Effect of laser heating temperature on coating characteristics of Stellite 6 deposited by cold spray

Laser-assisted cold spray (LCS) is a new coating and fabrication process which combines some advantages of CS: solid-state deposition, retain their initial composition and high build rate with the ability to deposit materials which are either difficult or impossible to deposit using cold spray alone. Stellite 6 powder is deposited on medium carbon steels by LCS using N 2 as carrier gas pressure. The topography, cross section thickness, structure of the coatings is examined by SEM, optical microscopy, EDX. The results show that thickness and fluctuation of coating are improved with increased deposition site temperature. Porosity of coating is affected by N 2 and deposition site temperature. In this paper, it presents optimal coating using N 2 at a pressure of 3 MPa and temperature of 450°C and deposition site temperature of 1100°C.

An improved three-dimensional model for growth of oxide films induced by laser heating

In this paper we consider a three-dimensional heat diffusion model to explain the growth of oxide films which takes place when a laser beam is shined on and heats a metallic layer deposited on a glass substrate in a normal atmospheric environment. In particular, we apply this model to the experimental results obtained for the dependence of the oxide layer thickness on the laser density power for growth of TiO2 films grown on Ti-covered glass slides. We show that there is a very good agreement between the experimental results and the theoretical predictions from our proposed three-dimensional model, improving the results obtained with the one-dimensional heat diffusion model previously reported. Our theoretical results also show the occurrence of surface cooling between consecutive laser pulses, and that the oxide track surface profile closely follows the spatial laser profile indicating that heat diffusive effects can be neglected in the growth of oxide films by laser heating. © 2001 Elsevier Science B.V.

One-dimensional analytical model for oxide thin film growth on Ti metal layers during laser heating in air

This paper presents the theoretical and experimental results for oxide thin film growth on titanium films previously deposited over glass substrate. Ti films of thickness 0.1 μm were heated by Nd:YAG laser pulses in air. The oxide tracks were created by moving the samples with a constant speed of 2 mm/s, under the laser action. The micro-topographic analysis of the tracks was performed by a microprofiler. The results taken along a straight line perpendicular to the track axis revealed a Gaussian profile that closely matches the laser's spatial mode profile, indicating the effectiveness of the surface temperature gradient on the film's growth process. The sample's micro-Raman spectra showed two strong bands at 447 and 612 cm -1 associated with the TiO 2 structure. This is a strong indication that thermo-oxidation reactions took place at the Ti film surface that reached an estimated temperature of 1160 K just due to the action of the first pulse. The results obtained from the numerical integration of the analytical equation which describes the oxidation rate (Wagner equation) are in agreement with the experimental data for film thickness in the high laser intensity region. This shows the partial accuracy of the one-dimensional model adopted for describing the film growth rate. © 2001 Elsevier Science B.V.

Laser heating of polymers

Zur Untersuchung von Effekten beim Laserheizen von Polymeren wurde ein Temperaturmessaufbau entwickelt. Das Messprinzip basiert auf der Auswertung der thermischen Emission. Der Messaufbau besteht aus einer hochauflösenden Kamera, ausgestattet mit Bildverstärker, sowie Interferenzfiltern um eine spektrale Auflösung zu gewährleisten und einem gepulster NIR-Heizlaser. Die Pulsdauer des Lasers liegt in der Größenordnung von 10 µs, der Strahldurchmesser durch entsprechende Fokussierung in der Größenordnung von 10 µm. Mittels Fit des Planck‘schen Strahlungsgesetzes an die aufgenommene thermische Emission konnten 2D Temperaturgraphen erhalten werden. Eine Ortsauflösung von 1 µm und eine Zeitauflösung von 1 µs konnten realisiert werden. In Kombination mit Finite-Elemente-Simulationen wurde mit diesem Aufbau die Laserablation verschiedener Polymere untersucht. Dabei hat sich gezeigt, dass bei Polymeren mit einem Glasübergang im Temperaturbereich zwischen Raum- und Zerfallstemperatur, photomechanische Ablation stattfand. Die Ablationsschwelle lag für diese Polymere mehrere 10 K über dem Glasübergang, weit unter der Zerfallstemperatur aus thermogravimetrischen Experimenten mit typischen Heizraten von 10 K/min. Bei hohen Laserenergien und damit verbundenen hohen Temperaturen konnte dagegen thermischer Zerfall beobachtet werden. Ein Übergang des Mechanismus von photomechanischer Ablation zu Ablation durch thermischen Zerfall ergab sich bei Temperaturen deutlich über der Zerfallstemperatur des Polymers aus der Thermogravimetrie. Dies wurde bedingt durch die kurzen Reaktionszeiten des Laserexperiments in der Größenordnung der Pulsdauer und steht im Einklang mit dem Gesetz von Arrhenius. Polymere ohne Glasübergang im Heizbereich zeigten dagegen keine photomechanische Ablation, sondern ausschließlich thermischen Zerfall. Die Ablationsschwelle lag auch hier bei höheren Temperaturen, entsprechend dem Gesetz von Arrhenius. Hohe Temperaturen, mehrere 100 K über der Zerfallstemperatur, ergaben sich darüber hinaus bei hohen Laserenergien. Ein drastisches Überhitzen des Polymers, wie in der Literatur beschrieben, konnte nicht beobachtet werden. Experimentelle Befunde deuten vielmehr darauf hin, dass es sich bei dem heißen Material um thermische Zerfallsprodukte, Polymerfragmente, Monomer und Zerfallsprodukte des Monomers handelte bzw. das Temperaturprofil der Zerfallsreaktion selbst visualisiert wurde.

