Papers by Author: Domenico Umbrello

Abstract: Cryogenic cooling is a new emerging cooling application in machining processes. Quantitative understanding of the effects of cryogenic cooling on the machining performance is important for continued applications. This study focuses on cryogenic machining of hard-to-machine material, AISI 52100, particularly with an analysis of cooling-induced chip morphology, chip hardness and the effect of workpiece hardness, etc., as these measures reflect the material`s thermo-mechanical behavior during the plastic deformation. AISI 52100 steel, with different initial hardness values, is selected as the work material for orthogonal cutting under dry and cryogenic cooling conditions, and the results are compared. The findings of this study show that cryogenic cooling affects the chip formation process, and the associated hardness produced on the machined surface.

Abstract: The phenomenological models for material flow stress and fracture, typically used in the Finite Element simulations of Inconel 718 alloy during machining processes, are often deemed to represent only certain metallurgical material states. In contrast, these models are not suitable to describe the constitutive behaviour of the workpiece for different metallurgical states (i.e., annealed, aged, etc.) and, consequently, different hardness values. Since the description of the material behaviour requires correct formulation of the constitutive law, new flow stress models which include also the hardness effect should be developed and, accordingly used, for computer simulation of machining Inconel alloy. This paper describes the development of a hardness-based flow stress and fracture models for machining Inconel 718 alloy, which can be applied for a wide range of work material hardness. These models have been implemented in a non-isothermal viscoplastic numerical model to simulate the influence of work material hardness on the chip formation process. The predicted results are being validated with experimental results available in literature. They are found to satisfactory predict the cutting forces, the temperature, the shear angle and the chip morphology from continuous to segmented chip as the hardness values change.

Abstract: The material grain size changes significantly during machining of hardened steels, and this must be taken into account for improved modeling of surface integrity effects resulting from machining. Grain size changes induced during orthogonal cutting of hardened AISI 52100 (62 HRC) are modeled using the Finite Element (FE) method; in particular, a user subroutine involving a hardness-based flow stress model is implemented in the FE code and empirical models are utilized for describing the phase transformation conditions to simulate formation of white and dark layers. Furthermore, a procedure utilizing the Zener-Hollomon relationship is implemented in the above-mentioned user subroutine to predict the evolution in material grain size at different cutting speeds (300, 600, 900 SFPM). All simulations were performed for dry cutting conditions using a low CBN-content insert (Kennametal KD050 grade, ANSI TNG-432 geometry). The model is validated by comparing the predicted results with experimental evidence available in the literature.

Abstract: Surface integrity of machined products can have a critical impact on their performance, such as corrosion, wear and/or fatigue resistance. It has been reported that reducing the grain size of AZ31B Mg alloys could significantly enhance its corrosion resistance, which is often the limiting factor for its wide application. Severe plastic deformation (SPD) has proved to be an effective way to induce grain refinement. In this study, the potential of cryogenic machining as a novel SPD method to induce grain refinement on the surface of AZ31B Mg alloys was investigated. The microstructures of the workpiece surface/sub-surface and the machined chips after both dry and cryogenic machining were studied. A surface layer where nanocrystallized grains exist was found in the machined surface under cryogenic conditions. Increasing the edge radius of the cutting tool resulted in a thicker grain refinement layer. In addition to the experimental study, an FE model based on the Johnson-Cook constitutive equation was developed and validated using experimental data in terms of chip morphology and forces. The capability of this model to predict critical deformation parameters for dynamic recrystallization (DRX), such as strain, strain-rate and temperature, was demonstrated. With further development, the model can be used to predict the onset of DRX and the grain size on the machined surface.

Abstract: Nowadays, numerical simulation of cutting processes receives considerable interest among the scientific and industrial communities. For that, various numerical codes are used. Nevertheless, there is no uniform standard for the comparison of simulation model with these different software. So, it is often not easy to state if a given code is more pertinent than another. In this framework, the present work deals with various methodologies to simulate orthogonal cutting operation inside two commercial codes Abaqus and Deform. The aim of the present paper is to build a common benchmark model between the two pre-cited codes which can initiate other numerical cutting model comparisons. The study is focused on the typical aeronautical material - Ti-6Al-4V - Titanium alloy. In order to carry out a comparative study between the two codes, some similar conditions concerning geometrical models and cutting parameters were respected. A multi-physic comprehension related to chip formation, cutting forces and temperature evolutions, and surface integrity is presented. Moreover, the numerical results are compared with experimental ones.