LASER GLAZING RAPIDLY SOLIDIFIED MICROSTRUCTURE OF DD8 SINGLE CRYSTAL Ni--BASED SUPERALLOY

The laser glazing rapid solidification can make the dendrite refined and reduce the segregation of alloying elements,so it is very favorable to mechanical properties of alloys,e.g.,the fatigue life of Ni-based superalloy can be prolonged by laser surface melting process due to improved resistance to stress corrosion cleavage.The laser surface melting process can be used for modification of surface or repair of casting defects(such as surface porosity,surface stray grains) in single crystal components.Recently,this technique has become an attractive research subject with latent application. A successful laser glazing rapid solidification to single-crystal should ensure the preservation of the single-crystal nature,i.e.,the re-solidified surface layer needs to be epitactic with the substrate.The microstructure after laser re-solidification is closely related to the processing parameters,however, the relationship between microstructure and re-solidification conditions or processing parameters is not well understood and needs further study.In the present work,the microstructure of laser glazing solidified Ni-based single crystal DD8 was investigated by OM,SEM as well as TEM.The results show that the surface width of the melted pool becomes narrow with an increase of scanning rate, so the melted pool has a relatively high interface/volume ratio which leads to a higher cooling rate. The primary dendrite arm spacing decreases with an increase of cooling rate,and gradually reaches a minimum about only 3μm,which is two orders of magnitude smaller than that of the untreated part of alloy.EDXA shows that the dendritic segregation is not obvious and the chemical compositions tend to be homogeneous after re-solidification,which is caused by the higher solidification rate and related partition coefficients of alloying elements.The solidified structures are composed of the quasiplane front,cellular and fine dendrites from substrate to surface of melted pool.The structures in the regions of the quasi-plane front and cellular consist ofγ-solid solution and dispersiveγ' precipitates with about 20 nm in size.In the regions of the cellular-dendrite and the dendrite,however,there is a eutectic structure with a higher Ti content in the interdendrites.These eutectic structures are small in size and look like leaves.In the transition region,bothγandγ'-phases are not completely melted,and there are some dislocations distributed on the interfaces of theγ'-phase particles induced by thermal stress betweenγandγ' phases at higher cooling rate.The unmeltedγ' particles can act as nuclei of the second precipitatedγ' phases which have a size of about 10 nm and a pile-up-like morphology.