Experimental and simulation based study on micro-scaled sheet metal deformation behavior in microembossing process

In microforming, the workpiece size is in microscale and has only a few grains involved in a deformation zone, leading to the deformation behaviors different from those in macroscale. The researches on micro-scaled plastic deformation behavior and microforming process are thus needed. In this research, the tensile test and the embossing of microchannels using pure copper foils with different grain sizes are conducted to investigate the material size effect on the flow stress, surface roughening and local deformation behavior. It is revealed that the surface roughness increases with strain and its change rate increases with grain size. This phenomenon results from the deformation incompatibility among grains with different properties in material surfaces. In addition, the size effect on the measurement of material properties in tensile test is analyzed based on the Monte Carlo simulation. It is found that the longer the gage length and the lesser the number of grains in the specimen section in tensile test, the higher the probability to have a significantly large fraction of soft grains in the section of specimen. The decrease of flow stress with the increase of grain size is partly caused by the decrease of Taylor factor, which leads to the underestimation of the averaged flow stress of the grains along the gage length. By using the flow stress curves obtained via tensile test to simulate the microembossing process, the simulation result shows an underestimation of the deformation load and the deviation tends to increase with the increase of grain size. This further validates the occurrence of size effect leading to the error of the measurement of material flow stress in tensile test.