Detection Of Near-surface Defects In Laser Additive Manufacturing Samples Based On High-frequency Ultrasonic Technology

Metal additive manufacturing technology has the characteristics of high material utilization rate and strong forming ability of special-shaped parts,which has attracted more and more attention of countries all over the world.Because the thermal process of metal additive manufacturing is very complicated,the melting and solidification of the material are completed in a very short time,resulting in defects such as pores and cracks in the parts.The existence of defects will greatly affect the performance of parts.Online testing of the additive manufacturing process can detect defects in time to take corresponding countermeasures,which has important application value.Due to non-contact,high-precision,non-destructive characteristics,laser ultrasonic testing is suitable for online testing of additive manufacturing.In this paper,the defect detection of laser additive manufacturing specimens is taken as the research object.The effects of laser parameters,detection angle,scanning step and detection distance on laser ultrasonic defect detection were systematically studied.On this basis,using optimized technology,laser ultrasonic surface wave technology was applied to achieve quantitative characterization of surface and near surface defects of additive manufacturing parts.The results show that the detection angle has no obvious effect on the detection of defects in the PBF specimen.However,due to the overlap between the inner lanes of the laser printing layer,the ultrasonic propagation will be reflected and scattered,thus the detection angle should be parallel to the printing direction when performing defect detection on DED samples.As the scanning step length decreases,the defect detection accuracy gradually improves.The 0.1mm scanning step can detect 0.2mm defects in PBF and DED samples;the 0.05 mm scanning step can detect PBF samples Defects of 0.1mm.As the detection distance increases,the detection accuracy generally shows a downward trend.With a detection distance of 1mm,defects of0.2mm and 0.3mm in PBF and DED samples can be detected respectively.Based on the systematic analysis of the impact of laser ultrasonic inspection technology on defect detection,this paper uses laser ultrasonic B-scan and C-scan diagrams to locate and quantitatively characterize different crack defect widths.The maximum positioning error is 2%,and the maximum error of width measurement is 7%.By analyzing the characteristic information of different ultrasonic signals,the quantitative characterization of surface defects with a minimum depth of 0.1mm is realized.The study shows that the interaction between surface waves and defects at different depths on the subsurface is an oscillating process that increases first and then decreases.For the PBF sample,the quantitative detection of the subsurface defect size within a depth of 0.2mm and the qualitative detection of all defects within1 mm were achieved by scanning images.For DED specimens,with the help of dynamic screenshots,defects within a depth of 0.5 mm can be identified.Combined with wavelet analysis,quantitative characterization of the depth of subsurface defects within 0.5mm depth of PBF samples.