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Overview gases for joining

Gases for joining technologies

Gas welding / oxyacetylene welding

The manual oxyacetylene welding process is one of the oldest joining procedures. It involves heating the metal to be joined to melting temperature in the joining area using a fuel gas/oxygen flame. The addition of a filler metal (welding wire) causes the components that are to be joined to melt and a strongly coalesced joint to be formed. Only acetylene is used as fuel gas. This process is still popular today in assembly and maintenance work.

The advantage of oxyacetylene welding is the fact that it has a reducing flame and that this flame can be adjusted to suit the particular welding requirements. Further benefits include good gap bridging, minimal groove preparation and the fact that the process can be used anywhere. This process can be used to weld steel as well as non-ferrous metals.

 

 

FLAME BRAZING

Flame brazing, too, involves the use of a fuel gas/oxygen flame. However, the surfaces of the parts to be joined are not themselves melted but heated to just above the melting temperature of the solder material. The solder, which is usually in the form of a wire, is added while the joint is being continuously heated so that it melts. A small gap must be maintained between the parts to be joined, into which the solder can flow by capillary action. The use of a flux improves the adhesion of the components with the solder. This also results in the formation of a strongly coalesced joint.

Soldering and brazing are among the oldest and, at the same time, most modern joining processes. Technological progress and its demands as well as cost-conscious production planning have led to the use of all common hydrocarbons and hydrogen as fuel gases.

By adding a flux to the fuel gas flow (flux brazing), the process can also be automated in either linear or rotary brazing machines.

GMA welding

GMA welding is the most popular welding process. Depending on the material to be welded and the shielding gases that are used, the processes are divided into the following categories:

  • Metal Active Gas welding (MAG)
  • Metal Inert Gas welding (MIG)

Both processes are similarly structured. An endless wire electrode is supplied to the arc by a wire transport device and melted away under a shielding gas. The image shows the structure of a GMA welding process.

The shielding gases have different properties depending on their composition and therefore influence the welding result in different ways. The main task is to shield the liquid melt from the atmosphere, which contains nitrogen, oxygen and moisture. Depending on the material to be welded, these can have an adverse effect on the weld or even result in the failure of the welding process.

Shielding gases influence the following aspects:

  • Metal transfer
  • Flow behaviour of the melt
  • Ignition behaviour of the arc
  • Stability of the arc
  • Heat transfer
  • Penetration profile
  • Chemical composition of the weld
  • Spatter frequency and size
     

For the joining of thin galvanised sheet metal (up to approx. 40 µm thickness), gas metal arc brazing, or GMA brazing for short, has important advantages compared with metal active gas (MAG) welding. It has a high level of process reliability, better quality of seams, very good joint strength and very good corrosion resistance. For this reason, GMA brazing has become firmly established in car manufacturing.

Gas metal arc brazing is similar to MAG welding. The only difference is that the filler metal is replaced by a wire consisting of suitable solder. Selecting the right parameters – current, voltage, wire feed – prevents melting of the surfaces of the components to be joined. A joint is formed in the same way as with flame brazing. Frequently used brazing materials include the following:

Name

Melting range

[°C]

Yield point

[N/mm²]

Tensile strength

[N/mm²]

Expansion

[%]

CuSi3

900 - 1025

250>120

340 - 460

40-46

CuAl8

1030 - 1040

180

380 - 450

40

CuAl8Ni2

1030 - 1050

290

530 - 590

>30

CuAl5Mn1Ni1

1043 - 1074

-

430

35

The standard shielding gas used in GMA brazing is argon. But this does not always lead to optimum results. Based on extensive experience, Messer recommends using a shielding gas mixture consisting of argon and small quantities of active gas for GMA brazing. This will result in seams with a smooth surface and good transitions between the seams and the base metal.

TIG welding

The main difference between TIG welding and GMA welding lies in the addition of the filler material, which is not continuously supplied to the process as an electrode, as it is with GMA welding. With TIG welding, the arc burns between the workpiece and a non-melting tungsten electrode. As with oxyacetylene welding, the filler material is added manually. The role of the shielding gas is to protect the electrode and the molten pool from the negative effects of the atmosphere. Oxygen, in particular, would lead to a deterioration of the electrode.

TIG welding is particularly well suited for welding high-alloy steels, aluminium and other non-ferrous metals. For high-alloy steels and nickel-based materials, a small amount (2% to 7.5%) of hydrogen is added as a reducing component. For light metals and copper, the addition of helium (up to 90%) has proved effective, depending on the thickness of the workpiece. The process can be operated with direct current as well as alternating current. Direct current with a positive electrode is generally used for welding steels, copper, nickel alloys, titanium and zirconium. Alternating current is used for aluminium.

Plasma welding

Plasma welding is similar to TIG welding. With this type of welding, the arc is covered by a narrow nozzle and constricted by the small aperture and the high outflow velocity of the gases.

Plasma welding differs from TIG welding by virtue of the arc that is constricted by a water-cooled nozzle. This arc exits the nozzle as a plasma jet with a high temperature and power density. An additional shielding gas layer surrounds the plasma jet and protects the melt from the surrounding air. In most cases, the gas surrounding the electrode is argon. In addition to this plasma gas, you also need a shielding gas to prevent oxidation of the weld pool (usually argon with 5% of hydrogen). Plasma welding is mostly used for butt welding of sheet metal and pipes. Its main advantages are controlled penetration and high weld quality.

 

When welding high-alloy steels, the root must also be protected against contact with atmospheric oxygen. Root protection is often used in MAG welding too. Generally, a residual oxygen content of less than 20 ppm is required at the root. The amount discoloration to be permitted depends on the intended use of the component in question. In the case of small pipes, the weld root is protected by passing shielding gas through them. The important thing here is the adjusted outlet opening. In the case of larger pipes, the backing gas is targeted at the weld with special equipment. The gas flow has to be applied for a sufficiently long period before welding is started.

Generally, so-called forming gases – nitrogen/hydrogen mixtures – are used. The hydrogen component provides greater security against residues of atmospheric oxygen. For this reason hydrogen content is always higher in building site applications than in workshops. Previous tests have shown that the presence of hydrogen in the backing gas has no negative effects, even on duplex steels.

Precise measurements can be carried out to check that conditions are oxygen-free. It is important to follow the correct procedure here.

Forming can also be used for welding plain steels or aluminium, where it produces an even, oxide-free root. The forming gas used here is welding argon.

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