AAM Joining 1 Fusion Welding

8/10/2019 AAM Joining 1 Fusion Welding

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Version 2002 European Aluminium Association ([email protected]) 1

Joining Fusion weldingTable of co ntentsFusion welding ........................................................................................................................... 2

1.1 Characteristics of aluminium in fusion welding ............................................................... 2

1.2 Benchmarking of fusion welding processes .................................................................... 3

1.3 Joint design for arc and beam welding............................................................................ 41.4 Joint design for beam welding w/r to loading direction ................................................... 51.5 Weld preparationThe aluminium oxide film ................................................................. 61.6 Weld preparation ............................................................................................................. 71.7 Weldability ....................................................................................................................... 8

1.7.1 Introduction .............................................................................................................. 8

1.7.2 The welding zones (definitions) ............................................................................... 9

1.7.3 Fusion zone ........................................................................................................... 10Solidification in the fusion zone .................................................................................. 10Crack formation in the fusion zone ............................................................................. 11Prevention of cracking in the fusion zone ................................................................... 12Filler materials ............................................................................................................ 13Selection of filler materials .......................................................................................... 14

Gas absorption and pore formation ............................................................................ 15

1.7.4 Heat affected zone ................................................................................................. 17Strength characteristics .............................................................................................. 17Characteristics of the HAZ in heat treatable alloys (6xxx and 7xxx alloys) ................ 18

1.8 Imperfections ................................................................................................................. 19

1.8.1 Imperfections in welds Introduction .................................................................... 19

1.8.2 Imperfections in welds External imperfections ................................................... 20

1.8.3 Imperfections in welds Internal irregularities ...................................................... 22

1.8.4 Imperfections in welds Mechanical pores ........................................................... 231.9 Gas shielded arc welding .............................................................................................. 24

1.9.1 Process definition by type of electrode .................................................................. 24

1.9.2 Shielding gases for welding aluminium ................................................................. 251.9.3 Effects of electrode polarity on weld penetration ................................................... 26

1.9.4 Oxide Removal ...................................................................................................... 271.10 MIG welding (to be developed) ................................................................................... 281.11 TIG welding ................................................................................................................. 29

1.11.1 TIG welding processes ........................................................................................ 29

1.11.2 Tungsten electrodes for AC and DC welding currents ........................................ 31

1.11.3 Influence of the type of current on the weld pool geometry ................................. 32

1.11.4 Equipment for TIG welding .................................................................................. 33

1.11.5 Equipment: Welding torch .................................................................................... 34

1.11.6 Influence of type of shielding gas ........................................................................ 35

1.11.7 Technical guidelines ............................................................................................ 36

1.11.8 AC-TIG welding.................................................................................................... 37

TIG Welding with Alternating Current ......................................................................... 37

Arc characteristics with Alternating Current ............................................................... 38Varying the AC current amplitudes and frequencies .................................................. 39Forming the tip of tungsten electrodes for AC welding .............................................. 40Guideline for TIG (AC) welding parameters ............................................................... 41

1.11.9 DC-TIG welding ................................................................................................... 42TIG-He-DC (EN) Process characteristics ................................................................ 42Guideline for TIG-He-DC (EN) welding parameters ................................................... 43Example of TIG-He-DC welded joints ........................................................................ 44

1.12 Plasma arc welding (to be developed) ........................................................................ 451.13 Stud welding ................................................................................................................ 46

1.13.1 Stud welding processes ....................................................................................... 46

1.13.2 Stud welding with lift ignition ................................................................................ 48

1.13.3 Capacitor discharge stud welding (tip ignition) .................................................... 50

1.14 Laser beam welding (to be developed) ....................................................................... 521.15 EB-welding .................................................................................................................. 53

1.15.1 Vacuum Electron Beam Welding (VEBW) ........................................................... 53


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1.15.2 Non-vacuum electron beam welding (NVEBW) ................................................... 56

1.15.3 Example of VEBW welded joints ......................................................................... 57

1.15.4 Example of NVEBW welded joints ....................................................................... 581.16 Hybrid techniques ........................................................................................................ 60

1.16.1 Definition of terms ................................................................................................ 60

1.16.2 Laser-MIG welding ............................................................................................... 61

Laser-MIG welding ..................................................................................................... 61Laser-MIG welding Example of a Laser-MIG joint................................................... 63

1.16.3 Plasma arc MIG ................................................................................................ 64Plasma arc - MIG welding .......................................................................................... 64Plasma arc MIG welding Example of a Plasma arc MIG joint ........................... 65

Fusion welding

1.1 Character ist ics of alum in ium in fusion weld ing

Fusion welding of wrought and cast aluminium componentsis the key joining technologyin automotive structural engineering. Various welding processes (s. "Benchmarking", nextpage) have been adapted to or developed specifically for aluminium, which are described insubsequent chapters.

Characteristics of aluminium relevant to welding performance are listed in the table below inrelation to welding of steels.

