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

Aluminium - lithium alloys are of technological importance to the aerospace industry. The advantages of aluminium - lithium alloys over conventional high-strength aluminium alloys include significant weight savings, increased stiffness, better fatigue resistance and broader temperature capability. This alloy series however, suffer low ductility and fracture toughness when produced using the ingot metallurgy (IM) method. This shortcoming can be diminished by powder metallurgy (PM) processing techniques. Ceramic reinforcement such as carbides, oxides and nitrides are added to the metal alloys to enhance strength, modulus, wear resistance and high temperature properties. Current PM processes however, are limited to reinforcement size of >5 mu m. When finer ceramic particles are added the metal matrix composites (MMC) suffer from agglomeration of the reinforcement phase leading to problems in metal-ceramic interface and poor mechanical properties. Agglomeration occurs in submicron ceramic particles as a result of Van der Waals forces and electrostatic charges. Moisture in the atmosphere also plays a part. This paper reports an alternate route in the preparation of MMC with fine ceramic particles (<2 mu m) as the reinforcement. The MMC is prepared by a process called Plasma Spray Atomization (PSA). This was carried out by melting the metal matrix and ceramic reinforcement simultaneously in a high temperature plasma flame (similar to 2000 degrees - 10,000 degrees C). The molten droplets are fragmented into very fine composite 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 are power, rotating disk speed and distance between the plasma torch and rotating disk were investigated. The results showed that this process 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 alloy particle. Quantitative analysis of individual composite particles using wavelength dispersive X-ray analysis (WDX) shelved that the concentration of SiC is low in the large particles (> 20 mu m) but high in particles <10 mu m.