Abstract
Despite the ongoing progress in metallurgical characterisation of machined surfaces, knowledge of the thermal conditions under which they originate during the workpiece-flank interaction is still lacking. When cutting advanced superalloys, little is known about temperature evolution in the machined part volume, where workpiece material interacts with the tool flank. In this work, the characteristics of the thermal field and the resulting surface metallurgy induced in a next generation nickel-base superalloy have been studied for cutting scenarios involving different combinations of thermo-mechanical boundary conditions. Analysis of the thermal field evolution in the workpiece subsurface has allowed the heating and cooling rates induced by cutting to be revealed, allowing description of two distinct types of thermal cycle, with a Heating-Peaking-Cooling (H–P–C) and a Heating-Quasi-isothermal Deformation-Cooling (HQC) structure depending on the process aggressiveness. Subsurface thermal history has been found to relate with the severity of the cutting-induced deformation, as it combines information on thermal field magnitude and on the process rates. Furthermore, thermal balance equations have been applied to study the rate of the heat generation in the machined subsurface due to its own plastic deformation while interacting with the tool flank. This has revealed that the highest rate of heat generation induced by plastic deformation occurred in thin surface layers at the beginning of the workpiece-flank contact, which has been associated to the conditions under which white layers (WLs) are generated. Energy balance analysis has furthermore indicated the development of a less severe and less impulsive deformation process at higher subsurface depths, which has been linked to the formation mechanism of material drag (MD) layers. In this way, the thermal history of machined surfaces has been related to their resulting metallurgical integrity, allowing in-depth understanding of the physical conditions developing when cutting next-generation superalloys.
Highlights
Understanding the mechanisms of microstructural modification induced by machining processes to high performance alloys represents a fundamental research topic with high relevance from both an academic and industrial perspective
International Journal of Machine Tools and Manufacture xxx (xxxx) xxx cycles and heat generation in TSZ. This will allow the thermal history induced by cutting to be linked to the formation mechanisms of microstructurally deformed layers (MD and white layers (WLs)) in the workpiece subsurface as a result of the tool-workpiece mechanical interaction induced in the tertiary shear zone (TSZ)
A representative temperature map at the tool-workpiece interface can be observed in Fig. 5a, where an intermediate thermal field severity was generated by means of cemented carbide (CC) insert employed at moderate surface speed (Test #1, VC = 30, f = 0.1)
Summary
Understanding the mechanisms of microstructural modification induced by machining processes to high performance alloys represents a fundamental research topic with high relevance from both an academic and industrial perspective. International Journal of Machine Tools & Manufacture, interface, where high temperature and strain locally develop at extreme rates [3]. These are: (i) primary shear zone (PSZ), constituted by fast shearing metal volumes transitioning from the workpiece material to the forming chip; (ii) secondary shear zone (SSZ), developing by friction at the tool-chip contact region; (iii) tertiary shear zone (TSZ), where plastic deformation occurs due to the interference of the workpiece new (machined) surface with the tool flank
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