites, while the middle and upper microstructures of the repair zone comprise both columnar and equiaxed dendrites (Fig. 2). A Nb-rich Laves phase is distributed between the dendrites in the repair zone (Fig. 3). Compared with the substrate, the heat-affected zone exhibits a lower number of γ′ phases and shows a coarsening trend, and the MC carbides decompose to form M23C6 carbides (Fig. 4). Upon the completion of aging heat treatment, fresh grain boundaries are established in the repair zone microstructure, accompanied by the fragmentation of certain Laves phases and the uniform precipitation of the γ′ and γ″ enhanced phases (Figs. 5 and 6). The average hardness of the repair zone of the sedimentary specimen is approximately 291 HV, which is lower than the average hardness of the substrate (361 HV). The average hardness of the repair zone of the heat-treated specimen is approximately 461 HV, which is higher than the average hardness of the substrate (388 HV) (Fig. 8). After the heat treatment, the average friction coefficient of the specimen repair zone is approximately 0.40, the average friction coefficient of the substrate is approximately 0.51, and the wear resistance of the repair zone is better than that of the substrate. The wear mechanism of the repair zone is abrasive wear and that of the substrate is abrasive and adhesive wear (Figs. 9 and 10).
In this study, the repair of GH738 alloy is performed using GH4169 alloy powder via laser deposition. The outcomes indicate that lower microstructures of the sedimentary repair zone exhibit columnar dendrites, while middle and upper microstructures of the repair zone comprise both columnar and equiaxed dendrites. After the aging heat treatment, the columnar dendrite structure in the repair zone demonstrates a tendency to transform into equiaxed dendrites, and the grains are refined. The hardness of the sedimentary specimen substrate decreases from the heat-affected zone to the repair zone, with the hardness of the repair zone being higher than those of the substrate and heat-affected zone after the aging heat treatment. The average hardness of the sedimentary repair zone is approximately 291 HV, that of the heat-treated repair zone is approximately 461 HV, and the hardness of the heat-treated repair zone is higher than that of the substrate. The wear resistance of the heat-treated repair zone surpasses that of the substrate, with the abrasive wear mechanism being prominent in the repair zone, while the substrate exhibits a combination of adhesive and abrasive wear mechanisms.
