7. welding of aluminium alloys - kaumercury.kau.ac.kr/welding/welding technology ii - welding... ·...

7.

Welding of Aluminium Alloys

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 2002br-er08-01.cdr

Figure 7.1


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-ElongationDiagrams of Al and Steel

Elongation

Al-alloy

Steel

Str

ess


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

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


Figure 7.4


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

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


Figure 7.5

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 7.6


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

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


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 containingaluminium solid solution

aluminium solid solutionand copper aluminide(Al Cu)2

copper content ofAlCuMg

Copper

Tem

pera

ture


Figure 7.8


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 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


Figure 7.9

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


Figure 7.10


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 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

longation

d2

0.2

% y

ield

str

ess

s0.2

400

300

200

110

135

180

Tensile

str

ength

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


Figure 7.11

Age Hardening of Al Alloys

0 30 70

100

200

300

400

N/mm²

% Strain

0

Tensile

str

ength

Rm

AlMg5

AlMg3

Al99,5


Figure 7.12


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

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

Ro

r R

mp

0,2


Figure 7.13

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


Figure 7.14


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


Figure 7.15

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

usceptibili

ty

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


Figure 7.16


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.

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


Figure 7.17

Excessive Porosity in a Al Weld


Figure 7.18

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


Figure 7.19


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


Figure 7.21

Figure 7.20

7. welding of aluminium alloys - kaumercury.kau.ac.kr/welding/welding technology ii - welding... ·...

Documents