Sustainable Drilling Strategies for Rivet Hole Formation in Nickel-Based Alloys for Aeronautical Applications

The formation of rivet holes is a critical step in aeronautical assembly, as hole quality directly influences the fatigue resistance and structural reliability of riveted joints. Nickel-based alloys, such as Inconel 625, present additional challenges due to their poor machinability and the stringent surface integrity requirements imposed by the aerospace sector. This study investigates innovative and sustainable drilling strategies for rivet hole preparation, focusing on the comparative performance of two environmentally friendly cooling and lubrication methods: minimum quantity lubrication with an eco-friendly fluid (MQL-Eco) and cold compressed air (CCA). A comprehensive experimental campaign was carried out to analyze the combined effects of spindle speed, S, feed rate, f, and cooling method, R, on hole surface roughness parameters (Ra and Rz). These values are measured inside the drilled hole using optical scanner 3D equipment. Statistical tools, including analysis of variance (ANOVA) and response surface methodology (RSM), were employed to identify the most significant factors and optimize cutting conditions. The results reveal that the interaction between spindle speed and coolant type is the dominant contributor to surface roughness variability, with MQL-Eco consistently achieving values within the aeronautical standard range (Ra = 0.8–1.6 µm), and the coolant factor is the second cause of variability in both roughness Ra and Rz. Moreover, correlations between roughness parameters and tool wear confirm the relevance of sustainable cooling methods in extending tool life while maintaining compliance with aerospace quality requirements. The findings demonstrate that innovative eco-friendly drilling approaches can effectively replace conventional lubrication, offering a viable pathway towards greener manufacturing practices in metal forming and joining technologies.