Vapor Recoil Research Articles

Contact angle evolution during selective laser melting

The researches about the connection of the geometrical feature and the melting and solidification behavior of the molten pool during selective laser melting (SLM) are necessary with the development of additive manufacturing (AM) technique. This research has paid attention to the contact angle evolution during SLM. A model has been built for analysis. The extreme state and the equilibrium state of the contact interface have been solved for the molten pool spreading. As the molten pool is solidified during non-equilibrium state, we propose the combined action of the fluid flow driving forces and the lifetime of the molten pool as the main reasons for the contact angle evolution. Based on the thermal simulation performed, it is confirmed that the shear stress induced by the surface tension gradient, the evaporation recoil pressure and the lifetime of the molten pool all increase first and then become stable as tracks increase. As a consequence, the contact angle decreases first and then becomes stable.

Droplet Transfer Behavior of Flux Cored Wire TIG Welding

This paper puts forward a new method to achieve flux cored wire TIG welding and uses high-speed photography to analyze the droplet transfer behavior and forces acting on the droplet. The droplet transfer forms include bridging transfer, slag column guided transfer, and non-contact transfer; each of these forms may be observed as the melting position of the welding wire changes. The important role of surface tension in the process of droplet transfer is proposed using static force balance theory and pinch instability theory. The phenomenon of droplet backward swing during welding process could be attributed to the vapor recoil force produced by vapors from the droplet. The welding experiments show that the proposed welding process is stable and that the weld quality produced is good.

Numerical simulation of multi-component evaporation during selective electron beam melting of TiAl

Powder-based additive manufacturing technologies such as selective laser melting (SLM) and selective electron beam melting (SEBM) hold great promise for producing high-value metal components with customized geometries. Development of scanning strategies and prediction of optimal process parameters thereby remain key factors for the fabrication of high-quality products without local inhomogeneities in alloy composition or porosity.In this contribution, the relation between energy input, evaporation and residual porosity is numerically investigated on the titanium aluminide alloy Ti-48Al-2Cr-2Nb. The numerical model is based on a lattice Boltzmann method and includes hydrodynamics, thermodynamics and multi-component evaporation. Simulation results show that the spatial distribution of alloying elements within the final part is dominated by the advection of melt which is driven by surface tension and evaporative recoil and influenced by the random arrangement of powder particles. Regarding evaporation losses, the line energy appears to be of central importance, as it strongly affects peak temperatures during processing. For a given melt strategy there is a trade-off between porosity and loss of aluminum due to evaporation. It is demonstrated that significant reductions in evaporation losses can be achieved by application of a suitable beam scanning strategy. The presented numerical findings are consistent with experimental data.

Swing-like pool boiling on nano-textured surfaces for microgravity applications related to cooling of high-power microelectronics

Here, we demonstrate that heat removed in pool boiling from a heater mimicking high-power microelectronics could be used to facilitate a swing-like motion of the heater before being finally dissipated. This swing-like motion could be beneficial for shedding a large vapor bubble that encapsulates high-power heaters in microgravity where buoyancy force is unavailable for vapor bubble removal. The swing-like motion is propelled by vapor bubble recoil, the force which exists irrespective of gravity and buoyancy. We also demonstrate that this force could be significantly enhanced by depositing on the heater surface supersonically blown polymer nanofibers with cross-sectional diameters below 100 nm. These nanofibers provide additional nucleation sites, resulting in much more frequent bubble nucleation and departure, and thus a higher overall vapor recoil force propelling the heater motion. Such nanofibers strongly adhere to the heater surface and withstand prolonged harsh pool boiling. The measured velocity of the model swing-like heater in Novec 7300 fluid is about 1 cm/s.

