Abstract
There is a popular and ever-growing interest in the manufacturing world for cost and lightweight reduction for sustainable development. This is especially true in developing nations. Dissimilar welding of relatively low-cost low carbon steel to stainless steel has a lot of economic advantages in these regions. The outstanding corrosion resistance property, high formability, weldability, and excellent mechanical properties make austenitic stainless steel arguably the most demanded engineering material. AISI 316 austenitic stainless steel was specifically developed for improved corrosion resistance. This addition of molybdenum in the development of AISI austenitic stainless steel gives it superior corrosion resistance property over other types of austenitic stainless steels such as the AISI 304 type. This makes it suitable material in the fabrication of pressure vessels, heat exchangers, furnace building and pharmaceutical equipment. Despite the advantages of AISI 316 austenitic stainless steel, its wide application is still limited due to its high cost compared to low carbon steel. This makes the welding of lower-cost mild steel and austenitic stainless steel inevitable. AISI 1008 mild steel with appreciable resistance to chloride pitting and strength at elevated temperature also find application in pressure vessels, furnaces, boiler, and marine engineering. However, welding dissimilar materials are challenging due to differences in thermal properties and precipitation of brittle intermetallic compounds. Solid-state welding process such as friction stir welding and laser welding have successfully achieved dissimilar welding of mild steel and stainless steel due to their several advantages such as narrow heat-affected zone, little or no precipitation of intermetallic compounds, high corrosion resistance and outstanding mechanical properties. However, their high equipment cost, the requirement of high technical ability and low suitability for outdoor welding make arc welding processes such as Tungsten Inert Gas (TIG) and Metal Inert Gas (MIG) the choice many fabrication industries. TIG and MIG welding processes require less technical ability, are relatively cheap, have high gap bringing capability, high energy density and suitable for outdoor welding. This work has studied the TIG and MIG dissimilar welding of AISI 1008 mild steel and AISI 316 austenitic stainless steel in both butt and lap joint configurations experimentally and numerically. Successful weld joints were obtained. Welding process parameters such as welding current, welding voltage and gas flow rate were optimized by Taguchi optimization. Welding current and gas flow rate were found to be the most significant factors for the ultimate tensile strength and ultimate tensile shear strength in the TIG butt and lap welded samples respectively while the welding voltage was the most significant process parameter in all the joint configurations v considered. In the TIG welded samples, the respective average optimal ultimate tensile strength and ultimate tensile shear strength in the butt and lap joints were 493.29 MPa and 507.36 MPa while in the MIG welding process, the corresponding values were 559.20 MPa and 683.05 MPa. The average ultimate tensile strengths of the base metals were 395.92 MPa and 638.13 MPa for the mild steel and stainless steel respectively. The ultimate tensile strength and ultimate tensile shear strength observed in the hybrid TIG-MIG welded samples were 513.33 MPa and 526.76 MPa respectively. The microhardness of the fusion zone of TIG-welded samples was higher than that of the MIG welded samples. The maximum weld zone hardness in TIG and MIG welded samples were 386.10 HV and 244.63 HV respectively. At high heat input except in the TIG lap joint weldments, widmanstatten ferrite structure was detected at the heat-affected zone on the mild steel side and growth in the austenite grain size on the stainless steel side. The microstructure of the weld zone in all the welded samples was comprised of equiaxed austenite grains with dark skeletal delta ferrite at the grain boundaries. The presence of silicon in ER309LSi welding wire and higher heat input led to the development of a bigger bead in the MIG welded samples. Greater resistance to pitting corrosion was observed in the MIG welded samples. From the Ansys numerical analysis, the MIG welding process with higher heating loads gave higher temperatures and thermal stresses, while the TIG welding gave lower temperatures and stresses. In addition to that, the thermal distribution within the base metal is directly affected by the thermal conductivity of the material. AISI 1008 mild steel materials with higher conductivity easily allow heat energy to flow through them, preventing the build-up of thermal stresses at any point within the material.
Ph.D. (Mechanical Engineering)