Domenico Umbrello

ABSTRACT The aim of the present work is to investigate the effect of a severe plastic deformation (SPD) process, cryogenic burnishing, on the surface integrity modifications of a Co–Cr–Mo alloy due to the burnishing-induced surface integrity properties. A set of experiments was conducted to investigate the influence of different burnishing parameters on distribution of grain size, phase structure and hardness of the processed material. The results from this work show that the proper selection of burnishing conditions can significant improve the surface integrity of the Co–Cr–Mo alloy due to refined microstructure, high hardness, and favorable phase structure on the surface layer, which could potentially lead to advanced wear performance of such material.

ABSTRACT Microstructural phase transformations, commonly named as the white layer on hard turned components, have in recent times become an interesting research topic in machining. Three main theories have been proposed to justify the mechanisms of white layer formation: (i) rapid heating and quenching; (ii) severe plastic deformation; (iii) surface reaction with the environment. Furthermore, coolant application also affects the surface microstructural alterations resulting from machining operations, which have a significant influence on product performance and life. The present work aims at understanding the effects of cryogenic coolant application on machined surface alterations during orthogonal machining of hardened AISI 52100 bearing steel. Experiments were performed under dry and cryogenic cooling conditions using cubic boron nitride (CBN) tool inserts with varying initial hardness and tool shape. Several experimental techniques were used in order to analyze the machined surface. In particular, optical and scanning electron microscopes (SEM) were used for characterizing the surface topography, whereas the microstructural phase composition analysis and chemical characterization have been performed using X-ray diffraction (XRD) and energy-dispersive spectroscopy (EDS) techniques. The experimental results prove that the microstructural phase changes are partially reduced or can be totally avoided under certain cryogenic cooling conditions. Therefore, cryogenic cooling has the potential to be used for achieving enhanced surface integrity, thus contributing to improved product life and functional performance.

ABSTRACT Microstructural phase transformations, commonly named white layer on hard turned components, are becoming one of the most interesting research subjects for the scientific community. Three main theories have been proposed to justify the mechanisms of white layer formation: rapid heating and quenching, which results in sudden microstructural phase transformation; severe plastic deformation, which produces a homogenous structure and/or a very fine grain size microstructure; and surface reaction with the environment. The present work aims to understand which of the above mentioned mechanisms is the main cause of the white layer formation when AISI 52100 hardened steel is machined by cubic boron nitride inserts. For this reason, an experimental campaign was carried out, and several experimental techniques were used in order to analyse the machined surface. In particular, optical and scanning electron microscope were utilised for surface topography characterisation, while microstructural phase composition and chemical characterisation have been performed by means of X-ray diffraction and energy dispersive spectroscopy techniques. The experimental results prove that the white layer is the result of microstructural alteration, i.e. the generation of a martensitic structure.

ABSTRACT Microstructural phase transformations, commonly known as white layer formation in hard turned steel components, have in recent times become an interesting research topic in machining as they are related to the surface integrity and functional performance of components. Three main theories have been proposed to justify the mechanisms of white layer formation: (1) rapid heating and quenching; (2) severe plastic deformation; and (3) surface reaction with the environment. Coolant application also affects the surface microstructural alterations resulting from machining operations, which have a significant influence on product performance and life. The present work aims at understanding the effects of cryogenic coolant application on the machined surface alterations during machining of hardened AISI 52100 bearing steel. Experiments were performed under dry and cryogenic cooling conditions using cubic boron nitride tool inserts with varying initial work material hardness, tool shape, cutting speed and feedrate. Optical and scanning electron microscopes (SEM) were used to analyse the affected layer in the machined subsurface, while X-ray diffraction technique was utilised to investigate the microstructural phase composition. The experimental results prove that the microstructural phase changes are heavily influenced by the cutting process parameters and the use of cryogenic cooling, in some cases leading to the total removal of martensite.

ABSTRACT Metallic foams are new materials mainly produced by expansion in a proper chamber and mainly characterized by internal voids: a material characterized by a very low density is obtained in this way. A lot of foamed components are commonly produced, directly by injecting a gas or foaming agent into molten metal inside a closed die. However, secondary operations on these materials can play an important role in order to enhance the foam production flexibility. From the above considerations, the deformation behaviour of an aluminium foam was investigated by compression tests. The study compares three different numerical analyses highlighting their points of strength and weakness in order to verify their applicability in process design. More in detail, two models based on the implicit formulation were investigated; in one case, the billet material was set as porous object with the material density which was calculated and updated as part of the simulation. The second implicit analysis, instead, was built using the plastic material formulation; the porosity, in this case, was physically created introducing voids within the workpiece.The latter simulation class was carried out through an explicit investigation; an efficient model construction was proposed introducing spherical surfaces connected each other with plans. Experimental data were used to validate the calculated results and a discussion concerning the three different numerical analyses was finally reported.

