Isothermal precision forging of Ti-6.5Al-3.5Mo-1.5Zr-0.3Si alloy impeller with twisted blades
SHAN De-bin(单德彬), SHI Ke(史 科), XU Wen-chen(徐文臣), L? Yan(吕 炎)
School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China
Received 28 July 2006; accepted 15 September 2006
Abstract: Ti-6.5Al-3.5Mo-1.5Zr-0.3Si (TC11) alloy impeller is an important part in the airspace engine that serves under severe working conditions, and it requires excellent mechanical properties and high dimensional precision. However, the integral titanium alloy impeller is difficult to precisely forge because of its complex shape and poor formability. In order to develop optimum forging process of this kind of complex parts, the deformation characteristics of TC11 alloy under isothermal compressing conditions were studied. Furthermore, an alternative material, namely pure lead, was selected to model the forming process of the impeller and investigate metal flow during forging. Based on the research, local loading method was determined to forge the TC11 alloy impeller precisely under isothermal condition. The dimensional accuracy, mechanical properties and microscopic structure of the forged product satisfy operating requirements.
Key words: titanium alloy impeller; isothermal forging; deformation behavior; physical modeling
1 Introduction
Due to Ti-6.5Al-3.5Mo-1.5Zr-0.3Si (TC11) alloy has improved elevated temperature strength, high creep resistance and good corrosion resistance[1], it has been widely used for airspace components such as blades and compressor discs. However, the alloy is difficult to form into complex shape because of its narrow forging temperature ranges and high deformation resistance. Therefore, it is necessary to study the deformation behavior under high temperatures for the process optimization. Isothermal bulk forming operation is advantageous in that the temperature is uniform and deformation tends to be homogeneous, thereby providing better microstructure control and optimum property combinations[2-5]. Obviously, this technology can decrease forging load and is suitable for forging components with complex shapes like gears, blades and rotors[6-9]. Titanium alloy impeller is one of the most important parts in the airspace engine that serves in severe working conditions. Due to the complexity of impeller shape and high demands for mechanical properties, isothermal precision forging of TC11 alloy impeller requires advanced tools and close control of the process variables. Compared with a final trial, physical modeling is low in cost and has been widely applied to the field of plastic forming[10-12], and it is helpful to examine the die structure, process parameters and metal flow rules.
The objective of this research is to characterize the flow behaviors of Ti-6.5Al-3.5Mo-1.5Zr-0.3Si alloy at high temperatures and provide fundamental data for isothermal forging process. Physical modeling of the impeller forging using pure lead was employed to study the metal flow and die structure. Based on the researches, isothermal precision forging of TC11 alloy impeller with twisted blades was carried out, microstructure and mechanical properties of the product were analyzed.
2 Deformation behavior
Hot rolled commercial Ti-6.5Al-3.5Mo-1.5Zr-0.3Si alloy rods were used in this investigation. Table 1 shows the chemical composition of the alloy. The α-β transus temperature of the alloy was determined by thermography as about 980 ℃. Specimens of 12 mm in height and 8 mm in diameter were electric spark machined from the hot rolled rods. Compression tests were carried out at 800-1 000 ℃ and strain rate range of 0.005-5 s-1 on Gleeble-1 500 system. All the speci- mens were resistance heated to compression temperature with a heating rate of 5 ℃/s and homogenized for 3 min before deformation. After the test, the specimens were water quenched immediately with a delay of less than 3 s.
Table 1 Chemical composition of Ti-6.5Al-3.5Mo-1.5Zr-0.3Si billet (mass fraction, %)
Fig.1 shows the true stress—strain curves at various forging temperatures and strain rates. When the forging temperatures are low, for example 800-900 ℃, the flow stress increases rapidly with the increase of strain to a peak value and then decreases gradually with the increase of strain to the end of forging. The flow curves display the characteristic of dynamic recrystallization. The flow softening may be attributed to the increase of proportion of soft β phase and the adiabatic heating. While the curves at 950 ℃ and 1 000 ℃ indicate that the flow stresses are nearly constant after maximum values because the flow softening is balanced by work hardening, which displays the characteristic of dynamic recovery. However, it is difficult to determine the deformation mechanisms only according to the shapes of the true stress—strain curves because different mecha- nisms can result in similar flow stress behavior [13].
Fig.2 shows the dependence of flow stresses on test temperatures and strain rates at the true strains of 0.1 and 0.6. In general, the stresses decrease largely with the increase of test temperatures and the decrease of strain rates. The decreasing tendencies of stresses are similar at different strains except for two curves at 5 s-1 and 0.5 s-1, which are shown in Fig.2 (b). It may be attributed to the flow softening caused by adiabatic heating at low temperature and high strain rate. However, it is worth noting that the variation of flow stress is different at various temperature ranges and shows little change when test temperatures are near the β transus temperature. It is also found that the variation of flow stress is inconspicuous when strain rates are lower. Therefore, it can be considered that the isothermal forging process of this alloy can be carried out under the conditions of higher temperature and lower strain rate, typically 950- 1 000 ℃ and 0.005 s-1, respectively.
