A theoretical study of the evaporation induced by a pulsed laser in a finite slab

The two-dimensional integral Laplace transform technique has been applied to get the temperature distributions in the molten layer, the solid part and in the backward surface of a finite target irradiated with a pulsed laser. Formulas for the time dependence of the evaporated part and the molten layer thicknesses of the target were found. This is accomplished by considering the temperature dependence of the absorption coefficient of the irradiated surface as well as the chemical reaction. As an illustrative example computations were carried out on an aluminum (Al) target.     

On the evaporation of a semi-infinite target induced by a pulsed laser

The two-dimensional Laplace integral transform technique has been applied to get the temperature distributions in the molten and the solid parts of a semi-infinite target irradiated with a pulsed laser. Formulas for the time dependence of the evaporated part and the molten layer thicknesses of the target were found considering the temperature dependence of the absorption coefficient of the irradiated surface as well as the chemical reaction. As the illustrative example computations were carried out on an aluminum target.     

Evaporation of a thin film coated on a substrate induced by a pulsed laser

The two-dimensional Laplace integral transform technique has been applied to get the spatial and temporal temperature distributions in both the molten layer thickness of a thin film coated on a substrate, the still solid part of the thin film of the target and the temperature distribution in the substrate. Also a formula for the time dependence of the evaporated part of the thin film of the target as well as the molten layer thickness of the thin film were obtained. Calculations of the obtained relations were carried out during the irradiation with a pulsed laser. The derivation has taken into account the temperature-dependent absorption coefficient of the irradiated surface and the chemical reaction in the vapor of the thin film. As an illustrative example, computations were carried out on an aluminum thin film coated on a glass substrate.     

A theoretical study on the melting of a finite slab with a pulsed laser

Calculations of the spatial and temporal temperature distributions in the molten layer thickness, the still solid part and in the backward surface of the finite slab were carried out during the irradiation with a pulsed laser. The two-dimensional Laplace integral transform technique has been applied to obtain the mathematical expressions for these temperature distributions and the molten layer thickness as a function of the melting time. The derivations have taken into account the temperature-dependent absorption coefficient of the irradiated surface and the cooling. As an illustrative example, computations were carried out on a finite aluminum (Al) target.     

On melting a semi-infinite target using a pulsed laser

The two-dimensional Laplace integral transform technique is used to solve the problem of melting a semi-infinite target induced by surface absorption of a laser pulse.Mathematical expressions for the temperature distribution within the solid part of the target and the thickness of its liquified part were obtained considering cooling, temperature dependent absorption coefficient of the irradiated surface and constant temperature in the molten layer. As an illustrative example computations were carried out on a semi-infinite aluminium (Al) target.     

The temperature profile in the molten layer of a thin-film coated on a substrate induced by irradiation with a pulsed laser

The two-dimensional Laplace integral transform technique has been applied to get the temperature distributions in the molten layer of a thin-film coated on a substrate, the solid part of the film and the substrate during the irradiation with a pulsed laser. The derivation has taken into account the cooling and the temperature-dependent absorption coefficient of the irradiated surface. As an illustrative example computations were carried out on an aluminum thin-film coated on a glass substrate.     

Estimation of laser solid forming process based on temperature measurement

By using a moving disc heat source model, an analytical model was developed to describe laser solid forming (LSF) process with the feedback of the surface temperature of the molten pool, which can be used to estimate the geometric characterizations (width and height) of the clad layer rapidly. An on-line temperature measurement system was established and some single-pass cladding experiments were conducted while the molten pool temperature was monitored. It was found that the estimated geometric characterizations agreed well with the experimental results. In addition, the power consumed by conduction, convection, radiation, evaporation and absorption during LSF were also estimated by the model. It was shown that the majority of the total absorbed power was conducted to the substrate. The effective model can not only be used to optimize the processing parameters but also potentially applied to the real-time feedback control.     

Laser melting of carbide tool surface: Model and experimental studies

Laser controlled melting is one of the methods to achieve structural integrity in the surface region of the carbide tools. In the present study, laser heating of carbide cutting tool and temperature distribution in the irradiated region are examined. The phase change process during the heating is modeled using the enthalpy–porosity method. The influence of laser pulse intensity distribution across the irradiated surface (β) on temperature distribution and melt formation is investigated. An experiment is carried out and the microstructural changes due to laser consecutive pulse heating is examined using the scanning electron microscope (SEM). It is found that melt depth predicted agrees with the experimental results. The maximum depth of the melt layer moves away from the symmetry axis with increasing β.     

