Zone Melting Research Articles

The influence of shielding gases on keyhole-induced porosity and nitrogen absorption in SS 304 stainless steel fiber laser welds

Regarding porosity formation and gas content, choosing the appropriate shielding gas for laser welding is essential for achieving high-quality joints. Keyhole-induced porosity formation tendency and nitrogen content in SS 304 stainless steel welds were investigated based on the nitrogen content in shielding gases during fiber laser welding. Beads-on-plate autogenous welds were made at 5 kW continuous wave (CW) fiber laser in N2 and Ar mixtures. Optical metallography, micro-focused X-ray, X-ray radiography, and high-speed images of the molten pool were used to investigate the porosity formation. In addition, a gas analyzer was used to study the weld metal nitrogen content. The results show that nitrogen significantly impacts the reduction of porosity in the melting zone and increases the dissolved nitrogen in the solidified weld metal, as using pure nitrogen leads to an increase in dissolved nitrogen by 36% higher than the nitrogen content in the base metal. In contrast, it has almost no significant effect on the keyhole mode.

Temperature distribution in the crust and upper mantle of the territory of Ukraine

The study aims to build a three-dimensional thermal model of the crust and upper mantle of the territory of Ukraine. Its basis is a scheme of deep processes in the tectonosphere, which, first of all, considers the results of heat and mass transfer during modern activation. They are superimposed on the models of the platform (except for the territory of the East European platform; the Donbas is also included in it), the Alpine geosyncline of the Carpathians, and the Hercynian-Cimmerian geosyncline of the Scythian plate. The incomplete process of modern activation cannot be accurately described by the geological theory used by the authors. Gravity modeling was previously conducted on a system of profiles around the northern hemisphere with a total length of more than 30,000 km, crossing Eurasia, North America, as well as the Atlantic and Pacific oceans to select an adequate scheme of heat and mass transfer. The paper distinguishes the most realistic scheme of the process . It is applied for Ukraine, and the more accurately the activated area is determined. Such a task was solved for the first time. In the south, the model is limited by the Black Sea depression, at a depth of 400 km. Temperatures in the transition zone to the lower mantle were not considered. Test thermal models are compared with geothermometers. The error (50°C) of calculation and cross-section of isotherms is determined (150°C for depths from 50 to 400 km, at a depth of 25 km the error is lower, the cross-section of isotherms is 100°C). Zones of partial melting of the rocks of the crust and upper mantle have been established. They are distributed in the middle part of the crust, in the upper horizons of the mantle (50-100 km). At a depth of about 400 km, partial melting occurs only under the non-activated part of the platform. Differences in the model from the presented one are described. They are associated with possible variations in the age of the process and its peculiarities at different levels of heat and mass transfer. Practical significance. The study emphasizes that mineral deposits are characteristic to thermal anomalies and other environmental parameters.

Surface defects related to polishing cycle in ß-Ga2O3 crystals grown by floating zone

We report an investigation on ß-Ga2O3 Schottky barrier diodes performed on substrates grown by floating-zone method using structural characterization techniques (secondary-ion mass spectrometry, inductively coupled plasma-mass spectroscopy, and atomic force microscopy) and electrical measurements (current-voltage, capacitance-voltage, Hall effect, and capacitance deep-level transient spectroscopy). Four distinct electron trap levels labeled ES, E1, E2, and E3 were found in the range of 1 eV below the Ga2O3 conduction band minimum. Among them, E1, E2, and E3 show signatures similar to those reported in the literature for Czochralski and edge-defined film-fed grown ß-Ga2O3 substrates. Trap ES was found near the surface, and we could establish a link between this defect and the damage induced by the substrate polishing technique. The level related to ES was identified at ∼0.31 eV below the conduction band minimum. An energy band above 0.31 eV was also detected and is associated with states at the metal–semiconductor interface. We demonstrated that the interface states and surface deep traps are not uniformly distributed on the ß-Ga2O3 surface. Furthermore, they contribute to the reverse leakage current and the on-state conduction degradation of the diodes.

