**Aleksander Lisiecki**

Silesian University of Technology, Faculty of Mechanical Engineering, Department of Welding Engineering, Konarskiego 18A street, 44-100 Gliwice, Poland; aleksander.lisiecki@polsl.pl

Received: 9 August 2019; Accepted: 20 September 2019; Published: 24 September 2019

**Abstract:** This study measured optical properties, such as specular, diffuse, and total reflection for 808 nm wavelength, characteristic for high power diode lasers radiation, from the surface of titanium alloy Ti6Al4V at delivery conditions, polished, and oxidized. Moreover, the optical properties of surface layers produced by high power direct diode laser (HPDDL) melting and nitriding were determined. Additionally, a methodology for determining the value of absorption for 808 nm wavelength of the HPDDL radiation on the surface of a melt pool during laser surface melting and nitriding of titanium alloy was proposed. The results show that the distinct differences in absorption affect the heat transfer, thermal conditions of laser heating and thereby the penetration depth during laser melting and nitriding of the titanium alloy.

**Keywords:** titanium nitrides; reflectivity; absorption; high power diode laser
