Crack detection in an aerospace turbine disk is essential for aircraft- quality detection. With the unique circular stepped structure and superalloy material properties of aerospace turbine disk, it is difficult for the traditional ultrasonic testing method to perform efficient and accurate testing. In this study, ultrasound phased array detection technology was applied to the non-destructive testing of aviation turbine disks: (i) A phased array ultrasonic c-scan device for detecting aerospace turbine disk cracks (PAUDA) was developed which consists of phased array ultrasonic, transducers, a computer, a displacement encoder, and a rotating scanner; (ii) The influence of the detection parameters include frequency, wave-type, and elements number of the ultrasonic phased array probe on the detection results on the near-surface and the far surface of the aerospace turbine disk is analyzed; (iii) Specimens with flat-bottom-hole (FBH) defects were scanned by the developed PAUDA and the results were analyzed and compared with the conventional single probe ultrasonic water immersion testing. The experiment shows that by using the ultrasonic phased array c-scan to scan the turbine disk the accuracy of the detection can be significantly improved which is of greater accuracy and higher efficiency than traditional immersion testing.
Superalloy not only has excellent creep resistance, corrosion, and oxidation resistance but also have an extraordinary combination of toughness, high-temperature strength, creep resistance, excellent thermal fatigue, and resistance to degradation in an oxidizing or corrosive environment, which are widely used in the aerospace turbine disk [
Phased array ultrasonic testing (PAUT) has been used in recent years for various non-destructive (NDT) applications in areas including aerospace, railway, oil and gas, nuclear power, and other fields [
Howards et al. [
In this study, ultrasound phased array detection technology is applied to the non-destructive testing of aviation turbine disks. First of all, the PAUDA integrated with PA ultrasonic transducer, PA board, coupling, displacement, encoder rotating, scanner, and computer are designed for accurate safe, and high-efficiency aerospace turbine disk inspection. Its performance test is conducted by Non-destructive characterization and verification of ultrasonic phased array equipment. Meanwhile, the influence of the detection parameters such as the frequency, wave-type, and elements number of the ultrasonic phased array probe on the detection results on the near-surface and the far surface of the aerospace turbine disk is analyzed. The detection parameter setting rules for near-surface and far surface defects in turbine disk are given, which provides a basis for scanning configuration for actual c-scan imaging detection of aviation turbine disks. To explore the ability and the accuracy of detection when a phased array detects an aerospace turbine disk, traditional immersion testing and phased array ultrasonic c-scan testing are performed. The result proved that for the disc forging the scanning efficiency and detection ability of the device developed in this paper are much higher than that of the conventional ultrasonic water immersion c-scan testing.
Phased array ultrasonic detection technology is developed based on radar technology, and its transducer working principle is based on the Huygens-Fresnel principle. Each wafer in the ultrasonic phased array is called a unit. When each unit is excited by the pulse signal of the same frequency, the sound pressure amplitude of the ultrasonic wave emitted by each unit in the array at certain points in the space is superimposed in phase due to the sound waves. While being enhanced, the sound pressure at other points is weakened due to the anti-phase cancellation of sound waves, thus forming a stable phased array ultrasonic sound field in space. When each unit in the phased array ultrasonic transducer is controlled by a certain rule and timing electronic system, the ultrasonic waves emitted by each unit in the array will be superimposed into a new wavefront. the ultrasonic phased array transducer is receiving the pulse return when the signal is waved, the receiving unit is controlled according to certain rules and timing to synthesize the signal and obtain the display image [
The c-scan shows the acoustic image of the cross-sectional echo of the sample perpendicular to the propagation direction of the sound beam, that is, the acoustic image of the cross-section of the tested workpiece at a certain distance from the surface where the probe is located. The probe moves with the scanning mechanism along the surface of the workpiece according to certain rules, and an acoustic image with a certain depth from the surface of the workpiece can be obtained [
The experiment was based on phased array ultrasonic electronic linear scanning and imaging algorithms combined with a self-built mechanical scanning device to carry out research c-scan imaging detection on aerospace turbine disk. The motion control system is composited of the computer, phased array detection instrument, phased array probe, placement encoder, and scanner, as shown in
The tested specimen was an aerospace turbine disk. For the convenience of description, the upper and lower sides of the aerospace turbine disk were marked as the A defect side, the B testing side.