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.

Numerical simulation of cutting process is today moving towards two different directions. The former concerns the development of high performance codes able to approach the 3D phenomena, the latter is already focused on the study of some fundamental aspects whose full understanding may be strategic for the knowledge enhancing in this very complex field. In the paper this second way

Numerical simulation of machining is now evolving toward new directions, in particular taking into account 3D applications. On the other hand, some aspects are still not well solved and different software are today available to carry out fundamental analysis on orthogonal cutting. Some of these codes are very user-friendly but don't allow any tuning of the parameters. Other ones, are

ABSTRACT Microstructure in subsurface is investigated to understand the effect of cryogenic coolant in hard machining of hardened AISI 52100 bearing steel. The cutting experiments were performed under dry and cryogenic cooling conditions using cubic boron nitride (CBN) tool inserts. The change in the microstructure was analyzed at varying of the tool shape and cutting speed for different initial hardness of the workpiece. The machined surfaces and subsurface were analyzed by several investigation techniques. The affected layer depth was observed with a scanning electron microscope (SEM), while the phase composition was analyzed by X-ray diffraction (XRD). The grain size has been directly measured with a focused ion beam and scanning ion microscope (FIB-SIM) and compared with an indirect method of analysis: X-ray diffraction peak profile (XDPP). The experimental results show that the microstructural phase strongly depends on the cutting process parameters. Furthermore, when the cryogenic cooling is applied, martensitic structure disappears in some cases. The grain size on the machined surfaces is also influenced by the material properties and the process parameters. In addition, it is demonstrated that the XDPP indirect method for grain size analysis is a valid alternative to the direct method.

Residual stresses can enhance or impair the ability of a component to withstand loading conditions in service (fatigue, creep, stress corrosion cracking, etc.), depending on their nature: compressive or tensile, respectively. This poses enormous problems in structural assembly as this affects the structural integrity of the whole part. In addition, tool wear issues are of critical importance in manufacturing since

ABSTRACT In the last years, the growing role of process flexibility in modern mechanical industries has driven a rising interest in optimisation of process/product design through innovative techniques. Moreover, the development of niche productions, which are characterised by low production volumes and small batches leads to the need of more flexible and rapid forming technologies. In this way, a great research effort is performed towards the study of new stamping processes: among them hydro forming finds a large interest in automotive industry since it allows to significantly reduce tooling costs and also to avoid some secondary operations. Different studies are available in the technical literature concerning the fundamentals of tube hydro forming processes as well as the industrial application of such operations. As process design issues are concerned, in the paper, the authors propose the integration of different tools, namely artificial intelligence techniques, numerical simulations and experimental knowledge to carry out tube hydro forming design. A fuzzy system was integrated with a FEM code for process simulation obtaining a sort of closed loop control system to be utilised in the process design phase. An experimental validation of the design procedure was developed as well proving the effectiveness of the proposed approach in order to obtain defect free components both as geometrical features and ductile fractures are concerned.

ABSTRACT The phenomenological models for material flow stress and fracture, typically used in the Finite Element simulations of machining Nickel-based alloys, are often deemed to represent only certain metallurgical material states. In contrast, these models are not suitable to describe the constitutive behavior of the workpiece for different metallurgical states (i.e., annealed, aged, etc.) and, consequently, different hardness values.

ABSTRACT Machining processes are frequently investigated by numerical simulations. Usually 2D analyses are carried out in order to reduce CPU times, considering orthogonal cutting conditions. In this way, the computational time sharply reduces and many process variables may be calculated (i.e. forces, chip morphology, shear angle, contact length). On the other hand, the analysis of thermal aspects involved in machining, for instance the temperature distribution reached in tool, still represents an open problem. Finite element codes are able to simulate a very short process time that is not sufficient to reach steady state conditions. Several approaches have been proposed to overcome this problem: in the paper some of them are applied and critically discussed.