Effect of Compound Energy-Field with Temperature and Ultrasonic Vibration on Mechanical Properties of TC2 Titanium Alloy

GAO Tiejun, WANG Kaixuan, LU Haitao, YANG Yong

(1. College of Aerospace Engineering, Shenyang Aerospace University, Shenyang 110136, China; 2. Key Laboratory of Fundamental Science for National Defense of Aeronautical Digital Manufacturing Process, Shenyang Aerospace University, Shenyang 110136, China;3. AECC Shenyang Liming Aero Engine Co., Ltd., Shenyang 110043, China)

Abstract: To solve the problem of the poor plasticity and to meet the requirements of high temperature for forming titanium alloy, mechanical properties of TC2 titanium alloy under the compound energy-field(CEF) with temperature and ultrasonic vibration were studied. The effects of CEF on tensile force, elongation,microstructure and fractography of the TC2 titanium alloy were compared and analyzed. The results show that,under the same thermal conditions, the deformation resistance of TC2 titanium alloy decreases with the increase of ultrasonic vibration energy. The formability is also improved correspondingly due to the input of ultrasonic vibration energy and its influence on the microstructure of the material. However, when the ultrasonic vibration energy is larger, the fatigue fracture will also appear, which reduces its formability.

Key words: TC2 titanium alloy; temperature and ultrasonic vibration; compound energy-field; tensile properties; microstructure

1 Introduction

With the continuous improvement of modern aircraft for over 100 years, the materials for airframe structural have also evolved from wood and cloth to aluminum alloys, titanium alloys, superalloys,composite materials,etc. Compared with the other metal materials, titanium alloys have excellent performance such as low density, high strength,high and low temperature resistance, good corrosion resistance and good compatibility with composite materials. Therefore, the proportion of titanium alloys used in aircraft and engines and the titanium alloy parts are increasing. And the titanium alloy parts structure is becoming more and more complex. At present,the proportion of titanium alloys has exceeded the aluminum alloys in the new jet fighter, for example,the American B2 is 26%, the F35 is 27%, and the F22 is 41%. It has become one of the leading indicators to evaluate the advanced nature of aircraft[1-3].

However, due to the low elongation, high forming resistance and low elastic modulus, the titanium alloy has poor formability at room temperature. It is difficult to form parts with complex shapes because of the large spring-back of the formed parts, low forming quality and accuracy. In order to improve the formability of titanium alloys and the quality of the formed parts, an auxiliary forming method that applying physical energy fields is generally used, such as thermoforming and laser forming with the temperature fields, electric pulse forming and electromagnetic forming with the electric or magnetic fields. Thermoforming with the temperature fields is the main forming method for the hard-todeform titanium alloys among the above methods. The sheet is heated to a specific temperature (generally above 500 ℃, the superplastic state should exceed 900 ℃), as a result the forming resistance reduces and the formability increases due to the softening effect induced by high temperature. Moreover, the springback can also be inhibited, thereby improving the forming accuracy of titanium alloy metal parts[4-6]. Guanet alstudied the changes in microstructure and formability of TB6 titanium alloys during hot forming. They found that when the temperature reaches theβphase transition temperature TB6 titanium alloys showed excellent forming performance. The constitutive equation of TB6 titanium alloys is obtained[7]. Chenet alinvestigated the bulging-drawing forming properties of TC4 titanium alloys at 460, 520 and 580 ℃. The results showed that the plasticity and formability of the material have been significantly improved under thermal condition and the optimum temperature is 580 ℃[8]. Wanget alperformed uniaxial tensile and compression tests on TC17 titanium alloys under different temperature conditions. They found that the flow stress increases rapidly with the increase of strain and shows strain hardening effect at 25-600 ℃[9].

