BOC 216312 GasShielding Arc Welding AUS LR FA

8/7/2019 BOC 216312 GasShielding Arc Welding AUS LR FA

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1

Gas-shielded Arc

Welding of Aluminium


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02

Contents

03 Gas-shieldedarcweldingofaluminummaterials

04 Theinfluenceofconductivity

05 Theinfluenceofhydrogensolubilityandsolidificationrange

06 Preventionofcracks

10 SeamPreparationinbuttwelding

10 Shieldinggases

11 Summary

12 TechnicalSpecifications

Pulsed Arc


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03

Table 1: Physical properties of various aluminium alloys.

Alloy Electricalconductivity@20C SolidificationrangeC

Symbol EN Number International Sm/mm W/cm K

Al 99.5 3.0255 1050A 33.5 35.5 2.26 2.29 659 658

Al Mg 5 3.3355 5056A 14 19 1.20 1.34 625 590

Al Mg 4.5 Mn 3.3547 5083 15 19 1.20 1.30 640 575

Al Mg Si 0,5 3.3206 6060 26 35 2.0 2.4 650 615

Al Mg 1 Si Cu 3.3211 6061 23 26 1.63 640 595

Al Zn 4.5 Mg 1 3.4335 7020 21 25 1.54 1.67 655 610

G Al Si 1.2 3.2581 6082 17 26 1.3 1.9 580 570

G Al Si 10 Mg 3.2381 6005 17 26 1.3 1.9 600 550

Gas-shielded arc welding of aluminium materials

The handling of aluminium differs from that ofsteel in many respects as each material requiresspecic treatment. To prevent corrosion, thetwo materials should be stored and processedseparately right from the beginning. Likewise,only appropriate tools should be used foreach material. Tools previously used for othermaterials, e.g. carbon steel, must be cleanedbefore use with aluminium. Aluminium

requires tooling with a larger rake angle,the right grinding wheels and wire brusheswith stainless steel bristles.

Thephysicalpropertiesofaluminiumanditsalloys

Anyone welding aluminium for the rst time, either on its own or

in conjunction with steel, should rst of all consider the physical

properties of aluminium alloys. Their excellent electrical and

thermal conductivity together with the solidication range are

ideal properties for the welding of this non-ferrous material.


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04

1 2 3

Electrical conductivity, Sm/mm 15 19 24 32 34 36

Current / voltage 250A / 26V 300A / 28V 340A / 29V

Depth of penetration 3mm 7mm 9mm

Wire diameter and feed rate 1.6mm; 8m/min

Travel speed rate 50cm/min

The inuence of electrical conductivity

The electrical conductivity of a wire electrodeis basically inuenced by the nature andcomposition of the alloy elements. The lowerthe alloy proportion, the better the electricalconductivity at the stick-out of the wire and thehigher the welding current and voltage (withconstant wire feed). This means the arc outputrises, improving the weld geometry in the process(Figure 1). Changing the type of wire electrode

because of the base material, therefore requiresadapted set values for the wire feed (= weldingcurrent) and welding voltage.

Theinfluenceofthermalconductivity

The thermal conductivity of aluminium (Table 1) which is

signicantly better than that of structural steel (app. 0.5W/cm.K),

makes rapid welding more difcult. It also reduces the depth

of penetration and leaves the melting pool with less time for

solidication, thereby affecting the degassing, which can lead

to lack of fusion and pores. To prevent these problems, the

following should be done:

Preheating

Increasing the arc output

Under certain circumstances, additional preheating during

the welding of thick cross-sections

Figure 1: MIG welding: Inuence of electrical conductivity on weld geometry.

Filler alloy:

1. SG-AlMg 5

SG-AIMg 4.5 Mn

2. SG-AlSi 5

3. SG-Al 99.5 Ti

Shielding gas Argon

Base metal AlMg3


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05

The inuence of hydrogen solubilityand the solidication range

Hydrogen is the main cause of pores duringthe welding of aluminium. In Figure 2, thehydrogen solubility of iron is compared to thatof aluminium. The signicant surge in solubilitywhen solidication takes place (at about 660C)and the low rate of solubility in the solid stateshow that hydrogen in aluminium (if the rateof cooling is too rapid) can easily be frozen in,thereby creating pores.