Transient Temperature Distribution in a Sheet Metal by Low-Energy Laser Scanning

Low-energy laser-heating techniques are widely used in engineering applications such as, thinfilm deposition, surface treatment, metal forming and micro-structural pattern formation. In this paper,under the conditions of ignoring the thermo-mechanical coupling, a numerical simulation on the spatialand temporal temperature distribution in a sheet metal produced by the laser beam scanning in virtue of thefinite element method is presented. Both the three-dimensional transient temperature field and thetemperature evolution as a function of heat penetrating depth in the metal sheet are calculated. Thetemperature dependence of material properties was taken into account. It was shown that, after taking thetemperature dependence of the material absorbance effect into consideration, the temperature change ratealong the scanning direction and the temperature maximum were both increased.

Time and space-resolved measurement of a gas-puff laser-plasma x-ray source

The dynamic interaction processes between a nano-second laser pulse and a gas-puff target, such as those of plasma formation, laser heating, and x-ray emission, have been investigated quantitatively. Time and space-resolved x-ray and optical measurement techniques were used in order to investigate time-resolved laser absorption and subsequent x-ray generation. Efficient absorption of the incident laser energy into the gas-puff target of 17%, 12%, 38%, and 91% for neon, argon, krypton, and xenon, respectively, was shown experimentally. It was found that the laser absorption starts and, simultaneously, soft x-ray emission occurs. The soft x-ray lasts much longer than the laser pulse due to the recombination. Temporal evolution of the soft x-ray emission region was analyzed by comparing the experimental results to the results of the model calculation, in which the laser light propagation through a gas-puff plasma was taken into account. (C) 2003 American Institute of Physics.

Study of thermal behaviors in CO2 laser irradiated glass

To understand mechanisms underlying laser-induced damage of BK7 and fused silica, we calculate the temperature field of the substrates with CO2 laser irradiating at a given laser power and beam radius. We find that the two glasses show different thermal behaviors. A model is developed for estimating the time t to heat the surface of the substrates up to a particular temperature T with cw CO2 laser irradiation. We calculate theoretically the duration t that the samples are irradiated, from the beginning to visual catastrophic damage, with the assumption of damage threshold determined by the critical temperature. The duration t that the samples are irradiated, from the beginning to visual catastrophic damage, is investigated experimentally as well. Here we take the melting point or softening point as the critical temperature, given the thermomechanical coupling properties, which is enough to cause damage for BK7. Damage features are characterized by the sound of visual cracks. Finally, we calculate stresses induced by laser heating. The analysis of stress indicates that the damage of BK7 is due to the stresses induced by laser heating. (c) 2005 Society of Photo-Optical Instrumentation Engineers.

Study of thermal behaviors in CO2 laser irradiated glass

To understand mechanisms underlying laser-induced damage of BK7 and fused silica, we calculate the temperature field of the substrates with CO2 laser irradiating at a given laser power and beam radius. We find that the two glasses show different thermal behaviors. A model is developed for estimating the time t to heat the surface of the substrates up to a particular temperature T with cw CO2 laser irradiation. We calculate theoretically the duration t that the samples are irradiated, from the beginning to visual catastrophic damage, with the assumption of damage threshold determined by the critical temperature. The duration t that the samples are irradiated, from the beginning to visual catastrophic damage, is investigated experimentally as well. Here we take the melting point or softening point as the critical temperature, given the thermomechanical coupling properties, which is enough to cause damage for BK7. Damage features are characterized by the sound of visual cracks. Finally, we calculate stresses induced by laser heating. The analysis of stress indicates that the damage of BK7 is due to the stresses induced by laser heating. (c) 2005 Society of Photo-Optical Instrumentation Engineers.

Recent developments of the Laser-assisted Cold Spray process and deposit characterisation

Laser-assisted Cold Spray (LCS) is a new coating and fabrication process which combines the supersonic powder beam found in Cold Spray (CS) with laser heating of the deposition zone. LCS retains the advantages of CS; solid-state deposition, high build rate and the ability to deposit onto a range of substrates, while reducing operating costs by removing the need to use gas heating and helium as the process gas. Recent improvements in powder delivery and laser energy coupling to workpiece have been undertaken to improve deposition efficiency (DE) and build rate, while real-time temperature logging allows greater management of deposition conditions and deposit characteristics.

