Microstructure of rapidly solidified laser molten AI-4.5 Wt Pct cu surfaces
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I.
INTRODUCTION
IN the past,
the major industrial interest in laser applications for materials technology was in "machining" processes such as cutting, drilling, and welding. The present commercial availability of high powered directed energy sources, like continuous 15 kW lasers or electron beam sources, has led to the development of some new rapid solidification processes (RSP). One such process is the rapid surface melting and subsequent solidification of metals and semiconductors. In such processes, cooling rates as high as 10 6 ~ per second are effected (compared to less than 10 2 ~ per second in conventional solidification by casting). This, in turn, reveals different characteristic features of solidification. The major microstructurat differences commonly observed in aluminum alloys are extension of the solid solubility and refinement of the microstructure.l Numerous investigations ~-6 have shown that such changes may have a tremendous effect on the strength, plasticity, fatigue, stress corrosion, and other related properties of aluminum alloys. Particularly in aluminum-copper alloys the following changes were reported: (1) extension of solid solubility, 1 7 (2) growth of coupled eutectics at off-eutectic compositions, 8 (3) refinement of microstructure,l" 9 and (4) formation of nonequilibrium phases such as A13Cu2.~0 Some work has been done on surface melting of A14.5 wt pct Cu by high powered directed energy sources.1112 ~3 Theoretical considerations suggest that the first 3 to 5 #m near the fusion line resolidify in a plane front mode, which is followed by a cellular dendritic resolidification mode. In this work we aimed to characterize the microstructure changes occurring after a laser surface treatment and to prove that indeed the 3 to 5 /~m near the fusion line actually solidified in a plane front mode.
II.
EXPERIMENTAL PROCEDURE
Only a brief description of the experimental procedure will be given here; a detailed description is given elsewhere. 13 A1-4.5 wt pct Cu was cast from 99.99 pct pure A1 and 99.999 pct pure Cu. The types of casts studied were A. MUNITZ is with Nuclear Research Center, Negev, Metallurgy Laboratory, P. O. Box 9001, Beer Sheva, Israel. Manuscript submxtted June 12, 1984. METALLURGICAL TRANSACTIONS B
either a directionally solidified cast with considerable precipitation of CuA12 or a uniform equiaxed cast. The latter was obtained from the former by a repetitive annealing, quenching, and rolling procedure. Annealing was performed at 535 ~ for about 24 hours. The subsequent quenching was performed by dropping the ingot into cold water. Rolling was carried out at room temperature, the thickness reduction being about 8 pct of the former thickness. The process ended when the thickness had reduced to about 50 pct of its initial value. Finally, the ingot was annealed again at 535 ~ for about 72 hours and quenched into cold water. The homogeneity thus obtained has been checked with an X-ray microprobe analyzer. Cu concentration was found identical (4.5 wt pct Cu) throughout the ent
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