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Vacuum Brazing Repair of Ni-based Turbine Blades

Modern gas turbines use directionally solidified (DX) and single crystal (SX) nickel-based alloy turbine blades. In actual operation, these blades have to meet extremely high working requirements. After a certain period of operation, obvious damage mechanisms will be displayed on the blade.
Since the production cost of components is too high, it is necessary to study suitable repair methods for more economical use of gas turbines.
Vacuum brazing is a viable repair method. It has been put into practical use, but currently only in parts with low mechanical strain.
1 The principle of vacuum brazing repair
Vacuum brazing of nickel-based alloys usually uses a brazing material that is close to the composition of the base metal. In order to reduce the melting point of vacuum brazing (compared with the base metal), boron and silicon are added to form eutectic and liquid phase at vacuum brazing temperature. Taking boron as an example here, during isothermal vacuum brazing (see Figure 1), boron, which lowers the melting point, diffuses from the solder to the base metal. In this way, the concentration of boron in the solder decreases from C0 to C1, and the concentration of boron at the base metal/solder interface causes local melting of the base metal. With the consumption of boron, the temperature of the liquid rises, and the vacuum brazing material solidifies at the equilibrium concentration Cs.

Since boron forms brittle intermetallic phases with nickel-based alloying elements such as tungsten and button, the metallographic structure of the weld zone is particularly noteworthy. By choosing the appropriate vacuum brazing parameters and subsequent heat treatment, we strive to achieve the best possible uniform distribution of boron in order to improve the toughness response of vacuum brazing.
In order to obtain high strength and high repeatability, the surface to be vacuum brazed must be fully cleaned. Oxidation cracks can be cleaned chemically or mechanically. Oxides, such as titanium and aluminum oxides must be removed; the base metal becomes locally depleted of the corresponding elements. In addition to their high affinity for oxygen, these elements stabilize the γ phase and to a considerable extent determine the strength of the base metal.
Subsequent work depends on the chosen cleaning method, if chemical methods are used to clean the crack, the surface can be repaired by vacuum brazing with wide gaps. Use box or paste solder. A typical brazing material is a mixture including base metal mixture powder, brazing powder and organic binder.
2 experiments
Vacuum brazing experiments were carried out on workpieces made of MAR M247 CC (polycrystalline), CM DS 247LC (directional solidification) and CMSX-4 (single crystal). Various solder powders mixed with powder and base metal powder. The chemical composition of different base metals and solders is shown in the table below. In addition, boron, an element that lowers the melting point, is added to the base metal powder for alloying to produce a melt-spun foil. In order to achieve the corresponding mechanical properties, tensile tests were carried out on the selected butt vacuum brazed samples at room temperature and 850°C, respectively. The properties of the vacuum brazed parts are shown in the metallography. Some samples were analyzed by means of scanning electron microscopy and electron probe microanalyzer (EPMA). The development of the gamma phase and the local distribution of elements that contribute to the stabilization of this phase are of particular value.

3 Conclusion
After experiments, it is concluded that vacuum brazing of nickel-based alloys (MAR M 247 CC, CMDS 247 LC and CMSX-4) can be successfully performed. The tensile strength was measured on the MAR M 247 CC vacuum brazed sample, and the value close to the strength of the base metal can be measured. Some specimens vacuum brazed with MAR M 247 CC foil failed on the base metal.
Tests using a commercial vacuum brazing powder mixture together with the base metal CMSX-4 partially showed very good wetting characteristics and a defect-free metallographic structure similar to the base metal. The tensile strength values obtained at room temperature and 850°C showed obvious dispersion.
For example, using melt-spun foil (CMSX-4 containing boron), vacuum brazing at 1250 °C for 20 min, and additional diffusion process (1200 °C, 24 h) will achieve better results.
Vacuum furnace equipment selection: A process technology must be matched with excellent equipment to play a role, and the vacuum brazing furnace selected in vacuum brazing is very important. The RHVB vacuum high-temperature brazing furnace produced by SIMUWU is an excellent product for handling such processes. With its excellent temperature control accuracy and temperature uniformity, it can realize the brazing of the workpiece evenly and evenly, so as to achieve a high level of brazing level, reduce the scrap rate, increase the efficiency of production and reduce the cost.