Solidification Of Molten Droplet Research Articles

Computational study on structures of vertical columns formed by successive droplets

• The positive enthalpy was developed to investigate the heat accumulation effect. • Shape parameters were established for modeling bonding states. • The ideal bonding structure was obtained by linear interpolation of shape parameters. • The average length of ravine was proposed to evaluate the surface quality. • The effects of superheat and Weber numbers on surface quality were investigated. Components fabricated by the deposition and solidification of molten droplets have the common defect of rough surface, which are mainly caused by the poor bonding and inadequate evolution of the interface. To address such a problem, the pileup process of vertical column by successive droplets was simulated under an axisymmetric system, where the evolution of interface was captured by Front Tracking Method (FTM). The positive enthalpy was calculated to investigate the heat accumulation effect on the structure of vertical column. Shape parameters were established for modeling bonding states, and the ideal bonding was obtained by linear interpolation of these parameters. The average length of ravine was proposed to evaluate the surface quality, and the effects of superheat and Weber numbers on surface quality of ideal structure were investigated. The results show that the positive enthalpy increases gradually under constant deposition rate, and the excessive bonding occurs on the head of vertical column due to the heat accumulation. The mathematic calculation model proposed to acquire the ideal bonding state and the variation rules of surface quality for ideal structure, which is affected by superheat numbers and Weber numbers, can provide access to improve the forming precision in droplet based manufacture.

Simulation of Erosion and Redeposition of Plasma Facing Materials Under Transient Plasma Instabilities

Deposition of plasma energy during off-normal fusion reactor operational events delivers a transient heat flux of up to 100 MJ/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> to the plasma-facing materials (PFMs). Understanding the exact material response to the extreme energy loading conditions plays a key role in establishing a realistic computational tool that simulates the fusion plasma-material interaction. Surface damage can occur due to vaporization, melting, spallation, and liquid splatter. However, splashing mechanisms such as boiling and splattering, which result from various liquid instabilities, appear to be the main mechanism contributing to the melt layer erosion. The primary focus of this article is melting and resolidification and the effect of redeposition of the eroded material on surface erosion. A set of selected PFMs was exposed to a plasma heat flux of up to 40 GW/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> over a deposition duration of 200 μs. The source of the high energy plasma used in this article is the Surface InteRaction Experiment at North Carolina State (SIRENS) plasma source, which used to simulate disrupted plasma conditions. The underlying erosion mechanisms involved in the formation, ejection, and solidification of molten droplets are investigated using the basic plasma equations and a plasma fluid model implemented in the simulation code. The net erosion and redeposition thickness due to erosion of the vapor and melt layer have been evaluated post-plasma exposure and compared to the experimental measurements.

Substrate Melting and Re-solidification During Impact of High-Melting Point Droplet Material

In this paper, impact, spreading, and solidification of molten droplet on a dissimilar substrate along with substrate melting and its re-solidification are investigated numerically. Volume of fluid surface tracking method coupled with the solidification model within a one-domain continuum formulation is used to model the transient flow during the droplet impact, its subsequent spreading, and solidification. Evolution of melting front in the substrate is modeled by solving the governing equations for solidification in the substrate, too. Simulations are performed for the impact of a heated droplet on a substrate. The model predicts substrate melting, which can give better insight of bonding between the coating material and substrate, where droplet and substrate are of different materials. It is observed that melting in the substrate in the present case starts soon after the impact of the heated droplet. The depth and the width of the melting front in the substrate increase with the time and after reaching a maximum they start to decrease because of start of re-solidification from the melted edge. In the central part of the splat droplet solidifies, while the substrate remains melted which can enhance the coating strength and its bonding with the substrate.

