Effect of ball nose flank wear on surface integrity in high-speed hard milling of AISI 4340 steel using MQL

Tool flank wear, owing to its direct interaction with the machined surface, can have detrimental effects on the workpiece surface integrity. This study investigates the impact of tool flank wear on surface integrity characteristics, particularly white layer thickness (WLT) and chemical corrosion resistance, during high-speed milling of AISI 4340 steel. Twenty-one experiments, ranging in 7 levels of flank wear widths (0-0.6 mm), were carried out under consistent cutting conditions in the presence of a minimum quantity lubrication (MQL) system. The results illustrate that up to a flank wear width of 0.4 mm, there is a modest increase in surface roughness, microhardness, and WLT. However, beyond this threshold, a significant escalation in these parameters is observed. Notably, a wear width of 0.6 mm induces non-uniform material flow, impacting microhardness up to 120 mm beneath the surface and causing a sudden increase in WLT. According to open-circuit potential analysis, the surface's tendency to electrochemical reactions increases slightly as the wear width increases up to 0.5 mm. The electrochemical impedance spectroscopy of the machined surfaces also revealed that utilizing tools worn to 0.4 and 0.6 mm, respectively, led to a decrease in R corr values by 35 % and 75 % compared to the specimen machined with a new tool. These insights underscore the critical importance of managing tool wear to maintain surface integrity in high-speed milling operations.