Ultrasonic Testing of Aeroengine Turbine Blades

Foreword: The Company (SIUI) and the Beijing Institute of Technology (BIT) have jointly carried out the scientific research project of "Automatic Non-destructive Testing System for Flaws and Thickness of Aero-engine Blades". In this project, SIUI undertook the research and development of ultrasonic testing instruments and probes for flaws of aero-engine blades, and BIT undertakes the research and development of ultrasonic testing and thickness measurement methods and processes for aero-engine blades. The complete set of automatic testing equipment for aero-engine blades including probes and other devices realizes the full-automatic ultrasonic testing and thickness measurement of engine blades. This study introduces the ultrasonic testing method of blade flaws, the application of SIUI instrument and probe in blade inspection, the automatic inspection scheme and complete set of equipment of engine blades. By detecting the simulated flaws of blade, the feasibility and effectiveness of ultrasonic longitudinal wave reflection method for detecting internal flaws of blade and ultrasonic surface wave method for detecting surface flaws of blade are demonstrated.

1. Basic Structure and Common Flaws of Aeroengine Turbine Blade

Aero-engine turbine blades are generally made of nickel-based super alloy, heat-resistant stainless steel, cobalt-based alloy steel or iron-based 169 alloy steel. In order to meet different types of air cooling and air film cooling technology requirements, the leaf body cavity channel is very complex. There are different shapes of air-cooled holes. In addition, in order to overcome the impact of thermal corrosion, high temperature oxidation and mechanical wear on the blade surface, the blade surface is attached with a special chromium-based or aluminum-based protective layer and ceramic thermal barrier coating that resists high temperature oxidation and ablation. Its basic structure is shown in Fig. 1.

Due to the complex internal structure of aero-engine turbine blades, generally precision casting molding without margin is adopted. In the process of molding, the blades have surface defects such as slag inclusions, holes or cracks, and the blade body shape is complex, the cross-section is different along the length of the blade, and the molding process is complex, resulting in defects such as porosity, slag inclusions, cracks and looseness inside the molded blades[1]. Due to the particularity of the blade structure, these defects are difficult to find through conventional non-destructive testing methods, and the potential harm is large, so it is of great significance to find effective testing methods to find defects of turbine blades and eliminate hidden dangers in time.

2. Ultrasonic testing method of blades

2.1 Ultrasonic longitudinal wave pulse reflection method

When ultrasonic waves reach the interface of different acoustic impedance in the medium, they will be reflected. The pulse reflection method is to use this principle to detect flaws. In that testing process, a probe is usually used as a transmitter and a receiver. Firstly, an ultrasonic instrument transmits pulse to excite the crystal of the probe to generate vibration and enter a tested workpiece through a coupling agent. According to the amplitude of the reflected wave and its position on the time-based axis, the size and position of the defect equivalent are judged.

When the longitudinal wave pulse reflection method is used to detect the workpiece, if the workpiece is intact, and the echo signal received by the ultrasonic probe only has the initial wave and the bottom wave; if there is a defect in the workpiece with an area smaller than the cross-section of the beams, a defect wave will appear between the initial wave and the bottom wave; If there is a defect in the workpiece with an area larger than the cross-section of the beams, the ultrasonic beam will be completely reflected by the defect, so there is only the initial wave and the defect wave, and the bottom wave disappears. The principle is shown in Fig. 2.

According to the principle above, the longitudinal wave probe is vertically placed above the blade through the coupling agent, and the ultrasonic longitudinal wave reflection method is used to detect the blade. If there are area flaws such as porosity, slag inclusion, cracks and shrinkage porosity in the blade, the reflection and transmission characteristics of ultrasonic wave will be changed. For example, If an area type defect is encountered, the reflected energy of the ultrasonic wave is larger and the transmitted energy is lower, and information such as the type, size, position and the like of the defect can be given by collecting and processing the reflected and transmitted ultrasonic wave signals and combining the position information between the probe and the blade and the position of the transmitted wave on the time base axis. If there is no defect inside the blade, there are only initial wave and bottom wave.

The advantage of longitudinal wave pulse reflection method is high sensitivity, which can find small flaws and determine the equivalent size of flaws; in addition, because the sound path from the testing surface to the defect can be obtained on the time base axis of the display screen, the defect can be located accordingly. Generally, the horizontal linearity error of the instrument is very small, so the positioning accuracy is high. However, the P-wave pulse emission method also has some disadvantages. Because of the existence of the initial wave, the defect signal on the surface and near surface is covered, and there is a blind area on the surface and near surface testing.

