7. Welding of Aluminium Alloys

7. Welding of Aluminium Alloys


84 Figure 7.1 compares basic physical properties

Property

Al

Fe

of steel and aluminium. Side by side with different mechanical behaviour, the following

Atomic weight

[g/Mol]

26.9

55.84

Specific weight

[g/cm³]

2.7

7.87

fcc

bcc

Lattice

differences are important for aluminium weld-

E-module

[N/mm²]

71*10³

210*10³

R pO,2 PO,2

[N/mm²]

ca. 10

ca. 100

R mm

[N/mm²]

ca. 50

ca. 200

spec. Heat capacity

[J/(g*°C)]

0.88

0.53

[°C]

660

1539

[W/(cm*K)]

2.3

0.75

Spec. el. Resistance

[nWm]

28-29

97

Expansion coeff.

[1/°C]

Melting point Heat conductivity

24*10

-6

12*10

Al2O 3

Melting point of oxydes

[°C]

2050

- considerably lower melting point compared with steel - three times higher heat conductivity - considerably lower electrical resistance

-6

FeO Oxydes

ing:

Fe 3O 4

- double expansion coefficient - melting point of Al203 considerably higher

Fe 2O 3

than that of Al; metal and iron oxide melt ap-

1400

proximately at the same temperature.

1600 (1455)

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© ISF 2002

Basic Properties of Al and Fe

Figure 7.2 compares some mechanical properties of steel with properties of some light metals. The important advantages of light

Figure 7.1

metals compared with steel are especially

shown in the right part of the figure. If a comparison should be based on an identical stiffness, 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 qualitatively the stress-strain diagram

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

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Deflexions and Weights of Cantilever Beams Under Load

not

subject to a lattice transFigure 7.2


85

formation during cooling, thus there is no structure transformation and consequently no danger of hardening in the heat affected zone as with steel.

4 cm 2

low carbon steel

200°C

400

1000 1200

600

800

1500

-2

Steel

-4

Stress

8 cm aluminium 6

100°C 200

4

Al-alloy

2 300 400 500 600 -2 -4 -6 -8 -18

Elongation br-er08-03.cdr

© ISF 2002

-16

-14

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Comparison of Stress-Elongation Diagrams of Al and Steel

Figure 7.3

-12

-10

-8

-6

-4

-2

0

2

cm

6

© ISF 2002

Isothermal Curves of Steel and Al

Figure 7.4

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 welding 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 compared 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 hardenable and non-hardenable (naturally hard) alloys.


86

Al Cu Mg

ing consumables. Al Mg Si

Cu

Aluminium alloys are often welded with conAl Zn Mg

sumable of the same type, however, quite Mg

often over-alloyed consumables are used to

Al Zn Mg Cu

678

Al alloys together with preferably used weld-

hardenable alloys

Figure 7.6 shows typical applications of some

Al

Zn

Al Si Cu

and to improve the mechanical properties of Al Si

the seam.

Si Al Mg

The classification of Al alloys into two groups

Al Mg Mn

Mn

is based on the characteristic that the group Al Mn

of the non-hardenable alloys cannot increase br-er08-05.cdr

the strength through heat treatment, in con-

678

Mg and Zn because of their low boiling point)

non-hardenable alloys

compensate burn-off losses (especially with

© ISF 2002

Classification of Aluminium Alloys

trast to hardenable alloys which have such a potential. The important hardening mechanism for this

Figure 7.5

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 sufficiently 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 Al - alloys Al99,5 AlCuMg1 AlMgSi0,5 AlSi5 AlMg3

AlMg2Mn0,8 AlMn1

Typical use electrical engineering mechanical engineering, food industries architecture, electrical engineering, anodizing quality architecture, anodizing quality architecture, apparatus-, vehicle-, shipbuilding engineering, furniture industry apparatus-, vehicle-, shipbuilding engineering apparatus-, vehicle-engineering, food industry

W elding consumable SG-Al 99,5Ti; SG-Al 99,5

tion, no precipitation will

SG-AlMg4,5Mn

take place. The alloy ele-

SG-AlMg5; SG-AlMg4,5Mn; SG-AlSi5 SG-AlSi5

ments are forced to remain dissolved, the crystal is out

SG-AlMg3; SG-AlMg4,5Mn SG-AlMg5; SG-AlMg3; SG-AlMg4,5Mn

of equilibrium. If such a structure is subjected to an

SG-AlMn1;SG-Al99,5T

age hardening at room or

base material - aluminium percentage of alloy elements without factor

elevated

temperature,

a

© ISF 2002

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Use and Welding Consumables of Aluminium Alloys

