A new criterion for the appearance of hot tears in metallic alloys is proposed. Based upon a mass balance performed over the liquid and solid phases, it accounts for the tensile deformation of the solid skeleton perpendicular to the growing dendrites and for the induced interdendritic liquid feeding. This model introduces a critical deformation rate ( p,max ) beyond which cavitation, i.e., nucleation ⅐ ε of a first void, occurs. As should be expected, this critical value is an increasing function of the thermal gradient and permeability and a decreasing function of the viscosity. The shrinkage contribution, which is also included in the model, is shown to be of the same order of magnitude as that associated with the tensile deformation of the solid skeleton. A hot-cracking sensitivity (HCS) index is then defined as . When applied to a variable-concentration aluminum-copper alloy, this HCS ⅐Ϫ1 ε p,max criterion can reproduce the typical ''⌳ curves'' previously deduced by Clyne and Davies on a phenomenological basis. The calculated values are in fairly good agreement with those obtained experimentally by Spittle and Cushway for a non-grain-refined alloy. A comparison of this criterion to hot cracks observed in ring-mold solidification tests indicates cavitation depression of a few kilo Pascal and tensile stresses in the coherent mushy zone of a few mega Pascal. These values are discussed in terms of those obtained by other means (coherency measurement, microporosity observation, and simulation). Even though this HCS criterion is based only upon the appearance of a first void and not on its propagation, it sets up for the first time a physically sound basis for the study of hot-crack formation.

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A microporosity model, based on the solution of Darcy's equation and microsegregation of gas, has been developed for arbitrary two-(2-D) and three-dimensional (3-D) geometry and coupled for the first time with macroporosity and pipe-shrinkage predictions. In order to accurately calculate the pressure drop within the mushy zone, a dynamic refinement technique has been implemented: a fine and regular finite volume (FV) grid is superimposed onto the finite-element (FE) mesh used for the heat-flow computations. For each time-step, the cells, which fall in the mushy zone, are activated, and the governing equations of microporosity formation are solved only within this domain, with appropriate boundary conditions. For that purpose, it is necessary to identify automatically the various liquid regions that may appear during solidification: open regions of liquid are connected to a free surface where a pressure is imposed, partially closed liquid regions are connected to an open region via the mushy zone, and closed regions are totally surrounded by the solid and/or mold. For partially closed liquid pockets, it is shown that an integral boundary condition applies before macroporosity appears. Finally, pipe shrinkage (i.e., shrinkage appearing at a free surface) is obtained by integration of the calculated interdendritic fluid flow over the open-region boundaries, thus ensuring that the total shrinkage (microporosity plus macroporosity and pipe shrinkage) respects the overall mass balance. This very general approach is applied to Al-Cu and Al-Si alloys.

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In previous work it has been shown that near surface residual stresses may be deduced from surface strains produced by making a cut of progressively increasing depth. The process of electric discharge wire machining (EDWM), by providing very narrow cuts, greatly improves the ability of the method to resolve a stress gradient near the surface. However, the EDWM process may also introduce residual stresses. In the present work a model for estimating the influence of EDWM is presented, and a procedure for eliminating its effect on residual stress measurement is developed. Experimental results validate the theoretical approach.

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In order to quantify the cooling conditions undergone by an ingot during direct-chill (DC) casting, thermocouples were immersed in the liquid pool and consequently entrapped in the solid, thus monitoring the temperature of the metal during its descent. Assuming steady-state thermal conditions, the time-dependent temperatures measured by these thermocouples were then converted into spacedependent temperature profiles. These values were the input of a Maximum A Posteriori (MAP) inverse method described by Rappaz et al., [1] which has been adapted in this case to steady-state thermal conditions. This MAP method permits the deduction of the temperature-dependent thermal conductivity of the alloy, initially, and then of the highly nonuniform heat-flux distribution along the ingot rolling faces, in a second step. The obtained values are in good agreement with literature and clearly reflect the widely different boundary conditions associated with primary cooling (contact with the mold) and secondary cooling (water jet).