PREPARATION OF AL-LI SICP COMPOSITE POWDER BY A PLASMA SPRAY ATOMIZATION (PSA) TECHNIQUE/

Aluminium - lithium alloys are of technological importance to the aero space industry. The advantages of aluminium - lithium alloys over conv entional high-strength aluminium alloys include significant weight sav ings, increased stiffness, better fatigue resistance and broader tempe rature capability. This alloy series however, suffer low ductility and fracture toughness when produced using the ingot metallurgy (IM) meth od. This shortcoming can be diminished by powder metallurgy (PM) proce ssing techniques. Ceramic reinforcement such as carbides, oxides and n itrides are added to the metal alloys to enhance strength, modulus, we ar resistance and high temperature properties. Current PM processes ho wever, are limited to reinforcement size of >5 mu m. When finer cerami c particles are added the metal matrix composites (MMC) suffer from ag glomeration of the reinforcement phase leading to problems in metal-ce ramic interface and poor mechanical properties. Agglomeration occurs i n submicron ceramic particles as a result of Van der Waals forces and electrostatic charges. Moisture in the atmosphere also plays a part. T his paper reports an alternate route in the preparation of MMC with fi ne ceramic particles (<2 mu m) as the reinforcement. The MMC is prepar ed by a process called Plasma Spray Atomization (PSA). This was carrie d out by melting the metal matrix and ceramic reinforcement simultaneo usly in a high temperature plasma flame (similar to 2000 degrees - 10, 000 degrees C). The molten droplets are fragmented into very fine comp osite particles when they impact on a solid rotating disk. The powder collected are characterised using standard metallographic techniques, scanning electron microscopy (SEI) and laser diffraction particle size analyser. Several parameters such as plasma gas composition, plasma a re power, rotating disk speed and distance between the plasma torch an d rotating disk were investigated. The results showed that this proces s produced fine Al-Li/SiC composite particles in the size range 1 - 35 mu m. SEM observation of polished cross sections showed the particles consist of submicron SiC particles dispersed within a single AI-Li al loy particle. Quantitative analysis of individual composite particles using wavelength dispersive X-ray analysis (WDX) shelved that the conc entration of SiC is low in the large particles (> 20 mu m) but high in particles <10 mu m.