Melt Hydrodynamics Research Articles

Numerical study of melt hydrodynamics in pulsed laser processing of nickel-based superalloy

Pulsed laser processing (PLP) of materials has wide-spread applications in the industry, yet the underlying complex material response to the incident laser pulse especially the melt flow mechanism is not fully understood. Knowledge of melt hydrodynamics can provide systematic theoretical guidance for the improvement of PLP quality. In this paper, a two-dimensional numerical model was built to simulate melt flow during PLP of nickel-based superalloy. The driving forces for melt hydrodynamics at different stages of laser pulse irradiation were studied comprehensively. Numerical results reveal that thermo-capillary stress drives radially-outward melt flow at the initial stage of laser pulse irradiation. At the middle stage, evaporation-induced recoil pressure is the dominant driving force for melt pool deformation, creating a depression at the center and a hump at the periphery. Thermo-capillary force has a minor effect on melt pool deformation but a large contribution to the flow velocity field. At the post-irradiation stage, the peripheral molten hump flows back rapidly and the melt pool experiences some gradually damping oscillations under the action of Young-Laplace stress. Moreover, it was demonstrated that laser power density plays an important role in deciding the melt flow characteristics and the relative contributions of different driving forces. The results in this study provide an unambiguous physical picture of the melt hydrodynamics in pulsed laser processing.

ФОРМУВАННЯ СТРУКТУРИ ЛИСТОВИХ ЗАГОТОВОК ПРИ ВАЛКОВІЙ РОЗЛИВЦІ МЕТАЛІВ

The physical modeling method was used to study the processes of formation of the structure of sheet blanks during ingotless casting-rolling of metals. At the same time, on the basis of theoretical analysis, the main parameters are determined, with the help of which it is possible to effectively control the processes of crystallization and the formation of the structure of sheet blanks. The studies carried out made it possible to establish the regularities of the influence of hydrodynamic and thermophysical conditions of pouring metal (the speed and nature of metal movement in the inter-roll space, the overheating temperature of the melt and the intensity of heat removal from it) on the processes of its crystallization and structure formation. A transparent organic alloy camphene (C10H16) was used as a modeling medium, which makes it possible to visually study these processes during the ingotless rolling of metals. Due to the transparency of camphene, the transformation of the sheet blank structure at different stages of its formation was clearly studied: the rough primary structure of the crusts solidifying on the rolls, due to their partial deformation in the rolling zone, passes into a fine-grained structure upon exiting it. The dependence of the linear rate of metal pouring on the overheating temperature of the melt and the intensity of heat exchange processes in the roller mold is shown. At the same time, the optimal ranges of values of these parameters are determined, which ensure the stability of the process of roll casting of metals. The results obtained were the basis for the development of practical recommendations for roll casting of real metals, in particular aluminum alloys of various grades. Keywords: roll casting-rolling of metals, roll mold, melt hydrodynamics, overheating temperature, intensity of heat exchange processes, sheet blank, casting speed, structure transformation.

Ultrashort pulsed laser induced complex surface structures generated by tailoring the melt hydrodynamics

We present a novel approach for tailoring the laser induced surface topography upon femtosecond-fs pulsed laser irradiation. The method employs spatially controlled double fs laser pulses to actively regulate the hydrodynamic microfluidic motion of the melted layer that gives rise to the structures formation. The pulse train used, in particular, consists of a previously unexplored spatiotemporal intensity combination including one pulse with Gaussian and another with periodically modulated intensity distribution created by Direct Laser Interference Patterning (DLIP). The interpulse delay is appropriately chosen to reveal the contribution of the microfluidic melt flow, while it is found that the sequence of the Gaussian and DLIP pulses remarkably influences the surface profile attained. Results also demonstrate that both the spatial intensity of the double pulse and the effective number of pulses per irradiation spot can further be modulated to control the formation of complex surface morphologies. The underlying physical processes behind the complex patterns generation were interpreted in terms of a multiscale model combining electrodynamic excitation with melt hydrodynamics. We believe that this work can constitute a significant step forward towards producing laser induced surface structures on demand by tailoring the melt microfluidic phenomena.

