Matthieu Schneider

Gas investigation for laser drilling

This article deals with the gas effect on percussion laser drilling in ms pulse duration range. On the one hand, the flow of assistance gas jet is investigated with and without a target using a strioscopy setup and Pitot’s tube. By this way, the position of shock waves in the supersonic jet and near the target surface is revealed. From this characterization, the distance between exit nozzle and target can be optimized to induce higher pressure on surface and protect optics from liquid ejection. On the other hand, metal liquid and vapor jets from irradiated target are observed with a high-speed camera (100 000 Img/sec). Without assistance gas, a surprising result on the video is a shock wave inside the metal vapor jet like a supersonic flow. The assistance gas limits the propagation of the vapor and facilitates the deposition of metallic liquid around the front surface holes.

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...

Recoil pressure and surface temperature in laser drilling

To drill sub-millimeters holes one of the way is laser drilling. Laser drilling is a well-established industrial process from decades; however several fundamental questions are still unsolved about the physical understanding of the laser matter interaction. The sequential description of the laser drilling process is as following. The irradiated surface is heated by absorption of laser energy. Once the surface temperature is higher than the vaporization one, a vapor flow is normal to the local surface. The recoil pressure generated by the evaporation exerts a force on the melted surface and starts expelling the melt layer out of the hole by the side. A Mach shock disc appears in the flow, the ejected vapor flow is then supersonic. The surface goes deeper in the target. This paper relates to an original way for measuring pressure and surface temperature on target from the observations of the supersonic vapor flow. From these observations and with the Prandtl-Meyer function applied on under-expanded supersonic jet one can correlate the surface temperature, the pressure on the melt layer and the Mach number on the supersonic jet. Results show below 22 MW.cm-2 of absorbed intensity the highest surface temperature reached, is in the range of 5000K, with a Mach number of 5 and a recoil pressure about 150 bars.To drill sub-millimeters holes one of the way is laser drilling. Laser drilling is a well-established industrial process from decades; however several fundamental questions are still unsolved about the physical understanding of the laser matter interaction. The sequential description of the laser drilling process is as following. The irradiated surface is heated by absorption of laser energy. Once the surface temperature is higher than the vaporization one, a vapor flow is normal to the local surface. The recoil pressure generated by the evaporation exerts a force on the melted surface and starts expelling the melt layer out of the hole by the side. A Mach shock disc appears in the flow, the ejected vapor flow is then supersonic. The surface goes deeper in the target. This paper relates to an original way for measuring pressure and surface temperature on target from the observations of the supersonic vapor flow. From these observations and with the Prandtl-Meyer function applied on under-expanded superson...

Parametric study of drilling with new innovative laser source: Application to percussion regime

To drill sub-millimeters holes, laser percussion drilling has been a well-established and recognized industrial process for many years. However, inherent factors such as the laser source instability, the number of parameters and the complexity of the phenomenon are still making the physical understanding of this process very difficult. And in particular the gas effect, which is in several models, an element of the dynamic drilling. This paper deals with the first experimental investigations concerning the drilling in percussion regime performed with a new laser HL201P (Nd:Yag) from Trumpf. The laser parameters and the gas influence over hole geometry are among the subjects discussed in this paper. The HL201P laser has some beam properties, which are essential to the understanding of the drilling phenomenon: a great energy and pulse duration stability, a ‘‘top-hat’’ intensity distribution and a constant focal plane location in accordance with the laser parameters.To drill sub-millimeters holes, laser percussion drilling has been a well-established and recognized industrial process for many years. However, inherent factors such as the laser source instability, the number of parameters and the complexity of the phenomenon are still making the physical understanding of this process very difficult. And in particular the gas effect, which is in several models, an element of the dynamic drilling. This paper deals with the first experimental investigations concerning the drilling in percussion regime performed with a new laser HL201P (Nd:Yag) from Trumpf. The laser parameters and the gas influence over hole geometry are among the subjects discussed in this paper. The HL201P laser has some beam properties, which are essential to the understanding of the drilling phenomenon: a great energy and pulse duration stability, a ‘‘top-hat’’ intensity distribution and a constant focal plane location in accordance with the laser parameters.

