The welding of aluminium and its alloys

Oxide film removal during welding

The need to remove the oxide film prior to welding to reduce the risk of porosity has been covered above. It is also necessary to disperse this film

during welding if defects such as lack of fusion and oxide film entrapment are to be avoided. Figure 2.9 illustrates oxide filming in a fillet weld that will obviously have a pronounced effect on joint strength.

Aluminium oxide (Al2O3) is a very tenacious and rapid-forming oxide which gives aluminium its excellent corrosion resistance. Aluminium oxide has a very high melting point, 2060 °C compared with the pure metal which melts at 660 °C. The oxides of most other metals melt at tempera­tures at or below that of their metals and during welding will float on top of the weld pool as a molten slag. Heating aluminium to its melting point without dispersing the oxide film will result in a molten pool of aluminium enclosed in a skin of oxide, rather like a rubber toy balloon filled with water. This skin has to be removed by some suitable means. With fluxed processes, soldering, brazing, MMA and SA welding, the flux needs to be very aggressive to dissolve the film. Failure to remove these fluxes on completion can give rise to service failures from corrosion and, in addition to porosity, is a further reason why MMA and SA welding are rarely used.

Fortunately, in gas shielded arc welding there is a phenomenon known as cathodic cleaning which can be employed to give the desired result. When the electrode is connected to the positive pole of the power source and direct current is passed there is a flow of electrons from the workpiece to the electrode with ions travelling in the opposite direction and bombard­ing the workpiece surface. This ion bombardment breaks up and disperses the oxide film and permits the weld metal to flow and fuse with the parent metal. The MIG welding process uses only DC electrode positive (DCEP) current - using DC electrode negative (DCEN) results in an unstable arc,

-ve 1/2 Cycle

1.10 Effect of polarity on cathodic cleaning and heat balance.

erratic metal transfer and poor weld quality. Oxide film removal is there­fore an intrinsic part of the MIG process.

TIG welding, on the other hand, conventionally uses DCEN, which, if used on aluminium, can result in poor weld quality. Using DCEP with TIG, however, results in the tungsten electrode overheating as some 60-70% of the heat generated in a TIG welding arc may be produced at the positive pole. (Conventionally a rule of thumb for the heat balance in a TIG arc is regarded as being two-thirds at the positive pole, one-third at the negative pole. This, however, varies widely depending upon the shield gas, current, arc length, etc.) This can cause melting of the electrode and bring the welding operation to a premature end. A compromise is therefore reached by using AC where oxide film removal takes place on the positive half cycle and electrode cooling on the negative half cycle as illustrated in Fig. 2.10. TIG welding of aluminium is therefore normally carried out with AC, although there are a couple of techniques that use either DCEP or DCEN. These will be discussed in Chapter 6 on TIG welding.