Mariette Nivard

Spallation generated by femtosecond laser driven shocks in thin metallic targets

Spallation induced by a laser driven shock has been studied for two decades on time scales of nanosecond order. The evolution of laser technologies now opens access to sources whose pulse duration is under the picosecond, corresponding to characteristic times of numerous microscopic phenomena. In this ultra-short irradiation regime, spallation experiments have been performed with time-resolved measurements of the free surface. These measurements, complemented with post-test observations, have been compared with numerical simulations to check the consistency of modelling of the laser–matter interaction, shock propagation and to the study of dynamic damage at this ultra-short time scale, inducing strong tensile stress states at very high strain rates.

State-of-the-art laser adhesion test (LASAT)

This paper proposes a state-of-the-art laser adhesion test. It consists of testing material interfaces with laser-driven shock wave. Since the first demonstration in the 1980s by Vossen, many studies and developments have been done. This paper presents recent experiments and developments on the basic physics involved. Results show the ability of the technique to perform a quantitative adhesion test for a wide range of materials and configurations. Edge effect principle and ultra-short shock wave give perspectives for new applications for multi-layer combination of material. Fundamental principles are evidenced through experiments on bulk ductile materials before demonstrating their application to coated systems.

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.

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.

Lasers and Thermal Spray

Basically, thermal spray and laser processing can be considered as half brothers since they show many common features due to the use of a (more or less) high-energy source for both. Their combination can therefore be very fruitful and prominent to achieve coatings, which results in their most recent and advanced applications. In the materials processing development story, the laser will thus have moved from cutting to coating. This keynote presentation focuses on the recently-developed coupling of laser processing to cold spray). In this dual process, a cold spray gun is combined to a laser head in a single device, e.g. on a robot. Series of coating experiments using various laser irradiation conditions, primarily pulse frequency, were carried out for Al-based and Ni-based alloys. Laser pre-treatment of the substrate just prior to cold spray, was shown to be beneficial for adhesion of cold-sprayed coatings. Adhesion improvement was exhibited and studied from LASATesting (LASAT for “LAser Shock Adhesion Test”). Incidentally, through LASAT also, the role of lasers in the development of thermally-sprayed coatings can be considered as major. Results are discussed in the light of a TEM (Transmission Electron Microscope) study of the coating-substrate interface with and without laser pre-treatment.

Adhesion study of cold-sprayed CoNiCrAlY-Mo coating of inconel 625 using the Laser Shock Adhesion Test (LASAT)

MCrAlY-typed coatings are conventional for applications to land-based turbines against hightemperature oxidation and corrosion. However, improvements are still currently expected from innovations in the coating process and/or in the selection of the starting materials. Both types of innovations were studied in the present work. The former consisted in cold spray as a substitute for plasma spray which is conventionally used as the coating process. The latter consisted in developing mechanically-alloyed powders to be suitable for the targeted application especially. In this study, coating-substrate adhesion was considered as the justice of the peace to assess improvements from these innovations. This was determined using the LAser Shock Adhesion Test, namely LASAT, which was recently developed as innovative adhesion testing of thermal spray coatings. Among the main results, mechanical alloying was shown to be satisfactory to result in an homogeneous powder from the mixing of CoNiCrAlY with Mo. This powder could be coldsprayed, all the more easily because of a fine grain size. Results were compared with those obtained from conventional commercial pre-alloyed powders. As a general result, it was shown that cold spray could lead to highly-dense and high-adhesion MCrAlY-typed coatings onto Inconel 625 even though the process is usually claimed to be convenient for high-ductility materials such as copper. Incidentally, LASAT was confirmed to be a flexible and powerful testing tool to study adhesion; which resulted in the ranking of the various types of coatings involved in the work. Results are discussed in the light of an experimental simulation of the impinging of cold-sprayed particles using so-called “laser flier impact experiments”. In this development of this simulation approach to cold spray, the flier was made of a 50μm-thick disc machined from HIP’ed CoNiCrAlY.

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.

New advances for laser adhesion test (lasat): From thick to thin coatings

This paper present recent developments in LAser Adhesion Test. This technique sollicitates material interfaces with laser driven shock wave. Results show the ability of the technique to do a quantitative adhesion test for wide range of material and configuration. Edge effect principle and ultra-short shock wave give perspectives for new applications for thick and thin coatings. Fundamental principles are evidenced through experiments on bulk ductile materials before demonstrating their application to coated systems.This paper present recent developments in LAser Adhesion Test. This technique sollicitates material interfaces with laser driven shock wave. Results show the ability of the technique to do a quantitative adhesion test for wide range of material and configuration. Edge effect principle and ultra-short shock wave give perspectives for new applications for thick and thin coatings. Fundamental principles are evidenced through experiments on bulk ductile materials before demonstrating their application to coated systems.

Laser drilling in percussion regime: Study of the scale time of the breakthrough

This paper concerns the laser drilling in percussion regime. Direct observation of the rear surface during the breakthrough is performed with high-speed camera. The breakthrough duration increases as function of time from 100 to 280 µs for laser power from 3 KW to 18 KW. For any incident laser power, breakthrough velocity is rather constant at 1.5 m/s. The assistant gas plays role in pushing liquid and vapor from the rear surface. The diameter is larger with assistant gas. Laser transmitted by the rear hole is measured. It is long and intense enough to damage surface located near the rear hole.This paper concerns the laser drilling in percussion regime. Direct observation of the rear surface during the breakthrough is performed with high-speed camera. The breakthrough duration increases as function of time from 100 to 280 µs for laser power from 3 KW to 18 KW. For any incident laser power, breakthrough velocity is rather constant at 1.5 m/s. The assistant gas plays role in pushing liquid and vapor from the rear surface. The diameter is larger with assistant gas. Laser transmitted by the rear hole is measured. It is long and intense enough to damage surface located near the rear hole.

Absorption in laser drilling in percussion regime

This paper presents some new results concerning absorption measurements in laser drilling in percussion regime used in aeronautical engine industries. This process consists in the irradiation of a metallic target with a laser tuned in the MW.cm-2 range (pulse duration in mus-ms range). The laser energy is absorbed by the surface for the heating, the melting and the vaporization of the target. This vapor spreads out and pushes the melt pool. In this case, the drilling is dominated by melt ejection induced by the pressure gradient between the irradiated area and the hole surroundings. Balance energy evaluation is an essential diagnostic to describe laser/matter interaction. Particularly, in laser drilling, the laser/target coupling is related to the penetration of the laser in the target due to the hole formation. Laser beam could be confined in the hole cavity. Measurements have been performed with HL201P laser from Trumpf and integrating sphere coupled with calibrated photodiode. Laser is focused on the target located at the center of the sphere.