Fabrication of a Gradient Nano-/Micro-structured Surface Layer on an Al–Si Casting Alloy by Means of Ultrasonic–Electropulsing Coupling Rolling Process

By using a novel surface modification technique named ultrasonic–electropulsing coupling rolling (UECR) process on an Al–Si casting alloy rod, the surface of material was smoothened significantly. Meanwhile, a strengthened layer with a gradient change in hardness was obtained in the outer surface, corresponding to a homogeneous gradient nano-/micro-structure. The thickness of nanometer-thick laminated structures was at least 40 μm, which was much thicker than conventional ultrasonic rolling process. During UECR, the formation of the well-defined nanocrystalline structure was attributed to the high strain rate and simultaneous annealing process realized by ultrasonic impact and electropulsing treatment.