To achieve sound and efficient weld joints of aluminium parts the choice of joint design,welding parameters, procedures and processes must consider the following: welding characteristics of the alloys control of imperfections characteristics of the fusion welding processes the joint configuration and weld preparation.

Thermophysical properties of aluminium compared to steel

Source: Vocational TALAT, 1999

Fusion welding of aluminium is today well understood and can be simulated by severalspecific software programmes (Sysweld, Weldsim).


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1.2 Benchm arking of fusion weld ing proc esses

Since there is often a choice between various processes, additional factors must be

considered, e.g.: the existing know how; the given infrastructure; welders knowledge and economics.


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1.3 Joint design fo r arc and beam weld ing

Gas shielded arc and beam welding require different types of joints, s. figures below.

The right choice of joint configuration depends on material thickness

accessible torch or beam positions technological requirements clamping possibilities required tolerances.


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1.4 Joint design fo r beam weld ing w /r to loading d irect ion

In particular for beam welded joints,the joint configuration must be selected with respect to

the principle stressesacting on the joint, i.e. tension, compression or shear loads. Thefigure below gives some recommended joint configuration for the main types of loading.

Shear stress should be largely avoided, because most joints are very sensitive to this kind ofloading.


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1.5 Weld preparat ion The alumin ium oxide f i lm

Literature:

Ostermann, F., Anwendungstechnologie Aluminium, Berlin, Heidelberg, London, NewYork, Tokyo: Springer-Verlag, 1998, ISBN 3-540-62706-5 p. 114

The thickness of the oxide layer covering technical aluminium surfaces is smaller than 0.01mm. The melting point of aluminium oxide is about 2000C in contrast to approximately 660Cof the aluminium metal. In order to form a proper weld the oxide film has to be removed orcracked in the welding process to allow the fusion of the metal.

Properties of the aluminium oxide film:

Low electrical and thermal conductivity Inhomogeneous structure / rough surface with affinity for gas adsorption and surface

contamination potential sources for gas porosity

Oxide density is higher than the density of molten aluminiumpotential source fornon-metallic inclusions in the fusion zone.

Therefore, the application of an appropriate preparation of the surfaces to be welded is mostimportant.


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1.6 Weld preparat ion

The production ofhigh-quality fusion welds of aluminiumparts and components depends

strongly on the quality of the surfaces to be welded. Special care is required to limit porosityand other internal imperfections of the weld zone. The most important factors are:

Correct storage of components, work piece, filler materials (dry, smut-, oil- and dust-free environment)

Avoiding dew point phenomena during transport and storage

Proper surface preparation of joints to remove oxide film, smut, oil, etc.

The amount of porosityin the weld seam will decrease as result of the applied preparationtechnique of the joint surface in the following order:

As-received state (sheared, plasma arc cut or laser cut )

Milling

Grinding

Brushing with rotating CrNi steel brushes

Etching

Etching and scraping (only joint surface)

In critical applications, the cleaned surface produced by etching is subsequently being

protected by a suitable conversion treatment.

The conversion treatment of components ensures ideal welding conditions. However, thisadditional process step proves to be unnecessary provided that the components are properlystored and handled before welding.


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1.7 Weldab il i ty

1.7.1 Introduction

Weldability, i.e. the suitability of an aluminium part or component for welding depends on:

Base material quality(alloy composition, surface characteristics,...)

Design(suitability of design for welding, joint design,...)

Welding process(equipment, welding conditions,...)


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1.7.2 The welding zones (definitions)

Fusion welding produces a locally modified microstructure. Different zones can be identified

as a result of local alloy composition changes and/or the temperature cycle during welding.Depending on the actual heat input and the geometry of the joint, the width of these zonescan vary considerably.

Characteristics of the welding zones:

Fusion zone Melting of materials A and B; if applicable also introduction of filler metal Solidification of the molten metal pool starting from the weld seam

Transition zone

Diffusion of alloying elements along the grain boundaries Melting of eutectic phases at the grain boundaries

Heat affected zone Thermally controlled solid state processes (segregation effects, precipitation

processes, recovery and recrystallisation processes)


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1.7.3 Fusion zone

Solidification in the fusion zone

Solidification in the fusion zone is generally characterised by rapid cooling and solidificationprocesses: Formation of a fine as-cast grain structure, Control of the resulting grain size by appropriate selection of the welding parameters.

Three types of weld pool solidification are being observed: Without eutectic formation pure aluminium, Low grain boundary eutectic content low alloyed aluminium, Sufficient grain boundary eutectic highly alloyed aluminium.

As a result of these different types of solidification, potential defects forming at the grain

boundaries vary.


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Crack formation in the fusion zone

Depending on the specific welding conditions (design, applied welding process and materialcombination), there is a risk for crack formation in the fusion zone. An important factorinfluencing the cracking sensitivity is the solidification range (temperature difference betweenthe liquidus and the solidus).

