Ways to Improve the Purity of Cast Superalloy and Its Finished Castings

Casting superalloys are key materials for the manufacture of aerospace engine's key components. Its quality directly affects the life of aircraft engines and relates to the safety of aircrafts. This has become a hot issue of concern.
The material of the alloy and its parts is mainly expressed in the gas and impurity content of the alloy because it is the origin of the initiation crack. To ensure the quality of materials, many countries have established corresponding quality control standards, such as GE's P29TF-S7 "allowable limit of trace elements in high temperature alloys", technology of British RR company Spey engine related materials (including raw materials used) and castings standard. With the development of aero-engines, it is required to use high-performance materials such as directional and single-crystal alloys as the turbine components of the engine. Its quality requirements have also increased. For example, the second-generation single crystal alloy CMSX-4 produced by the Cannon-Muskegon Company in the United States has the lowest gas impurity content in the current alloy.
In China, the content of gas and impurities in existing superalloys is generally higher than international standards due to limitations in equipment and process conditions. Therefore, the performance level of the alloy is limited, and the volatility is large, and the pass rate of the manufactured parts is low. The qualified rate of turbines and guide vanes produced by famous foreign engine companies is about 90% for solid blades and 70% for hollow blades (complex cavity hollow blades), which is still not reached in China. Therefore, improving the purity of alloys and castings is an important task for China's metallurgical workers, with greater social and economic benefits.

1 The basic requirements of alloy purity

Advanced aeroengines not only require materials with higher performance levels, but also have higher requirements for the gas inclusion content of materials to ensure the reliability of the engine. As far as the engine turbine blade material is concerned, with the development of the engine, the development of a deformed superalloy to the casting superalloy; the development of an equiaxed grain casting superalloy to a single crystal superalloy; the development from the first generation single crystal alloy to the second, Three generations of single crystal alloys. For a typical cast superalloy, the gas content in the alloy is generally (5~10)*10<-6>, and the single crystal alloy such as CMSX-4 gas content (1~4)*10<-6>.
Content of impurities in the alloy: The key impurity elements Bi, Te and Tl are less than 0.5×10-6; Ag, As and Pb, Sn are less than 5×10-6; and the impurity elements are up to about 40, except for the key impurities. Apart from the elements, the total amount of other impurity elements is less than 400×10-6; the maximum content of individual impurities does not exceed 25×10-6. This is a request from GE of the United States. Of course, the requirements for different materials and different parts are also different, but the general trend is that the limit on the amount of gas impurities in alloys has become more and more stringent, and it has been developed to require the preparation of high-purity alloys. For example, high-purity In718 alloy engine key parts have been prepared.

2 Ways to Improve Purity

Only by clarifying the sources of gases and impurities in the alloy, and taking measures to effectively remove the gases and impurities in the alloy, can the purity of the alloy be fundamentally improved.

2.1 Sources of impurities Gases and impurities in alloys are generally caused by the following reasons: Raw materials used for smelting alloys include gases and impurities, such as nickel, chromium, cobalt, and other metal raw materials, and metal raw materials selected according to certain standards. Alloys enter the melt during smelting, so alloys that have been refined using raw materials of different degrees of purity have differences in performance. For example, the cerium C6K niobium alloys refined with two different types of nickel have significantly different properties (see Figure 1). Different grades of nickel, cobalt, and chromium are used to refine the C6Φ alloy and its strength is not the same. Therefore, the use of relatively pure raw materials can refine high-quality superalloys.

Fig.1 Effect of purity of nickel on the permanent strength of ЖC6KΠ(a) and ЖC6Φ(b) alloys
Fig.1 Effect of Ni purity on stress rupture of
ЖC6KΠ(a)and ЖC6Φ(b) alloy

2.2 The influence of clean processing of alloy raw materials The cleanliness of raw materials used for refining alloys is an extremely important link. Since the preparation methods of raw materials such as nickel, cobalt, and chromium are different, the foreign substances remaining on the raw materials are also different. Therefore, when the alloy is refined with metal raw materials, it must be cleaned beforehand. For example, the aluminum block is washed with alkali, and the nickel block is pickled or rolled to remove unnecessary foreign substances. Some raw materials can also be pre-treated by smelting to increase purity. For example, using some raw materials to pre-refining low-P, low-S intermediate alloy, and then used to refine the alloy.

