EoW May 2011

technical article Analysis of surface flaws on drawn wire and wire breaks By Kazunari Yoshida, Tetsuo Shinohara, Tsutomu Yamashita and Atsuhiro Tanaka of Tokai University, Japan

Abstract: Using experiments and finite element method (FEM) analysis, this paper exa- mines whether surface cracks on wire rods grow or are removed during drawing. The deformation behaviour of V-shaped, concave, and U-shaped transversal cracks were observed upon repeated drawing. The authors clarified the conditions under which these transversal cracks are removed in order to achieve a surface condition similar to that of the area without cracks. 1 Introduction The diameter of wires and bars produced by drawing varies from about 10μm to 100mm. For any diameter of wire, breaking during drawing has a direct effect on productivity. Also, flaws on the surface of wires often cause fatigue breaking, the deterioration of mechanical properties, and loss of commodity value. This is why surface flaws are the most common complaint of users. Furthermore, finer and longer wires have recently been requested for use as the bonding wires of semiconductors and in medical devices. Therefore, wire breaking and surface flaws are problems that must be solved. Many researchers have investigated the optimal die shape to reduce drawing force, measures to prevent wires from internal cracking (through the formation of central burst defects or cupping), the optimal lubricant, measures for reduction of residual stress and so forth. Useful findings have been obtained, but there have been few studies of measures to prevent wire breakage and on the effects of inclusions and surface flaws on wire drawing 1, 4 . In this study, the causes of wire breakages in drawn wires of different diameters were examined. Analyses of wires with surface flaws and inclusions were performed experimentally and by a finite element method (FEM) to determine whether surface flaws and inclusions cause wire breakages or remain as surface flaws when drawing is repeated.

‘Inclusion’

‘Split End’

‘Contamination’

‘Tension’

‘Hollow End’

‘Pressed hollow’

▲ ▲ Figure 1 : Types of wire breaking

Inclusion Hollow end

Contamination Pressed hollow Split end Tension Others

▲ ▲ Figure 2 : Percentages of causes of wire breaking during drawing of superfine gold wires

2 Types of wire

One occurs when inclusions are found on the fracture surface, and the other, called “hollow end” breaking, occurs when traces of concave inclusions are found but the inclusions themselves are not found. Two types of breaking are caused by foreign materials introduced during a process other than the casting process. One is called “contamination” breaking, when foreign materials are found on the fracture surface, and the other is called “pressed hollow” breaking, when no foreign materials are found. Additionally, “split end” breaking is considered to be caused by surface flaws, and “tension” breaking is caused by applying a stress stronger than the wire strength 5 .

breakage caused by drawing

Recently, the occurrence of wire breaking during drawing has greatly decreased due to reduced particulate segregation and fewer inclusions in bars and wires brought about by effective material cleaning and because of the improvement of drawing conditions. However, the finer the diameter of a wire, the higher the frequency of occurrence of wire breakage during drawing. There are several causes of wire breaking ( Figure 1 ). Two types of breaking are caused by inclusions.

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EuroWire – May 2011