STUDY OF THERMAL AND METALLURGICAL ASPECTS OF FRICTION STIR WELDED JOINTS OF DISSIMILAR METALS

Abstract

Multi-material Dissimilar friction stir welding (FSW) of magnesium AZ91D and aluminium AA6061-T6 poses significant challenges due to metallurgical incompatibility. Despite their potential in lightweight applications, comprehensive optimization of process parameters in SiC-reinforced AZ91D and AA6061-T6 FSW joints remains limited. The present investigation comprehensively examines the combined effects of process parameters and nanoparticle reinforcement on the mechanical and metallurgical behavior of dissimilar AZ91D/AA6061-T6 joints. SiC particles were introduced into pre-configured grooves, and experiments were carried out by varying volume fraction (Vf) (5%,10%,15%) of SiC, tool rotational speed (TRS) (600,700,800 rpm), and traverse speed (TS) (20,30,40 mm/min). This study investigates the influence of Tool Rotational Speed (TRS), Traverse Speed (TS), and SiC Volume fraction (Vf) on the mechanical and metallurgical properties of these joints using Central Composite Design (CCD) of Response Surface Methodology (RSM) to mathematically model FSW input parameters with key mechanical responses such as ultimate tensile strength (UTS), strain, and microhardness. The analysis of variance (ANOVA) approach identified critical parameters. It validated the model’s prediction with a 95% confidence interval (CI), yielding R² values of 0.9973 for Ultimate Tensile Strength (UTS), 0.9319 for % strain, and 0.9951 for microhardness, indicating excellent predictive capability. Optimization revealed that the optimal microhardness, strain, and UTS in the stir zone (SZ) were 88.44 HV0.1, 6.45%, and 114.56 MPa, respectively. Microstructural analysis revealed that the SiC nanoparticles significantly refined the grains in the stir zone (SZ). This refinement was primarily due to the pinning effect of nano-sized SiC particles, which restricted grain growth and facilitated dynamic recrystallization (DRX) during FSW, ultimately leading to a substantial reduction in grain size. Increasing SiC Vf from 5% to 15% enhanced friction stir-welded (FSWed) joints and exhibited improved mechanical characteristics. Among systematically designed experiments, the condition with 700 rpm, TRS 30 mm/min, and Volume fraction 15% achieved the highest UTS (114.98 MPa) and strain (6.67%) along with microhardness of 88.9 HV0.1, closely matching the model’s predicted optimum and vi confirming its accuracy and robustness across the design space. In contrast, the lowest UTS of 71.46 MPa, 32.63% joint efficiency, and 67.87 HV0.1 microhardness was recorded at 600 rpm, 40 mm/min, and 5% Vf SiC, mainly due to inadequate mixing and particle clustering, confirming the strengthening role of SiC. These findings illustrate the potential of SiC nanoparticle reinforcement in enhancing the mechanical properties of FSWed joints and demonstrate that the weld quality in AZ91D and AA6061-T6 joints strongly depends on parameter selection to ensure uniform particle distribution and defect-free welds. Complementing the experimental investigation, a three-dimensional transient thermal simulation was performed using ANSYS, considering heat generation from both frictional sliding and plastic deformation at the tool-workpiece interface. The model was validated using thermocouple measurements placed on both the advancing and retreating sides of the joint, showing a deviation of less than 5% between the simulated and experimental temperature histories. The simulation results indicated that heat input increased with tool rotational speed and decreased with traverse speed, directly correlating with the observed material flow, grain refinement, and IMC morphology. The optimal condition (700 rpm, 30 mm/min) yielded a balanced heat input, which is sufficient for full plasticization without melting, thereby explaining the superior mechanical performance and defect-free weld morphology. Overall, this integrated experimental-numerical-statistical framework establishes a comprehensive process-structure-property relationship for dissimilar AZ91D/AA6061- T6 FSW joints reinforced with SiC nanoparticles. The study not only validates the predictive capability of CCD-RSM for optimizing FSW parameters but also highlights the vital role of thermal input and nanoparticle dispersion in achieving defect-free, AZ91D/AA6061-T6 dissimilar joints. The outcomes provide a reliable foundation for extending nanoparticle-assisted FSW toward advanced lightweight hybrid structures in automotive and aerospace applications.