Size effects on formability in microscale laser dynamic forming of copper foil

• Four different microchannels were fabricated simultaneously by one laser shock.
• Interactive effect of specimen/grain size, feature size on formability is studied.
• Roughening caused by grain size effect is improved by high speed sliding on die.
• Superplastic deformation of coarse grained specimen in μLDF is discussed.
• The formability and mechanical property of formed parts and is improved in μLDF.

Unlike quasistatic loading of traditional microforming technology, microscale laser dynamic forming (μLDF) provides an efficient approach to fabricate microparts with enhanced mechanical properties. However, the size effects phenomenon is still inevitable in high strain rate microforming. It is thus necessary to investigate the size effects and dynamic deformation behaviour in μLDF. In this work, four different microchannels using annealed copper foils with four different grain sizes are manufactured simultaneously to investigate the size effects on formability in μLDF. The ratio of sheet thickness to grain size (N = t/d) is used to represent the interactive effects of grain size and specimen size, whereas the ratio of channel width to sheet thickness (M = W/t) is employed to characterize the feature size effect. Experimental results show that the N value and M value have an interactive effect on the normalized forming depth and the surface roughness value. When the M value decreases to the critical point of 4.5, the normalized forming depth is dramatically decreased and greatly influenced by the N value. Furthermore, the formability characterized by surface quality, thickness reduction and forming accuracy declines with the decrease of N value. The irregular thickness distribution and the inhomogeneous material flow of coarse-grained microchannel are attributed to the anisotropy and uneven grain distribution of microstructure. Despite the decreased formability, the coarse-grained microchannel with high necking ratio is still fabricated without failure and experiences more enhanced hardening effect than the fine-grained specimen, presenting the superplastic deformation behaviour in μLDF. When laser energy increases, the surface quality of the formed parts is improved by the intensive high speed sliding of asperities on the micro-die. The surface roughness value of the coarse-grained specimen is significantly reduced and therefore, the localized necking can be suppressed.

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