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CONVECTIVE EFFECTS IN SOLIDIFICATION.

Exploitable results

A software package, named CONVECT-SOL, has been developed for the treatment of convective effects in solidification processing of materials. The 2 following configurations are considered: shaped castings in a foundry, more particularly problems of cold shuts, poor grain structure or detrimental distribution of inclusions or reinforcing particles; massive crystals, grown by Bridgman or floating zone techniques, with special attention to banding or striation phenomena. The package includes a lot of data resulting from collective experimental and theoretical work. CONVECT-SOL has the following attributes: Improved description of solidification coupled to flow: This is important in the present trend to thinner and lighter pieces. A special module detects conditions in which solidification stops the flow before the print is filled, leading to the defect named cold shot or cold shut. This involves microstructure formation mechanisms. Description of the behaviour of particles in relation to solidification: These particles can be inclusion particles in a piece of superalloy, or particles of inoculating ferro alloy in cast iron, or reinforcement particles in a cast metal matrix composite. A module is proposed for simulating the interactions of such particles with the solidifications front. Improved description of buoyancy flow: Commercial codes are mainly oriented to forced flow. In fact, buoyancy driven flow has important consequences on the growth of large crystals, in particular banding and striations. In shaped castings, buoyancy flow can strongly affect solidification in massive parts, creating segregation marks (ghost lines). An improved formulation will result in a more precise design of the feeder system, thus leading to savings in material, time between design and production, and finally cost. Surface tension driven flow (Marangoni flow): Implementation of thermocapillary flow concerns the zones of liquid free surface which are submitted to a high temperature gradient. An industrial case is floating zone crystal growth. Besides, it is known that thermocapillary flow strongly affects liquid pool and solidification in welding conditions. Improvement resulting from solidification in centrifuge machines: In this case the flow results from an increased gravity coupled to Coriolis effect. Solidification experiments in centrifuge machines have demonstrated a better stability for growth of single crystals. Moreover this part can give new possibilities for flow in centrifugal shaped castings, for industrial manufacture of tubes, automotive wheels, jewellery, dental prosthesis parts, etc.
A software tool has been built for treating convective effects in solidification. It is articulated in a numerical frame code, and in a series of modules dedicated to specific cases. It is a contribution to rapid prototyping in the industrial sectors of shape casting foundry, and of crystal solidification processing. As an example, it was applied to a piece in form of a 3-dimensionall loop, representing a massive cast iron piece with a core. The reslts showed how the flow separates, how solidification occurs during filling, and how paricles initially suspended in the liquid are distributed during solidification. This software is particularly orientated to 3 kind of problems: Solidification in thin wall castings, emphasizing the problems of fluid length and of cold shut. A criterion function was implementated and tested experimentally on a series of cast iron and aluminium-silicon thin plates. The fluid length was measured in a series of casting tests on an aluminium alloy in low pressure and high pressure conditions. The results were represented by a simple 1-dimensional model enabling to presict the ability to produce thin elongated casting pieces. Solidification in the presence of particles in suspension (inclusions, inoculants, reinforcing particles in metal-matrix-composites, equiaxed crystals). According to relative flw and solidification conditions, they can be either entrapped in the solid or pushed at the interface. Striations and banding phenomena in massive solidification products, including massive crystals. Dedicated modules treated coupled thermo-capillary and thermo-gravity flows, either by streamline diffusion method, or by Fourier transform spectral method. They were applied to floating zone crystal growth. The example showed the transition from stready axi-symetrical to 3D-steady, then to oscillatory growth.
Software has been developed to obtain numerical buoyant and/or capillary flows confined in 2-dimensional and 3-dimensional Cartesian or cylindrical containers. Accurate steady flows, accurate evaluation of transition thresholds to unsteadiness, and unsteady flows are simulated. Flows in porous media are also determined.