Microstructure of Iron Substrates Borided with Ni2B Particles by Laser-Induced Surface-Alloying

Armco iron substrates were laser-induced surface-alloyed using Ni 2 B particles. The injected particles dissolve in the melt pool created by irradiation of the substrate surface with a CW-CO 2 laser. Solidification of the Fe-Ni-B melt causes the formation of boride phases imparting hardness values of ∼ 600 Hv to the surface layer. The evolving microstructure of surface alloyed-layers prepared by single laser tracks and overlapping laser scans was analyzed for varying laser powers and laser scan velocities. The studies revealed that either fcc γ-(Fe,Ni) or bcc α-Fe(Ni) dendrites form the matrix of the alloyed layers, depending on the amount of Ni 2 B dissolved in the melt. In the first case a (Fe, Ni) 3 B-γ-(Fe,Ni) eutectic solidifies in the space between the γ-(Fe,Ni) dendrites, while in the latter case α-Fe(Ni) and the metastable Fe 3 B phase solidify eutectically in the interdendritic regions. The thickness and microstructure of the Ni 2 B laser-alloyed coatings determine their proneness to cracking.