Stray Grain Formation Mechanism During Seeding Process of Single Crystal Superalloy DD419
The single crystal(SC)castings of superalloy DD419 were prepared using seeds deformed in different degrees.The grain structure on the etched seed surface and on the longitudinal sections was examined,combined with the measurement of metallographic structure and crystal orientation of the corresponding parts,in order to identify the origin of the stray grains.The results show that on the surface of the slightly-deformed seeds,only slightly recrystallized grains are detected,which have no influence on the seeding process of the superalloy castings.In the moderately and severely deformed seeds,however,grains with different recrystallization degrees are observed.It is deduced that the recrystallization on the surface of deformed seeds occurs during preheating and holding stage before pouring,which is very similar to the recrystallization structure formed during the conventional solution heat treatment.These recrystallized grains are partially melted in preheating and pouring stage.In the subsequent solidification process,stray grains grow epitaxially from the unmelted residual recrystallized grains near the remelting interface.The experimental results show that the origin of the stray grains during seeding process is the recrystallization in the seed surface layer,instead of the nucleation of the new grains in the liquid phase.
single crystal superalloyseedcrystallizationstray grain
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