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Surface Layer Dynamics During E-Beam Treatment
Abstract
The interaction of a pulsed intense electron beam with a metal target leads to rapid heating and subsequent cooling of the surface layer, accompanied by a series of phase transitions among the solid, liquid, vapor, and plasma phase. As a consequence of the treatment, depending on the beam parameters, the metal target is eroded and a topographical pattern (waviness, craters, etc.,) evolves on its surface. Surface roughening, a major drawback of pulsed intense electron beam treatment, is well known but lacks comprehensive understanding. In this paper, the process of pulsed intense electron beam interaction with metal targets is studied with special attention to the dynamics of the target surface layer and the development of surface roughness. The pulsed electron beam facility GESA generates electron beams with power density 0.5-2 MW/cm2, electron energy up to 120 keV, and pulse duration up to 200 μs. Different fast in situ diagnostics are applied to study the various processes occurring at the target surface: 1) melting and resolidification are visualized by time and space resolved imaging of the surface specular reflectivity; 2) spectroscopy is used to characterize the plasma phase adjacent to the target surface; 3) the evolution of irregularities and bubbles at the surface is studied by high-resolution microscopy; and 4) a stroboscopic imaging technique is applied to catch the evolution of the surface topography. The experimental data are compared with numerical simulations of heat transfer. All results and processes involved in pulsed intense electron beam treatment are discussed with respect to the target surface layer dynamics.
... Intense pulsed electron beams with 120 keV electron energy, 1-2 MW/cm 2 beam power density and 20-50 μs pulse duration (20-80 J/cm 2 beam energy density) as generated by the GESA facility cause the development of a topographical pattern on the target surface with tens of micrometers in height and hundreds of micrometers on the lateral scale. In order to gain knowledge about the interaction of electron beams, with parameters typical for GESA, and metal targets, various fast in situ diagnostic tools were set up and applied in recent years [12][13][14]. From time-and space-resolved surface specular reflectivity measurements, the onset of melting and solidification was determined [12]. ...
... Impact ionization by beam electrons was found to result in a low-density low-temperature target plasma, excluding electric fields in the plasma sheath to be responsible for the growth of the surface features. High-resolution schlieren imaging and a stroboscopic imaging technique allowed the detailed observation of surface irregularities during treatment [14]. Around the onset of melting, bubbles and micro-irregularities were found at the target surface. ...
... Typical profilometer images [13,14] of various targets after treatment with an electron beam of energy density 56 ± 2 J/cm 2 are shown in Fig. 1. Note that the images were obtained in the center of the beam footprint, where also the energy density is taken. ...
Irradiation of metal targets with intense pulsed electron beams leads to rapid melting and solidification of the target surface layers. For beams generated by the GESA facility with 120 kV acceleration voltage, 20-80 J/cm2 beam energy density, and 20-50 μs pulse duration, the induced changes in material properties are accompanied by the evolution of a topographical pattern on the target surface, typically tens of micrometers in height and hundreds of micrometers laterally. In this work, GESA-modified surface layers of stainless steel, copper, and aluminum alloy targets are characterized by a post-treatment study. Larger surface roughness is found for treatment with an electron beam of higher energy density. Similarly, the roughness amplitude increases for the repetitive application of high energy density pulses (60-80 J/cm2, 40-50 μs). Several shorter pulses with lower energy density (~ 30 J/cm2, 25-30 μs) lead to a coarsening of the surface features. The target material inhomogeneity and the aspect of the melt front are investigated by cross-sectional analysis. Material homogenization is observed in the melted surface layer. Straight melt fronts are found for stainless steel, whereas aluminum alloy shows non-uniform melting. Based on the experimental findings, different processes and their influence on the surface layer dynamics during the melted stage are discussed. These include intense evaporation and dissolution of inclusions into the surrounding melt material.
... Therefore, the output waveform can be chosen step-wise arbitrarily. As the beam energy in the GESA process is directly dependent on the acceleration voltage, a flat-top pulse is desirable [8]. Choosing a small stage voltage of 1 kV allows for a voltage ripple of less than 1 % of the maximum output voltage. ...
