On modelling the influence of thermo-mechanical behavior in chip formation during hard turning o f 100Cr6 bearing steel G.Poulachonl, A.Moisan' (I), I.S. Jawahir' (1) LaBoMaP, ENSAM, 71250 Cluny, France CRMS, University of Kentucky, Lexington, KY. 40506-01 08, U.S.A. 1 2 Abstract This paper deals with turning of hardened alloy steels (up to HV800). First, the research focuses on the evaluation of flow stress in machining of 100Cr6 (AISI 52100) bearing steel. A material constitutive law including work hardening, thermal softening, and strain-rate sensitivity has been looked for. The work hardening effect has been determined with the help of quasi-static compression tests performed on standard test specimens. The dynamic compression tests performed at high strain-rates'' using the Hopkinson bars, have shown no tendency to viscosity effects. Hot compression tests show that thermal softening plays a significant role in the the process feasibility. Cutting tests performed under various cutting conditions have highlighted the conflicting work hardening-thermal softening processes. This balance is discussed with a shear instability criterion, presenti ng the work hardeni ng to thermal softening ratio through a revised material behavior law. Keywords: Hard turning, Work hardening, Thermal softeni ng 1 INTRODUCTION Machining is not an easy process to study and to model due to the inherent difficulty to know exactly what happens in the region around the tool tip. The two basic requirements for effective and accurate modelling of the metal m achining processes are: (i) knowledge on the flow stress effects in the primary and secondary deformation zones, and (ii) quantitative understanding of the friction laws at the tool-chip and tool-workpiece interfaces. In this paper, only the fi rst requir ement will be discussed, namely how to define and model the flow stress. The definition and the range of validity of a constitutive law are fairly well known in the field of metal forming and therefore is reasonably well applied in industry. Several commercial software codes have been developed in recent years. Although very limited in machining, many of the numerical codes utilize constitutive lawshelationships, which describe with sufficient accuracy the thermo- mechanical behavior of the material machined. Usually, the constitutive law is a functional relationship among the current local values of stress, strain, strain-rate, and temperature, and is of the form: f E, i ) The determination of an appropriate constitutive relation for a given material is often a semi-empirical process, involving both a theoretical understanding of the physical mechanism governing plastic deformation and the application of experimental data. Such data are commonly derived from standard specimen tests. However, in the case of machining, the combination of large strains (2-5), high strain-rates (105-106 s-'), high cutting temperatures is very difficult to achieve from conventional tests. The results of a good cutting model will depend essentially on the accuracy of the constitutive law rather than the sophistication of the numerical code. While recent attempts on FEM simulations of machining, including tool- chip contact friction and material behavior at large strains, strain-rates and temperatures, are directed towards the reality of the cutting process [I-91, it is however necessary to validate the chosen constitutive relation by comparing the predicted behavior by its use with that observed experimentally. Most notable constituti ve relationships are proposed by Johnson and Cook [l o], Zerilli and Armstrong [ I l l and Harding [12]. The aim of the present paper is to discuss the major issues and difficulty in developing a suitable constitutive law for difficult to machine hardened steels and to present a new method and promising directions for modelling the material behavior in hard turning. 2 EVALUA TION OF CONSTITUTIVE RELATIONSHIPS 2.1 State-of-the-art Researchers attempting to validate constitutive relations at large strains, strain-rates and, temperatures often use FEM simulations, incorporing the proposed constitutive relation, to predict cutting forces, cutting temperatures, stresses, etc. in chip formation. Generally consistent and experimentally verified results of FEM simulations are in direct relationship with the constitutive law. Much research has been done exploring ways to establish flow stress models. Johnson and Cook's law [lo] - a thermo-visco- plastic-hardening law - is among the most recognized tools in the field of machining because it represents a simple description of the deformation behavior for metallic materials and in consequence is a more robust behavior law. M aekawa and Shirakashi [13-141 defined an empirical expressi on of the flow stress by performing an incremental strain method with the split Hopkinson bar technique. This more complex expression takes into account the history effects of strain-rate and temperature and also the effects of blue-brittleness. They found that up to A1 temperature, no age hardening and anneal softening effects took place within 90 of heating. Accordingly, it is imperative that the flow stress depends not only on strain hardening, strain-rate and temperature, but also on the material microstructure such as the composition, the phases, the grain size, etc. Zerilli and Armstrong [I ] and Goldthorpe and Church [I51 proposed