2.2 Ultrasonic surface wave testing

The ultrasonic longitudinal wave reflection method can reduce the sensitivity and detection rate of defect testing because of the existence of initial wave, which makes the echo of surface and near-surface flaws partially or completely submerged. In order to compensate such issue, the ultrasonic surface wave testing method is adopted, and the surface wave length is about one half of the longitudinal wave, which has higher testing sensitivity for surface and near-surface flaws at the same frequency[2].

In order to obtain the surface wave, the waveform conversion method is adopted. Fig. 3 shows the schematic diagram of waveform conversion at liquid-solid interface. αL and αLL are the incident angle and reflection angle of longitudinal wave respectively, βLT and βLL are the refraction angles of refracted transverse wave and refracted longitudinal wave respectively, CLI is the longitudinal wave sound velocity in liquid (water), and CLT and CLL are the refracted transverse wave sound velocity and refracted longitudinal wave sound velocity in solid (blade) respectively.

According to Snell's law:

When the sound wave is incident on the interface of liquid-solid medium, the sound velocity usually satisfies CLL>CLT>CLI. There are two critical angles of total internal reflection in the incident angle of liquid, namely

According to the wave pattern conversion principle, when the incident angle αL=0, there is only one type of refracted longitudinal wave in the solid. If 0<αL<αⅠ, both refracted longitudinal wave and refracted shear wave exist in solid, and the refracted longitudinal wave becomes weaker and the refracted shear wave becomes stronger with the increase of angle. If αⅠ<αL<αⅡ, the longitudinal wave is totally reflected and only the refracted shear wave exists in the solid. If αL≥αⅡ, there is no refracted wave in the solid, the total reflection is achieved, and only the surface wave is produced on the surface.

Other methods of generating surface waves include cross-finger probes, comb probes and wedge probes, among which a wedge probe is simple to make, with the highest efficiency and the most widely application. The longitudinal wave generated by the crystal is incident on the surface of the workpiece (the second medium) at a certain angle through the solid wedge (the first medium), the acoustic wave will be refracted at the interface of the two media, and the surface wave will be generated through waveform conversion, as shown in Fig. 4. When the inclination angle of the wedge is greater than or equal to the second critical angle, the body wave excited by the probe will be completely converted into the surface wave propagating along the surface of the tested workpiece.

The surface wave is a non-dispersive wave, and its sound velocity CR is independent of frequency, mainly related to the transverse wave velocity CS in the material, generally about 87~95% of CS, and the approximate calculation formula is:

Where σ is the Poisson's ratio of the solid, calculated as the ratio of shear to longitudinal velocities, i.e.

where CS is the shear wave velocity and CL is the longitudinal wave velocity.

The amplitude of surface acoustic wave (SAW) attenuates rapidly with the increase of depth, and the energy is concentrated in the surface layer of about one wavelength depth, so it is easier to obtain high intensity than body wave. Since the propagation velocity CR of the surface acoustic wave in titanium alloy (such as Ti6A14V) is about 2.96×106 mm/s, if the working frequency fof the probe used is 5MHz, the wavelength is

It can be seen that the effective testing depth of SAW on titanium alloy (Ti6A14V) blade is about 0.592 mm for the probe with 5MHz working frequency.

According to the characteristics of surface wave, surface wave is used to test the surface and near-surface flaws of blade.

3. Test Protocol

Take a titanium alloy blade as an example, its structure is shown in Fig. 5. The blade is composed of blade body and mortise. The blade body includes blade basin, blade back, inlet and exhaust edge. The blade has complex space curved surface, because the blade wall thickness is not uniform, the conventional nondestructive testing method is difficult to realize the aviation blade curved surface tracking testing, which may easily lead to missed testing or the like. Therefore, the manipulator ultrasonic nondestructive testing system is adopted. The CAD data of the blade is input into the system, the manipulator clamps the blade to move along the set track. When the blade passes through the sound field generated by the probe from the testing starting point in turn, the ultrasonic echo signal of the current testing point is obtained. Use the acoustic time of this echo and the acoustic time of the bottom wave of the current detection point to identify whether there is a defect. That is, the bottom wave acoustic time t1 of the tested point is calculated through the wall thickness of the blade, and the identification range is set between the initial wave (t0) on the time base axis and t1, that is, between the initial wave and the bottom wave. If the echo signal after the initial wave (t0) is less than t1, it can be judged as a defect, and if it is equal to t1, it is a bottom wave signal.

