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Transcript of 8. Welding of Aluminiumdantn/WT/WT2-c8.pdf · 8. Welding of Aluminium 99 Figure 8.6 shows typical...
8.
Welding of Aluminium
8. Welding of Aluminium 97
Figure 8.1 compares basic physical properties
of steel and aluminium. Side by side with dif-
ferent mechanical behaviour, the following
differences are important for aluminium weld-
ing:
- considerably lower melting point compared
with steel
- three times higher heat conductivity
- considerably lower electrical resistance
- double expansion coefficient
- melting point of Al203 considerably higher
than that of Al; metal and iron oxide melt ap-
proximately at the same temperature.
Figure 8.2 compares some mechanical prop-
erties of steel with properties of some light
metals. The important advantages of light
metals compared with steel are especially
shown in the right part of the figure. If a comparison should be based on an identical stiff-
ness, then the aluminium supporting beam has a 1.44 times larger cross-section than the
steel beam, however only about 50% of its weight.
Figure 8.3 compares quali-
tatively the stress-strain dia-
gram of Aluminium and
steel. In contrast to steel,
aluminium has a fcc (face
centred cubic)-lattice at
room temperature. This is
why there is no distinct yield
point as being the case in a
bcc (body centred cubic)-
lattice. Aluminium is not
subject to a lattice trans-
Deflexions and Weights of Cantilever Beams Under Load
br-er-08-02.cdr
Figure 8.2
Property Al Fe
Atomic weight [g/Mol] 26.9 55.84
Specific weight [g/cm³] 2.7 7.87
Lattice fcc bcc
E-module [N/mm²] 71*10³ 210*10³
R PO,2 [N/mm²] ca. 10 ca. 100
R m [N/mm²] ca. 50 ca. 200
spec. Heat capacity [J/(g*°C)] 0.88 0.53
Melting point [°C] 660 1539
Heat conductivity [W/(cm*K)] 2.3 0.75
Spec. el. Resistance [nWm] 28-29 97
Expansion coeff. [1/°C] 24*10-6
12*10-6
FeO
Oxydes Al2O3 Fe3O4
Fe2O3
1400
Melting point of oxydes [°C] 2050 1600
(1455)
Basic Properties of Al and Fe
pO,2
m
© ISF 2002br-er08-01.cdr
Figure 8.1
8. Welding of Aluminium 98
formation during cooling, thus there is no structure transformation and consequently no
danger of hardening in the heat affected zone as with steel.
Figure 8.4 illustrates the effect of the considerably higher heat conductivity on the welding
process compared with steel. With aluminium, the temperature gradient around the welding
point is considerably smaller than with steel. Although the peak temperature during Al weld-
ing is about 900°C below steel, the isothermal curves around the welding point have a clearly
larger extension. This is due to the considerably higher heat conductivity of aluminium com-
pared with steel.
This special characteristic of Al requires a input heat volume during welding equivalent to
steel.
Figure 8.5 lists the most important alloy elements and their combinations for industrial use.
Due to their behaviour during heat treatment can Al-alloys be divided into the groups harde-
nable and non-hardenable (naturally hard) alloys.
Comparison of Stress-ElongationDiagrams of Al and Steel
Elongation
Al-alloy
Steel
Str
ess
© ISF 2002br-er08-03.cdr
Figure 8.3
Isothermal Curves of Steel and Al
4
2
-2
-4
4
2
-2
-4
low carbon steel
aluminium
400
600
200°C
800
10001200
1500
-6
-8
cm
8
6
-18 -16 -14 -12 -10 -8 -6 -4 -2 0 2 cm 6
500
600
400300
200
100°Ccm
© ISF 2002br-er08-04.cdr
Figure 8.4
8. Welding of Aluminium 99
Figure 8.6 shows typical applications of some
Al alloys together with preferably used weld-
ing consumables.
Aluminium alloys are often welded with con-
sumable of the same type, however, quite
often over-alloyed consumables are used to
compensate burn-off losses (especially with
Mg and Zn because of their low boiling point)
and to improve the mechanical properties of
the seam.
