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Porosity and the Mechanical Properties of Aluminium Alloys

Aluminium alloys are extremely susceptible to weld metal porosity. This is particularly associated with rejection of hydrogen from the solidifying weld pool, and there is substantial published literature on the sources of hydrogen, on the mechanism of pore formation, and on the roles of material and welding variables.

On exposure to the atmosphere, aluminium and its alloys rapidly form a hydrated oxide surface layer, and this effect renders it difficult or impossible to ensure complete freedom from weld metal porosity when arc welding is carried out. Benefit in avoiding the problem can be gained by optimising alloy composition, by using TIG rather than MIG welding and employing helium-rich shielding gases. From the practical viewpoint, it is essential that the joint area is rigorously cleaned immediately prior to welding and that a stable gas shroud is kept over the molten pool.

Figure 1. Aluminium Porosity
Figure 1. Aluminium Porosity

The effects of porosity on yield and tensile strength, ductility and fatigue properties are summarised below -

  • Increasing area fractions of porosity in a weld cross-section reduce strength. Fine porosity - when present in sufficient quantity to contribute a total area comparable to that of large pores - also causes a loss in strength
  • The loss in transverse tensile strength in the weld metal in aluminium welds is proportional to the loss of sound metal area in the plane of expected fracture
  • Fatigue life in transverse tension-tension loading also shows a degradation in failure stress at a given cycle life with increasing porosity. This reduction is also proportional to area loss in the failure plane
  • Porosity obtained in horizontal position welding is more damaging to mechanical properties than other positions because the pores are not randomly distributed throughout all possible failure planes, but tend to be concentrated in a given plane normal to a transverse load. This becomes a plane of weakness
  • Pore location, except as in above, has little effect on the relationship between mechanical strength and area loss due to porosity. Excessive concentration near one surface of the bead (most likely the top) can impose a bending component, which amplifies the stress somewhat. For example - in an artificial defect specimen, in which the entire porosity was contained in a surface layer 20% of thickness in depth, strength was reduced by less than 10% of the linear prediction value
  • Proof strength of the weld metal is not significantly affected by porosity.
  • Ductility of the aluminium weld metal is sharply reduced by the presence of small amounts of porosity
  • Fatigue life at high stress levels is markedly reduced by small amounts of porosity; these pores act as sites for crack initiation and growth
  • In resistance spot-welds, porosity up to 40% of the nugget diameter did not affect the static or fatigue performance of the welds in shear when maintaining a constant 6.3mm diameter
  • It is very likely that fracture resistance will also be affected negatively by large volume fractions of porosity, although no published references have been identified 
  • Defects of this type are not easily detected in fillet welds, unless they are surface breaking

 

 

TWI can offer its Members the benefit of long experience in this field, across multiple sectors and applications. Services may range from material selection, design and best practice, through to failure analysis and corrective action.

Avatar Chris Wiseman Industry Sector Manager - Corporate Development

Chris Wiseman is Corporate Sector Manager at TWI Ltd, for both the Aerospace and Equipment, Consumables & Materials sectors. He also has a specific geographic responsibility for TWI Industrial Members in India and in supporting TWI’s exciting growth there.

As a Materials Scientist, over many years, he has contributed to a number of technologies, including, corrosion prevention, thermo-mechanical forming, novel testing, process optimisation, joining and surface treatments.

Today his role is focused on developing technical and commercial opportunities for TWI and its Industrial Members around the World.  

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