Zhu Zhishou, Shang Guoqiang, Wang Xinnan, Zhu Liwei, Li Jing, Li Mingbing, Xin Yunpeng, Liu Gechen
(China Aviation Engines Beijing Aeronautical Materials Research Institute Advanced Titanium Aviation Science and Technology Key Laboratory, Beijing 100095)
Abstract: Because of its characteristics of solid-state phase change with diversity and complexity, the organizational performance relationship of titanium alloy has always been one of the hot topics of materials scientists. By adjusting the distribution ratio, processing process and heat treatment process parameters of titanium alloy, the tissue type and organization parameters of titanium alloy components can be adjusted within a certain range to achieve the best match of comprehensive performance such as strength, plasticity, toughness, fatigue and fatigue crack expansion rate. Based on comparing the four typical microstructure characteristics and control techniques of iso-axis, bi-state, net-basket and chip-layer tissue of titanium alloy materials, this paper takes aviation TC21 titanium alloy, TC4-DT titanium alloy and TC32 Titanium alloy and TB17 titanium alloy are reviewed as examples of the influence relationship between different microstructure characteristics and tensile performance, fracture toughness and fatigue crack expansion rate, which provides reference for titanium alloy to select suitable tissue parameters, achieve optimal comprehensive performance matching and stable mass production.
Titanium alloy materials have obtained a large number of applications in aerospace, petrochemical, automotive industry and sports and leisure products because of their high ratio strength, high ratio modulus, high toughness, good corrosion resistance and weldable comprehensive performance, but titanium alloys have increased the stability control difficulty of engineering applications to a certain extent because of the diversity and complexity of solid phase change. According to the relationship between the composition, process, organization and performance of titanium alloy material, the composition determines the alloy type, the process determines the microstructure structure, and the microstructure determines the comprehensive properties of the alloy. Therefore, by adjusting the distribution ratio, processing process and heat treatment system of titanium alloy, the organization type and tissue parameters of titanium alloy components can be adjusted within a certain range, so as to achieve the best match of the hydraulic properties of strength, plasticity, toughness and fatigue.
This paper analyzes the four typical microstructure characteristics and control techniques of titanium alloy materials, and discusses the correspondence between different microscopic tissue characteristics and tensile properties, fracture toughness and fatigue crack expansion rate, respectively, with TC21 titanium alloy, TC4-DT titanium alloy, TC32 titanium alloy and TB17 titanium alloy as examples, in order to better grasp the relationship between titanium alloy microstructure characteristics and hydraulic properties, and provide a reference for the actual production stability control.
1 Classification and main characteristics of titanium alloy microstructures
Titanium alloy according to the stable phase composition can generally be divided into α titanium alloy, α and β titanium alloy and β titanium alloy, of which β titanium alloy can also be further subdivided into near substabilisation according to Mo equivalent β titanium alloy, sub-stable β titanium alloy, stable β titanium alloy, the typical microstructure of commonly used titanium alloy can be divided into four types: isometric tissue, two-state tissue, net basket organization and chip layer organization, as shown in Figure 1.
The main characteristics of iso-axis organization are the uniform distribution of the iso-axis primary α phase with content of more than 40% on the transformation β substrate, and the isometric α phase is mainly spherical, oval, olive-shaped, rod-shaped, short-rod-shaped and other forms (Figure 1 (a)). The content and distribution of the α phase of the primary stage in the annealing state are also different due to different mo equivalents of different types of titanium alloy, for example, the content and distribution of the near-α titanium alloy are lower than the substation β titanium alloy, and the α phase size and quantity of the iso-axis primary phase are lower.
The main characteristics of the two-state tissue are the flaky β the transformation base tissue is distributed no more than 50% of the iso-axis primary α phase, β the transformation of the tissue’s α phase or secondary α phase form varies with the alloy type (Figure 1 (b)). The content and distribution of the two-state tissue α phase of different titanium alloy types are also different, compared to the near-α type and the α and titanium alloy β, the primary α of the substational β titanium alloy is relatively small.
The main features of the net basket organization are the original β grain boundary is broken to varying degrees, the crystal boundary α phase is discontinuous, the crystalline α phase becomes short and thick, the staggered distribution in the original β grain outline is woven into a net, belongs to the deformed β transformation organization (Figure 1 (c)). Different types of titanium alloy or different β processing process formed by the network tissue shape characteristics are quite different, generally there are broken crystal boundary α phase, intermittent crystal boundary α phase and crystal with large α-phase net tissue and other form characteristics.