Reactive wetting of TiC-Ni cermet by Ag-Cu-Zn alloy during evaporation

In order to investigate the effect of the evaporation on wetting, reactive wetting of TiC-Ni cermet by molten Ag-Cu-Zn alloys with different Zn contents was studied at 810 °C using a sessile drop method in vacuum. The vapor recoil force induced by the strong evaporation of Zn in the Ag-Cu-25Zn alloy drove the spreading of the molten Ag-Cu-25Zn alloy. Furthermore, the sequential evaporation of Zn in the Ag-Cu-25Zn alloy caused the formation of a (Cu, Ni) solid-solution layer at the interface between the TiC-Ni cermet and the Ag-Cu-Zn alloy. Thus, the (Cu, Ni) solid-solution layer could serve as a metallization layer on the surface of the TiC-Ni cermet, which promoted the non-wetting to wetting transition of the TiC-Ni cermet by the Ag-Cu-25Zn alloy. The equilibrium contact angle of the Ag-Cu-25Zn alloy on the TiC-Ni cermet was 19.9°, approximately equal to that of a Ag-28Cu eutectic alloy on a Cu-Ni alloy surface. The final contact angle of the Ag-Cu-Zn liquid alloy on the TiC-Ni cermet first decreased and then increased as the Zn content in the liquid was increased from 0% to 30%. It reached a minimum value in the Ag-Cu-25Zn alloy.

Boiling crisis controlled by capillary pumping and viscous friction: Liquid penetration length and dry spot diameter

A boiling crisis, or critical heat flux (CHF), is a condition that determines the upper bound on removable thermal energy at a boiling surface. In such situations, the liquid cannot wet the surface because a vapor film completely covers it. CHF is enhanced on micro-structured surfaces when under boiling conditions. CHF values were measured for surfaces with rectangular microchannel geometries of various channel widths, (10–30 μm) and generally increased in value as channel widths decreased. However, the CHF value for the 5-μm channel-width surface was found to be lower than the wider channel-width surfaces. This observation contradicts models based on vapor recoil and classical instability mechanisms. Hence, we present a fluid-dynamics model that considers capillary pumping and viscous friction. With a focus on the spatial distribution of the liquid penetration region and the local dry spot under a large vapor bubble, this model can accurately predict the CHF variation associated with different channel widths.

Interfacial stability and self-similar rupture of evaporating liquid layers under vapor recoil

We investigate interfacial stability of an evaporating viscous liquid layer above/below a horizontal heated substrate in the framework of a long-wave model that accounts for surface tension, positive/negative gravity, and evaporation effects of mass loss and vapor recoil. With the time-dependent linear stability analysis, it is found that the interface instability is enhanced by vapor recoil with time using an effective growth rate. The destabilizing mechanism of vapor thrust competes with the stabilizing surface tension, and the effects of the latter are not asymptotically negligible near rupture, reflected by a rescaled effective interfacial pressure. A two-dimensional nonlinear evolution is investigated for the quasi-equilibrium evaporating layers with different evaporative conditions for Rayleigh–Taylor unstable and sessile layers. For weak mass loss and strong vapor recoil, the well-defined capillary ridges emerge around a deepening narrow valley with increasing wavelength under a positive gravity, while, on the basis of initial condition, main and secondary droplets are either coalesced partially or separated by a sharp dry-out point under a negative gravity. The rupture location depends strongly on the characteristics of a given initial condition, except for the random perturbation. For both the cases, an increase in the modified evaporation number tends to reduce the rupture time tr and droplet thickness remarkably. Similarity analysis along with numerical strategy is presented for the final stage of touch-down dynamics, determined by a physical balance between the vapor recoil and capillary force. The evaporation-driven rupture with a significant vapor recoil and negligible mass loss is shown to contain a countably infinite number of similarity solutions whose horizontal and vertical length scales behave as (tr − t)1/2 and (tr − t)1/3. The first similarity solution represents a stable single-point rupture.