Seismic observation of an active detachment faulting system beneath the Longqi hydrothermal field at the ultraslow spreading southwest Indian ridge

Hydrothermal processes in detachment settings at slow and ultraslow spreading ridges differ greatly from those at melt-rich faster spreading ridges. Active detachment faulting provides the possibility for off-axis high-temperature hydrothermal vents located far away from the heat source beneath the axial volcanic ridge. Seismic data from the slow spreading Mid-Atlantic ridge revealed that hydrothermal fluids may exploit detachment faults to extract heat from a melt zone near the crust-mantle interface. However, knowledge of the subsurface structure and the kinematic processes of detachment faults, and their interaction with hydrothermal fields at the slowest spreading ridges is still insufficient. Here, we report new insights on the seismicity beneath the Longqi hydrothermal field at the ultraslow spreading southwest Indian ridge from three ocean bottom seismometer monitoring experiments. The seismicity outlines the subsurface geometry of a detachment faulting system (DF1 and DF2). The strongly flexed DF1 (45°) is a mature detachment fault with a domed-shaped OCC where ultramafic rocks are exposed on the seafloor. An active young DF2 with intense earthquake activity along the steep subsurface (67°) suggests the initiation phase of rotation. The diversity of hydrothermal activities in the Longqi detachment-hydrothermal systems is closely related to the evolution of detachment faults. Additionally, both the non-transform discontinuity on the western margin of the Longqi-1 field and local faults facilitate hydrothermal circulation. Our study provides baseline observations for a Longqi-type of hydrothermal circulation at the inside corner associated with detachment faulting and non-transform offsets.

Combustion Behavior and Microstructure of TC17 Titanium Alloy under Oxygen-Enriched Atmosphere

TC17 titanium alloy is widely used in the aerospace industry, but its combustion behavior and microstructure after combustion are rarely investigated. Herein, the ignition critical oxygen pressure, combustion velocity, and microstructure after the combustion of TC17 titanium alloy were investigated by promoted ignition combustion tests under an oxygen-enriched environment. The results indicated that there were three stages, ignition, splash, and flame propagation, for the combustion process of the TC17 alloy. As compared to TC11 titanium alloy, the TC17 titanium alloy exhibited a similar ignition critical oxygen pressure with the same size, but an obviously faster burning rate, which followed a power law relationship with the oxygen pressure. The segregation of Cr, Mo, and Al was observed in the interdendritic phase of the melting zone and the interface between the melting zone and the heat-affected zone. The segregation of Cr at the liquid/solid interface can be responsible for accelerating the burning kinetic of the TC17 alloy by decreasing the interfacial temperature.

Direct-write laser-assisted patterning of form birefringence in wave plates fabricated by glancing-angle deposition

The development of optics that provide spatial control of birefringence could enable better control of laser beam polarization, but available solutions are limited. Here we demonstrate a method to locally modify the birefringence of wave plates fabricated by glancing-angle deposition. The method employs localized melting of the anisotropic microstructure in a vacuum environment to alter the local birefringence. We demonstrate that this process is only possible under high vacuum to avoid trapping air within the melt zone. The direct-write method presented here can be readily utilized for coatings exhibiting form birefringence of virtually any chemical composition, size, and format.

Effects of Temperature Difference and Heat Loss on Oscillation Characteristics of Thermo-Solutocapillary Convection in Toluene/N-Hexane Mixed Solution

During the crystal growth process using the floating zone method, the uneven distribution of impurities on the surface of the melt can trigger a coupling mechanism between solutocapillary convection driven by the concentration gradient and thermocapillary convection driven by the temperature gradient, resulting in the Marangoni convection at the free surface. When the temperature and concentration gradients reach certain values, the crystal surface and interior exhibit time-dependent, periodic oscillations, leading to the formation of micrometer-scale impurity stripes within the crystal. This study focuses on the effects of temperature difference and heat loss in a liquid bridge under microgravity on the structure and interface oscillation characteristics of thermo-solutocapillary convection, aiming to explore the coupling phenomenon of this oscillation and provide valuable information for crystal growth processes. An improved level set method is employed to accurately track every displacement of the interface, while the surface tension is addressed using the CSF model. In addition, the area compensation method is used to maintain simulation quality balance. A comprehensive analysis is performed on the oscillation characteristics of thermo-solutocapillary convection at the free surface, ranging from the temperature, concentration, deformation, and velocity distributions at the upper and middle heights of the liquid bridge. The results indicate that under small temperature differences (ΔT = 1 − 3), the transverse velocity at the upper end exhibits a single-periodic oscillation, while the longitudinal velocity presents a double-periodic oscillation. At the intermediate height, both the transverse and longitudinal velocities display a single-periodic oscillation. Under a large temperature difference (ΔT = 6), the oscillation of velocities at the upper end and the middle position become multi-periodic. In addition, heat loss has certain regular effects on the oscillatory flow of thermo-solutocapillary convection within a certain range. The velocity, amplitude, and frequency of the upper end and the middle position at the free surface decrease gradually, and the oscillation intensity also weakens with the increase in heat loss (Bi = 0.2 − 0.6). These new discoveries can provide a valuable reference for optimizing the crystal growth process, thereby enhancing the quality and performance of crystal materials.

