K4169 high-temperature alloys exhibit high strength, plasticity, and heat-corrosion resistance in the middle and low-temperature ranges and are particularly suitable for manufacturing aircraft engines. The as-cast state of the K4169 alloy is prone to severe elemental segregation, resulting in deteriorated welding performance. The presence of liquation cracks in the heat-affected zone significantly reduces the safety and operational reliability of the product. To enhance the quality of repairs, laser pretreatment is employed to adjust the composition, structure, and phase distribution of the base material while reducing its strength and hardness to improve the liquation crack sensitivity of the matrix material. However, a systematic study of the pre-laser treatment process for K4169 nickel-based high-temperature alloys has not been carried out. In particular, the influence of the base material’s structure on crack initiation mechanisms is not well understood. Therefore, this study emphasizes the necessity of employing pretreatment processes to regulate the K4169 alloy base material before repair, and conducts in-depth research on the sensitivity and mechanisms of liquation cracks in the repaired specimen’s heat-affected zone. The results of this study have significant implications for the high-quality repair of nickel-based high-temperature-alloy components in aerospace.
This study employed a homogenization+solution+aging treatment and homogenization+hot isostatic pressing+solution+aging treatment on a K4169 alloy substrate prior to repair, followed by repair experiments using the laser deposition process with synchronized powder feeding for different substrate microstructures. The repaired specimens subjected to the homogenization+solution+aging treatment were denoted LDR, whereas those subjected to the homogenization+hot isostatic pressing+solution+aging treatment were denoted LDR-K9. The process parameters included a laser power of 1600 W, scanning speed of 8 mm/s, scanning speed of 1 rad/min, overlap rate of 40%, and laser diameter of 3 mm. Subsequently, the cross-sections of the heat-treated substrate and repaired specimens were ground and polished, followed by corrosion using a solution of hydrochloric acid, nitric acid, and hydrofluoric acid (80 mL HCl+7 mL HNO3+13 mL HF). The microstructures of the substrate specimen cross-sections and the distribution and characteristics of cracks in the repaired specimens were observed using an OLYMPUS GX51 optical microscope (OM) and a ZEISS Sigma300 scanning electron microscope (SEM). Energy-dispersive spectroscopy (EDS) was employed to characterize the distribution of elements in the substrate region. Phase analysis was performed using a Bruker d2-phaser X-ray diffractometer (XRD). Microhardness measurements of the repair, heat-affected, and substrate zones were conducted using an HVS-1000Z Vickers hardness tester under a 1.96 N load for 15 s. Tensile tests were performed using an INSTRON5982 universal testing machine at a strain rate of 0.5 mm/min.
The average width of the columnar dendrites in the homogenization+solution+aging treated substrate is 123.6 μm. Under the conditions of the homogenization+hot isostatic pressing+solution+aging treatment, the as-cast columnar dendrites dissolve and transform into large equiaxed grains (Fig.4). The homogenization+solution+aging treated substrate is mainly composed of Laves phase, δ phase, and carbides, while the homogenization+hot isostatic pressing+solution+aging treated substrate is primarily composed of δ phase and carbides (Figs.5 and 6). LDR-K9 exhibits cracks distributed on one side of the substrate with an average crack length of 0.68 mm and a maximum crack length of 0.72 mm (Fig.7). In the heat-affected zone of LDR, a liquation film with a small thickness and no cracks is formed by the liquefaction of the Laves phase components (Fig.8). In the heat-affected zone of LDR-K9, the liquation film forms by the liquefaction of δ phase and a small amount of γ' phase components, with a greater thickness, and cracks present (Fig.9). Simultaneously with the liquation of phase components, the segregation of S, P, and B towards the grain boundaries lowers the solidus line, accelerating the segregation liquation and crack formation in the heat-affected zone (Fig.10). The average hardness values of the substrate and heat-affected zone in LDR are 220 HV and 210 HV, respectively, which are higher than those of LDR-K9. The content of Laves and δ phases influences the microhardness of the substrate and heat-affected zone (Fig.12). The tensile strength, yield strength, and elongation of LDR are 870.7 MPa, 618.9 MPa, and 7.7%, respectively. In contrast, the tensile and yield strengths of LDR-K9 decrease by 20.3% and 38.4%, respectively, whereas its elongation increases (Fig.14).
Cracks are present in LDR-K9, distributed on one side of the substrate, with an average crack length across the cross-section of 0.68 mm and a maximum crack length of 0.72 mm. The occurrence of liquation cracking in the heat-affected zone is closely related to the composition and grain size of the substrate. The average hardness values of the substrate and heat-affected zone in the LDR are 220 HV and 210 HV, respectively, with tensile strength, yield strength, and elongation values of 870.7 MPa, 618.9 MPa, and 7.7%, respectively. In comparison, both the substrate and the heat-affected zones in LDR-K9 exhibit lower average hardness values (210 HV and 200 HV, respectively). The content of the Laves and δ phases affects the microhardness of both the substrate and heat-affected zone. The tensile and yield strengths decrease by 20.3% and 38.4%, respectively, whereas elongation increases.
