M7 GTAW

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ModulModul 77

MSMS--455 TEKNIK PENYAMBUNGAN LOGAM455 TEKNIK PENYAMBUNGAN LOGAM

Gas tungsten arc welding (GTAW) PrinsipPrinsip KerjaKerja GTAWGTAW ArusArus listriklistrik GTAWGTAW ElektrodaElektroda Gas Pelindung

Plasma arc welding (PAW)

Electroslag welding (ESW)

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Gas tungsten arc welding (GTAW)Gas tungsten arc welding (GTAW)

• Proses pengelasan dimana panas yang

digunakan berasal dari arc yang terbentuk

antara elektroda tungsten (non-

consumable) dan benda kerja.

• Sering juga disebut Tungsten Inert Gas

(TIG).

• Awalnya disebut Argonarc atau Heliarc.

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•Elektroda tungsten, logam cair, arc dan

daerah yang terpanaskan dilindungi oleh

gas pelindung untuk mencegah

kontaminasi dengan udara sekitarnya.

•Karena elektroda tungsten tidak

meleleh, maka diperlukan logam pengisi

(filler metal).

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GTAW power sourceGTAW power source

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Skematik proses GTAWSkematik proses GTAW

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Direct current electrode negative (DCEN)

is produced when the electrode is

connected to the negative terminal of the

power source. Since the electrons flow

from the electrode to the plate,

approximately 70% of the heat of the arc

is concentrated at the work, and

approximately 30% at the electrode end.

This allows the use of smaller tungsten

electrodes that produce a relatively narrow

concentrated arc.

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The weld shape has deep penetration and

is quite narrow. Direct current electrode

negative is suitable for welding most

metals. Magnesium and aluminum have

a refractory oxide coating on the surface

that must be physically removed

immediately prior to welding if DCSP is to

be used.

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Direct current electrode positive (DCEP) is

produced when the electrode is connected

to the positive terminal of the welding

power source. In this condition, the

electrons flow from the work to the

electrode tip, concentrating approximately

70% of the heat of the arc at the

electrode and 30% at the work.

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This higher heat at the electrode

necessitates using larger diameter

tungsten to prevent it from melting and

contaminating the weld metal. Since the

electrode diameter is larger and the heat

is less concentrated at the work, the

resultant weld bead is relatively wide and

shallow.

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Aluminum and magnesium are two metals

that have a heavy oxide coating that acts

as an insulator and must be removed

before successful welding can take

place. Welding with electrode positive

provides a good oxide cleaning action in

the arc. If we were to study the physics of

the welding arc, we find that the electric

current causes the shielding gas atoms to

lose some of their electrons.

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Since electrons are negatively charged,

these gas atoms now are unbalanced and

have an excessive positive charge. As we

learned in Lesson I, unlike charges

attract. These positively charged atoms

(or positive ions as they are known in

chemical terminology) are attracted to the

negative pole, in this case the work, at

high velocity.

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Upon striking the work surface, they

dislodge the oxide coating permitting good

electrical conductivity for the maintenance

of the arc, and eliminate the impurities in

the weld metal that could be caused by

these oxides.

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Direct current electrode positive is rarely

used in gas-tungsten arc welding. Despite

the excellent oxide cleaning action, the

lower heat input in the weld area makes it

a slow process, and in metals having higher

thermal conductivity, the heat is rapidly

conducted away from the weld zone. When

used, DCEP is restricted to welding thin

sections (under 1/8") of magnesium and

aluminum.

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Alternating current is actually a

combination of DCEN and DCEP and is

widely used for welding aluminum. In a

sense, the advantages of both DC

processes are combined, and the weld

bead produced is a compromise of the

two. Remember that when welding with

60 Hz current, the electron flow from the

electrode tip to the work reverses direction

120 times every second.

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Thereby, the intense heat alternates from

electrode to work piece, allowing the use of

an intermediate size electrode.

The weld bead is a compromise having

medium penetration and bead width. The

gas ions blast the oxides from the surface

of aluminum and magnesium during the

positive half cycle.

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KeuntunganKeuntungan::

• Kekuatan yang tinggi pada sambungan.

• Sambungan lebih ulet.

• Ketahanan korosi sambungan baik.

• Tidak menggunakan fluks.

• Lasan lebih rapih dan kehadiran cacatsangat rendah.

• Sedikit asap dan percikan.

• Lasan mudah diamati oleh juru las.

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• Dapat digunakan untuk berbagai posisipengelasan dengan sedikit percikan.

• Sedikit membutuhkan metal finisihing.

• Sedikit distorsi.

• Dapat menyambungkan logam yangberbeda (dissimilar metals).

• Dapat digunakan dengan atau tanpalogam pengisi (filler metal).

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Kerugian:

• Laju deposit rendah.

• Tidak ekonomis untuk elektroda denganukuran lebih besar dari 3/8 inch.

• Torch khusus.

