Effect of two-dimensional ultrasonic rolling on grain size and micro-hardness of 7075 aluminum alloy

Two-dimensional ultrasonic surface rolling process (TDUSR) is an effective method to improve fatigue, wear, and corrosion resistance of materials by producing strain hardening and changing stress state and microstructure. In this paper, the experiment and finite element simulation of two-dimensional ultrasonic surface rolling 7075 aluminum alloy were carried out to study the evolution of the microstructure and the relationship between the surface grain size and micro-hardness. Deform was redeveloped on the basis of grain refinement model to simulate the change of grain size of 7075 aluminum alloy after TDUSR treatment. The micro-hardness and grain size obtained by TDUSR were analyzed using micro-hardness tester and XRD diffractometer. The relationship between the grain size and micro-hardness was established based on the experimental results. The simulation and experimental results showed that the grain size in the rolled region decreased greatly after TDUSR treatment. And the grain size refined from micron to nanometer, forming a gradient nanocrystalline structure along the radial direction. The grain refinement might be caused by the comprehensive influence of the surface equivalent stress and the equivalent strain rate when the static load was less than 150 N. When the static load exceeded 150 N, the plastic deformation temperature also had a certain effect on the grain refinement. TDUSR treatment can promote the preferred grain orientation and change the microstructure in the rolled surface layer. The inverse Hall-Petch phenomenon existed in the nanostructured surface layer, and the surface micro-hardness increased with the increment of the grain size.