Abstract
This research work reports the study entitled Numerical and experimental investigation of TIG-MIG hybrid welded joint. Hybrid welding refers to the use of two different welding processes to produce a seamless weld. A typical example of hybrid welding is laser hybrid welding, where a laser beam is used together with an arc welding beam to produce a weld. Arc hybrid welding is another type of hybrid welding where TIG and MIG or TIG and MAG welding processes produce a weld seam. The research study focus is on TIG-MIG hybrid welding of mild steel plates while the research objectives were to investigate the mechanical and microstructural characterisation of hybrid welded steel joints.
Welded plates were obtained using two passes butt welding. The hybrid welded plates used the TIG welding process as the first pass and the MIG welding process as the second. Furthermore, both TIG and MIG welded plates were conducted for comparison purposes. The welded plates were subjected to characterization such as microhardness, microstructural evolution, corrosion and tensile test. The microstructure analysis was conducted using the Olympus GX71 machine, while the microhardness test was conducted using the Emcotest Dura Scan G5 based on the ASTM 384 standard. Furthermore, the tensile test was conducted following the ASTM E8 standard guideline. In addition to the experimental investigation of the TIG-MIG hybrid, a numerical analysis of the process was conducted. The Finite Element Analysis of the hybrid welding process focused on the thermal and structural properties of the weld i.e. the effective stress, welding deformation, and the fusion zone's prediction.
The microstructure for TIG and MIG samples exhibited a ferritic structure with a small presence of pearlite. The microstructure of TIG-MIG hybrid samples exhibited ferritic-martensitic grains with a small presence of pearlite. The microstructure for hybrid samples showed small grain growing uniform from the first run (TIG) to the second run (MIG). The microhardness results presented were obtained using the Vickers microhardness test. The samples for TIG, MIG and TIG-MIG hybrid welded plates had on average maximum values of 226HV, 229HV and 227HV, respectively. Thus, TIG has a lower hardness value, and MIG has the highest hardness value, while hybrid has a hardness in between the two welding processes. On average, the maximum UTS for TIG, MIG, and hybrid samples are 549MPa, 571MPa, and 566.7MPa, respectively.
The finite element simulation for each of the three welding processes was conducted for two welding runs. The finite element results have shown that the hybrid welding simulation produced a gradually distributed temperature profile with a longer rear length similar to the MIG welding simulation. The simulation also provided the structural effect due to welding. The maximum effective stress for TIG, MIG and hybrid welding processes is 473.58MPa, 545.88MPa and 515.07MPa, respectively. The
NUMERICAL AND EXPERIMENTAL INVESTIGATION OF HYBRID MIG-TIG WELED STEEL JOINT Page iii
plastic deformation experienced by the welded plate for TIG, MIG and hybrid welding simulation is 0.82mm, 0.87mm, and 0.93mm, respectively. The hybrid welding simulation resulted in less welding distortion than the MIG welding simulation whilst providing similar structural integrity.