The main feature of the chip layer organization is that the transformation α phase is arranged in a flaky pattern within the original β grain of the coarse isometric axis, and the α of the original β crystal boundary is generally formed into a clear and complete continuous mesh (Figure 1 (d)). Different titanium alloy types or different β heat treatment process parameters, will form a different thickness of the plate layer, especially, when the cooling rate of β heat treatment increased to a certain extent, the transformation of the plate layer α phase into a small needle, this layer organization is also known as “Wei’s tissue”, the general titanium alloy does not want this type of layer tissue.
2 Titanium alloy microstructure control technology and applications
2.1 Isometric tissue
Figure 2 shows a technical diagram of the control of shaft tissue such as titanium alloy. When the main deformation processing of titanium alloy and the subsequent heat treatment are carried out in the α and β phase area, and the heating temperature is lower than the phase change point more, the isometric tissue is generally available, the commonly used titanium alloy ingots open blank to the bar or forged blank semi-finished forging route mainly has the conventional process and high and low high process. By forging the three fires above the phase change point, the large grain of the ingot is fully broken, and then repeatedly pulls out the deformation in a wide temperature range below the phase change point (50 ±20) degrees C, in order to obtain fine crystallization and uniformization of the isometric tissue. For large forging blanks with large cross-sections, high and low deformation control process can be used, but the heart overheating problem caused by large thickness blanks due to large deformation should be avoided.
Iso-axis organization is suitable for bars, plates, wire and pipe and other semi-finished products, need to control the uniformization of ingot melting ingredients, open blank forging homogenization, nascent α phase balling, metallurgy and deformation defects and other technical key, to avoid β spots (Cr, Fe, Mo and other partial analysis) and low-fold uneven (fine crystal bright belt) tissue defects. As shown in Figure 2, when β spots appear in titanium alloy, its plasticity and fatigue performance will be greatly reduced, and when there is a low-fold uneven organization, its strength, plasticity and fatigue performance will also be reduced to a certain extent.
2.2 Two-state organization
The control technology of titanium alloy bimorphic tissue is shown in Figure 3. When the main forging deformation of titanium alloy is completed at the upper temperature of the two-phase zone or in the two-phase area, the heat treatment after forging heating to the upper temperature of the two-phase zone can generally get two-state tissue. Titanium alloy ingot blanks and semi-finished products commonly used forging process also have conventional processes and high and low high processes. Through the phase change point above the three fire open blank forging, the ingot thick grain is fully broken, in order to improve toughness and high temperature performance, in the subsequent α and β two-phase area repeatedly pulling deformation, the heating temperature as far as possible to control the upper temperature range below the phase change point forging (phase change point below 15 to 25 degrees C), in order to obtain fine crystallization and uniformity of the two-state organization. For large forging blanks with large cross-sections, high and low deformation control process can be used, and it is also necessary to avoid the overheating of the heart caused by large thickness blanks due to large deformations.
The two-state tissue is more difficult than the isometric tissue process control, so it is suitable for the organization of titanium alloy parts of the final finished blank, for example, most of the forgings, directly usable rods / thick plates, etc. The control temperature of two-state organization is in a narrow temperature range, therefore, the final performance is more sensitive to the initial α phase content, morphology and distribution of organizational parameters, in the actual production control, in addition to paying attention to the control points of the two-state organization, but also need to pay attention to the α phase sphericalization/refinement, crystal boundary α fragmentation, transformation β and other organizational parameters detail control.
As shown in Figure 3, in the typical application of two-state organization, there are mainly near-β forged “three-state” tissue control of TC11 titanium alloy and ta15 titanium alloy thick plate, forging two-state tissue control. Among them, “three-state organization” can also be understood as a two-state organization considering the transformation of β phase organization parameters.
2.3 Netball Organization
The control technology of the titanium alloy basket tissue is shown in Figure 4, when the titanium alloy is deformed near the phase change point (α and β) /β, or heated and started to deform in the β phase area, the deformation is completed at the temperature of the α-β phase area, and is controlled in the α-β phase zone When the total amount of deformation, can form a transformation of the α-phase basket weaving structure, compared to iso-axis organization and dual-state tissue forging technology, net basket tissue parameter control is more difficult, for this reason, titanium alloy forging technology has developed similar β forging process, cross-β forging and quasi-β forging process.
Due to the difficulty of process control for obtaining the best toughness match, the net tissue is suitable for the organizational status of the final product of the higher strength alloy, such as high strength or ultra-high strength finished forging blanks. The ideal netball tissue is made up of tiny β grains, broken crystal boundaries α phases, and woven β transformational tissues. The main control points of the net basket organization are the original β grain refinement, crystal boundary α fragmentation, and the optimization of the net tissue parameters.
