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Choice of Shielding Gases

CWA-Toronto 4th Nov 2003

Viwek Vaidya

Applications of Shielding Gases & Modes of Metal Transfer

Too many choices!

• Too many base metals• Too many processes• Too many brands• Variety of opinions!• Gas compositions

often unknown• A5.32 - shielding gas

standard

Shielding gas components

• CO2 - Dirty gas needs cleaning

• Argon – From cryogenic processing

• Oxygen - From cryogenic processing

• Helium – From mines of Natural gas

• Hydrogen – Chemical processing

• Nitrogen – From cryogenic processing

The functions of shielding gases are

• Protect the weld pool from atmosphere

• Provide a gas plasma - ionised gas

• Support metal transfer and bead wetting

Physical properties of pure gases

• Size and shape of the plasma column depends on:– thermal conductivity

• specific gravity:– determines the heat content of the gas

• Ionization potential:– ease of carrying electrical energy

High thermal conductivity gases

• CO2 and He have high thermal conductivities

– For equal energy into the plasma– less surface needed to transmit heat– Plasma column remains narrow– Localised under the molten tip of wire

• This produces globular transfer

Globular transfer

• High thermal conductivity

• Narrow plasma column

• CO2 and Helium produce globular transfer

• Impossible to produce spray transfer!

Thermal conductivity and plasma shape

• Thermal Conductivity is the ease with which the gas will dissipate heat

• Argon has low thermal conductivity– It is used for low R-Value windows

Argon

Low thermal conductivity gases

• Argon has low thermal conductivity– For equal energy into the plasma– more surface is needed to transmit heat– Plasma column expands – Reaches beyond the molten tip of wire– Rapid melting with high electron condensation

heating

• This produces spray transfer

Spray Transfer

• Low thermal conductivity

• Expanded plasma column

• Electron condensation heating

Spray Transfer

• Wire melts in a fast fine droplet stream

• Wire end becomes pointed

• Spray transfer results in high deposition and good penetration

• Argon gives spray transfer!

Penetration profiles

• Argon has a finger nail penetration profile consistent with spray transfer

• CO2 and He have elliptical penetration consistent with the globular transfer

The 90/10 rule

• To ensure a smooth spray transfer remember this rule!

• 90% Innert gas (Ar/He)• 10% Active gas (Active gas component)

– These could be CO2/O2 combinations

• For welding steel or stainless base materials some oxygen or CO2 is needed for arc stability

Spray transfer limits

• Below 85% Argon content, spray transfer is not possible

• Beyond 5% Oxygen levels, fume emissions become a problem

• Although He is innert, it promotes globular transfer

• Any gas mixture that will spray transfer, will generally pulse and short circuit

• Hydrogen will provide heat and clean oxides

Shielding gas choice

Quiz time!

Argon – Helium mix

• 75%Ar+25% He

• Applicable to what base metals?

Argon – Helium mix

• 50%Ar+50% He

• What is the bead shape?

• What is the advantage with respect to Blue Shield 1?

Argon – Helium mix*

• 25%Ar+75% He

• *Can one spray transfer with this gas? Why?

• What application ? Automatic or manual?

Argon – Oxygen mix

• 98%Ar+2%O2

• This gas can be applied to what base metals?

• What would be the bead shape?

Argon – Oxygen mix*

• 95%Ar+5% O2

• Can one spray transfer with this gas?

• *Can this be applied to Stainless steels?

Argon – CO2 mix*

• 90%Ar+10%CO2

• Bench mark composition for spray transfer.

• *What would be the weld bead shape compared to Ar-5%O2

Argon – CO2 *

• 85%Ar+15% CO2

• Can this gas be used for GMAW spray transfer?

• *Can this gas be used for FCAW mild steel, and stainless steel?

Argon – CO2 *

• 75%Ar+25% CO2

• Can one spray transfer with this gas?

• How will the bead shape look compared to CO2

• *What will the fillet weld penetration be like compared to CO2? What about fumes?

Helium – Argon – CO2 mix*

• 90%He+7.5%Ar+2.5% CO2

• *Can one spray transfer with this mixture? Why?

• What base metal should be welded with this mix?

Argon – H2 mix *

• 97.5%Ar+2.5% H2

• *Can this gas be used for GTAW of ferrous metals?

Argon – H2 mix *

• 65%Ar+35% H2

• *Is this a welding grade gas? Where is this applied?

Nitrogen – H2 mix *

• 95%N 2+5% H2

• *Can you weld with this gas? In what application?

Argon – CO2 – O2 mix*

• 91%Ar+5% CO2 +4% O2

• Can one short circuit and spray with this gas?• * How will it weld on non blasted plate, GMAW?

Why?

Argon – Helium – CO2 mix*

• 81%Ar+18%He+1% CO2

• Where would this gas composition be used?

• Magnesium, copper, mild steels or SS?

• *Can one short circuit and spray transfer with this mix? Why?

Argon – CO2 – Helium mix*

• 75%Ar + 15% CO2 + 10%He

• *What advantage would this gas have over a 85% Argon + 15% CO2?

Thank you for your attention!

Questions?