Mechanical Parameters and Microstructural Evolution of FDM-Printed PLA and PLA+CF Under Variable Infill Architecture and Lubricant Exposure

This study examines the influence of internal infill geometry, infill density, and short-term mineral oil exposure on the tensile and microstructural behavior of Fused Deposition Modeling (FDM) 3D-printed Polylactic Acid (PLA) and Carbon-Fiber-Reinforced PLA (PLA+CF). Standardized ISO 527-2 specimens were fabricated using linear, triangular, and hexagonal infill patterns at 30%, 60%, and 100% densities, followed by seven-day immersion in mineral oil. Mechanical testing and quantitative optical image analysis were performed to correlate porosity characteristics with tensile response. For PLA, the linear 30% infill achieved the highest tensile strength (31.5 MPa), while the hexagonal pattern exhibited the greatest ductility (ε = 4.9%). Oil exposure caused slight reductions in strength (-1.2%) and modulus (-4.1%) but increased elongation by 76%, indicating mild matrix plasticization. For PLA+CF, tensile strength and stiffness increased with density, reaching 33.4 MPa and 500 MPa at 100% infill, while oil exposure enhanced strength by 6.9% and reduced the average pore size from 475 µm² to 146 µm². Overall, the results demonstrate that optimizing infill topology, density, and fiber reinforcement significantly improves load transfer efficiency and environmental stability. These findings establish quantitative correlations between pore morphology and tensile behavior, providing a framework for the predictive design of environmentally resilient FDM polymer-composite components for semi-lubricated or tribological applications.