Supersonic laser deposition of Ti and Ti64 alloys

The importance of metal coating technologies drives the continuous improvement of metal deposition techniques for application in a wide range of industrial sectors. This work presents the foundations of a new process technology for the deposition of Ti and Ti64 coatings on various substrates using supersonic powder streams and impact site laser heating. Full density metallic deposits are obtained under appropriate impact conditions without the need for transiting the melting point of the deposited material or substrate leading to large energy savings. Details of the experimental approach will be presented along with the general characteristics of the titanium coatings produced using this novel coatings method.

High speed titanium coatings by supersonic laser deposition

The importance of metal coating technologies drives the continuous improvement of metal deposition techniques for application in a wide range of industrial sectors. This work presents the foundations of a new process technology for the deposition of titanium coatings on steel tube substrates using supersonic powder streams and impact site laser heating, known as Supersonic Laser Deposition (SLD). Metallic deposits are obtained under appropriate impact conditions without the need for exceeding the melting point of the deposited material or substrate leading to improved coating quality. Details of the experimental approach are presented along with the general characteristics of the titanium coating produced using this novel coatings method. © 2011 Elsevier B.V. All rights reserved.

Laser-heated high-temperature NMR - a time-resolution study

Submitted to Appl Magn Reson Sponsorship: EPSRC / EU

Fast plasma heating in a cone-attached geometry - towards fusion ignition

We have developed a PW (0.5 ps/500J) laser system to demonstrate fast heating of imploded core plasmas using a hollow cone shell target. Significant enhancement of thermal neutron yield has been realized with PW-laser heating, confirming that the high heating efficiency is maintained as the short-pulse laser power is substantially increased to a value nearly equivalent to the ignition condition. It appears that the efficient heating is realized by the guiding of the PW laser pulse energy within the hollow cone and by self-organized relativistic electron transport. Based on the experimental results, we are developing a 10kJ-PW laser system to study the fast heating physics of high-density plasmas at an ignition-equivalent temperature.

Atomistic-continuum modeling of ultrafast laser-induced melting of silicon targets

In this work, we present an atomistic-continuum model for simulations of ultrafast laser-induced melting processes in semiconductors on the example of silicon. The kinetics of transient non-equilibrium phase transition mechanisms is addressed with MD method on the atomic level, whereas the laser light absorption, strong generated electron-phonon nonequilibrium, fast heat conduction, and photo-excited free carrier diffusion are accounted for with a continuum TTM-like model (called nTTM). First, we independently consider the applications of nTTM and MD for the description of silicon, and then construct the combined MD-nTTM model. Its development and thorough testing is followed by a comprehensive computational study of fast nonequilibrium processes induced in silicon by an ultrashort laser irradiation. The new model allowed to investigate the effect of laser-induced pressure and temperature of the lattice on the melting kinetics. Two competing melting mechanisms, heterogeneous and homogeneous, were identified in our big-scale simulations. Apart from the classical heterogeneous melting mechanism, the nucleation of the liquid phase homogeneously inside the material significantly contributes to the melting process. The simulations showed, that due to the open diamond structure of the crystal, the laser-generated internal compressive stresses reduce the crystal stability against the homogeneous melting. Consequently, the latter can take a massive character within several picoseconds upon the laser heating. Due to the large negative volume of melting of silicon, the material contracts upon the phase transition, relaxes the compressive stresses, and the subsequent melting proceeds heterogeneously until the excess of thermal energy is consumed. A series of simulations for a range of absorbed fluences allowed us to find the threshold fluence value at which homogeneous liquid nucleation starts contributing to the classical heterogeneous propagation of the solid-liquid interface. A series of simulations for a range of the material thicknesses showed that the sample width we chosen in our simulations (800 nm) corresponds to a thick sample. Additionally, in order to support the main conclusions, the results were verified for a different interatomic potential. Possible improvements of the model to account for nonthermal effects are discussed and certain restrictions on the suitable interatomic potentials are found. As a first step towards the inclusion of these effects into MD-nTTM, we performed nanometer-scale MD simulations with a new interatomic potential, designed to reproduce ab initio calculations at the laser-induced electronic temperature of 18946 K. The simulations demonstrated that, similarly to thermal melting, nonthermal phase transition occurs through nucleation. A series of simulations showed that higher (lower) initial pressure reinforces (hinders) the creation and the growth of nonthermal liquid nuclei. For the example of Si, the laser melting kinetics of semiconductors was found to be noticeably different from that of metals with a face-centered cubic crystal structure. The results of this study, therefore, have important implications for interpretation of experimental data on the kinetics of melting process of semiconductors.