Multi-objective optimization of friction stir additively manufactured Al-7075 multi-layered laminates

Friction Stir Additive Manufacturing (FSAM) is an advanced technique for non-fusion aluminum alloys, enabling the production of large, irregular-shaped parts without the typical solidification defects associated with melting. FSAM relies on a critical interplay of parameters that significantly influence the microstructure and mechanical properties of the final fabricated structure. This study focuses on developing seven-layered Al-7075 laminates following the Taguchi L9 matrix array under forced cooling. Three input parameters, namely tool stirring speed (TSS), tool welding speed (TWS), and the number of tool passes (NPs) were selected for multi-objective optimization using Taguchi Grey Relational Analysis (GRA) for higher microhardness, UTS, and % elongation. Microstructural analysis revealed fine equiaxed grains within the stir zone, indicating a substantial 95.5% reduction in grain size compared to the base metal, along with a mixed ductile-brittle fracture in samples with higher UTS. Taguchi single-response optimization identified optimal settings in sample L6 (800 rpm, 90 mm/ min, and 1 NP) for increased hardness and reduced % elongation. Conversely, sample L7 (1000 rpm, 30 mm/min, and 3 NPs) exhibited the lowest hardness and UTS, along with the highest % elongation due to the high density of precipitate dissolution. Notably, sample L9 (1000 rpm, 90 mm/min, and 2 NPs) achieved the highest UTS of 415 MPa as a result of an even distribution of precipitates with low dissolution density, representing 74.3% of the base metal's strength. TWS emerged as the predominant input parameter, contributing 56.94%, 52.33%, and 47.33% to hardness, UTS, and % elongation, respectively. Moreover, Taguchi-GRA identified the optimal parameter set as TSS of 1000 rpm, TWS of 60 mm/min, and 2 NPs for higher hardness, UTS, and % elongation. ANOVA analysis of GRA demonstrated robust predictive capabilities with an R-squared value of 88.09%.