The risk for crack formation in the fusion zone increases with increasing solidification range ofthe alloy (or the alloy combination) to be welded: Pure aluminium has no solidification range and is resistant to cracking. Alloys containing copper have solidification ranges larger than 100C and therefore

show a high cracking sensitivity.


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Prevention of cracking in the fusion zone

Compensation of cracking tendency by alloying with filler metalThe cracking sensitivity within the fusion zone can be reduced by the use of a filler wire. Theaddition of the filler wire allows changing the alloy content in the fusion zone to an uncriticalalloy concentration.

Two possible ways of reducing the tendency for crack formation are indicated at right:

Alloying with Mg (R-5056A) Possibility A;

Alloying with Si (R-4043A) Possibility B.

Under the assumption of a 50% mixture of parent and filler metal in the weld pool, the graphicshows a significantly decreasing tendency for crack formation.


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Filler materials

Filler metals are generally introduced in the form of wires. The addition of filler metals has twogoals: To avoid the formation of cracks in the fusion zone through an adjustment of the alloy

composition of the molten pool, To provide additional material for gap bridging

Therefore, filler wire alloys have to be adapted to the alloy compositions(s) of bothcomponents to be welded and the strength and formability (as well as other properties) of theweld zone have to meet the product requirements for the final assembly.

The composition of the most important filler metals for aluminium welding are given at right: The most important are the three groups containing Si, Cu, or Mgas main alloying

elements. Depending on the specific alloy, varying amounts of other elements are used, like

Mn, Cr, Tiand Fe.


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Selection of filler materials

Depending on the specific welding technique and material combination, the application of afiller wire is not necessary. However, if a filler wire is being used, it is most important to selectthe proper filler metal composition.

Other measures which have to be considered when using filler metals are: Welding with the largest possible wire diameter reduces the relative surface to the

wire volume and introduces the minimum amount of hydrogen into the weld pool. The storage conditions at the wires have to be carefully controlled (cool and dry).


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Gas absorption and pore formation

Hydrogen and its influence on pore formation

Pores caused by hydrogen are a characteristic phenomenon observed when weldingaluminium and its alloys. The reason for the formation of hydrogen porosity duringsolidification is the ability of liquid aluminium to absorb a large amount of hydrogen. Duringsolidification, the hydrogen solubility decreases by a factor of 20 and the excess gas isreleased and forms pores.

Depending on the solidification morphology of the alloy, two types of hydrogen porosity areobserved: Pores distributed uniformly, Pores linked together in a chain-like fashion.

Low welding speeds (and high energy input) lead to a broad fusion zone and as a result of thelower solidification rate to an unfavourable porosity distribution.

Gas absorption during welding

The high solubility of molten aluminium for hydrogen leads to the absorption of additionalhydrogen in the molten metal pool during welding.

Hydrogen gas and other gases causing pores can be absorbed by Turbulences in the shielding gas envelope; the reason can be a too high or too low

flow rate of the shielding gas. Unstable arc conditions,

Unfavourable torch position, too high slanting angle Entrance of air into the shielding gas nozzle, Impurities on the work piece surface, filler wire, or in the used shielding gases, Inaccurate edge preparation.


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1.7.4 Heat affected zone

Strength characteristics

Every fusion welding process leads to the formation of a heat affected zone in theneighbouring parent metal where - depending on the original microstructure and the location -different thermally assisted microstructural processes can occur. The extension of the heataffected zone depends on the thermal input, which again depends on Heat input of the specific welding technique Welding speed and - for multi-pass welding - pass thickness, Thermal conduction determined by the work piece geometry (and alloy composition).

In general, the heat affected zone is characterised by a local softening (reduction of strength).The resulting strength change depends on the alloy type, the original microstructuralcharacteristics (local strengthening mechanisms) and the active softening mechanisms.


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Characteristics of the HAZ in heat treatable alloys (6xxx and 7xxx alloys)

AlZnMg-alloys (7xxx, Cu-free): The weldable EN AW-7020 offers unique possibilities due to: large solid solution range from 350 to 500 C, low quench rate sensitivity, significant age-hardening effect at room temperature.

Therefore, the HAZ reaches more or less the original strength after 90 days of natural ageing.

AlMgSi-alloys (6xxx): Full re-hardening of AlMgSi alloys requires complete solution heattreatment followed by quenching and artificial ageing; as a result, the welded assembly willmost likely be strongly distorted.Partial re-hardening after welding up to 40% of the parent metal strength is possible.Therefore, it is recommended: To keep the energy input per unit length of the weld as small as possible (reduce

width of HAZ), To weld AlMgSi alloys in the T4 temper followed by age hardening of the complete

assembly (if possible).

Source: Wauschkuhn and Klock, VAW (now Hydro)


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1.8 Imperfect ions

1.8.1 Imperfections in welds

Introduction

Imperfections in welds can be divided into externaland internalirregularities.

