Frederic Coste

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.

Study of CW Nd-Yag laser welding of Zn-coated steel sheets

The welding of Zn-coated steel thin sheets is a great challenge for the automotive industry. Previous studies have defined the main physical processes involved. For non-controlled conditions, the zinc vapour expelled from the interface of the two sheets violently expands inside the keyhole and expels the melt pool. When using CO2 lasers, we have previously shown that an elongated laser spot produces an elongated keyhole, which is efficient for suppressing this effect. We have adopted a similar approach for CW Nd : Yag laser welding and we observe that an elongated spot is not necessary for achieving good weld seams. Several diagnostics were used in order to understand these interesting results. High-speed video camera visualizations of the top and the bottom of the keyhole during the process show the dynamics of the keyhole hydrodynamic behaviour. It appears that the role of the reflected beam on the front keyhole wall for generating a characteristic rear wall deformation is crucial for an efficient stabilization of the process. Our dynamic keyhole modelling, which includes ray tracing, totally confirms this interpretation and explains the results for very different experimental conditions (effect of welding speed, laser intensity, variable sheet thickness, laser beam intensity distribution) that will be presented.

Metallic vapor ejection effect on melt pool dynamics in deep penetration laser welding

The hydrodynamics phenomena involved during the laser welding process are very complex. Among the possible involved mechanisms controlling the hydrodynamics of the weld pool, it is well known that the melt displacement resulting of the local pressure applied to the liquid surface as a result of the evaporation process, plays an important role. We would like to show in this article that, using specific experiments based on twin or triple spot interaction geometry, the friction forces of the metal vapor escaping from the keyhole are also shown to have a very important role for the hydrodynamics of the melt pool along and around the keyhole.

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...

Analysis of laser–melt pool–powder bed interaction during the selective laser melting of a stainless steel

The laser powder bed fusion (LPBF) or powder-bed additive layer manufacturing process is now recognized as a high-potential manufacturing process for complex metallic parts. However, many technical issues are still to overcome for making LPBF a fully viable manufacturing process. This is the case of surface finish and the systematic occurrence of porosities, which require postmachining steps. Up till now, the porosity origin remains unclear but is expected to be related to the stability of the process. As a LPBF part is made by the accumulation of hundreds of meters of small weld beads, it also appears to be important to understand all the phenomena that occur during the laser-powder-melt pool (MP) interaction for each single track. For this reason, in the first part of our study, using an instrumented LPBF setup and a fast camera analysis (>10 000 image/s), single tracks were fabricated and analyzed in real time and postmortem. Spatters ejections and powder denudation phenomena were observed together wit...

Laser-delayed double shock-wave generation in water-confinement regime

This paper investigates the different physical processes involved during laser-delayed double shock-wave generation in water-confined geometry. With this technique, two laser pulses, separated by a Δt duration, irradiate a target immersed in water at an intensity of a few GW/cm2 and form a high pressure plasma which results in a double shock-wave generation. This 2 pulses configuration is currently being investigated as an attractive method for improving the LASer Adhesion Test (LASAT) [L. M. Barker and R. E. Hollenbach, J. Appl. Phys. 43, 4669–4674 (1972)] technique by adapting the time delay Δt to the position of interfaces. The LASAT technique is a noncontact adhesion test allowing to generate a high-level tensile stress near interfaces with the use of laser-driven shock wave. The generation of two delayed high-intensity shock waves by laser plasma in the water-confinement regime has been investigated at 10ns@532 nm with the new Nd:YAG laser GAIA from Thales Laser company in the new facility HEPHAISTOS. For each incident Gaussian laser impulsion, the characterization of the high-amplitude laser-plasma-generated shock wave and its propagation through the target has been performed using a velocity interferometer system for any reflector [L. Berthe et al., “State-of-the-art laser adhesion test (LASAT),” Nondestr. Test. Eval. 26(3–4), 303–317 (2011)]. The new laser facility allows us a nanosecond-control of the time delay between the two laser pulses and a precise control of each laser maximum-intensity. Therefore, the influence of the first laser-induced plasma, on the second shock-wave generation has been studied by modifying different parameters such as the delay Δt and the intensity I1 and 12 of each pulse and different aluminum plate thicknesses from 0.2 to 1.5 mm. Preliminary tests show that the maximum pressure level of the second generated shock wave is sensitive to the time delay between the two impulsions and influenced by the plasma generated by the first laser pulse.

Experimental study of CO2 laser welding inside a groove— Application to high thickness laser welding

Laser welding inside a groove has to be considered when a high thickness material is welded. The use of a groove shape allows the access of the laser beam to the lower part of the groove. The choice of the groove geometry (groove width, depth, or aperture angle), as well as the assist gas setup (leading, trailing or backside protection) are then significant operating parameters for the process control. In this study, U- and V-shaped groove welding with a CO2 laser was investigated. The U-shaped groove welding generates an increase in penetration depth, a very strong reduction of the process noise and of the plasma plume volume, compared to a bead on plate or a V-shaped groove welding. Also, the penetration depth for a U-groove welding is 20% greater than for a V groove. The groove geometry and the surface tension effects are determinant for these improvements. The influence of shielding gas shows that a trailing protection (gas flow emitted from the nozzle towards the welding direction) should be adopted ...

Deep penetration laser welding with Nd:YAG lasers combination up to 11 kW laser power

The use of a combination of 2 or 3 Nd:YAG lasers having each an output power of 3 or 4 kW is presented in the case of heavy section welding. We discuss the main difficulties that are occurring for these conditions. The strategy leading to the welding of sample thickness up to 60 mm is presented.