Welding of Lightweight Aluminum-Lithium
Alloys
Information on the welding of various
aluminum-lithium alloys is reviewed
BY T. S. SRIVATSAN A N D T. S. SUDARSHAN
ABSTRACT. Lithium-containing aluminum
alloys have generated much interest in the
aerospace industry because lithium additions to aluminum can decrease the density and improve the stiffness compared
to conventional aluminum alloys of comparable strength. In this paper, published
studies of the structure and properties of
welded aluminum-lithium alloys are discussed. The weldability characteristics of
these lightweight alloys using gas metal
arc, gas tungsten arc, electron beam and
laser beams are summarized. While the
alloys appear to be easily fusion weldable,
they may be prone to problems of weld
porosity, cracking, poor penetration and
low joint efficiency. The effects of heat
treatment on postweld joint efficiency are
also highlighted. Intrinsic problems associated with conventional welding were partially eliminated through laser beam welding. The laser beam weld studies revealed
a low degree of surface degradation, an
absence of porosity and attractive joint
efficiency. The results delineated in the
published literature suggest that future
studies must concentrate on a more comprehensive and systematic approach to an
evaluation of properties if the use of
welded aluminum-lithium structures is to
be rapidly accepted in numerous industrial
applications.
Introduction
Selection of materials for commercial,
military and space-related applications often involves a compromise between performance and cost. The material that will
provide the least expensive component
while delivering the specified performance
is usually preferred. The need for a certain
level of performance is often dictated by
the mission and this justifies the use of a
certain type or class of material. For
example, aluminum alloys have been used
extensively for airframe structures. These
T. S. SRIVA TSAN is with the Department of
Mechanical Engineering, The University of
Akron, Akron, Ohio. T. S. SUDARSHAN is with
Materials Modification, inc., Falls Church, Va.
alloys, both conventional and advanced,
were developed for improved mechanical properties so that improvements in
aircraft performance could be obtained
by efficiently reducing structural weight.
This can be achieved through reductions
in density and improvements in strength,
stiffness, durability and damage tolerance
(Ref. 1). A NASA-funded study by the
Lockheed Corporation (Ref. 2) revealed
that reducing the density of aluminum alloys by 7 to 10% could provide cost-effective structural weight reductions when
compared to composite materials that
have high production costs. These alloys
could also potentially provide a unique
combination of properties that includes:
1)
2)
3)
4)
Decreased densities.
Increased stiffness.
High yield and tensile strengths.
Good resistance to the propagation of
fatigue cracks.
5) Resistance to exfoliation corrosion.
6) Resistance to stress corrosion cracking.
7) Improved thermal stability.
8) Attractive weight savings, which would
help lower operating costs by reducing
fuel consumption.
In recent years, the family of lithiumcontaining aluminum alloys has received
much attention because it offers the promise of substantial specific strength and
specific stiffness advantages over other
commercial 2XXX- and 7XXX-series aluminum alloys (Refs. 3-5) and carbon-fiber
composites. This advantage makes them
an ideal candidate for weight-critical and
KEY W O R D S
Al-Li Alloy Welding
Aerospace Welding
GMAW
GTAW
Electron Beam Welding
Laser Beam Welding
Weldability Studies
Postweld Heat Treat
Surface Preparation
Metallurgy
stiffness-critical structures for military,
space and commercial applications.
Most high-strength aluminum alloys
used in aircraft structures are mechanically
fastened, which has the drawbacks of
slow assembly and limitations in joining
thin sections. Fusion welding of lightweight aluminum-lithium alloys has been
evaluated by several investigators and has
resulted in various degrees of success.
However, there exists a critical need to
develop new joining methods in order to
extend the range of applications for these
alloys and to improve the overall performance, durability, damage tolerance and
life of safety-critical components and
structures. Furthermore, new joining
methods could facilitate use of these
alloys in marine hardware, lightweight
pressure vessels and lightweight armored
vehicles.
The objective of this paper is to review
published literature on the weldability of
various lithium-containing aluminum alloys. The study encompasses a range of
welding techniques and includes conventional arc welding and welding using highenergy-density laser beams. In particular,
relevant metallurgical issues that influence
weldability are examined. Results of recent research are emphasized, although
significant prior work is also included to
provide continuity. Initial discussions concern the influence of lithium additions to
aluminum.
Effects of Lithium Additions
to Aluminum
Magnesium and lithium are the only
t w o elemental additions which, when
added to aluminum, have the potent ability to decrease its density. Beryllium also
decreases the density of aluminum but it
is extremely toxic and is a health hazard.
Lithium is one of the few elements with
substantial solubility in aluminum (4.2 wt% at 600°C/1112°F in a binary aluminumlithium alloy). The potential for aluminum
alloy density reduction through lithium
additions is evident by comparing its
atomic weight (6.94) with that of aluminum (26.98). Lithium additions to alumi-
WELDING RESEARCH SUPPLEMENT 1173-s
10
ALLOYING
15
ELEMENT, WEIGHT %
2.0
3.0
4.0
A L L O Y I N G ELEMENT. WEIGHT *
Fig. 1 - The effect of solute additions on the modulus of aluminum alloys Fig. 2— The effect of solute additions on the density of aluminum alloy
(Ref. 73).
(Ref. 5).
num also cause a significant increase in
elastic modulus-Fig. 1. Each 1% lithium
addition of aluminum, up to 4 wt-% lithium, decreases the density by 3% (Fig. 2),
and increases the elastic modullus by 6%
(Ref. 6). The specific modulus (modulus of
elasticity by weight) of an alloy with 2.8
wt-% lithium is 2 1 % higher than that of
aluminum Alloy 2024-T351 and 26% higher
than that of aluminum Alloy 7075-T651.
The higher specific modulus reduces the
rate of fatigue crack growth. The decrease in density is far more effective in
reducing structural weight than improved
strength, modulus, toughness or fatigue
resistance (Ref. 7). For example, in an aluminum alloy having 3 wt-% lithium, structural weight savings of 10% on affected
structures could be realized by direct substitution, and over 16% by design modifi-
Table 1—Electrical Conductivity of Swiss
Alloys
Alloy
Composition
(wt-%)
Conductivity
(% IACS)<a>
AI-2.52 Mg-0.60 Li-0.55 Mn-0.14 Ti
AI-2.50Mg-0Li-O.54Mn-O.15Ti
22.5
32.3
{a) Percent of the conductivity of annealed copper.
Table 2—Spot Varestraint Test Results
Alloy
Weldalite® 049 Variants
6.1 Cu
5.2 Cu
4.8 Cu
2090
2219
2014
TCL in HAZ<a>
(mm) per Sample
0, 0, 0
0, 0, 1.6
0, 0, 0
0, 0, 0, 0
8.7, 7.7
11.1, 5.2,
11.2, 8.3
(a) Total crack length in weld heat-affected zone
From Ref. 34.
174-s | JULY 1991
cation based on improved mechanical
properties (Ref. 8).
The potential benefits of lithium additions to aluminum in terms of reduced
density and improved stiffness, combined
with high strength (0.2% yield strength >
500 MPa/72.5 ksi) was first recognized by
LeBaron (Ref. 9) and resulted in the development of the Al-4.45Cu-1.21Li-0.51Mn0.2Cd alloy designated as 2020 by the
Aluminum Association. Besides possessing
high strength, low density and an increased elastic modulus, aluminum Alloy
2020 offers freedom from exfoliation corrosion and stress corrosion cracking, thus
making it potentially superior to other
commercially available aluminum alloys
for use in high-performance military structures. Alloy 2020 was used in the wing
skins of the U.S. Navy RA-5C Vigilante aircraft. At approximately the same time,
design studies in the United Kingdom suggested significant mass savings as a result
of using aluminum Alloy 2020 on a large
scale in relatively thin sections. Weight reductions of 400 kg (880 Ib) were predicted, with 22% of the savings resulting as
a consequence of the lower density of the
alloy. The high elastic modulus and
strength of the alloy offered advantages in
areas where elastic stability was highly
critical. Interest in the commercial use of
Alloy 2020, however, was not pursued
further because of:
1) Segregation effects.
