Microstructure Evolution And Mechanism During Friction Stir Processing In Nickel-base Superalloys

Local modification of engineering materials can significantly enhance the local performance in the premise of remaining the same base materials,which possesses considerable economic benefits and promising applications.In this study,friction stir processing（FSP）was applied to complete surface modification of Waspaloy and Mar-M247nickel-base superalloys.Based on the study of FSP process,the microstructure of FSP region,the materials flow and the recrystallization mechanism during FSP were investigated.In addition,FSP was used as the pre-treatment of laser remelting to study the influence of FSP microstructure on liquation cracks.An optimized parameter window of FSP on Waspaloy was obtained via process experiments.The influence of the process parameters on the depth and defects distribution of FSP region was studied.A preliminary study of defects distribution during FSP on Mar-M247 was accomplished.Moreover,the stir tool tip,a hot isostatic pressed Si₃N₄ sphere,was found to be worn,broken and decomposed.The microstructure of the FPS region was characterized by optical microscope（OM）,scanning electron microscope（SEM）,electron backscatter diffraction（EBSD）and transmission electron microscope（TEM）.Combined with the properties of Waspaloy and Mar-M247 alloys,the microstructure evolution and mechanism were investigated during FSP at high strain rate,high strain and short-time high temperature.The results show that cooling conditions greatly influence the microstructure evolution during FSP.With the cooling condition of air,coolant water and liquid nitrogen,the matrix microstructures of Waspaloy FSP region are mainly randomly oriented equiaxed grains with average size of 1-2μm,200-700 nm,150-700 nm,respectively.Continuous and discontinuous dynamic recrystallization are both observed in the microstructure evolution during FSP on Waspaloy with above cooling conditions.The FSP region of Mar-M247 is consisted of equiaxed grains with size of 300-800 nm matrix and multi-scale disintegrated carbide particles.The as-cast carbide dendrites were mechanically disintegrated into micron and submicron carbide particles and dispersed in the matrix during stir.Metallurgical bonding is first found between the broken-up carbides and the matrix.Theγ?particles are completely dissolved in the matrix and their re-precipitation is suppressed during the rapid cooling.Continuous dynamic recrystallization is observed in the matrix microstructure evolution in Mar-M247.The heat-affected zone（HAZ）liquation cracks of laser remelting before and after FSP were compared to investigate the feasibility of FSP as the pre-treatment of fusion welding.The results show that after FSP,the HAZ liquation cracks of Waspaloy are eliminated completely.The amount of liquation cracks of Mar-M247 obviously decreases.On the one hand,the matrix grains were significantly refined and the grain boundary area increased correspondingly.Therefore,the content of liquid phase in grain boundaries and the thickness of liquid films were reduced.On the other hand,the as-cast continuous and massive carbides were disintegrated,refined and dispersed into the matrix during FSP,thus avoiding the liquid phase to concentrate and form liquid films rapidly.In addition,the constitutional ofγ?is completely eliminated due to theγ?dissolution.In a word,the liquation cracking of nickel-base superalloy is successfully suppressed via microstructure refinement and homogenization induced by FSP.