Laser controlled melting of Hastelloy X alloy with presence of B ₄C particles at surface

Laser controlled melting of pre-prepared Hastelloy X alloy surface is carried out at high nitrogen gas pressure environment. The pre-prepared workpiece surface consists of 40 μm carbon film with uniformly distributed 5%B₄C particles. The carbon film enhances the absorption of the incident laser irradiation and holds B₄C particles at the workpiece surface prior to the laser treatment process. The morphological and metallurgical changes in the treated layer are characterised using scanning electron microscope, energy dispersive spectroscopy, and X-ray diffraction. The microhardness at the treated surface and the residual stress formed in the surface vicinity are measured. It is found that dense layer consists of fine grains are formed at the surface because of the high cooling rates. Locally distributed undissolved B₄C particles are observed at the surface due to high melting temperature of B₄C. Although thermal expansion coefficients of B₄C and the base material are different, no microcracks are took place in the close neighbourhood of the particles. The microhardness of the surface increased almost 2·2 times of the base material hardness and the residual stress formed at the surface is on the order of-1·4 GPa, which is compressive.