Thermal analysis of the startup phase for D. C. casting of an AA5182 aluminum ingot.

摘要

The evolution of temperature and stress during the start-up phase of the direct chill (D.C.) casting process has been studied to determine the factors that make this phase of the process prone to surface crack generation. The analysis was carried out principally using a finite element based heat flow model, but a preliminary thermal stress model was also employed. Key to the study was the experimental measurement of temperature in the region of the cast prone to crack formation and the development of a technique to determine the surface heat fluxes as a function of surface temperature (system boiling curves) in the direct chill water.;An inverse heat transfer methodology was developed to calculate the boiling curves for direct chill water cooling. This method uses as input the data acquired from one embedded thermocouple transiting the water cooling region and involves the application of 1-D and 2-D finite element based heat conduction models in succession. The technique has been verified using hypothetical temperature data obtained from a transient casting simulation conducted with a known heat flux profile. The results of the inverse heat calculations on the industrial data indicate that a variation in surface morphology, occurring during the early stages of casting, influences the shape of the water cooling flux/temperature relations and has a bearing on the amount of heat extracted during startup. The intensity of the direct chill water cooling was found to be enhanced in the "lapped surface morphology" portion of the ingot relative to the remainder of the cast.;Based on the calculated system boiling curves, a FEM simulation of the cast start was undertaken. The thermal analyses was employed to identify conditions that may enhance the potential for surface crack generation. The simulation data, in conjunction with relevant industrial measurements, suggests that a combination of an increase in the solidified shell thickness (defined as the distance of the solidus isotherm of the ingot parallel to the water contact line), and a high surface temperature gradient in the vicinity of the water contact point, accounts for the high incidence of surface face cracking observed.;A preliminary thermal stress analysis qualitatively supports the association of a peak tensile stress with a peak in surface temperature gradient. Maximum values of both shell thickness and peak surface temperature gradient were observed to occur in the "lapped surface morphology" regime. The peak values in this region of the ingot were attributed to both an increase in the severity of water cooling (as calculated with the inverse heat transfer technique) and an enhancement in heat extracted by the mould. This observation indicates that events occurring in the meniscus region, with particular regard to the development of surface morphology, have a significant impact on subsequent cooling behaviour.