Macrosegregation in castings rotated and oscillated during solidification
- PDF / 1,459,811 Bytes
- 5 Pages / 612 x 792 pts (letter) Page_size
- 43 Downloads / 208 Views
J. S T E W A R T , L. C . M a c A U L A Y , A N D F . W E I N B E R G
The macrosegx'egation present in stationary, rotated, and oscillated castings of AI-3 wt pct Ag was determined by measuring the distribution of radioactive silver addedto the melt. Considerable scatter was observed in the measurements, the scatter being dependent on the sampling technique used. It was found that no significant macrosegregation was present in the stationary and rotated castings. Extensive macrosegregation was detected in the oscillated casting. For the oscillating case the macrosegregation can be accountedfor on the basis ofthe long range movement of dendrite fragments whichbreak and/or melt off in the solid-liquid interface region. This movement is a direct result of turbulent waves associated with the oscillation. The maximum silver concentration is shown to be related to the columnar-to-equiaxed transition.
CASTINGS which have a small equiaxedgrain structure are considered to be more homogeneous and to have better mechanical properties than equivalent castings with a partially columnar structure. One way of controlling the grain structure is by mechanically mixing the residual liquid during solidification. This can be done by moving the mold. Constant rotation of a cylindrical mold, radially cooled, will suppress the columnar to equiaxed transition (CET); oscillation of the mold will promote an earlier CET; and a stationary mold will have a structure between the rotation and oscillation cases. 1'2 the control of grain structure by mechanical mixing of the liquid during casting may cause macrosegregation--(a function of the kind and extent of liquid mixing). In addition, rotational forces might influence solute transport in the liquid if there is a large density difference between solute and solvent. If maerosegregation is enhanced by liquid mixing this could be detrimental to casting quality. The purpose of the present investigation is to determine the extent of macrosegregation in stationary, rotated, and oscillated castings, and relate the results to the cast structure. E X P E R I M E N T T h e m a c r o s e g r e g a t i o n in the c a s t i n g s w a s d e t e r m i n e d by a radioactive tracer technique. The alloy used w a s A I - 3 w t p c t A g m a d e u p of 9 9 . 9 9 p c t A 1 a n d 99.8 p c t A g . T h e i n g o t s w e r e cylindrical, 3.25 in. in d i a m a n d a p p r o x i m a t e l y 6 in. h i g h . T h e c a s t i n g a p p a r a t u s u s e d w a s s i m i l a r to that d e s c r i b e d b y C o l e a n d B o l l i n g ,z w h i c h e n a b l e d the c a s t i n g of A I - A g a l l o y s to b e m a d e in stat i o n a r y , rotating, o r oscillating m o l d s . M e l t i n g w a s d o n e in a g r a p h i t e c r u c i b l e in a r e s i s t a n c e f u r n a c e . T h e a l l o y w a s s u p e r h e a t e d to a p p r o x i m a t e l y 8 0 0 ° C a n d i m m e d i a t e l y p r i o r to c a s t i n g , a s m a l l a m o u n t of r a d i o a c t i v e A g ~'° w a s a d d e d into the m e l t . T h e c a s t i
Data Loading...
