Onderzoek naar de methode van lasinspectie van buisfittingen van titaniumlegering

In this paper, the author combined the specific work situation using the relevant technical knowledge; first of all, the specific hazards in the weld of titanium alloy pipe fittings are elaborated in detail, and on this basis, the two commonly used detection technologies-ray flaw detection image recognition technology and nonlinear ultrasonic detection technology are discussed and analyzed in detail. This study aims to improve the weld detection of titanium alloy pipe fittings in an all-round way and provide substantive help for production work.

Titaniumlegering is a commonly used metal in industrial production. Its high strength and corrosion resistance are highly valuable in some special industries. However, titanium alloy is a metal material that is difficult to weld. In the welding process, it is easy to be affected by the environment, facilities and equipment, lasproces, and various human factors, which leads to various problems in the welding work. Therefore, after welding, the staff needs to detect the weld of the titanium alloy buisfittingen, find the problem the first time, and solve it through the corresponding technical means. At this stage, there are two main types of welding seam
detection technology for titanium alloy pipe fittings: radiographic image
recognition technology and nonlinear ultrasonic detection technology. In the
following, the author will analyze and discuss these two detection technologies
in detail. 

1. The harm of titanium alloy pipe weld

(1) Cracks in the weld zone
The content of S, P, C, and other chemical components in titanium alloy is relatively small, so the low melting point of eutectic generated in the grain boundary is also very small. In addition, the crystallization temperature range of titanium alloy is very narrow, and the shrinkage of the weld during solidification is not enough, which leads to the low sensitivity of hot cracking.
In addition to hot cracks, there are also cold and delayed cracks in the weld zone of titanium alloy. Suppose the nitrogen and oxygen content in the titanium alloy weld is relatively high. In that case, the performance of the weld will be reduced, the phenomenon of brittleness will occur, and then cracks will occur under the action of a large amount of welding stress. This kind of crack is a cold crack, which is formed under the condition of too low temperature.
In titanium alloy welding, the heat affected zone sometimes also appears delayed cracks. The main influencing factor of delayed cracks is hydrogen. Delayed crack is the most harmful weld problem in the welding process of titanium alloy pipe fittings, which can only be treated in a preventive manner. The staff needs to reduce the hydrogen source at the position of the titanium alloy welded joint. It can be treated by vacuum annealing in combination with the actual situation to resolve the harm of hydrogen elements in the welded joint.
(2) Weld porosity
Weld porosity is the most common defect in the welding process of titanium alloy pipe fittings. N₂, O₂, H₂, and H₂O in the weld will lead to the emergence of weld porosity. The weld pores of titanium alloy pipe fittings are mostly concentrated in the fusion zone, a typical feature of titanium alloy welding pores. The pores in the weld of titanium alloy pipe fittings will increase the stress and reduce the plasticity of the metal around the pores, eventually leading to the welded joint’s fracture. Therefore, the staff must strictly control the weld porosity during the welding process.