The solidication range can be considered as a yardstick for

determining the formation of pores. If there is no solidication

range or only a slight one, the aluminium welding deposit

solidies so rapidly that degassing cannot fully take place,

thereby causing porosity. Table 1 shows that the solidication

range increases as the alloy content rises with a corresponding

reduction in the risk of pore formation. There are ve main

reasons for the occurrence of pores and lack of fusion in the

welding of aluminium materials:

High thermal conductivity

Good hydrogen solubility in the molten state

A noticeable surge in solubility when solidication occurs

at about 660C

Low solubility in the solid state

No or virtually no solidication range

Figure 2: The hydrogen solubility of aluminium and iron.

AI

Fe

Temperature [C]

Hydrogensolubility

[cm/100g]

100

50

10

5.0

1.0

0.5

0.1

0.05

0.01

659 1536

500 1000 1500 2000 2500

0.036

0.7

8

28

50


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06

Prevention of cracks

Heat cracks in the weld seam occur wheneverthere is a high silicon and /or magnesiumcontent (Figure 3). The size of the alloyproportion required in the wire electrodedepends on the base material and the degreeof agitation anticipated.

Shouldaluminiumoxide(AI2O

3)

onthesurfaceberemoved?

To make welding easier and prevent defects in the weld,

aluminium oxide close to the weld is usually removed. The

reasons for this are:

Al2O

3has a considerably higher melting point (app. 2050C)

than aluminium (about 500C 600C, depending on the alloy)

Al2O

3is heavier than aluminium and can lead to inclusions

Al2O

3is hygroscopic and binds to moisture, which leads

to the formation of pores

Tendencytoheat-cracking

0 1 2 3 4 5 %alloy

Mg

Si

Preventionofcratercracks

The considerable shrinkage which aluminium is prone to

frequently leads to crater cracks. This can be prevented if

the following action is taken:

Filling in the nal crater just before it solidies either

manually or through the application of a crater lling

program (Figure 4)

Place the craters on a drain plate, if possible

Return the arc at the end of the seam to the seam alreadywelded and then turn off

Crater cracking Crater lled

Figure 4: MIG welding of aluminium: avoidance of crater cracks.

Figure 3: Tendency to heat-cracking of aluminium.


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07

As the oxide skin reforms in a very short time, it has to be

removed not only before but also during welding while

the layers are building up. This can be done:

Mechanically by means of brushing and grinding

With the assistance of the cleaning effect, through the electrical

current, with the electrode having positive polarity (Figure 5)

With adequate heat concentration, using about 75 to 100%

helium in TIG welding, with DC and a tungsten electrode with

negative polarity

Wire brushes are used to remove the oxide skin and the smoke

precipitate. Brushes with worn-out ends can no longer cut and

brush the surface effectively. Far from removing the Al2O

3, it

simply presses it in, causing smudging (Figure 6).

However, a lack of aluminium oxide during TIG welding causes

the arc to burn erratically, resulting in:

A rather wide weld with an irregular delineation

A dark precipitate, and

Almost no cleaning zone (see Figure 7, page 8)

In Figure 7, the left hand side shows that because of the oxide

present, the arc has burnt steadily. A relatively wide cleaning

zone and a uniform weld delineation can be clearly seen.

The cleaning effect from the electrical current can be explainedas follows:

If the electrode has a positive polarity (whether it is a wire or

tungsten electrode), the positively charged gas ions coming from

the positive pole hit the oxide skin with great kinetic energy and

shatter it (Figure 5, below).

+

Ions

+

Electrons

AIO

1.77eV

3.95eV AIAI

Figure 5: Cleaning effect (Al2O

3elimination) through electrical current.


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AIO

rounded bristle ends (worn out)

no cleaning action

sharp-edged bristles

shaving effect

Figure 6: Inuence of bristle end form on removal of Al2O

3.

Figure 7: AC TIG welding: weld region with and withoutaluminium oxide.

withoutA120

3withA1

20

3

180A 180ACurrent

Base metal : AI Mg3, 3mm thick, square butt joint

Wire : SG AI Mg5, 1.6mm dia.

Shielding gas : Argon

The earlier theory about the cleaning effect relates to the

electrons escaping from the aluminium. The following fact

argues against this the electron emission is also subject to

physical laws. The electrons emerge from the aluminium oxide

at 1.77 electron volts, whereas they would have needed 3.95

electron volts from aluminium (Figure 5, right-hand side).

Corresponding to Figure 7, Figure 8 shows the current and

voltage patterns during TIG welding for the welds shown.

This conrms the view previously expressed concerning the

behaviour of the arc in terms of the presence or absence of

aluminium oxide.

The effect of brushing (particularly rotating brushes) which

should effectively remove all the oxide is questionable in many

cases, especially during TIG welding.

Figure 8: AC TIG welding: current and voltage patterns for Figure 7.

withoutA120

3withA1

20

3




08


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Alushields aresuperior in most

cases with regardsto economics

and quality.

