The present study aims to clarify the scale dependency of progressive adhesion behavior of work material on micro tools under the dry friction in microforming. Scaled progressive deep drawing test up to 300 times is conducted under dry condition in micro-and milli-scale. The process dimensions of 0.97mm and 5.82mm in drawn cup diameter are produced with the stainless steel foils of 0.05mm and 0.3mm in thickness, respectively. The experimental results show that the transition of maximum punch force has different tendencies in each scale. These tendencies are well corresponded to the transition in surface state of tools and drawn cups. While, the strong adhesive wear is observed for the milli-scale, there is a slight change in the surface state of the tools for micro-scale. To investigate this difference in the adhesion behavior of the work material in each scale, a finite element analysis considering surface asperities is conducted. The distribution of the adhesion volume on the die corner radius is evaluated with a semi-empirical wear model in which is calculated with a function of normal pressure and relative velocity between blank and die. The results show the low adhesion volume in micro-scale, due to the short sliding distance during the process. The progressive adhesion behavior of work material and the advantage of tool life in micro-scale metal forming are demonstrated.

In the miniaturization of dimensions for sheet metal forming, the relative ratio of the surface asperities of tools and blanks to the outside dimensions becomes larger than that in the case of the conventional macroscale process. This means that the surface asperities may affect frictional behavior, so that it would also affect processing characteristics and accuracy of products. In this report, micro-deep drawing for producing cups of 700µm diameter and 20µm thickness is conducted using microtools and stainless steel foils with different surface conditions. To evaluate the effects of surface properties on micro-formability and micro-forming accuracy, punch force, surface accuracy, and the thickness strain distribution of microcups are experimentally investigated. Additionally, using a finite element (FE) model that considers surface roughness, the effect of surface roughness on formability is analyzed under different tool and material surface conditions. Results show that the global forming behavior in microforming is subjected much more intensely to tribological contact behavior, which is caused by the difference of surface asperities, than that in the case of the macroscale region. Moreover, it is shown that predominant factor over this local tribological behavior is the interaction of both tool/material surface asperities that depends on the normal load condition.