The welding of aluminium and its alloys

Plasma-arc welding

As described in Section 4.3 a modification to the TIG welding torch enables a strong plasma jet to be produced that has some very desirable features for both welding and cutting. Despite these advantages the process found little application for the welding of aluminium when DC electrode nega­tive or electrode positive were used because of extensive lack of fusion

defects. Alternating current gave no better results as high currents were

required, resulting in rapid electrode deterioration. The pulsing of the current also caused weld pool instability, poor bead shape and lack of fusion defects. These limitations were overcome with the development of square wave power sources, already mentioned in Chapter 6, since which time plasma-TIG has become accepted as a viable production process.

8.2.1 Plasma-TIG welding

8.2.1.1 Main characteristics

As mentioned above, the basic principles of the plasma-TIG process (EN process number 15) have been covered in Section 4.3 which describes the use of the heat from the plasma-arc for cutting purposes. For welding the transferred arc plasma-jet is used as the heat source, the major difference

from cutting being that no cutting gas is introduced to blow away the molten metal (Fig. 8.1).

There are a number of advantages that plasma-TIG has compared with conventional TIG mainly because of the cylindrical and constricted plasma column. This provides less sensitivity to process variables than with the TIG process. The constricted plasma column means that the heat is confined to a smaller area than with TIG, enabling a very stable controllable arc to be produced at currents as low as 0.1 A. It is possible to weld without keyhol - ing at thicknesses less than 2.5 mm but there is little advantage to be gained in terms of productivity over TIG. The keyholing technique may be used in manual welding but it is more common to find it in mechanised or auto­mated applications.

The plasma is strongly directional and can be pointed in any given direc­tion even at very low currents. The cylindrical plasma column means that heat input is constant irrespective of torch to workpiece distance, unlike TIG with its conical arc. The tungsten electrode is recessed inside the torch nozzle, making tungsten contamination an impossibility. There is also an increase in weld quality with a reduced risk of porosity and distortion. Higher welding currents enable material as thick as 15 mm to be welded positionally in a single pass with a square edge weld preparation using the keyhole technique although in the flat position the maximum thickness is limited to around 8 mm without filler metal.

The process may be used in a melt-in mode using techniques similar to those that would be used for TIG. The weld may be made autogenously in those alloys that are crack-resistant; filler wire may be added to those that are crack-sensitive. This wire can be added manually but torches are avail­able equipped with automatic wire feed. This latter feature, however, makes the process sensitive to stand-off distance. A change in the stand-off will affect the position in which the wire enters the weld pool and this may give variable weld quality.

The problem with conventional plasma-TIG is that the process normally operates on DC negative polarity so that no cathodic cleaning takes place, an obvious disadvantage when welding aluminium. Welding without the facility to remove the oxide layer causes porosity. To overcome this a devel­opment of the DC positive plasma-TIG process, the variable polarity plasma process was developed. This utilises a square wave form with a suit­able balance of the DC negative and positive components to provide both melting and adequate oxide removal.

8.2.1.2 Variable polarity plasma-arc process principles

For the plasma to form, a pilot arc is first established within the torch annulus by means of a high-frequency discharge. As the plasma gas passes through this HF discharge it is ionised, allowing the welding current to flow and the plasma flame to be established. The plasma gas flow is very small, typically 1-5 litres/min. This is insufficient to provide adequate shielding and therefore needs to be supplemented with a secondary shield gas. The gases are generally high-purity argon similar in quality to that used for TIG welding, but helium or argon-helium mixtures may also be used.

Arc stability and oxide removal are better than with TIG or MIG pro­vided that the appropriate wave form is used. It is necessary to tailor each wave form and the balance between DC electrode positive and DC electrode negative to the individual alloy composition. Typical wave form characteristics for the keyhole welding of a number of alloys are given in Table 8.1. It is worth remembering that if the keyhole technique is used autogenously the alloys must be capable of providing crack-free weld metal.

Over 8 mm thick it is necessary to weld with a prepared edge and filler wire although in the vertical-up (PF) position material as much as 16 mm thick can be welded. The diameter of the filler wire is the same as would be used for TIG welding, generally 1.6 or 2.4 mm diameter.

8.2.2 Plasma-MIG welding

Plasma-MIG welding utilises a MIG wire, generally 1.6 mm in diameter, fed through a plasma-arc torch. This allows a higher combined welding current to be used than for the MIG wire alone with a high current density and a higher deposition rate than MIG being achieved. This enables welding speeds to be increased giving lower heat input and narrower heat affected zones with better mechanical properties.

The process is generally used in a mechanised or automated application although it is possible to use it in a semi-automatic manual mode. The thick­ness that has been welded ranges from 6 mm to 60 mm.