| As one of the five major nondestructive testing(NDT) methods, ultrasonic testing has been applied in the industry field extensively. However, there was few relevant research in defect detection in Semi-Solid Metal Processing (SSP). According to the research status in aluminum alloy in SSP and flaws in turbo impeller such as porosity, shrinkage and cracks with SSP thixoforming, this paper utilize both ultrasonic A-Scan and C-Scan testing modes to characterize the flaws in turbo impeller. Artificial defects used to simulate the natural defects in the turbo impeller such as flat bottom hole, through hole and notch are identified using spectral analysis and image analysis. The contributions are summarized as follows:Firstly, build up ultrasonic testing A-Scan-C-Scan system which includes hardware and software. Hardware is composed with GE USIP 40 ultrasonic detector, RIGOL DS1000Z digital oscillograph, artificial specimen, detector and couplant. Software is composed of MATALB program and Analysis module in K-Scan system.Secondly, Spectrum analysis on ultrasonic signal of A-scan is performed applying the Fast Fourier Transform (FFT) to the windowed signal indicating the defects, and then the defects were categorized according to parameters such as peak value in frequency domain, intervals between resonant frequencies, peak number, skewness and kurtosis. Peak value in frequency domain of through hole is maximal, followed by the flat bottom hole and minimal for the notch; the intervals between resonant frequencies of the flat bottom hole is maximal, followed by the notch and minimal for the through hole. Peak number of through hole is maximal, followed by the notch and minimal for the flat bottom hole. Skewness and Kurtosis of (?) 1.2mm notch is maximal, followed by the through hole and flat bottom hole is minimal. The forms of defection can be identified according to the features of C-Scan image edge. Also, by analysis of decay of C-scan signal with respect to the increasing burial depth of defects, the error between the measured defect size to the actual defect size can be estimated. The results show that the bias between measured and actual value of flat bottom hole, through hole, notch is 40 μm, 30μm and 50 μm successively, which provides the basis for esimating natural defects with C-scan imagery.Then, both ultrasonic A-Scan and C-Scan testing methods are performed to detect the natural defections in the turbo impeller. Flaw type can be diagnose primarily using parameters in artificial flaws. Defections can be further identified according to the ultrasonic C-Scan images. Defection I is zonal flaws or platelike flaws of strong reflection energy with large area, which might be the concentration gap results from two or more than two thixotropic slurry imcompleted fusion. Defection Ⅱ is high-density spot defection of smaller area with feak reflection, which might be the little concentration gap or several porosities splitting from a large pore under the pressure during the casting period. Defection Ⅲ is individual defect with strong reflection echo, which might be formed of gas evolution produced in the process of casting or the micro-crack.Finally, metallography analysis is using to test the results from images. The trend of defects morphology is consistent with image and accuracy in defections size testing is fairly high. Ultrasonic A-Scan with C-Scan shows its remarkable effect in defection size, depth and type.Experiments shows that the ultrasonic nondestructive testing system and data processing method can feasibly and efficiently identified the defections in the turbo impeller produced with SSP and solve the practical problems in project. |
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