Abstract:
Effective selection of appropriate shielding environment plays an important role in achieving a high quality weld, which in turn enhances deep penetration, surface appearance and mechanical properties of the weldments. In this work the appropriate percentage composition of CO2 in Argon- shielding environment that gives AISI 304 gas metal arc weldments of high quality in terms of hardness and tensile strength was investigated. The gas metal arc welding was done using inverter MIG- 200 welding machine equipped with an automatic wire feeder. AISI 304 plates were joined using gas metal arc welding (GMAW) technique and welding was performed using combinations of established process parameters. The microstructure of the weldments was examined using optical microscope while their micro-hardness and tensile strength of the at varying combinations of Argon/CO2 as were measured using Vickers micro-hardness tester and Instron universal testing machine respectively. Visual observation of the beads shows that a very smooth and continuous weld bead was achieved when 100% argon was used as the shielding gas. However, as the CO2 composition reached 30%, the weld bead became irregular in shape and discoloration was observed on the bead surface. In all the samples, except the sample formed with 100% CO2 shielding gas composition, the microstructure of the HAZ appears to be coarser than that of the weld zone. The hardness values of the weldments were found to range from 220 HV to 310 HV when the shielding gas compositions were 100% argon and 100% CO2 respectively. Also, it was noticed that the hardness at the welded joint increases with increase in the CO2 composition in the shielding gases. For example, a micro-hardness value of 247±6 HV was found for welded joint formed at 75% argon - 25% CO2 composition, whereas a lower value (220 ±8HV) was found at 100 % argon gas composition during the welding process. The ultimate tensile strength (UTS) value for each weldment is an average of three test results. Generally, the value ranged from 596 MPa to 133 MPa. It is obvious that the UTS decreased as the CO2 composition increases. Reasonably high strength was obtained when the shielding gas composition ranged between 100% argon and 70% argon-30% CO2. After, the UTS decreased significantly. The effects of the CO2 addition and the suitable composition of CO2 addition to Argon shielding environment during GMAW of AISI 304 stainless steel have been established.
