In many casting processes, some stages, such as the starting phase in the direct chill casting of aluminium alloys, are particularly critical because of the propensity of some alloys to develop either hot tears which initiate at non zero liquid fraction, or cold cracks which nucleate and grow exclusively in the solid metal. Figure 1 shows a cracked rolling sheet ingot: two hot tears initiated during the starting phase of casting have grown up as the ingot was further cast. For some very sensitive alloys, up to 10 % of the ingots present some cracks and have to be remelted.