-arc is established between the tip of a tungsten ...u12133.fs.cvut.cz/assets/subject/files/137/TJC-Lecture-05-GTAW.pdf · GTAW - advantages very regular and well controlled welding

GTAW – Gas Tungsten Arc Welding

-arc is established between the tip of a tungsten electrode and the workpiece to

melt the base material (possibly also filler)

-tungsten electrode is considered to be non-consumable

-inert shielding gas protects the molten weld pool

FSW – Friction stir welding

ČVUT, Fakulta strojní, Ústav strojírenské technologie, skupina svařování

GTAW welding

GTAW is defined as arc welding using

a non consumable tungsten electrode

and a shielding gas protecting the weld.

Method no. acc. ČSN EN ISO 4063

14 – Gas shielded arc with non consumable

tungsten electrode141

142

143

145

146

147

used names

GTAW –USA

TIG – Tungsten inert gas

WIG - Wolfram inert gas



Basic parameters

Positions : all

Base material : all

steels, alloyed steels, Al, Cu, Ni alloys

Welded thickness : 0,5 - 10 mm

Current, voltage short circuit : I = 10 -500 A

U = 16 - 21 V

Tungsten electrod diameter : 2 – 4 mm

Welding source : steep, AC, DC +/-

Equipment

Welding torch

GTAW - advantages

very regular and well controlled welding proces results in good

mechanical, chemical properties

High quality and precision

Pin point control

Aesthetic weld beads

No sparks or spatter

No flux or slag

No smoke or fumes, no spatter,

molten pool is well visible and so controlable

welding in all positions

No need for further machining

Easy automation

GTAW - disadvantages

Lower filler metal deposition rates

Good hand-eye coordination a required skill

Brighter UV rays than other processes

Slower travel speeds than other processes

Low productivity

Difficult, expensive

GTAW - use

Welds more metals and metal alloys than any other process

sealing joints

precision welding

atomic energy, aircraft – turbine blades

repair of dies, forming tools

Root welds

for thin sheets

especially for highly alloyed materials - SS, HSS, AHSS

especially for light alloys - Al, Mg

heat exchangers, exhausts, pressure wessels



Tungsten electrodes

normalized ČSN EN ISO 6848 –size, composition, color

Diameter 0,5 - 10 mm, standard 2-4 mm, lengths – 50-200 mm

Color + name = composition

Wolfram + alloying

- WP - pure W Tt = 3 420 °C

- WT – W+ThO2 - Tt = 3 300 °C

- WZ – W+ ZrO2 - Tt = 2 700 °C

- WL – W+ La2O3 - Tt = 2 300 °C

- WC – W+ CeO2 - Tt = 2 600 °C

- Electron emisivity –

- Longevity

- Stability

- Temperature decrase 1 000 °C



Electrode shape

Direct current Alternative current



Sharpening



Grinders



Filler wire – solid, cored wire

Basic weld joints

Butt Weld and Stringer Bead

Lap Joint

T-Joint

Corner Joint

Shielding gases

Most often inert gases used - Ar, He

• Ar + He mixtures

• Ar + H2

Hydrogen is reduction gas – reduces oxygen, High heat transfer, improved

penetration, stability, for austenitic steels,

• Ar + N2

for austenitic steels and duplex steels – nitrogen stabilizes austenite

Argon► Good arc starting

► Good cleaning action

► Good arc stability

► Focused arc cone

► Lower arc voltages

Helium► Faster travel speeds

► Increased penetration

► Difficult arc starting

► Less cleaning action

► Less low amp stability

► Flared arc cone

► Higher arc voltages

► Higher flow rates (2x)

► Higher cost than argon



Effect of shielding on the arc

Argon Ar + He Helium

I=240 A, material 1.4301



Ar-He mixtures

Argon

280 A / 25 V

75% Argon / 25% Helium

282 A / 27 V


285 A / 30 V


285 A / 34 V



Shielding quality – root shielding

Demands

Corrosion

Outlook

Shape

Proper shielding

Shielding when cooling

High purity gas

No oxygen presence - tubes



Shielding



Arc start

1) Electrode touch

2) Lift arc

3) High frequency start - standard

Non contact start is a must for non contamination

of electrode and WM.

Technique

Arc Length

Arc length normally one electrode diameter, when AC welding with a balled end electrode

When DC welding with a pointed electrode, arc length may be much less than electrode

diameter

Gas Cup SizeInside diameter of gas cup should be at least three times the tungsten diameter to provide adequate shielding gas coverage

Picture on right shows example of gas cup size and torch position

Electrode Extension

Refers to distance the tungsten extends out beyond the gas cup

May vary from flush with the gas cup to no more than the inside diameter of the gas cup

Longer the extension, the more likely it may contact something by accident

General rule would be to start with an extension of one electrode diameter

Gas cup size

El. Extension

Arc length

GTAW – change of polarity

non consumable electrode and invertor welder makes

possible change of polarity, square pulse, arc pulse, pulse

frequency atc.

GTAW – change of polarity



Polarity and heat input



Arc cleaning effect during Al welding

+ +

--

Kinetic energy, momentum Electron emission energy

IontyAl2O3

Elektronybez

Al2O3

AlAl

1,77 eV 3,95 eV



AC pulse



AC



Pulsing DC current

Average current is low, so heat input as well. Small HAZ, deformations etc.



DC pulse - U, I time curves



Arc Spot welding



Automation

TIG welding has low effectivity

Solution by:

Filler wire feed

Wire preheating

Torch movement

Narrow gap

Orbital welding

Robotic welding



Filler wire feed



Narrow gap welding



Orbital welding of tubes

Over 200 mm



Orbital welding of tubes

Diameter 10 - 220 mm





Orbital welding – heat exchangers

Documents

-arc is established between the tip of a tungsten ...u12133.fs.cvut.cz/assets/subject/files/137/TJC-Lecture-05-GTAW.pdf · GTAW - advantages very regular and well controlled welding