Aging characteristics of electron beam and gas tungsten arc fusion zones of Al-Cu-Li alloy 2090
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INTRODUCTION
THE alloy 2090-T81, with its combination of good cryogenic mechanical properties, high specific elastic modulus, and low density, is a promising candidate for weight-restricted cryogenic structures, t~,2,31An important possible application considered for 2090 is cryogenic fuel tankage for space vehicles, which requires a welded construction. Compatibility in strength, ductility, and toughness between the parent metal and the weld is desired. However, there are problems associated with producing a sound weld, along with the problems of strength mismatch and low ductility in the weldments, t4-81 Alloy 2090-T81 is a precipitation-strengthened alloy. Thus, the volume fraction, size, and distribution of strengthening phases are the contributing factors in obtaining the desired properties. The chemistry and thermomechanical processing have been developed to produce the following strengthening phases in 2090. t9-12]The small addition of Zr to 2090 results in the formation of fl' (A13Zr). The fl' is a metastable, coherent dispersoid that homogeneously precipitates during homogenizationtl2,~3] and functions as the grain refiner and recrystallization inhibitor during subsequent processing. The 8' is also a metastable, coherent phase that ordinarily forms through spinodal decomposition during the quench from solution heat treatment, t~4.~5.16] Three different morphologies of 6' have been documented: small discrete spherical particles, 6' that encapsulates the/3', and 6' that coats the broad faces of 0' (AI2Cu). till Both 0' and T~ (A12CuLi) are semicoherent phases that precipitate as platelets on the {100} and {111} habit planes, respectively. They nucleate heterogeneously on dislocations and low-angle grain boundaries during subsequent aging. Of the two, T~ is the predominant Cu-bearing strengthening phase present in 2090. Hence, in order to
A.J. SUNWOO, Staff Scientist, Center for Advanced Materials, Lawrence Berkeley Laboratory, and J.W. MORRIS, Jr., Professor, Department of Materials Science and Engineering, are with the University of California, Berkeley, CA 94552. Manuscript submitted March 12, 1990. METALLURGICAL TRANSACTIONS A
acquire T1 homogeneity in the matrix, 2090 is stretched prior to artificial aging. In addition to T1, the other equilibrium precipitate present in 2090 is the T2 (ml6CuLi3)I17] that tends to form on high-angle boundaries and has irregular morphology. During welding, the prior thermomechanical processing of 2090 is erased, and the fusion zone consists of an as-cast microstructure with Li and Cu solute gradients near the dendrite boundaries. Due to the nature of solidification, solute segregation to the dendrite boundaries is unavoidable but can be minimized by controlling the constitutional undercooling the welding process. Previous study has shown that there is a distinct difference in the Cu distribution between the electron beam (EB) and the gas tungsten arc (GTA) fusion zones, tTI Copper segregation to the dendrite boundaries was more evident in the GTA fusion zone. The Cu
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