QUANTITATIVE UNDERSTANDING OF THE EFFECT OF SOLID-PHASE PROCESSING TECHNIQUES ON MICROSTRUCTURE, MECHANICAL PROPERTIES, AND CORROSION BEHAVIOR OF LIGHTWEIGHT ALLOYS

Swapnil Sawalkar

The present manufacturing environment for high-performance structural alloys is undergoing a significant shift towards sustainability. Conventional melt-based processing methods are fundamentally energy-consuming and frequently result in solidification defects that hinder the performance of materials in critical industries such as aerospace, defense, and nuclear energy. To tackle these issues, Solid-Phase Processing (SPP) has emerged as a groundbreaking manufacturing approach. SPP techniques shape, join, or alter materials' properties entirely below their melting points by employing high strain, shear, and frictional heat to facilitate microstructural changes. Although the energy efficiency and mechanical advantages of SPP are acknowledged, a considerable knowledge gap persists regarding how these heavily deformed, non-equilibrium microstructures affect long-term electrochemical stability (corrosion) and mechanical reliability in extreme conditions. This thesis aims to bridge this gap by exploring how specific deformation paths influence the microstructure and subsequent electrochemical stability of advanced aluminum and magnesium alloys. Thesis contains a study on three solid-phase processing techniques, namely friction extrusion, hot isostatic pressing, and constrained groove uniform pressing.The initial solid phase processing method examined was friction extrusion, which allowed for the extrusion of AZ31 Mg alloy at speeds reaching 4.8 m/min, 10 m/min, and 12.9 m/min. By carefully controlling the thermomechanical history during the extrusion process, I was able to achieve high-speed processing while minimizing hot shortness and degradation of surface integrity. The microstructures produced showed notable grain refinement and a decrease in yield asymmetry, which can be directly linked to the severe plastic deformation pathways that modified the crystallographic texture of the magnesium rods. A thorough EBSD analysis was conducted to connect these microstructural alterations with improved mechanical yield symmetry and enhanced corrosion resistance, demonstrating that high strain rate processing can effectively optimize both structural and electrochemical characteristics. Further investigations concentrated on the dependability of Al6061 cladding for nuclear fuel plates, particularly examining the effects of Hot Isostatic Pressing (HIP) and boehmite coatings. The Hot Isostatic Pressing (HIP) process for the Al6061 cladding was performed under conditions of high temperature and pressure, specifically at 580 °C and 100 MPa for a period of 90 minutes. This thermo-mechanical cycle was essential for the bonding of the co-rolled U-10Mo/zirconium fuel foil within the aluminum sheets, yet it also caused considerable microstructural changes. Subsequently, the boehmite coating was developed. The elevated thermal conditions of the HIP cycle led to a noticeable decrease in mechanical strength, mainly due to the dissolution of fine Mg2Si strengthening precipitates. In the T6-tempered condition, these needle-like precipitates offer substantial resistance to dislocation movement through peak-aging. However, the HIP temperature of 580 °C surpasses the solvus for these phases, resulting in their dissolution back into the solid solution or significant coarsening, which fundamentally reduces the yield strength of the cladding. This microstructural change also resulted in heightened corrosion vulnerability. The HIP process facilitated grain boundary migration and modified the distribution of secondary phases, enhancing the micro-galvanic coupling effect between the aluminum matrix and the noble intermetallic particles. This increased potential difference at the micro-scale expedited localized pitting and intergranular attack. The introduction of a boehmite coating alleviated these issues by greatly enhancing the thickness of the passive layer. This dense, stable layer served as a superior physical and chemical barrier compared to the naturally occurring thin oxide film. In the third technique, the issue of microstructural inhomogeneity that is typical in traditional sheet metal processing was tackled through the creation and patenting of an innovative die system for Constrained Groove Uniform Pressing (CGUP). This unique severe plastic deformation (SPD) method was designed to apply consistent, repetitive shear and compressive strain to copper and aluminum sheets, promoting an even distribution of refined grains. The microstructural changes during the CGUP process were marked by significant subgrain formation and a substantial buildup of lattice defects. By the 30-pass stage, the dislocation density had increased by about 14 times. This strain accumulation compelled dislocations to self-organize into a dense network of low-angle boundaries, which was evident in EBSD orientation maps as a transition from a mottled appearance at 20 passes to a distinctly cellular mesh at 30 passes. From a mechanical perspective, the process led to considerable enhancements in properties; the tensile strength increased by 2.5 times, and the hardness nearly doubled after the final processing stages. These enhancements were mainly due to strain hardening and the high density of subgrain boundaries that obstructed dislocation movement. Furthermore, the material developed a weak octahedral texture, indicating a transition from a random orientation to a specific deformation-induced preferred orientation. It was noted that the high-energy reactive sites generated by the increased dislocation density were counterbalanced by the formation of corrosion-resistant (111) plane orientations. As a result, the processed material demonstrated a stable electrochemical response.

QUANTITATIVE UNDERSTANDING OF THE EFFECT OF SOLID-PHASE PROCESSING TECHNIQUES ON MICROSTRUCTURE, MECHANICAL PROPERTIES, AND CORROSION BEHAVIOR OF LIGHTWEIGHT ALLOYS

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