Ultrasonic vibration-assisted forming is a process that under the ultrasonic vibration with a certain direction, a certain frequency and amplitude is applied to the specimen or mold. Comparing with traditional forming methods, ultrasonic vibration-assisted forming can produce two special effects in the forming process:the bulk effect of ultrasonic vibration on the internal stress of the specimen and the surface effect of ultrasonic vibration on the friction between the mold and the specimen. Based on these two special effects,ultrasonic can not only reduce the forming force of the material and the friction coefficient between the mold and the specimen, but also can improve the forming performance and forming quality of the material[10-13]. In particular, as the power capacity of ultrasonic generator improves in recent years, the ultrasonic vibration energy can be used in much wider applications. For the metals, such as steel, aluminum alloys, magnesium alloys and titanium alloys, the ultrasonic vibrationassisted forming has been applied in the processes of blanking, deep drawing, bending and bulging. Small,or micro-sized parts, ultrasonic vibration-assisted forming shows unique advantages than other processes.As early as 1955, Blahaet alcarried out ultrasonic vibration-assisted tensile experiments on single-crystal zinc. It was observed that the flow stress has been reduced with the superposition of ultrasonic vibrations.This phenomenon is called “softening effect” (Blaha effect), which is also the earliest discovery of the impact of ultrasonic vibration on the plastic forming process of metal[14]. Wenet alapplied the ultrasonic vibration to the compression tests. And the larger the vibration amplitude, the greater the load reduction[15].Zhanget alstudied the effect of ultrasonic vibration on the microstructure of AS41 magnesium alloys.The results showed that the grain of AS41 magnesium alloys changes from coarse to uniform and fine, and this phenomenon is caused by acoustic cavitation and flows[16]. Pasierb used a special radial vibration die and attached a blank holder in the deep drawing of the cylindrical part. The deep drawing experiments on aluminum, copper, and zinc sheets showed that the load decreases significantly when vibration is applied[17].Gaoet alperformed the ultrasonic vibration-assisted bending process of TC1 titanium alloy sheet, and the results showed that ultrasonic vibration can effectively reduce the bending force. The residual stress and spring-back of the sheet ensure the dimensional accuracy and quality of the parts[18]. In this paper, based on the above research results, the forming process of CEF for the hard-to-deform titanium alloy is proposed.It is expected to further enhance the high temperature formability and forming quality of titanium alloy,thereby effectively reducing the production cost of titanium alloy metal parts.

2 Experimental

2.1 Material

TC2 is a kind of near-α titanium alloy which consists of α phase andβphase, with excellent oxidation resistance and corrosion resistance for longterm work at high temperatures. It is widely used in aviation, aerospace and other fields. TC2 titanium alloy sheet with a thickness of 1.0 mm that used in this work is produced by Baoji Titanium Industry. And the chemical composition is shown in Table 1.

Table 1 Chemical composition of TC2 titanium alloy/wt%

2.2 Experimental device and scheme

Tensile testing device assisted by CEF is shown in Fig.1. The electron universal testing machine is equipped with an ultrasonic vibration-assisted device and a temperature control system. It can be used for tensile tests under different temperature fields, as well as for the CEF. The ultrasonic vibration-assisted device is composed by an ultrasonic generator, an ultrasonic transducer, an ultrasonic amplitude amplifier pole and a fixture. The ultrasonic transducer converts the electrical signal generated by the ultrasonic generator into a mechanical signal, and then the mechanical signal which is amplified by the ultrasonic amplitude amplifier pole can transmit the longitudinal vibration to the fixture, thereby driving the specimen to vibrate.The temperature control system consists of the heating furnace, the water-cooling circulation system and the temperature control interface, which can effectively provide a stable temperature condition.

Fig.1 Compound energy-field with temperature and ultrasonic vibration-assisted tensile test device

Considering the existing TC2 titanium alloy hot forming temperature, the experiment was carried out at 500, 550, 600 and 650 ℃. The tensile speed was 5 mm/min. The ultrasonic generator with frequency of 20 kHz and rated power of 2 kW is selected for assisting tensile. The ultrasonic vibration power used for tensile tests was 1.0, 1.2 and 1.4 kW, respectively.The microstructure and fracture appearance of the fracture position of tensile specimens were analyzed by optical microscopy (Olympus) and scanning electron microscopy (ZEISS) after the tensile tests.

