Ejection Of Molten Material Research Articles

Morphological and ablation characteristics of brass and fused silica after interaction with ArF excimer laser

This study investigates the interaction of the 193 nm ArF excimer laser with brass and fused silica in the ambient air. The study compares the differences in surface morphology and ablation rate of craters produced in brass and fused silica at different values of fluence and number of laser pulses using optical and scanning electron microscopy. The ablation rate was found to be slightly lower in fused silica than in brass. The ablation rate increased from 180 to 370 nm/pulse in brass and from 120 to 240 nm/pulse in fused silica when increasing the fluence from 6.8 to 15.2 J/cm2. In contrast to fused silica, melting and ejection of molten material were dominant in the ablation of brass. Various shapes and sizes of particles were observed in the debris deposited around the craters created in brass and fused silica. Resolidified particles with sizes greater than 1 µm were observed at and around the crater edges in brass due to the deposition of the ejected liquid droplets. In fused silica, large cauliflower-like particles were observed around the craters. Nano-size particles were also formed in brass and fused silica at low fluence but with a density smaller than that of the large particles formed at high fluence. The threshold fluence calculated from the relationship between ablation rate and fluence was found to be 0.78 and 3.03 J/cm2 in brass and fused silica, respectively.

Experimental investigation on a new hybrid laser process for surface structuring by vapor pressure on Ti6Al4V

Besides conventional structuring processes such as turning, milling or photo-chemical etching, laser processes are increasingly being used for surface structuring of metals. These laser processes differ fundamentally in that structuring is carried out either by material removal or by material redistribution. In this study, a new hybrid process of material ablation by means of pulsed laser radiation and material redistribution based on a remelting process by means of cw laser radiation is experimentally investigated. Besides an introduction to this new hybrid process, we give a detailed description of the equipment and methods used as well as surface structures produced on Ti6Al4V. A melt pool was generated on a prepared Ti6Al4V surface using cw laser radiation with a laser beam diameter of 520 μm, laser power of 220 W, and a scanning velocity of 100 mm/s. In order to create surface structures, simultaneously, superimposed pulsed laser radiation with a laser beam diameter of 65 μm, pulse duration of 60 ns, a maximum pulse energy of 0.35 mJ, and a pulse frequency of 50 kHz was used to evaporate small amounts of molten material from the melt pool. This localized evaporation of molten material is assumed to create vapor pressure that deforms the melt pool surface and therefore leads to surface structures. Our results indicate that by pulsed laser radiation capillary surface waves with a wavelength of the doubled laser beam diameter are excited on the melt pool surface. This forced excitation of capillary surface waves result in surface structures that are analyzed after solidification by means of white light interferometry. Based on this analysis we derived an oscillation frequency of ν = 2.27 (± 0.16) kHz for the excited capillary surface wave as well as an effective kinematic viscosity of μ = 0.1328 cm2 s−1 for the damping of this surface oscillation during solidification. In terms of structural features, we achieved surface structures with heights of up to 100 μm. Furthermore, structure height controllably scales in dependence on pulse energy and number of laser pulses as long as no ejection of molten material takes place. Finally, a comparison of the redistributed material volume per time shows that we achieved a volume redistribution rate of 28.37 mm3/min, which is significantly bigger than has been achieved with other laser texturing techniques so far and demonstrates the high potential of this new hybrid technique not only for surface structuring purposes.

Phase Transformations in Metals Stimulated by a Pulsed High-Energy Electromagnetic Field

Processes occurring in metals with microdefects when metallic specimens are treated by short high-density electric current pulses are considered. The variations in the electric and temperature fields in the material and their influence on the phase transformations and the stress–strain state in the vicinity of microdefects in the form of plane cracks with linear sizes of the order of 10 μm are studied. A mathematical model of the effect of an electromagnetic field on a predamaged thermoelastoplastic material with an ordered system of defects is proposed. The model takes into account melting and evaporation of the material and the dependence of all of its physical and mechanical properties on temperature. The solution of the resulting system of equations is sought by the finite element method on moving grids with the use of the combined Euler–Lagrange method. The dependence of the processes on the boundary conditions of the model is considered. We estimate the error that occurs when solving the problem for one representative cell rather than for the whole sample with an ordered system of defects. The influence of the distance between the cracks on the deformation and healing of microdefects is investigated. Numerical modeling has shown that a high-density current with large field gradients arises in the vicinity of microdefects, which leads to intensive local heating accompanied by thermal expansion and melting of the metal on the tips of the microcracks. This results in high compressive stresses near microcracks, intensive plastic flow of the material and, as a consequence, clamping of microcrack sides, decrease in microcrack length, and ejection of the molten material into the crack. As a result, the microcrack is completely healed. The numerical results obtained by the proposed model agree with experiments. Computations showed that if the distance between microcracks is equal to or greater than ten of their lengths, then the time for complete defect healing weakly depends on the distance between defects and the micro-defect interaction can be neglected. The interaction between microcracks in the metal significantly affects their healing process if the distance between them is reduced to about 5÷6 of microcracks lengths. With further decrease in the distance between the defects up to one microcrack length, the healing process does not change qualitatively, but slows down significantly: the ejection of molten material into the crack still happens, but the crack size reduction, especially in the transverse direction, is substantially smaller.

