Simone Matteï

Diode laser welding of ABS: Experiments and process modeling

The laser beam weldability of acrylonitrile/butadiene/styrene (ABS) plates is determined by combining both experimental and theoretical aspects. In modeling the process, an optical model is used to determine how the laser beam is attenuated by the first material and to obtain the laser beam profile at the interface. Using this information as the input data to a thermal model, the evolution of the temperature field within the two components can be estimated. The thermal model is based on the first principles of heat transfer and utilizes the temperature variation laws of material properties. Corroborating the numerical results with the experimental results, some important insights concerning the fundamental phenomena that govern the process could be extracted. This approach proved to be an efficient tool in determining the weldability of polimeric materials and assures a significant reduction of time and costs with the experimental exploration.

Numerical modeling of molten pool formation during an interaction of a pulse laser (Nd:YAG) with an aluminum sheet

Laser welding offers many advantages over conventional techniques, but is subject to a great number of different parameters. Whereas a purely experimental study would be very expensive, modelling the process can optimize it more economically. Laser welding leads to the occurrence of physical phenomena that do not appear in conventional welding. Modelling presents several difficulties arising from the following: the strong thermal gradients present over short distances; the evaporation of the material, leading to a deformation of the free surface; the appearance of a keyhole-like cavity if the recoil pressure becomes particularly high. Marangoni convection, initiated by surface tension gradients, has a considerable effect on thermal transfer in the melted pool and, consequently, on the enlargement of the melted pool. The latent heat of fusion and vaporization are introduced into the model as sources in the heat equation. The Newton-Stokes equations are modified to model the flow in the solid-liquid zone. The model takes into account the non-linearities arising from the variations, with temperature, of thermophysical characteristics. The results of a 2D numerical calculation for a laser pulse (Nd : YAG) on a sheet of aluminium are presented here. FEMLAB, a software based on the finite element method, was used.Laser welding offers many advantages over conventional techniques, but is subject to a great number of different parameters. Whereas a purely experimental study would be very expensive, modelling the process can optimize it more economically. Laser welding leads to the occurrence of physical phenomena that do not appear in conventional welding. Modelling presents several difficulties arising from the following: the strong thermal gradients present over short distances; the evaporation of the material, leading to a deformation of the free surface; the appearance of a keyhole-like cavity if the recoil pressure becomes particularly high. Marangoni convection, initiated by surface tension gradients, has a considerable effect on thermal transfer in the melted pool and, consequently, on the enlargement of the melted pool. The latent heat of fusion and vaporization are introduced into the model as sources in the heat equation. The Newton-Stokes equations are modified to model the flow in the solid-liquid zone. T...

Effects of laser beam scattering on through-transmission welding of polymers

We present here a study of the scattering phenomena in semi-transparent polymers induced by their compositions in the through-transmission laser welding processes. In a 1st part the scattering of a laser beam into a polymer is explained as well as the mathematical modeling currently used. In a 2nd part a numerical study of the beam scattering in different inhomogeneous media is realized. We show here that this phenomenon is of prime importance for the welding process and that the simulation allows a good estimation of it. The results are used into a finite elements program, in order to determine the thermal field and the structure behavior under laser excitation and to simulate a through-transmission laser welding process on PMMA polymers and compared with experimental data. We show here the interest of a good understanding of the scattering phenomena and the possibilities to simulate it.We present here a study of the scattering phenomena in semi-transparent polymers induced by their compositions in the through-transmission laser welding processes. In a 1st part the scattering of a laser beam into a polymer is explained as well as the mathematical modeling currently used. In a 2nd part a numerical study of the beam scattering in different inhomogeneous media is realized. We show here that this phenomenon is of prime importance for the welding process and that the simulation allows a good estimation of it. The results are used into a finite elements program, in order to determine the thermal field and the structure behavior under laser excitation and to simulate a through-transmission laser welding process on PMMA polymers and compared with experimental data. We show here the interest of a good understanding of the scattering phenomena and the possibilities to simulate it.