2) The associated low ductility.
3) Inadequate fracture toughness in the
short transverse orientation, which
made its efficient use at high stresses
inadvisable.
4) Problems during melting, which were
caused by the high reactivity of lithium.
These limitations resulted in the early
withdrawal of 2020 as a commercial alloy
(Ref. 10). The need for lightweight structures resurrected much interest in the aluminum-lithium alloy system during the late
1970s. A major obstacle in the develop-
ment of these alloys has been the inherent
limited ductility (Refs. 11-13). The intense
planar deformation (Fig. 3), due to the
presence of ordered matrix precipitates
responsible for strengthening (Fig. 4), results in either planar slip or grain boundary
decohesion (Refs. 14, 15). Consequently,
deformation beyond the point of plastic
instability is seldom observed. Binary, ternary and quaternary alloys belonging to
the aluminum-lithium system have in recent years been the subject of increased
research activity aimed at understanding
their various metallurgical and mechanical
characteristics (Refs. 16-20). However, in
order to exploit the potential benefits
these alloys have to offer, methods of
joining these alloys must be examined and
well understood in order to derive maximum benefit in using an aluminum-lithium
alloy for a particular application.
Weldability of
Aluminum-Lithium Alloys
Weldability is defined as the ability of a
material to be welded free of defects. In
the case of aluminum alloys, it is a measure
of an alloy's resistance to hot cracking
during welding. While the theory of hot
cracking is indeed complicated and controversial (Ref. 21), there exists phenomenological data for binary aluminum alloy
systems showing compositional effects on
hot cracking (Refs. 22-25). The hot cracking susceptibility of an alloy can be quantitatively assessed by the Varestraint test.
The maximum crack length (MCL) possible in any weldability test where an
instantaneous strain is applied is governed
by the extent of the two-phase (liquid and
solid) region behind the weld pool (Ref.
26). The size of the two-phase region (0')
can be best approximated by the expression /?' = (TL — T E ) / G , where TL is the liquidus temperature, Tj is the temperature
of the eutectic having the lowest melting
point, and G the weld-temperature gradi-
ent. This assumes that tears, if any, will not
e x t e n d b e y o n d the p o i n t w h e r e liquid
films preexist, as d e t e r m i n e d b y the solidus, that is, the eutectic t e m p e r a t u r e of the
alloy. A l t h o u g h t h e r e exists n o simple correlation b e t w e e n m a x i m u m crack length
(MCL), t o t a l crack length (TCL) a n d w e l d ability, an alloy having a large M C L also will
h a v e a large TCL a n d , thus, s h o u l d b e susc e p t i b l e t o h o t tearing.
Predicting the m a x i m u m crack length
based o n the size o f the t w o - p h a s e region
has b e e n successful for aluminum-silicon,
a l u m i n u m - c o p p e r and a l u m i n u m - l i t h i u m
binary alloys, b u t n o t f o r a l u m i n u m - m a g nesium, w h i c h , o n a c c o u n t of its l o w e u tectic t e m p e r a t u r e , should exhibit the largest MCLs. That these alloys d o n o t b e h a v e
accordingly at l o w solute c o n t e n t (Fig. 5)
can be related t o the l o w quantity of e u tectic characteristic o f these alloys, w h i c h
makes t h e m m o r e resistant t o h o t tearing.
As magnesium c o n t e n t increases, h o w ever, m o r e eutectic is g e n e r a t e d and p r e d i c t e d MCLs are a p p r o a c h e d . For the alum i n u m - c o p p e r binary system, increasing
the solute c o n t e n t b e y o n d the 2 t o 3%
range reduces M C L , a b e h a v i o r that w a s
rationalized as being d u e t o backfilling
(Ref. 26).
Arc Welding
There h a v e b e e n only a f e w studies t o
c o m p r e h e n s i v e l y u n d e r s t a n d the t e c h nique and use of w e l d i n g t o join lightw e i g h t aluminum-lithium alloys. T h e w e l d ability o f high-purity aluminum-lithium b i nary alloys w a s investigated b y Cross a n d
c o w o r k e r s (Ref. 24) t h r o u g h the use of t h e
Varestraint test. In the Varestraint test, a
specific a m o u n t of strain is i n d u c e d o n the
Table 3—Tensile Properties of Weldalite" 049 Weldments in the As-Welded Condition
Variant
Filler
Process
(Current)
6.1 Cu
2319
CTA (DC)
6. I Cu' b)
li 19
CTA (AC)
6. I Cu
Base
CTA (DC)
6. I Cu
Base
VPPA
4.8 Cu
Base
CTA (DC)
0.2" , YS
mean
best
mean
best
mean
best
mean
best
mean
best
(of 5)
(of 3)
(of 3)
(of 4)
(of 3)
MPa
(ksi)
242
262
246
314
256
262
286
286
247
239
(35)
(38)
(36)
(46)
(37)
(38)
(42)
(42)
(36)
(35)
UTS
Elongation'3
MPa
(ksi)
(%)
325
344
333
353
318
332
340
353
303
315
(47)
(50)
(48)
(51)
(46)
(48)
(49)
(51)
(44)
(46)
3.7
2.7
4.4
4.5
2.4
2.8
1.6
2.4
2.3
3.1
ia) Elongation in 2.54 cm
(l>) Bar thickness = 0.b_ cm. all others 0.95 c m .
f r o m Ref. i-4
Table 4—Typical Tensile Properties of 2XXX-Series Aluminum Weldments in the As-Welded
Condition
Alloy
20 14-Tb
20 I4-T6
22 I9-T8I, T87. T6
11 I9-T85 1
25 I9-T87
2090
2090
2090-T8E41
Filler
Metal
4043
2014
2319
23 19
2319
2090
5356
23 19
0.2'V, YS
Process
GTA/CMA
GTA/CMA
GTA/CMA
GMA
GMA
GTA
GMA
GMA
MPa
193
I93
179
183
220
123
165
204
(ksi)
(28)
(28)
(26)
(27)
(32)
(18)
(24)
(30)
UlS
MPa
234
11 1
241
300
317
230
253
232
Elongation'3'
(ksi)
(34)
(32)
(35)
(44)
(46)
(33)
(37)
(34)
(%)
4
2
3
4.9
4.2
4.0
3.0
5.2
(a) Elongation in 5.08 cm.
From Ref iA
surface of a specimen d u r i n g w e l d i n g .
T h o s e evaluations w e r e p e r f o r m e d t o ascertain the susceptibility of the binary alloy
t o h o t tearing. H o t tearing is d e f i n e d as
cracking in the w e l d z o n e caused b y an
inability o f the liquid region t o s u p p o r t the
Fig. 3 — Bright-fieldtransmission electron micrograph showing planar deformation in Al-1.2 ILi-4.45Cu alloy.
strain resulting f r o m solidification shrinkage (Refs. 2 4 , 27). Cross a n d c o w o r k e r s
(Ref. 24) o b s e r v e d the susceptibility of the
binary aluminum-lithium alloy t o h o t tearing t o vary w i t h lithium c o n t e n t , w i t h the
peak o c c u r r i n g at 2.6% l i t h i u m - F i g . 6.
Fig. 4 - Bright-Held transmission electron micrograph showing
precipitates (A^Cu and T1Al2CuLi) distributed in matrix of
4.45Cu alloy.
plate-like
Al-l.21Li-
WELDING RESEARCH SUPPLEMENT 1175-s
6
•
Aluminum-magnesium
2.4
S
/
o
A-
___:
1.6
_^
1?
