Semester

Spring

Date of Graduation

2003

Document Type

Thesis

Degree Type

MS

College

Statler College of Engineering and Mineral Resources

Department

Mechanical and Aerospace Engineering

Committee Chair

Keh-Minn Chang.

Abstract

In Direct chill (DC) casting which is used in the manufacture of high strength aluminum alloys uneven cooling rates at different parts of the ingot cause thermal stresses. Due to these thermal stresses, cracks are initiated at the startup stage which may propagate as the stress elevates. Hence the evaluation of fracture toughness (cracking resistance) at different parts of ingot is very much essential to understand the propagation of cracks in ingots.;The objective of this research was to determine the cracking resistance of Al 2024 (Al-Cu-Mg) and Al 3004 (Al-Mn-Mg) alloys and to illustrate the relation of cooling rate during solidification and alloy composition on the cracking resistance of these alloys. The specimens were obtained from ALCOA and were first precracked and the cracking resistance was determined by quench cracking tests which simulated the propagation of cracks under thermal stresses. The software for these tests was developed at West Virginia University. They were performed on specimens obtained from different locations of directionally solidified ingots that exhibited different solidification rates along their length. The solidification rates were similar to the range of real time cast ingots. After the tests microstructures and fractured surfaces of the specimens were examined by optical microscope, SEM and HISCOPE.;The cracking resistance was observed to increase with the solidification rate (which decreases from surface to center in a real time cast DC ingot) for both the alloys and for a given solidification rate the cracking resistance of 3004 was found to be higher than 2024. The above variations were successfully explained based on the microstructural observation and fracture surfaces of the alloys and the effect of alloy composition and solidification rate on cracking resistance was successfully illustrated.

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