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Choice of Shielding Gases CWA-Toronto 4th Nov 2003 Choice of Shielding Gases Applications of Shielding Gases & Modes of Metal Transfer Viwek Vaidya

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 – CO2 – H2 mix 96.25%Ar+2.75% CO2+1%H2 Can this gas be used for short circuit and spray transfer for dissimilar welds?

Argon – H2 mix * 97.5%Ar+2.5% H2 *Can this gas be used for GTAW of ferrous metals?

Argon – H2 mix * 95%Ar+5% H2 *Can one spray transfer with this gas?

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?