Which shielding gas is used for arc brazing?
Pure argon always works. For CuSi3 and tin bronzes in MIG brazing, argon with a little CO₂ or O₂ is often better; for aluminium-containing filler metals such as CuAl8, stay with argon or argon-helium without an active component.
According to DVS 0938-1, brazing is usually done with argon (group I1 to ISO 14175) or with argon mixtures to which small amounts of CO₂ or O₂ have been added. The gas protects filler metal and sheet from air and replaces the flux that would be needed in flame brazing.
| Filler metal / process | Shielding gas | ISO 14175 | Note |
|---|---|---|---|
| CuSi3Mn1, MIG | argon or argon with a little CO₂ or O₂ | I1, M12, M13 | the active component steadies the arc and reduces pores, but adds more heat |
| CuSn filler metals, MIG | argon or argon with a little CO₂ or O₂ | I1, M12, M13 | as for CuSi3 |
| CuAl7 (CuAl8), MIG | argon or argon-helium, without active component | I1, I3 | without active component according to DVS 0938-1 |
| TIG brazing | argon | I1 | the tungsten electrode does not tolerate an active component |
| Plasma brazing | argon as shielding and plasma gas | I1 | plasma gas around 0.9 l/min according to DVS 0938-1 |
Assignment according to DVS 0938-1:2012, section 5.3.2. The ISO 14175 groups are examples: I1 argon, I3 argon with helium, M12 argon with 0.5 to 5 % CO₂, M13 argon with 0.5 to 3 % O₂.
What do carbon dioxide and oxygen do in the brazing gas?
Small amounts of CO₂ or O₂ stabilise the arc, reduce the tendency to porosity and lower the surface tension of the filler metal pool. In return, the heat in the base material increases.
- Steadier arc: the arc root jumps less, especially on a galvanised surface with zinc vapour.
- Better wetting: according to DVS 0938-3, active components can reduce the surface tension of the brazing pool. This helps when the filler metal does not flow well or the seam is too high.
- More heat: that is the price. With thin sheet or a tendency to braze cracking, you therefore stay with the smallest active component that keeps the arc steady.
As a guide: in the trials at SLV München on braze cracking, CuSi3 was brazed under argon with 2.5 % CO₂, CuAl8 and all TIG brazing under pure argon (Aicher and Cramer, DVS-Berichte 244, 2007).
Which gas for CuAl8?
Argon or argon-helium, without carbon dioxide and oxygen. This is what DVS 0938-1 recommends for filler metals containing aluminium, and this is how CuAl8 was brazed in the trials at SLV München.
Helium makes the arc hotter and wider, which improves the flow of the aluminium bronze filler metal. Pure argon also works. CuAl8 is not suitable for TIG and plasma brazing anyway, see TIG and plasma brazing.
Nitrogen and hydrogen: what needs to be considered?
Neither normally belongs in the brazing gas. Nitrogen does stabilise the arc and makes the seam wider, but it can cause very considerable porosity. Hydrogen increases the speed, but also leads to pores.
When brazing dissimilar joints with pure copper, especially with phosphorus-deoxidised copper Cu-DHP, no hydrogen-containing gases may be used according to DVS 0938-1.
How much gas do you need?
In the guide values of DVS 0938-1, it is 12 l/min argon with 1.0 mm wire, for MIG as well as plasma brazing. In plasma brazing, around 0.9 l/min argon is added as plasma gas.
More gas is not better. Too high a gas flow rate causes turbulence and draws air into the gas shield, which produces pores just as too little gas does. Adjust the gas flow rate to the nozzle diameter and torch distance and avoid draughts at the workplace. Settings for MIG brazing: MIG brazing.
Pores despite shielding gas: what to check?
With pores, check the gas supply first, then the heat and torch guidance. DVS 0938-3 lists a number of causes related to the gas.
- Check the gas hose for cracks and porous areas, and the connections for leaks.
- With water-cooled torches, check whether cooling water can get into the gas.
- Avoid draughts at the workplace.
- Do not set too high a gas flow rate; it causes turbulence.
- Choose a gas nozzle of suitable shape and diameter and keep it free of brazing spatter.
- Optimise the arc length and give the pool time to degas.
The most common source of pores when brazing galvanised sheet, however, is zinc vapour, not the gas. More on this under Seam defects and braze cracking.
Frequently asked questions
Can you braze with the mixed gas for steel, such as argon with 18 % carbon dioxide?
Not recommended. The high CO₂ content puts a lot of heat into the sheet and makes the zinc vaporise more. Only small active components are used for brazing.
Is pure argon enough for CuSi3?
Yes. The guide values in DVS 0938-1 apply to CuSi3Mn1 under pure argon. A small CO₂ or O₂ content can steady the arc if it burns unsteadily on galvanised sheet.
Do you need flux for arc brazing?
No. The shielding gas protects filler metal and sheet; flux is not needed.
Who can help with choosing the gas?
DVS 0938-1 recommends using the experience of the gas suppliers to match the gas to the brazing task. On the matching filler metal: Filler metals and standard designations, ML CuSi3, ML CuAl8.
Further reading: articles on arc brazing
The series in the order in which the questions arise in practice: from the process via filler metal, gas and base material to the finished seam. For an overview, see Arc brazing.
About the author
Robert Lahnsteiner, Ing., is an International Welding Engineer (IWE) and managing director of MIGAL.CO GmbH. After training in telecommunications and electronics at HTL Braunau, he spent eighteen years in welding technology at Fronius from 1983. Since 2001 he has run MIGAL.CO, the aluminium welding wire manufacturer in Landau an der Isar, together with Martin Erl. His focus is aluminium filler metals, wire manufacturing and wire feeding, and the monitoring of welding processes. Within DVS, the German Welding Society, he has served for around 30 years on technical committee FA03 “Arc welding” and chairs working group 2.4.4 “MIG welding of aluminium”.