Heat treating characteristics of high strength Al-Zn-Mg-Cu alloys with and without silver additions
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EFFECTS
of silver additions to AI-Zn-Mg-Cu alloys were examined at Alcoa Research Laboratories as early as 1947 as part of extensive investigations of effects of alloying elements on properties of aluminum. In early experiments, tensile properties of 7075 and related alloys in the form of sheet were compared with those of similar sheet containing additionally 0.5 to 1.6 pct Ag. After conventional isothermal and step-precipitation heat treatments, alloys containing silver generally developed strer~gths comparable to those of the control alloys, but in some instances they developed strengths up to 10 pct lower. After Polmear' reported in 1960 that 0.3 to 1 pct Ag additions substantially increased strengths of AI-ZnMg-Cu alloys isothermally precipitation-heat-treated above 250~ (121~ additional experiments were initiated at the Alcoa Research Laboratories. Effects of time at room temperature prior to elevated temperature isothermal precipitation heat treatment on tensile properties of 7075 and 7079 sheet and of comparable sheet containing 0.4 to 0.6 pct Ag were evaluated. Silver-free alloys and alloys containing silver developed equally high strengths after precipitation heat t r e a t m e n t s at 250~ (121~ or lower. Alloys containing s i l v e r p r e c i p i t a t i o n - h e a t - t r e a t e d above 250~ (121~ after a short time at room t e m p e r a t u r e , howe v e r , developed s u b s t a n t i a l l y higher strengths than i d e n t i c a l l y p r o c e s s e d s i l v e r - f r e e control a l l o y s . Strength of the s i l v e r - f r e e a l l o y s substantially inc r e a s e d when p r e c i p i t a t i o n heat t r e a t m e n t was p r e ceded by natural aging i n t e r v a l s of s e v e r a l days but strength of the alloys containing s i l v e r was a l m o s t unaffected. The effect of n a t u r a l aging i n t e r v a l was also r e p o r t e d by Rosenkranz. 2 Because s i l v e r additions provided no strength advantage and because r e s i s t a n c e J. T. STALEYis Senior Research Engineer, PhysicalMetallurgyDivision, Alcoa Research Laboratories,Alcoa TechnicalCenter, Pittsburgh, Pa. R. H. BROWN,formerly Assistant Director, Alcoa Research Laboratories, is now retired. R. SCHMIDTis Metallurgist,U. S. NavalAir Systems Command, Washington,D. C. Manuscript submitted January 19, 1971. METALLURGICALTRANSACTIONS
to s t r e s s - c o r r o s i o n cracking of sheet in the highest strength t e m p e r was adequate, no f u r t h e r work with sheet was indicated. Additional work with thick sections was indicated, however, because s i l v e r additions a p p e a r e d to provide their maximum strength advantage a f t e r precipitation heat t r e a t m e n t at t e m p e r a t u r e s which promote a high degree of 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 in the c r i t i c a l s h o r t - t r a n s v e r s e d i r e c t i o n of A1-Zn-Mg-Cu alloy products. To determine whether s i l v e r additions were beneficial in c o m m e r c i a l l y p r o c e s s e d m a t e r i a l ,
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