CD
C
CD
O
m
__
o
F
-!
E
X
co
* \
2.0 •
|
3
• /
Aluminum-copper
0.8
0.4
\
^Aluminun
Aluminum-silicon
2
-
1
-
\
/
•
•
•
\
0.0
1
O
Solute, weight %
Fig. 5 - Schematic showing the hot cracking susceptibility of aluminumbased binary alloys as determined by the Varestraint weldability tests using 4% augumented strain (Ref. 34).
F u r t h e r m o r e , they also f o u n d the d e p t h of
p e n e t r a t i o n t o increase w i t h increasing
heat input, as is o b s e r v e d in o t h e r highstrength precipitation h a r d e n a b l e alumin u m alloys (Ref. 27).
Cross a n d c o w o r k e r s (Ref. 24) also
f o u n d the d e p t h of p e n e t r a t i o n t o increase w i t h lithium c o n t e n t . T h e increase
in p e n e t r a t i o n w a s rationalized as b e i n g
d u e t o a decrease in t h e r m a l c o n d u c t i v i t y
w i t h increase in lithium c o n t e n t . These researchers o b s e r v e d that t h e high-purity
binary alloys w e r e readily w e l d a b l e a n d
possessed g o o d resistance t o h o t tearing.
T h e h o t tearing susceptibility of an alumin u m alloy having 2.6% lithium w a s o b served t o be less than that o b s e r v e d w i t h
t h e w e l d a b l e Al-4.4% M g c o m m e r c i a l A l l o y 5083.
Fig. 7—Total crack
length within the weld
vs. augumented strain
(",,) for the alloys §
investigated (Ref. 34).
At a p p r o x i m a t e l y t h e same time, an A l 2.5Mg-0.6Li-0.55Mn alloy having excellent f o r m a b i l i t y , m o d e r a t e strength, g o o d
e l e v a t e d - t e m p e r a t u r e strength and f a v o r able w e l d a b i l i t y characteristics w a s d e v e l o p e d (Ref. 28). These researchers f o u n d
that lithium a d d i t i o n t o a l u m i n u m d e creased electrical c o n d u c t i v i t y (Table 1),
t h e r e b y , facilitating resistance or s p o t
w e l d i n g . O t h e r researchers also suggested
that lithium c o n t r i b u t e d t o ductility (Ref.
23). This intrinsic i m p r o v e m e n t was attribu t e d t o the fact that at these levels lithium
exists in solid solution, a n d n o t as c o h e r e n t
5' (Al 3 Li) precipitate. T h e 8' precipitate is
d e t r i m e n t a l t o ductility because it p r o m o t e s h e t e r o g e n e o u s d e f o r m a t i o n , that
is, planar slip (Refs. 10, 15, 2 9 - 3 3 ) . These
A l - M g - L i - M n alloys w e r e f o u n d t o b e
70
2014
60
50
40
o
<
cc
o
30
<
20
^
to
Weldalite
049
Commercial alloys
0.25
0.50
0.75
AUGMENTED STRAIN (%)
176-s|JULY 1991
'
3
L±
5&
J
Fig. 6 — The peak hot tearing susceptibility of high-purity Al-Li binary alloys occurring at 2.6 wt-% lithium (Ref. 24).
I
_£_
1
2
wt
1.00
attractive candidates for a u t o m o t i v e a p plications w h e r e spot w e l d i n g c o u l d serve
as a cost-effective m e t h o d o f joining.
A n o t h e r alloy d e v e l o p m e n t e f f o r t inv o l v e d the d e v e l o p m e n t of alloys c o n taining c o p p e r . T h e h o t cracking resistance
of
an
Al-XCu-1.3%Li-0.4%Ag0.14%Zr ( w h e r e X varies f r o m 4.5 t o
6.3%) alloy, later t o n a m e d W e l d a l i t e ®
049, w a s assessed b y the Varestraint test
(Ref. 34). In this study, t h e researchers
tested t h r e e variations of W e l d a l i t e ® 0 4 9
in o r d e r t o d e t e r m i n e the e f f e c t of c o p p e r
c o n t e n t in the range f r o m 4.8 t o 6.1 w t - %
o n t h e alloy's weldability. T h e results of
W e l d a l i t e ® 049 w e r e c o m p a r e d w i t h
those for c o n v e n t i o n a l a l u m i n u m - c o p p e r
Alloys 2219 a n d 2014 and the Al-Cu-Li A l loy 2090. A variation in the weldability o f
the W e l d a l i t e ® 0 4 9 - t y p e alloys as a
f u n c t i o n of c o p p e r level w a s o b s e r v e d ;
w i t h the 6.1 w t - % c o p p e r variant revealing l o w e r total crack length at all strain
levels than the 4.8% c o p p e r variant. The
i n t e r m e d i a t e variant c o n t a i n i n g 5.2% c o p per revealed greater total crack length
than the o t h e r t w o variants at l o w strain
a n d l o w e r t o t a l crack length at a high
s t r a i n - F i g . 7. T h e Al-Cu-Li alloys W e l d alite® 049 and 2090, exhibited cracking
parallel t o t h e w e l d b e a d at t h e w e l d - b a s e
metal interface. H o w e v e r , Alloys 2219
and 2014 exhibited cracking in the heataffected zone and within the w e l d metal.
These researchers also o b s e r v e d that s p o t
Varestraint tests o n ternary Al-Cu-Li alloys
revealed little or n o cracking but a significant a m o u n t of cracking in the heat-
_J
o
if
6
•T2090-T8E41
5
/
/
sz
4
k
S
o
E
P
fc
60
ZJ
50
O
H
/
/
3
E
E
40
x:
1
A
XT.20-T851
30
C
.CD
-*_
2
?0
O
/
/
1
I
CO
__X^^^
o
^*^r^
S^^^""~
^ ^ ~
X
CO
0
10
£
& * ^
0
._
"CO
n—XT130-TB5.
XT130-T85.
1
0
1
Augmented strain, %
Augmented strain, %
F;g. 8 - The weldability of XT alloys (gas tungsten arc, direct current, electrode negative), as determined by the Varestraint tests on 12.5 mm-thick X
37.5 mm-wide samples machined from extrusions and degreased. etched and welded (Ref. 26).
affected zone of Alloys 2219 and 2014 Table 2. The difference in hot cracking
susceptibility of the alloys was attributed
to factors involving: amount of interdendritic eutectic products, the distribution of
interdendritic eutectic products, and
freezing range of the alloys. Table 3 outlines the results of weld tensile strength
determinations of Weldalite® 049 by
Kramer and coworkers (Ref. 34). The
fracture of welded tensile specimens of
Weldalite® 049 almost always occurred in
the base metal/fusion zone interface. The
weldment strengths of Weldalite® 049
were found to be significantly higher than
values reported for Alloys 2219, 2014 and
2090-Table 4.
mercially available Al-Li alloys or conventional aluminum alloys at similar strength
levels. There were six such alloys that
were designed as XT 110, 120, 130, 140,
150 and 160. Although exact compositions are proprietory, lithium content
ranges from about 2% (XT110, 120 and
130) to 3.3% (XT 150, 160), the latter being the most rigid and lightest in weight.
Alloy weldability was evaluated by the
Varestraint test using augmented strains of
0.5, 1.0 and 4.0%. The test subjects the
weld pool to controlled amounts of strain
via instantaneous bending of a specimen
around a curved mandrel during welding.
