Giampaolo Campana

Prediction of melt geometry in laser cutting

Abstract In this paper, an analytical model for the evaluation of the melt film geometry in laser cutting of steels is developed. Using as basis, a previous model for kerf geometry estimation developed by the authors, with both reactive and non-reactive process gases, the film thickness and velocity were determined as a function of the kerf depth in the cutting plate. Two criteria were then adopted to predict the quality of the laser cutting operation: the first is based on a minimum acceptable value of the ejection speed of the melt from the bottom of the kerf, the second on the occlusion of the kerf itself due to an excess of molten material in the boundary layer at the kerf width. These criteria determined a feasibility region in the domain of the process and material variables, such as cutting speed, assistant gas pressure, laser beam power and material characteristics. These factors may be successfully used to build a process-planning tool for parameters optimisation and setting, in order to achieve a satisfactory process quality. The model response is in excellent agreement with the feasibility regions reported from experimental data by various authors and demonstrates a relationship between the occurrence of dross adhesion and the two different mechanisms predicted for such a phenomenon were: unsatisfactory ejection speed of the melt film from the bottom of the kerf and occlusion of the kerf.

Prediction of mechanical properties in spheroidal cast iron by neural networks

Abstract An artificial neural network-based system is proposed to predict mechanical properties in spheroidal cast iron. Several castings of various compositions and modules were produced, starting from different inoculation temperatures and with different cooling times. The mechanical properties were then evaluated by means of tension tests. Process parameters and mechanical properties were then used as a training set for an artificial neural network. Different neural structures were tested, from the simple perceptron up to the multilayer perceptron with two hidden layers, and evaluated by means of a validation set. The results have shown excellent predictive capability of the neural networks as regards maximum tensile strength, when the variation range of strength does not exceed 100xa0MPa.

A Method for Laser Heat Treatment Efficiency Evaluation in Multi-Track Surface Hardening

Laser surface hardening is nowadays an industrial emerging technique, which is gradually substituting induction and flame surface hardening thanks to its advantages related to power saving and process versatility. This manufacturing technology is a challenging process especially when it has to be applied on surfaces larger than the beam spot. In this case several adjacent passes must be performed in order to scan the whole surface to be treated. This strategy involves inevitably an intrinsic tempering effect due to the re-heating of the previously hardened material. The extent of the softening occurring depends on several parameters. First of all, it depends on the material and its initial state, then on process parameters related to the laser source, such as type, optical path and spot dimension and further on the adopted surface scan strategy of the beam. This last set of process parameters is represented by: laser beam speed, number of tracks, pass overlapping degree and tracks sequence. The hardness uniformity of the heat treated layer and the consequent effectiveness of the process depend strictly on the tempering degree occurring in the material. According to this it is important to find a practical method devoted to quickly characterize the result of a laser surface treatment in terms of tempered zones extension and distribution. This article proposes, then, the definition of a “Covering Uniformity” index (CU) which represents an engineering approach to this problem and it allows to easily determine the effectiveness of a particular laser hardening treatment. The CU index is based upon hardness measurements and it is related to the ratio between the extension of the tempered zone and the total extension of the treated area. In order to underline and demonstrate the intrinsic value of this parameter a set of experimental trials was carried out on AISI 1070 carbon steel, AISI 1040 carbon steel and AISI 420B martensitic stainless steel.Copyright

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.

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.

Optimization Strategies of Laser Hardening of Hypo-eutectoid Steel

The interest towards LASER hardening of steels has been increasing since the last few years due to its undoubted advantages. The main drawback affecting this manufacturing technology is the tempering effect induced when multiple passes on the same surface must be carried out. In order to minimize the softening effect due to tempering and to speed up the process a numerical model for the simulation of the treatment is proposed. This model is able to detect the optimal LASER path trajectory according to the source parameters and the scanning velocity, and it is able to predict the resulting microstructures and the relating hardness. Some examples on an hypo-eutectoid steel are presented together with validation tests.

