Valdis Bojarevics

The ExoMet Project: EU/ESA Research on High-Performance Light-Metal Alloys and Nanocomposites

The performance of structural materials is commonly associated with such design parameters as strength and stiffness relative to their density; a recognized means to further enhance the weight-saving potential of low-density materials is thus to improve on their mechanical attributes. The European Community research project ExoMet that started in mid-2012 targets such high-performance aluminum- and magnesium-based materials by exploring novel grain refining and nanoparticle additions in conjunction with melt treatment by means of external fields (electromagnetic, ultrasonic, and mechanical). These external fields are to provide for an effective and efficient dispersion of the additions in the melt and their uniform distribution in the as-cast material. The consortium of 27 companies, universities, and research organizations from eleven countries integrates various scientific and technological disciplines as well as application areas—including automotive, aircraft, and space. This paper gives an overview of the project, including its scope for development and organization. In addition, exemplary results are presented on nanoparticle production and characterization, mixing patterns in metal melts, interface reactions between metal and particles, particle distribution in the as-cast composite materials, and mechanical properties of the as-cast composite materials. The application perspective is considered as well.

Progress in fluid flow research: turbulence and applied MHD

Transition Turbulence MHD Turbulence and Atmospheric Turbulence Astrophysics Experimental Methods and Data Analysis Energy Conversion Solidification Incompressible Flow Plasma MHD.

Contactless Ultrasound Generation in a Crucible

Ultrasound treatment is used in light alloys during solidification to refine microstructure, remove gas, or disperse immersed particles. A mechanical sonotrode immersed in the melt is most effective when probe tip vibrations lead to cavitation. Liquid contact with the probe can be problematic for high temperature or reactive melts leading to contamination. An alternative contactless method of generating ultrasonic waves is proposed, using electromagnetic (EM) induction. As a bonus, the EM force induces vigorous stirring distributing the effect to treat larger volumes of material. In a typical application, the induction coil surrounding the crucible—also used to melt the alloy—may be adopted for this purpose with suitable tuning. Alternatively, a top coil, immersed in the melt (but still contactless due to EM force repulsion) may be used. Numerical simulations of sound, flow, and EM fields suggest that large pressure amplitudes leading to cavitation may be achievable with this method.

MHD of Large Scale Liquid Metal Batteries

Liquid metal batteries are candidates for large-scale energy storage in a national energy grid. The attraction of the liquid batteries lies in the fast kinetics at liquid metal-electrolyte interfaces, simple assembly and recycling, while the major difficulties to implementation are their sensitivity to liquid motion and operation at elevated temperatures. The concept of liquid metal battery bears a close similarity to aluminium electrolytic production cells. The two liquid layer magnetohydrodynamic effects can be projected to the three liquid layer self-segregated structure of the batteries. The trend for commercial electrolysis cells is to increase their size instead of operating a large number of parallel small ones. Our aim is to develop a numerical model for the three density-stratified electrically conductive liquid layers using 3D and shallow layer approximation accounting for specific magnetohydrodynamic effects during periods of battery charge/discharge. A possibility to reuse infrastructure of an old aluminium electrolysis potline for a large scale liquid batteries facility is discussed.

Pseudo-spectral solutions for fluid flow and heat transfer in electro-metallurgical applications

The pseudo-spectral solution method offers a flexible and fast alternative to the more usual finite element and volume methods, particularly when the long-time transient behaviour of a system is of interest. The exact solution is obtained at grid collocation points leading to superior accuracy on modest grids. Furthermore, the grid can be freely adapted in time and space to particular flow conditions or geometric variations, especially useful where strongly coupled, time-dependent, multi-physics solutions are investigated. Examples include metallurgical applications involving the interaction of electromagnetic fields and conducting liquids with a free surface. The electromagnetic field determines the instantaneous liquid volume shape, which then affects the electromagnetic field. A general methodology of the pseudo-spectral approach is presented, with several instructive example applications: the aluminium electrolysis MHD problem, induction melting in a cold crucible and the dynamics of AC/DC magnetically levitated droplets. Finally, comparisons with available analytical solutions and to experimental measurements are discussed.

Dual frequency AC and DC magnetic field levitation melting of metals

Dual frequency AC magnetic field effects are analyzed in the case of levitated samples heated, melted and confined. The time dependent numerical models based on accurate spectral approximation give an insight to oscillation patterns and stability conditions for a copper sample both in solid and liquid state after the melting stage. Application to microgravity experiments planned at International Space Station and the full gravity magnetic levitation for the similar sample are presented.

Modelling of electromagnetic levitation – consequences on non-contact physical properties measurements

Electromagnetic levitation of electrically conductive droplets by alternating magnetic fields is a technique used to determine the physical properties of liquid metallic alloys such as surface tension, viscosity, heat capacity and thermal diffusivity/1/. To improve accuracy, it is mandatory to reduce electromagnetic stirring and shaping of the droplet, therefore experiments are conducted in microgravity. Properties are deduced from direct measurements of position or temperature using specific models. Our purpose is to check various assumptions on which those models are built by the use of adapted numerical codes. We first compare experimental and numerical results concerning the shape and mass centre oscillation frequencies of electromagnetically levitated Nickel droplets. Axisymmetric numerical model yields equilibrium shapes and positions of the droplets in a good agreement with experiment. Then, fluid flow effects on the measurement precision of surface tension and viscosity by comparing expecte and calculated properties values are charcterized. We determine critical values of initial droplet distortion or magnetic field intensity which can lead to an overestimate of the value of viscosity. We also calculate flow effects of heat capacity and thermal conductivity values.

Mathematical Modelling of Hall‐Héroult Pot Instability and Verification by Measurements of Anode Current Distribution

A software application based on the full MHD model of the aluminium electrolytic production cell is used to predict the liquid metal surface instability in a commercial Trimet operated potline. The results are compared with the electric current distribution variation in time over the anodes obtained from the measurement of magnetic fields by wireless sensors. The model incorporating full 3d busbar configuration predicts a critical instability excitation frequency 0.0259 Hz, which compares to the measured frequency of 0.0254 Hz. The mathematical software permits to analyse the sensitivity to the pot individual features like ACD, anode loads, ledge shape, bottom wear and busbar irregularities. The ability to monitor continuously the electric current distribution to high accuracy helps to control disturbances and to visualise the cell interior with the help of this numerical tool.

Modeling the Break-up of Nano-particle Clusters in Aluminum- and Magnesium-Based Metal Matrix Nano-composites

Aluminum- and magnesium-based metal matrix nano-composites with ceramic nano-reinforcements promise low weight with high durability and superior strength, desirable properties in aerospace, automobile, and other applications. However, nano-particle agglomerations lead to adverse effects on final properties: large-size clusters no longer act as dislocation anchors, but instead become defects; the resulting particle distribution will be uneven, leading to inconsistent properties. To prevent agglomeration and to break-up clusters, ultrasonic processing is used via an immersed sonotrode, or alternatively via electromagnetic vibration. A study of the interaction forces holding the nano-particles together shows that the choice of adhesion model significantly affects estimates of break-up force and that simple Stokes drag due to stirring is insufficient to break-up the clusters. The complex interaction of flow and co-joint particles under a high frequency external field (ultrasonic, electromagnetic) is addressed in detail using a discrete-element method code to demonstrate the effect of these fields on de-agglomeration.

Non‐Linear Stability Analysis of Cells Having Different Types of Cathode Surface Geometry

Different operational parameters are known to influence cell stability like ACD, ledge toe position and metal level.