Сравнение энергетической эффективности трехфазных линейных индукционных машин поперечного и продольного поля

The paper presents the results of energy efficiency and traction evaluation of linear induction machines for with molten aluminum recycling furnaces. Ansys Maxwell software was used for the synthesis of 3D parametric models of electromagnetic fields for a large air gap furnace. Interchangeable modules of three-phase transverse and longitudinal field inductors were used as a subsystem of a parametric model. Both inductor models include invertor low frequency power supply. Transverse and longitudinal field inductors have been designed with similarity principles. A stationary electromagnetic mode and power parameters has been modeled without taking melt hydrodynamics into account. Linear induction transverse-field machines have demonstrated higher efficiency.

Numerical Simulation of Effect of Different Initial Morphologies on Melt Hydrodynamics in Laser Polishing of Ti6Al4V.

As a surface finishing technique for rapid remelting and re-solidification, laser polishing can effectively eliminate the asperities so as to approach the feature size. Nevertheless, the polished surface quality is significantly sensitive to the processing parameters, especially with respect to melt hydrodynamics. In this paper, a transient two-dimensional model was developed to demonstrate the molten flow behavior for different surface morphologies of the Ti6Al4V alloy. It is illustrated that the complex evolution of the melt hydrodynamics involving heat conduction, thermal convection, thermal radiation, melting and solidification during laser polishing. Results show that the uniformity of the distribution of surface peaks and valleys can improve the molten flow stability and obtain better smoothing effect. The high cooling rate of the molten pool resulting in a shortening of the molten lifetime, which prevents the peaks from being removed by capillary and thermocapillary forces. It is revealed that the mechanism of secondary roughness formation on polished surface. Moreover, the double spiral nest Marangoni convection extrudes the molten to the outsides. It results in the formation of expansion and depression, corresponding to nearby the starting position and at the edges of the polished surface. It is further found that the difference between the simulation and experimental depression depths is only about 2 μm. Correspondingly, the errors are approximately 8.3%, 14.3% and 13.3%, corresponding to Models 1, 2 and 3, respectively. The aforementioned results illustrated that the predicted surface profiles agree reasonably well with the experimentally measured surface height data.

Tailoring submicrometer periodic surface structures via ultrashort pulsed direct laser interference patterning

Direct laser Interference Patterning (DLIP) with ultrashort laser pulses (ULP) represents a precise and fast technique to produce tailored periodic sub-micrometer structures on various materials. In this work, an experimental and theoretical approach is presented to investigate the previously unexplored fundamental mechanisms for the formation of unprecedented laser-induced topographies on stainless steel following proper combinations of DLIP with ULP. DLIP is aimed to determine the initial conditions of the laser-matter interaction by defining an ablated region while double ULP are used to control the reorganisation of the self-assembled laser induced sub-micrometer sized structures by exploiting the interplay of different absorption and excitation levels coupled with the melt hydrodynamics induced by the first of the double pulses. A multiscale physical model is presented to correlate the interference period, polarization orientation and number of incident pulses with the induced morphologies. Special emphasis is given to electron excitation, relaxation processes and hydrodynamical effects that are crucial to the production of complex morphologies. Results are expected to derive new knowledge of laser-matter interaction in combined DLIP and ULP conditions and enable enhanced fabrication capabilities of complex hierarchical sub-micrometer sized structures for a variety of applications.

Numerical Simulation of Melt Hydrodynamics in Laser Micro Processing of Metals

Laser material processing at micro-meter domain can be used for a plethora of applications such as micro drilling, micro welding, micro cutting, micro-texturing as well as micro polishing. In laser micro-cutting, welding and drilling, a high power laser (Intensity ~1010 W/m2) is used to melt and vaporize the material to produce high penetration welds, kerf cuts, and high aspect ratio micro holes. Whereas, in the case of laser texturing, conduction mode welding and polishing, the laser power density is sufficiently low to allow only melting of the material. With the irradiance of high-intensity laser, generation of highly dynamic melt pool occurs due to the presence of vaporization induced recoil pressure, Young Laplace surface tension force, and thermo-capillary shear stress. The melt pool convection results in geometrical defects such as recast layer, porosity, spatter formation, humping, protuberances and melt ripples. These defects pose a severe challenge towards industry utilization of laser micro processing, where precision and repeatability is of utmost importance. Therefore, a competent numerical model considering three co-existing phases (solid-liquid-gas) is developed to predict defects and characterize different regimes through insights on micro-scale melt hydrodynamics. This paper gives an overview of characteristic melt pool hydrodynamics and associated defects in laser micro-processing operations. The developed model has been validated using experiments obtained with 1.07 μm SPI fiber laser.

