METAL-MATRIX COMPOSITE LAYER FORMATION WITH 3 MU-M SICP POWDER ON IMI318 TITANIUM-ALLOY SURFACES THROUGH LASER TREATMENT

Metal surface modification is frequently carried out to increase the l ife and performance of engineering components that undergo severe serv ice conditions such as wear, erosion and corrosion, without the cost o f treating the entire component. The development of a suitable high pe rformance surface layer on substrate materials can combat many surface degradation mechanisms. The formation of an insitu surface layer MMC by injecting ceramic particulates on laser melted surfaces can in part icular, be used in applications requiring a localised treatment withou t changing the properties of the bulk material. Using a high volume fr action (25-60vol%) of large (50-150 mu m) ceramic particles (TiC, SiC) on titanium surfaces, the surface hardness was found to increase betw een 1.1 to 4 times the base hardness (150 Ky). In the present work, a 5kW continuous wave carbon dioxide laser was used under both helium an d nitrogen environments to introduce a smaller volume fraction of 3 mu m SiCp particles using the powder preplacement technique on a laser m elted IMI318 titanium alloy (Ti-6Al-4V) surface. By changing the laser processing parameters, graded microstructures with hardness gradients were obtained over a melt depth of about 1 mm. The tracks produced su rface cracks, but of low intensities (0.1 to 0.3 No./mm). However, in some glazing conditions cracks were absent. Under a helium environment , most of the ceramic particles dissolved in the melt pool. and the mi crostructure consisted mainly of thread-like particles; some dendrites were also present near the surface and at the edges of the melt pool. Under a nitrogen environment, the tracks produced mostly dendritic st ructure with no SiCp particles in the melt zone. The hardness develope d from a base hardness of 350 Hv to the range 600 to 1200 Hv and 600 t o 1450 Hv for tracks processed in helium and nitrogen atmospheres resp ectively.