The strain is augmented longitudinally
along the weld and its magnitude is determined by the radius of curvature. Total
crack length and the maximum crack
length were measured and plotted against
augmented strain to obtain comparative
Camalco's proprietory Al-Li alloys were
designed primarily to provide higher
stress-corrosion cracking (SCC) resistance
and impact resistance than either com-
weldabilities for the different alloys. The
Varestraint tests on extruded bars revealed the most weldable alloy to be XT
130, followed by XT 150, XT 110 and XT
140, with Alloy XT 120 being the least
weldable. These alloys showed improved
weldability over Alloy 2090-T8E41Fig. 8.
The weldability of commercial aluminum-lithium Alloys 2090, 2091, 8090 and
Weldalite® 049 were assessed on a TCL
contour map (Fig. 9) known as the Pumphrey-Moore diagram. Alloys high in either copper or magnesium solute content,
which are known to be readily weldable,
are situated near either axis: 2219 and
5083. Those alloys containing substantial
amounts of both elements, that is 2024
and 7075, are known to have poor weldability and are situated up the hill, toward
larger TCL's (Ref. 26). Of the four alumi-
Table 5—Compositions (wt-%) of AI-3 Li-X RS-P/M Alloys and Mechanical Properties of the Alloys before and after Welding'3*
loint
Efficiency
Base Metal
YS
AI-3
AI-3
AI-3
AI-3
AI-3
Alloy
Li
Li-0.2 Zr
Li-1 Co
Li-2 Mg-0.2 Zr
Li-2 Cu-0.5 Zr
MPa
312
418
-
444
507
Weld
Elong.
TS
(ksi)
(45)
(61)
-
(64)
(74)
MPa
388
481
-
556
569
(ksi)
(56)
(70)
-
(81)
(83)
{%)
5.7
8.2
-
7.9
5.4
YS
MPa
209
317
241
314
314
(%)
TS
(ksi)
(30)
(46)
(35)
(46)
(46)
MPa
272
365
298
340
355
Elong.
(ksi)
(39)
(53)
(43)
(49)
(52)
(%)
6.3
1.4
2.0
1.2
1.2
YS
67.0
75.8
-
70.7
61.9
TS
70.1
75.9
-
61.1
62.4
(a) All specimens were heat-treated to peak strength before tensile testing
Ref. 45.
WELDING RESEARCH SUPPLEMENT 1177-s
WELD *
150
3:
5356
FILLER
WELD
*• 5 : 0 1 4 2 0
FILLER
WELD
*
FILLER
100
50
5 5:
Contour lines:
• 5083
•
total crack length (TCL), in.
« •
».
100
g.
'a.
150-.
E
3
'tn
cu
c
7: 0 1 4 2 0
E
E
\
100
CT)
ca
Weldalite 049
2519*
2
3
4
5
:
2219
6
Copper, weight%
Fig. 9 - Total crack length contour map with aluminum-lithium
2090, 2091, 8090 and Weldalite^ 049 superimposed (Ref. 26).
n u m - l i t h i u m alloys, t h e o r d e r o f increasing
w e l d a b i l i t y (decreasing TCL) is 2 0 9 1 , 8 0 9 0 ,
2090, a n d W e l d a l i t e ® 049.
Gas Tungsten Arc Welding
Gas t u n g s t e n arc (GTA) w e l d i n g studies
o n the ternary AI-4.5Cu-1.21Li-0.51Mn0 . 2 C d alloy (designated as 2 0 2 0 by the
A l u m i n u m Association) d e v e l o p e d b y t h e
A l u m i n u m C o . of A m e r i c a (Alcoa) rev e a l e d t h e b u t t joints t o h a v e w e l d
strengths o f o n l y 230 M P a (33.4 ksi) in
c o m p a r i s o n w i t h base metal tensile
strength of 5 8 0 MPa (84.1 ksi) (Ref. 40).
H o w e v e r , heat t r e a t m e n t o f t h e w e l d ments w a s f o u n d t o restore t h e strength
o f the w e l d t o a b o u t 5 0 0 M P a . A n o t h e r
study w a s d i r e c t e d at w e l d i n g Alloy 2 0 2 0
that was p r o d u c e d by a rapid solidification
process a n d resulted in a large a m o u n t o f
p o r o s i t y w i t h the w e l d m e n t s e x p e r i e n c i n g
grain b o u n d a r y liquation at t h e fusion line
(Ref. 41). T h e p r o b l e m s associated w i t h
w e l d i n g the rapidly solidified AI-4.5Cu1.21Li-0.51Mn alloy w e r e n o t a t t r i b u t e d
t o alloy c o m p o s i t i o n , b u t m o r e t o p o w d e r
processing and degassing.
Rapid solidification processing in c o m bination w i t h p o w d e r metallurgy (PM)
processing techniques offers p o t e n t i a l i m p r o v e m e n t s in the p r o p e r t i e s of lithiumcontaining a l u m i n u m alloys (Refs. 4 2 - 4 4 ) .
T h e weldability o f a series of rapidly solidified, p o w d e r metallurgy processed b i nary a n d t e r n a r y a l u m i n u m alloys c o n t a i n -
1 7 8 - s | JULY 1 9 9 1
-2
-1
0
POSITION ALONG W E L D
Alloys
1
(cm)
Fig. 10- Vickers hardness profiles (using 2.5-g load) across transverse
cross-section of selected weldments in Al-5Mg-2Li-0.01Zr alloy 01420 in
the as-welded condition (Ref. 62).
ing 3 w t - % lithium w a s investigated b y
B o w d e n a n d M e s c h t e r (Ref. 45). In this
study, B o w d e n and M e s c h t e r f a b r i c a t e d
alloys f r o m v a c u u m - a t o m i z e d p o w d e r ,
and p e r f o r m e d w e l d i n g in t h e as-extruded
(i.e., F) t e m p e r . Electron b e a m b e a d - o n plate w e l d s w e r e m a d e using a b e a m current o f 125 m A , an accelerating voltage of
15 kV, and a w e l d i n g s p e e d of 0.18 m / s ,
at a v a c u u m o f less t h a n 1.3 x 10" 2 Pa. T h e
w e l d e d plates w e r e heat t r e a t e d t o peak
strength (solution heat t r e a t m e n t at 5 6 0 °
C / 1 0 4 0 0 F for one hour, water quenched
and artificially aged at 177° C / 3 5 1 ° F prior
t o tensile testing. Results of the tensile
tests p e r f o r m e d o n t h e a s - w e l d e d plates
w e r e c o m p a r e d w i t h similar tests perf o r m e d o n peak-strength plates that w e r e
n o t w e l d e d . T h e joint efficiencies r a n g e d
f r o m 6 0 t o 7 5 % of base material p r o p e r ties (Table 5). These researchers (Ref. 45)
rationalized t h e d e g r a d a t i o n in strength in
the w e l d z o n e t o be a result of destruction
of the substructure caused b y melting that
a c c o m p a n i e d w e l d i n g . Since substructure
c o n t r i b u t i o n s t o strength are significant in
t h e case o f P / M alloys, destruction o f the
substructure that results f r o m the t h e r m o mechanical processing used t o f o r m t h e
p r o d u c t directly results in a d e g r a d a t i o n in
strength. T h e essential o u t c o m e o f this
study b y B o w d e n a n d M e s c h t e r was that
t h e RS-PM Al-3 Li-X ( w h e r e X is the t e r n a r y
alloy addition) alloys are w e l d a b l e w i t h the
aluminum-lithium alloys containing zircon i u m exhibiting joint efficiencies o f o v e r
75%.
