Scripta METALLURGICA Vol. 24, pp. 1233-1238, 1990 Pergamon Press plc et ~TERIALIA Printed in the U.S.A. EFFECT OF HOT WORKING ON THE MICROSTRUCTURE AND PROPERTIES OF A CAST 5083 AI-SiCp METAL MATRIX COMPOSITE I. Dutta, C. F. Tiedemann and T. R. McNelley Department of Mechanical Engineering Naval Postgraduate School Monterey, California 93943 (Received January ii, 1990) (Revised April 23, 1990) Introduction Discontinuously reinforced metal matrix composites (MMC) are usually fabricated by powder metallurgy (P/M) methods and thus are high in cost and limited in product size. In contrast, solidification processing is potentially simple and economical, with an added advantage of large possible product size. This has led to considerable attention on the development of cast MMCs in recent years [1-7]. Currently used casting techniques, however, are all plagued by three recurrent problems: poor wetting of the reinforcement by molten metal; inhomogeneous reinforcement distribution due to flocculation; and reaction of the melt with the reinforcements [ I, 2, 7], resulting in poor as-cast microstructures and properties. Post-consolidation thermomechanical processing (TMP) of P/M products is often employed [8] and numerous studies have examined the influence of TMP on the microstructure and properties of MMCs [9-12]. Maclean, et al. [9] studied the effects of hot rolling and extrusion on aluminum alloys 6061 and 2024 reinforced with silicon carbide whiskers and particulates and reported slight improvements in the ultimate tensile stren~h and ductility but no improvement in yield strength. Pickens, et al. [10] performed hot torsion testing of SiC whisker reinforced aluminum matrix composites to identify deformation temperature and strain rate conditions which minimize surface cracking and whisker breakage. McDanels [11] obtained greater fracture strains in heavily hot- worked P/M aluminum-SiC composites and found higher fracture strains than those reported elsewhere for less heavily worked composites. This was attributed to reduced matrix porosity, a fine and uniform reinforcement particle distribution and the break-up of inclusion stringers as a result of the large hot-working strain. Hot working was also used successfully by Harrigan, et al. [12] to improve the particle distribution, ultimate tensile strength and strain to failure of silicon carbide particle-reinforced 6061 AI P/M composites. Improvements in ductility were found at rolling reductions of around 64 pct., while strength improvements became apparent only at reductions of 80 pct. or more [12]. To date, there has been no systematic evaluation of the effect of hot working on the microstructure and properties of cast metal matrix composites. In this work, TMP schedules involving forging followed by hot rolling with reheating between passes were used in order to improve the microstructure of a cast aluminum composite. The composite microstructures before and after processing were characterized using optical, scanning and transmission electron microscopy. The microscopic observations were correlated with a limited amount of mechanical test data. Experimental Aluminum alloy 5083 reinforced with 10 v/o SiC particulates ranging from 0.5 to 5 I.tm in size was obtained from Science Applications International Corporation, La Jolla, California. The composite was manufactured by a proprietory casting technique and a commercially available 5083 A1 remelting stock was used as the matrix material. The aluminum alloy 5083, which has a nominal composition of A1-4.5 pct. Mg, was chosen as the matrix material since magnesium is thought to improve wetting of SiC by molten aluminum [13, 14]. The as-cast composite was hot-forged to a reduction of 3:1 and was delivered in the form of a cylindrical billet of 0.15 m diameter and 0.025 m thickness. From the as-received material, two 0.075 m x 0.025 m x 0.025 m billets were produced such that their long dimension coincided with the radius of the cylinder. These billets were then processed in three steps. They were solutionized at 425oc for 24 hours, followed by hot-for~ng to a reduction of 2:1 along the original forging direction and subsequently rolled at 425oC to total hot-reductions of 62 pct. and 91 pct. The hot-working temperature was chosen to be above the solvus temperature (~260oc) but sufficiently below the eutectic temperature (451oc) to prevent remelting of any eutectic formed due to non-equilibrium 1233 0036-9748/90 $3.00 + .00