Laser repetitive pulse heating of tool surface

Laser heating of a cemented carbide tool is considered and the temperature field as well as phase changes in the heated region is modeled. Temperature rise, liquid layer thickness, and mushy size are predicted numerically. A control volume approach is introduced to solve the governing equations of heat transfer and phase change. Consecutive pulses with the duty cycle of 60% are accommodated in the simulations in line with the experimental conditions. An experiment is carried out to treat the cemented carbide tool surfaces using the CO₂ laser delivering consecutive pulses. The treated surfaces and their cross-sections are examined using the scanning electron microscope (SEM). It is found that the temperature gradient is high along the laser beam axis resulting in cracks at the irradiated surface. The rapid solidification of the surface causes compact structures with very fine grains in the surface region of the laser irradiated spot.     

Interaction of beam and coated metals at high power continuous irradiation

The beam-matter interaction with various coating effects has received continued attention in the high power laser community. Previous works suggest that coatings promote target damage when compared to beaming on uncoated surface. Three types of paint coatings (acrylic urethane, silicone alkyd and stealth blend) and a water coat on metals (Al, Ti and STS) are irradiated with a CO₂ laser. Both strain and temperature measurements are provided for assessing the instantaneous response characteristics of each coating on different metals. A selective combination of surface coats with metals has been proven to be effective in either preventing or enhancing damage, both thermal and mechanical, associated with focused beaming on a target.     

Near-critical nanosecond laser-induced phase explosion on graphite surface

Optical reflectivity, removal rate and ablative recoil pressure magnitudes were measured as a function of laser fluence during high-power UV nanosecond laser ablation of graphite. At low fluences only melting and weak surface vaporization of molten carbon were observed. At moderate fluences there is a very narrow fluence interval where the reflected fluence starts to saturate, while the removal rate and ablative recoil pressure rise drastically in a correlated manner, indicating the onset of a near-critical surface phase explosion. Then, at higher fluences the reflected fluence, removal rate and recoil pressure saturate with an appearance of a luminous plume, altogether indicating negligible specular reflectance and absorbance on the target surface due to its complete screening by the highly-absorbing laser plume. The overall strong correlation between the removal rate and recoil pressure magnitudes may indicate rather quasi-continuous removal of the near-critical superheated molten carbon layer by a propagating unloading wave in the absence of a crucial sub-surface temperature maximum in the layer.     

Laser interference metallurgy: A new method for periodic surface microstructure design on multilayered metallic thin films

Methods for micro- and nanostructuring are essential for functionalization of materials surfaces. In particular, photon-assisted methods for synthesis of functional surfaces have been intensively investigated in the last years. In this study, a new method for surface modification and production of long-range order periodical structures called “laser interference metallurgy” is explored. A metallic thin film sample consisting of three layers composed of Fe, Cu and Al (from top to bottom) on a glass substrate was irradiated with an interference pattern using a Nd:YAG laser (wavelength of 355 nm, 10 ns of pulse duration). For the interference pattern, a configuration producing a line-type energy distribution was chosen. The laser fluence was high enough to melt the aluminium and copper layers at the interference maxima but the iron layer remained in the solid state. Thus, diffusive and convective exchange occurred between aluminium and copper at the energy maxima positions leading to periodical alloy formation with a long-range order. Because it remained in solid state, the iron layer at the top acted as a protective layer effectively preventing removal of the molten layers. The interaction of the different layers was characterized using FIB, TEM and EDX in STEM mode.     

Laser short pulse heating of metal nano-wires

Non-equilibrium energy transfer takes place in a solid substrate during a short-pulse laser irradiation and temperature field can be obtained analytically in the irradiated region. In the present study, laser short-pulse heating of metal nano-wire is considered and the analytical solution for two-dimensional axisymmetric nano-wire is presented. Since the absorption of the incident beam takes place in the skin of the irradiated surface, a volumetric heat source resembling the absorption process is incorporated in the analysis. Three different nano-wire materials are introduced in the analysis for the comparison reason. These include silver, chromium, and copper. It is found that temperature decay is gradual on the surface vicinity and temporal variation of the surface temperature follows almost the laser pulse intensity profile at the irradiated center.     