In order to realize the testing covering the whole area of the blade, before scanning the blade, the blade is divided into four testing areas a, b, c and d, as shown in Fig. 6. Because the blade root area a is shielded by the blade clamp, the thickness of the blade at the inlet edge b and the exhaust edge d is very thin, and the three areas only need to use surface waves to detect near-surface flaws; In the middle area c, besides the surface acoustic wave testing, the ultrasonic longitudinal wave reflection method should be used to detect the internal flaws.

4.?Experimental verification

4.1 Ultrasonic longitudinal wave reflection method

See Fig. 7 for the schematic diagram of the system for testing the aeroengine blade by using the liquid immersion ultrasonic longitudinal wave reflection method. The inspection task is completed by using the scanning mode of clamping the blade by the manipulator. The following are the experimental results of detecting the blade crack and the flat bottom hole defects.

4.1.1 Crack defect testing experiment

Use the ultrasonic immersion point-focusing probe with center frequency of 20MHz and an SIUI 20M6SJ40Z probe for vertical incidence testing, and set the water distance as 18 mm. The flaw detector SIUI SyncScan is employed, with the excitation voltage set to 200V, the pulse width set to 30ns, the damping set to 2, the receiving bandwidth set to 2-20MHz, and the receiving gain set to 68dB. The crack defect area with width 0.15 mm and length 6mm is made on the blade test piece for scanning. The planned track point spacing is 1.25 mm, the row spacing is 0.10 mm, the distance from the gate to the bottom surface is 0.516 mm, and the gate width is 2.579 mm. The physical picture of blade crack defect (left) and the scanning result picture (right) are shown in Fig. 8, and the design size of crack defect is shown in Table 1.

From the C-scan images, it can be seen that all the designed crack flaws are detected by the ultrasonic longitudinal wave reflection method under the experimental conditions, and the minimum crack defect size in the blade specimen can be detected is 6mm (L) × 0.15mm (W)×0.2mm (D).

4.1.2 Experiment of flat-bottomed hole defect testing

Use the ultrasonic immersion point-focusing probe with center frequency of 20MHz and model of SIUI 20M6SJ40Z for vertical incidence testing, and set the water distance as 18 mm. The flaw detector SIUI SyncScan is used, with excitation voltage set to 250V, pulse width set to 30ns, damping set to 2, receiving bandwidth set to 2-20MHz, receiving gain set to 70dB, to scan the flat bottom hole defect area of Φ 0.15 mm, Φ 0.2 mm, Φ 0.3 mm and Φ 0.4 mm made on the blade test piece. The planned track point spacing is 1.25 mm, the row spacing is 0.10 mm, the distance from the gate to the bottom surface is 0.449 mm, and the gate width is 2.243 mm. The actual picture of flat bottom hole defect of blade (left) and the scanning result picture (right) are shown in Fig. 9, and the design size of defect is shown in Table 2.

From the C-scan images, it can be seen that all the flat-bottomed hole flaws can be detected by the ultrasonic longitudinal wave reflection method under the experimental conditions, and the minimum size of the flat-bottomed hole defect in the blade specimen can be detected is Φ0.15mm× 0.5mm.

4.2 Ultrasonic surface wave testing

In order to verify the surface wave testing method, the fixture and support frame for the curved blade and the probe holding device of the manipulator are designed and manufactured, so that the system can realize automatic ultrasonic surface wave testing. The instrument in the system includes an SIUI SyncScan instrument and an SIUI 5Z6*6BM surface wave probe to test a small crack defect with width of 0.1mm and depth of 0.3mm at the edge of blade. The waveform of testing result is shown in Fig. 10, and the designed defects can be detected.

In addition, For blade edge defects, liquid-immersed ultrasonic longitudinal wave focusing probes can also be used to detect surface waves at angles greater than the second critical angle to the blade. This method is used to carry out ultrasonic B-scan imaging experiments on the flat bottom hole with a diameter of 0.2mm and a crack with a width of 0.2mm on the inlet and exhaust sides, and the detection results are shown in Figure 11. It can be seen that the location and shape of the defect in the B-scan are consistent with the physical object, indicating that this method can effectively detect whether there are surface defects at the edge of the blade.

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