Research Of Nonlinear Ultrasonic Testing Method For Kissing Bond In Titanium Alloy Diffusion Bonding

As a precision welding technology, diffusion bonding has becoming more and more important in the field of aerospace, aeroplane, electronics industry and so on. But by the influence of welding process, the surface condition, the weld interface prone to micro-defects and contact type defects, which are difficult to find using conventional methods. These defects often bring security risks to the actual structure, and has restricted the further application of this technology.According to the characteristics of the diffusion bonding, titanium alloy diffusion bonding specimens with different sizes and shapes of defects were fabricated by adding the solder resist or bonding with abnormal parameters. These samples were detected by conventional ultrasonic C-scan technique to study the influence of defect size and shape, step length and ultrasonic field type of the probe to the detection ability. Finally, the failure test was used to verify the test. The experimental results show that the macroscopic defects in the diffusion bond can be detected by the conventional ultrasonic C-scan method, and the detection difficulty increases with the decrease of the defects’ size. In the same condition, the echo amplitude of the line-shape defect is bigger than that of the round-shape and the square-shape defects, so it is easier to be detected. Smaller the scanning step size is, clearer the detection image is, but lower the detection efficiency is. In the actual detection one should take both factors into account, the chosen length should be smaller than the size of defect detected. Simply improving the detection frequency does not necessarily improve the detection ability. Smaller the focal size is, the detection sensitivity is better, more advantage to the detection of small defects even the detection rate will be reduced.On the purpose of improving the detection capability of small defects in diffusion bonding. In this paper, the author designed two kinds of nonlinear ultrasonic detection scheme: double probes method and bimorph method. Double probe method using the existing probes center frequency of 10 MHz and 20 MHz, tilting the angle to the transmit-receive ultrasound signals. Also fabricated two bimorph probes by frequency 5MHz/10 MHz and 2.5MHz/5MHz. Excited by the original system with normal amplitude excitation and nonlinear ultrasound system with finite amplitude incentive. Then using MATLAB for extracting and calculating the nonlinear coefficientβ by processing the signal and finally imagingβ. The results show that the double probes method can get a clearer nonlinear imaging results. When the size of defect is large, the nonlinear coefficient in the center of the defect is relatively low, on the edge of the defect is high. As the small defect, the nonlinear coefficient of whole defect area increases. For the bimorph probes, small amplitude excitation may leads the larger blind areas on the surface, and it’s difficult to detect the defect in thin specimen. But the combination of the RITEC nonlinear ultrasonic equipment and the C-scan system makes a better detection results.