VIII. Proceso GTAW (Gas Tungsten Arc Welding)
El proceso GTAW (Gas Tungsten Arc Welding) también conocido como TIG (Tungsten
Inert Gas), es un procesos de arco eléctrico en donde el calor para soldar es
generado por un arco eléctrico entre el extremo de un electrodo de tungsteno no
consumible y el metal base. Metal de aporte puede, o no puede ser añadido y la
protección del arco es proporcionado por un gas, el cual puede o no, ser inerte.
1
Plasma
2
8.1. Aplicación
Actualmente el proceso TIG es ampliamente utilizado
para soldar aleaciones ferrosas y no ferrosas.
Aceros al carbono
Aceros inoxidables
Aluminio y aleaciones
Aleaciones de magnesio
Aceros de baja aleación
Cobre y aleaciones
3
8.1. Aplicación
Níquel y aleaciones
(Resistente a la corrosión y
a las altas Temperaturas)
Zirconio y aleaciones
Titanio y aleaciones
(Alta resistencia a la
corrosión)
(Más ligero que el acero y
tiene alta resistencia a la
corrosión y mecánica)
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8.2. Características del Proceso GTAW
• Es muy versátil, ya que puede soldar una amplia variedad de aleaciones
metálicas.
• Puede ser utilizado en todas posiciones
• Es bueno para soldar espesores delgados
• La pileta líquida es visible al soldador
• No produce escoria
• El metal de aporte no es transferido a través del arco
• No produce salpicaduras
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8.2. Características del Proceso GTAW
VENTAJAS
• Puede ser utilizado para soldar la mayoría de las aleaciones metálicas
utilizadas en la industria.
• No hay escoria, por lo que no es necesaria después de soldar
• No hay salpicaduras
• No es necesario metal de aporte
• Puede ser utilizado fácilmente en todas posiciones.
• Puede ser utilizada la corriente pulsante para reducir el aporte térmico
• El arco y la pileta de soldadura son visibles al soldador
• Debido a que el aporte de metal no es a través del arco, la cantidad añadida
no es dependiente del nivel de corriente utilizado
DESVENTAJAS
• La velocidad de soldadura es relativamente lento
• El electrodo es fácilmente contaminado
• No es recomendable para soldar grandes espesores debido a su bajas tasas
de depositación
• El arco requiere protección de las corrientes de aire
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8.3. Método de aplicación
• Manual
• Semiautomático
• Máquina
• Automático
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8.4. Equipo para el proceso GTAW
1.
2.
3.
4.
5.
6.
7.
8.
Fuente de potencia
Cables
Antorcha
Electrodo
Pedal (opcional)
Sistema de suministro de gas
Sistema de enfriamiento
Pinza tierra
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8.4.1. Fuente de potencia
Característica externa voltaje-amperes para el proceso GTAW
(corriente constante)
9
8.4.1. Fuente de potencia
Ciclo de trabajo
• 60% - Método de aplicación manual y semiautomático
• 100% - Método de aplicación a maquina y automático
% CT 
( I tasada )
2
( I c arg a )
2
( CT tasado )
Por ejemplo, si una máquina de soldar tiene tasada un ciclo de trabajo del
60% a 300 amp, el ciclo de trabajo de la máquina cuando es operada a 250
amp, será:
% CT 
( 300 )
2
( 250 )
2
( 60 )  86 %
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8.4.1. Fuente de potencia
Ciclo de trabajo
Ciclo de trabajo versus corriente de carga
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8.4.1. Fuente de potencia
Polaridad directa
(electrodo negativo)
Todos los metales. Para
las aleaciones de Al y
Mg,
deben
usarse
procedimientos
especiales
Corriente directa
Polaridad invertida
(electrodo positivo)
Poco utilizado, porque la
capacidad de conducción de
corriente del electrodo es
extremadamente baja
Direct
current,
straight
polarity
(electrode negative,
DC-EN) → deep
penetration
Direct
current,
reverse
polarity
(electrode positive,
DC-EP) → low
penetration
Polaridad directa e invertida
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8.4.1. Fuente de potencia
Corriente pulsante
Terminología de la corriente pulsante
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Corriente pulsante
The most important weld parameters
are:
pulse current lp
background current lG
pulse current time tp
background current time tG
pulse frequency fp = 1 / tc
Where: tc = duration of period
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Corriente pulsante
Concerning the equipment of TIG-pulsed arc welding, a relatively new pulsed arc
welding process has emerged, which only modifies by the current (pulse amplitude,
impulse frequency, mark-space ratio).
