Investigation of microstructure and mechanical properties of SPH440-OD steel combined with mag welding method

In this thesis study, the microstructure and mechanical properties of the formations resulting from the joining of SPH440-OD hot rolled automotive steel with Metal Activated Gas (MAG) welding, one of the gas arc methods, were examined. In the welding application, wire speed and welding voltage values were preferred as variable welding parameters and welding processes were carried out in 9 different combinations. Then, samples were cut from the welded plates obtained from different welding parameters, and microstructure images were obtained on these samples under an optical microscope on three different regions: the Base Metal (BM), the Weld Metal (WM), and the Heat Affected Zone (HAZ). In order to examine the mechanical properties of the structures, tensile tests and Vickers microhardness tests were applied. As a result, it was observed that the different preferred welding parameters affected the percent elongation and tensile strength within the scope of the tensile test, and also affected the quality of the weld seam. In the tensile sample of the 3C coded welded plate, the highest tensile stress value was obtained with 515.83 MPa under 6 m/min wire speed and 19 V welding voltage conditions. The lowest tensile stress value was realized in the tensile sample of 2C coded welded plate with 487.83 MPa under 5.5 m/min wire speed and 19 V welding voltage conditions. The highest percentage elongation value, 21.64%, was seen in the tensile sample of the 2B coded welded plate under 5.5 m/min wire speed and 17 V welding voltage conditions. It was observed that the lowest elongation value, 17.85%, occurred in the tensile sample of the 2A coded welded plate under 5.5 m/min wire speed and 15 V welding voltage conditions. In general, keeping the wire speed constant and gradually increasing the welding voltage enabled the weld seams to widen. It was observed that the fracture in the samples generally occurred in the BM. In metallography experiments, it was observed that the grains in the HAZ became denser and larger compared to the BM, and columnar grain structures were observed in the WM. The hardness of the samples increased from the BM to the HAZ and from the HAZ to the WM.