Abstract: The cooling strategy plays an important role during the machining of difficult-to-cut materials, such as titanium alloys. In this work, an oils on water (OoW) cooling was investigated with coated carbide end mills with AlCrN/AlTiSiN coatings during the Ti-6Al-4V alloy machining. Through various amounts of water (1.5, 3 and 4.5 L/h), oil (20, 30, 50 mL/h) and lubricant types (1000-20 and 2000-30 synthetic ester, 2000-10 fatty alcohol), tool wear, cutting force, cutting temperature, the surface roughness and chip morphologies were observed under the OoW cooling condition. The cutting performances were compared for all conditions, where a low amount of water (1.5 L/h) demonstrated significant improvement in the cutting performances of the coated tools, while the lubricants and amount of oil had low effects. The effect of OoW cooling on the cutting performance was also studied in comparison with dry cutting, flood cutting and minimal quantity lubrication with cryogenic air (CAMQL) cooling. For the optimized water amount at 1.5 L/h, the lower tool wear and surface roughness of the machined surface were observed. The wear rate of the coated tool through OoW cooling was also significantly slower compared to dry cutting, flood cutting and CAMQL cooling. The reason for improvement under the OoW cooling condition originated from the improved cooling effect compared to other cooling strategies, due to the excellent combined cooling effect of water and oil lubrication through OoW cooling. In addition, the chip morphology was also observed to be affected by both the cooling strategy and cooling parameters.

Key words: oils on water, lubrication mechanism, tool wear, Ti-6Al-4V alloy