Thermal stress prediction for direct-chill casting of a high strength aluminum alloy.

摘要

Direct chill (D.C.) casting is one of the most important semi-continuous methods for the production of high strength aluminum alloys. The enormous unevenly cooling of ingots during the casting process can cause significant thermally induced stresses, which may result in solidification cracking. The control of the cracking during DC casting is a state-of-art technology, and many finite element models have been applied to simulate the solidification process during ingot casting. So far, most of the simulations can predict the thermal fields of the ingot accurately, but very few works can get satisfactory thermal stress profiles. One of the major difficulties is the lack of valid thermo-mechanical properties for constitutive modeling of as-cast ingots.; The mechanical properties of a high strength aerospace aluminum alloy 7050 was studied in the as-cast ingot form. A thermo-elastic-plastic constitutive model was adopted to summarize the ingot strength and deformation behavior over a wide temperature range from the melting point to room temperature. In addition, the dependence of ingot properties on the casting structure as well as the cooling history at different ingot locations were determined. The cooling history of 7050 ingots can be divided into two portions at every location. The solidification rate between liquidus (635{dollar}spcirc{dollar}C/1175{dollar}spcirc{dollar}F) and solidus (524{dollar}spcirc{dollar}C/975{dollar}spcirc{dollar}F) decides the cast microstructure, which exhibits various coarse grain structures with notable dendrite segregation. After solidification, the cooling rate of solid ingots will influence the formation of the precipitation phases and their morphology. Both portions of the cooling history were considered as the parameters in the constitutive models. A finite element model (FEM) was developed to predict the thermal stress distribution in DC cast aluminum ingots by employing a commercial FEM code ABAQUS. The in-situ measured temperature profiles was input as the thermal conditions through a user subroutine, and the material constitutive model was employed in the modeling. In addition, fracture toughness of as-cast ingots was investigated experimentally through on-cooling K{dollar}sb{lcub}rm IC{rcub}{dollar} tests for material from the center and surface of Al-7050 ingot.