Francis Briand

Experimental study of the dynamical coupling between the induced vapour plume and the melt pool for Nd–Yag CW laser welding

We discuss the effects of the interaction between the vapour generated by the ablation process occurring on the front keyhole wall (KW) during deep penetration Nd–Yag laser welding and the surrounding metallic melt pool. It is shown that the inclination of the front KW determines the importance of this process. At low welding velocities, the front KW inclination is small and therefore the drag forces induced by the expanding vapour accelerates a liquid thin film around the keyhole and parallel to its axis. At high welding velocities, the front KW inclination becomes large, and the evaporation process is very important. Therefore the expanding metallic vapour impinges on the rear KW and opens the keyhole aperture. These effects localize the droplet generation process. By using an adequate side gas jet nozzle, we show that we can stabilize the melt pool fluctuations, and therefore suppress droplet generation and improve the weld seam quality.

Study of keyhole behaviour for full penetration Nd-Yag CW laser welding

The understanding of keyhole behaviour is still a topic of great importance. The keyhole dynamics can be rather easily analysed in the case of full penetration laser welding experiments. Moreover, this configuration can allow an easy access to the complete keyhole geometry, if thin metallic sheets are welded. Indeed, in this case, only one laser beam reflection occurs on the front keyhole wall (KW). We present the results of the study of these welding configurations, where a Nd : Yag laser with a top-hat intensity distribution is considered. Several diagnostics have been used. For different experimental conditions, the dynamics of the keyhole and its complete geometry (front wall inclination, top and bottom apertures) have been analysed by using on axis visualizations through the top and the bottom of the keyhole with a 4 kHz high speed video camera. The measurement of the transmitted laser power also gives access to the front KW reflectivity. These results are compared and are used to validate our dynamic keyhole modelling, where ray tracing is included. Finally, for these conditions, an analytical model can be proposed that relates the laser parameters (incident laser power, spot diameter) with the processing parameters (material, sheet thickness and welding velocity) for Nd–Yag laser welding.

Investigations of GMAW plasma by optical emission spectroscopy

We report on investigations of gas metal arc welding plasma operated in pure argon and in a mixture of argon and CO2 at a dc current of 326?A. The spatially resolved electron densities and temperatures were directly obtained by measuring the Stark widths of the Ar?I 695.5?nm and Fe?I 538.3?nm spectral lines.Our experimental results show a reduction of the plasma conductivity and transfer from spray arc to globular arc operation with increasing CO2 concentration. Although the electron density ne increases while approaching the core of the plasma in the spray-arc mode, a drop in the electron temperature Te is observed. Moreover, the maximum Te that we measure is about 13?000?K. Our experimental results differ from the Haidar model where Te is always maximum on the arc axis and its values exceed 20?000?K. These discrepancies can be explained as a result of underestimation of the amount of metal vapours in the plasma core and of the assumption of local thermal equilibrium plasma in the model.

Plasma diagnostics in gas metal arc welding by optical emission spectroscopy

The plasma column in a metal inert gas welding process is investigated by optical emission spectroscopy and high-speed imaging. The concentration and repartition of iron vapours are measured and correlated with the plasma and electrode geometric configuration. Plasma temperatures and electron densities are also measured for each studied position in the plasma. The temperatures are calculated using two different methods, allowing validation of the local thermodynamic equilibrium state of the plasma. The results show a maximum temperature of 12 500 K in the upper part of the arc, away from the arc axis. The iron concentration reaches a maximum of 0.3% close to the anode and strongly decreases along both the vertical and radial directions.The plasma thermophysical properties, calculated from this plasma composition, are then discussed regarding the metal transfer mode.

Measurement of atomic Stark parameters of many Mn I and Fe I spectral lines using GMAW process

The particular character of the welding arc working in pure argon, whose emission spectrum consists of many spectral lines strongly broadened by the Stark effect, has allowed measurement, sometimes for the first time, of the Stark parameters of 15 Mn I and 10 Fe I atomic spectral lines, and determination of the dependence on temperature of normalized Stark broadening in Ne = 1023 m−3 of the 542.4 nm atomic iron line. These results show that special properties of the MIG plasma may be useful in this domain because composition of the wire-electrode may be easily adapted to the needs of an experiment.

Analysis of the various melt pool hydrodynamic regimes observed during cw Nd-YAG deep penetration laser welding

