Granulation of Al-alloy composite powders for cold isostatic pressing

Metal-matrix composites combine the properties of the metal matrix such as ductility and toughness and the properties of the ceramic reinforcements, which include high thermal stability and high Young's modulus. The popularity of aluminium and its alloys as matrix material can be attributed to its low cost relative to other light structural materials such as magnesium and titanium, its present wide application as an aerospace structural material and its escalating usage in the automotive engine coupled with other intrinsic properties that provide ease in fabrication. This paper presents the preliminary results of the development of several methods of eliminating or diminishing the problem of ceramic agglomerates during the processing of MMCs. These include the use of a fluidised bed, a filtration method that involves an ultrasonic-treated ceramic slurry, ball milling in liquid media and spray drying. The microstructure of these microgranules is examined using metallographic techniques and scanning electron microscopy (SEM). The results revealed that the fluidised bed and atomisation methods were unsuccessful in granulating the composite powders. The ceramic powders were observed to exist as large agglomerates up to 0.5 mm in diameter. On the other hand, the filtration method and ball milling in liquid media produced aggregates that consist largely of Al-alloy powders thinly coated with the ceramic. This method has the apparent potential producing suitable aggregates for cold isostatic pressing (CIP). The pressure applied to CIP (up to 400 MPa) is sufficient to break up the thin ceramic film and distribute it evenly in the CIP compact. Therefore, during subsequent processing such as hot extrusion, the reinforcement will be well dispersed within the microstructure.