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J o u r n a l o fM a t e r i a l sP r o c e s s i n gT e c h n o
l o g yE L S E V IE R Journal of Materials Processing Technology49
(1995) 431-4 43
F r i c t i on w e l d i n g o f S t - A 1 an d A 1- C u m at e
r i a l sB e k i r S . Y i l b a ~ a '* , A h m e t Z . $ a h i n a
, N a f i z K a h r a m a n b , A h m e d Z . A 1 -G a r n i a
aKin g Fah d Universi ty o f Petro leum a nd Minerals, M
echanica l Engineering Department ,Dhahran 31261 , Sa udi
Arabia
b Erciy es University. E nginee ring Faculty, Kayseri,
TurkeyReceived 26 M ay 1993; acce pted 19 A pril 1994
Industr ia l SummaryT he prese nt stud y investigates the m echa
nical and m etallurgical pro pertie s of fr iction-weldedsteel-alum
inium and alum inium -cop per bars, a f r ic t ion welder having
been designed and buil tfor th is pu rpose. T he ef fects of the
three m ain param eters : speed of ro tat ion; f r ic tion load;
anddura t ion of welding; on the m etal lurgical and mec hanical
proper t ies of the weld such as theyield, tensile and brea king
strength s, are stud ied expe rim enta lly an d statistically. Th e
metallu r-g ical proper t ies of the weld are examined using
electron and optical microscopy.
1 . I n t r o d u c t i o nF r i c t i o n w e l d i n g i s c l
a ss i fi e d a s a s o l i d - s t a t e w e l d i n g p r o c e s
s , in w h i c h j o i n i n g
o c c u r s a t a t e m p e r a t u r e b e l o w t h e m e l t
i n g p o i n t o f t h e w o r k m e t a l . W h e n m e l t in go
c c u r s t h e r e is l i tt l e e v i d e n c e i n th e r e s u
l t i n g w e l d e d e n d - p r o d u c t . H e a t is g e n e r
a t e d b yd i r e c t c o n v e r s i o n o f m e c h a n i c a l
e n e r g y i n t o t h e r m a l e n e r g y a t t h e i n t e r
fa c e o f t h ew o r k p i e c e s d u r i n g t h e w e l d i n g
p r o c e s s . F r i c t i o n w e l d s a r e m a d e b y h o l d
i n g a n o n -r o t a t i n g w o r k p i e c e i n c o n t a c t
w i t h a r o t a t i n g w o r k p i e c e u n d e r g r a d u a l
l y i n c r e a s i n gp r e s s u r e u n t i l t h e i n te r f a
c e r e a c h e s t h e w e l d i n g t e m p e r a t u r e , a n d
t h e n s t o p p i n gr o t a t i o n t o c o m p l e t e t h e w
e ld . T h e h e a t d e v e l o p e d a t t h e i n t e rf a c e r
ai se s t h e t e m p e r -a t u r e o f t h e w o r k p i e c e s
r a p i d l y o v e r a v e r y s h o r t a x i al d i st a n c e ,
to v a l u e s a p p r o a c h i n gt h e m e l t i n g r a n g e ,
w e l d i n g o c c u r r i n g u n d e r t h e i n f lu e n c e o
f t h e p r e s s u r e t h a t is a p p l i e dw h e n t h e h e a
t e d z o n e i s i n t h e p l a s t i c t e m p e r a t u r e r a
n g e .
F r i c t i o n w e l d i n g c a n b e a c h i e v e d a t h i
g h p r o d u c t i o n r a t e s a n d t h e r e f o r e is e c o
n o m i -c a l in o p e r a t i o n , i n a p p l i c a t io n s w
h e r e f r i c t io n w e l d i n g h a s r e p l a c e d o t h e
r j o in i n gp r o c e s s e s , t h e p r o d u c t i o n r a t e
h a v i n g b e e n i n c r e a s e d s u b s t a n t i a l l y [ 1
] . S a v i n g s i nm a t e r i a l c a n b e r e a l i z e d a l
s o b y t h e u s e o f f r i c t io n w e l d i n g , t h is b e i
n g e s p e c i a l ly t r u e* Corresponding
author.0924-0136/95/$09.50 ~(3 1995 Elsev ier Scien ce S.A. All
righ ts reserved.S S D I 0 9 2 4 - 0 1 3 6 ( 9 4 ) 0 1 3 4 9 -
6
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432 B.S. Yi lba~ et al. / Journal o f Materials Processing
Technology 49 (1995) 43 1-44 3w h e n j o i n i n g l a rg e -d i
ame t e r ro t o r b o d i e s o r o t h e r ro t a t i n g me mb e
r s , o r j o i n i n g t h eco m p o n en t s o f v a l v e sy s t
ems I -2 ].