Stability of an evaporating meniscus: Part I – Theoretical analysis

The stability of an evaporating meniscus in a narrow channel is theoretically investigated. A non-linear differential equation is derived using the lubrication approximation and taking into account evaporation, long-range molecular forces, surface tension, vapor recoil, thermocapillarity and viscous forces. Using a linear stability analysis, the susceptibility of the film to instability is studied. Three different base state profiles are used: flat replenished thin film, modified Young-Laplace profile and curved thin film. The last base state most accurately reflects an evaporating meniscus; however, its implementation requires the numerical solution of the non-linear thin-film equations. A shooting technique together with a fourth order Runge-Kutta method is used to obtain the curved thin-film profiles. The results obtained for each base state are analyzed and limitations are discussed. The curved thin-film base profile predicts strong instability for higher wall superheats. The growth rates are found to be positive (destabilizing) in the thin-film region and become negative (stabilizing) towards the intrinsic meniscus. The results indicate that the instability most likely originates in the evaporating thin-film region. Part II of the presented paper is devoted to experimental investigation.

Experimental and theoretical characterization of deep penetration welding threshold induced by 1-μm laser

The deep penetration-welding threshold (DPWT) is the critical value that describes the welding mode transition from the thermal conduction to the deep penetration. The objective of this research is to clarify the DPWT induced by the lasers with wavelength of 1μm (1-μm laser), based on experimental observation and theoretical analysis. The experimental results indicated that the DPWT was the ratio between laser power and laser spot diameter (P/d) rather than laser power density (P/S). The evaporation threshold was smaller than the DPWT, while the jump threshold of the evaporated mass flux in the molten pool surface was consistent with the DPWT. Based on the force balance between the evaporation recoil pressure and the surface tension pressure at the gas–liquid interface of the molten pool as well as the temperature field, we developed a self-focusing model, which further confirmed the experimental results.

Critical heat flux triggering mechanism on micro-structured surfaces: Coalesced bubble departure frequency and liquid furnishing capability

This study presents a new critical heat flux (CHF) triggering mechanism for explaining the CHF enhancements of micro-structured surfaces. Differentiating the existing vapor recoil mechanism and hydrodynamic instability theory, this model focuses on the phenomenological observations of the boiling phenomena in which a number of small bubbles are generated, which then merge and coalesce into a large bubble. According to this model, the CHF phenomenon is governed by the frequency of the coalesced bubble departure and the liquid furnishing capability into a thin liquid–vapor layer (macrolayer). The microstructures on the surface change the evaporative mass flow rate owing to the liquid–vapor interface deformation near the triple contact line of the macrolayer, leading to a change in the coalesced bubble departure frequency. The liquid flow resistance, which increased owing to the microstructures, can affect the liquid volume flow rate available for evaporation in the macrolayer; thus, it directly alters the CHF behavior. The liquid flow resistance induced by the microstructures could contribute to the relative deterioration of the CHF value at the surface. The CHF can be directly modeled by means of the summation of the mean velocity of liquid flowing into the macrolayer due to the pressure potential induced from capillary and disjoining pressure effects, as well as the coalesced bubble departure frequency.

Numerical analysis of weld pool oscillation in laser welding

Volume of fluid (VOF) numerical simulation was used to investigate melt flow and volumetric oscillation of conduction-mode pulsed laser weld pools. The result is compared to high speed video stream of titanium laser spot welding experiment. The total simulation time is 10ms with the first 5 ms being heating and melting under constant laser irradiation and the remaining 5 ms corresponding to re-solidification of the weld pool. During the melting process, the liquid pool did not exhibit periodic oscillation but was continually depressed by the evaporation recoil pressure. After the laser pulse, the weld pool was excited into volumetric oscillation by the release of pressure on its surface and oscillation of the weld pool surface was analyzed. The simulation model suggested adjusting thermal diffusivity to match cooling rate and puddle diameter during solidification which is distinguishable from previous weld pool simulation. The frequency continuously increased from several thousand cycles per second to tens of thousands of cycles per second as the weld pool solidified and its diameter decreased. The result is the first trial of investigation of small weld pool oscillation in laser welding although there have been several reports about arc welding.

