Investigation on grain size effect and forming mechanism of laser shock hydraulic microforming of copper foil

Laser shock hydraulic microforming (LSHM) is a new microforming that combines the advantages of laser shock microforming and impact hydroforming. LSHM uses monopulse laser to drive the rubber layer for squeezing liquid medium to form metal foils. The size effect and forming mechanism under this process will surely introduce new features, and they were explored in terms of maximum bulge height, surface topography, thickness distribution, microhardness, microstructure, and elastic recovery. When N=t/d (thickness/grain size)>1, the N value decreases, the maximum bulge height and thickness thinning increase, and the roughening of surface topography is not obvious. When N<1, the maximum bulge height decreases, the topography becomes coarser significantly, and the thinning regularity decreases. The LSHM process can greatly improve the thinning at fillet area due to the pressure equalization effect of liquid and the replenishment of material flow. The maximum thinning occurs at the bottom transition area. The area with the highest hardness is the bottom because of the coupling effect of high strain rate and plastic deformation on the increase in dislocation density. When N<4.3, this case conforms to conventional Hall-Petch tendency because dislocation movement is the dominant deformation. When N>4.3, this case accords with inverse Hall-Petch tendency because twin nucleation is dominant. When N<1, the deep elastic recovery rate is much greater than that when N>1 due to the increase in the scale of grain atoms. The increase in laser power density can appropriately reduce the elastic recovery ratio because of grain refinement.

Automated summary

LSHM technology combines the advantages of laser shock microforming and impact hydroforming to improve thinning at the fillet area and achieve maximum thinning at the bottom transition area. The area with the highest hardness is the bottom due to the coupling effect of high strain rate and plastic deformation on the increase in dislocation density. The controlling factors in the forming mechanism are primarily the ratio of material thickness to grain size N.