Imperfections can also be distinguished with respect to their nature: Physical e.g. cracks in fusion zone Chemical e.g. oxide inclusions Geometrical e.g. edge misalignment


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1.8.2 Imperfections in welds External imperfections

External weld imperfections such as misalignment of edges, notches, poor weld geometry are

described for example for butt welds in EN 30042.

The different irregularities are listed together with an illustration and their specificclassification numbers according to EN 26520 (ISO 6520).

In some cases, e.g. for the irregularity with classification No. 507 "edge misalignment", thereis also a subdivision into different evaluation classes defining allowable limiting values,depending on the required geometrical tolerance.


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1.8.3 Imperfections in welds Internal irregularities

See also:

AAM

Joining

1 Fusion welding > Weldability > Fusion zone > Gas absorption andpore formation

AAM Joining 1 Fusion welding > Weldability > Fusion zone > Solidification in thefusion zone

There are also standards and guidelines for the description of internal imperfections, e.g.guidelines EN 1435, DIN 54109 Pt.2 and DVS pamphlet 1611 (evaluation of radiographs inrailway coach constructions - fusion welded joints in aluminium and its alloys).

The purpose of these standards and guidelines is to allow for a clear description of theobserved defects, e.g. type and distribution of porosity, inclusions, cracks and fusion defects.

The use of these standardised terms enables the establishment of application-oriented

consistent quality specifications for welds.


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1.8.4 Imperfections in welds Mechanical pores

See also:

AAM

Joining

1 Fusion welding > Weldability > Fusion zone > Crack formation inthe fusion zone

"Mechanical pores" are a specific type of internal imperfections which are caused by theentrapping of air during welding.

Formation of mechanical pores is favoured by: the use of high energy welding processes (high welding speed) alloys with small solidification ranges small tapering angle of the weld edges.

This effect can be compensated by: the creation of ventilation areas achieved by a narrow air gap between the work

pieces edge preparation with taper angles of 50-70 reduction of welding speed to enable proper degassing.

Source: VAW (now Hydro)


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1.9 Gas sh ielded arc w elding

1.9.1 Process definition by type of electrode

See also: AAM Joining 1 Fusion welding > MIG welding AAM Joining 1 Fusion welding > TIG welding AAM Joining 1 Fusion welding > Plasma arc welding

This introductory chapter to the following chapters on arc welding of aluminium summarisesthe characteristics common to the relevant processes: MIG welding TIG welding Plasma arc welding (PAW)

Specific information on these processes is described elsewhere (s. Links)

In gas shielded arc welding the weld pool is generated by the heat input of an electric arcdrawn between the electrode and the work piece and is shielded from the atmosphere by aninert gas.


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1.9.2 Shielding gases for welding aluminium

The aluminium weld pool must be shielded from the atmosphere by inert gases. Helium,

argonand their mixtures (Ar/He: 30/70; 50/50 or 70/30) are used in aluminium arc welding, s.EN 439.

The main functions of shielding gases are: shielding the weld pool from atmosphere generating a stable arc

Compared to argon, helium offers the following welding characteristics: lower arc stability higher arc power deeper penetration higher heat concentration higher heat dissipation

higher melting rate

Because of its lower density helium requires a higher flow rate than argon.

Process gases and gas supply system must be clean and free from moisture because evenminute traces of dirt or moisture can cause severe weld porosity.

Sometimes monomix shielding gases are used consisting of inert gases with minute amountsof active gases (O2, N2, NO in less than 0,1 vol.%). The effect of these additions is said toprovide a slight but measurable improvement of energy transfer and consequently of weldspeed. Monomix shielding gases are also said to produce a smoother transition between weldand base metal, thus improving the fatigue properties of welded components.


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1.9.3 Effects of electrode polarity on weld penetration

Both direct (DC) andalternating current(AC)are used for aluminium welding. The form of

the weld pool and of the weld seam can be influenced by current type and electrode polarity.

Three types of arc generation are feasible: Direct current, straight polarity (electrode negative, DC-EN)

deep penetration Direct current, reverse polarity (electrode positive, DC-EP)

low penetration Alternating current (AC)

medium penetration


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1.9.4 Oxide Removal

See also:

AAM

Joining

1 Fusion welding > TIG welding > DC-TIG welding

Aluminium surfaces are always covered by a thin oxide film. Due to its high melting point(2050C vs. 650 C of the metal) the oxide film does not dissolve in the weld bath, but mustbe destroyed to obtain a fused bond line in the joint. Destruction of the oxide is achieved bycleaning action of the electric arc and of the shielding gases. Argon and helium act differently:

Argon: Oxide removal is essentially achieved by ion impingement. The efficiency depends onelectrode polarity (see figure) and is therefore influenced by the type of current: Positive electrodecracking of the oxide film by ion impingement, Negative electrodeno oxide film removal by ion impingement.

Helium: Because of its higher ionisation energy and higher heat dissipation, helium provides

some cleaning action with negatively poled electrodes, see TIG welding (use Links).

