Welding Journal Research Supplement
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Residual Copper Effects on Resistance Spot Welded Low Carbon Steel
Increasing residual copper leads to a higher instance of expulsion and higher strength but lower ductility in resistance spot welds
Abstract
The use of scrap for steel making has increased worldwide and is expected to continue as steelmakers and users pursue sustainability efforts. Advances in steelmaking practices have enabled greater scrap recyclability and energy savings, but have come at the cost of reduced control over residual elements such as copper. This work studies the effect of increased copper content, up to 0.26 wt-%, on the base-metal microstructure and resistance spot-welding behavior of a low-carbon automotive sheet steel. Alloys were subcritically annealed prior to spot welding, tensile shear testing, electrical resistivity measurements, and microstructural analysis. The higher-copper alloys showed increased base-metal strength, likely due to copper solid-solution strengthening. The higher-copper content alloys also exhibited higher electrical resistivity, resulting in a greater incidence of expulsion during resistance spot welding. Despite increased expulsion at high welding currents, resistance spot welds of the higher copper content alloys showed higher strength and lower ductility in tensile-shear testing. This work shows that elevated residual copper content can be accommodated in welded automotive steel sheets, provided base metal processing and welding parameters are adjusted to account for its influence.