Aluminum, Oxidation, Hydrogen and Porosity

Aluminum, Oxidation, Hydrogen and Porosity.
Aluminum has a high maximum solubility for hydrogen atoms in the liquid form and a low solubility at the solidification point.

Hydrogen dissolved in the liquid weld metal will try to rise out of the weld during the aluminum solidification. Some hydrogen gas pores will be trapped and porosity will occur.

Aluminum combines with oxygen to form an aluminum oxide layer. This micro surface layer will form instantaneously if the oxide is removed by machining or grinding. The oxide layer is porous and can easily trap moisture, oil, grease and other materials. The aluminum oxide layer provides excellent corrosion resistance, but must be removed before welding as it prevents fusion due to its much high melting point point than the aluminum alloy .

Arc polarity, plasma molecular action, mechanical cleaning, solvents and chemical etching are all used to attack the oxide layer. When MIG welding if the layer is not removed sufficiently a black soot will appear either side of the weld. To eliminate the soot, first try to lower the arc length (voltage) as this makes the MIG plasma more dense which provides a more concentrated plasma cleaning action.

The majority of aluminum weld porosity results from entrapped hydrogen gas in the weld pool. Hydrogen is highly soluble in molten aluminum. Hydrogen can be derived from many sources.

[a] Hydrogen from base metal contaminates, hydrocarbons, lubricant, oils dirt, grease, moisture, paints and compressed air and contaminates from pneumatic cleaning tools or cleaning brushes.

[b] Hydrogen from lubricant contaminates on the alum weld wire surface.

[c] Hydrogen from moisture, water leaks in water cooled torches. Water from the gas cylinders. Water from the porous, hydrated, alum oxide layer on the base metal surface.

[d] Hydrogen that results from high humidity, condensation on parts and weld wires.

[e] Hydrogen that results from contaminates from grinding wheels.

To minimize hydrogen and weld porosity potential consider, cleaning, degreasing, stainless wire brushes or carbide wheels to remove the oxide surface. Remember you can always find porosity in the alum weld, the real question is how much is acceptable and what inspection and weld process control method will be used to control the porosity.

To reduce aluminum weld porosity potential, slow down the weld solidification rate to allow the hydrogen to exit. Reduce alum weld porosity with the following 11 points.

[1] To remove the alum surface oxide consider a die grinder (>30,000-rpm) rotary, coarse carbide file. An effective cleaning solution is acetone, beware highly flammable..

[2] Increasing weld parameters, with MIG increase the wire feed rate.

[3] Increase weld size.

[4] Avoid weaves.

[5] Slow down weld travel speed.

[6] Use smaller diameter MIG wires.

[7] Evaluate the weld procedure so that weld heat and weld sequence is used as a tool for porosity reduction.

[8] Use lowest possible MIG weld voltage. Low weld voltage results in short arc lengths which create more energy in the arc plasma providing improved arc cleaning action of the surface alum oxides.

[9]Use a higher energy gas mix like 60 helium - 40 argon. The helium requires higher weld voltage. The 60 helium mix is superior to the common 75 helium 25 argon mix, as the the higher argon content helps stabilize the arc and provides superior weld cleaning action.

[10] Don't use MIG wire wipes clipped on the wire.

[11] Don't use anti spatter within 2 inches of the weld. If you know how to set a weld you would not use anti-spatter.