During the impulses where high current is present in the pulse arc process, a large
amount of heat is generated in the welding area. This results in fusion of the work
material.
In the impulse pause where low current is preset, only a little heat is transmitted into
the workpiece, thus the weld pool stays comparatively cool. The low currents during
the background current time only serve to maintain the arc in order to avoid
interruptions and ignition difficulties. When welding with a filler, the filler will be fused
with the base material during the impulse phase. The impulse frequency is usually
between 0.5 Hz and 10 Hz.
15
Corriente pulsante
The weld heat input can be considerably changed by the choice of times and current
values. In the extreme case a weld seam can consist of fusion welding points which
lie next to each other or overlap.
Thanks to the TIG-pulsed arc welding, the area of application of the TIG-process can
be extended to low power values thus material thicknesses can be reduced and the
weld seam appearance can again be improved.
For welding aluminium with a DC supply it is only possible to use helium as shielding
gas.
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Corriente pulsante
Advantages:
• possibility of lower energy inputs
• better depth-to-width ratio in the case of higher thickness
• more stable arc
• more uniform root formation
• better ‘out of position’ weldability
• less workpiece distortion
• better modulation of the welding pool
• better gap bridging ability
Disadvantages:
• welding equipment is expensive
• equipment set up is more complicated
17
8.4.1. Fuente de potencia
Soldadura producida por la corriente pulsante
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8.4.1. Fuente de potencia
Corriente alterna
Aleaciones de Al y Mg
• Mejor acción limpiadora de los óxidos en la superficie
• Mejor acción y suave acción de la soldadura
• No hay reducción en la salida ajustada de un transformador convencional
• Deben utilizarse de electrodos de mayor diámetro
• Los sistemas del balanceo de onda lo hacen muy caro
Alternating current (AC) → medium penetration
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Corriente alterna
Destruction of the Oxide Layer
In the case with the electrode as cathode, the emitted electrons meet the anodic poled
workpiece and, by the conversion of kinetic energy, they generate a large amount of heat on
the point of contact and thus give a deep penetration. In comparison, the electrode tip is only
heating up a small amount, due to the upcoming gas ions, which in contrast to the electrons
show a larger mass, but generate less heat and are considerably slower than the electrons.
Operating using this polarity the oxide layer is not destroyed, which in reality means that this
polarity is not suitable for aluminium welding.
In the case with the electrode as anode, the emitted electrons meet the electrode and heat it
up rapidly. In comparison, the workpiece which is poled as a cathode is only heated a
20
relatively small amount, thus only a flat penetration arises.
Corriente alterna
Destruction of the Oxide Layer
At this polarity a cleaning effect comes up, i.e. the oxide skin is torn apart and
detached. This effect is explained by the fact that the relatively heavy ions meet
the oxide skin and destroy it. At this polarity however, the high thermal load on the
tungsten electrode leads to a rapid destruction of the tungsten.
By using this kind of polarity several welding procedures can carried out by using
disproportionately thick tungsten electrodes for thin plates. However, this kind of 21
polarisation is not generally used for TIG welding.
Corriente alterna
TIG welding with an alternating current is mostly used for practical
fabrication.
During the positive half wave a cleaning effect occurs and the
tungsten electrode rapidly heats up; during the negative half wave
the electrode is allowed to cool down.
Consequently, the advantages of both kinds of direct current
polarity are united. Since the arc goes out at every current zero
crossing, it was traditionally supplied by a high frequency overlay
(150 kHz at 1500 to 2000 Amps) in order to facilitate a re-ignition
of the arc.
These machines have now been replaced by pulse generators that
do not constantly send out high frequency stress impulses, but
instead supply a smooth sinusoidal voltage. This has the
advantage that they are far less like to influence radio and TV
receptions in the close environment and, as a consequence, do
not have to be signed up at the federal post office.
