Abstract
Mild steel is one of the most commonly used structural materials in the metal industry due to its excellent weldability, good malleability, and adequate hardness properties. Most of the mild steel fabricated structures in engineering applications are joined by the techniques of welding such as the Metal Inert Gas (MIG) also known as Gas Metal Arc welding (GMAW) being the popular choice. MIG is a reliable and efficient arc-welding process, which joins metals by heating them with an electric arc created between a continuously fed consumable wire electrode and the metal workpiece. However, the ease of welding mild steel has been a major challenge due to the complex metallurgical transformations, metal deposition, inhomogeneous heating, and cooling rates experienced across weld zone and this often result in the compromise of the structural integrity of mild steel weld joints. Thus, some degree of reinforcements on mild steel weldments is required to improve the weld integrity. In recent times metal matrix composites (MMCs) have been used to enhance the properties of weldments. This research work explores the prospects of using pure titanium and its based alloys metallic micro powders as reinforcements in improving the structural integrity of AISI 1008 mild steel MIG weld joints. Three sets of butt configured weld joints were fabricated via MIG welding. The first set was fabricated without any reinforcement, while the second and third sets were respectively reinforced at the welding zone with pure commercial titanium (CP-Ti) and α-β grade Ti-6Al-2Sn-2Mo-2Cr-0.25Si alloy micropowders to locally form an MMCs at the weld line. A weld matrix consisting of twelve welds was produced by varying the welding voltage between 19 and 25volts and the welding current between 90 and 131 Amps. The welding speed was varied from 5mm to 3mm in increments of 0.5mm per the three fabricated sets. The shielding gas flow rate was kept constant. In order to study the influence of CP-Ti and α-β grade Ti-6Al-2Sn-2Mo-2Cr-0.25Si alloy reinforcements have on the nugget of AISI 1008 mild steel butt welds, all the MIG fabricated welds samples (reinforced and unreinforced) were characterized and compared through X-ray radiographic testing, microstructural evaluation, tensile testing, microhardness profiling, X-Ray Diffraction analysis, sliding dry wear testing and electrochemical corrosion testing. vi The X-ray radiographic and visual inspections conducted on the MIG formed samples revealed acceptable welds defects such as minimal spatter, lack of penetration, and lack of fusion. From the microstructural evaluation, acicular ferrite formation was found to be more prominent in titanium-reinforced samples, suggesting an Improvement in impact toughness. The diffractograms of the X-Ray Diffraction analysis showed peaks of Fe-Ti intermetallic and the presence of martensite grains in all the reinforced weld samples. The scanning Electron Microscope (SEM) in combination with EDS micrographs revealed a uniform distribution of CP-Ti and α-β grade Ti-6Al-2Sn-2Mo-2Cr-0.25Si alloy microparticles were well-bonded to the matrix alloy in the welding zone. Experimental results showed that incorporating the of CP-Ti and α-β grade Ti-6Al-2Sn-2Mo-2Cr-0.25Si alloy microparticles into the welding zone led to high Vickers microhardness values of the composite welded joints as compared to the unreinforced welded joints, and this was attributed to the presence of the iron-titanium (Fe-Ti) intermetallic compounds and the formation of martensite grains in the reinforced weld samples. There was no significant improvement in the ultimate tensile strength (UTS) as a result of titanium reinforcements, as all the UTS values were obtained in the same range of between (391.63 MPa to 418 MPa)...
M.Eng. (Mechanical Engineering Science)