2.4 Slice organization
The control technology of titanium alloy sheet tissue is shown in Figure 5. In general, when the titanium alloy parts complete the two-phase area forging blank, reheat to the β phase area cooling, can get the layer tissue, the use of post-forged heat treatment method is mainly available β heat treatment and ordinary β heat treatment. Because the plastic margin of medium-strength titanium alloy is relatively large, it is allowed to use the layer tissue, at the expense of a certain plasticity, to obtain the highest fracture toughness and the lowest fatigue crack expansion rate, suitable for medium- and high-strength titanium alloy finished forgings or direct use of thick plate and other semi-finished products.
The key points of the control point of the chip layer organization are to refine the original β grain through two-phase area forging, optimize the organization parameters of the chip layer and the control of tissue uniformity. By controlling the cooling rate, adjust the thickness of the layer of the transformed α phase, the size of the original β grain, and the size of the transformation α the phase cluster, so as to obtain the best toughness matching relationship. As shown in Figure 5, TC4-DT titanium alloy through quasi-β heat treatment process, successfully solved the titanium alloy frame beam forging grain coarse grain, low plasticity of the sheet tissue, complex cross-sectional very large forging tissue uniformity and ordinary β heat treatment in the actual production control difficulties and other technical problems.
3 Relationship between the microstructure of titanium alloy and the performance of mechanics in aviation
Figure 6 to Figure 8 is the relationship between the different microstructure types and room temperature mechanics of several commonly used aircraft structures, such as medium-strength and high-tough TC4-DT titanium alloy, high-strength and high-tough TC21 titanium alloy, medium-high-strength and high-tough TB17 titanium alloy, and so on.
It can be seen that the two-state tissue of titanium alloy has the highest tensile strength and the best stretch plasticity, for example, the cross-sectional shrinkage rate of TC32 and TC4-DT titanium alloy is as high as nearly 50%, which fully shows the comprehensive advantages of the two-state tissue in static stretching performance (Figure 6 (d)). The quasi-β heat-treated plate tissue, which greatly improves the KIC value of fracture toughness (Figure 7 (a)) and reduces the fatigue crack expansion da/dN value (Figure 8) under the premise that the tensile performance meets the requirements for use. Although the TC21 titanium alloy bimorphic tissue is better in tensile strength and plasticity, but after the quasi-β forging to obtain the net basket tissue, it can also achieve the highest KIC value and the lowest da/dN value, to ensure that the damage tolerance design of the flying mechanism parts is required. In order to further improve the fatigue resistance of damage tolerance titanium alloy, TC32 titanium alloy obtained the best comprehensive match of two-state tissue, net basket organization and chip tissue state strength, plastic toughness, fatigue, etc., using TC32 titanium alloy trial large beam forging The gap fatigue limit of the piece at Kt s 3 can even be matched with the ultra-high strength TB17 titanium alloy medium-sized forging with a strength of 1350 MPa, showing the comprehensive toughness matching ability of this type of alloy (Figure 7 (b)). It can be further seen from the different tissue types shown in Figure 8 that the da/dN value of medium and high strength TC4-DT or TC32 titanium alloy can be the lowest when using layer tissue, and the da/dN value of high-strength TC21 or TB17 titanium alloy can be the lowest when net tissue is used. This also shows that, with the right tissue parameter control technology, different types of titanium alloys can obtain the right damage tolerance, fatigue resistance and other comprehensive performance, and better meet the aircraft structure for long life, high weight loss and low cost design requirements.
4 Conclusion
(1) In order to achieve the batch stability of titanium alloy components for aviation, organizational stability is the core of determining the stability of batch production performance.
(2) Titanium alloy microstructure type can be divided into four types of organization type, isometric tissue, two-state organization, net basket organization and chip layer organization, through the establishment of organization type rating standard, and in production using suitable curable thermal process control technology in order to achieve the stability of various micro-organizations precise control, is the key technology to achieve batch stability.
(3) It is an important technical basis to achieve stable mass production to establish the best matching relationship between titanium alloy tissue type and tissue parameters and comprehensive mechanics such as strength, plasticity, toughness, fatigue and fatigue crack expansion rate through experimental research.
(4) For similar TC4-DT, TC21, TC32 and TB17 and other commonly used aviation high-performance new titanium alloy semi-finished products and components, the use of appropriate tissue parameters control technology, different types of titanium alloy can obtain the appropriate damage tolerance, fatigue and other use of performance of the comprehensive matching, in order to better meet the aircraft structure for long life, high weight loss and low-cost design and use requirements.