Figure 7.6

temperature in this condi-

precipitation of a second phase takes place in ac-


87

cordance with the binary system, the crystal tries to get back into thermodynamical equilibrium. Depending on the level of

stable condition

solution heat treatment

repeated hardening

solidification of alloy elements in solid solution

hardening temperature, the

quenching

regeneration

oversaturated solid solution, metastable condition

precipitation takes place in

warm ageing

cold ageing (RT ageing)

ageing at slightly increased temperature coherent precipitations, cold aged condition

three possible forms: copartly coherent precipitations, warm aged condition

coherent and partly coherent precipitations, transition conditions cold ageing -- warm ageing temperature rise

temperature rise

herent particles (i.e. particles

longer warm ageing partly coherent and incoherent precipitations, softening

from

the

matrix in their chemical composition but having the

longer warm ageing stable incoherent equilibrium phase stable condition © ISF 2002

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deviating

Ageing Mechanism

same

lattice

structure),

partly

coherent

particles

(i.e. the lattice structure of the matrix is partly re-

Figure 7.7

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 temperature (i.e. enabling diffusion). The precipitations cause a restriction to the

700 liquid

dislocation movement in the matrix lattice, thus

liquid and solid Q

600

leading to an increase in strength. The finer the

copper containing aluminium solid solution 500

At an increased temperature (heat ageing, Fig-

Temperature

precipitations, the stronger the effect.

P

400

300

ure 7.7) a maximum of second phase has precipitated after elapse of a certain time. Consequently a prolonged stop at this tem-

aluminium solid solution and copper aluminide (Al2Cu)

200

100 copper content of AlCuMg

perature does not lead to an increased strength, but to coarsening of particles due to

0

1

2

3

4

5

mass-%

Copper

diffusion processes and to a decrease in strength (less bigger particles in an extended

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space).

© ISF 2002

Phase Diagram Al-Cu

Figure 7.8

7


88 After a very long heat ageing a stable condition is reached again with relatively large precipitations of the second phase in the matrix. In Figure 7.7 is this stable final condition iden-

Q

tical with the starting condition. A deteriorati-

solution heat treatment

500 P

on of mechanical properties only happens

°C

quenching

Temperature

400

during hot ageing, if the ageing time is excessively long.

300

200

heat ageing

The complete process of hardening at room

100

temperature is metallographic also called age age hardening

hardening, at elevated temperature heat age0

2

4

6

8

10

12

h

Time

14

ing. A decrease in strength at too long ageing time is called over-ageing. © ISF 2002

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Temperature - Time Distribution During Ageing

Figure 7.9 shows a schematic representation of time-temperature curves during hardening

Figure 7.9

Figure

with age hardening and heat ageing.

7.10

shows

the

380

strength increase of AlZnMg The difference between age hardening and heat ageing is here very clear. Due to improved

diffusion

condi-

tions is the strength increase

320 0.2% yield stress s0.2 in N/mm²

1 in dependence of time.

water quenching (~900°C/min) air cooling (~30°C/min)

260 120°C 200 RT 140

80 10-1

in the case of heat ageing much faster than in the case of

age

hardening.

quenched

100

101

10²

10³

Ageing time in h © ISF 2002

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Increase of Yield Stress During Ageing of AlZnMg1

The

strength maximum is also reached considerably ear-

Figure 7.10

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


89

second phase is precipitated from a single-phase structure. To initiate this process, the structure 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. 500 110

N/mm²

Tensile strength sB

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-

135

400

150 180

300

190 205

230

260°C

cubation time. The lower the ageing tempera-

200 110

N/mm² 400 0.2% yield stress s0.2

ture, the higher the resulting values of yield stress and tensile strength. If a low ageing temperature is selected, the ageing time as well as

135

300

150 180 190 205°C

200

the incubation time become extremely long.

230 260 Fracture elongation d2

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-

%

190

180

205

150

135

20 10

0

crease of the ageing temperature shortens the duration of the complete precipitation process

30

110°C 260

230 30 min

10

-2

10

-1

1 day 0

1

10 10 Ageing time

1 week

10

2

1 1 month year

103 h 104

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© ISF 2002

Influence of Ageing Temperature and -Time on Ageing

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.

Figure 7.11

As the lower part of the 400

figure shows, the fracture

N/mm²

elongation

Tensile strength Rm

300

is

counter-

AlMg5

proportional to the strength

AlMg3

values, i.e. the strength

200

increase caused by ageing is accompanied by an em-

100

brittlement of the material.

Al99,5

0 0

30

%

70

Age Hardening of Al Alloys

Figure 7.12

Strain © ISF 2002

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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 mechanisms of strength increase of such alloys. On 300

one hand, tensile strength increases with in-

N/mm²

creasing content of alloy elements (solid solu-

250

tion strengthening), on the other hand, this increase is caused by a stronger deformation

Rm or Rp0,2

200

of the lattice. 150

Figure 7.13 shows the effect of the welding process on mechanical properties of a cold-

0,7

100

worked alloy. Due to the heat input during

0,5 50 HV30

0,4

Rp0,2/Rm

0,6

(recovery), in addition, a grain coarsening will

0,3 0,2

0 80

60 40 20 0 20 40 Distance from Seam Centre

welding, the blocked dislocations are released start in the HAZ. This is followed by a strong

60 mm 100

drop in yield point and tensile strength. This

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strength loss cannot be overcome in the case

© ISF 2002

Non-Hardenable Al Alloy

of a welding process.

