aluminum welding a tungsten electrode in inert gas atmosphere

Fusion welding of aluminum

Welding of aluminum and aluminum alloy, as, however, and other metals - is the joining of two metal components by establishing metallurgical bonds at the interface therebetween. This physical phenomenon is called coalescence [1]. These metallurgic bonds may be achieved by melting the two surfaces, and then it is called fusion welding. Another way - the creation of a high pressure between the two parts, sometimes - with the application of heat, to form metallic bonds along the border between. This is called solid-phase welding. An example of such welding is friction welding of aluminum.

The main types of fusion welding, which are used for joining aluminum components, are the following [1]:

  • non-consumable electrode in an inert gas environment;
  • consumable electrode in an inert gas environment;
  • oxygen gas;
  • an electron beam;
  • laser;
  • electric gas;
  • electro-slag;
  • submerged arc.

Below is a brief introductory overview of arc welding of aluminum and aluminum alloys with a nonconsumable electrode in an inert gas environment based on the materials of the well-known manual [1], as well as the European standard for arc welding of aluminum and aluminum alloys [2]. To clarify the practical details of this method it is necessary to contact specialized guidance on the method of welding.

Arc welding of aluminum by TIG

This type of welding has the following definition: arc welding, that employs a non-consumable tungsten electrode and inert gas to protect the electrode, arc and weld pool (figure 1).

Figure 1 - circuit arc welding process in an inert gas atmosphere
with tungsten electrode [1]

Abroad, three designations are used for this welding process: TIG, TAGS и GTAW. The first two are mainly used in Europe, the third - in the US. These designations are abbreviations of various process items, which are various combinations of the first letters of the following keywords:

  • T: Tungsten - tungsten
  • I: Inert - intert
  • G: Gas - gas
  • S: Shielding - protective
  • W: Welding - welding
  • A: Arc - arc.

Below, for brevity and convenience, we will call this process: the TIG method or TIG welding.

aluminum TIG welding method Properties

  • The arc acts only as a heat source and a welder decides to apply or not the filler wire.
  • The weld pool is well controlled, It can therefore be carried out without the use of welds chocks.
  • The arc is stable with a very low welding currents, which enables welding of thin-walled components.
  • The process provides a very good quality weld, but to achieve maximum quality for an experienced welder.
  • The process has a lower speed of the welding seam and a lower feed rate of the filler wire, than with MIG welding method, that in certain situations it makes it less productive.
  • Method of TIG welding of aluminum tends to be limited to a small thickness, usually to 6 mm.
  • TIG method gives less deep penetration into the base metal, than the method MIG, that is, the same method of welding consumable electrode. Therefore, when TIG welding method sometimes encounter difficulties in the seam angular joints and T-. Recommended types of training components for TIG welding method are shown in Figure 2.

Equipment for TIG welding of aluminum by

The main equipment for TIG welding includes:

  • source of electric current;
  • welding torch;
  • source of inert gas;
  • a filler wire feeder, and
  • water cooling system (if necessary).

A typical workstation for welding of aluminum by TIG shown 2.


Figure 2 - Repair aluminum castings using manual TIG welding
with direct current with helium as shielding gas [1]

TIG method: DC or AC

For welding of aluminum alloys used most classical method of TIG welding using a source of direct electric current. When this electrode is connected to its negative pole. It is known, that welding at this polarity is not effectively remove the oxide film from the aluminum surface. Moreover, in this method of arc welding in an inert gas atmosphere a large amount of heat at the positive pole. TIG welding method with an electrode, connected to the positive pole, leads to overheating and melting of the electrode.

Therefore, the manual TIG welding is usually performed by using AC. In this case, removal of the oxide film occurs, when the electrode is in the positive half cycle AC. On the negative half-cycle there is a cooling of the electrode and the weld penetration. The arc is extinguished and ignited at each half-cycle, when the arc current passes through zero. If current source frequency 50 Hz is happening 100 times per second, ie twice in each cycle.

shielding gas

argon

The preferred shielding gas for AC-TIG welding is argon.. Helium, and mixtures of argon and helium may also be used. Argon provides a broad and deep penetration of the weld, and thus makes the weld shiny and silvery. The easiest and most ignition and stable arc is achieved with the use of argon.

Helium

Helium increases the arc voltage, increases weld penetration depth, but makes ignition more difficult, but also adversely affects the arc stability. Some modern welding machines have the ability to start welding with argon and then, when the arc established, automatically skips helium.

argon + helium

The addition of argon to helium improves the ignition and its stability. The welding speed and penetration weld will be less, than when welding with a pure helium, but better, than when welding with argon only. Therefore it is possible to adjust seam width and depth of its penetration by changing the proportion of argon in the protective gas. mixture is often used with 25 % helium in argon [1].