W h en w e l d i n g d is s imi l a r m e t a ls b y f r ic t io
n w e l d i ng , p ro b l em s a r i se n o t o n l y f ro mt h e d
i ff e r en t h a rd n es s e s an d me l t in g p o i n t s o f th
e m a t e r ia l s , b u t a l s o fro m t h eposs ib i l i ty o f
in te rac t ion p ro du cing e i ther b r i tt l e in te rm eta l l
ic ph ases o r low mel t ing-p o i n t eu t ec t i c s [3 ] . I n t
h e a l u mi n i u m/ s t ee l s y s t em, i n t e rme t a l l i c
co mp o u n d s a r ea ma j o r p ro b l em, i n g en e ra l t h e
fo rm a t i o n o f i n te rme t a l li c p h as es b e i n g co n
s i d e redu n d es i r ab le . D aw es [4 ] h a s i n t ro d u ce
d a r e l a t i o n s h i p b e t w een th e p ro p e r t i e s o f
j o i n t san d d i s s i mi l a r ma t e r i a l s w h i ch fo rm
b r i t t l e i n t e rme t a l l i c co mp o u n d s , an d t h e
t i meav a i l ab l e fo r th e fo rm a t i o n o f t h e co m p o
u n d s , co n c l u d i n g t h a t s a t i sf ac t o ry w e l d
sco u l d b e m ad e i f t h e w e l d i n g co n d i t i o n s w
ere s u ch t h a t t h e i n cu b a t i o n p e r i o d w asl o n g
e r t h an t h e w e l d ti me . H o w ev e r , th e ex i s ten ce
o f an i n cu b a t i o n p e r i o d fo ri n t e rme t a l l i c f
o rma t i o n i s q u es t i o n ab l e an d co n t ro l s h o u l
d b e b as ed o n l i mi t i n g t h ei n t e rme t a l l i c t h i
ck n es s r a t h e r t h an o n u s i n g an i n cu b a t i o n p
e r i o d .T h e p re s en t s t u d y i n v es ti g a te s t h e p
ro p e r t i e s o f f ri c ti o n w e l d s fo r s t ee l - a l u
mi n i u man d co p p e r - a l u mi n i u m b a r s , a f r i c t
i o n w e l d e r h av i n g b een d es i g n ed an d man u fac -t
u r ed fo r th i s p u rp o s e . A t emp e ra t u re s en s o r is
em p l o y ed i n th e s y s t em t o mo n i t o r t h eh ea t i n
g t i me an d p ro v i d e f eed -b ack t o t h e e l ec t ro n i c
co n t ro l l e r .
T h e n u m b er o f p a r am e t e r s i n v o l v ed in f r ic
t io n w e l d in g is q u i te l a rge , s o t h a ta s t a t i s
t i c a l ap p ro ach b as ed o n o n e - f ac t o r - a t - a - t
i me i s t ed i o u s an d u n s a t i s f ac t o ry .The refo re ,
s t a ti s t ica l des ign o f the expe r ime nt b ased on fac to r
i a l ana lys i s i s necessary .In t h e l i g h t o f p r ev i o
u s s t u d i e s [5 ,6 ] , t h e p r e s en t w o rk is ex t en d
ed t o i n c l u d e t h es ta t i st i ca l an a lys i s o f the
fac to rs a f fec t ing the f r i c tion-weld ing p rocess , these
fac to rsi n c lu d i n g s p eed o f ro t a t i o n , f o rg in g
l o ad a n d w e l d i n g ti me . T o ach i ev e a co mp l e t eb
loc k ana lys i s, t h ree l eve ls o f each fac to r have been se
lec ted .