Численное моделирование процесса получения мультикремния методом направленной кристаллизации

Numerical simulation of silicon multi-crystal growth by directional solidification with a square crucible is considered. We validate the use of 2D geometry of the vertical cross section as a computational domain. The model describes melt hydrodynamics, global heat transfer, thermal stresses and the evolution of the dislocation density in the crystal. The sensitivity of the stresses and dislocation density in the Si crystal to the parameters of the Alexander-Haazen model is analyzed.

Hydrodynamics and heat exchange of crystal pulling from melts. Part I: Experimental studies of free convection mode

This work is a brief overview of experimental study results for hydrodynamics and convective heat exchange in thermal gravity capillary convection modes for the classic Czochralski technique setup obtained at the Institute of Thermophysics, Siberian Branch of the Russian Academy of Sciences. The experiments have been carried out at test benches which simulated the physics of the Czochralski technique for 80 and 295 mm diameter crucibles. Melt simulating fluids with Prandtl numbers Pr = 0.05, 16, 45.6 and 2700 have been used. Experiments with transparent fluids have been used for comparing the evolution of flow structure from laminar mode to well-developed turbulent mode. Advanced visualization and measurement methods have been used. The regularities of local and integral convective heat exchange in the crucible/melt/crystal system have been studied. The experiments have shown that there are threshold Grashof and Marangoni numbers at which the structure of the thermal gravity capillary flow undergoes qualitative changes and hence the regularities of heat exchange in the melt change. The effect of melt hydrodynamics on the crystallization front shape has been studied for Pr = 45.6. Crystallization front shapes have been determined for the 1 × 105 to 1.9 × 105 range of Grashof numbers. We show that the crystallization front shape depends largely on the spatial flow pattern and the temperature distribution in the melt.

Hydrodynamics and heat exchange of crystal pulling from melts. Part I: Experimental studies of free convection mode

This work is a brief overview of experimental study results for hydrodynamics and convective heat exchange in thermal gravity capillary convection modes for the classic Czochralski technique setup obtained at the Institute of Thermophysics, Siberian Branch of the Russian Academy of Sciences. The experiments have been carried out at test benches which simulated the physics of the Czochralski technique for 80 and 295 mm diameter crucibles. Melt simulating fluids with Prandtl numbers Pr = 0.05, 16, 45.6 and 2700 have been used. Experiments with transparent fluids have been used for comparing the evolution of flow structure from laminar mode to well-developed turbulent mode. Advanced visualization and measurement methods have been used. The regularities of local and integral convective heat exchange in the crucible/melt/crystal system have been studied. The experiments have shown that there are threshold Grashof and Marangoni numbers at which the structure of the thermal gravity capillary flow undergoes qualitative changes and hence the regularities of heat exchange in the melt change. The effect of melt hydrodynamics on the crystallization front shape has been studied for Pr = 45.6. Crystallization front shapes have been determined for the 1 × 105 to 1.9 × 105 range of Grashof numbers. We show that the crystallization front shape depends largely on the spatial flow pattern and the temperature distribution in the melt.

One-step fabrication of multifunctional fusiform hierarchical micro/nanostructures on copper by femtosecond laser

In this paper, one-step fabrication of fusiform hierarchical micro/nanostructures on copper surface by femtosecond laser is proposed. According to the preferential valley ablation, Marangoni effect based on melt hydrodynamics and the coupling of the incident light and surface plasmon polaritons, the formation mechanism of the fusiform hierarchical micro/nanostructures is explained. And the effect of laser parameters including the defocusing distance, laser power, and scanning speed on the surface morphology is investigated. Also, the quantitative analysis of the fusiform tapers is carried out. It is demonstrated that the three-dimensional size of the fusiform hierarchical micro/nanostructures could be controlled by adjusting laser parameters. In addition, the potential applications of the fusiform hierarchical micro/nanostructures in microliter droplet directional transportation and light absorption are explored.

Investigation of Influence of Sapphire Crystals Shape Factor on Its Growth by Horizontal Directed Crystallization

The value of shape factor allows to determine what processes (the surface or in crystal volume) define the dynamics of sapphire crystal growth by horizontal directed crystallization method and, as a result, estimate the influence of these processes on crystal quality. We receive the Stefan problem closed solution for sapphire crystals growth by horizontal directed crystallization method. It allows to investigate the growth process and the related factors (thermal stresses on different stages of growth process), melt hydrodynamics and its influence on defects formation. 1.

