Hydrogen Solubility in solid and liquid aluminum, How to avoid porosity in aluminum welding

Solubility of hydrogen in Aluminum

Hydrogen is the only gas that is soluble into the aluminum. Although, comparing with the solubility of gases in iron alloys, the quantity of solubility is rather low. 

The solubility of Hydrogen in aluminum depends on the chemical compositions of alloys and on the temperature. The quantity of hydrogen solubility additionally depends on the availability of Hydrogen in the alloy (either from moisture in atmosphere or contamination), which is typically specified as the partial pressure and shown in Milliliters of the solved gas per 100 Grams of metal. (1013 mbar and 0° Celsius; 1 ppm = 1,1124 ml/100g).

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Hydrogen Solubility in solid and liquid aluminum

How hydrogen cause porosity in Aluminum during Welding?

The solubility of hydrogen increases with the temperature during heating of the aluminum and vice versa happen during cooling. It has to be noted that as the solubility of Hydrogen in aluminum quickly declines at a temperature of approx. 600° Celsius during cooling it often results in porosity caused by frozen gas bubbles. With pure aluminum (1XXX series) the propensity to porosity is more, whereas it is lower with aluminum alloys (2XXX- 6XXX).

Due to these conditions, it led to the understanding that the presence of porosity with MIG-welding of aluminum is highly unavoidable unless source of moisture is fully eliminated in MIG aluminum welding.

Pores & porosity have negative consequences on the static and dynamic strength of weld joints and can lead to weld failure. Pores open to the surface may reduce the adhesion of paint. Internal porosity, results in the rejections of the completed welds noticed by RT or UT examination.

Generally, welding inspectors have trouble to determine the level of acceptable porosity and both fabricators and customers consider it as just poor work without resolving the actual cause which are not related to the workmanship but more like technological causes.

SEM of weld pore
SEM image of a pore

How to avoid Porosity in Aluminum Welding

The basic solution to avoid porosity in aluminum during welding is to keep the level of available Hydrogen as low as possible. Generally, a Hydrogen content of approx. 0,2 to 0,3 ml/100g is considered to be the maximum permitted level in order to get low porosity in aluminum. This value is exceeded under practical conditions such as higher humidity level in the welding place, high due point of the shielding gas or surrounding or contamination of the filer wire or work piece quite frequently. As we know, sources of Hydrogen in the welding are base material, filler material, shielding gas and atmosphere. Clean storage- and fabrication conditions, preparation of the surfaces and prevention of all other sources of Hydrogen is the most important rule.

Click here to learn How to avoid weld cracking in 2000 Series Aluminum alloys welding

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