2.3 Control of impurities during metallurgical smelting A reasonable vacuum induction melting process can further remove gases and impurities from the alloy. For example, the vacuum induction furnace melting process can reduce the total amount of Pb and Bi in the alloy by 40%. Of course, vacuum induction melting cannot completely remove impurities. Vacuum induction melting removes non-ferrous metal impurities having a higher vapor pressure in the molten alloy by evaporation. Because when the impurity in the molten alloy is reduced to a certain concentration, ie, below the critical concentration, the pressure of the impurity gas on the surface of the fused alloy will be lower than the vacuum in the furnace, and the impurities will evaporate under the thermodynamic conditions of the smelting. FIG. 2 shows the vacuum. Under the condition of the molten alloy surface impurity evaporation conditions.

Fig. 2 Evaporation conditions of impurities on the surface of molten alloy under vacuum conditions
Fig.2 Vapor condition of melt alloy surface impurity

The gases in the alloy are also brought in from the raw materials used in the smelting. Gas mass spectrometry studies and the study of gas composition and partial pressure during smelting show that the partial pressure of hydrogen and water vapor in the furnace atmosphere exceeds that of the atmosphere in the furnace. 90% of the amount. Therefore, the increase of the peak steam and hydrogen oxygen intensity is in line with the increase in the intensity of the gassing when the charge is completely melted and stirred. While adding Ti, Al these elements are particularly strong when stirring. Therefore, the addition of alloying elements for electromagnetic stirring facilitates the precipitation of gas. However, the precipitation of nitrogen is not strong. Oxygen is removed in the carbon deoxidation reaction. Vacuum induction melting is beneficial to the CO reaction, and it also accelerates deoxidation in the alloy. Russia's experiments on antimony C26-B alloy have raised the temperature of the carbon-containing fusion alloy from 1500 to 1550°C to 1600°C, which has reduced the oxygen concentration in the antimony C26-B1 alloy by 50%. Strong stirring is also conducive to gas evolution.
Nitrogen kinetics studies have shown that 50% of the nitrogen added with the charge can be excluded when melting in a vacuum induction melt, which has a greater effect on the high temperature refining of vacuum molten alloys. The carbon in the alloy under vacuum has a greater activity and has a strong effect with easily reducible ceramics such as silica, sodium oxide, and potassium oxide, resulting in the accumulation of these elements in the molten alloy. Using high-temperature isothermal insulation in vacuum can effectively eliminate Na, K in the molten alloy. During isothermal isothermal incubation, evaporation of these elements in the form of complex compounds can be enhanced, such as loss of chromium in the alloy melt, evaporation of Pb, Bi, and other elements.
The use of slag blending in vacuum induction melting is an effective method of removing sulfur. And with the increase of slag basicity, sulfur removal effect is also enhanced, in the volume of 10kg vacuum induction furnace with different alkalinity or different CaO content of slag mixture refining yttrium C26-B alloy, the alloy sulfur concentration changes.

2.4 Improving the surface quality and internal quality of mother alloy ingots The surface and internal quality control of master alloy ingots is mainly a strict inspection of the surface state of mother alloy ingots. At the same time, the material ingots are cut to perform slag testing. According to the standards of the United States and the United Kingdom's famous aero-engine company, the mother alloy ingot does not allow a shrinkage cavity and the secondary shrinkage cavity shall not exceed 9mm (some companies require the secondary shrinkage hole must also be removed). When cutting the shrinkage hole and the secondary shrinkage hole once, it is not allowed to cut off once. It should be cut along the circumference, and the center part should be punched with the impact hammer to prevent the sand grains of the grinding wheel from entering the shrinkage hole during the cutting and contaminate the alloy. The surface of the spindle is polished and black and dark spots are not allowed.
The ingot must be inspected for molten slag. There are currently two methods: First, cut the charge according to RR's method, and then remelt in the vacuum induction furnace. The amount of scum on the surface of the bath should be observed at 1480°C and must not exceed the total amount. The surface (pool surface) 1% to 2%; otherwise it must be dealt with. The other is an EB button test using an electron beam furnace to check the slag area. Therefore, in order to ensure the internal quality of the ingot, the insulation cap and slag filtration and casting are generally adopted to reduce the inclusion in the ingot. At present, China also has corresponding standard requirements in the technical conditions of mother alloy ingots, and has formulated a buoy dregs inspection standard HB5406-88 suitable for China. However, the implementation is not good, so the spindle quality requirements have been relaxed.

