WCA July 2011

3.2 Effect of foreign particles In some cases foreign materials are present on the wire surface during drawing, or foreign materials may enter the die via the lubricant. These foreign materials are mainly formed by erosion of the wire or of the die or equipment, or may originate from dust in the air. Depending on the shape and hardness of the foreign material wire breakage, as shown in Figure 5 , may occur. As an example, Figure 6 shows scanning electron microscope (SEM) images and EDS photographs of a wire after drawing with foreign materials on the wire surface. The material of the wire is austenite stainless steel. Analysis by EDS revealed that the foreign material was composed of iron carbide, which included a scarce Ni component. The size was 0.53x0.27mm, and Di/ Do was about 0.2. It is presumed that the wire did not break because of the low value of Di/Do. Figure 7 shows the results, gained by FEM analysis, of drawing a

Mechanical constants for gold

Young’s modulus

80GPa

Poisson’s ratio

0.44

σ =475 ε 0.07

Work-hardening curve

Material condition for inclusion

O

3 , SUS304

Material

A1

2

Young’s modulus

300, 194GPa

Poisson’s ratio

0.23, 0.30

Yield stress

4.3, 0.205GPa

α =7º, R/P=10%

Die half angle, reduction

Friction factor

0.05μm

❍ ❍ Table 1 : Condition of materials and drawing for FEM

A) The case of a large or hard inclusion

B) The case of a small or soft inclusion

❍ ❍ Figure 5 : Schematic diagram of wire breaking caused by foreign material

wire with a foreign material near the wire surface. At the interface of the foreign material and the wire, they are bonded mechanically. Upon repeated drawing, stress acts on the interface and causes separation at the interface, generating an empty space.

The results are shown in Figure 3 . It was found that drawing stress moves rapidly upwards when an inclusion passes through the die. It can be seen that the higher the ratio of inclusion size to wire diameter, Di/Do, the more the drawing stress rises.

❍ ❍ Figure 6 : SEM image and componential analysis of wire with foreign material

In the case of a wire with an inclusion for which Di/Do is 0.7, the drawing stress reaches the strength of the wire. This means that there is a high probability that the wire will break. Taking the safety factor into account, it is thought that there is a danger that the wire breaks when Di/Do is higher than 0.4. The sizes of inclusions were measured at the fracture surface when gold wires with the diameters of 20-50μm were drawn. Figure 4 shows the frequency of wire breaking for different values of Di/Do. This figure suggests that there is a danger of wire breakage if Di/Do is 0.3 or higher, and the highest frequency of wire breakage is when Di/Do is approximately 0.7.

A) SEM image of the drawn wire

C) Ni content

B) Fe content

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