Figure 7.13 400

Figure

7.14

illustrates

the

90 days RT

N/mm²

Rm

350

mechanisms in the case of a

21 days RT

hardenable aluminium alloy. welding heat, the precipitations are solution heat treated

Rp0,2

250 90 days RT

Stress

As a consequence of the

1 day RT

300

21 days RT 200 4 mm plates of: AlZnMg1F32 start values: Rp0,2=263N/mm² Rm=363 N/mm² welding method: WIG, both sides, simultaneously welding consumable: S-AlMg5 specimens with machined weld bead

1 day RT

150

and the strength values de100

crease in the weld area. Due to the age hardening, a re-

50 80 br-er-08-14.cdr

strengthening of the alloys

40

20

20 60 0 40 Distance from seam centre

Hardenable Al Alloy

takes place with increasing time.

60

Figure 7.14

80

100

mm

140 © ISF 2002


91 Figure 7.15 shows another problematic nature of Alwelding. Due to the high thermal expansion of aluminium, high tensions develop during solidification of the weld pool in the course of the welding cycle. If the welded alloy indicates a high melting inter© ISF 2002

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val, Hot Cracks in a Al Weld

cracks

may

easily

develop in the weld.

Figure 7.15

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 explained using the right part of the figure. One can see

100 %

that the manganese content

maximum of this hot crack

2 60 1 40

X X

3

20

susceptibility is likely with

Mg

Cracking susceptibility

hot crack susceptibility. The

Weld cracking tendency

influences significantly the

80

Si

X X

about 1% Mg content (corresponds with alloy 1). With increasing MG content, hot crack

susceptibility

0

100

300

Preheat temperature

400

°C

500 0

1

2

3

%

4

Alloy content 1: AlMgMn 2: AlMg 2,5 3: AlMg 3,5

© ISF 2002

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de-

Influence of Preheat Temperature and Magnesium Content

creases strongly (see also alloy 2 and 3, left part).

200

Figure 7.16

To avoid hot cracking, partly very different preheat temperatures are recommended for the alloys. Zschötge proposed a calculation method which compares the heat conductivity conditions of the Al alloy with those of a carbon steel with 0.2% C. The formula is shown in Figure


melting point pure aluminium

Recommended preheat temperature

600 °C 500 400 300 200

Welding possible without preheating: AlMg5, AlMg7, AlMg4.5Mn, AlZnMg3, AlZnMg1

100

0

mild steel (0.2%C) without preheating

660

lated

temperature of melt start (solidus temperature) preheat temperature heat conductivity

Al Zn Mg Cu 0,5 Al Zn Mg Cu 1,5

in °C in °C in J/cm*s*K

Al Si 5 Al Cu Mg 1 Al R Mg 2 Al Cu Mg 0,5 Al Mn Al Mg 2 Al Cu Mg 2 Al Mg 3 Al Mg 3 Si Al Mg Mn

TS Tvorw. lAl-Leg.

7.17, together with the re-

745 l Al-Leg.; Al 99,98R Al99,9 Al99,8 Al 99,7 Al 99,5 Al 99 Al R Mg0,5 Al Mg Si 0,5 Al Mg Si 0,8 Al Mg Si 1 E Al Mg Si 1 Al Mg 1

TVorw. = TS -

92

calculation

result.

These results are only to be regarded as approximate, the individual application is subject to the information of the manufacturer.

Increasing better weldability © ISF 2002

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Figure 7.17

Recommendations for Preheating

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 © ISF 2002

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Figure 7.18

Excessive Porosity in a Al Weld

pool during solidification. irregular wire electrode feed

too thick and water containing oxyde layer by too long or open storage in non air-conditioned rooms

Solubility of hydrogen in

humid air (nitrogen, oxygen, water)

aluminium changes abrupt-

nozzle deposits and too steep inclination of the torch cause turbulences

poor current transition

VS

humid air

too thick oxyde layer (condensed water) dirt film (oil, grease)

dissolves many times more just forming crystal at the

H2 H2

festes Schweißgut base material

melt-crystal, i.e. the melt of the hydrogen than the

feuchte Luftpores Poren solid weld metal

ly on the phase transition

same temperature. Grundwerkstoff

© ISF 2002

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Ingress of Hydrogen Into the Weld

Figure 7.19


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 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 7.18. Figure 7.20

To suppress such pore formation it is therefore necessary to minimise the hydrogen content 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

wedge

flame

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. br-er08-21.cdr

© ISF 2002

Examples to Minimise Distortion

Figure 7.21