Research On Metal Internal Defect Detection Method Based On Laser Ultrasonic Technology

Metal materials are widely used in modern industry and scientific instruments and equipment,but the metal workpiece in the casting,processing and long-term use process,the workpiece may produce large or small internal defects,these defects in a certain degree to change the mechanical properties of the workpiece,there are quality and even safety risks,the accurate detection of defects is very critical.Laser ultrasound technology has the advantages of contactless and broadband compared to traditional ultrasound inspection methods,and the laser can excite a variety of modes of ultrasound,which is more suitable for detecting internal metal defects.At present,the conventional internal defect detection methods are mainly reflection and transmission methods,the former quantitative detection of defect size generally requires analysis of the connection between a variety of conversion waves,complex calculations,and long energy propagation paths,large attenuation,the latter is inadequate in the positioning of the defect depth,therefore,laser ultrasound metal internal defect detection methods need to be further improved.To address the above problems,this thesis establishes a finite element defectfree model to solve the temperature field and displacement field distribution of laser ultrasound under the thermoelastic mechanism,and compares the temperature,ultrasonic velocity and peak size relationships of three metals,Al,Cu and Fe,under the same laser parameters.Taking Cu as an example,the influence of laser parameters on the excited ultrasonic signal was analyzed,and the results showed that the laser energy,pulse width and power density would affect the ultrasonic signal,and suitable laser parameters need to be set for subsequent inspection.According to the different depths where the internal metal defects are located,the internal defects are divided into near-surface defects and deep defects,and the finite element models of the corresponding defects are established respectively.Based on the analysis of the interaction between defects and laser ultrasound,a detection method based on the combination of reflection and transmission(reflection-transmission)is proposed for the detection of internal porous defects,so that the detected defect information is richer and thus more effective detection can be achieved.For near-surface defects,the reflection-transmission surface wave(r R-t R)method is used to locate and detect the depth and size of defects within 5%error.For deep defects,the moving scan detection method using reflectiontransmission shear wave(r S-t S)successfully detected deep hole-like defects with a diameter of 1 mm.In addition,for deep crack defects,the detection of defect depth and length was completed by using the transverse echo signals from the left and right endpoints on the upper surface of the defect,with the depth error within5% and the length error within 10%.The research of this thesis enriches the detection method of metal internal defects,and the results provide a certain theoretical basis and reference value for further engineering practice.