Papers by Author: Serafino Caruso

Abstract: The increasing demand for reliable and high-performance heat-transfer components has stimulated the development of robust joining and forming strategies for thin-walled stainless-steel tubes. In this work, the Controlled Tube Expansion of Plasma Arc Welded AISI 316 stainless-steel tubes was investigated through a combined experimental and numerical approach. Welded tubes with an initial diameter of 130 mm were expanded to 180 mm using a three-step mechanical expansion process, and six different expansion sequences were experimentally evaluated. Finite element simulations were performed using a coupled thermo-mechanical model incorporating a damage-based fracture criterion to predict material failure during expansion. Numerical predictions were in good agreement with experimental observations and allowed the identification of a critical cumulative damage threshold governing tube failure. Based on these results, a processability domain was defined, clearly distinguishing safe and unsafe expansion paths. The study demonstrates that tube expandability is strongly dependent on the deformation path and highlights the importance of progressive expansion strategies for maximizing material ductility while preventing fracture.

Abstract: In this study, the mechanical properties of welded joints of AA 6005 aluminum alloy obtained with friction stir welding (FSW) and conventional metal inert gas welding (MIG) are studied. FSW welds were carried out on a semi-automatic milling machine. The performance of FSW and MIG welded joints were identified using tensile and bending impact tests, as far as the environmental aspects are also included in the discussion. The joints obtained with FSW and MIG processes were also investigated in their microstructure. The results indicate that, the microstructure of the friction stir weld is different from that of MIG welded joint. The weld nugget consists of small grains in FSW than those found in MIG weld. Taking into consideration the process conditions and requirements, FSW and MIG processes were also compared with each other to understand the advantages and disadvantages of the processes for welding applications of studied Al alloy. Better tensile and bending strength were obtained with FSW welded joints.

Abstract: Residual stress is one of the most important surface integrity parameter that can significantly affect the service performance of a mechanical component, such as: contact fatigue, corrosion resistance and part distortion. For this reason the mechanical state of both the machined surface and subsurface needs to be investigated. Residual stress induced by dry and cryogenic machining of hardened AISI 52100 steel was determined by using the X-ray diffraction technique. The objective was to evaluate the influence of the tool cutting edge geometry, workpiece hardness, cutting speed, microstructural changes and cooling conditions on the distribution of the residual stresses in the machined surface layers. The results are analysed in function of the thermal and mechanical phenomena generated during machining and their consequences on the white layer formation.

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.