Effect of electrode materials type on resistance spotwelding of AISI 430 ferritic stainless steel

In this study, AISI 430 grade ferritic stainless steel sheets with 0.8 mm thickness were joined by resistance spot welding using different electrode materials. The hardness and tensile shear load bearing capacity of welded joint was determined and the microstructure of welded samples was also evaluated. The welding was performed using with a timer and current controlled resistance spot welding (RSW) machine having 20 kVA capacity and operating at 50 Hz controlled by a programmable logic controller (PLC). Welding was conducted at the constant electrode pressure of 2 bar using CuCo2Be, CuCr and CuZr electrodes having a 45º truncated cone resistance tips with 6 mm face diameter. For welding, 5, 10 and 15 cycle welding times and 5.4, 6 and 6.6 kA welding currents were applied. Optical examination of the welded samples was carried out using a Leica microscope. The load of 100 g was applied for 10 s in Vickers microhardness test. Tensile-shear test samples were prepared according to ASTM: E8M and tested using a universal testing machine. The crosshead speed was kept constant 1 mm min-1. The welding parameters required for maximum strength were found different for each electrode in this study. For example The best welding parameters are 10 cycles at 6 kA for CuZr electrode, 5 cycles at 6 kA for CuCr electrode and 15 cycles at 6.6 kA for CuCo2Be electrode. This difference can be attributed to the electrical conductivities of the electrodes. Because, the electrical conductivities of the CuZr and CuCr electrodes are approximately twice that of the CuCo2Be electrode. Fussion Zone (FZ) is featured by columnar ferrite grains and almost evenly distributed chromium-rich carbides throughout the matrix. The HAZ is divided to two distinct zones: high temperature HAZ (HTHAZ) and little temperature HAZ (LTHAZ). The HTHAZ exhibited ferritic microstructure with excessive grain coarsening. The volume fraction of precipitates in LTHAZ is higher than HTHAZ resulting in higher hardness value. Microhardness tests showed that the microhardness values increased up to 400 HV with increasing welding current (6.6 kA) and welding time (15 cycle) in the HAZ when CuCo2Be electrode is used.