Elevated temperature flow strength, creep resistance and diffusion welding characteristics of Ti-6Al-2Nb-1Ta-0.8Mo
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IN g e n e r a l ,
titanium alloys a r e excellent examples of m a t e r i a l s that can be r e a d i l y diffusion welded. 1 Titanium alloys can have low flow s t r e n g t h s at r a t h e r modest t e m p e r a t u r e s on the o r d e r of 1200 K (homologous t e m p e r a t u r e ~0.63); t h e i r high solubility for oxygen and other s u r f a c e contaminants p r o v i d e s for e a s y cleaning of faying s u r f a c e s at modest t e m p e r a t u r e s in nonreactive environments; finally, p r o p e r t h e r m o m e c h a n i c a l p r o c e s s i n g can induce s u p e r p l a s t i c behavior in many titanium a l l o y s . Work r e p o r t e d by A p r i g l i a n o and Zanis 2 has shown that it is p o s s i b l e to diffusion weld the advanced titanium alloy T-6A1-2Nb1Ta-0.8Mo at t e m p e r a t u r e s ranging from 1090 to 1250 K and low s t r e s s e s ranging f r o m 0.35 to 3.0 MPa. Low s t r e s s levels were of p a r t i c u l a r i n t e r e s t as they can be produced by a t m o s p h e r i c p r e s s u r e loading of evacuated v e s s e l s . Thus, diffusion welding of large d i a m e t e r titanium p r e s s u r e v e s s e l s was envisioned as an a l t e r n a t i v e to conventional fusion welding. A p r i g l i a n o and Zanis utilized two different m i l l p r o c e s s e d forms of this alloy, and found they r e q u i r e d different diffusion welding schedules ( t e m p e r a t u r e applied p r e s s u r e - t i m e ) in o r d e r to produce s a t i s f a c t o r y weldments. They felt that the difference in diffusion welding c h a r a c t e r i s t i c s of the two m i l l - p r o c e s s e d f o r m s was r e l a t e d to the initial m i c r o s t r u c t u r e of the a l l o y s which affected the mecha_aical flow s t r e n g t h p r o p e r t i e s under diffusion welding conditions. The work d e s c r i b e d h e r e i n was undertaken to c h a r a c t e r i z e the mechanical p r o p e r t i e s in c o m p r e s s i o n of the two m i l l - p r o c e s s e d f o r m s of T i - 6 A 1 - 2 N b - I T a 0.SMo studied by A p r i g l i a n o and Z a n i s . z C o m p r e s s i v e flow c u r v e s and steady state c r e e p r a t e s were d e t e r mined over the t e m p e r a t u r e and s t r e s s range of i n t e r e s t for diffusion welding. In addition, s e v e r a l diffusion welds were made, and the quality of the weldments was d e t e r m i n e d as a function of initial laying
J. DANIELWHITTENBERGER and THOMASJ. MOORE are Material Engineers, Materialsand Structures Division,NASA-Lewis Research Center, Cleveland,OH 44135. Manuscript submitted October 4, 1978. METALLURGICALTRANSACTIONSA
s u r f a c e p r e p a r a t i o n , deformation a c r o s s the weld i n t e r face, and time under s t r e s s and t e m p e r a t u r e . EXPERIMENTAL PROCEDURE M a t e r i a l s and
Specimen G e o m e t r y
Two m i l l - p r o c e s s e d heats of the titanium alloy T i - 6 A 1 - 2 N b - I T a - 0 . 8 M o were examined in this study: forged m a t e r i a l and r o l l e d plate m a t
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