Impact of supercritical carbon dioxide cooling with Minimum Quantity Lubrication on tool wear and surface integrity in the milling of AISI 304L stainless steel

In this study, the effect of supercritical carbon dioxide cooling with Minimum Quantity Lubrication (scCO2+MQL) on tool wear and surface integrity of AISI 304 L austenitic stainless steel in milling was investigated. A series of machining experiments based on a Design of Experiments (DoE) was carried out at various combinations of cutting parameters to investigate the effect of cutting speed and feed rate on tool wear, near-surface residual stresses, surface roughness and microhardness. The results were compared with the experimental results obtained from milling with flood coolant. A significant improvement in tool life was observed in milling with the scCO2+MQL using multilayer coated tungsten carbide inserts. The tool life in terms of cutting time increased by ∼324%, in comparison to a baseline flood coolant. Further, a decrease in surface roughness value (Ra) by about 30%, from 1.09 µm for flood coolant to 0.78 µm after face milling with scCO2+MQL was seen. Additionally, the Ra value slightly increased after machining, for both cooling methods with the increase of cutting speed of ∼19%. The observed changes in Ra value were discussed in terms of a built-up-edge (BUE) formation. There were no apparent differences in surface microhardness between both cooling methods. However, the surface microhardness increased with feed rate after milling with both scCO2+MQL and flood coolant due to the increased strain hardening. Also, there was no significant difference in residual stresses after milling, neither with scCO2+MQL nor the flood coolant. The surface residual stress values obtained in the transverse and longitudinal directions were consistent with a predictive model with errors of around 3–8%.