To inves t iga te the me chan ica l p rope r t i es o f the d i
s s imi la r f r ic t ion welds , t ens il e andm i c ro -h a rd n
es s t e st in g o f t h e r e s u l t an t w e l d s a r e ca r r
ied o u t , me t a l l u rg i ca l ch an g es i nt h e w e l d i n
g zo n e b e i n g s t u d i ed a l s o u s i n g SE M an d o p t i
ca l mi c ro s co p y .
2. ExperimentalT h e ro t a t i n g w o rk p i ece is d r i v en
b y a 1 0 k W e l ect ri c mo t o r w i t h a v a r i ab l e
ro t a t i o n a l s p eed o f u p t o 3 5 0 0 rp m max i mu m .
In a l l te s t s th e f r i c ti o n p e r i o d isd e t e rm i n
ed b y a t em p era t u re r is e i n t h e w e l d i n g zo n e ,
a co n t ro l l in g d ev i ce ac t i v a ti n gt h e fo rg i n g l
o ad an d s e t ti n g t h e d u ra t i o n w h en t h e t emp era
t u re o f t h e w e l d i n g zo n ereach es t h e r eq u i r ed v
a lu e . F i g . 1 s h o w s t h e ex p e r i men t a l s e t -u p
em p l o y i n g t h ep u rp o s e -b u i l t f r i c ti o n w e l
d e r [7 ] .
In in i ti a l tr i al s , a wide range o f m ach ine se t t
ings wa s exp lo red , whi l e main ta in ingm an y o t h e r p a
ram e t e r s , i .e t h e c lean l in e s s o f th e s u r f aces
o f t h e s am p l e s an d t h ed u ra t i o n o f t h e w e l d
in g . It s h o u l d b e n o t ed t h a t e ach s am p l e w as c
l ean ed b e fo rew e l d i n g u s i n g ace t o n e w h i ch en s
u re g r ea s e - an d d i r t - f r ee s u r f aces . T h e a i m
w as t oex p l o re t h e e f fec ts o f d if f er en t co m b i n
a t i o n s o f ro t a t i o n a l s p eed , fr i c ti o n a l l o
ad an dt em p era t u re r is e in t h e w e l d zo n e . A co m b
i n a t i o n o f t h e mach i n e s e tt in g s r e s u lt in g i
nr ep ro d u c i b l e mech an i ca l p ro p e r t i e s w as s el
ec ted fo r t h e r em a i n i n g p a r t o f t h e s t u d y
.
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434 B.S. Y ilba~ et al. / Journal o f Materials Processing
Technology 49 (1995) 43 1-44 3Table 1Welding parameters and their
levels
Level 1 Level 2 Level 3Speed of rotation (rpm) 2000 2500
2800Applied load (kg) 5-12.74 6.37 17.83 7.64-22.93Welding duration
(s) 4 7 10
3 . F a c t o r i a l a n a l y s i sTo investigate the effects
of factors governing the friction welding process on the
resultant weld properties, it was necessary to employ factorial
analysis based ona statistical approach. In this case, the
influence of each factor could be examinedindividually, together
with that of their interactions. This required a complete
fac-torial design, in which the selected factors were varied at
appropriately selected levels.A mathematical model relevant to the
present analysis is given in Appendix A.The responses considered in
the factorial analysis included yield strength, tensilestrength and
breaking strength.
4 . D i s c u s s i o nThe welding parameters of applied load
for developing friction, speed of rotation
and welding duration determine the thermal phenomenon occurring
during the weld-ing process. The cleanliness of the samples may be
considered as secondary import-ance. Increased friction load
secures a slight reduction of weld-zone temperature, withthe
consequent production of less intermetallic compound at the
interface, whilst theforging load becomes important only when
considering consolidation at the interface.When examining the
effect of parameters on the resulting weld properties, it
isobserved that increasing the speed of rotation and the level of
friction load during theheating period, gives deformation
conditions that activate the contacting surfaceswhilst also
limiting the temperature/time conditions for joint heating, and
conse-quently minimize the formation of intermetallics. There is no
significant effect ob-served on the recorded parameters by changing
the surface finish. Reduced surfacecleanliness in terms of
contaminants, especially grease, reduces the quality of thewelded
joint, consistent with the findings of earlier-reported work
[8].Discussions based on the resultant weld properties will be
presented under twosub-headings, as follows.4 .1 . S t e e l - a l
u m i n i u m w e l d s
Tensile test results are shown in Fig. 3. Some of the welds show
poor strength,which may occur due to some accumulation of alloying
elements at the interface as
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B.S. Yilba~ et al. / Jou rna l of Materials Process ing
Technology 4 9 (1995) 4 31 ~ 43 4 3 5
AE0==
.CO~C
"00> .
