Influence of Ultrasonic Vibration on Longitudinal Critically Refracted (L_cr) Wave Acoustoelastic Coefficient for Evaluating Coating Stress

Stress control was very important for the application of laser cladding technology, but there was still not an effective method at present, so a stress control method, including longitudinal critically refracted (L_cr) wave method and ultrasonic vibration method, was proposed, but its correlation was still not clear. Therefore, based on the longitudinal critically refracted (L_cr) wave acoustoelastic theory, the ultrasonic vibration effect on L_cr wave acoustoelastic coefficient of laser cladding coating was discussed. In this study, laser cladding coatings applied with 0, 10, 50 and 90% of the maximum ultrasonic vibration amplitude were prepared on surface of carbon steel, and the L_cr wave system for measuring stress consisting of a ultrasonic wave generator, a digital oscilloscope, two L_cr wave transducers (including one transmitting transducer and one receiving transducer) and a self-designed transducer clamp was employed. To explain experimental results, the microstructure and fracture morphology of laser cladding coating were observed by metallurgical microscope and SEM. The results show that although the ultrasonic vibration amplitudes are different, the relationships between difference in propagation time of L_cr wave and stress are almost consistent with the L_cr wave acoustoelastic theory. As the ultrasonic vibration amplitude increasing, the L_cr wave acoustoelastic coefficient of laser cladding coating becomes higher and higher gradually. While compared with the L_cr wave acoustoelastic theory, there are some differences in experimental results including fluctuation of difference in propagation time of L_cr wave, the difference in turning point stress and yield strength of laser cladding coating. At last, the experimental results were discussed, and the changes of microstructure, including the dendrite broken and grain refinement, caused by ultrasonic vibration effect are seen as the main reasons.