Laser boronizing of some titanium alloys

Samples of titanium with a commercial purity grade 4 and of alloys Ti-6Al-4V and Ti-4Al-2Sn-4Mo were surface hardened by laser alloying with boron. This treatment was carried out by contemporaneously laser melting of the titanium alloy surface and powders containing boron which were injected on the surface of the specimens. The alloying powders were deposited on the metal surface either before or during laser treatment. Boronizing powders with different chemical compositions (B, B4C, TiB2, MoB, MoB2 and W2B5) were used to prepare surface coatings. Treatments were carried out with CO2 laser equipment (1.2 kW) under a protective argon atmosphere using specific power ranging between 10 and 14 kW/cm2 and interaction times between 0.3 and 0.6 s in order to assess the best processing conditions. Morphology, composition and microhardness of the layers were studied by microscopy, X-ray diffraction and Vickers microhardness analyses. Hardened layers showed a content of boride crystals and a progressively decreasing hardness according to the distance from the surface. These coatings proved to be well adherent to the substrate and showed a hardness up to 14 × 103 MPa. The results obtained on different alloys showed that the matrix composition can affect the coating features. Borided samples submitted to annealing treatment displayed a satisfactory thermal stability.