An Investigation into the Effect of Cutting Fluid Additives on Machining of Hard-to-Machine Metal

Lubrication and cooling are critical requirements for any machining operation of titanium alloys and are typically employed in great quantities regardless of their environmentally hazardous nature. This highlights the need for the increase in the employment of environmentally benign cutting fluids and environmentally benign additives, which requires an improved understanding of their tribological behavior and the fluids performance during machining operations. The present work aims to explore the application of environmentally benign polymeric and phosphate ester additives for the machining of titanium alloys. To accomplish this, two commercially available environmentally benign additive packages, a phosphorus-based and a polymer-based additive, were evaluated to gain an improved understanding of the performance of currently utilized environmentally benign additives during the drilling of a titanium alloy. Based on the understanding derived from these two commercially available additive packages, this study endeavored to examine additives that could improve the performance of the current cutting fluids additives and thus further increase the industry’s reliance on environmentally benign additives.

Drilling assessments of the two commercially available additive packages performed at a constant metal removal rate (MRR) of 4.2 mm³/s revealed that the drilling performance of the two additive packages depended on the drilling conditions. At lower spindle speeds combined with higher feed rates, the phosphorus-based additive displayed better drilling performance, while at higher spindle speeds and lower feed rates, the drilling performance of the polymer-based additive was preferable.

The tribological behaviour of the additive packages, examined using ball-on-disc experiments performed on Al-Mn and Ti-6Al-4V alloys, revealed the additive packages’ behavior was influenced by temperature. The examination of the polymer-based additive revealed that the carbon-rich tribolayer it formed to reduce friction possessed a relatively stable tribological behaviour with temperature increase. However, the tribolayer formed from phosphorus-based additive was sensitive to the increase in temperature and applied load. The friction-reduction behavior of the phosphorus-based additive package could be attributed to the activation of the phosphorus additive resulting in the formation of phosphorus-rich boundary layers.

The examination of the commercially available additive packages revealed issues with sustaining drilling conditions with high spindle speeds and low feed rates for both packages and low speeds at high feed rates for the polymer-based additives. Polymeric and phosphate ester additives were evaluated through drilling tests, to determine the possibility of their employment; environmentally benign cutting fluid additives revealed their drilling performance was also related to the drilling condition. The polymeric ester additives possess similar working mechanisms as the polymer-based additive. However, the phosphorus ester additives displayed two different working mechanisms which influenced their drilling performance.