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BEHAVIOUR OF WELDED PART OF Ti-Ni SHAPE MEMORY ALLOY
M. Nishikawa, H. Tanaka, M. Kohda, T. Nagaura, K. Watanabe
To cite this version:
M. Nishikawa, H. Tanaka, M. Kohda, T. Nagaura, K. Watanabe. BEHAVIOUR OF WELDED PART
OF Ti-Ni SHAPE MEMORY ALLOY. Journal de Physique Colloques, 1982, 43 (C4), pp.C4-839-C4-
844. �10.1051/jphyscol:19824138�. �jpa-00222124�
BEHAVIOUR OF WELDED PART OF Ti-Ni SHAPE MEMORY ALLOY
M. Nishikawa
,
H. ~ a n a k a *,
M. ~ o h d a *,
T . ~ a g a u r a * and K. WatanabeFacuZty of Engineering, Osaka University, Yamada-oka 2-2, Suita, Osaka, Japan
*Engineering Division, Sharp Corporation 2613-1 Ichinomoto, Tenri, Nara, Japan
(Accepted 9 August 1982)
A b s t r a c t
Thin w i r e s of Ti-Ni shape memory a l l o y a r e welded w i t h o u t f u s i o n o r c a s t zone by t h e r e s i s t a n c e b u t t welding. The weld c u r r e n t i s f e d t o t h e b u t t welded p a r t s with l i t t l e o f f s e t d u r i n g t h e time s h o r t e r t h a n t h a t i n t h e f o r g i n g p r o c e s s . The t e n s i l e s t r e n g t h of t h e welded p a r t i s a t t a i n e d over 80 p e r c e n t s of t h a t of t h e b a s e metal. For checking t h e shape memory e f f e c t , t h e recovery from bending deformation f o r t h e welded p a r t s i s t e s t e d i n comparison with t h a t f o r t h e b a s e m e t a l , and t h e r e s i d u a l s t r a i n a f t e r t h e recovery i s e x p e r i m e n t a l l y r e l a t e d t o t h e r a t i o of l a t e n t h e a t of m a r t e n s t i c t r a n s f o r m a t i o n t o t h a t of r e v e r s e t r a n s f o r m a t i o n , measured by zlhe d i f f e r e n t i a l scanning c a l o r i e - meter.
1. I n t r o d u c t i o n
The Ti-Ni shape memory a l l o y named N i t i n o l i s one of advanced m a t e r i a l s a s h e a t s e n s i t i v e working elements. I n o r d e r t o develop v a r i o u s a p p l i c a t i o n s of N i t i n o l , it becomes i n c r e a s i n g l y important t o s t u d y t h e p r o p e r t i e s of welded p a r t of t h i s shape memory a l l o y , however, n o t a g r e a t d e a l h a s been r e p o r t e d on t h e welding. There i s only a few r e p o r t s on f u s i o n weld of N i t i n o l using t h e e l e c t r o n beam welding and H e l i a r c
eld ding.''^'
These f u s i o n zone a r e c h a r a c t e r i z e d by t h e f i n e d e n d r i c s t r u c t u r e i n r e c r y s t a l l i z a t i o n n o t t o e x p e c t good shape memory e f f e c t . And it i s r e p o r t e d t h a t t h e t e n s i l e s t r e n g t h of t h e s e weld p a r t s i s r e l a t i v e l y low a s compared with t h a t of b a s e metal. By u s i n g r e s i s t a n c e u p s e t b u t t welding, it i s p o s s i b l e t o a t t a i n t h e weldrnent w i t h o u t f u s i o n zone o r c a s t zone t o keep t h e shape memory e f f e c t .I n t h i s experiments, t h e welding i s c a r r i e d o u t by u s i n g t h e s p e c i a l l y designed r e s i s t a n c e u p s e t b u t t welding and t h e p r o p e r t i e s of welded p a r t of Ti-Ni shape memory a l l o y a r e i n v e s t i g a t e d . Welding c o n d i t i o n f o r sound weldments i s o b t a i n e d by u l t i m a t e t e n s i l e t e s t . The shape memory e f f e c t i s checked by t h e recovery
from bending deformation f o r t h e welded p a r t s i n comparison w i t h t h a t from t h e b a s e m e t a l and an index of t h e shape memory e f f e c t f o r weldment i s r e p r e s e n t e d by t h e r a t i o of l a t e n t h e a t of m a r t e n s t i c t r a n s f o r m a t i o n t o t h a t of r e v e r s e t r a n s f o r m a t i o n , measured by t h e d i f f e r e n t i a l scanning c a l o r i e m e t e r .
