ZHENG Qiuyang, SHI Haohan, JIANG Zhiguo, LIN Xuanyi, LI Gengjun, ZHOU Zhenyu, PIAO Zhongyu
The service reliability of aluminum alloy components is strongly influenced by their near-surface microstructure; however, under high-load and long-life conditions, inadequate surface performance remains a critical limiting factor. To address this issue, an ultrasonic-assisted surface burnishing process (USBP) is introduced. USBP superimposes high-frequency, low-amplitude vibration onto conventional surface burnishing (SBP) and, under the same average load, enables stronger dislocation activation and deeper gradient refinement. This work aims to compare the strengthening responses of USBP and SBP systematically and to elucidate the intrinsic advantages of USBP in terms of energy evolution, dislocation dynamics, stress-field distribution, and crystallographic reconstruction. A hybrid methodology combining molecular dynamics (MD) simulations with comparative experiments is adopted under matched loading conditions. In the simulations, representative near-surface volumes are constructed, and USBP is modeled through periodic normal and tangential vibrations superimposed on a constant contact load. Time-resolved evolution of internal energy, microstructure, and dislocation behavior is tracked, and post-processing using the Dislocation Extraction Algorithm (DXA) is employed to quantify dislocation line length, character, and spatiotemporal evolution. Experimentally, cast aluminum alloys are processed using USBP and SBP under equivalent nominal load and feed conditions. Electron Backscatter Diffraction (EBSD) is used to characterize grain-size gradients, and surface topography measurements quantify roughness and morphological uniformity. A unified statistical protocol is applied to report refined-layer thickness and gradient descriptors, ensuring consistency between simulations and experiments. The results show that the high-frequency stress waves introduced by USBP markedly increase the internal energy in the contact region, facilitating dislocation barrier crossing and significantly promoting dislocation nucleation, glide, and multiplication. As a consequence, the accessible slip pathways across grains of different orientations are broadened, the plastic deformation zone extends deeper into the subsurface, and a finer, thicker gradient nanostructure is established. Simulations further demonstrate that, within the same observation volume, both the dislocation line length and the growth rate of dislocation content remain consistently higher under USBP than under SBP, indicating a higher turnover of dislocation activity and a wider slip influence range. Experiments corroborate these findings: under the same average load, USBP produces lower, more uniform surface roughness, a thicker, more refined layer, and a deeper region of crystallographic reconstruction. Group-wise comparisons demonstrate that merely increasing the burnishing depth in SBP intensifies local plastic deformation but has a limited effect on promoting defect evolution and expanding the plastic zone, and therefore fails to reproduce the gradient refinement achieved by USBP. This contrast highlights the decisive role of high-frequency stress modulation in governing dislocation kinetics and microstructural evolution pathways, rather than relying solely on static deformation amplitude. The advantages of USBP originate from a coupled pathway of “high-frequency stress wave, dislocation activation, defect architecture evolution.” The superposition of stress / strain and velocity pulses on a constant load intermittently elevates the instantaneous resolved shear stress above static thresholds, thereby reducing the effective activation energy and activation volume required for dislocation barrier crossing. The subsequent unloading facilitates depinning and shortens the residence time of dislocations at obstacles, driving rapid cycles of generation, annihilation, and rearrangement, while intermittently activating additional slip systems in grains of various orientations. This dynamic mechanism accelerates sub-grain-boundary formation and dynamic recovery, suppresses premature work hardening, and distributes deformation over a larger subsurface volume, ultimately yielding a thicker, more continuous gradient refined layer and improved surface integrity. The principal conclusions are as follows: (i) USBP achieves deeper gradient refinement and broader plastic-zone expansion than SBP, addressing a key bottleneck in extending the service life of aluminum alloys; (ii) a multi-dimensional, simulation-experiment framework coupling energy evolution, dislocation dynamics, and crystallographic reconstruction is established, enabling mechanism inference beyond post-mortem characterization; (iii) DXA-based metrics of dislocation line length and slip influence range substantiate a dislocation-dominated, dynamically assisted mechanism, rationalizing both the expansion and surface morphology improvement; and (iv) increasing burnishing depth in SBP cannot substitute the dynamic effects of ultrasonic cycling, underscoring the necessity of high-frequency stress modulation in steering defect evolution. In summary, USBP outperforms SBP in surface refinement and gradient strengthening by dynamically assisting dislocations, lowering effective barriers, and expanding the plastic deformation region. These findings provide mechanistic insight and a practical processing route for controllable, efficient surface modification of aluminum alloys, with direct engineering relevance for achieving robust surface integrity and deep, stable gradient structures in high-reliability, lightweight components operating under long-life service conditions.
