Abstract
D.Phil. (Mechanical Engineering)
Metal Inert Gas (MIG) welding and Tungsten Inert Gas (TIG) welding processes are the most economical and widely used welding processes employed for welding most metals, including mild steel. However, the process phenomena involved in these welding processes, such as heterogeneous heating and cooling rate across the weld zones, metallurgical phase changes, material pool, and deposition, often compromises the structural integrity in mild steel welds. While the MIG and TIG welding techniques have been applied for the conventional standalone welding processes, recent studies indicate that the hybridisation of the MIG and TIG arc will overcome the innate limitations of the individual techniques and improve the structural integrity of the welded joints. The introduction of hybrid arc welding processes has presented overarching benefits in recent years. Hence, the need to investigate the use of the hybrid process in overcoming the limitations encountered while welding mild steel. This study explores the prospect of improving the structural integrity of AISI 1008 mild steel by hybridising the MIG and TIG arc processes. It also details optimising the process parameters of standalone and hybrid models of TIG and MIG using the Taguchi method. Further investigation and comparative study of the emerging characteristics of the MIG, TIG and the TIG-MIG hybrid welding of AISI 1008 was carried out. An L-9 orthogonal matrix was adopted for the experimental design phase for the three categories of welding (TIG, MIG, and TIG-MIG). This design was optimised using the Taguchi method, and characterisation of the resulting weldments was performed. Secondly, parametric optimisation of the TIG, MIG, and TIG-MIG welding processes were carried out to maximise the tensile strength for each welding process through the Taguchi optimisation method using Minitab 17 software. First and second-order regression models were developed to revalidate the experimental data, which the second-order regression models showed closer predicted values to the experimental values. Thirdly, a confirmatory experiment for both standalone and TIG-MIG hybrid based on the optimum set of parameters derived from the optimisation process was carried out. The three categories of welds were further compared based on their characterised tensile test, hardness test, microstructural and macrostructural microscopy, energy dispersive spectroscopy (EDS), X-ray diffraction, scanning electron microscopy (SEM), and corrosion testing. The optimum parameter setting for the TIG process was 15 V, 180 A, and 15 L/mm, that of the MIG process was seen to be 30 V, 280, and 17 L/mm, and that of the TIG-MIG process was vii 25 V, 180, and 19 L/mm. Comparing the joint integrity of the three weld types based on the optimal process parameters, the TIG-MIG hybrid joint had better tensile properties compared to the standalone MIG and TIG welded joints. Also, the MIG welded joint had better tensile and yield strength compared to TIG welded joint. The fracture morphology of the optimal welds showed that the TIG-MIG joint experienced a more ductile failure compared to individual TIG and MIG welded joints. On the contrary, the hardness profile reveals that the TIG welded joint had the highest hardness property at optimal process parameters. Also, the hardness value of the MIG welded joint was observed to be higher than that of the TIG-MIG hybrid welded joint. The MIG weld bead was observed to be much larger than those obtained from the TIG and TIG-MIG joints, and this is due to the higher levels of the input factor combinations. The heat-affected zone for the TIG-MIG hybrid joint was seen to be wider than those for the individual TIG and MIG welding processes. This is credited to the relatively high energy input during the hybrid process as the second weld pass introduces more heat into the material, thus extending the heat-affected zone. The presence of Widmanstatten and acicular ferrite accounted for the improved tensile and hardness properties of the weldments. The MIG weld showed the best corrosion resistance, closely followed by the TIG-MIG weld joint, while the TIG welded joint had the least corrosion resistance. The temperature distribution, thermal stresses, and the tensile strength of the optimum weld for each welding type were further predicted by numerical analysis. The predicted outcomes were in consonance with the experimental result. Similarly, the simulated tensile strength at optimal welding parameters also closely agreed with the experimental values. In conclusion, the proposed research objectives were successfully accomplished. The experimental and numerical study of the hybrid welded component for enhanced structural integrity was successfully characterised. The process window for welding AISI 1008 mild steel via the TIG-MIG welding process was established. The study reveals that the hybridisation of the TIG and MIG arc improves the properties of AISI 1008 mild steel. The joint properties of the TIG-MIG process were seen to be superior to those obtained from conventional standalone MIG and TIG welding processes. In addition, the results of the numerical models are useful for simulating the process for other grades of mild steel. Therefore, the TIG-MIG hybrid welding process can be employed to weld AISI 1008 mild steel for typical industrial applications requiring improved mechanical properties and excellent corrosion resistance.