1 Origin
Porosity in fusion welds is caused by gases, which are absorbed in the molten weld pool, and form bubbles, which are not able to escape during cooling and solidification. The gas of primary concern for aluminium is hydrogen, which originates from a contaminant, either a hydrocarbon (e.g. grease, skin and oils) or moisture, both of which can break down in the arc plasma to produce atomic hydrogen.
Hydrogen has a high solubility in molten aluminium and can be easily absorbed into the weld pool but the solubility falls, as the liquid cools, and is much lower in solid aluminium (approximately 20x lower). Thus as the aluminium cools, hydrogen is evolved and forms gas bubbles in the melt. When these bubbles become trapped in the solidifying weld metal, gas pores are created.
There are a several possible ways these contaminants can be introduced into weld pools. For example: they can be derived from materials present within or on the parent material or the filler wire; might be present on tools or welders' hands and can be introduced by touching the surfaces which are melted into the weld pool and from moisture, in particular, which can be introduced by the shielding environment experienced during welding. The manifestation of porosity can also be influenced the way the molten metal is manipulated, i.e. the specific welding conditions employed.
2 Causes
2.1 Internal hydrogen
Hydrogen can originate from within the parent materials, which might have high inherent hydrogen levels associated with their manufacturing procedures. This is most likely to be encountered with cast or sintered products but it has occasionally been caused by inadequate de-gassing during the manufacture of wrought materials.
2.2 Surface contamination
An important characteristic of aluminium is that, immediately on atmospheric exposure, a thin, tenacious oxide film is formed on its surface and this is responsible for its good corrosion resistance. This oxide film becomes hydrated in normal atmospheric conditions, i.e. moisture is adsorbed on to the exposed surfaces from the surrounding air and, if this hydrated oxide is exposed to the welding arc, the moisture is liberated from the oxide and atomic hydrogen is released and readily absorbed into the weld pool.
If the shielding of the weld pool by an inert gas is inadequate or is temporarily interrupted, moisture from the atmosphere can be drawn, directly, into the arc and cause hydrogen to be absorbed into the weld pool.
2.3 Shielding gas
The flow of shielding gas, itself, can act as a source of moisture, either if there is contamination in the shielding gas source (e.g. from a gas cylinder) or if the transmission of shielding gas from source to welding torch allows moisture pickup. For example, gas transmission lines, which are porous or contain leaks, can allow atmospheric moisture to be entrained into the, otherwise, dry shielding gas. In addition in a cold environment, when shielding gas is not flowing (perhaps overnight), condensation can create a build-up of moisture within the gas lines.
3 Controlling Porosity
3.1 Background
The priority for avoidance of porosity in aluminium is therefore reducing the potential for hydrogen to be absorbed into the weld pool. This is achieved by taking account of all of the possible sources, above, as well as process features of the different welding methods.
3.2 Cleaning
The most important influence on weld porosity is generally considered to be cleanliness. Any possible contaminants need to be removed from the parent and filler materials, both in the form of direct contamination and also potentially-hydrated oxide. Prior to welding, any surface to be welded (and a portion of the parent material outside of this) needs to be thoroughly degreased, typically by a solvent cleaner such as acetone or similar. Solvent cleaners are comprised of hydrocarbons and/or water and need to be removed prior to welding, although they are typically volatile and will evaporate quickly. However, once a joint is assembled, it is very difficult to remove any cleaning solution, so cleaning should always precede joint assembly.
After degreasing, it is most common to employ a mechanical cleaning operation. This is most frequently performed by stainless steel wire brushing, although this is not the most effective oxide removal technique. Cleaning should be performed with dedicated tools which are kept in conditions which minimise any possible contamination. It is noted that it is sometimes stated that oxide removal should be carried out ‘immediately’ before welding, as re-hydration will commence as soon as the fresh oxide surface is established. The appropriate elapsed time between oxide removal and welding will depend on the quality required, i.e. the acceptable level for porosity, as well as the temperature and humidity in the storage environment. Stringent porosity requirements can require oxide removal procedures to be carried out within hours, rather than days, of making the welds.
