Alessandro Fortunato

A Comprehensive Model for Laser Hardening of Carbon Steels

This article illustrates the development of a complete and exhaustive mathematical model for the simulation of laser transformation hardening of hypo-eutectoid carbon steels. The authors propose an integrated approach aimed at taking into consideration all the the phenomena involved in this manufacturing process, with particular attention to implementing easy mathematical models in order to optimize the trade-off between the accuracy of the predicted results and the computational times. The proposed models involve the calculation of the 3D thermal field occurring into the workpiece and predict the microstructural evolution of the target material exploiting an original approach based on the definition of thermodynamic thresholds which can be considered as a physical constant of the material itself. Several parameters and phenomena are taken into consideration in order to accurately simulate the process: laser beam characteristics, fast austenization of the steel and tempering effect due to mutually interacting beam trajectories.Copyright

An improved model for cold metal transfer welding of aluminium alloys

An improved model is proposed for automated cold metal transfer (CMT) welding based on a time-dependent double-ellipsoidal volumetric heat flux distribution. Equations are evaluated numerically within COMSOL Multiphysics for CMT welding of a 3-mm-thick AA5754 Al–Mg alloy plate. The simulation calculates transient and steady-state temperature distributions within the weld seam and heat-affected zone (HAZ). Validation of the model is achieved by comparing simulated temperatures with measured values from thermocouples in the HAZ during welding experiments, as well as through comparison of the calculated fusion zone and microscope images of the weld seam. Under steady-state conditions, large differences between the peak and average calculated temperatures in the weld pool highlight the underlying phenomenon responsible for improvements in weld quality for thin sheets with CMT compared to conventional joining processes. The developed simulation provides opportunities for process optimisation and sensitivity analysis in many applications.

Laser Profiling of Aluminum Oxide Grinding Wheels

Laser profiling experiments are performed at normal incidence on fine grain medium density aluminum oxide grinding wheels with a pulsed nanosecond 1064nm fiber laser source with maximum pulse fluence 369J/cm2. In order to determine the incision depth and ideal laser pass separation distance, laser exposures are first performed on high purity, low porosity aluminum oxide blocks and subsequently analyzed with an optical profiler operating in confocal mode. This ablation data is then applied to path planning for grinding wheel profiling experiments, with division of the necessary removal depth according to the measured incision depth and ideal pass separation distance. X-ray computed tomography is utilized to determine the resulting profile accuracy as a function of process parameters. Test results indicate a maximum profile accuracy in the order of 200μm; however, in order to approach the accuracy of diamond dressing, some two orders of magnitude lower, it is likely that tangential laser incidence is necessary.Copyright

A Thermal Model for Laser Hardening Simulation

Laser hardening is a very flexible and useful process for surface treatment of medium carbon steels, capable of processing varied and complex geometries. In order to enlarge the range of industrial applications to which this process can be applied, a suitable model is necessary in order to reduce the setup time requested for the optimization of new components. The process model presented is based on the Arrhenius-like equation for estimation of the thermally induced process reaction time for microstructural transformations. By means of experiments, all unknown parameters in the equations have been determined, highlighting the accuracy and low computation time of the simulator.Copyright

Laser shock peening and warm laser shock peening: process modeling and pulse shape influence

Laser shock peening is a well-known technology able to enhance the fatigue life of mechanical components by means of the introduction of residual stresses on their surface. These stresses are induced by means of the recoil pressure caused by the abrupt expansion, in a confining medium, of a laser-vaporized coating layer. If high power densities are used the recoil pressure can be high enough to induce compressive residual stresses on the target surface and to modify its mechanical properties. These mechanical properties can be predicted if the recoil pressure of the ablating layer is determined. In this paper the influence of the laser pulse shape on the recoil pressure is determined by means of a proper modeling of the whole process and the difference between cold and warm laser shock peening is pointed out.

