Effect of variations in peak and background currents during pulsed current gas tungsten arc welding on the dissimilar welded joint of ASTM A105-AISI 316L: Microstructural changes, mechanical properties and fracture mechanism

Abstract

The aim of this study was to investigate the effect of variations in peak (I_p) and background (I_b) currents during Pulsed Current Gas Tungsten Arc Welding (PCGTAW) on microstructural changes, significant improvement of mechanical properties and fracture mechanism of dissimilar welded joints of ASTM A105-AISI 316L. For this purpose, 12 mm thick AISI 316L stainless steel sheets and ASTM A105 steel sheets were welded together by PCGTAW process with I_p values of 180, 220 and 260A, and I_b values of 70, 110 and 150A. Optical microscopy, Field Emission Scanning Electron Microscopy (FE-SEM), Energy-Dispersive X-Ray Spectroscopy (EDS), and X-Ray Diffraction (XRD) were used to analysisthe microstructural and phase changes. Similarly, tensile, Vickers microhardness and Charpy impact tests were utilizedto evaluate the effect of changes in I_b and I_p currents on mechanical properties. The fracture mechanism after tensile and Charpy impact tests was investigated by FE-SEM. Microscopic studies indicated that the microstructure of the weld metal (WM) contained austenite dendrites with a small amount of grain boundary delta ferrite, to the extent that with increasing I_b and decreasing I_p, the microstructure of the WM was changed from columnar dendrites to extremely small and coaxial dendrites. Likewise, the increased I_b and decreased I_p reduced the size of dendrites as well as the amount of grain boundary delta ferrite in WM and sodidthe width of the heat-affected zone (HAZ) and partial melted zone (PMZ). The results of XRD analysis indicate the predominance of the austenite phasein WM. In the tensile test, all welded joints were fractured from the ASTM A105 steel side. The welded joints built up yield strength of approximately 250 ± 8 MPa, a tensile strength of around 484 ± 9 MPa, and a fracture strain close to 30 ± 2 %.The results of Charpy impact and microhardness tests showed that with increasing I_b and decreasing I_p, the hardness (from 250 ± 5 HV to 289 ± 4 HV) and fracture energy (from 140 ± 3 J to 173 ± 3 J) of WM are increased. Fractography of the fracture surfaces indicates the occurrence of fully ductile fracture in both tensile and Charpy impact tests.