2. Experimental
M a t e r i a l of t e s t p i e c e s used i n t h i s experiment was t h e t h i n w i r e of Ti-50 a t % N i shape memory e f f e c t a l l o y o with t h e diameter of 0.73 mm a s r e c e i v e d and was i n t h e temperature range of 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 from 59 C t o 65 C. The t e s t p i e c e s were p i c k l e d a f t e r t h e h e a t t r e a t m e n t i n vaccum f o r 30 minutes a t 400 C.
The edge of t e s t p i e c e s was grinded i n f l a t . The t e s t p i e c e s were welded by t h e r e s i s t a n c e u p s e t welding machine s p e c i a l l y designed f o r t h i s w i r e , equipped with
-
produced by Furukawa E l e c t r i c Company
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19824138
JOURNAL DE PHYSIQUE
Clamp
TiNiWire
Fig. 1 Schematic of r e s i s t a n c e b u t t welding machine
Compressed Air
condenser bank a s shown i n Fig.1. The u p s e t f o r c e was a b l e t o be k e p t an a r b i t r a r y c o n s t a n t v a l u e from 50 N t o 200 N by t h e s p r i n g . The weld c u r r e n t was changed from 385 A t o 600 A a t t h e peak v a l u e by s e t t i n g t h e charge v o l t a g e . F i g u r e 2 shows t h e t y p i c a l o s c i l l o g r a p h s of t h e weld c u r r e n t , t h e v o l t a g e and t h e displacement between t h e e l e c t r o d e s . The weld c u r r e n t i s f e d t o t h e f a y i n g weld p a r t s with l l t t l e o f f s e t d u r i n g t h e time of 7 msec. s h o r t e r t h a n t h a t i n t h e folrging p r o c e s s where t h e f u s i n g zone is extruded outward from observing t h e displacement between t h e e l e c t r o d e s . A t t h e s t a r t of welding, t h e o s c i l l o g r a p h of displacement shows a l i t t l e n e g a t r v e value. I t means t h a t t h e e l e c t r o d e moves backward due t o thermal expansion. I n t h e f o r g i n g p r o c e s s it t a k e s about 1 5 msec.
a f t e r t h e J o u l e h e a t i n g near t h e b u t t p a r t s . The i n f l u e n c e of t h e o f f s e t on t h e u l t i m a t e t e n s i l e s t r e n g t h i s shown i n Fig.3. The t e n s i l e s t r e n g t h measured a f t e r
Time (rnsec)
Offset Ratio xlOO (%)Fig.:! O s c i l l o g r a p h s of weld c u r r e n t , F i g . 3 E f f e c t of t h e o f f s e t r a t i o on v o l t a g e and displacement u l t i m a t e t e n s i l e s t r e n g t h between t h e e l e c t r o d e s
Welded part Welded part Welded part
oFp=7j=-
Welding Bending in Recovery in hot water
room temperature at95.C
r.:
radius of test pieceFig.4 Bending t e s t f o r checking t h e memory e f f e c t of t h e welded p a r t
For checking t h e shape memory e f f e c t , t h e recovery from t h e bending deformation was t e s t e d i n h o t w a t e r a t 95OC and t h e r e s i d u a l s t r a i n of t h e weld p a r t was measured i n comparison w i t h t h a t of b a s e m e t a l , a s shown i n Fig.4. The r e l a t i v e l a t e n t h e a t s of m a r t e n s i t i c and r e v e r s e t r a n s f o r m a t i o n were measured by t h e d e f f e r e n t i a l scanning c a l o r i e m e t e r usuing t h e weld p a r t with 1 mm l e n g t h c u t by t h e s l i c i n g machine.