The classification of Al alloys into two groups
is based on the characteristic that the group
of the non-hardenable alloys cannot increase
the strength through heat treatment, in con-
trast to hardenable alloys which have such a
potential.
The important hardening mechanism for this
second group is explained by the figures 8.7 und 8.8. Example: If an alloy containing about
4.2% Cu, which is stable at room temperature, is heat treated at 500°C, then, after a suffi-
ciently long time, there will be only a single phase structure present. All alloy elements were
dissolved, Figure 8.8 between point P and Q.
When quenched to room
temperature in this condi-
tion, no precipitation will
take place. The alloy ele-
ments are forced to remain
dissolved, the crystal is out
of equilibrium. If such a
structure is subjected to an
age hardening at room or
elevated temperature, a
precipitation of a second
phase takes place in ac-
Classification of Aluminium Alloys
67
86
78
Mg
Si
Mn
Cu
ZnAl
Al Cu Mg
Al Mg Si
Al Zn Mg
Al Zn Mg Cu
Al Si Cu
Al Si
Al Mg
Al Mg Mn
Al Mn
non-h
ard
enable
allo
ys
hard
enable
allo
ys
© ISF 2002br-er08-05.cdr
Figure 8.5
Use and Welding Consumablesof Aluminium Alloys
Al - alloys Typical use W elding consumable
Al99,5electrical engineering
SG-Al 99,5Ti;
SG-Al 99,5
AlCuMg1 mechanical engineering, food
industriesSG-AlMg4,5Mn
AlMgSi0,5 architecture, electrical
engineering, anodizing quality
SG-AlMg5; SG-AlMg4,5Mn;
SG-AlSi5
AlSi5 architecture, anodizing quality SG-AlSi5
AlMg3 architecture, apparatus-, vehicle-,
shipbuilding engineering, furniture
industry
SG-AlMg3;
SG-AlMg4,5Mn
AlMg2Mn0,8 apparatus-, vehicle-, shipbuilding
engineering
SG-AlMg5; SG-AlMg3;
SG-AlMg4,5Mn
AlMn1 apparatus-, vehicle-engineering,
food industrySG-AlMn1;SG-Al99,5T
base material - aluminium
percentage of alloy elements without factor
© ISF 2002br-er-08-06.cdr
Figure 8.6
8. Welding of Aluminium 100
cordance with the binary system, the crystal tries to get back into thermodynamical equilib-
rium.
Depending on the level of
hardening temperature, the
precipitation takes place in
three possible forms: co-
herent particles (i.e. parti-
cles deviating from the
matrix in their chemical
composition but having the
same lattice structure),
partly coherent particles
(i.e. the lattice structure of
the matrix is partly re-
tained), and incoherent
particles (lattice structure completely different from the matrix), Figure 8.7. Coherent particles
formed at room temperature can be transformed into incoherent particles by increase of tem-
perature (i.e. enabling diffusion).
The precipitations cause a restriction to the
dislocation movement in the matrix lattice, thus
leading to an increase in strength. The finer the
precipitations, the stronger the effect.
At an increased temperature (heat ageing, Fig-
ure 8.7) a maximum of second phase has pre-
cipitated after elapse of a certain time.
Consequently a prolonged stop at this tem-
perature does not lead to an increased
strength, but to coarsening of particles due to
diffusion processes and to a decrease in
strength (less bigger particles in an extended
space).