The challenging demands of pulsed electron beam devices (such as the GESA device) with respect to their pulsed power supply have led to the development of a new semiconductor-based Marx generator. At a maximum output voltage of 120 kV and 600-A pulse current for a duration of up to 100 μS, stepwise arbitrary output waveforms are desired. A fast rise time of the generator is achieved by using fast switching circuitry, low inductance capacitors, and a low inductance stage arrangement. For low jitter triggering of all stages and efficient signal transmission, the generator uses an optical bus system for communication. Due to the inherent dynamic load characteristics of the GESA device, the generator features a fast overcurrent protection scheme. This paper presents selected design aspects of the generator and their validation in a small-scale assembly able of delivering up to 8 kV at 600-A load current.
The paper presents a nonlinear coupled nonisothermal model of the process of material implementation into the target surface under conditions of surface treatment by a particle beam in the form of one or few consecutive pulses. The model takes into account the interaction of impurity diffusion, heat propagation, and mechanical disturbances, as well as the chemical reaction between the introduced impurity and the target material. The problem is solved numerically using an implicit symmetric difference scheme of the second‐order approximation in time and spatial coordinates. The results of solution are obtained for different time intervals. It was found that the interaction of different processes leads to the appearance of distortions in the front of waves. The distributions of temperature and impurity concentration have a wave character. The increment of the reaction product in the dynamics for different numbers of pulses is presented. The interaction mechanisms of the studied processes do not change and do not depend on the number of pulses. However, a more complicated picture is observed under treatment by several successive pulses due to the appearance of new extremes and distortions.
A new pulsed electron beam facility was constructed, which is specially designed to investigate the cathode plasma on the microsecond timescale. The new triode type accelerator GESA-SOFIE is equipped with a large-area explosive emission cathode. The electron beam is focused on the target in a guiding magnetic field. Particle-in-cell code simulations were used to develop an enhanced and more flexible design compared with the previous facilities GESA I and GESA II. This concerned an improved adjustment of static electric field and magnetic field configurations, the possibility to tune shape and position of the electrodes, and an easy access for diagnostic tools to monitor the areas critical for operation. The performance of GESA-SOFIE was demonstrated for various operating conditions and the facility's versatile capabilities were shown. Typical values of GESA-SOFIE operation are electron energy 30-100 keV, beam current 50-180 A, beam cross-section 10-20 cm², and pulse duration up to 200 μs.
The performance of the converging electron beam generated in cylindrical triodes is systematically studied by particle-in-cell code simulations. Depending on the cathode and grid potentials applied, different operation regimes are identified. For low voltages between cathode and grid, laminar flow and homogeneous beam energy density at the target (anode) is obtained. This applies both to the case of unipolar electron flow and to bipolar flow with counter-streaming ions. Hereby, the electron emission current is enhanced by about 50% for bipolar flow compared with unipolar flow. A further increase by about 20% is obtained when electron backscattering at the target is enhanced due to a change of target material from aluminum to tungsten. For cathode-grid voltages exceeding a critical value, laminar flow is replaced by non-laminar flow regimes. For unipolar electron beams, a virtual cathode forms between grid and target, which leads to an inhomogeneous power density at the target. For the specific geometry investigated and the cathode potential fixed at −120 kV, the cathode-grid voltage threshold for the formation of the virtual cathode is ~32 kV for Al targets and ~28 kV for W targets. For bipolar flow, the laminar flow regime already ends at cathode-grid voltages of ~23 kV (Al target) and ~20 kV (W target), respectively, and is replaced by magnetic insulation at the beam edge. For increasing cathode-grid voltage, the magnetically insulated region extends until beam pinching occurs.
Electron beam have many special properties which make them particularly well suited for use in materials handling through melting, welding, surface treatment, etc., taking into account that this manufacturing is performed in vacuum. The use of electron beam for surface limited heat treatment of workpiece has brought about a noticeable extension of the beam technologies. Some theoretical aspects and simulation results are presented in this paper, considering a high power electron beam processing system and Matlab facilities. This paper can be used in power engineering and electro-technologies fields as a guideline, in order to simulate and analyse the process parameters.