Figure 9: AC TIG welding: current-voltage cycle with scrapedand brushed weld region.

WeldregionbrushedWeldregionshaved




The right-hand part of Figure 9 (the brushed weld area) shows

a relatively uniform current and voltage pattern. Although it had

been believed that the Al2O

3had been removed using a rotating

brush, the welding reaction and the appearance of the weld

showed that hardly any oxide had been removed, leaving the

arc to burn at a steady rate.

However, if no aluminium oxide or hardly any is detected in the

weld, this is basically due to the cleaning effect of the electrical

current. This incidentally occurs in MIG as well as TIG welding.

Whether the aluminium oxide skin should be removed in

the weld region and if so, to what extent, depends on thecircumstances of each case. A lack of oxide can make the arc

behave erratically, creating atmospheric humidity and eddying,

leading to the formation of pores. Compromise, therefore, is

often needed. Pores can also occur if the weld area still shows

signs of moisture. Simply drying the area, using a neutral

oxy-acetylene ame, is often the solution. At the same time,

the weld is preheated.


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Seam preparation inbutt welding

Figure 10 Butt welding: chamfering the root side plate edges.

Wrong

Plate edges

not chamfered

Oxides not properly removed

Oxide notch

In butt welding, the plate edges of the underside should be

chamfered (Figure 10). This avoids an oxide chamfer on the

root side, making it easier to scour out the oxides from the

front faces.

Shielding gasesAluminium requires gases of an inert nature, with argon being

frequently used. Although more expensive from a cubic metre

price point of view, in many cases, Argon-Helium mixtures

(Alushield) are superior to Argon in terms of economics and

quality. Two examples illustrate the point.

Example:MIGwelding

Both girth welds and the straight bead on the cylindrical

housings for some high-voltage switches (Figure 11) should as

much as possible be pore-free. With argon as the shielding gas,

this could only be achieved through costly re-nishing work

using the TIG process.

The use of the shielding gas mixture 50% argon and 50% helium

(Alushield Universal) led to a drastic reduction in the amount

of re-nishing work the welds were virtually free from pores,

together with a reduction in the consumption of wire electrodes.

In spite of the higher gas price for the mixture, there was a

production cost saving of approximately 60%.

Figure 11 MIG Welding of cylindrical housings for high voltage switches.

Correct

Plate edges chamfered

Oxides completely removed

Perfect root formation


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Example:TIGwelding

In a tender submitted for the manufacture of telescopic braces

masts (Figure 12), the calculations were on TIG welding with

argon. When the structure had been installed, a further

calculation was done, which showed a reduction of 10% in the

total manufacturing costs with regard to the tender price. This

gure was achieved because the replacement of argon with a

mixture of 50% argon and 50% helium (Alushield Universal)

increased the welding speed, leading to a reduction in the

welding time and labour costs. The overall result was a 10%

reduction in production costs.

SummaryAluminium and its alloys need to be treated quite differently

from steel. A knowledge of its physical properties such as its

electrical and thermal conductivity, as well as its solidication

range can be of great help in welding of aluminium materials.

The amount of aluminium oxide to be removed depends in

the rst place on what is required of the weld in terms of

quality and secondly on the welding process. Wire brushesare not particularly suitable for removing oxide. Argon-helium

(Alushield) mixtures are often better than argon with regard

to quality and cost where practical. Whenever practical, push

pull guns should always be used with the leads kept as short

as possible.

Figure 12 TIG welding of telescopic braced masts.

11


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CylinderSizes

Content

(m3@STP) GaugePressure(kPa@15C)

Outlet

Connection

J 13.8 30,000* AS 2473 Type 10

G 8.6 16,900E2 4.1 20,000

D2 2.0 20,000

D 1.6 15,800

Pack Sizes

KAY15 227.2 30,000* AS 2473 Type 10

JAY15 207.7 30,000*

KAY9 136.3 30,000*

KAY4 60.6 30,000*

Cylinder colour (to AS 4484): Peacock blue body, shoulder and neck.Not all cylinders and packs are available at all BOC outlets. Please check with your local BOC branch on 131 262. Other sized

cylinders and packs may also be available on request.* Indicates tted with a pressure regulating valve outlet connection, 6,0009,000 kPa.

Gas Purity

Argon >99.995%

Note: Higher grades and purities ofthis product are available from BOC.Specications are included in the ScienticGases and Equipment Manual. Contact1800 658 278 to obtain a copy.

