Gérard Lesoult

Solidification of spheroidal graphite cast irons : I. Physical modelling

It is well known that eutectic and slightly hypereutectic cast iron may present austenite dendrites which should appear only in hypoeutectic alloys according to the phase diagram. Such an experimental fact was difficult to explain within the framework of the currently available quantitative models of the solidification of these alloys. The aim of this work is thus to present a physical model of the solidification of SG cast irons which quantitatively accounts for the formation of non-eutectic austenite during cooling and solidification of hypereutectic as well as hypoeutectic cast irons. Amongst other features, this model includes the description of the nucleation and growth of the proeutectic graphite in hypereutectic alloys. However, the most important part of the model is the quantitative description of the so-called eutectic stage of the solidification of SGI. Numerical simulations based on the present model are presented and discussed in Part II.

Solidification of spheroidal graphite cast irons—II. numerical simulation

A model of the solidification of spheroidal graphite cast iron (SGI) has been detailed in Part I. It was designed to allow the description of the successive steps of solidification of hypereutectic as well as hypoeutectic alloys, i.e. the pro-eutectic deposits and the eutectic reaction. In the present paper, this model is used first as a tool to analyse the main features of the solidification of SGI as experimentally established. Calculations are then described which were carried out to simulate a series of test castings from which sufficient experimental information was obtained to discuss the validity and the limits of the present modelling and simulating approach. Particular emphasis is put on the nucleation and growth of graphite nodules and on the precipitation of non-eutectic austenite during the eutectic stage.

Free growth of equiaxed crystals settling in undercooled NH4Cl–H2O melts

Abstract Recently theoretical works concerning the effect of convection on the growth of isolated dendrites have been compared with experiments on NH 4 Cl settling equiaxed crystals. It was inferred that more accurate theories were still needed to describe properly the equiaxed crystal growth in the presence of convection. Some new results have been obtained using the following experimental set-up: in a tube containing an undercooled solution of NH 4 Cl–H 2 O, settling NH 4 Cl equiaxed crystals have been filmed with a video camera so as to determine the evolution with time of their size and of their settling velocity. After a careful comparison of the experimental results with some calculations involving the choice of a stability constant, no major discrepancy has been found to prevent the application of the theories in question to moving equiaxed crystals.

Experimental investigation of the development of microsegregation during solidification of an AlCuMgSi aluminium alloy

Abstract Chemical heterogeneities which build up during dendritic solidification of metallic alloys are of great importance to their service properties. In nominally single-phase alloys, off-equilibrium eutectic often appears which is associated with large compositional changes in the primary phase at the scale of the solidification microstructure. In order to obtain a precise description of solute heterogeneities, it has become common practice to draw distribution curves obtained from numerous microprobe point counts. The experimental distribution curves show differences with respect to the usual predictions, these differences being related to the sign of curvature at low solid fraction and to the amount of highly segregated material at high solid fraction. The present study is an attempt to investigate experimentally microsegregation build-up during solidification, and was carried out on an aluminium alloy quenched during directional solidification.

Equi-axed growth and related segregations in cast metallic alloys

Abstract Full-scale trials of DC ingots and laboratory scale directional solidification experiments have been performed to study the effect of grain structure on macro-segregation in industrial cast products. An Al alloy sheet ingot was cast with constant casting conditions (speed, superheat, cooling rate) except for the grain refiner: the first half of the ingot was non-inoculated, while the second half was inoculated. The results indicate that the extent and intensity of the centreline segregation is modified via the grain-refinement treatment: the finer the grains are, the more intense is the macro-segregation. Numerical simulations of directional solidification of binary Al-Cu alloys have been carried out with the help of a 2D finite volume software which takes account of the movement of the liquid with respect to the solid in the mushy zone. It is possible to account for the segregation pattern of the directionally solidified ingots that exhibit columnar or coarse equi-axed grain structures. Contrarily, the intense segregation of the fine-grained ingots is not yet understood.

Numerical model for prediction of the final segregation pattern of bearing steel ingots

Abstract Chemical heterogeneities arise during solidification of steel ingots due to long range solute transport. A model taking into account heat, mass and momentum coupled transports during solidification of multicomponent alloys in a mold, is briefly presented. A comparison between experimental and numerical results illustrates its abilities and limits in the prediction of segregation pattern in bearing steel ingots.

Simulation of thermal analysis applied to the description of the solidification of hypereutectic SG irons

Thermal analysis is widely used in foundry to control melt preparation of cast irons. Its great versatility explains that many attempts have been made in the past to use it not only for controlling...

Effect of Grain Refinement on Macrosegregation in Direct Chill Semi-Continuous Casting of Aluminium Sheet Ingot

An experimental investigation of the macrosegregation occurring in a commercial size direct chill cast of an 5182 alloy ingot is described. The extent of the chemical homogeneity in a sheet ingot depending of the grain refining practice is discussed. A complete analysis of 2D macrosegregation maps and of the associated grain structure in a non grain refined part and a grain refined part of the ingot is compared and discussed. The intensity of the centerline segregation is modified via the grain refining practice. The change of macrosegregation measured in the slices with and without grain refiner may be attributed to the effects of the grain motion but also the liquid movement.

Modelling of microporosity formation in aluminium alloys castings

Abstract The solidification of Al-7wt.%Si alloy in sand mold geometries has been simulated using a 2D-finite volumes model which takes full account of coupled heat, mass and momentum transport phenomena. The shrinkage boundary condition used in this model is detailed, and results of the simulation are presented in terms of pressure drops which are viewed sometimes as one of the major causes of micropore formation in aluminium castings. Solidification of plates of different thicknesses in sand mold has been investigated. Calculations give a maximum absolute pressure drop of 27 000 Pa in the 7.5 mm thick casting whereas this maximum is only 300 Pa in the 30 mm thick casting. Considering the condition of microporosity formation, these results point out that pressure drops are far too low to account for the formation of micropores which are observed experimentally. Hydrogen segregation has to be considered and integrated in a model to give a good prediction of micropore appearance.

Spongy Behaviour of Alloys during Solidification: Flow of Liquid Metal and Segregation in the Mushy Zone

Etude theorique du processus de solidification. Interface liquide solide. Phenomenes de segregation, influence de la viscosite et de la temperature du metal liquide