Directional growth whisker reinforced Ti-base composites fabricated by laser cladding

New type Ti-base composite coatings with in-situ synthesized directional growth whiskers were successfully fabricated from a precursor mixture of titanium alloy and B4C powder by laser cladding. The typical microstructures, formation mechanism and properties of the reinforced composite coatings were experimentally studied for the purpose of developing novel hard facing coatings of titanium alloy. The results show that, there are two typical microstructures obtained in the composite coatings, i.e. the equiaxial composite structures and the columnar composite structures. The novel and interesting TiB whiskers, namely the directional growth TiB whiskers, are founded in the columnar composite structure zones. These TiB whiskers show uniform growth directions and display the characteristics of large slenderness ratio. The lengths of the whiskers are in the range of 10-30 μm, but the diameters are of extremely fine size in the nanometer range. The microhardness of the coating is in the range of 430-520 HV and is about 150 HV higher than that of the substrate, which implies that these composite coatings are effectively reinforced by in situ synthesized TiB phases. The analysis shows that, solidification velocity R, temperature gradient G and direction of heat flow were the key factors to determine what kind of whiskers can be obtained.