Strengthening of wrought aluminum alloys by fractional melting
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IN recent years, much effort has been spent on improving the mechanical properties of high strength alloys, with the main emphasis being control of the amount of second phases. Process routes which have recently been intensively studied are l) consolidation of splat quenched or atomized liquid droplets, ~-5and 2) modification of conventional ingot working practice by reduction of impurities, composition optimization, and thermal-mechanical treatments. 6-1~ The techniques of splat quenching and atomization rely on rapid solidification to produce very fine structures which can be easily homogenized. Also any second phases remaining are small in size, minimizing any deleterious effect on mechanical behavior. In some cases, the very high cooling rate results in supersaturated solid solutions, especially of transition elements such as Fe, Ni, Co, and Mn. 3~This effect may be useful to achieve improved modulus or improved high temperature properties via dispersoid formation. However, supersaturation of major alloy elements such as Cu, Mg, Zn in 7000 series A1 alloys is not useful since subsequent processing results in the formation of coarse second phases from much of the solute instead of the desired fine precipitates. One disadvantage of rapid solidification processing is that a high surface-to-volume ratio (small liquid droplet) is needed to effect the rapid heat transfer required. Thus, the alloy is susceptible to oxidation on a fine scale during solidification and subsequent processing. When the powders or splatted foils are compacted into ingots, the potential exists for containment of the oxide within the ingot. 1 Some selected "best" results of tests on specimens produced in this way are shown in Table I. Properties of aluminum alloys produced by conventional ingot casting can be greatly improved by reducFRANK E. GOODWIN is Metallurgist, Chambersburg Engineering Co., Chambersburg, PA, PARVIZ DAVAMI is Visiting Scientist at MIT, Tehran University of Technology, Iran, and MERTON C. FLEMINGS is Ford Professor of Engineering, Director, Materials Processing Center, Massachusetts Institute of Technology, Cambridge, MA 02139. Manuscript submitted January 21, 1980.
ing impurities which form insoluble second phases, 6-1~ and also by intensive thermal-mechanical treatments. The latter treatments break up and dissolve the soluble second phases. 7,8,HTensile and yield strengths can be increased by increasing total solute content, but working and solutionization become more difficult and ultimately uneconomical as solute content is increased. The relatively slow cooling rates inherent to ingot metallurgy allow nonequilibrium second phases to grow to sizes where, even after mechanical working, they cannot be fully dissolved in reasonable times. Examples of mechanical properties of selected 7000 series specimens produced from ingots are also shown in Table I. A new process has been developed at MIT, designed to circumvent the disadvantages of the above processes. The work follows from earlier studies on fractional melting whic
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