THE INFLUENCE OF PROCESS PARAMETERS ON CONSOLIDATION EFFICIENCY WHEN FORMING COMPOSITES BY SPRAYING ONTO MONOFILAMENTS

Composites have been produced by spraying titanium powder onto parallel arrays of tungsten monofilaments, using the vacuum plasma spray technique. The main variables studied were powder particle size, chamber pressure, fibre spacing and plasma gun operating variables. Porosity levels were measured by a gravimetric technique and also studied metallographically. It was found that relatively high porosity levels, largely located in the ''shadow'' of the fibres, persisted in all of the composites produced. These were measured as ranging from 10-15% to over 30%. Similar trends were apparent from metallographic sections, although the void contents appeared lower. It was found that lower porosity levels were favoured by larger particle sizes and lower chamber pressures, although the latter trend was not very clear-cut. Porosity increased sharply as the ratio of the gap between the fibres to the droplet diameter became less than a value of about 5. These results have been considered in terms of the mechanisms by which droplet impingement leads to envelopment of an array of fibres. It is concluded that it will be very difficult to identify a combination of operating parameters which will allow production of composite material with both fibre contents of commercial interest and suitably low porosity levels. The technique does not at present appear to represent an attractive option for commercial exploitation.