Oxide removal by argon arc is essentially caused by ion impingement (above left) and little - if

any - by electron penetration through the oxide barrier (above right).


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1.10 MIG welding (to be developed)

Process principles

Conventional MIG welding

Pulsed MIG welding

MIG welding procedures

Flat wire MIG welding

Multiple wire MIG welding

Preliminary information are available in the alumatter website (see aluMATTER > Processing> Joining > Welding > MIG Welding)


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1.11 TIG weldin g

1.11.1 TIG welding processes

Literature: Schoer, H., Schweien und Hartlten von Aluminiumwerkstoffen, Dsseldorf: Verl. fr

Schweien und Verwandte Verfahren, DVS-Verl., 1998, ISBN 3-87155-177-5 s.Table 10-1, page 200

In TIG welding, an electric arc is maintained between a non consumable tungsten electrodeand the work piece under an inert atmosphere (argon, helium, argon/helium mixtures).

It is possible to work with or without filler metal depending on the hot cracking sensitivity ofthe alloy and on the type of joint geometry.

Filler metal feeding is done either manually by rod or mechanically by wire.

TIG welding generally provides a smooth weld bead with little undercut. MIG joints can betherefore dressed by TIG for improved fatigue life.

The minimum static joint strength of TIG welds, however, is 10 to 15 % lower than that ofcomparable MIG welds of alloys in strain-hardened or age-hardened tempers (s. Lit. above).


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1.11.2 Tungsten electrodes for AC and DC welding currents

Electrodes are usually available as pure tungsten or tungsten with oxide additions of thorium,

zirconium, lanthanum, and cer.Oxide additions reduce the electron emission energy of the electrode: pure tungsten 5,36 eV thoriated tungsten 2,62 eV

For AC welding pure tungsten or zirconiated tungsten is used, while for DC welding thoriatedtungsten or tungsten electrodes with rare earth oxide additions are recommended (thorium isradioactive).

Thoriated electrodes are not recommended for AC welding, since the electrode tip does notform the required hemispherical bulb and gives an unstable arc and unsatisfactory cleaningaction. For DC welding thoriated electrodes provide for a stable arc, improved electrodelifetime, greater current carrying capacity and better arc ignition.


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1.11.3 Influence of the type of current on the weld pool geometry

Literature:

Aluminium-Taschenbuch 14. Aufl., Dsseldorf: Aluminium-Verlag, 1988, ISBN 3-87017-169-3

The form of the electrode tip and of the type of current influence the weld pool profile:

Negatively poled electrode (DC, EN): higher current loading compared to EP polarity, improvement of the electrode's life cycle, large heat input into the work piece, unsatisfactory cleaning action, narrow and deep weld pool.

Positively poled electrode (DC, EP):

excessive heating of the electrode, large diameter electrodes with wide angled tips necessary, good cleaning action of the arc, wide and shallow weld pool in comparison to EN polarity.

Alternating current (AC): compromise of the properties of positive and negative electrode polarity.


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1.11.4 Equipment for TIG welding

TIG welding equipment

Modern power sources are able to deliver direct or alternating current (sinus and square wave), vary the current stepwise or continuously, work with a constant current characteristic for control of the length of the arc.


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1.11.5 Equipment: Welding torch

See also:

AAM

Joining

1 Fusion welding > Weldability

Features: Gas nozzle and burning spot are cooled by the shielding gas. Air tightness of gas and cooling water channels must be assured, since humidity of

the shielding gas will produce porosity. Gas nozzle is made of metal or ceramics and has to be insulated against electricity

conducting parts. Electrode protrudes about 2 to 4 mm beyond nozzle.

Welding torch cooling:Depending on the severity of the thermal load torches are either air cooled (for light duty) or

water cooled (for currents>100 A cooling of torch and current cable is necessary).

Welding torches need space for welding operation!


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1.11.6 Influence of type of shielding gas

See also:

AAM

Joining

1 Fusion welding > Gas shielded arc welding > Shielding gases forwelding aluminium

The choice of shielding gas (argon, helium, Ar/He mixtures and additives) as well as thecurrent type and polarity influences the heat transfer of the arc and affects the profile of theweld seam.

The cross sections show a flat and broad penetration during positive polarity under argon gasshielding (bottom macrograph). Mixing of helium to argon in alternating current welding,produces a broader penetration profile.


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1.11.7 Technical guidelines

General guidelines:

Depending on the alloy type, TIG welding can be carried out without filler wire (e.g.for EN AW-5xxx alloys) or an appropriate filler wire must be used (e.g. for EN AW-6xxx alloys).

Generally, welding with filler wire improves gap bridging. For joining thick parts (> 8 mm), preheating is required. Preheating also helps to avoid hot cracking when welding higher gauge crack

sensitive alloys. Welding of butt-joints can be improved by the use of weld root supports and sloping

edges with an opening angle of approx. 60.


