Fatigue Properties of Zinc Coated Low Carbon Steel Sheet Joints by the Means of Spot Welding

EN 10346:2015 DX52D+Z steel is a low-carbon, zinc-coated steel suitable for cold forming, widely used in the automotive industry due to its superior formability despite its low strength. Sheets produced in different sizes and forms are annealed by passing through the continuous galvanizing line after cold rolling. In this study, fatigue properties of these steels which were joined by spot welding with low welding current and high welding cycles, were measured and failure characterization was made, too. Tensile testing was made using a number of lap joints with different parameters and the variation of hardness within the weld area was also investigated. During the fatigue testing, crack initiation and propagation have been experimentally determined. A significant increase in hardness was observed from the base metal to the weld nugget, and the welding process caused a 38% reduction in the strength of the base material due to the heat affect from the joint. In the microhardness test, the highest hardness value was measured in the weld nugget heat affected zone (228.4 HV), and the hardness of the base metal was 169.4 HV. It has been determined that welded joints have infinite life below approximately 20% of the maximum tensile load.