**4. Effect of surface topography on micro-deep drawabilty**

By using the developed micro-deep drawing experimental set-up and proposed surface roughness model, the effect of the surface topography of tools and materials are experimentally and numerically investigated. The impact and sensitivity of the difference in surface properties during micro-scaled forming is demonstrated.

Impact of Surface Topography of Tools and Materials in Micro-Sheet Metal Forming 123

**50μm 50μm 50μm**

4 4.2 4.4 4.6 4.8 5 5.2 5.4 5.6 5.8 6

7 7.2 7.4 7.6 7.8 8 8.2 8.4 8.6 8.8 9

laser scanning confocal microscope (LEXT OLS-3000, Olympus Co.).

(a) Untreated tool (b) Air-blasted tool (c) Ion-irradiated tool

**(a) (b) (c)**

8 8.2 8.4 8.6 8.8 9 9.2 9.4 9.6 9.8 10

> 30 35 40 45 50 55 6 0 65 70 Ra=0.15μm Rz=2.16μm (**Standard deviation** σ=0.30μm)

property can be obtained.

*4.1.2. Punch load-stroke curves* 

ironing force in each load-stroke curve.

30 35 40 45 50 55 60 65 70

For the untreated tools, machined traces on the circumferential direction of punch surface are observed. While, for the air-blasted tools, the maximum height of the surface roughness is remarkably rough and the dispersion of the maximum height roughness is also large. In contrast, for the ion-irradiated tools, though machining marks can slightly be observed, peak of the asperities are removed by ion sputtering and smooth surface with no directional

The tests were conducted under a nonlubricated condition. The drawing speed was 0.4mm/s and no blank holder force (BHF) was applied as mentioned. To evaluate the formability under each surface condition, punch force was measured with a micro-load cell. Additionally, to evaluate the drawn microcups, the cup surface roughness was measured by

In order to compare the punch force-stroke curve in each condition, the punch force is normalized. Normalized punch force, *P* , during the deep drawing process can be described as

> = ⋅ ⋅⋅ π

,where, *d*p is punch diameter, *t*0 is initial foil thickness, and *τ*Y is shear yield stress of the blank material (Hu et al., 2007). The punch stroke is normalized with the drawing punch diameter, *d*p. In this normalization, higher normalized force indicates the higher fraction of friction force to the whole forming force during the process. Fig.14 shows the comparative data of normalized punch load-stroke curves between the three tools of different surface asperities. The error bar of the curves indicates the standard deviation of the normalized punch force. Fig 15 summarizes the standard deviations of the maximum drawing and

 τ

<sup>0</sup> / ( ) *<sup>p</sup> <sup>y</sup> PP dt* (1)

**Figure 13.** 3D surface images of the micro drawing punch with different surface characteristics,

Ra=0.45μm Rz=3.32μm (**Standard deviation** σ=0.60μm)

0 5 10 15 20 25 30 35 40

Ra=0.11μm Rz=1.27μm (**Standard deviation** σ=0.62μm)
