Bruno Martin

Characterisation of cw Nd : YAG laser keyhole dynamics

Abstract The paper concerns laser–matter interaction characterisation. In this work, we use a rapid CCD camera located coaxially to the laser beam and we compare recorded images with those obtained by numerical modelling. Because images are difficult to understand, we compute thermal radiation emitted by a keyhole of fixed geometry and we adjust it trying to approach the camera record. The modelling treats radiative heat transfer within the keyhole and determines the sensor illumination map. By adjusting the geometrical characteristics of the hole, we seek to obtain the image that corresponds as well as possible to the realised experiment. Results are compared with other experimental methods simultaneously performed plume characterisation with an electric probe and spectrometric analysis. They show the existence of two distinct behaviours of the keyhole: a pseudo-steady state associated with regular and pseudo-constant keyhole shapes, low frequencies of electric current in the plume, and generally good welding results, and a highly dynamic mode associated with irregular and rapidly varying keyhole shapes, high frequencies in the plume current and generally poor welding results.

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...

Numerical investigation of laser beam shaping for heat transfer control in laser processing

This article presents a thermal finite volume model adapted to investigate laser processes. It is specially developed to treat moving heat sources with phase changes, melting and vapourisation. Heat transfer control in laser processing is particularly useful when processes must respect prescribed temperatures, or more generally, prescribed constraints. These processes involve generally several laser sources, or non-conventional power distribution. Hence, they have numerous parameters to set. Experimental optimisation is difficult and may be expansive. The numerical model is a useful and cheaper tool for development of those complex processes. For example in this article, two special laser processes are investigated: control of high carbon steel welded line cooling, zinc-coated sheets lap welding.

A study of noise transmission through cylindrical shell using modal truncation approach

The study of the vibroacoustic behavior of cylindrical structure like aircraft fuselage requires the establishment of very large models, and the numerical results which can be obtained are often difficult to interpret. On the other hand, the study of simple structures such as cylindrical shells is useful to validate the criteria and tools that can be used to simplify the models of complex structures. The presented study concerns the sound transmission through a cylindrical shell coupled to a cavity and subjected to an incident acoustic wave. Noise reduction is obtained and validated by a numerical method which uses structural and acoustical modes. A criterion for selecting eigen modes for the modal truncation is presented. Based on a principle of modal coupling, this criterion allows a judicious choice of modes that have an important influence on noise reduction. This criterion is used to reduce the length of acoustical and mechanical bases and to decrease time calculation.