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8.4.1. Fuente de potencia
Fuentes programables que permiten el método de aplicación automático
23
8.4.1. Fuente de potencia
Tipo de fuentes de potencia según sus características constructivas
Estáticas
Tipo transformador-rectificador
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8.4.1. Fuente de potencia
Tipo de fuentes de potencia según sus características constructivas
• Rotativas
Tipo motor generador de
combustión interna
Tipo motor generador
25
8.4.2. Cables
26
8.4.3. Antorcha
27
8.4.3. Antorcha
28
8.4.4. Electrodos
• Tungsteno puro (W)
• Zirconio -Tungsteno (Zr-W)
• Thorio – Tungsteno (Th-W)
•Lantano -Tungsteno (La-W)
• Cerio – Tungsteno (Ce-W)
• Itrio -Tungsteno (I-W)
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8.4.4. Electrodos
Clasificación
AWS
% W (mínimo)
por diferencia
Th
%
Zr
%
Total de otros
elementos
EWP
99.5
-
-
0.5
EWTh-1
98.5
0.8 – 1.2
-
0.5
EWTh-2
97.5
1.7 – 2.2
-
0.5
EWTh-3 (a)
98.95
0.35 -0.55
-
0.5
EWZr
99.2
-
0.15-0.40
0.5
Clasificación de los electrodos de Tungsteno (AWS A5.12)
Composition
EN-Classification
Tungsten (pure)
WP
Tungsten with 1% thoria
WT 10
Tungsten with 2% thoria
WT 20
Tungsten with 3% thoria
WT 30
Tungsten with 4% thoria
WT 40
Tungsten with 0.8% zirconia
WZ 8
Tungsten with 1% lanthana
WL 10
Clasificación de los electrodos de Tungsteno (EN-Norma europea)
30
8.4.4. Electrodos
31
8.4.4. Electrodos
W
The pure tungsten electrode is the one without any addition of oxide. It has a low
current capacity and is easily burnt. It is suitable for application under the
condition of AC and in the situation of low welding requirement
32
8.4.4. Electrodos
Zr-W
Zirconiated Tungsten Electrodes have good performance in AC welding.
Especially under high load current its excellent performance can not be
replaced by any other products.
33
8.4.4. Electrodos
Th-W
Thoriated tungsten electrode is a good general use tungsten for DC applications,
because it operates well even when overlload with extra amperage, thus improves
the performance of welding.
34
8.4.4. Electrodos
La-W
The lanthanated tungsten electrode has outstanding welding performance without radiation
hazard. And its electric conductivity is the most close to that of 2% thoriated tungsten
electrode. It enable welders to replace the thoriated tungsten electrode by lanthanated
tungsten electrode easily and conveniently without any change in welding procedure.
Therefore, lanthanated tungsten electrode is the most popular replacement of 2% thoriated
tungsten in Europe and Japan.
Lanthanated tungsten electrode is normally applied in DC (Direct Current) welding, it also
works well in AC (Alternate Current) welding.
35
8.4.4. Electrodos
Ce-W
Cerium tungsten electrodes have easy arc starting performance and
low are keeping current. It is especially used to weld pipes, stainless
steel articles and fine miniparts
36
8.4.4. Electrodos
Características de la punta del electrodo
37
8.4.4. Electrodos
Características de la punta del electrodo
Electrode taper is usually called out in degrees of included angle, usually
anywhere between 14 and 60 degrees. Grinding an electrode to a point aids
arc starting when depositing short-duration welds on small parts. However, in
most cases a flat spot or tip diameter at the end of electrode works best.
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8.4.4. Electrodos
Afilado de la punta
39
8.4.5. Pedal
40
8.4.6. Suministro de gas
41
8.4.7. Sistema de enfriamiento
42
8.4.8. Pinza tierra
43
8.5. Gases protectores
44
8.5. Gases protectores
In the case of aluminium, the argon offers a calm and stable metal transfer.
However it has a lower penetration intensity and its security against porosity (due
to hydrogen) is not as resistant as argon-helium mixtures.
Helium is not an appropriate shielding gas because of its very uneven coarse
drops and often with background current burdened metal transfer.
Effective combinations of helium and argon have been found to lie between 3070% of each respective gas. Most commonly used is a mixture of 50% argon and
50% helium.
The pre-heat expenditure can be reduced or totally avoided by using heliumbearing mixtures.
For increasing the weld penetration it is possible to add in O2 or CO2 between 150
and 300 vpm instead of N2.
