TEM observations of void-lobed defects and the origin of stringers in copper rod and drawn magnet wire

Abstract

Utilizing transmission electron microscopy (TEM), we have discovered a defect, which we call a void-lobed defect, in copper precursor rod and in sections of failed copper magnet wire. This defect is the origin of, or basis for, stringers or stringer defects in copper rod and wire, and consists of a contaminated, solidified copper microdroplet which is entrained in the solidifying rod, and is disconnected at its ends in the rod drawing direction, forming voids and elongated void lobes at the ends of the copper inclusion. Utilizing a novel technique to build up fine wires by copper electroplating, we have followed their progress in the magnet-wire drawing process using TEM. Detailed examination of wire failures showed that these defects are forced to the wire axis by successive drawing stages forming coalesced void/debris channels which provide a mechanism for failure. There was no evidence for copper oxides either as stringer components or in connection with debris channels in failed wires. These defects and therefore stringers in precursor rod are created as contaminated or reacted “copper rain” prior to the solidification stage, and entrained in the rod. The process is usually intermittent and may be reduced or eliminated by properly adjusting the equilibrium chemistry through hydrogen control for oxygen reduction and steam formation; thereby producing high-quality rod for magnet wire produciton. This is an extraordinary example of TEM application in the solution of a contemporary industrial problem which has been otherwise intractable.