Microstructure evolution and liquid phase separation in Ta-O hypermonotectic melts during laser-cladding

A three-layer Ta2O5-containing coating was successfully fabricated by laser cladding on a pure Ta substrate. The maximum thickness of such a coating is about 1.6 mm. The microstructure, phase constitution and elemental distribution in the coating were investigated. Results show that the coating has been metallurgically bonded to the Ta substrate and the microstructure exhibits a graded change along the deposition direction from Ta substrate to the top of coating. In the layers I and II of the graded coating, the microstructure evolution can be confirmed as a result of hypomonotectic reaction, but in the layer III it was formed by hypermonotectic reaction. At the top of coating, the microstructure was still homogeneous although liquid phase separation had occurred, which can be attributed to the fact that the O-rich droplets do not have enough time to float at high cooling rate. The theoretical calculation results show that during laser cladding, the solidification time of the melt pool was less than 0.1 s, which fits well with the results from the experiment.