Preaging effects in Al-Mg-Si alloys
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is well known that heat t r e a t i n g v a r i a b l e s in addition to the final aging time and t e m p e r a t u r e can have a m a r k e d effect on the hardening r e s p o n s e of h e a t - t r e a t able aluminum alloys. E x a m p l e s of such v a r i a b l e s a r e : 1) delay time between solution heat t r e a t i n g and aging (natural aging), 2) heating r a t e to the aging t e m p e r a t u r e , and 3) aging at an i n t e r m e d i a t e t e m p e r a t u r e p r i o r to final aging (preaging). A g r e a t deal of work has been published about the effects of v a r i a b l e s (1) and (3) on the aging c h a r a c t e r i s t i c s of A1-Zn-Mg a l l o y s . G e n e r a l l y , n a t u r a l aging and preaging t r e a t m e n t s a r e beneficial; they p r o m o t e fine, uniform p r e c i p i t a t e d i s p e r s i o n s and high strength. The situation a p p e a r s to be m o r e complicated in the A1-Mg-Si s y s t e m . It has been known for many y e a r s that alloys containing about 1.3 pct Mg2Si* have a nega*The Mg: Si content of an alloy is a convenient means of expressing the silicon and magnesium concentrations which are available for age hardening (quasibinary). The weight ratio of magnesium to silicon in the hardening phase is assumed to be 1.73:1. All concentrations are expressed in wt. pct.
rive r e s p o n s e to n a t u r a l aging, i.e., strength d e c r e a s e s as a r e s u l t of room t e m p e r a t u r e aging p r i o r to a r t i f i c i a l aging. C o n v e r s e l y , alloys with lower Mg2Si contents (---0.80 pct) have been r e p o r t e d to show a positive r e sponse to n a t u r a l aging. 1'2 The effects of e l e v a t e d - t e m p e r a t u r e preaging also a p p e a r to be inconsistent. I r e aging of alloys with 1.2 to 1.5 pct Mg2Si has been found to be beneficial; 3-s but is d e t r i m e n t a l in an A1-1 pct Mg2Si alloyfl At a somewhat lower Mg2Si level (0.90 pct), preaging has been r e p o r t e d to have a positive effect. 2 We p r e s e n t here the r e s u l t s of a study showing the effects of a number of aging v a r i a b l e s (natural aging, preaging, heating rate) on the aging c h a r a c t e r i s t i c s and p r e c i p i t a t e d i s p e r s i o n s in a c o m m e r c i a l aluminum a l loy containing from 0.60 to 0.90 pct Mg2Si (AA6006). This composition range has not yet been c l o s e l y e x a m ined. PROCEDURES
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RESULTS
Three 4 in. diam c o n t i n u o u s l y - c a s t ingots of the c o m positions shown in Table I were homogenized for 6 h at R. C. DORWARD is Staff Research Metallurgist, Center for Technology, Kaiser Aluminum and Chemical Corp., Pleasanton, Calif. 94566. Manuscript submitted May 25, 1972. METALLURGICAL TRANSACTIONS
565~ and extruded (31 : 1 reduction ratio) at 475~ into a fluted r e c t a n g u l a r shape. The e x t r u s i o n s were " p r e s s quenched" using f o r c e d - a i r cooling ( ~ 5 0 ~ and then s t r e t c h e d 1 pct for straightening. The aging v a r i a b l e s studied and the r e s u l t s obtaine
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