On the nature of green strength of compacted metal powders
- PDF / 823,238 Bytes
- 3 Pages / 612 x 792 pts (letter) Page_size
- 32 Downloads / 231 Views
gate s i z e . The d i s c r e t e size f r a c t i o n s used were 100/ 140 mesh, 200//325 mesh, and 400 m e s ~ / 1 0 pm.* They *The mesh designations refer to the U.S. Standard Sieve Series, the 10pm designation to micromesh sievesmanufactured by Buckbee Mears Co.
were obtained from the same nitrogen atomized, e l e c t r o l y t i c grade copper powder. T h e r m a l a g g l o m e r a t i o n was achieved by loosely pouring the v a r i o u s powders into a boat to a bed depth of 3.8 cm and heating at 815~ under Hz for 10 rain. The r e s u l t i n g porous cakes w e r e h a m m e r milled to y i e l d an a g g l o m e r a t e powder. Selected s i z e fractions* of the a g g l o m e r a t e d *The following U.S. Standard Sieve Size fractions were used: 325/400; 200/325; 140/200; 100[140; 80/100; 50/80; 35/50; 20/35.
powder w e r e d i r e c t l y used for g r e e n strength t e s t s . Green s t r e n g t h was m e a s u r e d a c c o r d i n g to s t a n d a r d p r o c e d u r e 1 on specimens compacted to a density of 7.00 g//cm ~. Die wall l u b r i c a t i o n only was used. The r e s u l t s a r e s u m m a r i z e d in F i g s . 1 and 2. P i c t u r e s of p r i m a r y p a r t i c l e s and of a g g l o m e r a t e s a r e shown in F i g . 3. The data points f a r t h e s t to the left of the c u r v e s shown in Fig. 1 r e p r e s e n t the p r o p e r t i e s
l
I
|
400
60 0
800
~ ' E I0
On the Nature of Green Strength of Compacted Metal Powders ERHARD KLAR AND WILLIAM M. SHAFER By definition, the t e r m green strength r e f e r s to the m e c h a n i c a l s t r e n g t h of a p a r t made from a p a r t i c u l a t e m a t e r i a l . 1 Green s t r e n g t h is an i m p o r t a n t technological p r o p e r t y . In powder m e t a l l u r g y it is well known that i r r e g u l a r p a r t i c l e shape and p o r o s i t y within individual p a r t i c l e s a r e d e s i r a b l e c h a r a c t e r i s t i c s for high green strength, and that solid p a r t i c l e s of s p h e r i c a l shape produce the lowest green strength. F o r a long t i m e the p r e v a i l i n g t h e o r y on green s t r e n g t h was based on the assumption that during p r e s s i n g neighboring p a r t i c l e s b e c a m e i n t e r l o c k e d with each o t h e r . Green strength was a s s u m e d to depend on total p o r o s i t y but not on pore size. 2 This theory is s t i l l being offered in r e c e n t textbooks on powder m e t a l l u r g y 3 in spite of e x p e r i m e n t a ! evidence that green strength can be explained in t e r m s of i n t e r p a r t i c l e contacts o r , to use a b e t t e r known t e r m , cold welding between neighboring p a r t i c l e s , and that it i n c r e a s e s with i n c r e a s ing p a r t i c l e p o r o s i t y and with d e c r e a s i n g p o r e s i z e . 4-5 The data r e p o r t e d h e r e i n provide f u r t h e r support for the new theory. D i s c r e t e s i z e f r a c t i o n s of n e a r l y s p h e r i c a l l y shaped copper powders w e r e t h e r m
Data Loading...