Data-driven automated control algorithm for floating-zone crystal growth derived by reinforcement learning

The complete automation of materials manufacturing with high productivity is a key problem in some materials processing. In floating zone (FZ) crystal growth, which is a manufacturing process for semiconductor wafers such as silicon, an operator adaptively controls the input parameters in accordance with the state of the crystal growth process. Since the operation dynamics of FZ crystal growth are complicated, automation is often difficult, and usually the process is manually controlled. Here we demonstrate automated control of FZ crystal growth by reinforcement learning using the dynamics predicted by Gaussian mixture modeling (GMM) from small numbers of trajectories. Our proposed method of constructing the control model is completely data-driven. Using an emulator program for FZ crystal growth, we show that the control model constructed by our proposed model can more accurately follow the ideal growth trajectory than demonstration trajectories created by human operation. Furthermore, we reveal that policy optimization near the demonstration trajectories realizes accurate control following the ideal trajectory.

Growth and phase transition of Sr3Zr2O7 single crystals

The melting behavior of Ruddlesden-Popper type hybrid improper ferroelectric Sr3Zr2O7 phase in the ZrO2–SrO pseudo-binary system was investigated, and its single crystals were successfully grown. A series of the slow-cooling floating zone experiments revealed that Sr3Zr2O7 melts incongruently into SrZrO3 phase and a liquid and that the compositional range where Sr3Zr2O7 and a liquid coexist is located around 70 mol% SrO composition. Based on the results, we attempted to grow Sr3Zr2O7 single crystals by the traveling solvent floating zone method using SrO-excess solvent and feed. Consequently, many small single crystals of Sr3Zr2O7 phase with several millimeters in size were discovered in the as-grown boules covered with SrO phase. The phase transition behavior of the grown crystals was investigated by differential thermal analysis with polarizing optical microscopy as well as by optical second harmonic generation measurements. We directly observed a reconstruction of orthorhombic twin domains in Sr3Zr2O7 single crystals accompanied by the first-order ferroelectric transition at about 410 °C.

Single-Crystal Growth of a Cubic Laves-Phase Ferromagnet HoAl2 by a Laser Floating-Zone Method

The successful growth of single crystals of a cubic Laves-phase material HoAl2 with the space group Fd-3m is reported in this study. The crystals were grown by a floating-zone method with five laser diodes as a heat source. Al-rich feed rods were prepared as compensation for heavy evaporation during the growth. The nominal ratio for the feed rods was optimized as Ho:Al = 1:2.5. Single crystals of HoAl2 with a length of 50 mm were first grown in this technique. Obtaining the large-sized crystal by the floating-zone method enabled us to systematically explore the physical properties using the same batch crystal. The crystal possessed a second-ordered ferromagnetic transition at 29 K and a first-ordered spin-reorientation transition at 20 K. The bulk physical properties, such as specific heat, magnetic susceptibility, isothermal magnetization, and thermal expansion measurements, were measured. Additionally, a magnetocaloric effect was evaluated by the magnetic entropy change. We demonstrate that anisotropic physical properties along the principal axes ([100], [110], and [111]) emerged below the magnetically ordered states, in contrast to the isotropic behavior in the paramagnetic state.

Investigation of radio-photoluminescence properties in Sm-doped Al4SiO8 single crystals

We report radio-photoluminescence (RPL) properties of Sm-doped Al4SiO8 single crystals as a new RPL material. The Sm-doped Al4SiO8 single crystals were synthesized by the floating zone method. In PL emission spectra, the emission peaks due to the 4f-4f transitions of Sm3+ ions and 2EJ–4A2 transitions of Cr3+ ions were observed before X-ray irradiation, and the ones due to the 4f-4f transitions of Sm2+ appeared as well as Sm3+ after X-ray irradiation. The RPL dose response functions showed linear response from 1 mGy to 10 Gy for the 2.0% Sm-doped sample and from 10 mGy to 10 Gy for the others. Sm2+ ions of the new emission center completely disappeared by heat treatment at 500 °C for 10 min.

Functional Prototype for the Validation of the Innovative Metal Additive Manufacturing SLEDM Process

Additive manufacturing has found its way into industrial series production. However, market growth has only been possible through the introduction of new AM-technologies and the continuous improvement of existing processes. This contribution covers the practical implementation of a prototype for research and validation of the unique SLEDM process patented by TU Graz. The demonstrator includes a feature for powder coating for the melting process as well as the concept of power input via an LED-based light source. The prototype enables a now simple and fast melting of different low-melting powder materials, such as tin, zinc, and aluminum, within an inert gas atmosphere. In addition, the experimental setup forms the basis for the development and adaptation of a suitable light source for the next generation SLEDM process. The goal is to test the additive manufacturing concept with the innovative LED light source and analyze the produced metal parts. The evaluation of the recorded measurement data and the analysis of the test results provide important knowledge about the potential of the SLEDM process for further research and industrial manufacturing. In the future, the individual control of the LED-cells in the matrix can be utilized for optimized temperature profiles in the melting zone and for the reduction of the energy demand of the machine. Overall SLEDM helps to fulfil the targets of sustainable production addressing both the material and energy aspect.