2.5 Reasonable Use of Returned Charges Casting high temperature alloys contain more precious metal elements, so the use of return materials is a common problem to be solved. Although there are differences between the use of different alloy return materials and the smelting process, in general, according to the test results of many foreign companies, the proportion of return materials used is in the range of 50% to 80%.
The most critical factor in the use of returned materials is the increase in nitrogen concentration. The nitrogen in the casting alloy return material is essentially brought into the casting stage and is generally concentrated on the casting surface. The study shows that the nitrogen content is greater than 0.01 at the gate surface depth of 400 μm. Therefore, in the return material smelting, we must pay attention to the charge surface cleaning process, generally roller sanding, due to the roller rotating block friction with each other up to 4.5h, so that the surface clean and then into the furnace smelting.
The process for preparing the returned materials and new materials is generally to first re-melt the returned materials into ingots in a vacuum. After the chemical components are qualified, the mother alloy ingots are re-melted and cast in proportion to the new material ingots. The same shall be used for the charge after the components are qualified, the performance is qualified, and the gas and impurities meet the requirements. It is generally required that the content of N2 and O2 in the returning ingot be (5 to 10) 10-6; H2 (1 to 2) 10-6.

2.6 Improve Casting Conditions and Reduce Impurities in the Process of Ingots Purity of Parent Alloy Materials In scouring parts, the contamination of the core material will affect the purity of the alloy due to the helium and shell materials, affecting the casting qualification rate. Directional, single crystal hollow blades, alloy liquid in a relatively long time, the higher the temperature of sequential solidification and shell type, core material and contaminate the alloy. Therefore, special attention should be paid to the quality of melting crucibles, cores, and shells when casting parts. In terms of germanium, isostatically-pressed magnesia preforms are generally used. Of course, Al2O3 and ZrO2 have been used abroad and their prices are relatively higher. Due to its abundant resources, magnesium oxide is widely used in China. When using, care should be taken to avoid mechanical bruising of alloy ingots and niobium. Before some factories in foreign countries perform secondary remelting casting parts, the material block must be chamfered 100% before it can be remelted into the furnace. In general, a prefabricated crucible is used 10 times, and the impurity content in the alloy increases after 10 furnaces. The important part stipulates that the maximum life of the control cymbals is 5 times. The shell strength, especially the quality of the inner surface and the second layer, directly affects the quality of the casting. Because the quality of the inner and outer layers of the shell is not good, the casting qualification rate is only 35%. After the process is improved, the inner surface of the shell is improved. The finish and the strength of the first and second floors increased the casting pass rate to 55%. It should be said that many factors affect the quality of precision cast parts. Due to the complexity of the unfinished casting process, especially the hollow blades, effective measures should be taken from each process to prevent possible contamination of the alloy with impurities.

3 Advanced Casting Technology

A new casting technique is currently being used. This is cold wall casting. As the conventional method of casting a superalloy will be contaminated by the smelting thorium refractories, even if the mother alloy used has a high degree of purity, it easily reacts with the ceramic and contaminates the alloy at a high temperature. This reduces the purity of the alloy casting and affects the casting yield. Therefore, in recent years, the use of cold-wall crucible casting alloys at home and abroad, such as the use of cold-wall crucible casting cast directional, single-crystal alloy parts. According to reports, a cold-wall crucible melting furnace ISP2/IICC suitable for high-temperature alloys is a relatively advanced capacity cold wall crucible furnace. Due to the high cost of cold wall crucible smelting furnaces, it is generally used for high purity single crystal oriented alloy casting parts.