Technology Briefing 625 - October 1997
A B D Gingell and T G Gooch
FULL REPORT
It is well recognised that aluminium alloys are highly sensitive to the development of weld metal porosity, and indeed the problem may be more acute than in most other structural steels.
Background
The occurrence of weld metal porosity may directly affect the service properties of a completed joint and further may render it difficult or impossible to undertake reliable non-destructive testing for other types of weld flaws. It is therefore important to identify the controlling factors in order to minimise its occurrence in practice, and this review was undertaken to define the present understanding of the problem. Relevant published literature was surveyed with attention being paid to the mechanism of pore formation and to the influence of welding parameters.
Objective
- To review published literature on porosity in aluminium alloy arc welds and assess the present level of understanding, so that recommendations can be made to minimise its occurrence during fabrication.
Approach
Published studies into the problem of weld metal porosity were surveyed. Appropriate references were obtained via Weldasearch, and discussions were held with welding engineers experienced in welding aluminium and its alloys.
Results and discussion
Porosity in aluminium alloy arc welds is particularly associated with hydrogen. This can stem from several sources, including hydrogen dissolved in base material or consumable, hydrogen or water present in the shielding gas, or hydrated oxides or other contaminants on the base or filler metals. The last of these appears to be of particular significance, and, in MIG welding, the electrode wire has been considered to be a major contributor to porosity through the presence of hydrated oxides on the surface of the wire which has a high surface area to volume ratio. Given the presence of hydrogen in the arc atmosphere, pick-up by the weld metal occurs primarily in regions under the arc root, which are heated to well above the melting point of the material. Since solubility in the solid material is very much lower than in the molten state, hydrogen rejection on solidification takes place to give porosity.
To some degree, the incidence of porosity can be reduced, if not eliminated, by optimisation of welding conditions. Rapidly freezing weld pools tend to give fine scattered porosity, whereas high heat input may give sufficient time for hydrogen to evolve from the solidifying pool. Manual TIG welding can give less porosity than MIG welding because the lower travel speed in the former case promotes hydrogen release from the molten pool. Release of gas from the molten metal depends upon the welding position, and porosity is particularly likely in overhead welds. In terms of MIG welding, porosity can be reduced by optimising the shield gas composition, 65% helium/35% argon mixture having been recommended. Porosity depends also on the material composition, via the influence on hydrogen solubility in the solid state. High magnesium alloys may display reduced porosity levels in consequence of a higher hydrogen solubility in solid metal.
The major point to be observed in fabrication is stringent attention to joint cleanliness and avoidance of extraneous contamination. Mechanical cleaning of the joint area immediately prior to welding is in principle to be advised, while consideration must be given to maintenance of stable welding conditions particularly in regard to the gas shroud around the fused metal.
Concluding remarks
The major factors contributing to porosity in aluminium arc welds have been well defined. The problem is recognised as stemming primarily from hydrogen pick-up and rejection by the molten pool, and various investigators have studied the effects of material type, welding parameters etc. on the occurrence of porosity. Available work does not indicate that porosity can be completely avoided solely by control of welding procedure. From the practical standpoint, the best defences are careful attention to maintaining a high standard of joint cleanliness and avoidance of atmospheric pick-up by the gas shroud.
Recommendations
It should be clearly recognised that aluminium alloys are singularly sensitive to porosity, and thus appropriate controls should be considered at the outset of fabrication. While porosity is alloy-dependent, the major palliative is stringent attention to cleanliness of the joint area immediately prior to and during welding. From the point of view of process variables, porosity is normally found to be less with TIG than with MIG welding, while in the latter case helium-rich gas mixtures are recommended.
Member Report No. 625-1997
Review of factors influencing porosity in aluminium arc welds
