Scripta METALLURGICA Vol. 24, pp. 1419-1424, 1990 Pergamon Press plc et ~TERIALIA Printed in the U.S.A. All rights reserved STABILITY OF NON-WE~TING DISPF/RSOID SUSPENSIONS IN METALLIC MELTS Geetha Ramani, T.R. Ramamohan, R.M. Pillai and B.C. Pal Regional Research Laboratory (CSIR) Trivandrum 695 019, India (Received April 4, 1990) (Revised May 18, 1990) 1. Introduction The solidification route is becoming increasingly popular for synthesising metal matrix composites (I). This involves the uniform dispersion of second phase dispersoids in molten metals by means of an impeller, followed by solidification of the melt. The introduction of the second phase dispersoids, such as graphite, AIgO 3 and SiO~, into the molten aluminium alloys, either by introduction through the vortex ~r by injection below the melt surface through an inert gas stream, met with limited success (2-4) because of poor wetting of the above dispersoids with molten metal. The reported recoveries of uncoated graphite in al~inium by either introduction technique were less than 0.05 wt %. Metal coatings such as nickel (2,3) and copper (5) upon the powders facilitate the introduction due to in,)roved wetting. Additions of reactive alloying elements like Mg (6), as well as preheat treatment (7) prior to the introduction, enabled incorpo.~tion of uncoated alumina and graphite particles to the limit of about 5 vol %. For superior mechanical properties and or for making cocm~site master alloys (8), 15 to 50 vol % dispersions of the second phase are desired. It has been obser- ved (9) that for a given metal/particulate system and a given set of mixing conditions, wetta- bility, particle size and buoyancy conditions, there is a critical volume percentage of the dispersoids above which all the dispersoids segregate out of the melt. This phenomenon, which is called "rejection", is quite sharp and happens at a specific vol % of the dispersoid. The study of this phenomenon is important since it places an upper limit on the amount of dispersion. Walter (I0, ii) observed a similar phenomenon of phase separation under microgra- vity conditions during the r~nelting of Al-glass, AI-Pb, AI-Bi and Al-ln multicomponent systems prepared by the powder metallurgy route. The segregation was reported to have taken place (I0, ii) above 12 vol % content of the second phase as the result of agglomeration, chain and skeleton formation. This paper reports a preliminary study of the rejection phenomenon and its dependence on particle size under a given set of mixing conditions for non-wetting graphite in A1 alloy melts. Here, the second phase dispersoids, viz. graphite, were continuously introduced into the melt while stirring and the rejection was observed at a certain critical volume fraction. A new mechanism has also been proposed for rejection under these circumstances. 2. Experimental Details Fig. 1 shows the schematic diagram of the experimental set up. 0.25 kg of Al-12 wt % Si alloy (LM 6) was melted in a clay-graphite crucible using a 0.5 kW resistance heating furnace. 3 wt % Mg was added to the melt to improve its wettability with graphite. The graphite powder was preheated at 775°C for 2 hours. The hot powder was slowly introduced 1419 0036-9748/90 $3.00 + .00 Copyright (c) 1990 Pergamon Press plc