Download - 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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GTAW power sourceGTAW power source
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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