Laser-induced thermal stresses on steel surface

In laser heat treatment of steels, a thin surface layer of austenite forms during heating and subsequent phase change process in the cooling period. However, thermal stress develops due to high-temperature gradient attainment in the surface vicinity which in turn results in microcrack development at the surface. The present study is carried out to compute the temperature profiles due to step input pulse laser radiation and determine the resulting thermal stresses. The study is extended to include three-step input pulses having the same energy content. This provides the comparison for the influence of the pulse length on the resulting thermal stresses. To validate the theoretical predictions, an experiment is conducted to irradiate the AISI 4142 steel surface by an Nd–YAG laser. Microphotography and EDS analysis of the heated regions are carried out. It is found that considerable thermal stress is eveloped at the workpiece surface due to attainment of high-temperature gradient in this region. In addition, microcracks are observed at the surface of the irradiated spot.     

Supersonic laser spray of aluminium alloy on a ceramic substrate

Applying a ceramic coating onto a metallic substrate to improve its wear resistance or corrosion resistance has attracted the interest of many researchers during decades. However, only few works explore the possibility to apply a metallic layer onto a ceramic material. This work presents a novel technique to coat ceramic materials with metals: the supersonic laser spraying.In this technique a laser beam is focused on the surface of the precursor metal in such a way that the metal is transformed to the liquid state in the beam-metal interaction zone. A supersonic jet expels the molten material and propels it to the surface of the ceramic substrate.In this study, we present the preliminary results obtained using the supersonic laser spray to coat a commercial cordierite ceramic plate with an Al-Cu alloy using a 3.5 kW CO₂ laser and a supersonic jet of Argon.Coatings were characterized by scanning electron microscopy (SEM) and interferometric profilometry.     

激光辐照下金属／炸药温度场的计算

 用有限元模型分析了激光辐照下金属/炸药双层材料的温度分布,得到了炸药表面中心温升与激光强度、光斑尺寸以及激光作用时间的关系,还得到了激光对金属/炸药装置点火的初步规律,分析了亚音速气流的对流换热对靶面温度场的影响。     

激光辐照下金属／炸药温度场的计算

用有限元模型分析了激光辐照下金属/炸药双层材料的温度分布,得到了炸药表面中心温升与激光强度、光斑尺寸以及激光作用时间的关系,还得到了激光对金属/炸药装置点火的初步规律,分析了亚音速气流的对流换热对靶面温度场的影响。     

Surface microrelief smoothing mechanisms in a target irradiated by an intense charged particle beam

Physical mechanisms and a mathematical model are proposed to describe the smoothing of the microrelief of an irradiated surface. Before melting, smoothing is caused by elastoplastic deformation of the material; after the melting of the surface layer of the irradiated target, smoothing is caused by the dynamics of the target boundary under the action of surface tension forces, viscous forces, and inertial forces. The results of numerical simulation agree qualitatively and quantitatively with experimental data. The proposed mathematical model can be used to predict changes in the microrelief of the irradiated surface and to choose irradiation conditions when solving the problems of radiation technologies.     

Modification of surface oxide layers of titanium targets for increasing lifetime of neutron tubes

The peculiarities of interaction of hydrogen ions with a titanium target and its surface oxide layer were studied. Two ways of modification of the surface oxide layers of titanium targets for increasing the lifetime of neutron tubes were proposed: (1) deposition of an yttrium oxide barrier layer on the target surface; (2) implementation of neutron tube work regime in which the target is irradiated with ions with energies lower than 1000 eV between high-energy ion irradiation pulses.     

The surface modified composite layer formation with boron carbide particles on magnesium alloy surfaces through pulse gas tungsten arc treatment

A novel fabrication process of surface modified composite layer by pulse current gas tungsten arc (GTA) surface modification process was used to deposit B₄C particles on the surface of magnesium alloy AZ31. This method is an effective technique in producing a high performance surface modified composite layer. During the pulse current GTA surface modification process, considerable convection can exist in the molten pool due to various driving forces and the pulse current could cause violent stirring in the molten pool, and the large temperature gradient across the boundary between the GTA modified surface and matrix metal resulted in rapid resolidification with high cooling rates in the molten pool, so that the process result notable grain refinement in the GTA surface modified composite layer. The hardness and wear resistance of the GTA surface modified composite layer are superior to that of as-received magnesium alloy AZ31. The hardness values and wear resistance of GTA surface modified composite layer depend on the GTA process parameters and the B₄C particles powder concentration and distribution. The optimum processing parameters for the formation of a homogeneous crack/defect-free and grain refinement microstructure were established.