Reducing the environmental impact of manufacturing processes by means of natural based lubricants

Cold rolling and machining processes are significant in the manufacturing of products, as they are utilized to modify the product's geometry and to influence its mechanical properties. The process of cold rolling involves a reduction in the thickness of the strip below its recrystallization temperature, resulting in an improvement in surface quality and an enhancement in strength due to the work hardening. Friction and wear are intrinsic factors that contribute to increase the energy consumption and equipment degradation. While friction is a necessary component for the proper grip of the strip, it also affects the distribution of force and can accelerate the wear of the rolls. Therefore, it is essential to utilize lubricants to minimize excessive friction. On the other hand, machining is a process that involves the controlled removal of material using computer numerical control technology. The use of metalworking fluids plays a primary role as they reduce friction at the tool/workpiece interface, thereby enhancing dimensional accuracy, surface finish and reducing the required power. Effective lubrication also facilitates an efficient chip removal, thereby optimizing the overall machining process. Mineral-based oils persist as the dominant lubricant in engineering applications due to their accessibility, cost-effectiveness and compatibility with a range of materials. However, the extensive exploitation of mineral oils results in environmental concerns due to their non-biodegradable, soil-contaminating, rapidly depleting and volatile nature. Mineral oils have the potential to persist in ecosystems, thereby contaminating groundwater for up to a century and impacting both plant growth and aquatic life. A single liter of mineral oil has the potential to pollute up to one million liters of water, making it imperative to address these environmental impacts with urgency. In addition, in the period between 2010 and 2015, approximately 33000 tons of oil were released into the environment, with 700 tons of this being the result of major spill incidents. This has led to an intensification of global environmental efforts, driven by rising petroleum prices, depletion of oil reserves and increasingly strict regulations on the use of mineral oil. This study was conducted in collaboration with the Italian company Kimya and proposes the use of eco-friendly lubricants as a viable alternative to traditional formulations. The objective of the research is to evaluate the performance of these innovative lubricants from multiple perspectives with the aim of determining whether they can be considered a solution that is at least as effective as - or potentially more effective than - conventional lubricants. For the cold rolling process, two formulations of eco-friendly lubricants, derived from animal and vegetable sources, were examined and evaluated in comparison with a third commercial semi-synthetic formulation. The comparison was conducted both in the industrial environment by means of a test conducted on the two-stand reversing cold mill (RCM) at the Marcegaglia plant in Ravenna, Italy, and at the laboratory scale. The industrial test revealed that the use of the green formulations exhibited advantages in terms of reduced rolling forces, decreased electrical consumption and lower work roll's wear for a wide range of coils and conditions. Results were corroborated by tribological tests conducted using the CSM high-temperature pin-on-disk tribometer (which demonstrated a reduction in the coefficient of friction CoF of approximately 10%) and the Falex tribometer (which showed a 10% improvement in extreme pressure performance and a 15% reduction in the CoF). Furthermore, two novel methodologies based on inverse analysis were presented for the investigation of the friction condition at the roll/strip interface, utilizing experimental data obtained from the RCM. The first methodology integrates a plane strain analytical model of the process within an optimization procedure managed by the multi-objective genetic algorithm MOGA-II. The optimization loop was capable of assessing four different values of the CoF for each stand of the two passes of the RCM by minimizing the discrepancy between the experimental and analytical rolling forces. The obtained values served to confirm the capability of the green formulations to consistently minimize the contact conditions. With regard to the second methodology, the process was simulated using the commercial finite element (FE) software Abaqus/CAE. The objective was to determine the CoF values in the two stands of the first pass by minimizing the difference between the experimental and numerical values of the rolling forces and inter-stand tension. Specifically, accurate metamodels were trained on the calculated error values using interpolating algorithms and integrated into the optimization procedure managed by the MOGA-II genetic algorithm. The results demonstrated that variations in the CoF not only affect the contact pressures in the roll bite of the same stand but also affect the friction conditions in the other stand. As part of the environmental assessment, the treatment of the emulsion was investigated in two distinct scenarios: the first involved fresh emulsions, which were treated with a powder composed of bentonite and activated carbon, and the second involved waste emulsions from the RCM. For waste emulsions, two different treatment modalities were explored: the physical and the chemical treatment. The efficacy of the treatments was demonstrated by the low chemical oxygen demand (COD) and total surfactant content in the resulting aqueous phase, which permitted its reuse for the production of new emulsions with lubricating properties comparable to those of fresh emulsions. For the machining processes, the animal-based lubricant was comparatively investigated with a commercial semi-synthetic one in terms of environmental impact and lubricating/anti-wear performance. With regard to the former aspect, the values of non-ionic surfactants and COD in the aqueous phase obtained after the chemical breaking of the emulsions were evaluated. For the second aspect, the performance of the emulsions at different concentrations was evaluated in both laboratory scale (using the Falex tribometer) and in the industrial production (tool flank wear assessment during turning operations). The chemical breaking revealed that the aqueous phase of the natural emulsion, both fresh and after several months of use, exhibited minimal non-ionic surfactant and COD content. From the perspective of the machining processes, tribological and wear tests demonstrated that the natural emulsion consistently resulted in a reduction in tool wear compared to the semi-synthetic emulsion.