7. Welding of Aluminium Alloys 84 Figure 7.1 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 7.2 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 7.3 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 7.2
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 2002 br-er08-01.cdr
7. Welding of Aluminium Alloys 85 formation during cooling, thus there is no structure transformation and consequently no danger of hardening in the heat affected zone as with steel.
Figure 7.4 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 7.5 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-Elongation Diagrams of Al and Steel
Elongation Al-alloy Steel S tress © ISF 2002 br-er08-03.cdr Figure 7.3
Isothermal Curves of Steel and Al
4 2 -2 -4 4 2 -2 -4
low carbon steel
aluminium 400 600 200°C 800 1000 1200 1500 -6 -8 cm 8 6 -18 -16 -14 -12 -10 -8 -6 -4 -2 0 2 cm 6 500 600 400 300 200 100°C cm © ISF 2002 br-er08-04.cdr Figure 7.4
7. Welding of Aluminium Alloys 86 Figure 7.6 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 7.7 und 7.8. 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 7.8 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
678
678
Mg Si Mn Cu Zn Al 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 n o n -h a rd e n a b le a llo y s h a rd e n a b le a llo y s © ISF 2002 br-er08-05.cdr Figure 7.5Use and Welding Consumables of Aluminium Alloys
Al - alloys Typical use W elding consumable
Al99,5
electrical engineering SG-Al 99,5Ti;
SG-Al 99,5
AlCuMg1 mechanical engineering, food
industries SG-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 industry SG-AlMn1;SG-Al99,5T
base material - aluminium
percentage of alloy elements without factor
© ISF 2002 br-er-08-06.cdr
7. Welding of Aluminium Alloys 87 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 7.7. 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 7.7) 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
rise longer 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 2002 br-er-08-07.cdr
Figure 7.7
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 containing aluminium solid solution
aluminium solid solution and copper aluminide
(Al Cu)2 copper content of AlCuMg Copper T emperature © ISF 2002 br-er08-08.cdr Figure 7.8
7. Welding of Aluminium Alloys 88 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 7.7 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 7.9 shows a schematic representation of time-temperature curves during hardening with age hardening and heat ageing.
Figure 7.10 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 Distribution During Ageing
solution heat treatment
quenching heat ageing age hardening 2 4 6 8 10 12 h 14 500 °C 400 300 200 100 0 Q P T emperature Time © ISF 2002 br-er08-09.cdr Figure 7.9
Increase of Yield Stress During Ageing of AlZnMg1
quenched Ageing time in h
0.2%yieldstress
inN/mm²
s0.2
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 2002 br-er-08-10.cdr Figure 7.10
7. Welding of Aluminium Alloys 89 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 7.11 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 7.11 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
elongation is counter-
proportional to the strength values, i.e. the strength increase caused by ageing is accompanied by an em- brittlement of the material. Influence of Ageing Temperature
and -Time on Ageing 260 0 10-2 10-1 100 101 102 103 h 104 190 180 150 135 110°C 230 260 30 min 1 day 30 20 10 Fractureelongation d2 0.2%yieldstress s0.2 400 300 200 110 135 180 T ensilestrength sB 110 135 150 180 230 500 400 300 200 Ageing time % N/mm² N/mm² 205 260°C 190 150 190 205°C 230 205 1 week 1 month 1 year © ISF 2002 br-er08-11.cdr Figure 7.11
Age Hardening of Al Alloys
0 30 70 100 200 300 400 N/mm² % Strain 0 T ensile strength Rm AlMg5 AlMg3 Al99,5 © ISF 2002 br-er-08-12.cdr Figure 7.12
7. Welding of Aluminium Alloys 90 Figure 7.12 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 7.12 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 7.13 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 7.14 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
HV30 80 60 40 20 0 20 40 60 mm 100 0,7 0,6 0,5 0,4 0,3 0,2 0 50 100 150 200 250 300 N/mm² R /R p0, 2 m R orR m p0, 2 © ISF 2002 br-er08-13.cdr Figure 7.13 Hardenable Al Alloy
4 mm plates of: AlZnMg1F32
start values: R =263N/mm²
R =363 N/mm² welding method: WIG, both sides,
simultaneously
welding consumable: S-AlMg5
specimens with machined weld bead
p0,2 m
Distance from seam centre
S tress 90 days RT 21 days RT 1 day RT 80 400 N/mm² 350 300 250 200 150 100 50 80 60 40 20 0 20 40 60 100 mm 140 90 days RT 1 day RT 21 days RT Rp0,2 Rm © ISF 2002 br-er-08-14.cdr Figure 7.14
7. Welding of Aluminium Alloys 91 Figure 7.15 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 inter- val, cracks may easily develop in the weld.