The high-temperature turbine impeller is one of the hot end parts of an aero engine. Its grains are relatively large, and the turbine disk itself is thick and complex. Therefore, choosing the best ultrasonic phased array inspection process parameters to improve defect detection sensitivity and imaging resolution is a key step. To compare the imaging quality of defects under different inspection parameters and give the optimal parameter settings for turbine disc phased array ultrasonic inspection, the probes are placed on the three defective inspection surfaces of surface B, E2, E3, and E4, respectively. Move the probe in the circumferential direction of the turbine disc to observe the scanned image of the phased array FBH defects sector. Since the number and position of the excitation array elements will affect the detection signal, to enable the main sound beam to gather at the FBH defects to obtain higher energy, when an obvious defect echo signal is found during detection, move the probe back and forth to find the highest defect echo place, save the image recording inspection data. Analyze the influence of test parameter settings on 1.2 mm diameter FBH defects detection with a depth of 50 mm on the far-surface and a depth of 2.5 mm on the near-surface. By comparing and analyzing the image quality of ultrasonic phased array detection different parameters, the optimal parameter settings for scanning detection in the near-surface area and the far surface area are determined.
The image quality evaluation standards mainly include system gain, signal-to-noise ratio, and near-surface image resolution. The gain of the system is the gain required when the reflected echo of the defect on the image reaches a certain amplitude. The image-to-noise ratio is the ratio of the highest amplitude of the defect to the highest amplitude of the noise. The image resolution is the decibel difference between the peak and valley of the characteristic signal. During the test, change a single parameter to obtain near-surface and far-surface detection images and extract A-wave signals under the same scanning line. Analyzing and comparing image gain, signal-to-noise ratio, and image resolution. Finally, give the optimal combination of near and far surfaces of the turbine disk.
Parameter optimization experiments show that frequency, wave type, and element number will affect the detection results of the near-surface and far surface area. Configuring a 7.5 MHz probe, set up 16 elements, and exciting T-waves are beneficial for near-surface area detection. Configuring a 5 MHz probe or 2.25 MHz probe, set up 32 elements, and exciting L-waves are beneficial for far-surface area detection. Detection parameters of different areas for the aerospace disc are listed in
Type | |
Wave-type | Coupling | ||
---|---|---|---|---|---|
Near | 1.20.8 | 7.5 | L | 16 | Wedge |
Far | 1.20.8 | 5 2.5 | T | 32 | Contact |
In the experiment encoder fixing device was placed in the hollow disk hub of the disc forging and the probe was fixed on the probe holder mounted. Because the c-scan shows the echo sound image of the cross-section of the sample perpendicular to the sound beam propagation direction the array elements should be arranged along the radial direction of the turbine disc when the probe is scanned along the circumference of the disc forging the testing. The probe was respectively pushed down on the three detecting sections of disc forging B side E2, E3 and E4 by adjusting manually telescopic and lifting rotary trusses. According to different diameters testing-side, the encoder needs to be calibrated before each testing. Slowly push the truss to drive the phased array probe to make a uniform circular motion along the surface of the disc forging, meanwhile, rotary encoder records the rotation angle to collect the position information of the detection probe, so that the ultrasonic phased array equipment can complete the real-time scanning and imaging work.
First, the disc forging specimen was tested using a conventional single probe ultrasonic water immersion C-scan testing. The testing results were shown in
To verify the performance of the fabricated scanner for aerospace turbine disk and the ability of phased array detection artificial defective specimens with FBH defects were scanned by the developed PAUDA. The results are shown in
Type | |||||
---|---|---|---|---|---|
Near | 2.5 | 0.81.2 | 41 | 2.423.20 | 15 |
Far | 34 | 0.81.0 | 21 | 3.313.58 | 34 |
50 | 0.81.2 | 31 | 3.223.45 | 17 |
(1) The phased array ultrasonic inspection capability is superior to conventional ultrasonic, and it can perform accurate and efficient non-destructive inspection of various defects in the complex structure of the superalloy aerospace turbine disk.
(2) Optimization of ultrasonic phased array detection parameters of aerospace turbine disk different depth areas can effectively improve detection accuracy and improve imaging quality. In the actual detection of the near-surface area of the aviation turbine disc, using a high-frequency probe, set a small element number, and excite the T-wave can obtain higher image resolution and high image quality, which is convenient for subsequent qualitative and quantitative research. Using a medium and low-frequency probe, set more array elements, and exciting L-waves are beneficial to the detection of the far surface area of the aviation turbine disk.
(3) The developed phased array ultrasonic C-scan device for detecting aerospace turbine disk cracks has greatly improved the detection efficiency and the detection ability compared to the conventional ultrasonic water immersion C-scan device. Moreover, it has the advantages of convenient operation, high precision, and low cost.