HEAT-FLOW-BASED ANALYSIS OF SURFACE CRACK FORMATION DURING THE START-UP OF THE DIRECT CHILL CASTING PROCESS .2. 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. Measure ments have been made on an AA5182 rolling ingot instrumented with embe dded thermocouples placed at key locations in the vicinity of the ingo t face near its base. The resulting temperature data have been input t o a two-dimensional (2-D) inverse heat-transfer model, developed in pa rt I of this two part study, in order to calculate heat flux vs surfac e temperature curves in the direct water impingement regime. The findi ngs indicate that the flow of heat is influenced by changing surface m orphology and water flow conditions during the start-up phase. A finit e element based simulation of the cast start, employing the calculated flux/surface temperature relations, reveals that the ingot shell at t he point of water contact reaches a maximum thickness early in the cas ting process. The location of this maximum was found to coincide with the position where surface cracks are routinely found to initiate. Fur ther, this maximum was found to also coincide with position at which t he 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 star t-up conditions and that their occurrence could be reduced by an optim al combination of water flow rate and casting speed during start-up.