E
C
eCI -
E0
==
e .
!an
1400
1300
1200
1100
10001 8 0 0
170016001500 -1 4 0 0 -13001200
11001 8 0 0
1300
1200 -
1 1 0 0
1000
9001 8 0 0
I I I I2 0 0 0 2 2 0 0 2 4 0 0 2 6 0 0
Load ap plied5-12.74 kg6.37-1783 kg7.64-22.93 kg
I2 8 0 0 3 0 0 0S p e e d ( r p m )
2 0 0 0 2 2 0 0 2 4 0 0 2 6 0 0 2 8 0 0S p e e d ( r p m )
Load Appl ied5-12.74 kg6.37-1783 kg7.64-22.93 kg
I3 0 0 0
Load Appl ied/ J ~ 5-12.74kgd J ~ 6 .37-1783 kg
d 7.64-22.93 kgI! I I I2 0 0 0 2 2 0 0 2 4 0 0 2 6 0 0 2 8 0 0 3
0 0 0
S p e e d ( r p m )Fig . 3 . Tensi le-test results (S t -A1)
.
a resu l t o f t emp erature r i se an d th e ex is t ence o f
in term eta l l ic layers such as FeA1 . Anin t e r me t a l li c t
h ic k n e s s o f 0 . 1 - 1 ktm i s a c c e p t a b le f or t h e
s o u n d w e ld s , a b o v e t h i s v a lu ew e ld s w i t h l o
w s t r e n g t h b e in g p r o d u c e d . I n t h e l a t t e r
c a s e , i t i s n o t e d t h a t t h ein t e r me t a l li c l a
y er i s t h i c k in t h e m id - r a d iu s o f t h e w e ld , a
n d t h in a t t h e c e n t e r a n d
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436 B.S. Y ilba~ et al. / Journal o f Materials Processino T
echnolooy 49 (1995) 431 -44 3
A>"r -
4 0 0
3 0 0
Ww 2 0 0G )r =
l Uz 1 O0
II n t e r f a c e
S t
I- 2 - 1
] g ~ A I
I I1 2
D i s t a n c e ( m m )Fig. 4. Microhardness-test results
(St-AI).
p e r i p h e r y o f t h e w e l d , t h e r e l a t i v e a b
s e n c e o f i n t e r m e t a l l i c l a y e r a t t h e c e n t
e r b e i n gc o n s i s t e n t w i t h t h e l o w r e l a t iv e
v e l o c i t y i n t h is r e g i o n .
M i c r o h a r d n e s s t e st r e su l ts a r e s h o w n i n
F i g . 4 , d e c r e a s e in m i c r o h a r d n e s s b e i n gn
o t e d i n t h e s te e l z o n e . T h e w i d t h o f t h e h e
a t - a f fe c t e d z o n e is v e r y sm a l l a s c o m p a r e
dt o t h a t f o r o t h e r t h e r m a l - w e l d i n g p r o c
e ss e s .
M i c r o p h o t o g r a p h s o f t h e i n t e rf a c e z o n
e a r e s h o w n i n F i g. 5 , i n w h i c h i t is o b s e r v e
dt h a t h e a v y d e f o r m a t i o n o c c u r s a t th e i n t
e r fa c e . I n t h is c a se , i t is m o s t p r o b a b l e t h
a t t h ea l u m i n i u m o x i d e f ilm i s b r o k e n u p , a
l l o w i n g t h e e x p o s u r e o f u n af f e c te d s u rf a
ce . A sa l u m i n i u m is r e l a t iv e l y s o f t, t h is r e
s u l ts i n a l a r g e a m o u n t o f d e f o r m a t i o n , s
o t h a ts u r fa c e c o n t a c t a t t h e i n te r f a c e s h
o u l d n o t b e a s ig n i fi c a nt p r o b le m . H o w e v e r
, t h es t a b le a l u m i n i u m o x i d e p r e s e n ts a b a
r r i e r to b o n d f o r m a t i o n , a n d c o n s e q u e n t
l y t h et i m e s a n d t e m p e r a t u r e s r e q u i r e d i
n d i f f u s i o n i s c o n d u c t i v e t o t h e f o r m a t i
o n a n dg r o w t h o f i n t e r m e t a l li c s .
