1, casting metallurgical process
At present, various advanced casting manufacturing technologies and processing equipment are constantly developing and improving, such as thermal control solidification, fine grain technology, laser forming repair technology, wear-resistant casting technology and so on. The original technical level has been continuously improved and perfected, thus improving the quality consistency and reliability of various superalloy casting products.
Superalloys containing little or no aluminum and titanium are generally melted by electric arc furnace or non-vacuum induction furnace. When the superalloy containing aluminum and titanium is melted in the atmosphere, the burning loss of elements is difficult to control, and more gases and inclusions enter, so vacuum melting should be adopted. In order to further reduce the content of inclusions, improve the distribution of inclusions and the crystal structure of ingots, a duplex process combining melting and secondary remelting can be adopted. The main means of smelting are electric arc furnace, vacuum induction furnace and non-vacuum induction furnace; The main means of remelting are vacuum consumable furnace and electroslag furnace.
Forging cogging can be used for solid solution strengthening alloys and alloy ingots with low aluminum and titanium content (the total amount of aluminum and titanium is less than 4.5%); Alloys with high aluminum and titanium content are generally extruded or rolled into blanks, and then hot rolled into products, and some products need further cold rolling or cold drawing. Alloy ingots or cakes with larger diameter need to be forged by hydraulic press or rapid forging hydraulic press.
2. Crystalline metallurgical process
In order to reduce or eliminate grain boundaries and pores perpendicular to the stress axis in cast alloys, directional crystallization technology has been developed in recent years. In this process, grains grow along the crystallization direction during the solidification of the alloy to obtain parallel columnar crystals without transverse grain boundaries. The first technological condition to realize directional crystallization is to establish and maintain a large enough axial temperature gradient and good axial heat dissipation conditions between liquidus and solidus. In addition, in order to eliminate all grain boundaries, it is necessary to study the manufacturing process of single crystal blades.
3. Powder metallurgy process
Powder metallurgy process is mainly used to produce precipitation strengthened and oxide dispersion strengthened superalloys. This process can make the cast superalloy which can not be deformed generally obtain plasticity or even superplasticity.
4, the strength improvement process
(1) solid solution strengthening
Adding elements (such as chromium, tungsten, molybdenum, etc. ) Different atomic size from that of the base metal will cause lattice distortion of the base metal. Adding elements (such as cobalt) that can reduce the stacking fault energy of the alloy matrix and adding elements (such as tungsten and molybdenum). ) can slow down the diffusion rate of matrix elements, thus strengthening the matrix.
⑵ precipitation strengthening
Through aging treatment, the second phase (γ′, γ″, carbide, etc. ) precipitates from supersaturated solid solution to strengthen the alloy. γ' phase is the same as the matrix, which is a face-centered cubic structure, and its lattice constant is close to that of the matrix, which is also a lattice with the crystal. Therefore, γ′ phase can be uniformly precipitated in the matrix in the form of fine particles, which hinders dislocation movement and has a significant strengthening effect. γ′ phase is A3B intermetallic compound, where A represents nickel and cobalt, B represents aluminum, titanium, niobium, tantalum, vanadium and tungsten, and chromium, molybdenum and iron can be both A and B. The typical γ′ phase in nickel-based alloys is Ni3(Al, Ti). The strengthening effect of γ′ phase can be enhanced by the following ways:
① increasing the number of γ′ phases;
(2) Make the γ′ phase have a proper mismatch with the matrix, so as to obtain the strengthening effect of lattice distortion;
(3) Adding niobium, tantalum and other elements to increase the antiphase domain boundary energy of γ′ phase to improve its dislocation cutting resistance;
④ Adding elements such as cobalt, tungsten and molybdenum to improve the strength of γ′ phase. γ "phase is a body-centered cubic structure, and its composition is Ni3Nb. Because of the large mismatch between γ "phase and matrix, it can cause a large degree of lattice distortion and make the alloy obtain a high yield strength. But above 700℃, the strengthening effect is obviously reduced. Cobalt-based superalloys generally do not contain γ phase, but are strengthened with carbides.