Heat Treatment Effects on Mechanical Properties of Atmospheric Plasma Sprayed FexB Coatings on Al Substrate

In this work, we draw attention to determination of heat-treatment effects on mechanical properties of atmospheric plasma sprayed (APS) FexB coatings on aluminum substrates by micro-indentation technique. With this regard, iron boride powders of Fe-18.8B-0.2C-0.5Si-0.8Al (wt.%) were deposited onto Al substrates by APS in order to improve the mechanical properties of Al surface. As-sprayed coatings are composed of mainly FexB and iron matrix supersaturated with boron owing to the rapid solidification of molten droplets flattened on a substrate. It was observed that APS coatings exhibited characteristic wavy layered structure having porosity, inclusions, and semi-melted particles. The postfurnace treatment of APS coatings was carried out at temperatures ranged from 450 to 550°C in an argon atmosphere. The post-treatment applied for APS deposits led to increase in hardness of 40% without showing cracks. Furthermore, micro-mechanical properties of FexB coatings were examined by Shimadzu Dynamic Ultra-Microhardness Tester for estimating Young’s modulus and hardness due to load—unload sensing analysis by applying different loads such as 160, 320, and 640 mN to determine load and indentation depth dependency of APS FexB on Al substrate for each samples, in details.

An Experimental Study of Molten Microdroplet Surface Deposition and Solidification: Transient Behavior and Wetting Angle Dynamics

The basic problem of the impact and solidification of molten droplets on a substrate is of central importance to a host of processes. An important and novel such process in the area of micromanufacturing is solder jetting where microscopic solder droplets are dispensed for the attachment of microelectronic components. Despite the recent appearance of a few numerical studies focusing on the complex transient aspects of this process, no analogous experimental results have been reported to date to the best of our knowledge. Such a study is reported in this paper. Eutectic solder (63Sn37Pb) was melted to a preset superheat and used in a specially designed droplet generator to produce droplets with diameters in the range 50–100 μm. In a first series of experiments, the size, temperature, and impacting speed of the molten droplets were maintained constant. The primary variable was the temperature of the substrate that was controlled in the range from 48°C to 135°C. The dynamics of molten solder microdroplet impact and solidification on the substrate was investigated using a flash microscopy technique. The time for the completion of solidification from the moment of a solder droplet impact on the substrate varies between 150 μs and 350 μs. The dynamic interaction between the oscillation in the liquid region and the rapid advance of the solidification front was visualized, quantified, and presented in this paper. In a second series of experiments, the evolution of the wetting angle between the spreading drop and the substrate was recorded and analyzed. No quantitative agreement with Hoffman’s correlation for wetting was found. It was established that the wetting angle dynamics is strongly coupled with the evolution of the droplet free surface. Two successive regimes were distinguished during the spreading. The influence of the initial impact velocity and substrate temperature on the dynamics of the measured wetting angle was described in both regimes. To the best of our knowledge, this study presents the first published experimental results on the transient fluid dynamics and solidification of molten microdroplets impacting on a substrate at the above-mentioned time and length scales that are directly relevant to the novel solder jetting technology. [S0022-1481(00)01403-1]

Theoretical analysis of spreading and solidification of molten droplet during thermal spray deposition

A theoretical model for droplet spreading and solidification has been developed which accounts for simultaneous effects of surface tension, solidification and thermal contact resistance. Based on this model, an analytic solution for splat flattening ratio has been obtained as a function of Reynolds, Weber, Prandtl and Jakob numbers. Two different regimes, in terms of Reynolds and Weber numbers, have been identified where either the viscous dissipation or surface tension dominates. A map for splat flattening ratio has been generated for Reynolds number in the range of 10 4–10 6 and Weber number in the range of 5–5000.

A CURVILINEAR LEVEL SET FORMULATION FOR HIGHLY DEFORMABLE FREE SURFACE PROBLEMS WITH APPLICATION TO SOLIDIFICATION

A curvilinear level set formulation has been developed for highly deformable free surface problems. In this new scheme, the grid lines follow the irregular domain generated by the multizone adaptive grid-generation (MAGG) scheme [1] and free surfaces are captured by level set functions among the curvilinear grids. Navier-Stokes equations are discretized and solved based on a multiphase curvilinear finite-volume method [2], and the level set function is solved based on a finite-difference method using the second-order essentially nonoscillatory (ENO) scheme [3]. The scheme is capable of accurately and efficiently representing the deformation, oscillation, merger, and separation of free surfaces. The effectiveness and robustness of the algorithm are demonstrated by using it for problems involving merger of bubbles, mold filling, and the spreading and solidification of molten droplets on a cold substrate where both free surface and solidification interfaces move and the mass of the liquid phase is continuously decreased.