I t is f o u n d f r o m T a b l e 2 t h a t t h e m a i n e f
fe c t s o f a ll th e p a r a m e t e r s o n y i e ld , t e n s
il ea n d b r e a k i n g s t r e n g t h s a r e n o t s i g n if
ic a n t . O n t h e o t h e r h a n d , f i r s t - o r d e r i n
t e r a c -t i o n s o f t h e s e p a r a m e t e r s h a v e a s
i g n i f ic a n t e ff e c t o n t h e t e n s i le p r o p e r t
i e s o f t h er e s u l t i n g w e ld s . T h i s m a y i n d i c
a t e t h a t e a c h f a c t o r h a s l i t tl e i n d i v i d u
a l e f f e ct o n t h et e n s il e p r o p e r t i e s , b u t t
h e c o u p l i n g o f f r ic t i o n lo a d , s p e e d o f r o t
a t i o n a n d w e l d i n g t i m eh as s i g n i f i can t e f f
ec t o n t h e s e p r o p e r t i e s . I n t h i s c a s e , i t
is d if f icu l t t o m ak e ana s s e s m e n t a s t o w h i c h
p a r a m e t e r h a s a g r e a t e r e f fe c t t h a n t h e o
t h e r s o n t h e s ep r o p e r t i e s .4 .2 . A l u m i n i u
m -co p p er we l d s
T e n s i l e te s t r e s u lt s a r e s h o w n i n F i g . 6
. S o m e j o i n t s f a il e d d u e t o t h e l a c k o f b o n
d i n g(i.e . v e r y l it tl e b o n d i n g o f a l u m i n i u m
o n t o t h e f r a c t u r e f a c es o f t h e c o p p e r ) a n
d t h ec o p p e r f r a c t u r e f ac e s s h o w a p a l e g r e
y a l u m i n i u m l a y er , w h i c h m a y c o n t a i n a c o
n -s i d e ra b l e a m o u n t o f i n t e rm e t a l l ic c o m p
o u n d a t t h e in t e rf a c e. W h e n t h e s p e ed o f
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B.S. Y ilba~ et al. / Journal of Materials Processing T
echnology 4 9 (1995) 431 -44 3 437
Fig . 5 . M i c roph ot ogra phy of t he we l d i ng zone (S
t-A1).
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438 B.S. Y i lba~ et al . / Journal o f Materials Processing
Technology 49 (1995) 43 1-44 3Table 2Statistical effects of w eld
parameters on mec hanical proper-ties for steel-alum inium
weldsParameters Stee l -a luminium
Yield Tensile Breakings t re n g t h s t r e n g t h s t re n g
t hn 0.1 > 0.1 > 0.1 >P 0.1 > 0.1 > 0.1 >t 0.1
> 0.1 > 0.1 >n x P 0.01 < 0.01 < 0.1 >n x t 0.01
< 0.01 < 0.1 >P x t 0.01 < 0.01 < 0.1 >
r o t a t i o n i n c r e a s e s , m i c r o p o r e s a t t h
e i n t e r f a c e d e c r e a s e , r e s u l t i n g i n a h o m
o g e n e o u sb o n d .
