Soldadura gtaw, saw y electroescoria (1) soldadura mixta

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VIII. Proceso GTAW (Gas Tungsten Arc Welding)

Soldadura Mixta

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.

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Plasma

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8.1. Aplicación

Actualmente el proceso TIG es ampliamente utilizado

para soldar aleaciones ferrosas y no ferrosas.

Aceros al carbono

Aluminio y aleaciones Aleaciones de magnesio Cobre y aleaciones

Aceros inoxidables Aceros de baja aleación

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8.1. Aplicación

Titanio y aleaciones

Níquel y aleacionesZirconio y aleaciones

(Resistente a la corrosión y

a las altas Temperaturas)

(Más ligero que el acero y

tiene alta resistencia a la

corrosión y mecánica)

(Alta resistencia a la

corrosión)

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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. Fuente de potencia

2. Cables

3. Antorcha

4. Electrodo

5. Pedal (opcional)

6. Sistema de suministro de gas

7. Sistema de enfriamiento

8. Pinza tierra

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8.4.1. Fuente de potencia

Característica externa voltaje-amperes para el proceso GTAW

(corriente constante)

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Ciclo de trabajo

• 60% - Método de aplicación manual y semiautomático

• 100% - Método de aplicación a maquina y automático

)()(

)(%

2

arg

2

tasado

ac

tasada CTI

ICT

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:

%86)60()250(

)300(%

2

2

CT

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Ciclo de trabajo

Ciclo de trabajo versus corriente de carga

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Corriente directa

Polaridad directa e invertida

Polaridad directa

(electrodo negativo)

Todos los metales. Para

las aleaciones de Al y

Mg, deben usarse

procedimientos

especiales

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

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Corriente pulsante


Terminología de la corriente pulsante

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The most important weld parameters

are:

pulse current lpbackground current lGpulse current time tpbackground current time tGpulse frequency fp = 1 / tcWhere: tc = duration of period

Corriente pulsante

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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.

Corriente pulsante

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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.

Corriente pulsante

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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

Corriente pulsante

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Soldadura producida por la corriente pulsante

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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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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

relatively small amount, thus only a flat penetration arises.

Corriente alterna

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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

polarisation is not generally used for TIG welding.

Corriente alterna

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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.

Corriente alterna

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Fuentes programables que permiten el método de aplicación automático

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Tipo de fuentes de potencia según sus características constructivas

Tipo transformador-rectificador

Estáticas


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• Rotativas


Tipo de fuentes de potencia según sus características constructivas

Tipo motor generador de

combustión interna

Tipo motor generador

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8.4.2. Cables

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8.4.3. Antorcha

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8.4.3. Antorcha

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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

Composition EN-Classification

Tungsten (pure) WP

Tungsten with 1% thoria WT 10




Tungsten with 0.8% zirconia WZ 8

Tungsten with 1% lanthana WL 10

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)

Clasificación de los electrodos de Tungsteno (EN-Norma europea)

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8.4.4. Electrodos

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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

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Zr-W

8.4.4. Electrodos

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.

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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.

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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.

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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

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8.4.4. Electrodos

Características de la punta del electrodo

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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.

8.4.4. Electrodos

Características de la punta del electrodo

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8.4.4. Electrodos

Afilado de la punta

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8.4.5. Pedal

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8.4.6. Suministro de gas

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8.4.7. Sistema de enfriamiento

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8.4.8. Pinza tierra

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8.5. Gases protectores

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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 30-

70% 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 helium-

bearing mixtures.

For increasing the weld penetration it is possible to add in O2 or CO2 between 150

and 300 vpm instead of N2.

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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.


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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.

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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.

Argon HeliumArgon 4.6

+300 vpm NO

+70 vpm N2

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8.6. Metal de aporte

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Especificación para aleaciones de Al (AWS A5.10)

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Especificación para aleaciones de Mg (AWS A5.19)

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Especificación para aceros inoxidables (AWS A5.9)

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Soldadura

por fusión

Arco eléctrico Oxigas (llama)

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Soldadura oxiacetilénica

Three basic types of oxyacetylene flames used in oxyfuel-gas welding and cuttingoperations: (a) neutral flame; (b) oxidizing flame; (c) carburizing, or reducing, flame. Thegas 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 oxyacetylenewelding. The acetylene valve is opened first; the gas is lit with a sparklighter or a pilot light; then the oxygen valve is opened and the flameadjusted. (c) Basic equipment used in oxyfuel-gas welding. To ensurecorrect connections, all threads on acetylene fittings are left-handed,whereas those for oxygen are right-handed. Oxygen regulators areusually painted green, acetylene regulators red.

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Proceso FCAW (Flux Cored Arc Welding)

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Proceso GMAW (Gas Metal Arc Welding)

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Proceso SAW (Submerged Arc Welding)

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Proceso SMAW (Shielded Metal Arc Welding)

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Proceso GTAW (Gas Tungsten Arc Welding)

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Plasma

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Electroslag

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Principales procesos de soldadura por fusión

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Particularidades de la soldadura por fusión

•Ocurre a alta temperatura de calentamiento

•Transcurre con gran velocidad.

•Se caracteriza porque los volúmenes del metal

calentado y fundido son muy pequeños.

•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. Modificaciones químicas

2. Absorción de gas

3. Precipitación de compuestos

4. 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

Soldadura gtaw, saw y electroescoria (1) soldadura mixta

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