Residual Stress Analysis of Laser-Drilled Thermal Barrier Coatings Involving Various Bond Coats

The gas turbine combustion chamber of aero-engines requires a thermal barrier coating (TBC) by thermal spraying. Further heat protection is achieved by laser drilling of cooling holes. The residual stresses play an important role in the mechanical behaviour of TBC. It could also affect the TBC response to delamination during laser drilling. In this work, studies of the cracking behaviour after laser drilling and residual stress distribution have been achieved for different bond coats by plasma spray or cold spray. From interface crack length measured pulse-by-pulse after laser percussion drilling at 20° angle, the role of the various bond coats on crack initiation and propagation are investigated. It is shown that the bond coat drastically influences the cracking behaviour. The residual stresses profiles were also determined by the incremental hole-drilling method involving speckle interferometry. An original method was also developed to measure the residual stress profiles around a pre-drilled zone with a laser beam at 90°. The results are discussed to highlight the influence of TBCs interfaces on the resulting residual stresses distribution before laser drilling, and also to investigate the modification around the hole after laser drilling. It is shown that laser drilling could affect the residual stress state.

A fast method for morphological analysis of laser drilling holes

This paper presents an original method for analyzing laser drilled holes. The so-called Direct Observation of Drilled hOle (DODO) method is introduced and its applications. The hole characterization thats been made is compared with x-ray radiography and cross-section analysis. Direct Observation of Drilled hole provides instantaneously surface state, geometric shape, as well as recast layer structure, without additional operation. Since no mounting resin is used to embed the sample, the preparation for analysis is simplified and, gives access to a 3D analysis of hole morphology. The principle of this technique consists in positioning the drilling axis on the joint plane of a butt configuration. Surfaces of the two parts of the sample to be joined are polished beforehand, to increase the contact surface, and then holes are drilled in the joint plane. Once the sample is drilled, the two parts are split so that one half of the hole is in each part of the sample. The preparation time of DODO method samples is shorter than the polishing time of the classical method. Moreover the implementation of the DODO method is much easier, for quality control as well as process development in laser drilling.

Laser impulse coupling measurements at 400 fs and 80 ps using the LULI facility at 1057 nm wavelength

At the Ecole Polytechnique « LULI » facility, we have measured the impulse coupling coefficient Cm (target momentum per joule of incident laser light) with several target materials in vacuum, at 1057 nm and 400 fs and 80 ps pulse duration. A total of 64 laser shots were completed in a two-week experimental campaign, divided between the two pulse durations and among the materials. Our main purpose was to resolve wide discrepancies among reported values for Cm in the 100 ps region, where many applications exist. A secondary purpose was to compare Cm at 400 fs and 80 ps pulse duration. The 80 ps pulse was obtained by partial compression. Materials were Al, Ta, W, Au, and POM (polyoxymethylene, trade name Delrin). One application of these results is to pulsed laser ablation propulsion in space, including space debris re-entry, where narrow ranges in Cm and specific impulse Isp spell the difference between dramatic and uneconomical performance. We had difficulty measuring mass loss from single shots. Imparted ...

