Heat-flow-based analysis of surface crack formation during the start-up of the direct chill casting process: Part II. experimental study of an AA5182 rolling ingot

The flow of heat during the start-up of the direct chill (DC) casting process has been studied with the aim of determining the factors that make this phase of the process prone to face crack generation. Measurements have been made on an AA5182 rolling ingot instrumented with embedded thermocouples placed at key locations in the vicinity of the ingot face near its base. The resulting temperature data have been input to a two-dimensional (2-D) inverse heat-transfer model, developed in part I of this two part study, in order to calculate heat fluxvs surface temperature curves in the direct water impingement regime. The findings indicate that the flow of heat is influenced by changing surface morphology and water flow conditions during the start-up phase. A finite element based simulation of the cast start, employing the calculated flux/surface temperature relations, reveals that the ingot shell at the point of water contact reaches a maximum thickness early in the casting process. The location of this maximum was found to coincide with the position where surface cracks are routinely found to initiate. Further, this maximum was found to also coincide with position at which the rate of deflection of the base of the ingot (“butt-curl”) begins to slow. Based on the heat-flow analysis, it is believed that the face cracks form due to an excessive shell thickness during transient start-up conditions and that their occurrence could be reduced by an optimal combination of water flow rate and casting speed during start-up.