A relief can be afforded by preheating of the material, Figure 7.16. With an increasing preheat temperature, the amount of fractured welds decreases. The different behaviour of the three displayed alloys can be ex- plained using the right part
of the figure. One can see that the manganese content influences significantly the hot crack susceptibility. The maximum of this hot crack susceptibility is likely with about 1% Mg content (corre- sponds with alloy 1). With increasing MG content, hot
crack susceptibility de-
creases 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 2002 br-er-08-15.cdr
Figure 7.15
Influence of Preheat Temperature and Magnesium Content 1: AlMgMn 2: AlMg 2,5 3: AlMg 3,5
Preheat temperature W eldcrackingtendency Crackingsusceptibility Alloy content X X X X 100 80 60 40 20 % 0 100 200 300 400 °C 500 1 2 3 0 1 2 3 % 4 Si Mg © ISF 2002 br-er-08-16.cdr Figure 7.16
7. Welding of Aluminium Alloys 92 7.17, together with the re-
lated calculation result.
These results are only to be regarded as approxi- mate, the individual appli- cation is subject to the information of the manufac- turer.
Another major problem dur- ing 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 solidification. Solubility of hydrogen in aluminium changes abrupt- ly on the phase transition melt-crystal, i.e. the melt dissolves many times more of the hydrogen than the just forming crystal at the same temperature.
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.
Tvorw. in °C preheat temperature l ; 745 T T . Leg Al S Vorw. - l - =
melting point pure aluminium
Increasing better weldability
Recommendedpreheattemperature Al99,98R Al99,9 Al99,8 Al99,7 Al99,5 Al99 AlRMg0,5 AlMgSi0,5 AlMgSi0,8 AlMgSi1 E AlMgSi1 AlMg1 AlSi5 AlCuMg1 AlRMg2 AlCuMg0,5 AlMn AlMg2 AlCuMg2 AlMg3 AlMg3Si AlMgMn AlZnMgCu0,5 AlZnMgCu1,5 mildsteel(0.2%C)withoutpreheating 660 600 °C 500 400 300 200 100 0 © ISF 2002 br-er-08-17.cdr Figure 7.17
Excessive Porosity in a Al Weld
© ISF 2002 br-er-08-18.cdr
Figure 7.18
Ingress of Hydrogen Into the Weld too thick and water containing oxyde layer by too long or open storage
in non air-conditioned rooms
nozzle deposits and too steep inclination of the torch cause turbulences
VS
too thick oxyde layer (condensed water) dirt film (oil, grease) H2 H2 Grundwerkstoff Poren festes Schweißgut feuchte Luft poor current transition irregular wire electrode feed humid air humid air (nitrogen, oxygen, water)
pores
solid weld metal
base material
© ISF 2002 br-er-08-19.cdr
7. Welding of Aluminium Alloys 93 This leads to a surplus of hydrogen in the melt due to the crystallisation 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 conductiv- ity, 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 so- lidifying front and remain in the weld metal as pores, Figure 7.18.
To suppress such pore formation it is there- fore necessary to minimise the hydrogen con- tent in the melt. Figure 7.19 shows possible sources of hydrogen during MIG welding of Al.
Figure 7.20 and 7.21 show the effect of pure thermal expansion during Al welding. The large thermal expansion of the aluminium along with the relatively large heat affected zones cause in combination with a parallel gap adjustment a strong distortion of the welded parts. To minimise this distortion, the workpieces must be set at a suitable angle before welding, Figure 7.21.
Examples to Minimise Distortion
wedge flame
© ISF 2002 br-er08-21.cdr
Figure 7.21 Figure 7.20