Research On Microstructure And Stress State Of Fe314 Laser Cladding Layer By Dual Mode Ultrasonic Regulation/evaluation

Laser cladding technology has been widely used in surface repair due to its high energy density.However,the deformation or cracking of the cladding layer caused by residual stress induced by local non-equilibrium rapid cooling and solidification in the laser cladding process is one of the difficulties restricting the application of laser cladding technology in this field.Therefore,effectively controlling the stress state of the laser cladding layer has become one of the fundamental core problems in this field.The ultrasonic impact is a technique of stress regulation by introducing residual compressive stress into the metal surface through high-frequency impact,which provides an effective means to solve the problem of laser cladding tensile stress.The ultrasonic method is a stress evaluation method based on the sonoelastic effect of ultrasonic waves in the stress field of solid media.It realizes the non-destructive,rapid,and quantitative evaluation of stress by measuring the propagation velocity of ultrasonic waves.This provides a reliable method to solve the problem that restricts the effective regulation of laser cladding stress.Therefore,if the closed-loop loop of stress "regulation + evaluation" is taken as the idea,the integration of ultrasonic shock(regulation stress)and ultrasonic(evaluation stress)technology can provide support for the effective control of laser cladding stress by dual-mode ultrasound.Because of this,the laser cladding layer of Fe314 alloy is taken as the research object in this study.The mechanism of dual-mode ultrasonic regulation/evaluation of laser cladding layer stress is studied based on the ideas of stress regulation and induction and stress evaluation.Based on the analysis of the microstructure gradient change of the laser cladding layer,the internal relationship between ultrasonic impact energy and stress of laser cladding layer was revealed.The microstructure,hardness,and wear properties of the laser cladding layer in the impact state were studied experimentally and theoretically using various analysis and testing methods.The results show that: For single-layer ultrasonic impact laser cladding,the higher the ultrasonic impact coverage rate,the grain size of the laser cladding layer decreases,and the cross-section hardness increases,but the influence of ultrasonic impact on the microstructure and hardness of the laser cladding layer decreases with the increase of the thickness.When the ultrasonic impact coverage rate is more significant than 300%,obvious plastic deformation occurs on the surface of the laser cladding layer.The thickness of the plastic deformation layer is positively correlated with the ultrasonic impact coverage rate.In addition,the ultrasonic impact can significantly improve the wear resistance of laser cladding.According to the EBSD results of the ultrasonic impact laser cladding layer,with the increase of ultrasonic impact coverage,the small Angle grain boundaries increase,and most of them concentrate on the surface of the cladding layer,the local orientation difference increases,and the dislocation density increases.The influence of ultrasonic impact on the microstructure of the double-layer laser cladding layer is mainly reflected in the boundary zone of the two layers.The larger the ultrasonic impact coverage rate is,the thickness of the remelting zone decreases,the boundary line of the two layers moves down,and the columnar crystals on both sides decrease.The thickness of the double-layer laser cladding layer increases first and then decreases and reaches the maximum value when 300% ultrasonic impact is applied,which is consistent with the laws of hardness and wear resistance.Based on the critical refraction longitudinal wave( wave)acoustoelasticity theory,the influencing factors of ultrasonic signal acquisition(ultrasonic stress evaluation)are studied,and the influence law of ultrasonic impact coverage on the acoustic elasticity coefficient of wave in laser cladding layer is discussed by mechanical experiments.The results show that the wave acoustic elastic coefficient of single-layer laser cladding layer firstly increases and then decreases with the increase of ultrasonic impact coverage.When the ultrasonic impact coverage rate is 400%,the wave acoustic elastic coefficient of the single-layer laser cladding layer reaches the maximum value.In addition,the time difference between ultrasonic signals and the stress turning point of the stress curve of the single-layer laser cladding layer changed from 570 MPa to 475 MPa.The analysis of the results of the double layer laser cladding shows that the wave acoustic elastic coefficient increases with the increase of the ultrasonic impact coverage rate.When the ultrasonic impact coverage rate reaches 300%,the wave acoustic elastic coefficient increases rapidly with the increase of the ultrasonic impact coverage rate.