Fig.1 Flow curves of Ti-6.5Al-3.5Mo-1.5Zr-0.3Si alloy at different strain rates and temperatures: 1—800 ℃; 2—850 ℃; 3—900 ℃; 4—950 ℃; 5—1 000 ℃
Fig.2 Variation of flow stresses with different test temperatures and strain rates: 1—5 s-1; 2—0.5 s-1; 3—0.05 s-1; 4—0.005 s-1
3 Physical modeling of impeller forging
It is a difficult job to precisely forge an integral titanium alloy impeller with many twisted blades because of its particular geometry shape and high requirements for die structure. Physical modeling is a useful method for economical process evaluation and optimization. According to similarity theory, pure lead that has excellent plasticity is selected to model the practical forging process of the titanium alloy impeller. The metal flow rules, which are similar to those of titanium alloy, are studied. The optimum billet shape, loading mode and die structure are determined.
Fig.3 shows the schematic diagram of the die device that is made of 5CrNiMo alloy. The surface of the female die is complex and irregular. Particularly, there are many cavities in the female die for the formation of twisted blades. Billet dimension used for the physical modeling is 110 mm in diameter and 230 mm in height. Forging experiment under ambient temperature was carried out on 50 MN hydraulic press with an average speed of 1 mm/s so that the strain rates imposed were of the order of 10-3 s-1. Fig. 4 shows the photographs of the deformed specimens at different height reductions. It can be seen that the main deformation is produced near the counter punch at the early stage of forging. With the increase of deformation, the deformed area becomes large. The metal flows sideward to fill the female die and the meshes deform dramatically. In general, deformation process shows the forging of an impeller is similar to that of upsetting of a cylinder and the billet is compressed in the vertical direction and radial extruded in the lateral direction. Fig.5 shows the forged impeller with a perfect shape. The forged impeller indicates the rationality of die structure and loading mode.
Fig.3 Schematic diagram of die device(unit: mm)
Fig.4 Photographs of deformed specimens at different height reductions: (a) 0 mm; (b) 32 mm; (c) 65 mm
Fig.5 Photograph of a pure lead impeller
4 Isothermal precision forging of TC11 alloy impeller
Based on compression tests and physical modeling researches, isothermal precision forging of the TC11 alloy impeller can be carried out. The billet shape and die structure are the same as that used for the physical modeling. Nickel-based superalloy K403 is employed for die material because of its high mechanical properties at elevated temperature. After resistance heating to about 200 ℃, the billet and die were coated with glass lubricant and then reheated to 950 ℃ with a holding time of 3 h. When the forging operation with the average speed of 1 mm/s was carried out, it was important to keep the isothermal condition to improve metal plasticity. Therefore, the die system was surrounded by heating system to avoid heat dissipation. The whole forging process is finished within 10 min, including a pressure holding time of 3 min to ensure that the ends of the die cavities can be filled up.
Fig.6 shows the forged TC11 impeller under the pressure of 14.5 MN. It can be seen clearly that even the ends of blades are fully filled up. According to the measurement results, the dimensional accuracy meets the requirement for subsequent process. No forging defects such as crack and folding occur on the impeller surface. The structure features and mechanical properties of forged impeller were analyzed in as-forged condition as well as heat treated condition. The optical micrograph in the middle region of forged impeller blade is shown in Fig.7. The microstructure is composed of a small quantity of β morphology and a majority of equiaxed α morphology. Fig.8 shows the macrostructure of forged blade, and the metal flow lines distribute along the geometric contour of the product, which can improve the fatigue life of the blade. After isothermal annealing, the mechanical properties including tensile strength, extensibility, contraction of area, Brinell hardness and impact ductility are tested, as shown in Table 2. The results show that all the data satisfy the operating requirements. It should be pointed out that the properties of forged impeller in both tangential and radial directions show little difference because of the microstructure uniformity, as a result of the isothermal forging with low strain rate. The acceptable results clearly demonstrate the reliability of selected process parameters and forming scheme.
Fig.6 Photograph of TC11 alloy impeller
Fig.7 Microstructure of TC11 impeller blade
Fig.8 Macrostructure of TC11 impeller blade
Table 2 Mechanical properties of forged TC11 impeller
5 Conclusions
1) TC11 alloy flow curves show that the flow stress variation is less remarkable at higher temperatures and lower strain rates. It is reliable to develop the isothermal forging operation under the conditions: 950-1 000 ℃ with the strain rate of the order of 10-3 s-1.
2) The physical modeling indicates the rationality of die structure and loading mode. The formation of the impeller with twisted blades depends mainly on the compression in the vertical direction and radial extrusion in the lateral direction of the metal.
3) The microstructure and mechanical properties of the forged TC11 alloy impeller satisfy the application requirement. The research can be applied to form other types of titanium alloy components with complex shapes.
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(Edited by CHEN Can-hua)
Corresponding author: SHAN De-bin; Tel: +86-451-86416221; E-mail: shandb@hit.edu.cn
