Investigation On High Frequency Ultrasonic Testing Technology Of Surface And Subsurface Defects In Metal Additive Manufactured Parts

The selective laser melting technology of metal additive manufacturing has developed rapidly in recent years.Due to its advantages such as high flexibility,rapid processing and ability to process complex shapes,it has been extensively used in the production of functional components.However,the rapid cooling during the printing process and the characteristics of low absorption rate and high reflectivity of the aluminum alloy to the laser make the printing surface roughness high.At the same time,the printing process is often accompanied by various small defects.Such defects will directly affect the performance of the material and may bring great safety hazards and cause huge losses.Therefore,detection of surface and subsurface defects during the additive manufacturing process is of great significance.Laser ultrasonic testing technology is widely used in complex industrial testing due to its non-contact,high frequency band,suitable for harsh environments,and capable of exciting various wave types.In this paper,a systematic study was carried out on the laser ultrasonic detection technology for the surface and subsurface defects of metal additive manufacturing parts.Based on laser ultrasonic technology,the influence of surface roughness on laser ultrasonic testing was firstly studied the interaction relationship between surface waves and surface and subsurface defects was analyzed and the propagation path and amplitude change of surface waves when encountering defects were discussed.According to the ultrasonic C-scan,the surface and subsurface defects were quantitatively detected.Moreover,the wavelet threshold noise reduction technology was used to reduce the noise of the ultrasonic signal to improve the accuracy of the defect detection.Because the surface roughness of additive parts will affect the accuracy of defect detection,the influence of roughness on laser ultrasonic excitation,reception and propagation was firstly studied and the results indicated that the peak value of the surface wave first increases and then decreases as the surface roughness increases.The relationship between roughness,surface wave energy attenuation and wave velocity was analyzed,and the best detection distance was determined.On this basis,the interaction relationship between surface and sub-surface defects of different widths and depths of burying and surface waves was also studied,and the quantitative detection accuracy of laser ultrasonic testing technology for surface and sub-surface defects with different surface roughness was explored.For surface defects,the width did not have much effect on the arrival time of the reflected wave,but had a greater impact on the amplitude of the transmitted wave.The larger the width,the smaller the amplitude of the transmitted wave.For sub-surface defects,the buried depth of the subsurface defect had a linear relationship with the peak-to-peak value of the reflected echo.The deeper the buried depth,the smaller the peak-to-peak value of the reflected echo.The width of the surface defect that can be detected by the ultrasonic C-scan technology increases when the surface roughness of the additive product increases.Surface defects with a minimum width of0.2mm can be detected when the roughness is less than 11.1?m and surface defects with the minimum width of 0.4mm can be detected when the roughness is 11.1?m.When the roughness is less than10.4?m,subsurface defects with a buried depth of 0.5mm can be detected.Finally,through the wavelet denoising technology with the wavelet basis function selected as "db6",the decomposition layer of 2,the threshold of "rigrsure" and soft threshold function as the final denoising parameter,The A-scan signal is denoised and reconstructed into the C-scan,which Improved the signal-to-noise ratio and detection accuracy.