We present a experimental study of the analysis of the hydrodynamics evolution of the melt pool during laser Yag welding of stainless steel. The main diagnostic that has been used in this study is a high-speed video camera (10 to 30 kHz) that allowed us to analyze the main different parts that can be observed on such melt pools. For our operating conditions (incident laser power: 4 kW, focal spot diameter: 0.6 mm, stainless steel), we have shown that five characteristic, different and complex hydrodynamic behaviors can be defined when the welding speed varies from typically a few m/min to a few tens of m/min. At low welding speed, the keyhole is quite vertical embedded inside a large pool that fluctuates due to friction effects induced by the quite vertical ejected plume. At high welding speeds, laser interaction is only localized on the keyhole front whose inclination increases with the welding speed. Induced melt flow then dominates and can generate the humping regime, with severe undercuts. For intermediate welding speeds, it is the interaction of the vapor plume with the melt pool that controls its stability. These experiments allow us to confirm that the interaction of the melt pool with the vapor plume, which is emitted with a variable dynamic and direction, perpendicularly from the inclined keyhole front, has an essential role for the melt pool stability and its dynamics in laser welding.We present a experimental study of the analysis of the hydrodynamics evolution of the melt pool during laser Yag welding of stainless steel. The main diagnostic that has been used in this study is a high-speed video camera (10 to 30 kHz) that allowed us to analyze the main different parts that can be observed on such melt pools. For our operating conditions (incident laser power: 4 kW, focal spot diameter: 0.6 mm, stainless steel), we have shown that five characteristic, different and complex hydrodynamic behaviors can be defined when the welding speed varies from typically a few m/min to a few tens of m/min. At low welding speed, the keyhole is quite vertical embedded inside a large pool that fluctuates due to friction effects induced by the quite vertical ejected plume. At high welding speeds, laser interaction is only localized on the keyhole front whose inclination increases with the welding speed. Induced melt flow then dominates and can generate the humping regime, with severe undercuts. For interme...

Study of keyhole behavior for full penetration Nd-YAG cw laser welding

The understanding of keyhole behavior is still a topic of great importance. The keyhole dynamics can be rather easily analyzed in case of full penetration laser welding experiments. Moreover, this configuration can allow an easy access to the complete keyhole geometry, if thin metallic sheets are welded. In that case, one or two laser beam reflections occur on the front keyhole wall. We will present the results of the study of these welding configurations. Several diagnostics have been used. For different experimental conditions, the dynamics of the keyhole and its complete geometry (front wall inclination, top and bottom apertures) have been analyzed by using on axis visualizations through the top and the bottom of the keyhole with a 4 kHz high speed video camera. The measurement of the transmitted laser power gives also an access to the front keyhole wall reflectivity. These results are compared and are used to validate our dynamic keyhole modeling, where ray tracing is included. Finally, for these conditions, an analytical model can be proposed that relates the laser parameters (incident laser power, spot diameter) with the processing parameters (material, sheet thickness and welding velocity) for Nd-Yag laser welding.The understanding of keyhole behavior is still a topic of great importance. The keyhole dynamics can be rather easily analyzed in case of full penetration laser welding experiments. Moreover, this configuration can allow an easy access to the complete keyhole geometry, if thin metallic sheets are welded. In that case, one or two laser beam reflections occur on the front keyhole wall. We will present the results of the study of these welding configurations. Several diagnostics have been used. For different experimental conditions, the dynamics of the keyhole and its complete geometry (front wall inclination, top and bottom apertures) have been analyzed by using on axis visualizations through the top and the bottom of the keyhole with a 4 kHz high speed video camera. The measurement of the transmitted laser power gives also an access to the front keyhole wall reflectivity. These results are compared and are used to validate our dynamic keyhole modeling, where ray tracing is included. Finally, for these cond...

Study of the spray to globular transition in gas metal arc welding: a spectroscopic investigation

The gas metal arc welding (GMAW) process is strongly influenced by the composition of the shielding gas. In particular, addition of CO2 increases the threshold current for the transition from unstable globular to more stable spray transfer mode. We report on the diagnostics—using optical emission spectroscopy—of a GMAW plasma in pure argon and in mixtures of argon, CO2 and N2 while operated in spray and globular transfer modes. The spatially resolved plasma parameters are obtained by applying the Abel transformation to laterally integrated emission data. The Stark widths of some iron lines are used to determine both electron density and temperature, and line intensities yield relative contents of neutral and ionized iron to argon.Our experimental results indicate a temperature drop on the arc axis in the case of spray arc transfer. This drop reduces with addition of N2 and disappears in globular transfer mode when CO2 is added. Despite the temperature increase, the electron density decreases with CO2 concentration. The highest concentration of iron is observed in the plasma column upper part (close to the anode) and for GMAW with CO2.Our results are compared with recently published works where the effect of non-homogeneous metal vapour concentration has been taken into account.

Experimental investigations of the arc MIG‐MAG welding

The type of the applied shielding gas has a strong influence on quality of the welding process. In particular, increase of the percentage of carbon dioxide in argon, causes increase of the transition current value from the globular to spray mode of metal transfer. Observations by fast camera allows to better characterize the arc column shape in the different working modes. The spectroscopic diagnostic of the welding arc is also necessary to understand the observed changes in the mode of droplet transfer. The use of an original diagnostic method allows to estimate the temperature and the electronic density distributions in the plasma without hypothesis on its equilibrium state. Results of this work seem to show that the observed effects could be linked to the microstructural modifications of the anode tip during the MIG‐MAG welding process as a function of the gas composition, and especially to the existence and disappearance of an insulating oxide “gangue” at the wire extremity.

Explorative approach of the spectral analysis tools to the detection of welding defects in lap welding

The study of the voltage and current signals during welding can give information on the process, especially in case of defects. A signal processing software using Fourier and Wavelet transforms has been used to analyse the signals produced during welding under normal conditions and during artificially created defects. Then the fundamental frequency shift, or pitch, has been measured and the frequency variation for each case has been studied. Results showed specific features for each defect type.