Weld outline comparison between various pulsed Nd:YAG laser welding and pulsed Nd:YAG laser–TIG arc welding

In this study, four welding methods, pulsed laser welding, defocused pulsed laser welding, laser–arc tandem welding, and laser–arc hybrid welding, were tested on 304 stainless steel sheets, and the differences between the weld cross-section outlines, especially the weld depth, were compared under different welding conditions. The author suggests that during the hybrid welding process, the upper part of the keyhole was surrounded by a large amount of melt such that the evaporation recoil pressure caused the gas–liquid interface to move towards the solid–liquid interface near weld pool surface. This flow behavior promoted both heat transfer from the central region of the weld pool to the solid–liquid interface near the weld pool surface, which resulted in an increased weld width, and increased the diameter of the upper part of the keyhole, which reduced the number of times the laser beam is reflected inside the keyhole and the coupling efficiency of laser energy. During pulse Nd:YAG laser–TIG arc tandem welding, there was a smaller amount of melt around the keyhole, which made it easier to form a U-shaped keyhole, which benefited the multiple reflection of laser beam in the keyhole and promoted the coupling of laser energy. The weld depths obtained from tandem welding and laser–arc hybrid welding were similar, which may be related to the difference in the keyhole shapes produced from the two welding processes.

나노 구조가 형성된 열전달 표면에서의 임계 열유속 증진 메커니즘

The critical heat flux (CHF) on a heat transfer surface with nanostructures is known to be significantly better than that on flat surfaces. Several physical mechanisms have been proposed to explain this phenomenon. However, almost all studies conducted so far have been qualitative, and a generalized theory has not yet been established. In this study, we developed a quantitative mechanism for CHF enhancement on a surface with nanostructures, based on vapor recoil and surface adhesion forces. We focused on the increase in the length of the triple contact line owing to the formation of nanostructures and the adhesion force between them and the liquid.

ELM-induced melting: assessment of shallow melt layer damage and the power handling capability of tungsten in a linear plasma device

Tungsten samples were exposed to combined steady state and edge localised mode transient replication experiments in a linear plasma device; either in combined hydrogen plasma and high powered laser exposures at Magnum-PSI or steady state hydrogen plasma and superimposed plasma pulses created using a capacitor bank in Pilot-PSI. With each transient the surface temperature of the sample was elevated above the melting point creating a shallow molten layer. An apparent heat transport reduction in the near surface layer increases as a function of pulse number. Scanning electron microscopy analysis of the samples shows large scale grain enlargement down to several hundred to thousand micrometers below the surface as well as melt layer motion due to evaporative recoil from the molten surface. In the ITER divertor this would lead to severe embrittlement and enhanced erosion of the tungsten surface leading to reduced lifetimes for the plasma facing material.

Critical phenomena at evaporation in a thin liquid layer at reduced pressure

This research is concerned with the problem of heat transfer in a thin liquid layer on a horizontal surface, which evaporates at reduced pressure, when structures shaped as “funnels” and “craters” appear on its surface under the action of vapor recoil force. An approximate model that takes into account the surface tension force, gravity force, vapor recoil force, and disjoining pressure is developed. For the experimentally realized shape of curved surface, in the frames of the model, the distribution of vapor recoil force, temperature, pressure, shear stresses, and local heat fluxes along the interface is found. The density of the heat flux corresponding to appearance of a crater at the place of an array of funnels is estimated. The results are in good agreement with the experimental measurements and the estimates by the Kutateladze formula for the first critical heat flux density.

Simulation of laser penetration efficiency

The results of numerical simulation of laser beam interaction with a hypothetical metallic material with properties similar to a steel alloy are reported. The numerical simulation was performed using a physical model that includes detailed consideration of surface evaporation, evaporative cooling of the surface and evaporation recoil induced melt ejection. The laser beam ‘penetration’ is considered in terms of melting through the sample or drilling through the sample due to both evaporation and recoil ejection of material. As a demonstration of the predictive capabilities of the model, the average velocity of penetration through a material with steel-like properties is numerically predicted for various laser interaction parameters such as, laser beam radius, laser pulse duration (including CW regime), laser pulse energy and pulse repetition. In particular, the average penetration velocities through a sample due to melting are compared for pulsed and CW lasers of the same power. For the sake of another demonstration of penetration simulation, the temporal dynamics of the position of melt front relative to the sample surface irradiated by a laser beam was computed for different laser pulse repetition rates and constant average laser power. An illustration of the penetration efficiency (W parameter) defined as the amount of energy per unit volume delivered into a target in order to achieve either melting of drilling through a target wall is shown in a wide range of laser pulse parameters covering regimes corresponding to domination of melting through and drilling through.

