Effect of surface nanocrystallization on welding liquation cracking of nickel-base superalloy

Friction stir processing (FSP) of Waspaloy superalloy was carried out to prepare surface nanocrystallite layers as a pre-treatment of laser remelting to explore the effect of FSP on liquation cracking. The microstructures of FSP and laser remelted regions were analyzed through scanning electron microscope and electron backscatter diffraction. The results showed that the equiaxed grains with size of 200 ~ 700 nm were successfully prepared in Waspaloy superalloy by FSP. The refinement mechanism was discontinuous recrystallization. From the topmost surface to the bottom of FSP region grains size increased gradually. The ultrafine grains were found to be effective to suppress liquation cracking in laser remelting of Waspaloy superalloy. Surface nanocrystallization changed the carbide redistribution and avoided more liquid phase in grain boundaries. The grain refinement made grain boundary area increase and reduced the thickness of liquid film, thus the liquation crackings were effectively suppressed.