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1.11.8 AC-TIG welding

TIG Welding with Alternating Current

In conventional TIG welding alternating current is generally used because of the balancedheat input to electrode and work piece. The sine wave AC power requires an additional HFgenerator to ignite and stabilize the arc.

The filler metal is added to the weld pool separately from the torch.

Characteristics of TIG (AC) welding: Welding speeds from 0.15 to 0.5 m/min, Filler wire diameter from 2 to 6 mm, Electrode diameter from 1.0 to 6.4 mm, Welding current from 40 A to 380 A,

Material thickness from 0.5 to 8 mm; single or multi pass welds, Use of pure tungsten electrodes or thoriated electrodes with spherical tip, High cleaning effect.


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Arc characteristics with Alternating Current

The use of a balanced alternating current in TIG welding is a compromise between DC (EP)and DC (EN) welding characteristics. It distributes the heat evenly among electrode and weldpool extending the electrode's life cycle and provides sufficient cleaning, although penetrationis less than with DC (EN).

The burning process of the arc is characterised as follows: During each current cycle the arc extinguishes and re-ignites twice. To re-ignite and

stabilise the arc a HF-power generator gives high voltage pulses at the beginning ofthe positive and negative half cycles.

During the positive half cycle arc cleaning occurs by intensive gas ion bombardmentof the work piece removing the oxide film. Electrons emitting from the work piece heatthe electrode tip.

During the negative half cycle electrons emitting from the electrode generate a highheat input to the work piece. Ions striking the electrode cause no heating so that itcan cool down.


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Varying the AC current amplitudes and frequencies

Pulsating Square Wave Alternating Current

Modern welding power sources permit the generation of pulsating square wave alternatingcurrents.

The main advantages are: due to the form of the square wave the process does not require a HF power source

for arc ignition (no interference with radio frequency), variability of the frequency and amplitude of each half cycle (s. figure below) for more

cleaning action or higher penetration.


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Forming the tip of tungsten electrodes for AC welding

The choice of the electrode diameter is determined by the thickness of the work piece and thecurrent magnitude (s. Table on page 5). For a stable arc a hemispherical form of the electrodetip is required. The procedure of electrode tip forming is as follows: The electrode is briefly loaded with an excessive current, which melts the tip to form a

hemispherical point. The current is then reduced to permit the tip to solidify. Overheating or underloading the electrode will produce unsatisfactory tip geometry,

see figure below. If a larger than normal electrode diameter is chosen, the electrode tip must be

tapered by grinding.


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Guideline for TIG (AC) welding parameters


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1.11.9 DC-TIG welding

TIG-He-DC (EN) Process characteristics

Process characteristics:

Polarity of the tungsten electrode: negative (EN) Shielding gas type: helium. The arc ignition occurs under argon. When a stable arc is

achieved, the shielding gas is switched to helium. No cleaning effect by oxide cracking Fusion of the joint faces occurs by melting of the oxide film High penetration due to high heat concentration High welding rates of 1.5 to 2.0 m/min. Risk of oxide inclusions and fusion defects in the root Only mechanised welding possible because of the required short arc of < 1 mm

Use of a tapered electrode tip is recommended.


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Guideline for TIG-He-DC (EN) welding parameters


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Example of TIG-He-DC welded joints

Pressure Vessel for Passenger Car Airbag

Circumferential joining of the two vessel parts.

Alloy: EN AW-6060 (AlMgSi);Wall thickness: 0.5 mm

Welding speed. 1 m/min


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1.12 Plasma arc weldin g (to b e developed)


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1.13 Stud w elding

1.13.1 Stud welding processes

Literature: Schmitt, K.G.: Strength and limits of short duration drawn arc stud welding in light

weight automotive applications. IIW Seminar "Trends in welding of lightweightautonotive and railroad vehicles". Wels, Austria Febr. 1997

Aluminium-Taschenbuch 15. Aufl., Bd. 3: Weiterverarbeitung und Anwendung,Dsseldorf: Aluminium-Verlag, 1997, ISBN 3-87017-243-6

DIN EN ISO 14555 (12.98): Welding; Stud welding of metallic materials. N.N.: Lichtbogenbolzenschweien mit Hubzndung. DVS-Merkblatt 0902 (07.1988),

Dsseldorf: DVS-Verlag N.N.: Lichtborgenbolzenschweien mit Spitzenzndung. DVS-Merkblatt 0903

(06.1989). Dsseldorf: DVS-Verlag Jenicek, A., Nentwig, A.W.E.: Bolzenchweien von Aluminiumwerkstoffen. Metallbau

Kurier. Pt. 1 Heft 10/2000 p. 80-83. Pt 2 Heft 11/2000 p. 49-51 Welz, W., Nentwig, A.W.E., Jenicek, A.: Bolzenschweien mit Hubzndung an

Aluminiumwerkstoffen. ALUMINIUM 67 (1991) 2 p. 153-159 Nentwig, A.W.E., Jenicek, A.: Qualittsbestimmende Einflufaktoren beim

Kurzzeitbolzenschweien von Stahl und Aluminium. Schweien und Schneiden 48(1996) 10, p. 763-769

Stud weldingallows the joining of bolts and similar attachments to sheet, extrusions or(weldable) castings without drilling holes, with single side access and without back-side support.

