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This Presentation is provided to you by: WeldCanada.com
Industry Standard Welding Procedures Software CSA, AWS, ASME and API Welding Codes
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?
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