Shielding Gases & Flux

8/10/2019 Shielding Gases & Flux

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IONIZATION POTENTIAL

• Ionization: is the process of converting an atom or

molecule into an ion by adding or removing charged

particles such as electrons or ions.

• Ionization potential: is the energy required to remove

electrons from gaseous atoms or ions.



Shielding gases for welding:

Protects the molten metal or arc area from the atmosphere.

It also influences the metal transfer mode and deposited weld

characteristics.

Shielding efficiency- depends on design of nozzle, gas flow rate, side

winds, purity of shielding gas etc.

Shielding gases

Active (O2, Co2, H2 etc) Inert (Ar, He, Ne, Kr etc)

Used with GTAW and for welding non-ferrous

metals with GMAW



Properties of shielding gas:

1. Ionization potential (IP):

Ionization potential ∞ 1

- Argon has low IP and helps to initiate and maintain the arc in a

stable operating mode.- Helium has high IP

2. Thermal conductivity:

- Pure Ar has low thermal conductivity.

Hence weld fusion area has wider top and a narrow ‘finger’ at the root.

- Helium has higher thermal conductivity. Therefore results in

broader, hotter arc.

Molecular weight of shielding gas



Argon Carbon

Dioxide

Heium Hydrogen Nitroge

n

oxygen

Chemical symbol Ar Co2 He H2 N2 O2

Atomic Number 18 6 2 1 7 8

Molecular weight 39.95 44.01 4.00 2.016 28.01 32.00

Specific Gravity,

Air = 1

1.38 1.53 0.1368 0.0695 0.967 1.105

Density(lb/cu ft) at

0 C, 1 atmosphere

0.1114 0.1235 0.0111 0.0056 0.0782 0.0892

Ionization

Potential(ev)

15.7 14.4 24.5 13.5 14.5 13.2

ThermalConductivity

(10-3 ×Btu/hr-ft-F)

9.69(32 F)

8.62(32 F)

85.78(32 F)

97.22(32 F)

13.93(32 F)

14.05(32 F)

Cubic ft/lb 9.67 8.73 96.71 192 13.8 12.08



3. Specific gravity:

weight of gas with respect to air.

- Helium is a light gas, so floats away and will not be an efficient shield.

Hence gas flow rate must be increased.

4. Dissociation and recombination:

Co2,H2,O2 are multi atom molecules.

- at higher temperatures, they disintegrate or dissociate into component

atoms.

- when dissociated atoms come into contact at the relatively cool work

surface , they recombine and release heat at that point.



5.Reactivity:

- Ar, He – completely non –reactive

- Co2,O2 reactive gases – react with FM or base plate enhance

stability of the arc and affect the type of metal transfer

6. Surface tension: influences bead shape

- if high, convex & irregular bead will result

- low values promote flatter beads with minimal susceptibility to

undercutting.

- Pure Ar produces sluggish weld puddle and high crowned bead

- if O2 added to Ar, lowers surface tension and promotes fluidity and

better wetting of base material.



7. Gas Purity:

- gas impurities affect welding speed, weld surface appearance, porosity level

etc.

Argon: Colorless, odorless, tasteless and non-toxic, 1.4 times heavier than air-

promotes good arc starting characteristics and arc stability due to low IP.

- The arc voltage of the tungsten arc in argon is lower than in helium.

Helium: lighter than air.

Has high thermal conductivity and high IP-hence used when increased heat input

is desired and also for Al, Mg welding.

Helium light weight causes it to float away from the arc zone, producing an

inefficient shielding.

Helium is expansive for welding and is sometimes in scarce supply.



Co2: is a compound of about 27% carbon and 72% oxygen.

- reactive gas, 1.5 times heavier than air. Odorless, colorless with a slightly

pungent acid taste.

- Co2 can be used alone for GMAW shielding gas applications

-Co2 + Ar is used to improve productivity and penetration in GMAW.

- oxidising gas – so use GMAW wires with high contents of Si & Mn.

- 100% Co2 shielding produces broad, deep penetration welds.

- inexpensive compared to inert gas

- drawback: arc can be somewhat violent- leads to spatter and makes welding

of thin materials difficult- so go for Ar- Co2 mixtures



H2: lightest element known, has high thermal conductivity and reactive nature.