The fusion heat is supplied by a high intensity arc struck between the tip of the stud and thework piece.

Stud welding processesapplicable to aluminium are characterised in the table at right. Themain distinction between the listed methods is the type of ignition of the arc: Lift ignition or Tip ignition.


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Stud welding methods applicable to aluminium:

Source: A. Jenicek and A.W.E. Nentwig, 2000


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1.13.2 Stud welding with lift ignition

AlMg3 stud weld, 8 mm diam., surface fine ground

Source: Welz, Nentwig, Jenicek, 1991

Shielding gas: Stud welding of aluminium with lift ignition requires inert gas shielding of theweld pool in order to achieve an acceptable quality of the weld: mixtures of Ar + 25-50% He)at 2-4 l/min. are recommended.

Surface preparation: Removal of oxide layer by mechanical (brushing, grinding) or chemical(10% NaOH) means is necessary to avoid imperfect welds.

Porosity in the weld zone: Porosity in aluminium arc stud welding cannot be avoided due torapid cooling of the weld. However, the amount of porosity depends greatly on surfacepreparation and optimised gas shielding procedure.

Short duration stud weldingdoes not need ceramic ferrules because of the smaller weldpool. It is, therefore, easier to adapt to automatic processes.

Stud materials: non-age-hardening alloys EN AW-Al99.5, -AlMg3, -AlMg5.

Substrate materials: low and medium strength age-hardening and non-age-hardening

wrought and weldable casting alloys.

For lift ignition stud weldingthe minimum wall thickness of the substrate is approx. 1.0 mm(0.8 - 1.5 mm, depending on alloy and temper).


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Schematic of stud welding with lift ignition, ceramic ferrule and shielding gas


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1.13.3 Capacitor discharge stud welding (tip ignition)

Capacitor discharge stud weld

Source: A. Jenicek and A.W.E. Nentwig, 2000

Because of the low thermal input of the capacitor discharge stud weldinglower wallthicknesses are possible in comparison with arc stud welding with lift ignition. In fact, evendiscolouration of the lacquered backside can be avoided.

Shielding gas is not necessarybecause of the extremely short arc duration.

Surface preparationby mechanical (brushing, grinding) or chemical means is extremelyimportant for good weld quality.

The process can be automated.Quality controlcan only be performed destructively, e.g. by bending the stud.

Stud materials: work-hardening alloys EN AW-Al99.5, -AlMg3, -AlMg5.

Substrate materials: low and medium strength age-hardening and work-hardening wroughtand casting alloys.


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Schematic of stud welding with tip ignition


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1.14 Laser beam welding (to b e developed)

See also the aluMATTER website: aluMATTER > Processing > Joining > Welding > LaserWelding)


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1.15 EB-welding

1.15.1 Vacuum Electron Beam Welding (VEBW)

Characteristics of the VEBW fusion process: The concentrated energy of the narrow electron beam penetrates the aluminium to

great depth, vaporisation of the metal leads to a keyhole surrounded by molten metal.When the beam moves forward, it melts the metal in front of the keyhole, which thenflows to the rear of the keyhole and solidifies to form the weld.

The high energy density of the beam of > 106W/cm

2destroys and melts the oxide

film. Vacuum hinders the reaction of molten metal with reactive gases. The beam energy is directly coupled into the metal. The heat affected zone is very narrow, and distortion is low.

Weld thickness can be up to 150 mm in a single pass with depth-to-width ratios up to45:1. High welding speeds (>>10 m/min.) are possible. Occurrence of hot cracking is reduced, many alloys can be welded autogenously

(without filler). Joint properties are excellent, but depend on alloy composition.

Main drawbacks: High capital cost, Pump cycle time (may be reduced by twin chamber machines), Exclusively automated welding, Protection from the resulting X-rays necessary, Tight fitting tolerances of joint.

EB welding under vacuum causes a low divergence of the electron beam: Spot diameter between 0.1 and 1 mm. Beam length up to 1000 mm. Beam power up to 100 kW. Beam travel over work piece either by electromagnetic deflection or mechanical

motion or both. Possibility of rapid changes of weld direction combined with high welding speeds. It is important to maintain the cavity long enough for optimum degassing to keep

porosity to a minimum. Beam oscillation by modulation of the frequencies of the free programmable deflector

coils in a range between 1 Hz and 100 kHz improves out-gassing of the weld and gapbridging.

Simultaneous mechanical motion and beam defection is possible at high speeds.


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For VEBW the preferred joint geometries are Lap joints

Simple butt joints Weld pool supported joints Joints with positioning guides Geometries for penetration improvement Thick sheets can be welded with square butt joints


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1.15.2 Non-vacuum electron beam welding (NVEBW)

Main features of NVEBW (compared to VEBW):

In NVEBW emergence of the beam into the atmosphere is made possible by passingthe beam through differential pressure chambers and small orifices.

