Superplastic behavior of welded specimens of the titanium alloy TA6V
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J. P. LECHTEN,
AND
B. BAUDELET
S u p e r p l a s t i c i t y allows m e t a l l i c s h e e t s to be t h e r m o f o r m e d . 1 F o r p a r t i c u l a r l y c o m p l i c a t e d s h a p e s , it is n e c e s s a r y e i t h e r to weld t o g e t h e r i n i t i a l l y f o r m e d p a r t s o r to t h e r m o f o r m d i r e c t l y a s t r u c t u r e c o m posed of p r e w e l d e d s h e e t s . T h i s last technique p o s e s the p r o b l e m of the weld zone b e h a v i o r d u r ing d e f o r m a t i o n in the s u p e r p l a s t i c r a n g e . We p r o pose to study the case of the s u p e r p l a s t i c t i t a n i u m alloy TA6V.2 The m a t e r i a l used for this i n v e s t i g a t i o n is a c o m m e r c i a l t i t a n i u m alloy T i 6 pct A1 4 pct V which was r e c e i v e d as 2 m m thick s h e e t s . It is a t w o - p h a s e alloy with a roughly equiaxed a - p h a s e of m e a n g r a i n s i z e 5 ~ m . P r e c i s e o b s e r v a t i o n s have shown that the a - p h a s e is s u r r o u n d e d by the ~ - p h a s e and the oz'phase, the l a t t e r b e i n g f o r m e d by the m a r t e n s i t i c t r a n s f o r m a t i o n of the fl-phase. 3 The s p e c i m e n s a r e cut out of the sheet and then welded t o g e t h e r by the TIG p r o c e s s , u s i n g a n o n c o n s u m a b l e e l e c t r o d e and with the weld being l e v e l with the s h e e t s u r f a c e s . To study the p l a s t i c b e h a v i o r of the welded zone u n d e r d i f f e r e n t s t r e s s conditions, t h r e e s p e c i m e n types w e r e p r e p a r e d (Fig. 1). The weld zone s t r u c t u r e c o n t a i n s p r i n c i p a l l y the m a r t e n s i t i c phase ~ ' which is c h a r a c t e r i s t i c of the t i t a n i u m alloy TA6V p r e p a r e d by s o l i d i f i c a t i o n and r a p i d cooling. 3,4 It is i m p o r t a n t to note that d u r i n g the r e h e a t i n g p e r i o d p r i o r to the test, the o~'-phase t r a n s f o r m s to a and ~p h a s e s . 3,s The s p e c i m e n s a r e d e f o r m e d in t e n s i o n in a i r at a c o n s t a n t s t r a i n r a t e . A coating c o n t a i n i n g g r a p h i t e is applied to the s p e c i m e n s b e f o r e the t e s t s to r e d u c e the oxidation effects at high t e m p e r a t u r e . The t e s t conditions a r e i d e n t i c a l to those which a l lowed the s u p e r p l a s t i c b e h a v i o r to be obtained for the TA6V alloy. 2 The s p e c i m e n s a r e d e f o r m e d at t h r e e t e m p e r a t u r e s (850, 900 and 950~ and t h r e e s t r a i n r a t e s (2.7 • 10-3 m n -~, 2.7 • 10 -2 m n -~ and 2.7 • 10 -~ mn-~). The m a x i m u m elongation of the s p e c i m e n s is l i m i t e d to 150 pct by the useful length of the f u r n a c e . A f t e r each test, the s p e c i m e n s a r e a i r cooled. All the t e s t s p e r f o r m e d show the s a t i s f a c t o r y b e h a v i o r of the weld, w h a t e v e r the type of s p e c i m e n , the t e m p e r a t u r e and the s t r a i n r a t e : no d e c
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