• Jika ada angin perlindungan gas buruk.

• Tidak mentolelir adanya kontaminasiterhadap logam pengisi atau logaminduk.

• Jika pendingin bocor akan menyebabkanporositas pada sambungan.

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• Saat menggunakan DC dapat terjadi arc

blow.

• Pada AC menggunakan generator

frekuensi tinggi

• Harus mahir (OFW skills).

• Peralatan permanen (not portable).

• Dapat terkontaminasi oleh elektroda

(tungsten contamination) pada lasan.

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The American Welding Society has

published Specification AWS A5.12-80 for

tungsten arc welding electrodes that

classifies the electrodes on the basis of

their chemical composition, size and finish.

ElektrodaElektrodaElektroda

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1. Pure Tungsten (AWS EWP)

Color Code: Green

Used for less critical applications. The

cost is low and they give good results at

relatively low currents on a variety of

metals. Most stable arc when used on AC,

either balanced wave or continuous high

frequency.

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2. 1% Thoriated Tungsten (AWS EWTh-1)

Color Code: Yellow

Good current carrying capacity, easy arc

starting and provide a stable arc. Less

susceptible to contamination. Designed for

DC applications of nonferrous materials.

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3. 2% Thoriated Tungsten (AWS EWTh-2)

Color Code: Red

Longer life than 1% Thoriated

electrodes. Maintain the pointed end

longer, used for light gauge critical welds in

aircraft work. Like 1%, designed for DC

applications for nonferrous materials.

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4. .5% Thoriated Tungsten (AWS EWTh-3)

Color Code: Blue

Sometimes called "striped" electrode

because it has 1.0-2.0% Thoria inserted in

a wedge-shaped groove throughout its

length. Combines the good properties of

pure and thoriated electrodes. Can be

used on either AC or DC applications.

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5. Zirconia Tungsten (AWS EWZr)

Color Code: Brown

Longer life than pure tungsten. Better

performance when welding with AC. Melts

more easily than thoriam-tungsten when

forming rounded or tapered tungsten

end. Ideal for applications where tungsten

contamination must be minimized.

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Gas pelindungGasGas pelindungpelindung

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Plasma arc welding (PAW)Plasma arc welding (PAW)

Salah satu proses pengelasan, dimana panas yangdigunakan merupakan perpaduan antara arc denganplasma (menggunakan superheated plasma).

Arc dan plasma diselubungi oleh gas inert yangberfungsi pelindung keduanya dari kontaminasi udarasekitarnya.

Secara operasional PAW mirip dengan GTAW.Perbedaan utamanya terletak pada komposisi busurplasmanya, kerapatan campuran bsur plasma lebihtinggi dibandingkan pada GTAW karena lubang noselrelatif lebih kecil

Proses awal PAW ini dengan cara mengatur bentukaliran plasma dan gas pelindung.

Yang pertama terbentuk adalah terjadinya nyala busurpemandu (pilot arc) diantara elektroda dan nosel.

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Electroslag welding (ESW)Electroslag welding (ESW)

Electroslag Welding (ESW) deposits the weld metal into the weld

cavity between the two plates to be joined. This space is enclosed by

water cooled copper dams or shoes to prevent molten slag from

running off. The weld metal is produced from a filler wire that forms

an initial arc with the workpiece until a sufficient pool of liquid metal

is formed to use the electrical resistance of the molten slag.

This process requires special equipment used primarily for horizontal

welds of very large plates up to 36 inches or more by welding them

in one pass as in large machinery and nuclear reactor vessels.

There are also variations of ESW where shielding is provided by an

appropriate gas and a continuous arc is used to provide weld metal.

These are termed Electrogas Welding or EGW machines.

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Equipment used for electroslag weldingoperations.Equipment used for electroslag weldingoperations.

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Similar to Electro gas weldingSimilar to Electro gas welding

Difference is Arc is started betweenDifference is Arc is started betweenelectrode tip andelectrode tip and bottombottom part of thepart of thepart to be weldedpart to be welded

Flux added first and then melted by theFlux added first and then melted by theheat on the archeat on the arc

Molten slag reaches the tip of theMolten slag reaches the tip of theelectrode and the arc is extinguishedelectrode and the arc is extinguished

Heat is then continuously produced byHeat is then continuously produced byelectrical resistance of the molten slagelectrical resistance of the molten slag

Single or multiple solid as well as fluxSingle or multiple solid as well as flux--cored electrodes may be usedcored electrodes may be used

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used for thick plates (13used for thick plates (13 -- 900 mm.)900 mm.)

resistance heating raised temp. toresistance heating raised temp. to18001800ooC. Molten slag melts the edgesC. Molten slag melts the edgesofof workpieceworkpiece and the electrode whichand the electrode whichsupply the filler materialsupply the filler material

best for vertical jointsbest for vertical joints

edge preparation minimaledge preparation minimal

M7 GTAW

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