45
8.5. Gases protectores
Argon
Concerning TIG-welding with negative polarisation of the electrode, a method has been
developed which, instead of the usual inert-gas argon, makes use of the helium gas.
This is based on special characteristics of this gas.
Due to the higher ionisation energy of helium compared to argon, a greater welding
voltage (approximately 75% greater) provides the same amount of amperage and this
leads to a higher thermal input into the workpiece. The higher heat conductivity of
helium is another advantage compared to the argon.
Because of its lower conductivity of electricity, a disadvantage of helium is the
production of a turbulent arc and difficult arc ignition when TIG-welding.
46
8.5. Gases protectores
Argon
In a lot of cases, mixtures of argon and helium result in a usable compromise. From an
economical point of view it also has to be considered that helium is more expensive than
argon. In addition, due to its lower specific weight, comparatively more helium than
argon has to be used for gas shielding purposes.
The higher energy input by helium results in an increased welding speed, lower pre-heat
temperatures at the same penetration, and a lower tendency for porosity by a hotter
weld pool with lower viscosity and better degasification possibilities.
TIG-welding of aluminium workpieces with an increasing usage of helium will be
introduced in future, particularly at machinery welding.
47
Helium
The shape of the arc is also mainly influenced by the type of inert gas used. This is
predominately due to the physical characteristics of the gas and respective thermal
conductivities, also the dissociation of the active gases have an influence.
The figure shows the influence of the inert gas on the penetration profile on plate
TIG welds in aluminium using different shielding gases.
+300 vpm NO
+70 vpm N2
Argon Helium
Argon 4.6
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49
8.6. Metal de aporte
50
8.6. Metal de aporte
Especificación para aleaciones de Al (AWS A5.10)
51
8.6. Metal de aporte
Especificación para aleaciones de Mg (AWS A5.19)
52
8.6. Metal de aporte
Especificación para aceros inoxidables (AWS A5.9)
53
Soldadura
por fusión
Arco eléctrico
Oxigas (llama)
54
Soldadura oxiacetilénica
Three basic types of oxyacetylene flames used in oxyfuel-gas welding and cutting
operations: (a) neutral flame; (b) oxidizing flame; (c) carburizing, or reducing, flame. The
gas mixture in (a) is basically equal volumes of oxygen and acetylene.
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(a) General view of and (b) cross-section of a torch used in oxyacetylene
welding. The acetylene valve is opened first; the gas is lit with a spark
lighter or a pilot light; then the oxygen valve is opened and the flame
adjusted. (c) Basic equipment used in oxyfuel-gas welding. To ensure
correct connections, all threads on acetylene fittings are left-handed,
whereas those for oxygen are right-handed. Oxygen regulators are
usually painted green, acetylene regulators red.
56
Proceso FCAW (Flux Cored Arc Welding)
57
Proceso GMAW (Gas Metal Arc Welding)
58
Proceso SAW (Submerged Arc Welding)
59
Proceso SMAW (Shielded Metal Arc Welding)
60
Proceso GTAW (Gas Tungsten Arc Welding)
61
Plasma
62
Electroslag
63
Principales procesos de soldadura por fusión
64
Particularidades de la soldadura por fusión
•Ocurre a alta temperatura de calentamiento
•Transcurre con gran velocidad.
•Se caracteriza
porque
los
volúmenes
calentado y fundido son muy pequeños.
del
metal
•Durante la soldadura tiene lugar una rápida transferencia de
calor del metal fundido, del baño de fusión a las zonas del
metal de base en estado sólido, adyacentes a él.
•Sobre el metal depositado en la zona de soldadura actúan
intensamente los gases y escorias que le rodean.
•El
metal de aportación, cuya
composición
química
puede
diferenciarse considerablemente de la composición
química del metal base se emplea en una serie de casos para
formar el metal del cordón.
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ZONA FUNDIDA
1.
2.
3.
4.
Modificaciones químicas
Absorción de gas
Precipitación de compuestos
Transformaciones estructurales
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1. Modificaciones químicas
Las variaciones en la composición química
de la zona fundida pueden tener una
influencia favorable o desfavorable sobre
las propiedades de la unión soldada.
Pérdidas por oxidación
- Si, Mn y C.
Fijación de elementos
-Elementos sólidos: C, P y S
-Elementos gaseosos: N, O e H
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