Defects and failure in ultra-fine copper magnet wire

Abstract

The flow processes in wire drawing and extrusion are extremely nonhomogeneous and depend prominently on the die half-angle, the fractional reduction, the velocity vector, the friction shear factor between the die and the wire surface, and the strain hardening capacity of the drawing metal. Structural damage during wire drawing for a given microstructure has been found to correlate with the amount of hydrostatic stress developed. Nonmetallic inclusions such as oxide particles and more recently hard metallic inclusions can nucleate voids and cracks along with an insidious phenomenon called central burst or cupping. In addition, there appears to be a microstructural distinction around the wire centerline which has not been investigated in any systematic manner. In fact, microstructural characterization of drawn copper magnet wires seems to have received little or no consideration, especially utilizing transmission electron microscopy (TEM). This study represents an effort to rectify these shortcomings by applying light metallography (LM) and TEM to the study of a wide range of copper rod and drawn wire microstructures, including ultra-fine wire microstructures. The authors define ultra-fine wire to be smaller than 44 gauge (<50 {micro}m diameter; which is roughly one-third the diameter of a human hair).