I n F i g . 7 m i c r o h a r d n e s s t e s t r e s u lt s a r
e s h o w n . S m a l l c h a n g e s o c c u r in m i c r o h a r
d -n e s s i n t h e c o p p e r z o n e , w h i c h m a y b e d u
e t o th e c o m p a r a t i v e l y h ig h h e a t - t r a n s f e
rc o e ff i ci e nt o f c o p p e r . I n F i g. 8, t h e m i c r o
p h o t o g r a p h y o f t h e A l - c o p p e r z o n e i s s h o
w n ,i n w h i c h a r e s o l v ed l a y e r o f i n t e r m e t a
l li c c o m p o u n d is s e en t o o c c u r a l o n g t h ei n
te r fa c e . A c o m p o u n d l a y e r d e v e l o p s a t t h e
i n te r fa c e , t h e d e p t h o f w h i c h l a y e ri n c r e
a s e s w h e n t h e r o t a t i o n a l s p e e d i n c r e a s e
s a n d t h e f r i c t i o n l o a d r e d u c e s . A ni n c re a
se i n t h e c o m p o u n d l a y e r r e su lt s in p o o r m e c
h a n i c a l p r o p e r ti e s. T h e s m o o t h -n e s s o f t
h e c o p p e r f a c e g i v e s a g o o d i n d i c a t i o n o n
b o n d i n t e g r it y , si n c e t h e b u l kd e f o r m a t i
o n i s c o n f i n e d t o t h e a l u m i n i u m . S o m e d e f
e c t s o c c u r a t t h e i n t e r f a c e a n dl a r g e p a r
t ic l e s o f c o p p e r a r e e m b e d d e d i n t h e a l u m
i n i u m , s h o w i n g t h e e v id e n c e o fs w i r l f l o w
i n t h e a l u m i n i u m .
S t a ti s ti c a l a n a l y s is s h o w s i m i l a r r es u
l ts t o t h o s e o b t a i n e d f o r s t e e l - a l u m i n i
u mw e l d s . A g a i n , i t i s d i ff i cu l t to d i s t i n g
u i s h t h e e f f e ct o f e a c h i n d i v i d u a l p a r a m
e t e r i n t h ep r o c e s s. C o u p l i n g e ff ec ts o f f r
ic t io n l o a d , s p e e d o f r o t a t i o n a n d w e l d i n
g t i m e o n t h et e n s i le p r o p e r t i e s a r e s i g n i
f ic a n t . T a b l e 3 s h o w s t h e r e s u l t s o f t h e s
t a t is t i c a l a n a l y s is .
5 . C o n c l u s i o n s
T h e c o n c l u s i o n s d e r i v e d f r o m t h e p r e s
e n t w o r k m a y b e l i s t e d a s f o l l o w .5 . 1 . S t e
e l - a l u m i n i u m w e l d s
1. T e n s i le p r o p e r t i e s i m p r o v e f o r s t e e
l - a l u m i n i u m w e l d s w h e n t h e i n t er m e t a l l
ict h i c k n e s s e x t e n d s o n l y t o 0 . 2 - 1 la m, a b o
v e th i s v a l u e w e l d s w i t h p o o r s t r e n g t h s b
e i n gp r o d u c e d . T h e i n t e r m e t a l l ic l a y e r t
h i c k e n s a t th e m i d - r a d i u s a n d b e c o m e s t h
i n a t t hec e n t e r a n d p e r i p h e r y o f t h e w e ld
.
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7.64-22.93 kgI 1 I ' I I ' I '
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Fi g . 6. T en s i l e - t e s t r e s u l t s ( A I Cu ) .
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Technology 49 (1995) 431 -44 3 44 1
Fig . 8 . M ic r o p h o to g r ap h y o f t h e we ld in g zo n
e ( A1- Cu) .
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4 4 2 B . S . Y i l b a ~ e t a l. / J o u r n a l o f M a t e r
i a l s P r o c e s s i n g T e c h n o l o g y 4 9 ( 1 9 9 5 ) 4 3
1 - 4 4 3
T a b l e 3S t a t is t ic a l e ff e ct s o f w e l d p a r a m
e t e r s o n m e c h a n i c a lp r o p e r ti e s fo r a l u m i
n i u m - c o p p e r w e ld s
P a r a m e t e r s A l u m i n i u m - c o p p e rY i e ld T e
n s i l e B r e a k i n gs t r e n g t h s t r e n g t h s t r e n
g t h
n 0.1 > 0.1 > 0.1 >P 0.1 > 0.1 > 0.1 >t 0.1
> 0.1 > 0.1 >n x P 0 .1 < 0 .01 < 0 .1
