Investigating the interactions of metalworking fluids and its additive with aerospace alloys

The global manufacturing landscape is experiencing unprecedented demands, driven by expanding industries and growing world population. The aerospace sector, in particular, has seen exponential growth and is projected to continue this upward flight. This growth highlights the need for advanced manufacturing techniques that underscore sustainability, precision, quality, and efficiency. At the core of aerospace manufacturing lies machining, a process vital to produce complex geometries with high quality finishes and minimal tolerances across an array of materials. However, machining aerospace alloys stages considerable challenges, such as low thermal conductivity, high chemical reactivity, elevated cutting temperatures, and pressures. These factors often lead to reduced tool life and poor surface finish, therefore compromising the machining processes. In response to these challenges, metalworking fluids serve as an integral component in machining processes, offering cooling, lubricating and chip flushing capabilities in harsh conditions.
Validating the compatibility of a metalworking fluid and a machining operation occurs at the testing and development stage. This stage consists of tribological benchtop tests, which provide little-to-no correlation with full-scale production trials. This disparity creates a critical knowledge gap in machining tribology.
This project aims to address this critical knowledge gap by developing a novel, tribological benchtop test that mimics metal cutting, providing a more representative assessment of metalworking fluid performance in benchtop conditions. Furthermore, this research adds a novel dimension by investigating fundamental fluid behaviour through rheological and wettability tests. The goal is to establish a multi-level testing methodology that increases in complexity, progressing from fundamental benchtop tests to actual machining tests. This comprehensive approach will facilitate an investigation of the chemical interactions and fluid behaviours of metalworking fluids across different testing environments.
To achieve this, a multi-level testing methodology was developed using existing standardised industry test methods (Reichert test and tapping torque test), a well-known tribological test method (MTM-SLIM test), and custom-made machining tests for this methodology (Brüker UMT cutting test and desktop milling test). These tests formed an increasing complexity testing methodology which would assess the performance of three cutting fluids – fluid 1 (naphthenic basestock fluid), fluid 2 (naphthenic basestock + phosphate polymer additive package fluid), and fluid 3 (commercial aerospace fluid).
Industry standardised testing methods showed conflicting results due to the use of improper contact geometries and materials, while conditions and parameters were predetermined. The use of additives yielded a negative response in the tapping torque, with higher torque values. Whereas the Reichert test produced results favouring the use of additives, producing higher load-carrying capacities, reduced wear, and quicker noise reduction.
The MTM-SLIM test results also favoured the addition of additives reducing the overall friction coefficient. This was supported by tribofilm imaging from the SLIM attachment and quantitative tribofilm thickness measurements. Fluid 2 provided superior film thickness compared to other fluids.
The novel Brüker UMT cutting test produced differentiation between friction forces for each cutting fluid, deeming fluid 3 as the better cutting fluid. The test setup featured features from machining operations, such as using a cutting tool, using the same workpiece materials, using the same cutting fluids, and the same fluid application method.
The desktop milling test was used to validate the Brüker UMT cutting test using the exact same materials, cutting tools, cutting fluids, and fluid application method. Fluid 2 displayed superior performance across all aerospace alloys. This highlighted the extreme conditions in machining, which are hard to replicate in a benchtop environment.
This project demonstrates the development of a multi-level testing methodology with increasing complexity, to provide a comprehensive assessment of cutting fluid performance. The results are used to create a fluid performance indicator, which describes fluid rankings in each test setting, to be utilised in accelerating product development further.