Effect of additive manufacturing of SUS316L using selective laser melting

Process parameters, i.e., laser power and scanning speed, play a key role in the fabrication of high-quality parts fabricated by selective laser melting. In this work, the microstructure, microhardness and porosity of selective laser-melted additive manufactured parts are studied. The porosity of the finished product depends largely on the energy density used, much more strongly on the scanning speed and less so on the applied laser power. The microstructure shows features attributable to a difference in cooling rates close to and farther away from the melting zone of the laser.

Constraints on ground deformation processes at the Tulu Moye volcanic complex, Main Ethiopian Rift

Tulu Moye is an actively deforming volcanic complex with a geothermal field in the Main Ethiopian Rift. We use InSAR between 2014 and 2022, integrated with other geophysical data, to investigate the temporal and spatial characteristics of the deformation signal in the area, and to model its source. Velocity maps and time series analysis show a deformation signal consistent with uplift at a velocity of up to 50 mm/yr in the satellite Line-of-Sight (LOS) in 2014–2017, then decreasing to 12 mm/yr until 2022. The centre of deformation is located about 10 km west of a main geothermal drilling site at Tulu Moye, between the Bora, Berecha, and Tulu Moye volcanoes, with a NW-SE elongation direction. Our best-fit model suggests that the deformation is caused by an 8.7 km by 1.2 km sill situated ∼7.7 km below the surface (∼5.9 km below sea-level), elongate in the N54°W direction and dipping S11°W, and experienced an average velocity of volume change of ∼8.9 × 106 m3/yr in 2014–2017. The surface projection of the sill overlaps with local transverse faults and hydrothermal manifestations. The sill is ∼1–2 km below clusters of microseismic swarms and a region of high resistivity, both indicating hydrothermal fluid flow. The location and geometry of the sill correlates with the upper edge of high conductivity interpreted as a zone of partial melt, and we therefore attribute the uplift at the Tulu Moye volcanic complex to inflow of magma in the sill. We also suggest that the transverse caldera rims faults may restrict magma flow, and also facilitate both vertical and lateral hydrothermal fluid flow.

Thermal stress characteristics of monocrystalline silicon induced by ns pulse laser under auxiliary heating

Aiming at the problems of unsmooth notch and even crack caused by thermal stress in the laser processing of monocrystalline silicon, the temperature and thermal stress under auxiliary heating (AH) induced by the ns pulse laser (NPL) and their distribution characteristics are studied. Based on the theory of heat conduction and elastic–plastic mechanics, a two-dimensional axisymmetric geometric model for the temperature and thermal stress distribution of monocrystalline silicon irradiated by the NPL under AH is established, and the effect of AH on laser-induced temperature and thermal stress field is calculated and analyzed. The results show that the temperature of monocrystalline silicon irradiated by the NPL increases over auxiliary heating temperature (AHT), but it is not a simple superposition between the AHT and the temperature induced by the NPL, but the temperature change rate gradually decreases. When the temperature is lower than the target melting point, the thermal stress at the target surface is always negative as compressive stress, and the change law is the same as that of the temperature. When the temperature is more than the target melting point, the thermal stress in the melting zone is released immediately. The thermal stress decreases with the AHT increasing, and the change rate gradually decreases too. The negative effect of thermal stress can be overcome by using the correlation between temperature and thermal stress. Finally, an experiment was introduced to validate the theoretical model and calculation result preliminary. This study can provide a theoretical basis for the new technology of laser processing-monocrystalline silicon.

