Case study: Investigation of shorter lifetime of upper Cu weld wheels in an industrial mash seam welding setup

Abstract

We present a case study investigating the failure mode of upper copper electrode weld wheels being used in a mash seam welding process in an industrial production line. The upper electrode wheels had a much shorter lifetime than the lower electrode. Published literature on the degradation mechanisms of electrodes has focused on resistance spot welding. There are far fewer studies of failure modes in mash seam welding failure modes. This study presents the compositional and mechanical analysis of a failed copper upper electrode taken from a mash seam welding production line. Optical and scanning electron microscopy with x-ray compositional results showed the formation of steel-copper mixing regions on the surface of the electrode in contact with the steel, and within the electrode itself. Existence of mixing regions in the electrode are further supported with localized hardness mapping by nanoindentation, which confirms the steel-copper mixed regions exist up to 0.8 – 1.2 mm below the knurled electrode’s surface. In addition to the mixed region, there was clear evidence of copper grain fragmentation as the steel was worked into the copper electrode. We conclude that failure occurred because intermittent air gaps between the electrode and steel during rolling caused insufficient cooling of the steel piece, causing a melt pool at the workpiece surface. This molten steel adhered to the copper electrode and was gradually pushed into the copper leading to electrode failure. Solutions to extend the service life were proposed, based on a thorough understanding of electrode degradation mechanism and deliberate electrode design changes.