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International audience; In the present research work, mechanical aspect of transient state in drilling operation is investigated by analyzing together thecutting forces and the chip formation. Particularly, a sudden peak occurring on cutting forces during transient state is observedand deeply studied. To do so, new experimental methodologies to analyze local cutting forces when drilling heat-resistantaustenitic stainless steel are introduced. The chisel edge and the cutting edge of each studied drill are numerically discretizedas series of elementary cutting edges. Therefore, local cutting force evolution is correlated with the local cutting geometryvariation along the cutting edge. On one hand, results have shown that for high ductile materials, local cutting geometry affectsdeeply not only the chip shape but also the chip flow direction. This strongly disrupts the monotony of the cutting forcesevolution in transient state and lead to a sudden peak occurrence on cutting forces. On the other hand, linear local cutting forcescould be determined by numerical decomposition of the global cutting forces measured during the drill tip engagement.

The aim of the investigations presented in this research work is to optimize the cutting geometry for drilling of Heat Resistant Austenitic Stainless Steel (HRASS) based on the tool life and strain hardening generated on the work material during cutting process. In particular, an attempt has been made to understand the effect of cutting edge preparation on both the cutting forces and the strain hardening of the work material. Besides, chips in flutes are also analyzed by tomography technique to better understand their evacuation in-situ in correlation with the cutting forces. A Quick-Stop Device (QSD) in drilling is also employed to analyze the influence of the cutting edge radius, the coating material, and the cutting fluid nature on the strain hardened layer using micro hardness filiations. Les travaux menés s’inscrivent dans le cadre du laboratoire commun ANR LUMIERE entre le LaBoMaP et Saint Jean Tooling

This study presents a novel analysis of the machined subsurface layer formation dealing with strain hardening phenomenon which results from complex mechanisms due to cutting edge multiple passes in drilling. On the one hand, the hardened layer during drilling is characterized in relation with the local cutting geometry and thanks to a quick-stop device (QSD) to suddenly interrupt the operation. Micro hardness is used to determine the hardened thickness of the machined subsurface layers along the local cutting edge geometry. On the other hand, orthogonal cutting performed with a complex self-designed planing experiment is used to investigate in details the hardening accumulation aspects. Then, dedicated methodologies are proposed to quantify the strain hardening as well as the incremental plastic strain generated by consecutive tool passes. In addition to the subsurface hardness evolution, the work material strain is observed during the steady-state cutting process thanks to the high-speed camera. The digital image correlation technique is exploited to analyze not only the plastic strain remaining on the workpiece after the cut but also the effect of the incremental plastic strain generated by the consecutive planing passes as the cutting edges in drilling do. One of the outcomes is that the hardened layer thickness can reach from two to three times the cut thickness in drilling or in planing. As a consequence, this work demonstrates that the cutting process affects itself by hardening. Thus, the studied austenitic stainless steel in such a way that this last is never cut in its initial state.

International audience; Machining of difficult-to-cut-materials like heat resistant stainless steels leads to the rapid tool wear and tool failure. The analysis of drilling process of these steels is even more difficult as it is not easy to investigate the cutting process at the drill tip. Therefore for this critical operation, modelling of cutting forces is very important. The study of the mechanical loading is based on the cutting tool geometry variation along the cutting edge. This study aims to investigate the drilling of heat resistant stainless steels with two geometries of twist drills, by analyzing specific cutting energy and forces at a global and a local scale.