In this paper the available data for the measurement of viscosity of aluminium and its alloys are reviewed. Most measurements are performed with an oscillating vessel technique and the merits of this technique are discussed. The purity of the aluminium affects the measured viscosity values and we recommend a value of between 1.0–1.4 mPa·s for the pure element at the melting point. Although studies of the viscosity of aluminium alloys are limited, the effects of elemental additions to the alloy are similar to those for additions to the base metal. Thus an increase in concentration of Ti, Ni, Cr, Mn, Mg tends to increase the viscosity whereas the viscosity decreases with increasing Zn and Si concentrations. Also purification of an alloy decreases the viscosity. There is a wide variety of models ranging from those based on empiricism to thermodynamic methods. With the present quality of input data it is probably better to use a simple rather than a sophisticated model.
This article reviews experiments on the production of low-energy, high-current electron beams (LEHCEB) and their use for surface modification of materials. It is shown that electron guns with a plasma anode and an explosive emission cathode are most promising for the production of this type of beams. The problems related to the initiation of explosive emission and the production and transportation of LEHCEBs in plasma-filled diodes are considered. It has been shown that if the rise time of the accelerating voltage is comparable to or shorter than the time it takes for an ion to fly through the space charge layer, the electric field strength at the cathode and the electron current density in the layer are increased. Experimentally, it has been established that the current of the beam transported in the plasma channel is 1–2 orders of magnitude greater than the critical Pierce current and several times greater than the chaotic current of the anode plasma electrons. Methods for improving the uniformity of the energy density distribution over the beam cross section are described. The nonstationary temperature and stress fields formed in metal targets have been calculated. The features of the structure-phase transformations in the surface layers of materials irradiated with LEHCEBs have been considered. It has been demonstrated that in the surface layers quenched from the liquid state, nonequilibrium structure-phase states are formed.
The process of electron beam interaction with metal targets was characterized using electrical and optical diagnostics. Electron beams with current density of 5–10 A∕cm2, electron energy up to 120 keV, pulse duration up to 200 μs, and cross-sectional area of 8–30 cm2 at the target surface were generated by GESA I and GESA II facilities. Streak imaging of the target surface specular reflectivity was used to determine the onset of melting and re-solidification of the target surface. Using time- and space-resolved schlieren imaging, the evolution of surface irregularities was studied. Experimental and numerical investigations of the neutral flow evaporated from the target surface showed a neutral density of ∼1019 cm−3 in the vicinity of the target and neutral velocities up to 2 × 105 cm∕s. Framing and streak images of visible light emission were used to study the temporal evolution of the target surface plasma and vapors. Time- and space-resolved spectroscopy was applied to determine the surface plasma density and temperature, which were found to be ∼1014 cm−3 and ≤1 eV, respectively. Because of this small plasma density, electric fields in the plasma sheath are not sufficient to cause electrohydrodynamic instability of the liquid target surface. However, hydrodynamic instabilities due to the intense neutral flow observed in experimental and numerical studies are likely to be responsible for the growth of wavelike irregularities.
Experimental evidence of transient self-dewetting of metallic surfaces is presented. Steel surfaces are melted by a pulsed electron gun, and the subsequent fast cooling against its substrate gives rise to the formation of characteristic patterns that we attribute to the dewetting of the liquid film. The patterns formed are similar to those obtained by spinodal dewetting, that is, when the dewetting action develops from a nonlinear instability on the liquid surface, and not from holes nucleation. High-purity iron does not show a similar behavior, indicating that the origin of the instability is due to the influence of the sulfur in the temperature dependence of the surface tension of the melt, which gives rise to a Be’nard-Marangoni instability.
The mechanism of microcrater formation under the action of an intense electron beam is theoretically studied for the clean
surface of a flat solid target with small perturbations in the form of protrusions or cavities. It is shown that the formation
of microcraters is related to the development of instabilities of the Rayleigh-Taylor and Richtmyer-Meshkov types and has
a threshold character with respect to the energy deposited in the target. The laws of microcrater formation in the regimes
of irradiation corresponding to typical experimental conditions are considered.
Metal surfaces treated with pulsed electron beams having a. power density of some MW/cm(2) show a wavelike surface after solidification. For several applications, the waviness of the surface-modified layers has to be limited and, more importantly, controlled. Therefore, the development of such treatment-induced roughness is investigated systematically. Time- and space-resolved imaging of the target surface and measurement of the intensity modulation of a reflected laser beam are the diagnostic tools applied to evaluate the origin and development of the roughness during melting and solidification. The intensity modulation measurements of the reflected laser beam are performed using a HeNe-laser (632.8 nm) in normal reflection. A streak camera (Hamamatsu C7700) with high dynamic range is used, as well as a framing camera by PCO Imaging. This allows the time evolution to be measured and the target surface to be visualized with high spatial resolution. Metals with significantly different physical (paramagnetic and diamagnetic) and thermodynamic properties, like Ti, Ta, stainless steel, Al, Ag, and Au, are investigated in these experiments. The results indicate that the surface waviness mainly depends on the physical and the thermodynamic properties of the target materials and that the electron beam parameters play only a secondary role.