Through nanohole formation in thin metallic film by single nanosecond laser pulses using optical dielectric apertureless probe

Separate nanoholes with the minimum size down to 35 nm (~λ/15) and nanohole arrays with the hole size about 100 nm (~λ/5) were fabricated in a 50 nm optically "thick" Au/Pd film, using single 532 nm pump nanosecond laser pulses focused to diffraction-limited spots by a specially designed apertureless dielectric fiber probe. Nanohole fabrication in the metallic film was found to result from lateral heat diffusion and center-symmetrical lateral expulsion of the melt by its vapor recoil pressure. The optimized apertureless dielectric microprobe was demonstrated to enable laser fabrication of deep through nanoholes.

The Stability of Evaporating Thin Liquid Film Containing Insoluble Surfactant on Heated Substrate

Flow of evaporating thin liquid film containing insoluble surfactant on a uniformally heated substrate is considered in this paper. Coupled nonlinear evolution equations for the film thickness and surfactant concentration are derived on the base of lubrication theory and reasonable boundary conditions. The flow stability of the thin liquid film has been studied using normal mode method according to the linear stability theory. The results show that the film stability is promoted by increasing the Capillary number and the surfactant Peclet number, while increasing the Marangoni number, the interface resistance number, the vapor recoil number and the evaporation number can reduce the stability of the system.

Investigation of Pool Boiling Critical Heat Flux Enhancement on a Modified Surface Through the Dynamic Wetting of Water Droplets

Dynamic wetting behaviors of water droplet on the modified surface were investigated experimentally. Dynamic contact angles were measured as a characterization method to explain the extraordinary pool boiling critical heat flux (CHF) enhancement on the zirconium surface by anodic oxidation modification. The sample surface is rectangular zirconium alloy plates (20 × 25 × 0.7 mm), and 12 μl of deionized water droplets were fallen from 40 mm of height over the surface. Dynamic wetting movement of water on the surface showed different characteristics depending on static contact angle (49.3 deg–0 deg) and surface temperature (120 °C–280 °C). Compared with bare surface, wettable and spreading surface had no-receding contact angle jump and seemed stable evaporating meniscus of liquid droplet in dynamic wetting condition on hot surface. This phenomenon could be explained by the interaction between the evaporation recoil and the surface tension forces. The surface tension force increased by micro/nanostructure of the modified zirconium surface suppresses the vapor recoil force by evaporation which makes the water layer unstable on the heated surface. Thus, such increased surface force could sustain the water layer stable in pool boiling CHF condition so that the extraordinary CHF enhancement could be possible.

Simulation and validation of the dynamics of liquid films evaporating on horizontal heater surfaces

In this study a non-linear governing equation based on lubrication theory is employed to model the thinning process of an evaporating liquid film and ultimately predict the critical thickness of the film rupture under impacts of various forces resulting from mass loss, surface tension, gravity, vapor recoil and thermo-capillary. It is found that the thinning process in the experiment is well reproduced by the simulation. The film rupture is caught by the simulation as well, but it underestimates the measured critical thickness at the film rupture. The reason may be that the water wettability of the heater surfaces is not taken into account in the model. Thus, the minimum free energy criterion is used to obtain a correlation which combines the contact angle (reflection of wettability) with the critical thickness from the simulation. The critical thicknesses predicted by the correlation have a good agreement with the experimental data (the maximum deviation is less than 10%).