Structure and mechanical properties of welded joints from alloy based on VTI-4 orthorhombic titanium aluminide produced by pulse laser welding

Ti2AlNb-based alloys are promising materials for operation at high temperatures in aerospace industry. Meanwhile, the existing difficulties of weldability restrict opportunities of their application. This work is devoted to studies of welded joints from Ti2AlNb-based VTI-4 alloy, obtained using pulsed laser welding (PLW). The optimum PLW modes have been determined providing uniform faultless joint. The features of formation of external defects, internal pores, cracks and non-uniform penetration depth were detected depending on welding conditions. The main PLW parameters influencing on formation of welded joint are voltage and duration of laser pulse. It was demonstrated that at insufficient medium and high peak powers sawtooth seam roots and internal pores can be formed. However, at higher rates of energy input thermal hydraulic processes in welding bathe are violated, accompanied by metal splashing (spattering), heterogeneity of pulse imposition is observed. This leads to formation of cracks, higher porosity, heterogeneity of melting zone, and as a consequence, poor mechanical properties. Microstructure analysis of the welded joints obtained by means of PLW has demonstrated that the melting area is comprised of long dendritic grains of β phase, and the heat affected zone from two regions of β + α2phases and β + α2+ O phases. Herewith, the achieved joint strength equals to ~80 % of the base metal produced using the optimum PLW mode.

Effects of laser floating zone processing on electrical and magnetic properties of (Ca,Pr)MnO3 system

Manganites present fascinating physical properties which can be tuned according with the method and processing conditions. In this work, Ca1-xPrxMnO3 (x = 0.03, 0.06, 0.1) materials were prepared by the Laser Floating Zone (LFZ) technique, under different pulling rates (25–100 mm/h) in air and argon atmospheres, obtaining fibres with 2.5 mm diameter. Such processing conditions promote the formation of secondary phases in (Ca,Pr)MnO3 fibres. The formation of the perovskite phase at slow pulling rates was observed. The effects of the LFZ conditions on the phase composition, microstructural features, electric and magnetic properties of as-grown and thermally post-processed fibres are analysed and discussed. The results indicate that the LFZ processing conditions have a great impact on the electrical and magnetic properties of the prepared manganites. Secondary phases increase with pulling rate and the effects imposed by their formation can be minimized by additional heat treatment at high temperatures, improving the electric conductivity and magnetization of the thermally-treated fibres.

Analysis on phase distribution and flow field morphology in double side blown gas-liquid mixture flows with high temperature and high density melt

In view of the gas-liquid two-phase flow process in the oxygen-enriched side-blown molten pool, the phase distribution and manifold evolution in the side-blown furnace under different working conditions are studied. Based on the hydrodynamics characteristics in the side-blown furnace, a multiphase interface mechanism model of copper oxygen-enriched side-blown bath melting was established. The results show that the existence of slag baffle makes the velocity distribution range of melting mixing zone wider, the mixing range of melt wider, the mixing effect better, and the role of melting zone strengthened. The average speed of case MY in the melting zone is the highest, nearly 5.2% higher than that of case EY, and 16.7% higher than that of case EN. Compared with the average turbulent kinetic energy in the melting zone, case MY has a significant advantage of 2.635 m2/s2, which is increased by 47%, 34%, and 14% respectively compared with case EN, case EY, and case MN.

Effects of Preparation Atmosphere and Doping Concentration on Scintillation and Photoluminescence Properties of Lu2O3:Eu Scintillation Single Crystals

Lu2O3:1%Eu and Lu2O3:8%Eu series single crystals were grown by optical floating zone method under air, N2, and H2-Ar mixed atmosphere (5 vol%H2 and 95 vol%Ar). The effects of preparation atmosphere and doping concentration on the scintillation and photoluminescence properties have been investigated and discussed. Correlated measurements of afterglow curves, X-ray excited luminescence spectra, photoluminescence excitation and photoluminescence, photoluminescence decay, and thermal stimulated luminescence curves were performed. Although Eu(S6) luminescence can be significantly depressed under oxidizing atmosphere, Lu2O3:8%Eu grown under weak reducing atmosphere presents the lowest afterglow intensity. This phenomenon was explained by different interstitial oxygen atom concentrations in the samples.

Removing of Fe, Pb and Hg from Crude Selenium by Fractional Crystallization

This paper focuses on a fractional crystallization methodology using a rotating and internally gas-cooled crystallizer to purity crude selenium. Experiments using a rotating and gas-cooled crystallizer (cooled finger) were performed. The distribution coefficients of the main impurities (Pb, Fe and Hg) in selenium were presented as a polynomial function of concentration. The experimental parameters such as crystallization temperature and rotation rate were determined and discussed. The appropriate crystallization temperature is 222 °C and the rotation rates are 120 and 300 rpm, respectively. The purity of crude selenium increased from 99.9% to over 99.997%. Compared with the traditional method such as zone melting, this method only takes less than one day to complete several purifications, and the purification effect is better than the former. The removal rates of Hg, Pb and Fe in Se are 28.70%, 97.63% and 96.28%, respectively. The direct yield of Se purified is 92.5%. This study provides an efficient process for high-purity selenium, which has important industrial applications.