Focal plane array infrared camera transfer function calculation and image restoration

Infrared images often present distortions induced by the measurement system. Image processing is thus an essential part of infrared measurements. A distortion model based on a convolution product is presented. The analytical form of the convolution kernel has been obtained from an image formation theory, along with an analysis of the sampling of the focal plane array camera detectors matrix. Image restoration is an ill-posed problem, and its solution can be obtained using regularization methods. In this work, image restoration is performed using a variation of Tikhonov regularization that makes use of the particular form of the convolution kernel matrix, which is built as a block-circulant matrix that admits a diagonal form in the 2-D Fourier space. The restoration procedure is used to restore a knife-edge infrared source image.

Laser brazing of steel-aluminum assembly

New anti-pollution and energy saving laws impose reduction of fuel consumption for vehicles of the automotive industry. This reduction may be provided through the lightening of vehicles by introducing Aluminum parts into the Steel car-body. Laser-brazing represents an adequate process than can be used to realize this kind of new structure. The main problem with thermal joining processes is the formation of inter-metallic phases which decreases the ductility of the joint. Laser-brazing allows a localized fusion which limits the development of brittle phases. This paper presents the results of a feasibility study for different joining-configurations with different filler materials: an Aluminum based alloy and a Zinc based alloy. The study shows that using zinc alloy for filler material gives better mechanical performances. Simultaneously, observations by optical and electron microscopy were carried out to characterize the joints. In order to understand the influence of the temperature on the formation of inter-metallic phases, a thermal modeling by FEM of the process was also carried out.New anti-pollution and energy saving laws impose reduction of fuel consumption for vehicles of the automotive industry. This reduction may be provided through the lightening of vehicles by introducing Aluminum parts into the Steel car-body. Laser-brazing represents an adequate process than can be used to realize this kind of new structure. The main problem with thermal joining processes is the formation of inter-metallic phases which decreases the ductility of the joint. Laser-brazing allows a localized fusion which limits the development of brittle phases. This paper presents the results of a feasibility study for different joining-configurations with different filler materials: an Aluminum based alloy and a Zinc based alloy. The study shows that using zinc alloy for filler material gives better mechanical performances. Simultaneously, observations by optical and electron microscopy were carried out to characterize the joints. In order to understand the influence of the temperature on the formation of in...

Laser braze welding using hot (88%-aluminum,12%-silicon) filler material to join steel with aluminum

New anti-pollution and energy-saving regulations will require the automobile industry to reduce the rate of fuel consumption per vehicle. Part of the solution could be to make car bodies lighter, which can be achieved by introducing some aluminum parts in place of steel. Laser braze welding is an appropriate method for realizing such structures. The main problem in thermal joining processes is the formation of intermetallic phases, which are detrimental to the mechanical performance of the joint. Laser brazing localizes the fusion and inhibits the development of brittle phases. Another problem encountered in brazing is poor wetting of the liquid material on the sheet-joint surface. This problem is often solved by the use of a brazing chemical flux or by working in a controlled atmosphere or vacuum. In an industrial context, however, such techniques are very difficult to implement. This paper presents the results of joining a 6016 aluminum alloy with hot dip galvanized low carbon steel using an aluminum-based filler material. This study demonstrates that the use of a chemical flux is not necessary. In order to improve the mechanical performance of the joint, the use of a hot wire, rather than a chemical brazing flux, appears a good solution. Optical and scanning electron microscopy was employed to characterize the joints. The temperature of the wire, just before illumination by the laser beam, was measured with a pyrometric device. It was found that preheating the wire has a strong influence on the mechanical properties of the joints.New anti-pollution and energy-saving regulations will require the automobile industry to reduce the rate of fuel consumption per vehicle. Part of the solution could be to make car bodies lighter, which can be achieved by introducing some aluminum parts in place of steel. Laser braze welding is an appropriate method for realizing such structures. The main problem in thermal joining processes is the formation of intermetallic phases, which are detrimental to the mechanical performance of the joint. Laser brazing localizes the fusion and inhibits the development of brittle phases. Another problem encountered in brazing is poor wetting of the liquid material on the sheet-joint surface. This problem is often solved by the use of a brazing chemical flux or by working in a controlled atmosphere or vacuum. In an industrial context, however, such techniques are very difficult to implement. This paper presents the results of joining a 6016 aluminum alloy with hot dip galvanized low carbon steel using an aluminum-ba...

