Melt Flow Dynamics Research Articles

Influence on the dilution by laser welding of aluminum with magnetic stirring

The Aluminum 6xxx alloys are well-known for their high susceptibility to hot cracking. It has been reported that this problem can be effectively resolved by introducing silicon into the molten pool. However, the high welding speed by laser welding process results in an inhomogeneous dilution of the silicon, which may lead to hot cracks in the Sipoor area. The idea of applying a low frequency magnetic field during laser welding process has been already carried out recently in order to make a stir effect inside the molten pool and therefore to improve the element dilution in the weld joint.In this paper, the effects of the magnetic field on the melt flow as well as the element dilution are shown by applying copper as “tracer” inside the molten pool. Several methods were designed with different forms of copper at different positions in the workpieces to visualize the melt flow and the corresponding element dilution in different areas of the molten pool. In addition, laser welding with AlSi18 filler wire was also conducted to realize a real welding process comparing to that with “tracer”. WDX analysis was then carried out to investigate how silicon distributed inside the welds with the help of magnetic stirring. The results demonstrate that a DC magnetic field, with its direction coaxial to the laser, tends to modify the melt flow dynamics at flux density up to 100 mT. The results of laser welding with filler wire showed that by the help of an AC magnetic field with a frequency up to 10 Hz and flux density above 100 mT, the element dilution could be improved in the weld joint. The WDX analysis showed that under the influence of an alternating magnetic field a periodicity of the silicon distribution can be achieved, which depends greatly on the magnetic frequency.

Structure of melt flow channels in the mantle

Structural events during the formation of the mantle peridotite section in the Voikar-Syn’ya massif of the Polar Urals are considered. The structural units of the mantle section were formed during several deformation stages. Dunite bodies in restitic peridotites were formed in the course of deformation that completed the formation of large-scale folds of high-temperature plastic flow of mantle material. The final stage of deformation accompanied by migration of melt through harzburgite occurred in the shallow mantle in the setting of suprasubduction spreading related to the ascent of a mantle diapir. The rate of plastic deformation was relatively low. As a result, the intracrystal translation gliding of dislocations in olivine was the main mechanism in both harzburgite and dunite. The paths of focused melt flow are marked by dunite veins and associated pyroxenite and chromitite. It is suggested that stress concentration in fold hinges and their abrupt relaxation with formation of orthogonal network of weakened zones with high permeability was one of the possible mechanisms of the formation of melt conduits. The dispersed melt ascending from a great depth spontaneously migrated toward these zones. The distribution and structure of chromitite bodies reflect multistage formation of dunite, nonstationary dynamics of melt flow through restite, and abrupt variations of local stress fields in the areas adjacent to melt conduits.

Surface Tension Measurement at High Temperatures by using Dynamics of Melt Flow

Surface tension of melts at high temperature has significant effects on different industrial processes. In a new containerless method for surface tension measurement, an atmospheric radio-frequency inductively coupled plasma melts metallic or ceramic rods and a high-speed charge-coupled device records the drop formation caused by melting. Pendant drops produced by the melt flow are compared with the theoretical Young–Laplace (YL) profiles. Moreover, the dynamics of the melt flow is mimicked by using numerical simulations of drop injection from a nozzle. The numerical model solves the axisymmetric Navier–Stokes equations for both the melt and the surrounding gas by using the finite volume method. Since the YL equations provide theoretical pendant drop profiles based on an inviscid quasiequilibrium condition, a detailed study of the differences between experimental, numerical, and theoretical profiles demonstrates some of the hydrodynamic effects influencing the surface tension measurement methods, which are based on drop profiles. Results from this surface tension measurement method, in addition to a discussion on the hydrodynamic effects, are presented.

Determination of the lower complexity limit for laser cut quality modelling

In this paper the authors address the modelling of surface profiles resulting from the laser cutting process. Despite the attention paid to this problem by the scientific community, the phenomena of striation formation is still only marginally understood. The models used in the literature are often highly simplified and focused on single large scale effects such as exothermic reactions of the melt with the assist gas. In this work a three-dimensional fully coupled model for the laser cutting process is used. It will be shown by the authors that, if details of the cut face surface profile, dross and burnout are sought, simplifications like reduction in model dimension and the neglect of gas dynamics, viscous shear, convection effects and melt flow dynamics are not admissible, and that those ‘small’ effects induce identifiable differences within the cut surface profile. However, it has also been found by the authors that currently available computer hardware is not capable of producing results at the necessary rate and cost required for parametric studies. Results from nine simulation runs amounting to 17 300 cpu hours on a SGI origin 3400 system are presented. These are intended to be taken as cautionary examples with regard to the numerical models applied and thus establish a lower limit of modelling complexity. This lower limit can then be built upon by introducing exothermic reaction, plasma formation, multiple reflections and the like.

Turbulent flow dynamics, heat transfer and mass exchange in the melt of induction furnaces

Experimental investigations of the turbulent flow velocities measured in the melt of experimental induction furnaces show, that beside the intensive local turbulence pulsations, macroscopic low‐frequency oscillations of the recirculated toroidal main flow eddies play an important role in the heat and mass exchange processes. Traditional numerical calculations of the flow and transfer processes, based on wide spread commercial codes using various modifications of the k‐ε turbulence model show that these models do not take into account the low‐frequency oscillations of the melt flow and the calculated temperature and concentration distributions in the melt essentially differs from experimental results. Therefore, the melt flow dynamics in an induction crucible furnace was numerically simulated with help of transient three‐dimensional calculations using the large eddy simulation turbulence model. This leads to a good agreement between calculated and measured periods of low‐frequency oscillations and heat and mass transfer between the toroidal flow eddies.

An investigation into melt flow dynamics during repetitive pulsed laser drilling of transparent media

This paper presents an investigation into the dynamics of repetitive pulsed laser drilling of a visually transparent media using a CO 2 laser source. This enabled the use of a high-speed imaging system for observing, in real time, the behaviour of the drilling process in the laser drilled cavity of 1.5 mm diameter holes of up to 18.5 mm in depth. The work revealed that the instantaneous drilling velocity within each laser pulse can vary considerably from the average drilling velocity as a result of the non-uniform temporal pulse shape and the oscillation of the melt ejection rate. During beam breakthrough, both upward and downward melt ejections were observed to occur inside the drilled hole for a short period of time, after which the material was ejected through the exit end of the holes. It has been shown in this work that the downward melt flow velocity increases with hole depth for a positively tapered hole (from 0.09 to 1.43 m/ s) and decreases with hole depth for a negatively tapered hole geometry (from 0.4 to 0.1 m/ s ), as a result of the change in the assist gas velocity inside the drilled hole with respect to the hole taper geometry. The mechanisms of forming the positively and negatively tapered holes in the transparent media have been correlated with the hole geometry and melt flow velocity. The work has demonstrated a new method of studying the melt dynamics in laser drilling.

Dynamics of ripple formation and melt flow in laser beamcutting

The dynamical behaviour of the laser beam fusion cutting process of metals is investigated. Integral methods such as the variational formulation are applied to the partial differential equations for the free boundary problem and a finite dimensional approximation of the dynamical system is obtained. The model describes the shape of the evolving cutting kerf and the melt flow. The analysis is aimed at revealing the characteristic features of the resultant cut, for example, ripple formation and adherent dross. The formation of the ripples in the upper part of the cut, where no resolidified material is detectable, is discussed in detail. A comparison with numerical simulations and experiments is made.