Interfacial Reaction Of Single Crystal Particulate Reinforced WC_p/Ti-6Al-4V Graded Composites Coating Produced By Laser Melt Injection

Protecting titanium alloys with a particulate reinforced metal matrix composites (MMCs) coating is a promising solution to improve their surface properties while keeping the advantageous bulk properties unaffected, which exhibits great advantage in academic study and practical application. However, there are two main problems to be resolved in the preparation of MMCs coating, namely excessive melting of the reinforced particle and the high crack tendency of the MMCs coating. Graded transition in composition between the MMCs coating and the substrate has been proved to be one of the most effective ways to restrain the crack of the coating. A graded MMCs coating can be produced by introducing an interlayer, which can decrease the crack tendency of the MMCs coating to some degree. Nevertheless, this solution can not resolve the excessive melting of reinforced particle. Moreover, introducing an interlayer clearly increases the difficulity of the production process.To solve above problems, laser melt injection (LMI) was introduced, by injecting the single crystal WC particles (WC_p) directly into a laser melt pool, the LMI process successfully realized the fabrication of the graded WC_p/Ti-6Al-4V MMCs coating without the introduction of the interlayer. Based on the study of the LMI process, the formation mechanism of the graded MMCs coating was investigated by means of numerical simulation of the flow and temperature field of the melt. In addition, the solidification process of the laser melt pool, the fracture behaviour of the MMCs coating and the WC_p/Ti interfacial reaction were studied.A suitable parameter window of LMI was obtained after a series of injection experiments. The influence of experimental parameters on the apearence of the laser track, the distribution and volume fraction of WC_p was studied. The injection velocity (v_p) of injected particles and the flow and temperature field of the melt were calculated to investigate the effect of the viscosity (η), Marangoni flow and the solidification front of the melt pool on the WC_p distribution. The results show that WC_p are freezen by the process of the solidification front, which leads to the formation of the graded MMCs coating. The solidification front of the pool is the most sensitive parameter that dominates the WC_p distribution, whereas the viscosity and Marangoni flow of the melt pool do not play a key role in the WC_p distribution.The phase composition and microstructure of the MMCs coating was analyzed by using XRD and scanning electron microscope (SEM). At the same time, a transmission electron microscopy (TEM) was used to characterize the microstructure in more detail. Phase identification and crystal orientation determination were investigated with select area electron diffraction (SAED). Based on the W-C, Ti-C, Ti-W binary and Ti-W-C ternary phase diagrams, the complicated crystallization behavior of the the laser pool during the rapid solidification process was studied. The element distribution of the Ti matrix, the volume fraction of WC_p and the content of TiC dendrite exhibit a graded distribution in the depth direction of the MMCs coating. By injecting the WC_p into the extended part of the melt pool, the amount of reaction products is greatly controlled. Moreover, a thin and regular cellular shape reaction layer is formed around WC_p. The WC_p/Ti reaction is composed of W₂C and TiC layer. It is important to note that for the first time a continue nano W layer with thichness of 200 nm between the W₂C and TiC layers is found.Base on the standard mechanics performance tests, the in situ tensile tests were accurately investigated inside a SEM to reveal the crack initiation and propagation in the MMCs coating during the loading in microscopical scale. Two distinguishable modes of crack nucleation are observed: cracking of the WC particle and decohesion of the WC/W₂C interface. Cracks usually propagate along the TiC dendrites in the matrix and the main crack usually follows the path with high density of WC_p. During the tensile test, the maximum stress (above 2000 MPa) inside WC_p is much higher than the fracture strength of single crystal WC (1487 MPa). This is the reason why high strength single crystal WC can crack. The observed failure mechanisms seem to do not exhibit distinct difference from the composites reinforced with granular WC_p, but the tensile strength of the composites is increased by at least 18 % when the use of granular WC_p was changed to single crystal WC_p.The WC_p/Ti interfacial reaction was studied by means of thermodynamics analysis and thermal simulation experiment. In addition, the twin crystals observed inside the W₂C layer well explaind the absence of orientation relationships between the W₂C layer and the parent phase WC during the solid phase transformation, and the existing controversies were clarified. The comparative analysis of WC_p/Ti interfacial reaction during the rapid and slow solidification shows that the reaction layer formed at the case of rapid solidification can restrain the interfacial reaction. Thermal simulation experiment further confirms that the W₂C layer is formed from the solid state of WC. The twinning deformation occurred inside the W₂C layer can not only damage the orientation relationship between WC and W₂C, but can split W₂C into many small grains. The swift formation of the TiC layer during the rapid cooling can intercept the dissolution of WC_p in the Ti melt and can act as a barrier against the interfacial reaction.