Ageing Mechanism
solution heat treatment
quenching
ageing at slightly
increased temperature
coherent
precipitations,
cold aged
condition
temperature
rise
temperature
riselonger warm
ageing
longer warm
ageing
partly coherent
precipitations,
warm aged
condition
partly coherent
and incoherent
precipitations,
softening
stable incoherent
equilibrium phase
stable condition
stable condition
solidification of alloy elements
in solid solution
oversaturated solid solution,
metastable condition
coherent and partly coherent
precipitations, transition conditions
cold ageing -- warm ageing
repeated hardening
regeneration
cold ageing (RT ageing)
warm ageing
© ISF 2002br-er-08-07.cdr
Figure 8.7
Phase Diagram Al-Cu
700
600
500
400
300
200
100
0 1 2 3 4 5 mass-% 7
Q
P
liquid
liquid and solid
copper containingaluminium solid solution
aluminium solid solutionand copper aluminide(Al Cu)2
copper content ofAlCuMg
Copper
Te
mp
era
ture
© ISF 2002br-er08-08.cdr
Figure 8.8
8. Welding of Aluminium 101
After a very long heat ageing a stable condi-
tion is reached again with relatively large pre-
cipitations of the second phase in the matrix.
In Figure 8.7 is this stable final condition iden-
tical with the starting condition. A deteriorati-
on of mechanical properties only happens
during hot ageing, if the ageing time is exces-
sively long.
The complete process of hardening at room
temperature is metallographic also called age
hardening, at elevated temperature heat age-
ing. A decrease in strength at too long ageing
time is called over-ageing.
Figure 8.9 shows a schematic representation
of time-temperature curves during hardening
with age hardening and heat ageing.
Figure 8.10 shows the
strength increase of AlZnMg
1 in dependence of time.
The difference between age
hardening and heat ageing
is here very clear. Due to
improved diffusion condi-
tions is the strength increase
in the case of heat ageing
much faster than in the case
of age hardening. The
strength maximum is also
reached considerably ear-
lier. The curve of hot ageing shows clearly the begin of strength loss when held at a too long
stoppage time. This figure shows another specialty of the process of ageing. During ageing, a
Temperature - Time DistributionDuring Ageing
solution heat treatment
quenching
heat ageing
age hardening
2 4 6 8 10 12 14h
500
°C
400
300
200
100
0
Q
P
Te
mp
era
ture
Time
© ISF 2002br-er08-09.cdr
Figure 8.9
Increase of Yield Stress DuringAgeing of AlZnMg1
quenched Ageing time in h
0.2
% y
ield
str
ess
in N
/mm
²s
0.2
water quenching (~900°C/min)air cooling (~30°C/min)
10-1
100
101
10² 10³
380
320
260
200
140
80
120°C
RT
© ISF 2002br-er-08-10.cdr
Figure 8.10
8. Welding of Aluminium 102
second phase is precipitated from a single-phase structure. To initiate this process, the struc-
ture must contain nuclei of the second phase. However, a certain time is required to develop
such nuclei. Only after formation of nuclei can the increase in strength start. The period up to
this point is called incubation time.
Figure 8.11 shows the effect of the height of
ageing temperature level on both, mechanical
properties of a hardenable Al-alloy and on in-
cubation time. The lower the ageing tempera-
ture, the higher the resulting values of yield
stress and tensile strength. If a low ageing tem-
perature is selected, the ageing time as well as
the incubation time become extremely long.
Figure 8.11 shows that a the maximum yield
stress is reached after a period of about one
year under a temperature of 110°C. An in-
crease of the ageing temperature shortens the
duration of the complete precipitation process
by a certain value raised by 1 to a power. On
the other hand, such an acceleration of ageing
leads to a lowering of the
maximum strength. As the
lower part of the figure
shows, the fracture elonga-
tion is counter-proportional
to the strength values, i.e.
the strength increase
caused by ageing is ac-
companied by an embrit-
tlement of the material.
Influence of Ageing Temperatureand -Time on Ageing
260
0 10-2
10-1
100
101
102
103
h 104
190180
150 135
110°C
230260
30min
1day
30
20
10
Fra
ctu
re e
lon
ga
tio
nd
20
.2%
yie
ld s
tre
ss
s0.2
400
300
200
110
135
180
Te
nsile
str
en
gth
sB
110
135
150
180
230
500
400
300
200
Ageing time
%
N/mm²
N/mm²
205
260°C
190
150
190205°C
230
205
1week
1month
1year
© ISF 2002br-er08-11.cdr
Figure 8.11
Age Hardening of Al Alloys
0 30 70
100
200
300
400
N/mm²
% Strain
0
Te
nsile
str
en
gth
Rm
AlMg5
AlMg3
Al99,5
© ISF 2002br-er-08-12.cdr
Figure 8.12
8. Welding of Aluminium 103
Figure 8.12 shows a method of how to increase the strength of non-hardenable alloys. As no
precipitations are present to reduce the movement of dislocations, such alloys can only be
strengthened by cold working.
