*4.1.1. Materials and experimental conditions*

The material used was stainless steel ultra-thin foils (JIS: SUS304-H, 20μm in thickness). Table 2 shows the mechanical properties of the used stainless steel ultra-thin foils, which was obtained by the tensile test.


**Table 2.** Mechanical properties of stainless steel foils (JIS: SUS304-H, 20μm in thickness) used in the experiment.

As for the micro tools for the test, the same tools as mentioned in section 2.2 were used. In order to simplify the process, only the first stage process was compared between different surface conditions. To fabricate the microtools of different surface properties, air-blasting treatment and ion beam irradiation treatment were conducted for both micro-drawing die and punch. For the air-blasting treatment, glass powder of 53-63μm size was irradiated for 5-10min on the microtool surface to produce the rough surface. An air-blasting apparatus PNEUMA BLASTER SGK-DT (FUJI Manufacturing Co.) was used for the treatment. The ion-irradiation was performed by electron cyclotron ion shower system, EIS-200ER (ELIONIX Inc.), on the condition as shown in Table 3. Fig.13 shows the 3D surface images and the surface profile data of untreated, air-blasted and ion-irradiated micro-drawing punch obtained by laser scanning confocal microscope (LEXT OLS-3000, Olympus Co.).


**Table 3.** Conditions for ion beam irradiation

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

**Figure 13.** 3D surface images of the micro drawing punch with different surface characteristics, (a) Untreated tool (b) Air-blasted tool (c) Ion-irradiated tool

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 property can be obtained.

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

#### *4.1.2. Punch load-stroke curves*

122 Metal Forming – Process, Tools, Design

**4.1. Effect of tool surface properties** 

effect of tool surface properties are discussed.

*4.1.1. Materials and experimental conditions* 

was obtained by the tensile test.

experiment.

Olympus Co.).

Ion gas Acceleration

**Table 3.** Conditions for ion beam irradiation

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

surface properties during micro-scaled forming is demonstrated.

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

Firstly, the effect of the tool surface properties is experimentally investigated. In order to study the sensitivity of tool surface properties on microformablity, micro-deep drawing test is carried out with three different tools with the different surface properties. By comparing the forming results of the forming force and the surface quality of the micro-drawn cups, the

The material used was stainless steel ultra-thin foils (JIS: SUS304-H, 20μm in thickness). Table 2 shows the mechanical properties of the used stainless steel ultra-thin foils, which

Young's modulus Yield stress Tensile strength Elongation 193[GPa] 1192[MPa] 1460[MPa] 1.5[%] **Table 2.** Mechanical properties of stainless steel foils (JIS: SUS304-H, 20μm in thickness) used in the

As for the micro tools for the test, the same tools as mentioned in section 2.2 were used. In order to simplify the process, only the first stage process was compared between different surface conditions. To fabricate the microtools of different surface properties, air-blasting treatment and ion beam irradiation treatment were conducted for both micro-drawing die and punch. For the air-blasting treatment, glass powder of 53-63μm size was irradiated for 5-10min on the microtool surface to produce the rough surface. An air-blasting apparatus PNEUMA BLASTER SGK-DT (FUJI Manufacturing Co.) was used for the treatment. The ion-irradiation was performed by electron cyclotron ion shower system, EIS-200ER (ELIONIX Inc.), on the condition as shown in Table 3. Fig.13 shows the 3D surface images and the surface profile data of untreated, air-blasted and ion-irradiated micro-drawing punch obtained by laser scanning confocal microscope (LEXT OLS-3000,

Voltage Vacuum Ion current

Ar 800eV 1x10-4Pa 1.2mA/cm2 5-10min 45o

density

Irradiation time

Irradiation angle

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

$$\overline{P} = P \left( (\pi \cdot d\_p \cdot t\_0 \cdot \pi\_y) \right) \tag{1}$$

,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 ironing force in each load-stroke curve.

Impact of Surface Topography of Tools and Materials in Micro-Sheet Metal Forming 125

**Ra=0.163**μ**m Rz=1.63**μ**m**

**Ra=0.242**μ**m Rz=2.40**μ**m**

**Ra=0.153**μ**m Rz=1.41**μ**m**

The air-blasted tool shows the maximum drawing force, while the maximum ironing force is indicated by the ion irradiated tools. In addition, the both the lowest drawing and ironing force are shown by the untreated tool. In comparison of the dispersion of the force in each tool condition, the widest variation in maximum drawing force is observed for the untreated tool and the larger dispersion in maximum ironing force is indicated by the ion-irradiated tool. The wider variation of the ironing force data for every tool conditions seems to be attributed to the high contact pressure induced by the ironing process. Thus, the difference

Fig.16 shows the surface images of the upper and the lower sides of the microcup wall of outer surface (Shimizu et al., 2009). The surface of the microcup has a large difference, as shown in the figure. In particular, for the microcup drawn by the ion-irradiated tool, scratches and chippings of the material caused by the adhesion are remarkable. In contrast, although the tool surface has the roughest surface for the air-blasted tool, the surface of the microcup is smoothened by sliding with the die, and better surface quality is observed. Furthermore, a roughened surface similar to an orange peel surface is observed at the lower part of the microcup drawn by the untreated tool. While, sliding traces with the die are

Air-blasted

**30μm**

**30μm**

**30μm**

Untreated

A

B

A

B

Ion irradiated

B

A

in the effect of tool surface properties on friction is markedly observed.

observed under the conditions with the air-blasted and ion-irradiated tool.