Numerical simulation of melt hydrodynamics induced hole blockage in Quasi-CW fiber laser micro-drilling of TiAl6V4

Laser drilling in the melt expulsion regime often inherits defects such as melt debris around the hole vicinity, recast layer deposition, melt shadow, taper, barreling and melt flow induced hole blockage. Thus, it is imperative to study and scrutinize the mechanism responsible for defects in laser drilling. In this paper, a two-dimensional free surface numerical model of quasi-CW drilling is presented which describes complex time-varying melt flow patterns considering the effects of recoil pressure, surface tension, Marangoni shear stress which are all dependent on surface temperature. It has been found out that in case of laser heating during initial pulses, recoil pressure is the dominating factor resulting in the expulsion of melt near hole entrance. However, during solidification surface tension induced backflow of melt creates a shadow at hole entrance. In the later stages, the drag force induced by surface tension constraints the recoil pressure, limiting the penetration depth due to which significant amount of melt remains inside the drilled hole, leading to melt re-closure induced hole blockage. The predicted hole depth dimensions and defects such as recast layer, melt shadowing, and hole blockage agrees reasonably well with experimental micrographs and literature data.

A Single-domain Approach to Simulate the Effect of Convective Flow on the Mushy-zone Structure in Czochralski Growth of Gadolinium Gallium Garnet Crystal

A numerical study was carried out to describe the effect of the melt hydrodynamics on the crystallization front shape in the Czochralski growth of a semitransparent oxide crystal. In the present model calculation, the enthalpy-porosity method was used to solve the phase change problem with the convection due to buoyant, thermocapillary and centrifugal forces. It was shown that the rotationally-driven flow protrudes into the mushy zone when the crystal rotation rate was increased to a certain critical value corresponding to Gr/Re2=0.89 as the ratio between the intensity of buoyancy and forced convection flow in the melt. The ratio between the vertical and horizontal temperature gradients beneath the mushy zone was found to be decreased by increasing the crystal rotation rate. It was shown that the shape of the zone deforms abruptly when the ratio between the axial and radial temperature gradients decreased to the values smaller than the unity. The Burger's number condition was found to be violated in the case Gr/Re2<0.89, at which the onset of geostrophic instability is expected.

Numerical modeling of power-down directional solidification process of Ti–46Al–8Nb refractory alloy

The work represents a brief outline of developed GIGAN computing code and its application for 2D-description of power-down directional solidification (DS) process applied recently in the course of laboratory R&D of new structural TiAl-based alloys. The temporal temperature field mapping, macro-scale study of melt hydrodynamics, heat-mass transfer, segregation effects, mushy zone evolution and solidification dynamics of TiAl–Nb melt/solid system have been performed numerically for set of DS trials being implemented virtually at different operational regimes and gravity environment. As the example of noteworthy numerical study results, the explanation of centerline peritectic segregation effect in Ti–46Al–8Nb ingots is mentioned, as well as the applied model verification using benchmark DS of TiAl–Nb alloy in microgravity aboard the MAXUS-8 ESA sounding rocket.

Heat-Mass Transfer and Related Microstructures in TiAl-Based Alloys Directionally Solidifying in the Earth Gravity and Microgravity

The first results are presented of ESA MAXUS 8 sounding rocket benchmark experiment on γ-TiAl (Nb,B) intermetallics solidification, that was a part of the IMPRESS EU FP6 project. Having the aim to investigate the columnar and equiaxed primary microstructures formation at diffusion controlled melt growth, an experiment was designed applying the power-down directional solidification technique with fixing quench in automatic spaceborne furnace module TEM03-5M. Two related alloys were tested: one inoculated with boron grain refiner Ti-44Al-7.5Nb-2.7B (at.%); and the other Ti-45.5Al-8Nb (at.%) without grain refinement. The series of terrestrial reference processes has been performed accordingly in counter-gravity solidification direction. The numerical studies of heat-mass transfer, melt hydrodynamics and temporal solidification dynamics of these alloys have been implemented under the earth-and zero-gravity approximations using the GIGAN software package (IPPE). The comparison is performed of numerical model findings with the real microstructure and composition of samples, whose analyses were carried out by backscattered scanning electron microscopy (SEM) and X-ray microprobe (EDX) techniques. The convection-induced peritectic macrosegregation effect appearance and development in ground-based reference ingots is analytically observed and numerically studied. The achieved microstructure formation in space benchmarks is discussed.