Figure 8.12 illustrates two essential mecha-
nisms of strength increase of such alloys. On
one hand, tensile strength increases with in-
creasing content of alloy elements (solid solu-
tion strengthening), on the other hand, this
increase is caused by a stronger deformation
of the lattice.
Figure 8.13 shows the effect of the welding
process on mechanical properties of a cold-
worked alloy. Due to the heat input during
welding, the blocked dislocations are released
(recovery), in addition, a grain coarsening will
start in the HAZ. This is followed by a strong
drop in yield point and tensile strength. This
strength loss cannot be overcome in the case
of a welding process.
Figure 8.14 illustrates the
mechanisms in the case of a
hardenable aluminium alloy.
As a consequence of the
welding heat, the precipita-
tions are solution heat treated
and the strength values de-
crease in the weld area. Due
to the age hardening, a re-
strengthening of the alloys
takes place with increasing
time.
Non-Hardenable Al Alloy
Distance from Seam Centre
HV
30
80 60 40 20 0 20 40 60 mm 100
0,7
0,6
0,5
0,4
0,3
0,20
50
100
150
200
250
300
N/mm²
R/R
p0
,2m
Ror
Rm
p0
,2
© ISF 2002br-er08-13.cdr
Figure 8.13
Hardenable Al Alloy
4 mm plates of: AlZnMg1F32start values: R =263N/mm²
R =363 N/mm²
welding method: WIG, both sides,simultaneously
welding consumable: S-AlMg5specimens with machinedweld bead
p0,2
m
Distance from seam centre
Str
ess
90 days RT
21 days RT
1 day RT
80
400
N/mm²
350
300
250
200
150
100
5080 60 40 20 0 20 40 60 100 mm 140
90 days RT
1 day RT
21 days RT
Rp0,2
Rm
© ISF 2002br-er-08-14.cdr
Figure 8.14
8. Welding of Aluminium 104
Figure 8.15 shows another
problematic nature of Al-
welding. Due to the high
thermal expansion of alu-
minium, high tensions de-
velop during solidification
of the weld pool in the
course of the welding cy-
cle. If the welded alloy indi-
cates a high melting
interval, cracks may easily
develop in the weld.
A relief can be afforded by
preheating of the material, Figure 8.16. With an increasing preheat temperature, the amount
of fractured welds decreases. The different behaviour of the three displayed alloys can be
explained using the right
part of the figure. One can
see that the manganese
content influences signifi-
cantly the hot crack suscep-
tibility. The maximum of this
hot crack susceptibility is
likely with about 1% Mg con-
tent (corresponds with alloy
1). With increasing MG con-
tent, hot crack susceptibility
decreases strongly (see also
alloy 2 and 3, left part).
To avoid hot cracking, partly very different preheat temperatures are recommended for the
alloys. Zschötge proposed a calculation method which compares the heat conductivity condi-
tions of the Al alloy with those of a carbon steel with 0.2% C. The formula is shown in Figure
Hot Cracks in a Al Weld
© ISF 2002br-er-08-15.cdr
Figure 8.15
Influence of Preheat Temperatureand Magnesium Content
1: AlMgMn 2: AlMg 2,5 3: AlMg 3,5
Preheat temperature
Weld
cra
ckin
g tendency
Cra
ckin
g s
usceptib
ility
Alloy content
X
X
X
X
100
80
60
40
20
%
0 100 200 300 400 °C 500
1
2
3
0 21 3 % 4
Si
Mg
© ISF 2002br-er-08-16.cdr
Figure 8.16
8. Welding of Aluminium 105
8.17, together with the related calculation result. These results are only to be regarded as
approximate, the individual application is subject to the information of the manufacturer.