Application of Laser in Joining Aluminum Foam Hybrid Materials

This article illustrates an experimental campaign aimed at assessing preliminary guidelines for the application of the laser in joining cellular-structured hybrid materials. In particular the target specimens exploited were all characterized by the presence of an aluminum foam core and by an external skin, made in aluminum or in stainless steel. The goal of the present paper is to underline a global feasibility of laser joining of these materials pointing out the role of the main process parameters and to suggest some original techniques which could be adopted in order to improve the overall quality of the joint. The experience described pointed out that, when dealing with this kind of materials, the role of the laser can be dual: in case of high energy density applications it can be used for local fusion of the workpiece, as in traditional welding, while in low energy density ones the radiation can be exploited as a controlled heating source for promoting local thermal actions particularly on the cellular portion of the material.Copyright

Investigation on Porosity Formation in AA6082 Hybrid Laser-GMAW Welding

This paper refers to the results obtained in the investigation on bubbles formation in hybrid Laser-GMAW welding. Bead on plate weldings were obtained on 8 mm thick AA6082 plates and the process parameters investigated were: GMAW current, arc transfer mode and mutual distance between arc and laser sources. Bubbles were observed by means of X-ray inspection techniques and measured exploiting an image analysis software. Finally a correlation, based on analysis of variance, between process parameters and porosity formation is proposed.Copyright

3D transient model for CO2 laser hardening

A 3D numerical model for the surface hardening process simulation carried out by means of a CO2 laser source is presented. The model is able to predict the extension of the treated area into the workpiece, the type of the resulting micro-structure and the optimal laser path strategy in order to minimize the micro-structural softening due to the tempering effect. The Fourier equation is solved using the Finite Difference Method (FDM) applied on a generical grid obtained by means of the domain discretization. The resulting time dependent temperature distribution into the workpiece is used for the evaluation of the induced heating cycle. By calculating the cooling velocity, the micro-structure transformation is determined together with the hardness in every point of the domain. The hardness reduction due to the tempering effect is also predictible. The computational times are small and the software is very suitable in industrial environment in the early stage of the process planning when several simulation runs must be performed. The modeling activity was developed by considering the class of the hypo-eutectoid steel. The experimental tests were realized on a C43 steel plate. The good agreement between the theoretical and experimental results is shown.

A New Computationally Efficient Method in Laser Hardening Modeling

Laser hardening is a laser assisted process devoted to the surface hardening of the mechanical components. This process is highly suitable for medium carbon steels with carbon content comprised between 0.2 – 0.6% or for low alloy steels which are usually surface hardened during their manufacturing process. Laser hardening technology is gaining a great industrial interest in the last years in fact, the possibility of integrating the heating source directly on the production line, together with the absence of the quenching medium, meets the production needs of modern industries. Laser hardening optimization could be complex especially when tempering due to multiple passes effects must be considered. Many research studies have been proposed in the last years aimed at predicting the optimal laser process parameters such as beam power density, beam velocity and scanning strategies. Many Authors agree with the assumption that the whole austenite resulting from the heating is transformed into martensite during the quenching. This is a valid approximation for single pass but could be a rough hypothesis in multiple-passes when the cooling rate could be not so high. Moreover hysteresis phenomena, due to the severe heat cycle occurring in laser hardening, should be taken into account for pearlite to austenite and martensite to austenite transformations during heating and for martensite tempering during multiple passes. In this paper the crucial problems to be faced regarding laser surface hardening modeling are discussed with respect to current literature. In particular, partial austenitization of the pearlite is suggested as a solution of the hardness prediction of the profile depth. Then three transformation parameters are proposed in order to take into account the hysteresis phenomena in martensite and pearlite transformations into austenite and in martensite tempering. Finally several experimental examples are proposed in order to validate the mentioned assumptions.© 2008 ASME