Tool wear mechanisms and failure modes in side milling of ultra-high strength steel under different sustainable cooling conditions

Ultra-high strength steels (UHSSs) are extensively utilized in the aerospace field because of their exceptional material properties. However, in green manufacturing processes, the issue of rapid tool wear hampers machining efficiency and raises production costs. The atomization modes of cutting fluids are considered a sustainable and effective method for cooling and lubrication, significantly reducing tool wear. The current paper carried out the comparative milling experiments on UHSSs to identify appropriate machining conditions, including dry machining, high-pressure air cooling (HPAC), air atomization of cutting fluid (AACF), and ultrasonic atomization of cutting fluid (UACF). The wear behavior of the coated carbide tool was investigated under various sustainable cooling conditions, focusing on tool life, tool wear types, wear mechanisms, and failure mode. Results showed that the lubrication ability in the milling process was more crucial than the cooling performance for improving the tool life. The UACF process extended the tool life by 19.2 % compared to AACF conditions. This was attributed to the fact that ultrasonic atomization enables the production of small droplet diameter, uniform droplet distribution, and high droplet number density, thereby enhancing the cooling and lubrication capabilities of the droplets. The predominant wear types on the coated tool encompassed abrasion, adhesion, coating detachment, and chipping. Additionally, the primary wear mechanisms observed on both the rake face and flank face were abrasive wear, adhesive wear, and oxidative wear regardless of cooling conditions. The failure mode of the coated carbide tool was attributed to the transgranular fracture of WC grains and ductile fracture of Co binder phase, leading to sudden tool breakage.