WCp/Ti–6Al–4V graded metal matrix composites layer produced by laser melt injection

The laser melt injection (LMI) process was explored to produce WC particles (WCp) reinforced Ti–6Al–4V metal matrix composites (MMC). In particular monocrystalline WC powder was used as injection particles to avoid the intercrystalline cracking often observed in granular or cast WCp reinforced MMC. WCp were injected into the extended part of the melt pool just behind the laser beam. The process allowed for the minimization of the WCp dissolution caused by the direct irradiation of the laser beam, and the decomposition reaction between WCp and Ti melt. Different parameters were applied, and a processing window of LMI was obtained. WCp exhibit a graded distribution along the depth direction of the MMC layer. New phases such as TiC and W2C are observed in the MMC layer, in which TiC is the predominant phase. TiC grains present a continuous decrease in both amount and size with the distance from the surface to the bottom of the MMC layer. Two types of reaction layers around WCp can be distinguished, namely an irregular reaction layer and a cellular reaction layer. The growth and final morphology of reaction layers are most likely being dominated by the composition of the neighbouring melt pool. A gradual hardness distribution in the depth direction of the composites layer is observed. Moreover, the transition from the MMC layer to the substrate also exhibits a gradual change in the hardness.