Dynamic Hybrid Modeling of the Vertical Z Axis in an HSM Machining Center: Towards Virtual Machining

The paper describes the dynamical real-time simulation of the Z axis of a five axis machining center for high speed milling. The axis consists of a mechanical structure: machine head and electro-mandrel, a CNC Control System provided with feed drives and a Pneumatic System to compensate the weight of the entire vertical machine head. These three sub-systems have been studied and modeled by means of: • FEM modeling of the mechanical structure; • an equation set to represent the main functions of the CNC; • an equation set to represent the functioning of the Pneumatic System. These different modeling sub-systems have been integrated to obtain the entire actual dynamical behavior of the Z axis. A particular analysis was developed to represent the friction phenomena by a specific analytical model. Experimental activity was developed to test and validate the different modeling sub-systems, and other experimental tests were performed on the machining center to compare simulation outputs with experimental responses.Copyright

Characterization of Lattice Structures for Additive Manufacturing of Lightweight Mechanical Components

Tensile and compression test specimens comprising lattice structures with simple cubic, crossing-rod and body-centered cubic (BCC) unit cells are produced via SLM additive manufacturing (AM) of AISI 316L stainless steel and CoCr powder. Equivalent stress-elongation curves are obtained, with equivalent strength, specific strength, stiffness modulus and specific stiffness calculated based on specimen density and sample cross-section. The obtained results highlight the fact that analogous structures can behave very differently depending on the chosen material. While large differences are obtained in strength and stiffness between the different unit cell types, specific strength and specific stiffness vary to a lesser extent. Two case studies are presented, including a porous structure suitable for bone implants in the field of biomedical engineering and an AISI 316L food packaging machine component. The results obtained in this study provide useful guidelines and equivalent properties for designers wishing to exploit the advantages of internal lattice structures in AM.Copyright

Non-conventional laser surface hardening for axisymmetric components

A new process, based on ring spot geometry, is presented for laser surface hardening of large cylindrical com-ponents. The proposed technique leads to a very hard, deep and uniform treated area along the entire work piece surface without introducing a tempered zone, making the process very attractive compared to conventional induction hardening that exhibits both low energy efficiency and poor flexibility. A complete physical model is presented for the process, together with a study of the influence of process parameters on the final outcome. The results of an extensive validation campaign, carried out following the AISI1040 standard, are also reported.

Laser Assisted Cold Bending of High Strength Steels

This paper presents the feasibility of an innovative application of laser-assisted bending process. The high strength steel sheets bending, carried out after a laser heat treatment, is studied. Several strategies aimed at obtaining a ductile structure along the bending line, suitable for cold forming, are investigated. The influence of laser processing parameters on the microstructure, hardness and strength of the sheets are discussed and analyzed. In order to predict the temperature and ensure the repeatability and reliability of the process, a model for heat treatment simulation is developed. The study of the experimental data and the integration with the simulation of the heating phase lead to the definition of specific process parameters suitable for achieving a crack-free cold bending of high strength steels.Copyright

A Multi-Axis Deep Drawing Servo Press With Non-Overconstrained Architecture

Servo actuated presses can provide maximum pressing force at any slide position in the same manner that hydraulic presses do, while offering several benefits in terms of precision, energy conversion efficiency and simplicity due to their lack of hydraulic circuitry and oil. Several press builders have developed electric-spindle actuated presses; however, issues relating to multi-axis architecture have been neglected. The present study proposes an innovative method of avoiding overconstrained architecture by implementing a kinematic mechanism that connects multiple servo axes to one slide. Servo axis design is developed by creating a dynamic model of a kinematic chain composed of a servo-motor, gearbox reducer and ball screw transmission. A study of a biaxial industrial servo press prototype with non-overconstrained architecture, currently under construction, is presented as proof of concept. It is shown that such a non-overconstrained multi-axis press can be constructed from commercially available components, achieving high energy efficiency at high load with relatively simple construction.Copyright