3 . R e s u l t s
The e f f e c t of t h e weld c u r r e n t (peak v a l u e ) on t h e u l t i m a t e t e n s i l e s t r e n g t h i s shown i n Fig.5. The u l t i m a t e t e n s i l e
s t r e n g t h i n c r e a s e s with t h e weld c u r r e n t u n t i l 500 A and n e a r 500 A,
shows t h e peak v a l u e , followed by 1000
-
d e c r e a s i n g a s f u r t h e r i n c r e a s e of
t h e weld c u r r e n t . When t h e J o u l e
-
h e a t i n g a t t h e b u t t p a r t s becomes 0 e f f e c t i v e , t h e welded p a r t i s
a
extruded outward by t h e p l a s t i c flowing t o appear a c l e a n s u r f a c e a t t h e b u t t p a r t , and t h e v a l u e of u l t i m a t e t e n s i l e s t r e n g t h i s a l s o
i n c r e a s i n g u n t i l t h e weld c u r r e n t Upset Force
of 500 A. When t h e J o u l e h e a t i n g
*--* 180
Ni s more e x t e n s i v e l y , t h e weld p a r t ---O 140 is f l a s h e d o u t t o reduce t h e t e n s i l e
$
s t r e n g t h . The u p s e t f o r c e i s &-A
100
necessary t o b e a p p l i e d a s matching t h e p l a s y i c flowing. I f the u p s e t
- 60
f o r c e i s s e t t o a high v a l u e , t h e welded p a r t i s b u c k l i n g n o t t o appear t h e c l e a n f a y i n g s u r f a c e .
On t h e o t h e r hand, i f t h e u p s e t Weld Current ( A ) f o r c e is s e t t o a low v a l u e l e s s
t h a n 50 N , t h e welded p a r t i s F i g . 5 R e l a t i o n between t h e u l t i m a t e t e n s i l e f l a s h e d o u t . Fig.6 shows t h e s t r e n g t h and t h e weld c u r r e n t (peak welding c o n d i t i o n on t h e sound v a l u e ) f o r v a r i o u s u p s e t f o r c e
JOURNAL DE PHYSIQUE
Weld Cur rent ( A Fig.6 Welding c o n d i t i o n on t h e sound
weldment w i t h u l t i m a t e t e n s i l e s t r e n g t h more t h a n 80 % of t h e b a s e metal s t r e n g t h
Fig.7 T y p i c a l macroscopic welded p a r t
weldment w i t h t h e u l t i m a t e t e n s i l e s t r e n g t h more t h a n 880 Mpa i . e . 80 % of t h e base metal s t r e n g t h . The macroscopic s t r u c t u r e of t h e sound weldment is shown i n Fig.7. The weld i s found t o be formed a s t h e p r e s s u r e welding between c l e a n f a y i n g s u r f a c e s e x t r u d e d by t h e u p s e t f o r c e . The d e n d r i c s t r u c t u r e is n o t observed i n t h i s welding, a s opposed t o t h e e l e c t r o n beam welding and H e l i a r c welding. For checking t h e shape memory e f f e c t , t h e recovery from t h e bending deformation was t e s t e d a s shown i n Fig.4. The a p p l i e d maximum s t r a i n was 1 0 p e r c e n t (RB=3.65 mm). A f t e r t h e recovery from t h e bending deformation, t h e r e s i d u a l s t r a i n of t h e b a s e m e t a l was about 0.6 t o 0.7 %. The r e s i d u a l s t r a i n of t h e weld p a r t a f t e r t h e recovery i s p l o t t e d i n Fig.8. The c i r c l e , t h e
t r i g o n o m e t r i c and t h e quadrangular p o i n t s r e p r e s e n t group of t h e r e s i d u a l s t r a i n l e s s t h a n 1.35 %, group of t h e r e s i d u a l s t r a i n from 1.35 % t o 1.50 % and group of t h e r e s i d u a l s t r a i n more t h a n 1.50 %,
r e s p e c t i v e l y . The l i n e AB
- z
'\\.
r e p r e s e n t s t h e boundary curve of
-
t h e group of t h e r e s i d u a l s t r a i n Q, l e s s t h a n 1.35 %. The r e g i o n
surrounded t h e s o l i d l i n e s i s t h e LL welding c o n d i t i o n s f o r t h e sound Y
weldments. The r e g i o n above t h e a
Stain
d o t t e d l i n e AB is found t o be t h e
'\ B
welding with good shape memory A 1.35-1.50
e f f e c t ( t h e r e g i o n marked t h e 1.50- c i r c l e p o i n t s ) .
