Interrelation of TEM microstructure, composition, tensile properties, and corrosion resistance of Al-Cu-Mg-Mn alloys
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T H E A1-Cu-Mg-Mn alloys of the Aluminum A s s o c i a tion 2XXX s e r i e s have been widely used for many y e a r s p a r t i c u l a r l y in a e r o s p a c e applications. P r e c i p i t a t i o n o c c u r s r e a d i l y in these a l l o y s , with the r e s u l t that they must be cooled r a p i d l y f r o m the solution heat t r e a t i n g t e m p e r a t u r e to develop good r e s i s t a n c e to i n t e r g r a n u l a r c o r r o s i o n . With thick s e c t i o n s , however, even the most r a p i d quench does not r e t a r d p r e c i p i t a tion sufficiently to p r e v e n t s u s c e p t i b i l i t y to i n t e r g r a n u l a r attack, exfoliation, or s t r e s s - c o r r o s i o n c r a c k i n g in n a t u r a l l y aged t e m p e r s . A r t i f i c i a l aging i m p r o v e s m e c h a n i c a l p r o p e r t i e s and c o r r o s i o n r e s i s t a n c e but is not always completely effective in developing high r e s i s t a n c e to c o r r o s i o n . The p r e s e n t investigation was initiated to develop m o r e s y s t e m a t i c information concerning effects of a l loying element contents, individually and in c o m b i n a tions, on the extent of p r e c i p i t a t i o n during quenching at different cooling r a t e s and to c o r r e l a t e c o m p o s i tions and s t r u c t u r e s with type of c o r r o s i o n attack in the n a t u r a l l y - a g e d t e m p e r . F r o m these o b s e r v a t i o n s it was hoped to obtain a b e t t e r understanding of the e f fect of s t r u c t u r e on type of c o r r o s i o n attack and d e velop information concerning the solute and p r e c i p i tation r e a c t i o n s . MATERIAL
AND
PROCEDURE
To evaluate the alloying element effects, particularly in combinations, two groups of alloys were p r e pared. One group included binary AI-Cu alloys with 2 to 5 pct Cu and ternary Ai-Cu-Mg alloys having the D. L. ROBINSON and M. S. HUNTER are Research Engineer and ScientificAssociate,(Retired), respectively,Alcoa Technical Center, Aluminum Co. of America,Pittsburgh, Pa. Manuscript submitted August 6, 1970. METALLURGICALTRANSACTIONS
same copper contents with magnesium additions of 0.5 to 1.5 pct. Compositions of this group a r e l i s t e d in Table I. The second group had copper contents of 3 and 4 pct and included A1-Cu-Mn, A1-Cu-Mg and A1-Cu-Mg-Mn alloys as l i s t e d in Table II. Small laboratory ingots were fabricated to 0.064 in. sheet. A solution heat treating temperature of 932~ (500~ was employed. To simulate quenching rates of sections of different thickness when quenched in cold water, panels of the sheet were quenched in different media to obtain average cooling rates of 7500~, 1200~ 180r and 40~ per sec for the critical temperature range 750~ to 550~ (399~ to 288~ Respective quenching media were cold water (black NaOH film), 20 pct NaSiO3 at 80~ to 120~ Ethyl Alcohol at 75~ and boiling water. All rates were determined from Speed-O-Max measurements using imbedded thermocouples. Tensile properties were determined from
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