Directional solidification of aluminum-nickel eutectic alloys using electroslag remelting
- PDF / 2,663,266 Bytes
- 10 Pages / 612 x 792 pts (letter) Page_size
- 8 Downloads / 235 Views
ALLOYS
which freeze with a eutectic reaction can be directionally solidified t o produce fibrous or l a m e l l a r composites in which one of the eutectic phase reinforces the other phase. Frequently the techniques generally used for directional solidification are s i m i lar to those used t o grow single crystals. T h e s e involve e i t h e r n o r m a l freezing, which is the Bridgeman or Stockbarger method, or zone melting. All these processes r e q u i r e very high p u r i t y starting materials a n d / o r very low solidification r a t e s . Moreover special experimental techniques are needed t o grow eutectic composites of nearly i d e a l microstructures. T h e s e factors tend to r e n d e r the production of directionally solidified eutectics relatively difficult and expensive on an industrial s c a l e . T h e r e have also been some a t tempts1,2 to study the properties of eutectic alloys where t h e r e is only a preferred alignment of f i b e r s instead of perfect alignment. This study explores the possible use of laboratory s c a l e electroslag remelting equipment for growing directionally solidified composites from eutectic melts. The A1-AlaNi eutectic system was selected for this study, s i n c e this system has been studied quite extensively, for producing directionally solidified composites by the conventional techniques. The ESR method of growing directionally solidified composites is likely t o have the following advantages over the n o r m a l freezing or zone melting methods heretofore employed:
1) Very steep temperature gradients can be readily obtained due t o the l a r g e superheat in the molten m e t a l droplets and high slag temperatures. 2) Large d i a m e t e r samples could be readily produced. The l a r g e t h e r m a l capacity of the system could make it relatively free from t h e r m a l fluctuations. 3) The possibility of some refinement of alloy droplet as it falls through the slag l a y e r is an attractive feature. This may allow the use of relatively i m p u r e starting materials in some c a s e s . 4) It may be possible t o start with a mechanical mixture of the elements, which constitute the eutectic. It is not necessary to have it in the prealloyed form. 5) Electroslag r e m e l t i n g variables can be changed t o readily get a l a r g e change of freezing r a t e s . 6) Arrangement t o withdraw the ingot can be readily made, t o make the process continuous or semicontinuous. 7) It is also possible, that with modification of the mold, a far g r e a t e r d e g r e e of directionality of solidification would be obtained. In s p i t e of t h e s eobvious advantages, the production of perfectly aligned i d e a l structures is not likely t o be an easy task by the ESR technique, specially when the starting materials are not high p u r i t y . The objectives of the present work were limited a) t o observe the kind of directionality obtained d u r i n g n o r m a l ESR, b) to evaluate the possible methods which can be used for improving the directionality of solidific
Data Loading...