GMAW & GTAW of 300 series of stainless steel use H2.

N2 : colorless, odorless, tasteless, slightly lighter than air.

N2 is inert except at welding temperature when it reacts with Al, Mg and Ti.

Not recommended for 1˚ shielding gas with GMAW but used as assist gas in laser

cutting on Stainless steel.

O2: Highly oxidizing & vigorously supports combustion

Oxygen normally added in amounts of 1% to 2%, or 3% to 5%.This provides for spry

transfer.

O2 added with Ar for GMAW to increase arc stability and to improve the shape of weld

bead.

also used to increase cutting speeds with plasma and laser processes

More oxygen will leads to porosity.



Different gas mixtures

-2 gases –duplex mixtures and 3 gases – Ternary Mixtures of gases

-Mixtures combine inert + active gases

- for GTAW--- inert gases normally used

Argon + O2: for GTAW, O2< 1% used to help stiffen the arc

- used for thin steels, S.S, DCEN of Al.

-In GMAW, it decides metal transfer mode.

- Pure Argon produces poor bead contour and penetration pattern.This is eliminated

by adding O2, 1% to 2%, 3-5%O2 (spray transfer)

--the more oxidizing the shielding gas – use electrode with sufficient deoxidizers to

overcome loss of Mn, Si, Al.

Advantage: good penetration pattern by broadening the deep penetration finger,

eliminates undercut due to better wetting action.

Disadvantage: more O2 leads to porosity



Shielding gas related to weld profile for DCEP





Ar+N2:

-In some countries pure N2 used for GMAW of Cu- quality of weld

is not good as desired.

-adding 50-75% Ar to N2 produce higher quality weld

Ar+Co2:

-75% Ar +25%Co2 used in GMAW

-widely used on thin steel where deep penetration is not

necessary and where bead appearance is important.

-spatter is reduced.

- it is also helpful for out-of-position welding.



Ternary mixtures of gases:

- three-component mixtures :Ar + O2 +Co2 and sometimes Ar + Co2 +He.

70%Ar + 2% O2 + Co2—used for welding steels.

- small amount of He to Ar-O2 mixture increases arc voltage and

provides higher deposition rates or higher travel speeds.



Flux

A substance that prevents formation of oxides and other

contaminants in welding, or dissolves them and facilitatesremoval

• Provides protective atmosphere for welding

• Stabilizes arc

•Reduces spattering

Flux Application

Pouring granular flux onto welding operation

Stick electrode coated with flux material that melts during

welding to cover operation

Tubular electrodes in which flux is contained in the core

and released as electrode is consumed



Electrode coatings

Electrode coatings can consist of a number of different compounds,

including Rutile , calcium fluoride, cellulose, and iron powder.

Rutile electrodes, coated with 25% –45% TiO2, are characterized by ease of

use and good appearance of the resulting weld. Helps to form a highly fluid but

quick freezing slag. Also provides ionisation for the arc.

However, they create welds with high hydrogen content,

encouraging embrittlement and cracking.

Electrodes containing calcium fluoride (CaF2), sometimes known as basic or

low-hydrogen electrodes, are hygroscopic and must be stored in dry conditions.

They produce strong welds, but with a coarse and convex-shaped joint surface.



Electrodes coated with cellulose, especially when combined

with rutile, provide deep weld penetration, but because of their

high moisture content, special procedures must be used to

prevent excessive risk of cracking.

Iron powder is a common coating additive, as it improves the

productivity of the electrode by increasing deposition rate,

sometimes as much as doubling the yield.

Metal Carbonates: adjusts the basicity of the slag & provides a

reducing atoms.



Coating Materials -Partial List

Arc StabilizersTitania TiO2

Gas-Forming Materials

Wood Pulp

Limestone CaCO3

Slag-Forming Materials

Alumina Al2O3

TiO2 SiO2

Fe3O4

Binding Agents

Sodium Silicate

Asbestos

StarchSugar

Alloying and Deoxidizing

Elements

Si, Al, Ti, Mn, Ni, Cr





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Shielding Gases & Flux