EoW May 2011

technical article

and rolling are used as mother wires and drawn repeatedly. The growth and removal of these cracks are examined in the experiments and by FEM analysis. Stainless steel (SUS304) rod wires were mechanically scratched in the axial direction using a lathe and analysed by experiments and FEM. Rod wires that were mechanically marked to form V-shaped, concave, and U-shaped cracks in the circumferential direction were used as specimens. FEM software, MSC/Marc Mentat 2008 R1, was used in this study. Figures 8 and 9 and Table 3 show the model used in the FEM analysis, its material constant and the parameters of the V-shaped crack, respectively. Coefficient of friction(μ) was set at 0.05. Moreover, the model was assumed to be axis-symmetric in the FEM analysis to save calculation time. 4.1 Comparison of results of experiment and FEM analysis A crack with a depth of h=0.8mm (8%) was cut on an 8mm diameter wire and the change in its shape was experimentally and analytically examined after each drawing pass. The initial crack on the mother wire was asymmetrically V-shaped. The shape of the initial crack was observed using a microscope, and a mother wire with a crack of the same shape was modelled in the FEM analysis. It is clear that the FEM analysis result agrees with the experimental result. As shown in Figure 10 , the crack appears to be removed because side AB is pushed up into the wire; however, side BC of the crack is tilted so that it overhangs side AB, forming an overlapping crack (defect).

Mechanical constants for gold

Young’s modulus

80GPa

Poisson’s ratio

0.44

Work-hardening curve

σ=475ε 0.07

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

rods during upstroke rolling are classified as shown 7 in Table 2 ; however, there is no clear solution to this problem. In particular, only a small number of studies have reported on surface cracks formed during drawing 8–11 . In this study, wire rods that developed circumferential cracks during casting

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 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. Three dimensions FEM code MSC/Marc Mentat 2008r1 was used in this study. The results of FEM analysis are consistent with the experimental results. Regardless of whether the foreign material is located in or on the wire, it does not undergo deformation because of its hardness, even if drawing is repeated. This leads to a high value of Di/Do, increasing the drawing stress and increasing the chance of wire breakage. 4 Analysis of a wire drawing with surface cracks Surface cracks develop on rods or wires because of mishandling during casting, hot rolling, drawing or transport, or because of the improper winding of wires 6 . The surface cracks that develop on wire

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

A) SEM image of the drawn wire

B) Fe content

C) Ni content

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EuroWire – May 2011