FABRICATION, CHARACTERIZATIONS AND OPTIMIZATION OF FUNCTIONALLY GRADED MATERIAL FOR LEAF SPRING PLATE

Abstract

The present research investigates the development, characterization, modelling, and optimization of a functionally graded Al7075/B₄C composite leaf-spring plate for lightweight and high-performance automotive suspension systems. To overcome the drawbacks of conventional steel and homogeneous MMC leaf springs, such as excessive weight, poor fatigue resistance, and non-uniform stress distribution, a five- layer reinforcement gradation (16/12/8/12/16 wt% B₄C) was strategically designed to match the bending-stress profile of the leaf plate. A hybrid stir-casting and gravity die-casting approach, supported by K2TiF6 as a wetting agent, enabled uniform particle incorporation, strong interlayer metallurgical bonding, and accurate layer- wise gradation during sequential semi-solid pouring. This fabrication route proved cost-effective, scalable, and suitable for large plate-type FGM components. Detailed microstructural characterization through FESEM, EDS, and XRD confirmed the intended gradation, uniform particle dispersion, defect-free interfaces, and the absence of undesirable reaction products. Post-fabrication T6 heat treatment further enhanced matrix strengthening and particle–matrix adhesion. Mechanical and dynamic tests revealed significant improvements in hardness, tensile strength, and storage modulus in layers with higher B₄C content, while the ductile mid-layer effectively maintained impact resistance. Dynamic Mechanical Analysis demonstrated enhanced damping performance, indicating suitability for vibration- sensitive automotive environments. vi A comprehensive finite element analysis (FEA) was conducted to evaluate stress distribution, von-Mises stress variation, and deflection behaviour under realistic suspension loading conditions. The functionally graded plate exhibited reduced peak stresses, smoother stress transition across layers, and significantly lower central deflection compared with homogeneous MMC designs. Six different gradation configurations were analysed, and strong agreement between experimental and numerical results validated the modelling strategy and the mechanical behaviour of the graded system. Optimization results identified the 16/12/8/12/16 wt% B₄C configuration as the most effective design, providing an optimal balance between stiffness, ductility, stress redistribution, load-bearing capacity, damping response, and structural reliability. This configuration demonstrated the best stiffness-to-weight performance and minimal deflection, making it ideally suited for heavy-duty suspension applications. Overall, this thesis establishes a complete and integrated framework, from material selection, fabrication, and microstructural analysis to mechanical evaluation, computational modelling, and optimization-for designing functionally graded aluminium composite leaf springs. The research demonstrates that FGM architecture, combined with an economical stir-casting route, can significantly enhance the performance, durability, and energy efficiency of suspension components. The work contributes a scientifically validated and industry-ready pathway for next-generation lightweight automotive systems, supporting the broader goals of sustainable engineering, improved fuel economy, and indigenous advanced-material development.