Research on Stress Evaluation of Laser Cladding Coating after Ultrasonic Impact under Fatigue Load with Metal Magnetic Memory Method

During the laser cladding process, rapid cooling and solidification in localized uneven areas generate residual stress, which can cause deformation or fracture of the cladding layer during use, thereby limiting the development and application of laser cladding technology. This study employs ultrasonic impact and high-frequency impact techniques to introduce residual compressive stress on the metal surface for stress regulation. The metal magnetic memory technique is used to evaluate the stress state of laser-clad specimens before and after cladding, as well as under different ultrasonic impact coverage rates. Through fatigue tests with varying numbers of fatigue cycles, the normal component of the magnetic memory signals on the specimen surface, the characteristic parameter K, and the fitting slope are analyzed. The results indicate that the signals obtained from offline detection more accurately reflect the characteristics of material stress changes with the number of fatigue cycles. Under different numbers of fatigue cycles, the response process of the fitting slope can be divided into three stages: a slow growth stage, a rapid growth stage, and finally a decline stage. As the ultrasonic impact coverage rate increases, the characteristic parameter K of the laser-clad specimens gradually increases, but the growth rate slows down. A moderate level of ultrasonic impact increases dislocation density, which can enhance fatigue life, while excessive ultrasonic impact leads to work hardening, thereby reducing fatigue life. Under the experimental conditions of this study, a coverage rate of 200% results in relatively better overall fatigue fracture resistance and a higher fatigue life compared to other coverage rates.