Effect of natural and forced convection in materials crystallization

Resume : Iron is an inevitable element in all Al castings, either added deliberately to improve strength or entering through the recycling chain when re-melting. During subsequent solidification, dissolved Fe in the liquid reacts with other alloying elements to form complex intermetallic phases, which degrade the downstream processability (e.g. in extrusion) and final properties, particularly elongation and toughness. To overcome the detrimental effects of tramp Fe, a cost and energy consuming homogenisation step is used in an attempt to break-up the intermetallic compound (IMC) network and to spheroidize IMCs to a less damaging morphology. In order to minimise homogenisation costs and possibly to eventually overcome the requirement, it is essential to understand and control the initial IMC formation during solidification. In this study, we report the in-situ observation of the Fe intermetallic formation in Al alloys under constrained and un-constrained solidification conditions, and in the presence of natural convection. The constrained (in the presence of extended primary Al columnar dendrites) and un-constrained (in the presence of “free floating” primary Al equiaxed dendrites) growth solidification conditions were obtained by directionally solidifying the alloy without and with addition of grain refiner. Under constrained primary Al growth conditions upwards and opposite to gravity, the Fe IMCs nucleated and grew in lengthy branched morphologies that passed across primary Al columnar dendrites. As the cooling rate increased, the IMCs then grew progressively only between the primary Al columnar dendrites. Under un-constrained primary Al growth conditions, the Fe IMCs nucleated and grew to much shorter lengths, between the equiaxed grains at all cooling rates studied. Consequently, we show how primary solidification morphologies and growth conditions can be used to manipulate IMC morphology. Finally, the effect of various convection effects in IMC development is also discussed.

Resume : The time-dependent VOF (Volume Of Fluid) formulation is used to track the shape of the free surface and the flow field inside the silicon crucible. The VOF formulation relies on the fact that two fluids (liquid silicon and argon gase) are not interpenetrating. The volume fraction is introduced as a new variable of the phase in the computational cell. In each control volume the volume fractions of all phases sum to unity. The fields for all variables and properties are shared by the phases and represent volume-averaged values, as long as the volume fraction of each of the phases is known at each location. In order to understand the influence of the crucible rotation on melt flow pattern and free surface in silicon melt, a set of numerical simulations are conducted using the finite volume method. The analysis of the obtained results led to conclude that the forced convection affects strongly the free surface shape and the melt flow.

Resume : Liquid metal engineering refers to the treatment of liquid metals by either chemical or physical means to enhance heterogeneous nucleation through manipulation of the chemical and physical nature of both endogenous (naturally occurred) and exogenous (externally added) nucleating particles prior to solidification processing. A prime aim of liquid metal engineering is to produce solidified metallic materials/components with fine and uniform microstructure, uniform chemical composition and reduced/eliminated cast defects. In this paper I offer an overview on our current understanding of enhancing heterogeneous nucleation through liquid metal engineering by intensive melt shearing and our current progress on development of solidification technologies by implementation of liquid metal engineering principles.

Resume : Crystal growth from the melt is non-equilibrium process affected by the time and spatial behavior of temperature and concentration gradients near the interface. Many methods of melt activation during crystal growth optimize and stabilize these gradients through the hydrodynamical impact on liquid [1]. Axial vibration control (AVC) technique is one of the most energy efficient for these purposes [2]. However besides thermal and chemical diffusion under non-isothermal conditions of a growing setup, the phenomenon of structure evolution of the melt should be taken into consideration. Melt structural changings have been declared for metals, oxides, chalcogenides, silicon, germanium, and complex semiconductors. In present work we analyzed temperature changing of melt structures for modeling materials NaNO3 and LiNO3 by XRD and Raman spectroscopy. To analyze the AVC impact on melt structure Raman spectrum have been analyzed for different temperature and for different vibrating regimes. Physical experiment has been conducted together with numerical simulation, from which flows’ parameters and viscous dissipation rate in boundary layer have been withdrawn. Single crystals grown by AVC-CZ technique for the melt structure control demonstrated high perfection comparing to the conventional CZ-grown crystals. 1. Semiconductors and Dielectrics Eds. P. Capper, P. Rudolph, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany, 2010 2. Avetissov, I. Ch. et al. Doklady physics, 54 (2009) 410-412

Resume : Compression plasma flows generated by a magnetoplasma compressor are a unique technique for surface materials modification. Specific configuration of electric and magnetic fields allows to reach the period of stable existence of a directed plasma flow of up to 100 microseconds. The influence of such plasma flows with the absorbed by the surface energy density up to 40 J/cm2 on the “coating/substrate” system results in surface layer melting and penetration of the coating element into the metal substrate. The atoms of the coating (Mo, Cr, Zr, Ni, Ti) are distributed homogeneously along the depth of 10 – 30 micrometers in the metal (Ti, Fe, Al) matrix. The main mechanism of mass transfer providing deeper penetration than the diffusion depth can be attributed to convective fluxes that appear in the liquid phase because of the Kelvin-Helmholtz hydrodynamic instability development. The plasma flow interacting with the melt surface results in the liquid motion with both tangential and normal components of the velocity, which obeys to the Navier-Stokes equation. Due to viscous friction between the layers in the melt the motion is spread to deeper layers causing mixing of coating and substrate elements in the liquid state. The model describing the experimental data is proposed.

Resume : The introduction of magnetic field in the solidification process is one of the effective methods to improve the microstructure and mechanical performance of alloys (by controlling the defects as freckles or segregated channels). At SIMaP-EPM laboratory, we have proposed to control the fluid flow with a travelling magnetic field (TMF). With this kind of electromagnetic field the control of the intensity and the direction of Lorentz force are easy (by changing the order of electric phases). Since ten years, EPM team developed some experiments and models around the TMF. The present work deals with a experimental and numerical studies of solidification process under forced convection induced by a travelling magnetic field (TMF). The impact of TMF and gravity on segregations of an Tin-Lead alloy has been examined.

Resume : In this paper we present experimental results showing the influence of the thermal Raleigh number on the CET transition during the solidification of a transparent alloy. In a cast cell of 10*3*40cm3, different solidification experiments of a 29,52 wt% hypereutectic alloy NH4Cl-H2O were performed. Wall?s temperature was measured during the solidification experiment. The starting temperature was 42?C and the ending temperature 5?C. The cooling rate was 1.2K/min. The liquid solution was filled in the cell at different heights: 8 cm, 12.5 cm, 20cm and 35cm. The influence of the Rayleigh number on the thermosolutal convection was studied in the cast cell. A PIV system (laser light and CCD camera) was used to record pictures during the experiments. The thermal Rayleight numbers varies from 107 for the 8 cm height to 109 for 35 cm height. The experiments were very different in terms of solidification type (Fig.1). Almost purely columnar and no apparent equiaxed crystals were observed for the height of 8 cm (Ra=107). For the 12.5 cm height (Ra=107) large number of equiaxed crystals settling was observed. The mush at the end of the experiment showed a large area of equiaxed in the centre of the cell (Figure 1 b). For the experiment with the 35 cm filling height (Ra~109 ) the solidification took place almost in a purely equiaxed and the mush had a equiaxed structure. The influence of the Rayleigh number on the CET will be discussed in term of undercooling temperatures and hydrodynamics.