Toward laser welding of glasses without optical contacting

The welding of transparent materials with ultrashort laser pulse at high repetition rates has attracted much attention due to its potential applications in fields such as optics, microfluidics, optofluidics and precision machinery. One demanding issue is the stable and reliable welding of different materials without the utilization of an intermediate layer or an optical contact. In this work, we maximized the size of the molten volume in order to generate a large pool of molten material which is able to fill an existing gap between the samples. To this end, we used bursts of ultrashort laser pulses with an individual pulse energy of up to $$10\,\upmu \hbox {J}$$ . The laser-induced welding seams exhibit a base area with a size of up to $$450\,\upmu \hbox {m}\,\times 160\,\upmu \hbox {m}$$ . Using these large modifications, we are able to overcome the requirement of an optical contact and weld even gaps with a height of about $$3\,\upmu \hbox {m}$$ . Bulging of the sample surface and ejection of molten material in the gap between the two samples allow to bridge the gap and enable successful welding. We also determined the breaking strength of laser-welded fused silica samples without an optical contact by a three-point bending test. The determined value of up to 73 MPa is equivalent to 85 % of stability of the pristine bulk material.

Interface melding in cold spray titanium particle impact

A focused ion beam (FIB) was used to characterize deformation and bonding behavior of titanium particles, cold sprayed onto a titanium surface. By milling away half of each particle a cross-section was obtained, revealing the particle–substrate interface. It was found that in some sections, voids lined the interface due to mismatch in particle- and substrate-surface topologies. In other places, the interface could not be discerned due to closure of the voids. It is shown that this effect was related to adiabatic shear instability, which caused violent jetting, ejection of molten material from the particle, and thermal softening of the interfacial layers. Deformation characteristics and variation in splat profiles due to microstructural influences, and the effect of particle size are also discussed.

Study on the Mechanism of High Speed Small Hole Drilling by EDM

In this paper, a single-pulse discharge experimental device has been developed to study the mechanism of high speed small hole EDM drilling. It is found that the emulsion used in high speed small hole EDM drilling as dielectric fluid is easier to be broken down than the kerosene which is always used in the traditional EDM. The injection flushing is contributive to the ejection of molten material in the crater produced by discharge, and can greatly improve the evacuating condition. Additionally, the effective quenching ability of emulsion and injection flushing can greatly reduce the necessary pulse interval and increase the effective pulse frequency.

Creation of micro-holes on glass surface by femtosecond laser through the ejection of molten material

The interaction of femtosecond laser pulses (800 nm, 150 fs, 1 kHz) with the surface of fused silica glass was investigated. The morphology and chemical composition of unirradiated and irradiated glass surfaces and of the material ejected from the surface were examined by secondary ion mass spectroscopy (SIMS), scanning electron microscopy (SEM) and X-ray diffraction (XRD). The experimental results show that (i) femtosecond laser pulses induced damage on the surface of the glass by the formation of micro-holes. Material was ejected from the glass surface during laser irradiation while the glass was melted, (ii) the interaction of the laser beam with glass was initially fast but decreased rapidly with time and eventually no change was observed, (iii) no significant changes in the chemical composition of the irradiated glass surfaces were observed. The chemical composition of the ejected and original material was essentially identical, and (iv) the presence of crystalline phase was detected in the ejected material.

Apparatus for the floating-zone refining of gallium arsenide

The design and operation of equipment to purify and to grow single crystals of gallium arsenide are described. Features of the design include a demountable quartz system, adjustment of the relative position of the ends of the gallium arsenide rod during operation, excellent visibility, a traversable heater system and close control of the arsenic pressure in the tube. The latter feature has reduced the problem of ejection of molten material from the zone found in previous work.