Investigation on initial grain size and laser power density effects in laser shock bulging of copper foil

Laser shock bulging process is a promising flexible microforming method in which the laser shock wave pressure is employed to cause plastic deformation and fabricate microfeatures on thin sheet metals. However, the influence of initial grain size and laser power density on the forming quality of bulged parts has not been well understood. In this paper, three various initial grain sizes as well as three levels of laser power densities were provided, and then, laser shock bulging experiments of T2 copper foil were conducted. The characteristics of bulging height, thickness distribution, microhardness, surface morphology, and microstructure were examined. It is revealed that the bulging height increases with the increase of grain size and the enhancement of laser power density. The exhibiting good formability of pure copper foil in laser shock bulging process is attributed to the inertial effect on necking stabilization and strain-rate effect on material constitutive behavior. The overall thickness of bulged parts decreases compared with its original thickness. Moreover, the microhardness in the laser-shocked region increases due to the strain hardening effect, but the microhardness distribution is nonuniform because of the inhomogeneous plastic deformation. The grain sizes of bulged parts are slightly refined with the increase of laser power density, especially for the coarse-grained part. The grain refinement in laser shock bulging forming process is attributed to the large plastic deformation at high strain rates under laser shock.