The t y p i c a l behaviours of
l a t e n t h e a t of m a r t e n s i t i c
4' rbo
500600
t r a n s f o r m a t i o n and t h e r e v e r s e Weld
Current ( A
t r a n s f o r m a t i o n a r e shown f o r
t h e welded p a r t and t h e base Fig.8 Maximum r e s i d u a l s t r a i n a f t e r t h e metal r e s p e c t i v e l y i n Fig.9. recovery from bending deformation f o r For t h e b a s e met-a1 annealed a t checking t h e shape memory e f f e c t 400°C f o r 30 minutes, t h e f i r s t
exothermic peak appeared i n t h e 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 near
Cooling Cooling
Fig.9 Endothermic and exothermic phenomena a s s o c i a t e d w i t h m a r t e n s i t i c and r e v e r s e t r a n s f o r m a t i o n s
55'C, followedby t h e second exothermic peak near -20°C a s cooling. On t h e r e v e r s e p r o c e s s a s h e a t i n g , t h e endothermic peak was n o t observed n e a r -20°C except t h e endothermic peak n e a r 55OC. On t h e o t h e r hand f o r t h e welded p a r t , o n l y t h e f i r s t exothermic peak was observed a s c o o l i n g and on t h e r e v e r s e p r o c e s s a s h e a t i n g only one endothermic peak was a l s o observed n e a r 60°C. The exothermic c a l o r i e (QM) and t h e endothermic c a l o r i e (QA) a r e p o s t u l a t e d t o b e r e l a t e d t o t h e shape memory e f f e c t due t o m a r t e n s i t c t r a n s f o r m a t i o n o f t h e metal i n working o r forming. I f t h e r a t i o of l a t e n t h e a t of m a r t e n s i t i c t o r e v e r s e t r a n s f o r m a t i o n (I=QM/QA) w i l l b e a index of t h e shape memory e f f e c t , t h e index of t h e base m e t a l annealed a t 400°C f o r 30 minutes i s c a l c u l a t e d t o b e 0.7. F i g u r e 10 shows t h e r a t l o of l a t e n t h e a t of m a r t e n s i t i c t o r e v e r s e t r a n s f o r m a t i o n f o r t h e p a r t s welded i n v a r i o u s welding c o n d i t i o n s . The welding c o n d i t i o n s surrounded t h e d o t t e d l i n e i n Fig.9 correspond t o t h o s e with t h e index v a l u e from 0.6 t o 0.8. The curve AB i s found t o b e agree with one of t h e boundary curves on t h e welding c o n d i t i o n s w i t h t h e index v a l u e of 0.8. The welding c o n d i t i o n s with good shape memory e f f e c t is found t o t h e correspond t h o s e with t h e index v a l u e l e s s than 0 . 8 on t h e welding c o n d i t i o n s f o r sound weldments.
Upset Force
*---o 180 N
*--a 140
F i g . 10 R e l a t i o n between t h e r a t i o of l a t e n t h e a t of m a r t e n s i t i c t o r e v e r s e t r a n s f o r m a t i o n and t h e weld c u r r e n t
7 -
i?
2
Weld Current ( A )JOURNAL DE PHYSIQUE
4.Conclusion
Thin w i r e of Ti-Ni shape memory a l l o y was welded without f u s i o n o r c a s t zone by t h e r e s i s t a n c e b u t t welding i n t h e atmosphere. The u l t i m a t e t e n s i l e s t r e n g t h was a t t a i n e d t o be more t h a n 880 Mpa i . e . 80 % of t h e base metal s t r e n g t h . The shape memory e f f e c t of t h e welded p a r t was found t o remained a f t e r t h e welding by checking t h e recovery from a bending deformation. The r e s i d u a l s t r a i n a f t e r t h e recovery was experimentally r e l a t e d t o t h e r a t i o of l a t e n t h e a t of 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 t o t h a t of r e v e r s e t r a n s f o r m a t i o n .
Refferences
1. C.M.Jackson, H.J.Wanger, and R.J.Wasilewski: 55-NITINOL-The a l l o y with a memory: Its p h y s i c a l m e t a l l u r g y , p r o p e r t i e s , and a p p l i c a t i o n s (19721, NASA-SP-5110
2 . " N i t i n o l Heat Engine" p u b l i s h e d by United S t a t e s Department of Energy