Several techniques of gas tungsten arc
(CTA) w e l d i n g u n d e r c o n d i t i o n s of m e chanical constraint w e r e used b y Pickens
and c o w o r k e r s t o evaluate the w e l d a b i l ity o f alloy W e l d a l i t e ® 049 a n d its variants
(Ref. 46). T h e quality of t h e w e l d m e n t s
w e r e assessed t h r o u g h their tensile p r o p erties. T h e tensile strengths of t h e G T A
w e l d m e n t s m a d e using 2319 a n d base
metal filler metal w e r e f o u n d t o be c o m parable t o c o n v e n t i o n a l a l u m i n u m alloys,
that is, 3 1 0 M P a (45 ksi) f o r 7075, a n d 225
MPa(32.6 ksi) f o r 2219. T h e tensile strength
of t h e W e l d a l i t e ® 049 w e l d m e n t s w a s
found t o be independent of copper c o n t e n t in the range o f 4.6 t o 6.3 w t - % , w i t h
l o w e r c o p p e r levels exhibiting higher d u c tility. Pickens a n d c o w o r k e r s (Ref. 46), in
their c o m p r e h e n s i v e study o n W e l d a l i t e ®
0 4 9 , also f o u n d t h e alloy t o h a v e g o o d
w e l d a b i l i t y w i t h an absence o f h o t cracking susceptibility despite the constraints
used during welding.
Weld Characteristics of the Soviet Alloy
01420
A survey o f the published literature o n
w e l d a b l e lithium-containing a l u m i n u m alloys reveals t h e Soviet Alloy 0 1 4 2 0
(AI-5Mg-2Li-X), w h e r e X d e n o t e s either
z i r c o n i u m or manganese, t o be t h e o n l y
lightweight aluminum-lithium alloy in w i d e spread c o m m e r c i a l use. Both the manganese-containing variant a n d t h e zirconi-
Table 6— Composition of Filler Metals Used to Weld Alloy 01420
Filler
Metal
01420
Sv AMg6
1557
Sv AMg5
Compo< ition (wt-%)
Mg
5.0
6.5
5.32
5.4
Mn
0.01
0.6
0.4
0.69
Ti
0.0015
0.10
Zr
0.11
Be
Cr
Li
-
0.21
0.0037
0.0021
0.0032
-
2.2
-
-
-
0.10
um-containing variant with ancillary alloying addition of titanium were designated
as 01420. The zirconium in the alloy is effective in refining grain size, and also improves corrosion resistance. Manganese
also refines the grain size and improves
the corrosion resistance of Alloy 01420
although not as effectively as zirconium.
Alloy 01420 was developed in 1967 by
Fridlyander (Ref. 47) and its density 2.47
g/cm 3 (Ref. 48) is lower than that of other
high-strength commercial aluminum alloys. The alloy displays a Young's modulus of about 73.5 GPa, compared with
about 71.7 GPa for the high-strength
Al-Zn-Mg alloys, and 70.3 GPa for highstrength Al-Mg alloys (Ref. 48). Alloy 01420
is widely used by the Soviets in airframe
and rocket construction (Refs. 48, 49).
Fridlyander and coworkers (Ref. 50)
claimed that use of aluminum-lithium alloy
01420 resulted in weight reduction of 11%
in compression-critical applications and
20% in stiffness-critical applications when
compared
with
conventional
highstrength Soviet aluminum alloys.
The fusion weldability of Alloy 01420
using various filler metals, the compositions of which are listed in Table 6 was
evaluated by Mironenko and coworkers
(Ref. 51). These researchers found the alloy to have extremely good weldability
when using argon-arc welding on 2-mm
(0.08 in.) thick sheet that was solutionized
at 450°C (842°F), air-cooled and aged to
—
—
0.09
—
peak strength. To eliminate porosity effects, the alloy surfaces were chemically
milled in 20 to 30% aqueous solution of
sodium hydroxide at 40° to 60°C (104° to
140°F) and followed by a cleansing etch in
a 30% nitric acid solution. They observed
all filler metals to contribute to the good
hot cracking resistance of the Alloy 01420.
They also found that filler metals containing various amounts of manganese, titanium, zirconium or chromium increased
grain refinement in the weld and consequently, provided stronger joints than
those produced by the base metal filler
metal (Alloy 01420). Based on their intensive study using a spectrum of filler metals, they concluded that Alloy 01420 had
good weldability.
Fridlyander and coworkers (Ref. 52) in
their study on the weldability of aluminum
Alloy 01420 obtained joint efficiencies
(i.e., strength of the weld divided by
strength of the base metal) of 70% without postweld heat treatment, and as high
as 99% after postweld heat treatment, i.e.,
resolutionizing, air-cooling and artificial
aging, joint efficiencies of 80% were obtained for gas tungsten arc welds that received no postweld heat treatment using
an AI-6.3Mg-0.5Mn-0.2Zr filler metal alloy.
The ability of Alloy 01420 to be airquenched from the welding temperature
and naturally aged to 80% of base metal
properties enabled it to be used in structures where resolutionizing, quenching
50
36
0.08
0.61
0.25
0.62
•
Q
Impurities
(Fe + Si)
(Fe + Si + Cu + Zn)
(Fe + Cu 4- Si)
(Fe 4- Si 4- Cu + Zn)
Al
Balance
Balance
Balance
Balance
and artificial aging were not practical.
However, numerous other studies using
conventional arc welding by Soviet researchers revealed weld-zone porosity to
be a key problem with Alloy 01420 (Refs.
52-54). The high porosity in the weld
zone was attributed to the surface reactivity of the alloy with ambient moisture
resulting in the formation of lithium containing compounds such as lithium-oxide
(Li0 2 ), lithium hydroxide (LiOH), lithium
carbonate (Li2CC>3) and lithium nitride
(LiN). In the case of an Al-Mg-Li alloy, Fridlyander found these phases to produce
absorption of ambient moisture on the
weld surface, resulting in hydrogen penetrating into the weld pool. To minimize
weld-zone porosity, prewelding surface
preparation was found to be essential
(Ref. 55).
The influence of surface preparation
techniques on weld-zone porosity was
also evaluated (Ref. 55). The techniques
used included:
1) Degreasing with organic solvents followed mechanical scraping.
2) Chemical pickling followed by cleaning
to remove reaction products.
3) Chemical pickling such that a passive
film developed.
4) Combination of chemical pickling and
mechanical scraping.
5) Mechanically milling up to 0.5 mm (0.02
in.) from the surface.
6) Chemical milling in an alkaline solution
1
1
Mechanically'Milled
Chemically Milled
34
40
Limit for commercial weldability
3_
>>
30
=
30
•-
__:
28
26
-X -a
-
20
5
10
.05
.10
.15
.20
.25
Surface Removal per Side (mm)
4047
4145
2319
Filler Alloys
4043
5356
Fig. 12— joint ultimate tensile strength of gas tungsten arc welds on
1.6mm thick 2090-T8E41 plate that was surface milled (Ref. 38).
Fig. 11 - Weld crack sensitivity of Alloy 2090-T8E41 with various filler
metal alloys (Ref. 38).
WELDING RESEARCH SUPPLEMENT 1179-s
tearing susceptibility of both gas tungsten
arc and electron beam (EB) welding was
examined. The vacuum heat treatment
was found to appreciably reduce weldzone porosity for both gas tungsten arc
and electron beam welding. However,
the material that had not been vacuum
heat treated was found to contain a continuous chain of pores in the fusion zone,
while the vacuum heat treated material
had pores which were small in size and
isolated in distribution. The vacuum heat
treatment was observed to: improve tensile strength and ductility (measured by
bend angle), have absolutely no influence
on impact toughness, and improve joint
efficiency to as high as 80%. Heat treatment in the temperature range of 450° to
500°C (842° to 932°F) and in a vacuum
environment not less than 1.33 Pa was
found to reduce weld-zone porosity due
to electron beam welding (Ref. 59). The
essential conclusion of this study was that
vacuum heat treatment is an effective
preweld preparation technique for minimizing weld-zone porosity in the Soviet
Alloy 01420.
