The paper presents results concerning an assessment of the microstructure 600 and 617 nickel alloys. Test alloys delivered as plates were exposed to welding arc applied in the TIG method. The assessment involving the microstructure of the fusion zone, heat affected zone and of the base material was performed using light microscopy and scanning electron microscopy. The results revealed significant differences in the structures of the fusion zone in 600 and 617 alloys resulting from various chemical compositions as well as from the significant segregation of the alloying elements between dendrites and interdendritic zones.