3 Results and discussion

Fig.2 shows the tensile force and displacement curves of TC2 titanium alloy under the CEF. It can be seen from Fig.2 that in initial stage of tensile (elastic deformation stage), the tensile force increases with the increase of displacement. The force and displacement curves are less affected by the CEF. With the increase of the displacement, it changes from elastic deformation to plastic deformation and the increase rate in tensile force starts to slow down. Compared with single temperature fields (the ultrasonic vibration power is 0 kW), the tensile force and displacement curves appear an inflection point in advance and start the plastic deformation, and the greater the ultrasonic vibration power, the earlier TC2 titanium alloy enters the plastic deformation stage. For example, the maximum tensile force is 1.8 kN under the single temperature fields at 500 ℃, and when the ultrasonic vibration power is 1.0, 1.2 and 1.4 kW, the maximum tensile force is 1.5,1.4 and 1.3 kN and the reductions of the tensile force is 16.7%, 22.2% and 27.8%, respectively. Moreover,under the same thermal condition, the greater the ultrasonic vibration power, the greater the reduction in tensile force, as shown in Fig.3.

There are two main reasons for the reduction of the tensile force under the CEF. On the one hand, a part of the energy generated by ultrasonic vibration converts into deformation energy and superimposes with the tensile force under the CEF. Thus, the tensile force of the TC2 titanium alloy decreases. On the other hand, a part of the energy generated by the ultrasonic vibration converts into thermal energy, which leads to the change of microstructure because the internal temperature of the tensile specimen increase. Fig.4 shows the metallographic structure of the TC2 titanium alloy under the CEF with 500 ℃. When the magnification is low, the grain size of TC2 titanium alloy is less different. The average grain size is 15.1 μm under the CEF. When the magnification is high, there is a trend of grain refinement because the substructure is caused by the ultrasonic vibration energy fields. It can also be seen from Fig.4 that under a single 500℃ temperature field condition, theβphase are pellets and the maximum grain size is 3.2 μm. The size ofβphase increases because the internal activity of material increases under the CEF, and the maximum size ofβphase is 6.8 μm under the CEF of 500 ℃/1.4 kW.

Fig.5 shows the effect of the CEF on the elongation of TC2 titanium alloy. It can be seen that compared to the single temperature fields, the elongation of the TC2 titanium alloy increases to varying degrees. But the increment of the elongation increases first and then decreases. For example, the elongation of TC2 titanium alloy is 41.3% under the single temperature field at 650 ℃, and when the ultrasonic vibration power is 1.0, 1.2 and 1.4 kW,the elongation is 47.4%, 45.8% and 43.4%, and the increment is 14.8%, 10.1% and 5.1%, respectively. On the one hand, the formability of the material improves because the internal activity of material increases by ultrasonic vibration. On the other hand, when the ultrasonic vibration power is larger or mismatching the material performance, the material is also prone to fatigue fracture, which decreases its formability. Fig.6 shows the fractography of the TC2 titanium alloy under the CEF with 650 ℃ and ultrasonic vibration. Under a single 650 ℃ temperature field condition, the fracture surface is uneven. There are obvious tearing ridges and the dimples are small and shallow. When the ultrasonic vibration power is 1.0 kW, the fracture surface is smooth and the tearing ridge decreases obviously.The dimple size is larger and the depth is deeper. At the same time, due to the growth of theβphase, small dimples are evenly distributed in the large dimple, and this kind of dimple can better resist the propagation of cracks and improve its formability. However,when the ultrasonic vibration power is larger, steps appear in the edge area of the fractography, which is a typical cleavage pattern. And with the increase of the ultrasonic vibration power, this phenomenon becomes more and more obvious, which causes a decrease in the formability of the TC2 titanium alloy.

4 Conclusions

a) Under the CEF with the increase of ultrasonic vibration energy, the deformability of TC2 titanium alloy increases and the deformation resistance decreases. However, when the ultrasonic vibration energy is too large, fatigue fracture is observed, which indicates that the plastic formability decreases.

b) Under the CEF with the increase of ultrasonic vibration energy, the internal temperature of the TC2 titanium alloy increases leading to certain changes of the microstructure, and the tendency of grain refinement andβphase growth appears.

c) The above research results show that in a certain stage during plastic deformation, the required temperature of hot forming for the titanium alloy can be decrease by the CEF.