Oxide removal can also be achieved using chemical cleaning solutions, such as sodium hydroxide or nitric acid. Systems employing an appropriate array of cleaning tanks can produce excellent results but account must be taken of the health and safety requirements associated with handling and maintaining aggressive chemical baths.
Finally, where possible aluminium welding should be kept separate from other material fabrication, to avoid cross-contamination via fume, grinding dust or tool use.
3.3 Shielding environment
To avoid introduction of contaminants through the shielding gas, a number of good practices can be employed. The distance the gas has to travel from source to the torch should be minimised and piping materials with a low permeation characteristic should be used (stainless steel being the most expensive but most effective for dealing with long distances). Mechanical joints are generally considered more prone to leaks than brazed or welded joints. Gas systems should be checked for leaks and the gas quality should be checked at the torch, to achieve a dew point preferably below -50degC. When systems have been left idle for some time, they should be pre-purged by flowing the gas for long enough to remove condensation and reduce porosity in the first welds to be produced.
Introduction of environmental moisture into the weld pool can be minimised by avoiding disruption of the arc or shielding gas flow. The welding area should be set up to avoid draughts and the wire feed should be smooth and consistent to reduce the risk of turbulence in the weld pool or arc plasma. The wire should contain no kinks and be fed by drive rolls, which are correctly adjusted, and be delivered through a contact-tip which is of the correct size and in good condition. Renewing a wire feed liner, or re-positioning of a poorly-placed torch cable can reduce some of these problems.
3.4 Welding conditions
Welding parameters can also have an impact on both the creation of pores and their probability of release from the molten pool. Thus, a long arc length can give molten metal droplets more time to pick up environmental moisture and introduce it into the weld pool, whereas, a higher welding current can increase the temperature of the weld pool, increasing the hydrogen solubility and allowing it to absorb more hydrogen which can be ejected on solidification. However, an increase in welding current, as well as a reduction in travel speed, can also potentially reduce porosity by giving hydrogen more time to "bubble out" of the weld pool, if the rate of gas evolution is higher than the rate of gas absorption. This can also be aided by preheat, heating the backing bar (if one is used), or by using an Ar-He shielding gas, rather than pure argon.
During the arc welding process, oxide is also stripped from the workpiece surface due to the nature of the welding process. In TIG welding, an AC welding current is used, which rapidly flips the polarity of the welding current between the tungsten electrode and the workpiece. When the electrode is positive, the workpiece surface has electrons stripped from it and it is bombarded with ions, which break-up and disperse the oxide film. However, this setup of the polarity causes significant heating of the tungsten electrode and so cannot be used continuously. Thus, AC current is employed conventionally, as a compromise solution, in which the electrode is maintained positive for approximately 20-40% of the arc time. In MIG welding, the electrode is always positive and so cleans by default. It should be noted that this dispersal of oxide remains an essential part of the welding process to achieve metal-to-metal contact, following any mechanical or chemical process employed to remove hydrated oxide. This is an important aspect of the welding process because an unhydrated oxide will have re-formed, initially, after removal of a hydrated oxide.
4 Concluding Remarks
Porosity is a well known phenomenon in aluminium welds but its significance is probably less well understood. In some applications, very careful control of the welding process is required to achieve the acceptance levels demanded. However, in other, less-critical applications, porosity might not be monitored, routinely, allowing less-stringent procedures to be followed. Thus, before instituting onerous procedures to reduce porosity, it is worth assessing the significance of the small spherical voids on the performance of a product. For example, one investigation came to the conclusion that porosity in aluminium welds became a problem when the density of voids on radiograhs obstructed the detection of more serious linear flaws, before the volume fraction was high enough to cause significant loss in strength.
Gas pores in aluminium welds can be avoided by the adoption of processes which avoid melting the material, e.g. friction, ultrasonic and pressure welding techniques.