Leonardo Orazi

Laser Ablation of Metals: A 3D Process Simulation for Industrial Applications

A model for laser milling simulation is presented in this paper. A numerical model able to predict the physical phenomena involved in laser ablation of metals was developed where the heat distribution in the work piece, the prediction of the velocity of the vapor/liquid front, and the physical state of the plasma plume were taken into account. The model is fully 3D and the simulations makes it possible to predict the ablated workpiece volume and the shape of the resulting craters for a single laser pulse or multiple pulses, or for any path of the laser spot. The numerical model was implemented in C+ + and an overview of the code capacities is presented.

High-speed manufacturing of highly regular femtosecond laser-induced periodic surface structures: physical origin of regularity

Highly regular laser-induced periodic surface structures (HR-LIPSS) have been fabricated on surfaces of Mo, steel alloy and Ti at a record processing speed on large areas and with a record regularity in the obtained sub-wavelength structures. The physical mechanisms governing LIPSS regularity are identified and linked with the decay length (i.e. the mean free path) of the excited surface electromagnetic waves (SEWs). The dispersion of the LIPSS orientation angle well correlates with the SEWs decay length: the shorter this length, the more regular are the LIPSS. A material dependent criterion for obtaining HR-LIPSS is proposed for a large variety of metallic materials. It has been found that decreasing the spot size close to the SEW decay length is a key for covering several cm2 of material surface by HR-LIPSS in a few seconds. Theoretical predictions suggest that reducing the laser wavelength can provide the possibility of HR-LIPSS production on principally any metal. This new achievement in the unprecedented level of control over the laser-induced periodic structure formation makes this laser-writing technology to be flexible, robust and, hence, highly competitive for advanced industrial applications based on surface nanostructuring.

Mechanisms of high-regularity periodic structuring of silicon surface by sub-MHz repetition rate ultrashort laser pulses

Silicon is one of the most abundant materials which is used in many areas of modern research and technology. A variety of those applications require surface nanopatterning with minimum structure defects. However, the high-quality nanostructuring of large areas of silicon surface at industrially acceptable speed is still a challenge. Here, we report a rapid formation of highly regular laser-induced periodic surface structures (HR-LIPSS) in the regime of strong ablation by infrared femtosecond laser pulses at sub-MHz repetition rate. Parameters of the laser-surface interactions and obtained experimental results suggest an important role of electrostatically assisted bond softening in initiating the HR-LIPSS formation.

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

Constrained free form deformation as a tool for rapid manufacturing

In this study an application of free form deformation for the editing of clouds of points or tessellated surfaces representing industrial parts is presented. The application is based on an efficient implementation of NURBS based free form deformation and the implementation of functions for imposing constraints to position, directions and curvatures of embedded objects. The developed functions permit the direct manipulation of triangulated surfaces eliminating the surface reconstruction stage, reducing the time requested in the reverse engineering process and, at the end, reducing the time to market.

A New Computationally Efficient Model for Tempering in Multitrack Laser Hardening in Medium Carbon Steels

The bottleneck in laser hardening principally occurs when large surfaces have to be treated because this process situation leads to multitrack laser scanning in order to treat all the component surfaces. Unfortunately, multitrack laser trajectories generate an unwanted tempering effect that depends on the overlapping of two close trajectories. To reduce the softening effects, a simulator capable to optimize the process parameters, such as laser power and speed and number and types of trajectories, could sensibly increase the applicability of the process. In this paper, an original model for the tempering is presented. By introducing a tempering time factor for the martensitic transformation, the hardiness reduction can be predicted according to any laser process parameters, material, and geometry. Experimental comparisons will be presented to prove the accuracy of the model.

Laser hardening process simulation for mechanical parts

In this paper a numerical simulation of laser hardening process is presented. The Finite Difference Method (FDM) was used to solve the heat transfer and the carbon diffusion equations for a defined workpiece geometry. The model is able to predict the thermal cycle into the target material, the phase transformations and the resulting micro-structures according to the laser parameters, the workpiece dimensions and the physical properties of the workpiece. The effects of the overlapping tracks of the laser beam on the resulting micro-structures is also considered. The initial workpiece micro-structure is taken into account in the simulation by a digitized photomicrograph of the ferrite perlite distribution before the thermal cycle. Experimental tests were realized on a C43 plate and the good agreement between the theoretical and experimental results is shown.

A New Computationally Efficient Model for Tempering in Multi-Tracks Laser Hardening

The bottleneck in laser hardening principally occurs when large surfaces have to be treated because this process situation leads to multi-tracks laser scanning in order to treat all the component surface. Unfortunately, multi-tracks laser trajectories generate an unwanted tempering effect that depends on the overlapping of two close trajectories. To reduce the softening effects, a simulator capable to optimize the process parameters such as laser power and speed, number and types of trajectories, could sensibly increase the applicability of the process. In this paper an original model for the tempering is presented. By introducing a tempering time factor for the martensitic transformation, the hardness reduction can be predicted according to any laser process parameters, material and geometry. Experimental comparisons will be presented to prove the accuracy of the model.Copyright

3D Modelling of Laser Hardening and Tempering of Hypo-eutectoid Steels

In this paper a mathematical model solved by means of the finite differences method (FDM) for laser surface hardening of complex geometries is presented. The 3-D transient model characterizes a software package named Laser Hardening Simulator (LHS), which makes it possible to predict the extension of the treated area into the mechanical components and thus the hardened depth into the bulk material. The obtained microstructures and the resulting hardness with respect to the laser parameters and to the laser beam path strategy can be determined by considering the quenching and the tempering effects due to the overlapping trajectories. The initial workpiece microstructure is taken into account in the simulation by a digitized photomicrograph of the ferrite-pearlite distribution before the thermal cycle. In order to show the accuracy of the model, experimental trials were conducted on the keyway for spline machined on a hub made of SAE 1043. The domain discretization for the solution of the heat flux problem into the workpiece and for the diffusion of the carbon is carried out by means of a mesh generator strategy implemented into the code.

3-D Modelling of Laser Ablation of Metals in Mould Manufacturing

An original model for laser milling characterization is presented in this paper. A 3-D numerical model able to simulate the physical phenomena involved in laser ablation of metals was developed where the heat distribution in the work piece, the prediction of velocity of the vapour/liquid front and the physical state of the plasma plume were taken into account. The numerical model was implemented in C++ and an overview of the code capacities is presented.Copyright