ArgonWeldingGrade,Compressed(Ar)GasCode061

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Technical Specications

Applications

All the major applications

of argon are related to the

production, processing

and fabrication of metals.

The role of argon is

nearly always to excludeatmospheric air from

contact with metal alloys.

An example of this is as a

purging gas to protect weld

areas, such as the inside of

pipes during welding

The predominant gas used

in GMA and TIG welding

applications. Argon is used

alone or mixed with other

gases such as helium, carbon

dioxide, oxygen, nitrogen orhydrogen

Used on a wide range of

ferrous and non-ferrous

materials for welding

A major component in most

shielding gas mixtures for

arc welding due to low

ionising potential and low

density

Each mixture is often specic

to welding a particular

material or to a particular

process. For example, argon

is used as a shielding gas

for TIG welding aluminium,

titanium and copper

Features

Colourless, tasteless and

odourless

Inert to all materials at all

temperatures and pressures

Heavier than air, argon will

collect in low-lying areas,

ducts and drains

An asphyxiant

Benefits

Argon produced by BOC has

a minimum purity level of

99.995% ideal for welding

applications

The low ionising potential

allows easy forming of awelding arc without reacting

with the metal components

being welded

When welding thicker

materials, other gases are

added to the argon base

to produce a more uid

weld pool

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Applications

Boat and ship building

Tankers

Truck body work

Water heaters and heat

exchangers

Piping and balustrades

Light busbars

Features

Excellent arc stability

High heat input efciency

Low distortion

Flatter weld bead with low

reinforcement

Faster welding speed

Good fusion characteristics

Little or no spatter

Used in dip, spray and

pulsed transfer modes

Benefits

Lower spatter reduces

clean-up time

Improved weld metal

properties

Easy to use

Good appearance and nish

with low reinforcement

levels

Increased productivity due

to fast weld speeds

Can be used on robotic

machines

AlushieldLightGasCode079

Gas Composition

Helium 27%

Argon Balance

CylinderSize

Content

(m3@STP)

GaugePressure

(kPa@15C) OutletConnection

G 7.8 16,900 AS 2473 Type 10

Pack Size

KAY4 55.6 30,000* AS 2473 Type 10

Cylinder colour (to AS 4484): Peacock blue body, brown shoulder and neck.Not all cylinders and packs are available at all BOC outlets. Please check with your local BOC branch on 131 262.* Indicates tted with a pressure regulating valve outlet connection, 6,0009,000 kPa.



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AlushieldUniversalGasCode507

Applications

Shop tting and frames

Heat exchangers

Tanks and vessels

Repairing vehicle body work

Rail carriages

Copper work

MIG Brazing: dissimilar

joints, automotive joints

Features

Ideal for automatic

and applications

Can be used for copper and

stainless steel TIG welding

Stable welding arc

Excellent appearance

and nish

Excellent fusion

characteristics

Suitable for applications

where penetration is critical

Benefits

Faster welding speed

Low defect levels

Strong weld

No post weld cleaning

Improved productivity

Reduced porosity

Gas Composition

Helium 50%

Argon 50%

CylinderSize

Content

(m3@STP)

GaugePressure


G 7.3 16,900 AS 2473 Type 10


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Applications

Aluminium castings

Ship building and

armoured vehicles

Heavy aluminium fabrication

Road and rail transport

Chemical and

petrochemical plants

Copper and aluminium

busbars

Features

Excellent arc stability

High heat input efciency

Low distortion and

oxidisation potential

Wide bead shape with

low reinforcement

Faster welding speeds

Good fusion characteristics

Reduced spatter

Benefits

Lower spatter reduces

clean-up time

Improved weld metal

properties

Easy to use

Reduced reject rates

Lower risk of defect levels

Good appearance and high

quality nish with low

reinforcement levels

Increased productivity

due to fast weld speeds

Can be used on robotic

machines

AlushieldHeavyGasCode069

Gas Composition

Argon 25%

Helium Balance

CylinderSize

Content

(m3@STP)

GaugePressure


G 7.3 16,900 AS 2473 Type 10

Currently not available in packs.

Cylinder colour (to AS 4484): Brown body, peacock blue shoulder and neck.

Not available at all BOC outlets. Please check with your local BOC branch on 131 262.



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Australia

[email protected]

www.boc.com.au

BOC is a trading name of BOC Limited, a member of The Linde Group. BOC Limited 2010. Reproduction without permission is strictly prohibited.

Details given in this document are believed to be correct at the time of printing Whilst proper care has been taken in the preparation no liability

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Documents

BOC 216312 GasShielding Arc Welding AUS LR FA