The effect of the cutting edge’s surface chemical composition on stabilizing the acoustic emission represented conditions of the cutting’s transient phase during drilling with a twist drill

The transient phase of cutting is the term used to describe the conditions during which the material processing by the cutting edge transitions from ploughing-dominated deformation to shear-dominated, stable chip formation. The transient phase occurs when the cutting edge works at an uncut chip thickness of the scale, which is comparable to or below that of the cutting edge radius, a condition that is typically met when the cutting edge enters and leaves the material. Since changes in material processing affect the energetics of cutting, tool life, and the resulting surface quality, the transient phase of cutting is unignorable in cutting process planning and the tool’s cutting ability. One of the most conventional process indicators for identifying the transient phase is the force of cutting. In drilling, however, the force is dominated by a large thrust component originating at the chisel edge, and the process is significantly affected by torsional vibrations, which obscure changes during tool entry into and exit from the material. Acoustic emission (AE) is widely used in machining research because it sensitively indicates changes in deformation and friction in the cutting zone. Thus, AE has become an effective asset for detecting and analyzing the chip formation mechanism and the tool–workpiece interactions. Furthermore, the root-mean-square value of the acoustic emission signal ( $$\:{AE}_{RMS}$$ ) represents the overall energy content of the AE signal within a given time interval, and correlates to the cutting energy consumed by the material processing. The aim of the current research is to identify the nature of and the conditions affecting the transient phase of cutting during drilling with a twist drill before any significant tool wear appears. The present study introduces drilling experiments performed on S235 structural steel under flooded cooling, with AE signals and the thrust force component (Fz) recorded simultaneously. The $$\:{AE}_{RMS}$$ signal exhibits well-distinguishable peaks during the tools’ entry and exit, which coincides with the expected induction of the transient phase of cutting. However, as the number of drilled holes increased, the magnitude of the $$\:{AE}_{RMS}$$ peaks gradually decreased and eventually stabilized at a nearly constant level. This implies that the energy consumption of the transition from ploughing-dominated material deformation to shear-dominated cutting during the transient phase has stabilized. Scanning Electron Microscope (SEM) and Energy Dispersive X-Ray Spectroscopy (EDS) investigations revealed that the chemical composition on the tool’s surface, which has been altered as a result of the tool and the workpiece being in physical contact during the cutting process, has an impact on stabilizing the $$\:{AE}_{RMS}$$ peaks during the transient phase of cutting.