EXPERIMENTAL STUDY OF CMT WELDING ON ALUMINIUM ALLOY BASED HYBRID COMPOSITE

Abstract

This research presents a comprehensive investigation into the fabrication, characterization, and welding behavior of a novel aluminium metal matrix composite (AMMC) based on AA6061- T6 alloy, reinforced with micro-sized Titanium Diboride (TiB2) and nano-sized Lanthanum Oxide (La2O3). The composite is fabricated via the stir casting process, employing 2 wt.% TiB2 as the primary reinforcement and varying amounts (0.5 wt.%, 1.5 wt.%, and 2.5 wt.%) of La2O3 as the secondary reinforcement. TiB2 is selected because of its high hardness, thermal stability, and strengthening potential, while La2O3 is incorporated to explore its rare-earth-induced grain refinement and enhanced thermal and wear resistance properties. Microstructural and mechanical characterization is carried out to understand the influence of La2O3 addition on the composite’s performance. The combined presence of TiB2 and an optimal level of La2O3 (0.5 wt.%) significantly refined the grain structure and improved reinforcement dispersion, resulting in superior hardness (88.91 HV) and tensile strength (243.77 MPa), reflecting enhancements of 27.3% and 40.66%, respectively, over the base alloy. X-ray diffraction (XRD) and Energy-Dispersive Spectroscopy (EDS) confirmed the formation and homogeneous distribution of reinforcement phases. However, excessive La2O3 content led to agglomeration, reduced mechanical strength, and diminished wear resistance. Following composite fabrication, Cold Metal Transfer (CMT) welding, a low heat input fusion welding technique, is employed to join similar AMMC sheets. Prior to welding the AMMC sheets, a bead-on-plate experimental study is performed using ER4043 and ER5356 filler wires under two different sets of welding parameters to determine the most suitable wire for composite welding. Comparative analysis revealed that welds made with ER5356 exhibited finer equiaxed grains in the fusion zone and better columnar-to-equiaxed transition in the HAZ due to the formation of Mg2Si. XRD results confirmed the presence of Si-rich phases in ER4043 welds and Mg2Si in ER5356 welds. Welds made with ER5356 also showed improved mechanical performance, with a 20.37% increase in microhardness and an 8.82% improvement in tensile strength compared to ER4043. Porosity and dilution levels are also lower in ER5356 welds. After selecting the appropriate filler wire, CMT welding of the AMMC is carried out using a butt joint configuration. The influence of welding parameters-current, travel speed, and gas flow rate, on the weld joint strength is analyzed using the Response Surface Methodology (RSM) approach. A quadratic regression model is developed, and Analysis of Variance (ANOVA) revealed that welding current is the most influential parameter (61.31%), followed by travel speed (10.78%) and gas flow rate (0.89%). The optimal parameter combination-142 A current, V 9 mm/s travel speed, and 14 L/min gas flow rate-is identified through desirability analysis. Experimental results confirmed the successful application of CMT welding to the hybrid composite, with all welds displaying continuous and defect-free bead formation. Microstructural characterization using optical microscopy, FESEM, and EBSD showed significant grain refinement in the fusion zone (FZ) and the presence of columnar dendritic grains in the heat-affected zone (HAZ). XRD and EDS analyses confirmed the retention of TiB2 and La2O3 reinforcements, along with the formation of a few intermetallic compounds. Microhardness testing revealed the highest hardness in the FZ, whereas tensile strength decreased by 25.9% compared to the unwelded base composite, with the HAZ identified as the weakest region. Fractography of tensile specimens indicated predominantly brittle fracture characterized by cleavage facets, shallow dimples, and tear ridges. In the final phase of the study, the effect of post-weld thermal exposure at elevated temperatures (ranging from 50°C to 250°C) is examined. It is observed that tensile strength decreased while ductility improved with rising temperature. The specimen tested at 50°C recorded the highest tensile strength (165.92 MPa), whereas the one tested at 250°C exhibited the greatest elongation (11.76%). The reduction in strength is attributed to grain coarsening, reduced dislocation density, and weakening of strengthening mechanisms such as Orowan looping. Fractographic analysis confirmed a transition from brittle to ductile fracture modes with increased thermal exposure. SEM and EDS analyses further validated matrix softening, particle retention, and microstructural evolution across the weld zones. Microhardness profiles showed the highest values in the fusion zone and a steady decline with increasing test temperature, particularly in the HAZ. Overall, this research demonstrates the successful development and joining of a novel hybrid aluminium composite using CMT welding. The findings establish a solid foundation for the use of such hybrid composites in advanced applications across aerospace, automotive, and structural industries, where high strength-to-weight ratio, wear resistance, and weld integrity are critical.