Cold-Cracking Assessment in AA7050 Billets during Direct-Chill Casting by Thermomechanical Simulation of Residual Therma
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DURING direct-chill (DC) casting of aluminum alloys, large thermal stresses develop inside the slabs (rectangular ingots) and billets (round ingots). These stresses are due to temperature gradients in the metal, and to the thermal contraction, which may result in cracking and failure.[1] The 7xxx series aluminum alloys are more vulnerable to cracking mainly because of poor thermal and mechanical properties in the as-cast condition. Low thermal conductivity values compared to other aluminum alloys result in high-temperature gradients, which in turn lead to accumulation of thermal stresses with different signs and magnitudes in different locations of the billets during DC casting. A large solidification range (470 °C to 630 °C), on the other hand, makes these alloys prone to solidification cracking called hot cracking. The presence of non-equilibrium phases, with low melting points, especially at grain boundaries and interdendritic spaces provides favorable paths for crack initiation and propagation. Investigation of as-cast mechanical properties of DC-cast 7050 and 7475 alloys has shown that such alloys lose their M. LALPOOR, Ph.D. Researcher, and D.G. ESKIN, Fellow M2i, are with the Materials Innovation Institute, 2628 CD Delft, The Netherlands. Contact e-mail: [email protected] L. KATGERMAN, Professor, is with the Department of Materials Science and Engineering, Delft University of Technology, Delft, 2628 CD, The Netherlands. Manuscript submitted March 12, 2009. Article published online October 27, 2009 3304—VOLUME 40A, DECEMBER 2009
ductility below 200 °C and become extremely brittle,[2] which can make the material prone to cold cracking. Numerical simulation of thermal stresses during DC casting of these alloys can reveal those stages and locations at which the materials susceptibility to cracking is high. The state of residual thermal stresses during DC casting have been determined by some researchers using various commercial and noncommercial finite element packages.[1,3–6] Ludwig et al.[7] and Boender et al.[8] went one step further and used the maximum principal stress and fracture mechanics to assess the critical crack size leading to catastrophic failure. For slabs (width: 1150 mm, thickness: 360 mm, length: 1500 mm) of Al-4.5 pct Cu[7] with the maximum principal stress 80 MPa, the critical crack size was assessed to be 8 mm. In AA2024 slabs (width: 2000 mm, thickness: 510 mm, length: 2000 mm),[8] critical crack sizes appeared to be between 5 and 50 mm for various maximum principal stress values found in different locations. However, such study has not been performed for highly prone to cold cracking 7050 or 7075 alloys over the critical temperature range (200 °C down to room temperature) mainly because of lack of necessary data. On the other hand, the derived critical crack sizes can only be realistic providing that the mechanical properties and constitutive parameters used to obtain and evaluate the simulation results are extracted from the samples in the genuine as-cast condition. In the homogenize