Another major problem
during Al welding is the
strong porosity of the
welded joint. It is based on
the interplay of several
characteristics and hard to
suppress.
Pores in Al are mostly
formed by hydrogen,
which is driven out of the
weld pool during solidifica-
tion. Solubility of hydrogen
in aluminium changes
abruptly on the phase
transition melt-crystal, i.e.
the melt dissolves many
times more of the hydro-
gen than the just forming
crystal at the same tem-
perature. This leads to a
surplus of hydrogen in the
melt due to the crystallisa-
tion during solidification.
This surplus precipitates in
form of a gas bubble at the
solidifying front. As the
melting point of Al is very
low and Al has a very high heat conductivity, the solidification speed of Al is relatively high.
As a result, in the melt ousted gas bubbles have often no chance to rise all the way to the
surface. Instead, they are passed by the solidifying front and remain in the weld metal as
pores, Figure 8.18.
Recommendations for Preheating
Welding possible without preheating:AlMg5, AlMg7, AlMg4.5Mn,AlZnMg3, AlZnMg1
T in °C temperature of melt start (solidus temperature)
in J/cm*s*K heat conductivity
S
Al-Leg.
T in °C preheat temperaturevorw.
l
;
745TT
.LegAl
SVorw.
-l
-=
melting point pure aluminium
Increasing better weldability
Recom
mended p
reheat te
mpera
ture
Al 99,9
8R
Al9
9,9
Al9
9,8
Al 99,7
Al 99,5
Al 99
Al R
Mg0,5
Al M
g S
i 0,5
Al M
g S
i 0,8
Al M
g S
i 1
EA
l M
g S
i 1
Al M
g 1
Al S
i 5
Al C
u M
g 1
Al R
Mg 2
Al C
u M
g 0
,5A
l M
nA
l M
g 2
Al C
u M
g 2
Al M
g 3
Al M
g 3
Si
Al M
g M
n
Al Z
n M
g C
u 0
,5A
l Z
n M
g C
u 1
,5
mild
ste
el (0
.2%
C)
without pre
heating
660
600
°C
500
400
300
200
100
0
© ISF 2002br-er-08-17.cdr
Figure 8.17
Excessive Porosity in a Al Weld
© ISF 2002br-er-08-18.cdr
Figure 8.18
8. Welding of Aluminium 106
To suppress such pore
formation it is therefore
necessary to minimise the
hydrogen content in the
melt. Figure 8.19 shows
possible sources of hydro-
gen during MIG welding of
Al.
Figure 8.20 and 8.21 show
the effect of pure thermal
expansion during Al welding.
The large thermal expansion
of the aluminium along with
the relatively large heat af-
fected zones cause in com-
bination with a parallel gap
adjustment a strong distor-
tion of the welded parts. To
minimise this distortion, the
workpieces must be set at a
suitable angle before weld-
ing, Figure 8.21.
Ingress of Hydrogen Into the Weld
too thick and water containing oxyde layerby too long or open storagein non air-conditioned rooms
nozzle deposits and too steep inclinationof the torch cause turbulences
VS
too thick oxyde layer(condensed water)
dirt film(oil, grease)
H2
H2
Grundwerkstoff
Poren
festesSchweißgut
feuchte Luft
poorcurrent transition
irregularwireelectrodefeed
humid air
humid air(nitrogen, oxygen, water)
pores
solid weld metal
base material
© ISF 2002br-er-08-19.cdr
Figure 8.19
Weld Gap Adjustment
parallel gap
overlap
weld pool
weld pool
opening gap
© ISF 2002br-er-08-20.cdr
Figure 8.20
8. Welding of Aluminium 107
Examples to Minimise Distortion
wedge flame
© ISF 2002br-er08-21.cdr
Figure 8.21