Papers by Keyword: Laser Shock

Abstract: Cold spray utilizes supersonic jets of compressed gas to accelerate powder particles at high velocities. A coating is formed on a substrate by the impact and deformation of particles. Laser Shock consists in illuminating a sample with a pulsed laser to generate a high pressure shock. Cold spray and laser shock are extremely dynamic processes (time scales of about 10-100 ns). In this paper, applications of laser shock for the study of cold spray are presented. A powder particle of a given size and morphology can be laser shock accelerated at cold spray velocities, finally hitting a substrate in a controlled experimental simulation of the process. Results allow also the characterization of powder materials, through the comparison of deformed particles with numerical impact simulations and the fitting of a plasticity model. Two main advantages compared to the split-Hopkinson pressure bar emerge: deformation rates are closer to cold spray conditions and powders are directly tested, rather than macroscopic cylinders. Laser shock can also be used to measure adhesion and internal cohesion of cold-sprayed coatings (LAser Shock Adhesion Test, LASAT). Cold LAser Shock Spray (CLASS), consisting in laser shocking a coating to re-spray it, can be used to characterize property gradient within a coating or as a new spraying process. Laser shock techniques can prove beneficial for the knowledge of powder materials, which is key for advances in cold spray and other powder based processes. Moreover, the combination of the two techniques could lead to hybrid processes.

Abstract: Based on the laser-driven flyer micro forming and laser high-speed impact welding, this paper put forward the laser high-speed impact synchronous welding and forming new process, and builds the compound welding experiment platform. The three-dimensional deep field digital microscope of KEYENCE VHX-1000C was used to measure the surface morphology and the maximum deformation depth of the welding and forming samples. By observing the surface morphology of the sample, it was found that strong plastic deformation occurred on the surface of the materials and well reproduced the shape of the mold. When the laser energy was below 4.5J, the maximum deformation depth of the samples increased with the laser energy. However, the maximum deformation depth decreased due to the spring back phenomenon when the laser energy was larger than 4.5J. The Axio CSM 700 confocal microscope was used to measure the morphology of the welding interface. The cross profile of the welding interface showed that most regions had been welded and the welding interface was nearly flat.

Abstract: Dynamic fragmentation in the liquid state after shock-induced melting, usually referred to as micro-spallation, is an issue of great interest for both basic and applied science. Recent efforts have been devoted to the characterization of the resulting ejecta, which consist in a cloud of fine molten droplets. We present laser shock experiments on tin and aluminium, to pressure ranging from about 50 to 300 GPa, with complementary diagnostics including a Photonic Doppler Velocimeter (PDV) set at a small tilt angle from the normal to the free surface, which enables probing the whole cloud of ejecta¹, and a soft recovery device consisting of a low density gel to collect debris. Optical microscopy of these gel collectors reveals the presence of droplets which confirm shock-induced melting prior to fragmentation. To quantify size distribution of the collected debris, 3D micro-tomography has been performed, using the consistent and high-energy X-ray irradiation available at the ESRF synchrotron facility in France (similar to Japan SPRING-8), where sub-micrometer spatial resolution could be achieved. In this paper, the resulting size distributions are presented and compared with theoretical predictions based on a one-dimensional description accounting for laser shock loading, wave propagation, phase transformations, and fragmentation. Discrepancies between measured and calculated size distributions are discussed. Finally, combining size and velocity data provides access to the ballistic properties of debris and their kinetic energy, which are key issues for anticipating the damage produced by their impacts on nearly equipments.

Abstract: The μLSF is a micro-plastic forming method with the laser-induced shock load generated by the plasma in confined medium. It is characterized in the high-pressure and ultra-high strain ratio. This paper developed the model of metal micro-bulk forming by laser shock, simulated the process of forming and compared the influence law of forming depth under the conditions of different laser energy ranging from 3J to 10.6J and sheet thickness ranging from 0.13mm to 0.30mm on. It showed that there exists a threshold for magnitude of the laser energy. The forming depth increased with the increase of magnitude when the magnitude was smaller than the threshold. However, it decreased with the increase of magnitude when the magnitude was larger than the threshold. When the magnitude of the laser energy was chosen, the forming depth presented non-linear reduction with the increase of sheet thickness.

Abstract: The residual stress distribution of fillet of the diesel engine crankshaft on laser shock processing was analyzed by ANSYS software. The simulation results compared with the experimental data, to get results as follows: the simulation results were basically consistent with the experimental data. This fully proved that the finite element simulation method and related simulation parameters were correct. The parameters of laser impacting were optimized by ANSYS simulation, and the residual stress favorable distribution of the crankshaft fillet was obtained.

Abstract: Damage prediction, adhesion strength and remaining lifetime of TBC are highly important data for understanding and preventing TBC spallation on blades. LAser Shock Adhesion Test (LASAT) is a powerful method to measure adhesion of coating due to its rapidity, simplicity and capabilities to distinguish different strength levels and the easy damage observation in case of TBCs. A new protocol of LASAT has been introduced in order to measure the adhesion level of the ceramic coating from the exploitation of the two-dimensional effects that promotes a shock wave pressure-dependent size of the damage. Finite element modeling, taking into account the TBCs dimensions, showed the edges effect on interfacial stress applied by laser shock.

Abstract: The fatigued specimens have attested the surface hardening of the 2024-T351 aluminum alloy based on the laser shock peening (LSP). Tests were carried out with simple regular specimens bending. It was shown that each crack started from or near to the specimen-hardened surface of the thickness 4mm. The hardened surface layer influenced material fatigue cracking on the Stage I of crack growth as well as on the Stage II where fatigue striations formation mechanism takes place. Parameters of S-N curves were analyzed for tested specimens and bimodal distribution of the fatigue durability (BDFD) was revealed. The left branch of the BDFD attested fatigue behavior of not hardened material. The right branch of S-N curves attested the material behavior after surface-hardening procedure by the LSP technology. The paper discussed mechanisms of material fracture in the bifurcation area for the stresses interval ranged from 270-295 MPa where transition from the left to the right branch of the discovered BDFD takes place. The subsurface crack origination occurs for the right branch of BDFD because of twisting of material volume under combination of material compression in a local area of crack origination with gases diffusion in the volume of crack origin.

Abstract: Most lithography-based micro-forming technology has the limitations in materials selection, forming efficiency and the complexity of micro-parts etc. It is urgent to develop the efficient low-cost micro-forming method in batch. This paper introduced the state-of-the-art of sheet metal micro-forming technology with laser-induced ultra-high strain rate involving the loading modes, processes and forming fundamental. Then, the application of the micro-forming of ultra-high strain rate induced by laser shock is extended into the bulk forming domain, micro bulk forming. It is pointed out that the further investigation on micro bulk forming method focuses on scale effects, forming mechanism, loading and control modes.

Abstract: In Laser Shock Processing when a material is irradiated with short laser pulses (ns)of very high densities(>GW/cm2), a high intensity shock wave is generated. This treatment can reduce the rate of fatigue cracking and stress corrosion cracking in structural metals or alloys needed for aerospace, nuclear power plants, and military applications. And laser shock processing has been shown to be a viable method of strengthen metallic components. Transformation on the characters of aerial engine blade by laser shock processing and influence on the fatigue life with these transformations were studied. And the relatively fatigue life experiment of aerial engine blade was done to validate the influence on the fatigue life of aerial engine by laser shock processing. It was found that laser shock processing could bring residual compressive stress and high-density dislocation on the surface of the blade. All these transformation greatly increase the fatigue life of aerial engine blade.