Behavior and mechanism of abnormal diffusion induced by micro-strain ultrasonic impacting during diffusion bonding

Strain-induced diffusion significantly enhances mass transport at low temperatures, has found valuable applications in areas such as forming joining, deformation alloying, and solid-phase additive manufacturing. Despite its practical utility, the fundamental mechanism governing this phenomenon has been a subject of academic debate for over half a century. A key challenge is to decouple the specific contribution of strain rate from the overall influence of large plastic strain. To address this, the present study introduces ultrasonic vibration with micrometer-scale amplitude into the diffusion bonding process of Zn/6063Al alloys. This approach successfully creates a high-strain-rate–low-strain condition at temperatures of 240–360 °C. The interdiffusion coefficients under these experimental conditions were determined from concentration profiles using the Boltzmann-Matano method. A quantitative model linking the strain rate to the evolution of crystal defects (dislocations and vacancies) was established. Based on a phenomenological model integrating defect density and diffusion kinetics, the incremental contribution of ultrasound to the diffusion coefficient was calculated. This analysis reveals a "acoustic diffusion" mechanism, where dislocation-mediated short-circuit diffusion predominates at lower strain rates, while the contribution of deformation-induced supersaturated vacancies becomes dominant at higher strain rates, with both mechanisms operating synergistically. This high-strain-rate and low-strain method effectively promotes atomic interdiffusion without causing macroscopic deformation of the base metal, which is an ideal strategy for practical engineering applications.