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The freckling mechanism of superalloys

University of British Columbia

Freckles are macro-segregation defects which are usually found in nickel-base superalloys or specialty steels and which occur during solidification. They are presently one of the major defects encountered in advanced casting technology of superalloys. Two modified Rayleigh criteria have been suggested which seem best adapted to freckling prediction. However only three superalloys have been previously tested for the application of the modified Rayleigh numbers and fAirther experiments on industrial alloys and in casting conditions similar to actual industry conditions are therefore needed. There are* conflicting reports as to the cause of freckling in IN718 alloy. One possible explanation is that freckling is caused by a downward interdendritic flow along the slope of solidification front, another possible explanation is that freckle may be caused by the density inversion between the interdendritic liquid and bulk liquid in the same way as in other superalloys. The Si content is believed play a critical role in influencing the freckle formation in IN718, without however any confirming experiment data. A vacuum induction furnace was built so as to directionally solidify superalloys at various angles to the vertical under typical industrial conditions (thermal gradients ranging from 500 to 4000°C/m (5 to 40°C/cm) and growth rates ranging from 1.6 x 10"5 to 10 x 10"5 m/s (1 to 6 mm/min). A comprehensive thermal modeling of this furnace was carried out with the FEM package ProCAST in order to accurately evaluate the solidification conditions (G and R) for each casting. Totally 10 alloys were tested and the modified Rayleigh criteria are shown to be able to predict freckling in all these alloys under different casting conditions and different growth front angles. Si content is shown to influence the density of the interdendritic liquid and hence the freckling mechanism in IN718.

Thesis/Dissertation

application/pdf

2009-06-15

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http://hdl.handle.net/2429/9144

Materials Engineering

University of British Columbia

UBCV

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