Powder metallurgy approach for control of microstructure and properties in high strength aluminum alloys
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A N D W. S. C E B U L A K
High-strength products made from atomized A1-Zn-Mg-Cu-Coalloy powders have good combinations of strength, ductility,resistance to stress-corrosion cracking and fracture toughness. Powder Metallurgy (P/M) methods produce fine metallurgical structures and compositions which cannot be produced by Ingot Metallurgy (I/M) methods. Fine structures result from very rapid solidification and from the effect of fine dispersoids in restricting grain growth. Stress-corrosion cracking (SCC) performance is favored by grain morphology of P/M products. CozAl9 particles in P/M products are 0.02 to 2.0 pm spheroids occurring frequently on grain boundaries where they may serve several functions in slowing SCC attack. Oxide particles are irregular shapes, 0.01 to 0.04 pm in size, occurring in clusters at grain boundaries and in grain bodies. Some of the oxide particles are magnesium oxide and alter the environment in a SCC crack to arrest attack. P o r o s i t y is not a s i g n i f i c a n t f a c t o r in the s t r u c t u r e of P / M p r o d u c t s made by a vacuum compacting process. P / M wrought p r o d u c t s have s u p e r i o r c o m b i n a t i o n s of high s t r e n g t h and s t r e s s - c o r r o s i o n c r a c k i n g r e s i s t a n c e c o m p a r e d to I / M 7075 and 7050 alloys. While equaling the f r a c t u r e toughness of 7075 alloy, the P / M p r o d u c t s at p r e s e n t have s o m e w h a t lower f r a c t u r e toughn e s s than 7050 alloy, due in p a r t to a l a r g e r a m o u n t of s e c o n d - p h a s e p a r t i c l e s in the f o r m of CozA19 and oxide.
S I G N I F I C A N T efforts a r e b e i n g made to f a b r i c a t e h i g h s t r e n g t h a l u m i n u m alloy p r o d u c t s f r o m r a p i d l y s o l i d i fied p a r t i c l e s . The objective of t h e s e efforts is to achieve b e t t e r c o m b i n a t i o n s of p r o p e r t i e s than can be obtained in c o n v e n t i o n a l l y produced 7075 wrought p r o d ucts. E x t r u s i o n s , f o r g i n g s and plate having s u p e r i o r c o m b i n a t i o n s of s t r e n g t h , r e s i s t a n c e to s t r e s s - c o r r o sion c r a c k i n g , r e s i s t a n c e to exfoliation c o r r o s i o n , and fatigue p r o p e r t i e s have b e e n made f r o m a t o m i z e d alloy p o w d e r s . 1-5 An example is given in T a b l e I to show that when P/M* alloy MA87 e x t r u s i o n was heat t r e a t e d and aged *TheabbreviationP,/Mwillbe usedin thispaperin referenceto wIought products fabricatedfrompowder and I/M for those fromcast ingots. to the s a m e yield s t r e n g t h as an I/M* 7075-T6 control, it had s u p e r i o r r e s i s t a n c e to s t r e s s - c o r r o s i o n cracking, s u p e r i o r elongations in both t e s t d i r e c t i o n s , s u p e r i o r NTS/YS in the t r a n s v e r s e d i r e c t i o n , and equal NTS/YS in the l o n g i t u d i n a l d i r e c t i o n . Although KQ v a l u e s for P / M alloy MA87 were not v a l i d Kic per ASTM E399 due to e x c e s s
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