Dissimilar steels laser welding: Experimental and numerical assessment of weld mixing

Upcoming strict CO2 regulations lead car manufacturers to look for mass saving solutions. The use of advanced high strength steel (AHSS) solutions enable optimizing both crash performances and mass saving. Particularly, the use of laser welded blanks made of dissimilar high strength steels is an efficient weight optimization solution. To support the joining of AHSS in car body design, a 3D model of heat transfer, turbulent flow and transport of species in the laser weld pool has been developed. It aims at providing a better understanding of diffusive-convective mixing in the weld and its influence on the weld mechanical properties. The presented model allows predicting the weld geometry and the element distribution. To validate the model, experimental tests were carried out. Welding of two dissimilar steels with different laser beam offset from the joint line was performed. Numerical and experimental investigations of dissimilar butt laser welding between high Mn and dual phase steels were carried out. The cross sections of the welds were characterized by scanning electron microscope (SEM) with energy-dispersive X-ray spectroscopy (EDX) elemental analysis. Quantitative mappings of Mn distribution in the melted zone offer an overview of mixing intensity. The results of the simulation have been found in good agreement with the experimental data. To go further and to assess the effect of weld mixing on mechanical performances, tensile tests were done. It was found that tensile behavior of the welds is determined by level of Mn and C dilutions. For attaining maximal joint performances, it is necessary to comprehend the elements distribution in the melted zone and to be able to control it through an accurate choice of operational parameters.Upcoming strict CO2 regulations lead car manufacturers to look for mass saving solutions. The use of advanced high strength steel (AHSS) solutions enable optimizing both crash performances and mass saving. Particularly, the use of laser welded blanks made of dissimilar high strength steels is an efficient weight optimization solution. To support the joining of AHSS in car body design, a 3D model of heat transfer, turbulent flow and transport of species in the laser weld pool has been developed. It aims at providing a better understanding of diffusive-convective mixing in the weld and its influence on the weld mechanical properties. The presented model allows predicting the weld geometry and the element distribution. To validate the model, experimental tests were carried out. Welding of two dissimilar steels with different laser beam offset from the joint line was performed. Numerical and experimental investigations of dissimilar butt laser welding between high Mn and dual phase steels were carried out. Th...

Determination of an empirical law of aluminium and magnesium alloys absorption coefficient during Nd?:?YAG laser interaction

Welding laser modelling requires knowledge about relative changes of many thermo-physical parameters involved in the interaction. The absorptivity of the material is one of the most important. In this study, experimental measurements of absorptivity with an integrating sphere on two alloys (aluminium and magnesium) were made. These results were compared with an analytical calculation that takes into account the trapping of the beam by multiple reflections inside the keyhole. Based on a statistical method, an empirical law is proposed connecting absorptivity with the peak power of the laser and the duration of interaction. During the interaction, two distinct phenomenologies are brought to light. A threshold is then defined after which the physical process becomes stable. Below this threshold, the measurement of absorptivity is problematic, and the values are widely dispersed.

Through-transmission welding of polymers: Effects of particles on laser beam scattering

We present here a study of the scattering phenomena in semi-transparent polymers induced by their compositions in the through-transmission laser welding processes. In a 1st part; an optical model based on Mie theory and Monte Carlo method is used to describe the laser beam behaviour in semi-transparent media. This model permits to estimate the laser power distribution at the interface of the two materials to be welded. A second model based on finite element method allows the temperature field estimation into the both components. The results are validated by infrared thermography in the case of a PMMA and ABS-PC alloy couple.We present here a study of the scattering phenomena in semi-transparent polymers induced by their compositions in the through-transmission laser welding processes. In a 1st part; an optical model based on Mie theory and Monte Carlo method is used to describe the laser beam behaviour in semi-transparent media. This model permits to estimate the laser power distribution at the interface of the two materials to be welded. A second model based on finite element method allows the temperature field estimation into the both components. The results are validated by infrared thermography in the case of a PMMA and ABS-PC alloy couple.

The numerical simulation of heat transfer during a hybrid laser-MIG welding of duplex steel

The present study is dedicated to the numerical simulation of an industrial case of hybrid laser-MIG welding of high thickness duplex steel UR2507Cu with Y-shaped gap geometry. It consists on stimulating the heat transfer during the welding by COMSOL Multiphysics software using heat equivalent source approach. A numerical exploratory designs method is used to identify and to optimize the heat sources parameters in order to obtain a small relative error between the numerical results and the experiment.