(a)

(b)

(c)

**Figure 16.** Surface images of microcup wall using 3 tools with different surface properties

A B

A B

A B

**Ra=0.133**μ**m Rz=0.86**μ**m**

**30μm**

**30μm**

**30μm**

**Ra=0.133**μ**m Rz=1.01**μ**m**

**Ra=0.147**μ**m Rz=1.18**μ**m**

*4.1.3. Surface quality of drawn cups* 

**Figure 14.** Comparison of normalized punch force-stroke curves between 3 tools with different surface properties

**Figure 15.** Comparison of standard deviation of maximum drawing and ironing force in each tool surface condition

The air-blasted tool shows the maximum drawing force, while the maximum ironing force is indicated by the ion irradiated tools. In addition, the both the lowest drawing and ironing force are shown by the untreated tool. In comparison of the dispersion of the force in each tool condition, the widest variation in maximum drawing force is observed for the untreated tool and the larger dispersion in maximum ironing force is indicated by the ion-irradiated tool. The wider variation of the ironing force data for every tool conditions seems to be attributed to the high contact pressure induced by the ironing process. Thus, the difference in the effect of tool surface properties on friction is markedly observed.

#### *4.1.3. Surface quality of drawn cups*

124 Metal Forming – Process, Tools, Design

properties

surface condition

**0**

**0.5**

**1**

**1.5**

**Standard deviation** 

σ**/ N**

**2**

**2.5**

**3**

**3.5**

**4**

**Figure 14.** Comparison of normalized punch force-stroke curves between 3 tools with different surface

**Drawing force Ironing force**

**Maximum ironing force**

**Maximum drawing force**

**Figure 15.** Comparison of standard deviation of maximum drawing and ironing force in each tool

**Untreated Blasted Ion-irradiated**

Fig.16 shows the surface images of the upper and the lower sides of the microcup wall of outer surface (Shimizu et al., 2009). The surface of the microcup has a large difference, as shown in the figure. In particular, for the microcup drawn by the ion-irradiated tool, scratches and chippings of the material caused by the adhesion are remarkable. In contrast, although the tool surface has the roughest surface for the air-blasted tool, the surface of the microcup is smoothened by sliding with the die, and better surface quality is observed. Furthermore, a roughened surface similar to an orange peel surface is observed at the lower part of the microcup drawn by the untreated tool. While, sliding traces with the die are observed under the conditions with the air-blasted and ion-irradiated tool.

**Figure 16.** Surface images of microcup wall using 3 tools with different surface properties

The higher value and wider dispersion of the ironing force under the ion-irradiated tool appears to be due to the strong plowing of the wear particles. In fact, Yang et al. reported that WC particles would be exposed by the ion irradiation of WC-Co hard alloy, due to the difference in sputtering rate between the WC and Co (Yang et al., 2008). Since Co particles are removed from the surface, the WC particles lose the binding agents and are easily to drop off from the surface. Therefore the dropped WC particles seem to scratch and plough the surface of work material. Thus, remarkable difference in interfacial behaviour between the tool and material are experimentally demonstrated.

Impact of Surface Topography of Tools and Materials in Micro-Sheet Metal Forming 127

modulus Yield stress Tensile strength Elongation

**1N30-H\_Mt 1N30-H\_Br**

1N30-H 70[GPa] 159[MPa] 177[MPa] 2.7[%] 1N30-O 70[GPa] 69[MPa] 88[MPa] 22.0[%] **Table 4.** Mechanical properties of pure aluminium foils (JIS: 1N30, 20μm in thickness) used in the

**1N30-H\_Br**

**1N30-O\_Br**

(a)

**Normailzed punch stroke S/dp**

**1N30-O\_Mt 1N30-O\_Br**

**Normalized punch stroke** *S***/***d***<sup>p</sup>**

**Pure aluminum (1N30-H)** 

**Pure aluminum (1N30-O)** 

**0 100 200 300 400**

**1N30-H\_Mt**

**Figure 18.** Punch force-stroke curves for comparing different material surface conditions

(b)

**Normalized punch stroke** *S***/***d***<sup>p</sup>**

**1N30-O\_Mt**

**0 100 200 300 400 Normailzed punch stroke S/dp**

Material Young's

**0**

**0**

**0.1**

**0.2**

**0.3**

**0.4**

**Normalized punch force**

**Normalized punch force** 

**0.5**

*P***/π·***t***0·***d***p·***τ***Y**

**0.6**

**0.7**

**0.1**

**0.2**

**0.3**

**0.4**

**Normalized punch force**

**Normalized punch force** 

**0.5**

*P***/π·***t***0·***d***p·***τ***Y**

**0.6**

**0.7**

experiment.

(a) 1N30-H, (b) 1N30-O