Numerical simulation of thermocapillary convection at surface modification by impulse laser radiation

Numerical simulation of metal surface alloying with impulse laser radiation has been performed. Impulse intensity influence on melt hydrodynamics and alloying substance distribution has been evaluated. For substance material, the authors used data on iron including dependence of surface tension on melt temperature and admixture concentration.

Numerical Study of Concentration and Thermocapillary Melt Convection under Pulsed Laser Alloying

The mathematical model of capillary thermal-concentration convection is proposed; on its basis, a numerical simulation of the process of metal surface alloying with the aid of pulse laser radiation was done. The influence of the pulse intensity on melt hydrodynamics and distribution of the alloying substance was evaluated. In the published works on this theme results of researches are not full enough and the extremely limited. For the substrate material, the data of titanium and iron were used, including the dependence of the surface tension on the melt temperature and admixture concentration.

Convection-induced peritectic macro-segregation proceeding at the directional solidification of Ti–46Al–8Nb intermetallic alloy

Authors extended the research on TiAl–Nb solidification/modeling reported in Kartavykh et al. (2010) [1]. Unidirectional counter-gravity (upward) melt solidification of Ti–46Al–8Nb (at.%) intermetallic alloy was performed in the three-zone resistive tube electro-furnace TEM 01-3M with power-down thermal profile operation. The laboratory refinement of this cylindrical ingot growth technique was developed in course of terrestrial preparation experiments in the furnace facility specially designed for a sounding rocket flight. Although Ti–46Al–8Nb is nominally slightly pro-peritectic composition, an axial elongated channel-like area with peritectically transformed microstructure was firstly observed in solidified ingots by SEM-BSE and EDX analyses, where Al content locally exceeds 47 at.%. For revealing the reasons of this microstructural inhomogeneity formation, the numerical modeling was applied. The real-time-scale 2D temperature field mapping, macro-scale study of melt hydrodynamics, heat-mass transfer, segregation effects, mushy zone evolution and solidification dynamics of TiAl–Nb melt/solid system have been performed accordingly. It was found that appearance of peritectic axial “spindle” in the solid is induced by the joint action of two factors in the melt: (i) rejection of Al solute ahead of the concave growth interface (dendrite tips front); (ii) development of weak laminar thermo-gravitational convective flow that picks up, pulls upward and stratifies an Al-enriched stream along the axis of melt column. The driving force of convection is a radial thermal gradient that depends on the furnace operational thermal conditions. To prevent the segregation, it was shown numerically that single regular convective cell could be broken into several cells by the appropriate variation of power-down cooling rate. The resulting uniform as-solidified microstructure of alloy produced confirms the modeling findings. Valuable data are obtained for the optimization of cast processes of TiAl-intermetallics.

Numerical study of convection-induced peritectic macro-segregation effect at the directional counter-gravity solidification of Ti–46Al–8Nb alloy

Unidirectional counter-gravity (upward) melt solidification of Ti–46Al–8Nb (at%) intermetallic alloy was performed in the three-zone resistive tube electro-furnace with power-down thermal profile operation. The laboratory refinement of cylindrical ingots growth technique was developed in course of terrestrial preparation experiments in the facility specially designed for a sounding rocket flight. Despite Ti–46Al–8Nb is nominally slightly pro-peritectic composition, an axial elongated channel-like area with peritectically-transformed microstructure was observed in solidified ingots, where Al content locally exceeds 47 at%. For revealing the reasons of this microstructural inhomogeneity formation, the numerical modeling was applied. The real-time-scale 2D temperature field mapping, macro-scale study of melt hydrodynamics, heat–mass transfer, segregation effects, mushy zone evolution and solidification dynamics of TiAl–Nb melt/solid system have been performed accordingly. It was found that appearance of peritectic axial “spindle” in the solid is induced by the joint action of two factors in the melt: (i) rejection of Al solute ahead of the concave growth interface (dendrite tips front); (ii) development of weak laminar thermo-gravitational convective flow that picks-up, pulls upward and stratifies an Al-enriched stream along the axis of melt column. The driving force of convection is a radial thermal gradient that depends on the furnace operational thermal conditions. To prevent the segregation, it was shown numerically that single regular convective cell could be broken into several cells by the appropriate variation of power-down cooling rate. The resulting uniform as-solidified microstructure of alloy obtained confirms the modeling findings.