Reducing surface roughening in microscale laser dynamic flexible bulging by using laser pre-shocking

Abstract

Roughening of the free surface in micro-forming results in the local necking and subsequent fracture, which seriously affects the formability of sheet metals. In this work, laser pre-shocking (LPS) was introduced into microscale laser dynamic flexible bulging (μLDFB) to reduce surface roughening in terms of reducing the surface roughness and refining the internal grains. In LPS experiments, when the laser energy is 447 mJ, the rebound of the copper foils can be negligible. LPS can be expected to suppress the surface roughening by reducing the initial roughness R0, increasing the N value and decreasing the rate of surface roughening. The copper foils subjected by 447 mJ LPS were used for the μLDFB experiments. The 50 μm T2 copper foils with four different grain sizes were used in μLDFB under different laser pulse energies. When laser pulse energy ranges from 325 mJ to 575 mJ, compared with μLDFB without LPS, the surface roughness on the bottom surface of the bulging sample is decreased. And the nano-hardness, elastic modulus and forming uniformity are significantly improved. In μLDFB experiments without LPS, for fine-grained samples, the cracks initiate in the bulging transition zone under 725 mJ laser energy due to surface roughening. As laser energy increases to 1050 mJ, the cracks initiate in the mold entrance zone due to the strong stress concentration. For coarse-grained samples, cracks initiate under 575 mJ laser energy, and the propagations of the cracks are difficult to predict due to non-uniform distribution of grain size and the difference of grain orientation. The experimental results suggest that LPS can reduce the possibility of the premature fracture caused by the local necking, and alleviate the uncertainty in the fracture mode.