Influence of incident angle on laser drilling

To drill sub-millimeter holes, laser percussion drilling has been a well-established industrial process for tens years. However, physical understanding is still quiet difficult because laser source can be instable with low pulse reproducibility, correlation between experimental parameters and hole morphology are not clearly identified. This paper deals with the study of hole morphology in function of peak power and incident angle. Holes are drilled with a millisecond laser source, their morphologic characteristics are essentially diameter, recast layer thickness, and depth. Results are based on a new and very fast hole analysis method (called DODO for Direct Observation of Drilled hOle). The influence on hole morphology of incident angle and peak power in a string of pulse are shown (drilled depth, diameter, recast layer thickness,…). Results reveals incident angle is not a determinant parameter in percussion drilling. The influence of peak power is fitted in the string pulse to eliminate the recast layer cracking. It comes from a solidification of a melt layer on a previous recast layer. To eliminate it from the hole, it is essential to melt the previous recast layer with higher peak power pulse than the previous one. With this drilling method, hole drilled has only one single recast layer at the end of the drilling, so hole cannot present some decohesion.To drill sub-millimeter holes, laser percussion drilling has been a well-established industrial process for tens years. However, physical understanding is still quiet difficult because laser source can be instable with low pulse reproducibility, correlation between experimental parameters and hole morphology are not clearly identified. This paper deals with the study of hole morphology in function of peak power and incident angle. Holes are drilled with a millisecond laser source, their morphologic characteristics are essentially diameter, recast layer thickness, and depth. Results are based on a new and very fast hole analysis method (called DODO for Direct Observation of Drilled hOle). The influence on hole morphology of incident angle and peak power in a string of pulse are shown (drilled depth, diameter, recast layer thickness,…). Results reveals incident angle is not a determinant parameter in percussion drilling. The influence of peak power is fitted in the string pulse to eliminate the recast layer...

Gas investigation on laser drilling

To drill sub-millimeters holes one of the way is laser drilling. Laser drilling is a well-established industrial process for tens years. However this process doesn’t get well under control. The influence of the inherent factors are not still understood. This article deals with the gas effect on drilling. We investigate the flow of an assist gas without and with a target with the help of a strioscopy set up. By this way, we see the position of the shock waves in the supersonic jet and on target surface. We inspect nature of the metal vapor jet from irradiated target and the influence of these different flows on drilling process.To drill sub-millimeters holes one of the way is laser drilling. Laser drilling is a well-established industrial process for tens years. However this process doesn’t get well under control. The influence of the inherent factors are not still understood. This article deals with the gas effect on drilling. We investigate the flow of an assist gas without and with a target with the help of a strioscopy set up. By this way, we see the position of the shock waves in the supersonic jet and on target surface. We inspect nature of the metal vapor jet from irradiated target and the influence of these different flows on drilling process.

Analysis and possible estimation of keyhole depths evolution, using laser operating parameters and material properties

The authors propose an analysis of the effect of various operating parameters on the keyhole depth during laser welding. The authors have developed a model that uses the analysis of the thermal field obtained in 2D geometry, which is mainly defined by the characteristic Peclet number. This allows us to show that the dependence of the aspect ratio R of the keyhole with the operating parameters of the process is a function of two parameters: a normalized aspect ratio R0, controlled by the incident laser power and the spot diameter, and a characteristic speed V0 related to the process of heat diffusion. The resulting general law R = f (R0, V/V0) appears to be very well verified by different experimental data and allows to define mean thermophysical parameters of the used materials. These data can then be used for keyhole depths prediction for any subsequent operating parameters of the same material. This model also allows us to define precisely a criterion for a keyhole threshold generation. The authors will apply the derived procedure to successfully analyze experiments on materials with very different thermophysical properties (such as steel alloys and copper), with various focal spots, incident laser powers, and welding speeds.The authors propose an analysis of the effect of various operating parameters on the keyhole depth during laser welding. The authors have developed a model that uses the analysis of the thermal field obtained in 2D geometry, which is mainly defined by the characteristic Peclet number. This allows us to show that the dependence of the aspect ratio R of the keyhole with the operating parameters of the process is a function of two parameters: a normalized aspect ratio R0, controlled by the incident laser power and the spot diameter, and a characteristic speed V0 related to the process of heat diffusion. The resulting general law R = f (R0, V/V0) appears to be very well verified by different experimental data and allows to define mean thermophysical parameters of the used materials. These data can then be used for keyhole depths prediction for any subsequent operating parameters of the same material. This model also allows us to define precisely a criterion for a keyhole threshold generation. The authors will...