*4.3.2. Simulation results*

130 Metal Forming – Process, Tools, Design

**Punch force /N**

**Punch force /N**

**Punch force /N**

**Figure 20.** FEM results of load-stroke curve under different surface conditions between tool and material (a) Effect of tool surface amplitude (b) Effect of blank surface amplitude with smooth tool, (c)

**0 100 200 300 400 500** 

**Punch stroke /mm**

(c)

(a)

Smooth Blank (Rz=0.05μm) **Smooth blank** Blank

Die

Die

Die

Die

Die

Die

Die

Blank

Die

**Rough tool** Blank

**Smooth tool** Blank

Tool roughness **0.5**μ**m Rz**

Tool roughness **0.05**μ**m Ra**

Blank roughness **0.05**μ**m Ra**

**Rough blank** Blank Blank roughness **0.5**μ**m Rz**

**Smooth blank** Blank roughness **0.05**μ**m Ra**

**Rough blank** Blank Blank roughness **0.5**μ**m Rz**

**0 100 200 300 400 500** 

Rough Blank (Rz=0.5μm)

Rough Tool (Rz=0.5μm) Smooth Tool (Rz=0.05μm)

**Punch stroke /mm**

(b)

**0 100 200 300 400 500** 

Rough Blank (Rz=0.5μm) Smooth Blank (Rz=0.05μm)

**Punch stroke /mm**

Effect of blank surface amplitude with rough tool

Fig.20 (a) shows the effect of roughness amplitude of a tool with same smooth material surface on formability (Condition No.2 and 4) (Shimizu et al., 2009). The curve obtained for the rough tool indicates the higher punch forces than those of the smooth tool. The difference in ironing force is particularly large. This tendency was also observed in the experimental results of the comparison between the untreated tool (smooth) and the airblasted tool (rough), as shown in Fig. 14. From the deformation history in the analysis, it can be seen that, if the tool surface asperity are sharper than the blank surface asperity, the plastic deformation of the blank surface asperity would be easily occurred, due to the intensive surface pressure. In the actual contact behavior during the process, this phenomenon in FE analysis could be translated to the fracture of oxide film layer due to the high normal pressure. It would easily induce the adhesion or the plowing and increases the friction.

Fig. 20 (b) and (c) show the effect of blank roughness amplitude under the same tool surface condition (Shimizu et al., 2009). As for the condition with the smooth amplitude tool (Condition No.3 and 4) as shown in Fig. 20 (b), the punch force of the rough material is lower than that of the smooth material. The similar tendency in the experiment is already shown in Fig. 18(a) and (b). As mentioned from the observed surface images of micro-drawn cup in Fig.19, since the real area of contact under the smoother amplitude blank is much larger than that of rough amplitude blank, the friction force will increase and it results in the higher forming force.

While for the condition with a rough amplitude tool (Condition No.1 and 2) shown in Fig.20 (c), the maximum drawing and ironing force indicates almost the same value between the conditions. This shows the higher impact of the tool surface roughness than the material surface roughness. Since the harder tool surface asperities would plow the softer material surface, the material surface roughness seems to be less influence on the friction resistance under the contact with the rough tool surface.

Thus, local interfacial behavior in micro-deep drawing could be explained on the basis of classic theory of conventional tribology. However, a feature of the micro-scale region appears to be the higher sensitivity of the global forming behavior to the microscopic surface properties. Therefore, the proper surface design of tools and work materials in micro-scale becomes significantly more important than in conventional macro-scale.
