ASK THE EXPERT 25 ASK THE EXPERT
Phil Hughes, welding industry expert at Air Products, sheds light on how to minimise porosity when welding aluminium.
What causes porosity when welding aluminium?
It is not unusual for pores and blackening of the joint to occur when welding aluminium. This porosity is due to hydrogen gas that becomes trapped in the weld metal as it solidifies.
There are many potential sources of hydrogen when welding aluminium. These may include moisture in the surrounding air, moisture trapped in or on the oxide layer of the workpiece or contamination due to the presence of oil and grease containing hydrocarbons.
When working with aluminium as oppose to steel, for example, the solubility of hydrogen increases significantly, which means the potential for it to become trapped in the weld is greater. The weld pool also solidifies much more quickly when welding aluminium, due to its higher thermal conductivity, which also heightens the risk of hydrogen becoming trapped.
To check that the degree of porosity in the weld falls within industry guidelines, welding engineers should consult EN970 for visual inspection of the weld and EN1435 for radiographic interpretation.
What can I do to minimise the risk of porosity occurring?
There are many things that can be done to reduce the risk of porosity occurring when welding aluminium. Research has shown that using a shielding gas that contains helium, such as Alumaxx® Plus, will produce a larger and hotter weld pool, allowing it to cool and solidify more slowly thus reducing the risk of hydrogen becoming trapped.
However, the most effective thing you can do is to minimise the amount of hydrogen before welding starts in the following ways:
.. All aluminium components should be thoroughly de-greased/solvent cleaned and dried before the welding operation. .. Cut, saw or machine aluminium parts that need to be welded without using cutting fluids. .. Use the biggest diameter of filler material possible.
If the design allows, brush the material with a stainless steel brush, to remove the oxide layer on the surface. Start to weld as soon as possible after cleaning (within 4 hours). More about Ask the Expert…
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.. Preheat the joint and its immediate surround in order to remove any residual moisture. Ensure filler wire and base material is stored in a warm, dry environment, preferably heated. ..Welding wire should be supplied in a ‘shaved’ or ‘double/triple shaved’ condition, to ensure Hydrogen content is low. Remove filler wire from the welding machine after use and store in a temperature-controlled area.
. When manual TIG welding, the tip of the filler wire should not be removed from the shielding gas column between puddling as this may introduce oxidation to the wire and then the weld pool.
For MIG welding, wire feed speed is critical and should be balanced with the amps, volts and travel speed used as this may contribute to erratic deposition and atmosphere absorption. Wire should be cut before striking.
What should I do if porosity is still a problem even after I have taken all the obvious precautions? This may be due to residual moisture inside the welding gun or in the pipes used between the welding machine and the gas delivery point. More often than not this is due to air entering the welding gun while not in use or air entering the system via leaks in the gas supply pipes or at connections. The pipes may not be suitable for the purpose and may be permeable, allowing moisture to enter the pipes and diffuse into the gas.
Is it possible that the moisture is in the sheilding gas I am using?
All shielding gases conform to BS14175 and the moisture content of the shielding gas has a guaranteed concentration of less than 10 ppm, no matter whether it is supplied to you in gaseous or liquid form.
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