2. The application of radiographic image recognition in the weld detection of titanium alloy pipe fittings

(1) Working principle of the radiographic inspection image
The X-ray flaw detection system comprises a computer, an optical camera, an image acquisition card, an image storage device, an X camera, and an image intensifier, and the detection is completed by photoelectric conversion. From the specific working mode, the X camera embedded in the X-ray flaw detection image table emits an X-ray. The X-ray will pass through the titanium alloy pipe to be detected, and then the image receiver receives the detection ray. In this way, the invisible light can be converted into visible light, and then the camera performs signal conversion to convert the optical signal into an electrical signal. The resulting image will be displayed in a 256-color grayscale on the display.
Once there is a problem in the weld position of titanium alloy pipe fittings, technicians can see some bright spots and bright lines in the image display equipment, and through the image processing system, according to the different characteristics of the image to determine the nature of the weld problem.
(2) Processing of radiographic image
The image processing of radiographic inspection of titanium alloy pipe fittings is mainly divided into two steps. The first step is to convert the optical image through an image intensifier and a camera. The second step is to complete the A/D conversion by computer and image acquisition card.
The image processing work must first eliminate the noise in the image. In the actual operation process, the low-pass filter neighborhood averaging method can be used for filtering processing. However, the edge contour of the radiographic image often contains much information, leading to the edge needing to be clearer when using the neighborhood averaging method. Relevant researchers have considered using high-pass filters to protect image edges, but the filtering effect of high-pass filters on noise could be better. After continuous exploration and practice, technical personnel will use median filtering for noise filtering.
In median filtering, a sliding window with odd points is generally used, and then the gray value of each point in the window is replaced by the gray value of the fixed value. The median value is arranged in descending order among the odd number of elements, and the middle value is taken out. In the even number of elements, the median value refers to the average gray value of the two elements in the middle position after the arrangement from large to small. In essence, the median filtering of digital images is the median filtering of two-dimensional sequences. In practical work, the weld image is processed by median filtering → adaptive threshold processing → solitary point filtering processing → edge detection → weld extraction.
After eliminating the noise in the key weld detection image, the technicians need to binarize the image, convert the gray image into black and white, and perform edge detection and curve-fitting on the subsequent image. In processing, to better show the problem information in the image, the staff often use the maximum variance method to clarify the adaptive threshold.

3. The application of nonlinear ultrasonic testing technology in the weld detection of titanium alloy pipe fittings

(1) Technical Principles of Nonlinear Ultrasonic Testing

Due to many contact defects in titanium alloy pipe fittings, it is difficult to determine faults using linear ultrasonic methods. Therefore, during the testing process, workers will choose nonlinear ultrasonic testing technology that is very sensitive to contact defects to determine faults. There are four main types of nonlinear ultrasonic testing techniques for titanium alloy pipe fittings: high-order harmonic, subharmonic, resonance frequency shift, and mixing modulation. The commonly used detection method in practical work is the high-order harmonic method. When ultrasonic waves propagate in nonlinear elastic media, i.e., fault areas, the waveform will experience certain distortion, leading to second or third harmonics in the received signal spectrum.

(2) Imaging Methods of Nonlinear Ultrasonic Testing Technology
Similar to linear ultrasound imaging, the imaging system needs to include signal excitation and reception devices, energy conversion devices, mechanical scanning devices, signal processing modules, and imaging modules to achieve nonlinear ultrasound imaging. During the design process, staff can utilize the ultrasonic excitation components of the original detection system to fully utilize the motion scanning, signal acquisition, and storage functions, thereby saving system development time.
In addition, the design of transducers is also crucial to ensure the stability of ultrasonic signal transmission and reception. Based on the actual situation of titanium alloy pipe fittings inspection, the author mainly discusses two transducer schemes applied in practical work, namely the dual probe method and the dual wafer probe method, which are based on the nonlinear harmonic method. The signal excitation system generates a fundamental wave signal with a frequency off during operation, which is then converted into the corresponding sound wave under the action of a transducer. Under the reflection of the workpiece, the presence and amplitude of the second harmonic in the received signal are evaluated, and the center frequency of the receiving transducer is 2f.
In the dual probe method, when an incident wave with a frequency of f enters the welding site of a titanium alloy pipe fitting, the sound beam will form a focus at the weld position, and the probe will receive the reflected echo with a frequency of 2f; The bimorph method innovatively places the emitting and receiving chips in the same acoustic lens during the design process, and the receiving chip’s acoustic frequency is also twice that of the emitting chip.

4. Summary

In summary, the inherent characteristics of titanium alloy pipe fittings during welding and their specific applications in industrial production are addressed. Relevant technical personnel should start from both the overall and detailed perspectives during the testing process, and make reasonable use of radiographic image recognition technology and nonlinear ultrasonic testing technology, in order to ensure the scientific and effective testing results and effectively improve the application level of titanium alloy key in industrial production.
Author: Yu Pei