Welding torch and welding cables

There are many different types of burners for welding current of several tens amperes to 450 ampere. Selection of the burner depends on the thickness of the welded material. Most modern burners (figure 3) have a current regulator, which is integrated into the torch handle. All burners, except, which operate at a current below 200 ampere, are water-cooled. The same water can be used for the cooling of power cables, It is making them lighter and more flexible.


Figure 3 - Modern burner for TIG welding

Overheating may cause the burner to melt the solder joints, or inside a plastic tube, which isolates the power cable. It is therefore important to choose a burner in accordance with the current strength, which will be used in the manufacture of welding, including taking into account, a current is applied, DC or AC.

Most of the burners is provided with metal or ceramic nozzle for forming a jet of gas. Ceramic nozzles are more popular, but they are more easily damaged, than metal. Nozzle diameter may range from 9,5 to 25 mm depending on the amount required for welding shielding gas, as well as the type of gas. It recommended for use in so-called gas burners lens. Gas lens comprises a mesh disc, which is inserted into the burner in order, to make the gas flow more laminar (figure 4). This helps to provide a more efficient gas protection for the formation of the weld.

tungsten electrodes

There are several types of electrodes for TIG welding methods. They include:

  • pure tungsten
  • wolfram, doped with thorium (ThO2)
  • wolfram, alloyed with zirconium (ZrO2)

These compounds are added, to improve starting characteristics of the arc, to stabilize the arc and to increase electrode life. Electrodes with zirconium considered preferable for TIG welding with alternating current, since they have a higher melting temperature, than pure tungsten and tungsten electrodes with thorium additives. So they can carry higher welding currents, They are more resistant to contamination and damage.

The end face of the electrode during welding must have a hemispherical shape. This contributes to its shape arc stability. The end of the electrode should be slightly sharp, to help shape its rounded end (Figure 4).


Figure 4 - Typical electrode for TIG welding

the electrode diameter is too small will lead to overheating and, perhaps, melted. This will lead to contamination of the weld pool tungsten. The electrodes are of a diameter of 0,3 to 6,4 mm. The electrode should not protrude from a burner nozzle, than 6 mm. This value can be increased to 10 mm, if the gas lens is used in the burner.

Manual TIG welding

Handling burner

It is necessary to keep the arc length as short as possible. In practice, the length of the arc is approximately equal to its diameter (Figure 5). If the arc is too long, the weld penetration decreases and increases the risk of defects due to insufficient penetration, poor quality weld and over its width. Moreover, in air may enter the gas cloud protection area forming the weld. This will lead to the introduction into the weld oxide inclusions.


Figure 5 - The angle of inclination of the torch and the welding rod for welding by TIG aluminum

The burner should be kept well, as it shown on the picture 5 - inclined 80º to the solidified weld. In the case of butt welding of elements of different thicknesses arc is directed more towards the thicker element. For fillet welds burner directed middle angle between two planes.

filler wire

If filler wire (filler rod) is used, it should be made uniformly and consistently at an angle 10-20 degrees, as it shown on the picture 5. The wire should not be fed directly into an arc, as this may lead to splashing and contamination of the electrode. Rod at an angle of more than 10-20 degrees prevents visual inspection of the weld pool. The tip of the filler wire should be inside the protective gas cloud until, while it remains hot, to avoid oxidation. By increasing the thickness of the welded component filler wire diameter also increase, which also causes an increase in the arc length, and. We must always remember, too long arc can cause problems with getting into the weld oxides. A large diameter bar can also block the material in front of the weld pool and interfere with the cleaning action of the arc, and this can lead to seizure by the weld of oxides.

Welding completion

Very important is the controlled completion of welding.. Abrupt shutdown of the welding current can lead to the formation of craters, sinks (elongated pores) and cracks in the last part of the weld pool. At the end of welding, it is necessary to gradually reduce the welding current and reduce the length of the arc as it attenuates, adding filler wire then, until the arc disappears.

Mechanization and automation of TIG welding

Mechanization and automation of TIG welding can have several advantages:

  • the ability to apply higher welding speeds, which gives a reduction in warpage and narrower heat-affected zones of welding;
  • tighter control of welding parameters, which allows you to weld thinner materials;
  • more thorough quality control of welding;
  • the ability to weld personnel with a lower degree of qualification, than is usually required for manual welding.

At the same time, the use of mechanization and automation has some disadvantages, including, significantly more time-consuming preparation of welded components for welding.

Sources:

  1. The welding of aluminium and its alloys / Gene Mathers – Woodhead Publishing, 2002
  2. The European standard EN 1011-4:2000 Welding – Recommendation for welding of metallic materials – Part 4: Arc welding of aluminium and aluminium alloys