Thermomechanical treatment of high purity 6061 aluminum
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has been known for some time that the performance of many aluminum alloys can be improved by a combination of plastic deformation and t h e r m a l treatments. Various c o m m e r c i a l practices utilize cold working in conjunction with aging.1 These include the T3 (solution heat treating, cold working and natural aging), T8 (solution heat treating, cold working and artificial aging), and T9/T10 (artificial a g i n g and cold working) temper conditions. It has been suggested2 that deformation p r i o r t o a g i n g m i g h t be used t o minimize the possible deleterious effects of precipitate free zones, that is the dislocations which r e s u l t from cold working may act as nucleation s i t e s for precipitation as well as mechanically strengthening the g r a i n boundary region. 3 In contrast, deformation after a g i n g generally produces a high strength, low-ductility product, the decreased ductility b e i n g ascribed t o the presence of nonmobile dislocations within the m a t r i x . More recent investigations have shown that propercontrol of the thermomechanical procedures, e . g . , warm r a t h e r than cold working, can r e s u l t in improved strength,4-7 fatigue resistance, 8-1° high temperature stability,n'x~ fracture toughness a n d / o r s t r e s s corrosion resistanceP '7'1.16 These studies have however been mainly limited to the high strength 2000 and 7000 s e r i e s aluminum alloys. The application of t h e s e techniques t o other alloy systems, e.g., A1-Mg-Si-Cu, r e m a i n s t o be established. Available information 17-21 does indicate that thermomechanical processing procedures quite s i m i l a r t o those previously used for 2000 and 7000 s e r i e s aluminum alloys should result in improved performance of other a l u m i num alloys. The object of the present investigation was then t o define those deformation and t h e r m a l practices which, when applied t o 6061 aluminum, would achieve increased strength levels without seriously affecting either the tensile ductility or notch toughness. EXPERIMENTAL
PROCEDURES
had been produced from a 136 kgm, 34.6 cm × 9.2 cm semicontinuously cast ingot. The chemical composition of this alloy is given in T a b l e I. The ingot was homogenized for 12 h at 833 K, scalped t o 8.27 cm and finally hot rolled at 755 K t o the 2.54 cm s t a r t ing thickness. Subsequent annealing was c a r r i e d out as follows: heat at 10 K per h t o 686 K, hold for 2 h, then cool at 24 K per h t o 505 K followed by air cooling. Four thermomechanical treatment (TMT) schedules were examined d u r i n g the course of this investigation, Fig. 1. Solution treatment p r i o r t o processing was standardized at 798 K for 1 h. The primary g o a l s of Schedules A and B were t o investigate the influence of a) deformation temperature, and b) the d e g r e e of deformation on the strength characteristics of high p u r i t y 6061 aluminum. A secondary objective of these schedules was to establish the differences in response t o thermomechanical treatment that might be occa
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