Surface layers made of FeCrAl alloys on T91 steel have shown their capability as corrosion protection barriers in lead bismuth. Pulsed electron beam treatment improves the density and more over the adherence of such layers. After the treatment of previously deposited coatings a surface graded material is achieved with a metallic bonded interface. Creep-rupture tests of T91 in lead-alloy at 550 °C reveal significant reduced creep strength of non-modified T91 test specimens. Oxide scales protecting the steels from attacks of the liquid metal will crack at a certain strain leading to a direct contact between the steel and the liquid metal. The negative influence of the lead-alloy on the creep behavior of non-modified T91 is stress dependent, but below a threshold stress value of 120 MPa at 550 °C this influence becomes almost negligible. At 500 °C and stress values of 200 MPa and 220 MPa the creep rates are comparable between them and significantly lower than creep rates at 180 MPa of original T91 in air at 550 °C. No signs of LBE influence are detected. The surface modified specimens tested at high stress levels instead had creep-rupture times similar to T91 (original state) tested in air. The thin oxide layers formed on the surface modified steel samples are less susceptible to crack formation and therefore to lead-alloy enhanced creep.
Melt layer erosion by melt motion is the dominating erosion mechanism for metallic armours under high heat loads. A 1-D fluid dynamics simulation model for calculation of melt motion was developed and validated against experimental results for tungsten from the e-beam facility JEBIS and beryllium from the e-beam facility JUDITH. The driving force in each case is the gradient of the surface tension. Due to the high velocity which develops in the Be melt considerable droplet splashing occurs.
Results of numerical calculations and experimental studies of the thickness of modified layers, distribution of implanted materials, microstructure and phase composition inside the layers following pulsed electron beam treatment are presented. Possible physical mechanisms of the alloying process are discussed. The improvements of surface properties, especially corrosion resistance against liquid lead, are presented. It is shown that corrosion of OPTIFER IVc steel by liquid lead containing 8×10−6at% oxygen is avoided by alloying Al into the surface.
This letter reports an interesting phenomenon associated with the high-current pulsed electron beam treatment:selective surface purification. The treatment induces crater eruptions that preferentially occur at irregular composition and structure sites. The eruptions of second phase inclusions naturally lead to the purification and homogenization of the melted surface layer. This improves significantly the corrosion resistance of NiTi and 316L alloys.
A literature survey of the experimentally determined values for the surface tension γ of molten pure metals has been carried out. It is intended to provide an assessed data base for use with a mathematical model currently being developed at the National Physical Laboratory, UK, to predict the surface tension of molten metal alloys.
The levitating–drop method has been used to determine surface tension as a function of temperature for four transition metals, and also for samples of type 316 stainless steel which exhibited variable weld penetration during tungsten-inert-gas welding. The results obtained verify the predictions of weld-pool models in that the differences in weldability of the steels can be related to the surface properties of the melts. To obtain good weldability in type 316 steels, it is necessary to ensure that the concentration of uncombined surface-active elements (such as sulphur) should exceed a specified minimum level. An indication of the amounts required for consistent weld penetration is given.MST/194
The paper describes the new pulsed electron beam facilities GESA designed for material surface treatment. These facilities ensure the possibility to realize melting depths of materials in the range of 10–100 μm. Parameters of the electron beam produced are as follows: electron energy 50–400 keV; beam current 200–500 A, beam cross-section 50–100 cm2; pulse duration 5–250 μs, maximum power density at the target −6 MW/cm2, maximum energy density 500 J/cm2. All beam parameters are controlled independently.
The viscosity of aluminium and its alloys-A review of data and models
- A T Dinsdale
- P N Quested
A. T. Dinsdale and P. N. Quested, "The viscosity of aluminium and its
alloys-A review of data and models," J. Mater. Sci., vol. 39, no. 24,
pp. 7221-7228, 2004.