Scattering of electrons at gas molecules causes a strong beam divergence andreduces the deep welding effect.

Working distance between work piece and orifice limited to 5-25 mm. Beam energy up to 30 KW. Use of shielding gas (He, Ar) required. No productivity loss caused by evacuation, but X-ray protection system needed. Weld penetration up to 25 mm. High welding speed (>10 m/min.). Reduced loss of alloying elements with low evaporation temperature (e.g. Mg, Zn). Tolerance for joint gap up to 0.5 mm.


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1.15.3 Example of VEBW welded joints

Vacuum Electron Beam welding of a multi-part piston for automotive use.

Due to the enclosed cooling channel this piston can't be produced by a single casting.

Parts to be welded are The piston base The upper segment including

a special piston ring and

a circumferential cooling channel.

Note that the VEBW welding achieves a depth-to-width ratio of 45:1.


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1.15.4 Example of NVEBW welded joints

See also:

AAM

Applications

3 Car body > Dash board / carrier

Cross beam of the instrument panel supportfor the VW Group's PQ24 platform.

Cross section of NVEBW welded support beam for an instrument panel

Source: Alcan (Constellium since 2011), Singen

Source: Alcan (Constellium since 2011), Singen


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The beam is produced by NVEBW from two stamped EN AW-5754 half shells.Apart from the high weld quality and good productivity ensured by this technology, the bigadvantage is the round shape of the weld (safe manual handling on the assembly line, nodanger of scrubbing of electric wire insulation during service, etc.).


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1.16 Hybr id techn iques

1.16.1 Definition of terms

Hybrid welding techniquesare a combination of two different welding processes with theintention to achieve an optimum in weld quality and welding speed by combining theadvantages of the individual processes.

The term "Hybrid welding" generally describes Laser-MIG welding, but could be used for anyother combination of welding techniques.

The combination of Plasma arc welding and MIG welding can also be considered a hybridwelding technique.

Characteristics of the individual processes:

Laser-Beam Welding High welding speed, High flexibility, Low thermal distortion of the parent metal, Low rate of equipment wear, Sensitive to gap bridging, High demand for edge preparation.

MIG, TIG, Plasma arc Welding High efficiency, Good gap bridging (further improved by filler wire),

Lower welding speeds, High thermal input (thermal distortion).


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1.16.2 Laser-MIG welding

Laser-MIG welding

Advantages of Laser-MIG hybrid welds: Lower tolerance requirements for edge preparation (lower sensitivity for gap width

variation) vs. Laser welding, Deep welding effect of the laser beam, Higher welding speed than MIG, Improved weld seam quality (less blowholes, porosity, undercuts, solidification

cracks, better seam surface quality) vs. Laser or MIG welding, Smoother thickness transition and bead surface are obtained than by laser alone.

Possible process combinations:

CO2-Laser MIG weldingNd:YAG-Laser and TIG welding( Diode-Laser ) Plasma arc


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Laser-MIG welding Example of a Laser-MIG joint

Overlapped welding of extruded and cast aluminium alloys with a varying thickness from 2,3to 4 mm.

Parameters: Shielding gas (Laser + MIG) 27 l/min argon Welding speed 1,8 to 2,5 m/min

Laser unit: Nd:YAG-Laser / Laser power 2,9 KW Beam quality [k] 0,016 Focus 150 mm

MIG unit: Used filler was SG-AlSi5; diameter 1,6 mm Average current 210 A Welding voltage 23 V Wire feeding rate 5 m/min

In a test of fatigue strength the endurance was three times higher compared to simply MIGwelded components.

Source: AUDI AG


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1.16.3 Plasma arc MIG

Plasma arc - MIG welding

Plasma arc - MIG welding is a combination of the MIG and Plasma arc welding process,achieving an increase of the filler wire melting rate by adding the plasma arc, preheating the work piece and filler wire, thereby

avoiding cold shut defects at the weld start

higher melting rate. Square butt joints can be welded even in thicker sheets Unsuitable for manual welding due to the required large sized welding torches.


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Plasma arc MIG welding Example of a Plasma arc MIG joint

Joining of a wheel disk to the rim band: Improvement of the starting and ending areas byPlasma arc-MIG welding provides better fatigue strength of the weld.

Parameters: Shielding gas 70% Ar / 30% He ; 6 l/min Welding speed 1.5 m/min Wire feeding rate 13.9 m/min Used filler was SG-AlSi5 diameter 1,6 mm

Plasma unit: Average current 180 A Welding voltage 28.4 V Plasma gas mix 70% Ar / 30% He ; 18 l/min

MIG unit: Average current 230 A Welding voltage 20.2 V

Source: Hydro Aluminium Deutschland

Documents

AAM Joining 1 Fusion Welding