Effects of heat-affected zone peak temperatures on the microstructure and properties of 2090 Al alloy
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INTRODUCTION
THE 2090-T81
AI alloy, with its combination of good cryogenic properties, low density, and high modulus, is of interest to the aerospace industry for weight-restricted aerospace applications. ~ Although Al components are primarily joined by fasteners such as rivets, cryogenic fuel tanks require a welded construction. These are three distinct regions in weldments: the fusion zone (FZ), the heat affected zone (HAZ), and the unaffected base metal (BM). The area of interest for this study is the HAZ. The HAZ is created by the high local temperature that was present during welding. Heat conduction from the weld pool causes the microstructuml changes in the HAZ, which, in turn, cause the changes in the mechanical properties. The HAZ properties of 2090 alloy are a concem since the properties are obtained through precipitation strengthening, and any thermal cycle will affect the precipitation behavior. In addition, as the strength of the fusion zone increases with the alloy additions, the HAZ will be the next weakest region in the weldment. The HAZ microstructure of welded 2219 and 6061 alloys has been characterized by Dumolt,/4] who found that the HAZ transformation in 2219 is dominated by the dissolution of precipitate, while changes in 6061 are dominated by precipitate coarsening. A brief HAZ studytS] of 2090 has shown that when the peak temperature of the HAZ exceeds the phase solvus temperature, dissolution of the strengthening phase occurs. However, no detailed study has been conducted to correlate these microstructural changes to the mechanical properties. Other studies [6,7] have focused more on the hot ductility
A.J. SUNWOO, formerly Graduate Student Research Assistant, Staff Scientist, and E.L. BRADLEY III, Graduate Student Research Assistant, are with the Lawrence Berkeley Laboratory and University of California at Berkeley. J.W. MORRIS, Jr., Professor of Metallurgy, is with the Department of Materials Science and Mineral Engineering, University of California at Berkeley, CA 94720. Manuscript submitted November 10, 1989. METALLURGICAL TRANSACTIONS A
and hot cracking of the HAZ. Since 2090 is being considered for cryogenic welded applications, knowledge of the HAZ properties at low temperature is essential. The purpose of this study is to investigate the effects of the HAZ thermal cycles on the microstructure and properties of 2090 A1 alloy in the T3 and T8 tempered conditions. Because the HAZ is an intricate part of the weldment, it is difficult to isolate and test only the HAZ. The approach used in this study is to experimentally obtain the temperature history of the HAZ and to simulate it using a Gleeble. The thermal cycle can be divided into three regimes: heating, peak temperature, and cooling. The effects of the three regimes have been studied independently, and only the peak temperature was found to have a significant influence on the response of precipitates, t41Emphasis is placed on the effects of the HAZ peak temperatures on the microstructure and mechanical properties of 2090.
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