Effect of ultrasonic impact treatment on surface stress evaluation of laser cladding coating by using critically refracted longitudinal wave

Abstract

Based on the Lcr wave acoustoelastic effect, surface stress of laser cladding Fe314 alloy coating after ultrasonic impact treat (UIT) was measured, and relation between Lcr wave acoustoelastic coefficient and repetition rate of UIT was discussed in this paper. The laser cladding Fe314 alloy coating was prepared on surface of carbon steel and impacted with different repetition rates of UIT by using UIT-300 system. After that, the experimental system of stress measurement with Lcr wave, consisting of a ultrasonic wave generator, a digital oscilloscope and two Lcr wave transducers (including one transmitting transducer and one receiving transducer) with 2.5MHz frequency, was used to collect Lcr waves of coating with calibration test, and then the difference in time of flight between Lcr waves was calculated with the cross correlation theory. To explain experimental results, the microstructure and three dimensional surface topography of coating were observed by using SEM and LSCM. Results show that as repetition rate of UIT increases, the Lcr wave acoustoelastic coefficient increases, when it reaches 300%, the Lcr wave acoustoelastic coefficient also reaches the maximum, after that the Lcr wave acoustoelastic coefficient decreases irregularly as repetition rates of UIT increases further. Compared with the Lcr wave acoustoelastic theory, when stress is in the range of turning point stress, there is a deviation on experimental result, and that deviation becomes more obvious as stress and repetition rate of UIT increase. It is analyzed that the change of microstructure, deformation state and initial stress of coating caused by UIT and different repetition rate are main reasons.