Table 7—Typical Tensile Results, 2090-T8E41 Room-Temperature Tests, Specimens Machine
Flush
UTS
TYS
Elongation
Gauge
0.12
0.12
0.12
0.21
0.32
Process(es)
Condition
(aKb)
(ksi)
AW
AW
CTA
VPPA
VPPA
VPPA
VPPA
(36.2)
(34.9)
(62.1)
PWHT
AW
(36.6)
(36.5)
AW<C>
MPa
250
241
429
252
252
(ksi)
(23.6)
(22.0)
(47.6)
(21.4)
(21.7)
MPa
163
152
328
148
150
(%)
4.3
4.2
9.9
7.0
6.2
(a) AW = As-welded; PWHT - Solution H/T plus artificial age.
(b) AW Failures-weld metal; PWHT Failures - HAZ/BM.
(c) Horizontal position; others vertical- ER2319 filler metal.
of NaOH.
7) Vacuum degassing followed by mechanical scraping.
Materials whose surfaces were prepared
using techniques 1 to 4 contained numerous pores mostly at the fusion line, with
few pores at the center of the weld.
However, Feedoseev and coworkers
(Ref. 56) found that mechanical milling 0.5
mm (0.02 in.) from the alloy surface or
chemical milling 0.3 mm (0.01 in.) from the
surface in a 200 g/L alkaline aqueous solution prior to welding reduced weldzone porosity to acceptable levels. The
chemical milling technique was most practical since it could be performed on a wide
variety of shapes, that is, sheet and extrusion.
The weldability of Alloy 01420 using
GTA with argon shielding gas and Alloy
01420 filler metal was also examined (Ref.
57). In this independent study, small spher1.02
I
I
I
0.890
ical pores were observed in the weld
zone. The shiny internal surfaces of the
pores indicated the presence of gaseous
pockets. The porosity was rationalized as
being due to hydrogen, which was adsorbed onto the weld surface during the
process of solution heat treatment of the
alloy in an environment containing moisture. The adsorbed moisture was effectively removed by chemically milling in a
sodium hydroxide aqueous solution (Ref.
57). Alloy 01420 was observed to be susceptible to moisture adsorption at ambient temperature, resulting in weld porosity in material that was chemically milled
but stored prior to welding.
The use of vacuum heat treatment to
minimize weld zone porosity in Alloy
01420 was studied by Mironendo and coworkers (Ref. 58). The effect of vacuum
heat treatment prior to welding on weldzone porosity, weld strength and hot
I
I
•
Penetration
•
Width
95
_c
>
-
0.762
|
—
0.635
o
2
p
cn
co
0.254
-
_kN
CO
LU
1 1
90 — •
1
1
•
•
•
•
1
1
•
•
•
•
1
•
•
•
l
l
_
•
—
c
cn
<
x
85
CO
80
—
-
75
-
-
111
_£-
O
\«
—
"
_L
118
70
i
0
I
0.2
Upper Surface
•
0.122
-
I
I
I
I
152
186
220
254
TRAVERSE SPEED (mm/sec)
Fig. 13 —Effect of laser beam traverse speed on weld penetration
weld width (Ref. 65).
180-slJULY 1991
1
1 I 1
"2090-T8E41
co
__:
p
WELD WIDTH (mm)
WELD PENETRATION
S
Flash welding of Alloy 01420 using high
current densities and rapid deformation
velocities was also successfully conducted
(Ref. 60). The strength of the flash welds
was found to be a function of initial sheet
thickness. Joint efficiencies as high as 90%
were obtained for 2 to 4-mm (0.008 to
0.16-in.) thick sheet with no postweld heat
treatment. However, with increase in
sheet thickness, the joint efficiency de-
and
i
I
0.4
i
I
0.6
l
1
1
1
1.0
l
1
1.2
i
1.4
Lower Surface
Fig. 14 — Vickers hardness profile across transverse section of laser weld
fusion zone in the as-welded condition (Ref. 65).
10pm
Fig. 15—Microstructure of bead-on-plate welds (Ref. 65). A — Laser beam tra verse speed of 103 mm/s; B — laser beam tra verse speed of 203 mm/s.
graded and an efficiency of only 65% was
obtained for a 10-mm (0.4 in.) thick sheet.
Postweld heat treatment was observed to
improve both the strength and ductility of
the flash welds to those of the base metal.
Spot welding of Alloy 01420 was explored by Ryazantsev and coworkers
(Ref. 61). These researchers found that the
force required for successfully spot welding the 01420 alloy to be lower than that
for other high-strength aluminum alloys.
However, the time period of application
of the spot welding force was longer and
the alloy was found to be more forgiving
to transient changes in spot welding parameters than other aluminum alloys.
Other research efforts of the Soviet Alloy 01420 found it to be weldable and that
good joint efficiencies could be easily obtained provided adequate care was taken
during surface preparation prior to welding. The purpose of surface preparation
was to ensure removal of the hydrated
layer that formed on the surface.
Most recently, Pickens and coworkers
(Ref. 62) investigated the weldability of
the Soviet-developed AI-5%Mg-2%Li0.1 %Zr alloy using gas tungsten are welding (GTAW). They found the alloy to be
weldable with tensile strength joint efficiencies as high as 64% when using base
metal filler metal, and without postweld
heat treatment. With postweld heat treatment of the alloy, tensile strength joint efficiencies as high as 85% were obtained.
The susceptibility of the alloy to weldzone porosity was minimized to acceptable levels by either machining or chemical milling in a 30% sodium hydroxide
(NaOH) solution followed by rinsing in a
30% nitric acid solution. Hardness profiles
across the weldments were found to be
consistent with joint efficiency values,
with a decrease occurring in the weld
bead —Fig. 10. The softening of the weld
bead with respect to the base material
was attributed to: the effects of lithium
depletion in the weld bead when using
base metal filler metal, and microstructural
coarsening.
nated by chemically or mechanically milling the surface prior to welding. The joint
tensile strength was found to improve
with increasing surface removal —Fig. 12.
Hardness profiles of the heat-affected
zone of Alloy 2090-T8E41 revealed a
gradual decrease in hardness at the fusion
zone. This was rationalized as being due
to the modification in the heat-affected
zone caused by the dissolution of strength-
Table 8—Tensile Strength and Joint
Efficiency of Laser Beam Welded 2090-T8E41
Weldability of Alloy 2090
The potential fusion weldability of Alloy
2090-T8E41 (AI-2.2Li-2.7Cu-0.12Zr) using
gas metal arc (GMA), gas tungsten arc and
electron beam welding processes was examined by Martukanitz and coworkers
(Ref. 38). These researchers also evaluated the crack sensitivity of Alloy 2090 in
the maximum strength (T8E41) condition
using various filler metal alloys. The weld
crack sensitivity of the alloy is exemplified
in Fig. 11.
Filler Alloys 4047, 4145 and 2319 were
observed to produce the least amount of
weld cracking during testing. Alloy 2090
was found to be easily weldable with a
low sensitivity to weld solidification cracking. Weld porosity in the alloy was elimi-
Alloy
and
Temper
2090T8E41
(asreceived)
2090T8E41
(asreceived)
2090T8E41
(asreceived)
2090T8E41
Condition
Tensile
Strength
Ultimate
loint
Tensile EfficienStrength
cies
MPa (ksi)
<%)
Longitudinal
408 (59)
—
Transverse
302 (44)
-
No prior
225 (33)
surface
preparation
55.10
Prior surface
335 (49)
preparation
82.10
WELDING RESEARCH SUPPLEMENT 1181-s
In recent years, few researchers have
attempted to evaluate the weldability of
Alloy 2090 in the T8E41 condition using
laser beam (Refs. 64-67). It is believed that
this particular aluminum alloy will show a
greater potential for laser beam welding
than most other commercial grades due
to the relatively low thermal conductivity
of this alloy (Ref. 67). It is known that lithium additions to aluminum decreases the
thermal conductivity of the material (Ref.
