Design and Evaluation of Lightweight Aluminum-Titanium Matrix Composites via Friction Stir Processing

Thesis Abstract

Friction stir processed aluminum-titanium matrix composites (Al-Ti MMCs) offer a promising route to achieving high specific strength and improved damage tolerance for aerospace and automotive applications, yet their mechanical performance hinges on the homogeneous distribution of Ti particles and the suppression of brittle intermetallics at the matrix–reinforcement interface. The study aims to design, fabricate, and evaluate lightweight Al-Ti MMCs produced by friction stir processing (FSP), with the objective of maximizing strength-to-weight ratio, impact resistance, and thermal stability while ensuring manufacturability at scale. Specific objectives include (1) optimizing process parameters (tool rotation rate, traverse speed, plunge depth, and pin geometry) to minimize Ti clustering and interfacial porosity; (2) characterizing microstructural evolution, including grain refinement, Ti dispersoid distribution, and interfacial reaction products using EBSD, TEM, and XRD; (3) assessing mechanical performance through tensile, fatigue, and notch-impact tests at room and elevated temperatures; (4) evaluating tribological behavior under low- and high-load regimes; and (5) developing a predictive model linking process conditions to mechanical properties via response surface methodology (RSM) and regression analysis. The population comprises Al 7075 substrate reinforced with Ti particles (?5 wt%) prepared by mechanical alloying, with a sample matrix of 60 FSP passes across three processing windows to explore parameter space. Data collection employed a combination of microstructural characterisation (EBSD, SEM–EDS, TEM), phase analysis (XRD), mechanical testing (room-temperature and 200°C tensile tests following ASTM E8/E23, fatigue testing per ASTM E466, and Charpy impact testing per ASTM E23), and tribological assessment (pin-on-disk, ASTM G99). Validity and reliability were ensured through replication (n=6 per parameter set for mechanical tests), calibration of measurement instruments, and cross-validation of microstructural quantifications via multiple imaging modalities. Data analysis integrated descriptive statistics, analysis of variance (ANOVA) to identify significant process-property relationships, regression modeling with RSM to predict optimal FSP conditions, and Weibull analysis for failure probability. A theoretical framework combining materials-by-design and the Hall–Petch strengthening mechanism, complemented by the Orowan dispersion theory for particles, guides interpretation of grain refinement and dispersion effects, while the rule-of-mixtures informs density and specific strength assessments. Anticipated findings indicate that optimized FSP parameters yield ultrafine equiaxed grains (<2 µm) and homogeneous Ti particle distribution, reducing crack initiation sites and enhancing yield strength above 520 MPa with specific strength above 150 kN·m/kg for a 5–8 wt% Ti reinforcement at 25–200°C. Improved wear resistance and reduced coefficient of friction are expected due to refined microstructure and face-centered cubic Ti-rich interfacial phases, with fatigue life improvements of up to 2× under low-cycle loading. The study is expected to contribute to knowledge by (i) providing a robust, process-structure-property map for Al-Ti MMCs produced via FSP, (ii) identifying critical interfacial phases and their role in mechanical performance, (iii) validating a predictive model linking FSP parameters to performance metrics, and (iv) informing scalable manufacturing guidelines for aerospace-grade lightweight MMCs. The main conclusions are expected to highlight the viability of FSP as a scalable route to uniform Ti reinforcement in Al matrices, achieving a balanced envelope of strength, toughness, and wear resistance without excessive processing costs. Recommendations include adopting the optimized parameter set for industrial trials, extending the study to alternative Al alloys (e.g., 6061 and 7075 variants) and different Ti forms (powder vs. whiskers), and pursuing in-situ monitoring of temperature and material flow during FSP to further enhance process control and repeatability.