INVESTIGATION ON MICROSTRUCTURE AND MECHANICAL CHARACTERISTICS OF STAINLESS STEEL USING COLD METAL TRANSFER (CMT)

Abstract

Gas Metal Arc Welding (GMAW) is an effective process of joining of various materials such as stainless steel, aluminum, nickel etc. The cold metal transfer (CMT) welding is the latest innovation of GMAW process with lower heat input for specially joining of lower thickness materials. CMT welding is much suitable for defect free welding with superior quality of thin materials. The microstructural characterization and mechanical properties of the CMT welded joints is found beneficial over conventional GMAW welding processes. Stainless steel metal is widely using in various industrial and structural applications due to its excellent mechanical, wear and corrision properties. In this research CMT welding process have been applied for studying the the weld bead geomentry and fabrication of thin AISI 304 stainless steel sheets butt welded joints. Studies were carried out to examine the effect of various input welding parameters such as welding current, welding speed and contact-to work-distance (CTWD) on the bead geometry, dilution and heat input of CMT weld bead. Taguchi L9 orthoganal design matrix was applied for finding the optimal process parameters on weld bead geometry. Results shows that welding speed is the most significant welding process parameter, followed by welding current and CTWD on weld bead geometry. Box-Behnken design matrix under Response Surface Methodology (RSM) technique was applied for finding the optimal input welding parameters to in order to obtain v maximum tensile strength of welded joints. Welding speed, welding current, CTWD and arc correction factors are chosen as input welding parameters and analysed the relationship with tensile properties of welded joints. The optimal welding parameters are welding current of 95 A, a welding speed of 8 mm/sec, CTWD 5 mm and arc correction factor of - 10 for obtaining maximum tensile strength of the welded joints. The results show that there is no fracture on the weld joint; the tensile residual stress levels of all the samples are in a controlled manner. FESEM images reveal a dimpled morphology and crack-free tensile fracture surfaces in welded joints. Ultrasonic vibrations combined fabrication techniques enhances the structural efficiency of manufactured products. It gives an appreciable advantages in microstructural and mechanical properties of products. Ultrasonic assisted Cold Metal Transfer (U-CMT) welding technique is used to join the AISI 304 stainless steel. The welded joints are analyzed using optical microscopy, FESEM and XRD. The mechanical properties of the U-CMT welded joint are evaluated using microhardness, tensile and residual testing and compared CMT welded joints. Two different ultrasonic vibrational amplitude 50 µm & 99 µm and three different welding currents 85A, 90A and 95A were chosen in this study. The microhardness and tensile results of U-CMT welded joint shows considerable improvement in their properties. The U-CMT welded samples experiences enhanced mechanical properties due to refinement of grains in welded region, which is produced by ultrasonic vibrations. The joint prepared by welding current of 95A with ultrasonic vibrational amplitude 50 µm shows better welding properties than other welded samples.