68).
Fig. 16 - Optical micrograph showing coarse
(Ref. 65).
ening phases, which contributed directly
to the degradation in strength. The extent
of degradation was related to the amount
of heat input used to produce the weld.
Lower heat inputs achieved through electron beam welding were found to produce joints having higher strength.
The weldability of Alloy 2090 by the
variable polarity plasma arc (VPPA) welding technique was examined by Criffee
and coworkers (Ref. 63). Alloys 2090 and
8090 sheet were welded using a direct
current 300-A variable polarity power
supply and a square-groove configuration. Welding was performed in both the
horizontal and vertical positions. Fracture
of the as-welded 2090 specimens was
found to consistently occur in the weld
metal. In their study, Criffee and coworkers (Ref. 63) observed that postweld
Fig. 17 -Optical
micrograph showing
pores at the weld/base
metal interface (Ref. 65).
182-s | JULY 1991
Laser beam welding has shown great
promise as a conventional welding technique since the development of multikilowatt continuous wave carbon dioxide lasers around 1970. A primary factor in establishing its potential for welding is the
laser's ability to generate power densities
greater than 106 W / c m 2 , whereas, conmicrostructures resulting from gas metal arc welding ventional arc welding processes achieve
only 102 to 104 W / c m 2 . In addition, the
heat source can be focused to spot sizes
on the order of 0.76 mm (0.03 in.) in
heat treatment to a T62 heat treatment
diameter producing welds with extremely
temper doubled the ultimate tensile
small heat-affected zones (HAZ). Earlier
strength and yield strength — Table 7, with
studies on aluminum alloys have revealed:
failure occurring at the heat-affected
high thermal diffusivity, incident beam rezone/base metal region. Both visual and
flection, and porosity as problems or obpenetrant tests revealed that with the exstacles to achieving acceptable aluminum
ception of few crater cracks, Alloy 2090
alloy laser beam welds (Refs. 69, 70).
welds revealed an absence of cracks in
Thermal diffusivity is a measure of the
either the weld metal or the heat-affected
material's ability to conduct heat. For aluzone. However, radiography tests deminum and its alloys, the thermal diffusivtected the presence of fine and coarse
ity is high, and consequently, the energy
pores along the fusion line. The results on
imparted by the incident beam is diffused
Alloy 8090-T851 were found to resemble
throughout the workpiece resulting in inthose for the 2090 weldments with failure
efficient use of energy and reduced weldoccurring in the heat-affected zone. This
ability. At ambient room temperature, the
study also revealed that the lightweight
thermal diffusivity of aluminum and its alaluminum-lithium Alloys 2090 and 8090
loys is approximately 0.90 cm 2 /s, and at its
could be welded effectively using the
melting temperature it is 0.80 cm 2 /s. Howvariable polarity plasma arc (VPPA) proever, a weldable metal such as Type 304
stainless steel has a thermal diffusivity of
0.04 cm 2 /s at both ambient temperature
and at its melting temperature (Ref. 69).
The thermal diffusivity of Alloy 2090 is
30% less than that of most other commercial aluminum alloys (Ref. 71). All
known metallic additions to aluminum reduce its thermal diffusivity to some extent.
Metals in solid solution will decrease diffusivity to a greater extent than when out
of solution. The intrinsic problem of incident beam reflection in aluminum alloys is
less severe than the problem of high thermal diffusivity, and can be overcome by
careful control of the laser welding parameters and process. In the conventional
methods of welding aluminum alloys, such
as GMA or GTA welding, filler metals are
used. These filler metals may be responsible for hydrogen-reiated porosity. Also,
while electron beam welding (EBW) offers
the advantages of a high energy density
welding process, in most cases, a vacuum
is desirable, which is not always practical.
Lasers provide an opportunity for welding
in a controlled atmosphere and offer the
advantage of localized heating coupled
with rapid cooling. This results in a small
heat-affected zone (HAZ) and a fine fusion
zone microstructure. An advantage of laser beam welding is that it can be accomplished without the use of a filler metal,
thereby, reducing the probability of porosity-related defects. However, for alloys
that display hot cracking sensitivity during
laser beam welding, it is both essential and
advantageous to use filler metal.
Molian and Srivatsan (Refs. 64-66) evaluated the weldability of Alloy 2090-T8E41
using a carbon dioxide (CO2) laser beam.
Weld penetration and weld depth were
found to decrease exponentially with an
increase in laser beam traverse speed (Fig.
13) indicating a transition from deep penetration-type (keyhole) to conductiontype weld. The amount of porosity was
observed to be unaffected by the depth
of penetration and the heat-affected zone
was found to be negligible. Microhardness of the welds were found to be in the
range 85 to 90 k g / m m 2 (Fig. 14) irrespective of the traverse speed of the laser
beam, while the hardness of the base
metal was 140 k g / m m 2 and the hardness
of the heat-affected zone was 110 k g /
mm 2 . The hardness values were found to
be consistent and unaffected by the presence of various microstructural features
resulting from welding. Optical microscopy of the fusion zone of the laser beam,
bead-on-plate welds revealed cellular
structures, with the presence of fine precipitates or particles inside the cells —Fig.
15. The fineness of the cell structure was
found to decrease as the distance from
the surface into the bulk material of the
fusion zone increased. Extremely fine and
almost featureless structures were observed as the laser beam traverse speed
was increased. In contrast to the fine microstructures resulting from laser beam
welding, CMA welding resulted in coarse
microstructures —Fig. 16.
The tensile strength of the as-welded
2090-T8E41 metal was lower than that of
the as-received, unwelded metal. The
lower tensile strength of the as-welded
metal was attributed to the presence of
pores, on the as-polished surface, at the
weld/base metal interface —Fig. 17. The
pores in the laser welds are thought to be
caused by the vaporization of volatile alloying elements such as lithium. Lithium
vaporizes at relatively low temperatures
of 1330°C (2426°F). Another possible
cause for the presence of pores in laser
beam welding is the entrapment of ambient gases absorbed during the welding
process. The tensile strength of the butt
joint weld with prior surface preparation
(335 MPa/48.6 ksi) was found to be better than the tensile strength of the weld
with no prior surface preparation (225
MPa)-Table 8. Figure 18 highlights a
comparison of joint ultimate tensile
strength between laser beam welding and
other welding procedures used for this al-
2090
|Lo"jiw"m I
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Fig. 18-Joint ultimate
tensile strength as a
function of weld
condition (Ref. 66).
CONDITIONS
loy. The improved tensile strength and
joint efficiency were attributed to the
small heat-affected zone and the fine microstructures in the weld zone.
Marsico and Kossowsky (Ref. 67) in
their study on laser beam welding of Alloy
2090-T8E41 found no cracking to occur in
the weld metal or in the heat-affected
zone. In high-strength aluminum alloys
weld cracking can occur as a result of:
aluminum's relatively high thermal expansion, large change in volume upon solidification, and wide solidification temperature range. High heat inputs through high
incident power and/or slow welding
speeds coupled with the use of preheating can contribute to weld cracking. These
researchers also found that processes that
characteristically employ relatively low
heat inputs such as laser beam welding
reduce or minimize the risk of weld cracking. Furthermore, since Alloy 2090 has a
narrow solidification range (560°-649°
C/1040°-1200° F) weld cracking was absent. During laser beam welding of Alloy
2090, the temperature in the heat-affected zone is high enough to cause dissolution of the major metastable strengthening precipitate (8', 8' and T-|). This
resulted in softening of the weld zone
with concomitant degradation in strength.
The weld heat-affected zone hot cracking susceptibility of Alloy 2091 (AI-2.3U2Cu-1.3Mg) has also been examined (Ref.
72). In this study, the Cleeble technique
was used and the strength and ductility of
the alloy were compared with commercial aluminum Alloy 2024 (Al-4Cu-1.4Mg).
It was found by Zacharia and Aidun that
the 2091 alloy was (Ref. 72): crack resistant in the weld metal heat-affected zone,
and exhibited excellent recovery of ductility over a range of temperatures. The
resistance of 2091 alloy to hot cracking in
the weld metal heat-affected zone was
rationalized based on the improved ductility of the alloy at temperatures close to
the solidus temperature.
Conclusion
A survey of the published literature on
weldability of lightweight aluminum-lith-
ium alloys reveals that the binary and ternary alloys are weldable by gas tungsten
arc (CTA), electron beam [EB], gas metal
arc (GMA), resistance, flash, spot, variable
polarity plasma arc and laser beam welding. Research efforts on the Soviet Alloy
01420 (Al-Li-Cu-Mg) have demonstrated
attractive welding characteristics for both
electron beam and gas tungsten arc welding. Surface preparation prior to welding
minimizes weld zone porosity, and
thereby, improved the quality and properties of the weld. Varying joint efficiencies have been obtained by the various
investigators with joint efficiencies as high
as 70 to 75% without postweld heat
treatment. However, the joint efficiency
improved to 90 to 99% following postweld heat treatment. Commercial Alloys
2090 and 2091 (Al-Li-Cu-Zr) and 8090 (AlLi-Cu-Mg) were also found to be weldable
by electron beam and gas tungsten arc
welding techniques. Surface preparation
prior to welding improved joint efficiencies to as high as 85% of base metal tensile properties.
In laser beam welding these lightweight,
high-modulus alloys, the heating and cooling rates are relatively high when compared to conventional welding processes.
As a result, the heat-affected zones in laser beam welds are dramatically smaller
than in welds made using conventional
techniques. Also, laser beam welds provide: a precise control of heat input
resulting in minimal damage to the material, and respond well to postweld heat
treatment.
Recommendations for Future
Research
All of the results summarized in this paper suggest that aluminum-lithium alloys
are weldable by different techniques with
surface preparation prior to welding and
postweld heat treatment improving both
the quality and performance of the welds.
However, the primary problem observed
in all of the above research efforts is the
restricted nature of the studies. If broad
generalizations are to be formulated for
the aluminum-lithium family of alloys, a
WELDING RESEARCH SUPPLEMENT 1183-s
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detailed characterization of each alloy
designation would become necessary.
Such studies would involve a careful evaluation of parameters such as the influence
of type of welding, weld speed, filler materials and their subsequent evaluation for
properties such as tensile strengths or
hardness and characterization of the heataffected zone and microstructure. While
much of the above information may be
available in company reports or in unpublished literature, an urgent need exists for
the dissemination of the above information so that the use of different aluminumlithium alloys in various welding applications can be readily expanded.
Acknowledgments
One of the authors, T. S. Srivatsan,
would like to acknowledge the Ohio
Board of Regents for partial support of
work carried out during this study, under
Grant No. OBR 5-34021. Thanks and appreciation are extended to the unknown
reviewers whose comments and suggestions have helped strengthen the manuscript.
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184-s|JULY 1991
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Pergamon Press, pp.1025-1039.
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WRC Bulletin 357
September 1990
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Calculation of Electrical and Thermal Conductivities of Metallurgical Plasmas
By G. J. Dunn and T. W. Eagar
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There has been increasing interest in modeling arc welding processes and other metallurgical processes
involving plasmas. In many cases, the published properties of pure argon or helium gases are used in calculations of transport phenomena in the arc. Since a welding arc contains significant quantities of metal
vapor, and this vapor has a considerably lower ionization potential than the inert gases, the assumption
of pure inert gas properties may lead to considerable error. A simple m e t h o d for calculating the electrical
and t h e r m a l conductivities of m u l t i c o m p o n e n t plasmas is presented in this Bulletin.
Publication of this report was sponsored by the Welding Research Council. The price of WRC Bulletin
357 is $20.00 per copy, plus $5.00 for U.S. or $10.00 for overseas postage and handling. Orders should
be sent with payment to the Welding Research Council, 345 E. 47th St., New York, NY 10017.
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February 1991
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This Bulletin contains two reports that c o m p a r e the French RCC-M Pressure Vessel Code and the U.S.
ASME Section III Code on Design of Nuclear Components and Piping Design Rules.
( 1 ) Improvements on Fatigue Analysis Methods for the Design of Nuclear Components
Subjected to the French RCC-M Code
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( 2 ) Framatome View on the Comparison between Class 1 and Class 2 RCC-M Piping
Design Rules
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By C. Heng and J. M. Grandemange
Publication of this report was sponsored by the Pressure Vessel Research Council of the Welding Research Council. The price of WRC Bulletin 3 6 1 is $30.00 per copy, plus $5.00 for U.S. or $10 for overseas
postage and handling. Orders should be sent with payment to the Welding Research Council, 345 E. 4 7 t h
St., Room 1 3 0 1 , New York, NY 10017.
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WELDING RESEARCH SUPPLEMENT 1185-s
WRC Bulletin 345
July 1989
Assessing Fracture Toughness and Cracking Susceptibility of Steel Weldments—A Review
By J. A. Davidson, P. J. Konkol and J. F. Sovak
The literature survey reviews the domestic and foreign literature to determine, document and evaluate: 1) the parameters of welding that control weld-metal and HAZ cracking; 2) tests for assessing the
susceptibility of structural steel to weld-metal and HAZ cracking; 3) the parameters of welding t h a t control HAZ toughness; and 4) tests for measuring the toughness of weld metal and HAZ. The work was perf o r m e d at the United States Steel Corporation Technical Center in Monroeville, Pa., and was sponsored
by the Offices of Research and Development, Federal Highway Administration, U.S. Department of
Transportation, Washington, D.C.
Publication of this bulletin was sponsored by the Weldability C o m m i t t e e of the Welding Research
Council. The price of WRC Bulletin 345 is $30.00 per copy, plus $5.00 for U.S. and $10.00 for overseas,
postage and handling. Orders should be sent with payment to the Welding Research Council, Room 1 3 0 1 .
345 E. 4 7 t h St., New York, NY 10017.
WRC Bulletin 347
September 1989
This bulletin contains two reports:
( 1 ) Welded Tees Connections of Pipes Exposed to Slowly Increasing Internal Pressure
By J. Schroeder
( 2 ) Flawed Pipes and Branch Connections Exposed to Pressure Pulses and Shock
Waves
By J. Schroeder
Publication of these reports was sponsored by the Subcommittee on Reinforced Openings and External Loadings of the Pressure Vessel Research Committee of the Welding Research Council. The price of
WRC Bulletin 347 is $25.00 per copy, plus $5.00 for U.S. and $10.00 for overseas, postage and handling.
Orders should be sent with payment to the Welding Research Council. Room 1 3 0 1 . 345 E. 47th St.. New
York, NY 10017.
WRC Bulletin 356
August 1990
This Bulletin contains three reports involving welding research. The titles describe the contents of the
reports.
( 1 ) Finite Element Modeling of a Single-Pass Weld
By C. K. Leung, R. J. Pick and D. H. B. Mok
( 2 ) Finite Element Analysis of Multipass Welds
By C. K. Leung and R. J. Pick
( 3 ) Thermal and Mechanical Simulations of Resistance Spot Welding
By S. D. Sheppard
Publication of the papers in this Bulletin was sponsored by the Welding Research Council. The price of
WRC Bulletin 356 is $35.00 per copy, plus $5.00 for U.S. and $10.00 for overseas postage and handling.
Orders should be sent with payment to the Welding Research Council, 345 E. 4 7 t h St., Room 1 3 0 1 , New
York, NY 10017.
186-s | JULY 1991