Porosity is one of the main defects or discontinuities that may be encountered when welding aluminum and its alloys. The phenomenon is caused by the hydrogen gas that cannot escape from the molten metal, while still remaining entrapped in the microstructure after cooling. In fact, the hydrogen is very soluble in liquid pure aluminum with a concentration degree in the order of 2.2 (cm3 per 100 gr. of metal). However, this high concentration drops drastically to 0.069 cm3 per 100 gr at the boundary of the solidification before falling up to (0.036 cm3/100 g of metal) after cooling. The following graph illustrates the evolution of this phenomenon. It should be noted that the high thermal conductivity of aluminum, 210 W/m-K, promotes also the porosity formation especially in thick joint assemblies.
Hydrogen solubility in aluminum
Thus, since it is no longer soluble in solid aluminum after complete cooling, this free hydrogen will form voids air pockets, referred to as porosity. In function of the cooling conditions, the porosity may be open to emerge on the surface and/or stay embedded in the metal. A coarse or clustered porosity may adversely affect certain metal properties such as tensile strength and toughness.
SOURCES OF HYDROGEN
The presence of hydrogen is originated primarily from humidity and contamination, and to a certain extent, fabrication condition and welding methods.
- Humidity : the humidity comes from the following:
- Hydrated oxide layer on the metal surface;
- Storing conditions of welding consumable (covered electrodes, rods and filler wire);
- Condensation in water-cooled welding torches (leaking joints and fittings)
- Shielding gases pipes (defective or non-hermetic material);
- Inadequate purity in shielding gases ;
- Temperature change between the storing area of filler materials and the welding station (condensation- dew point)
- Contamination :
Hydrogen is the decomposition product of hydrocarbon products such as machining oil, grease, solvents, solid lubricants, dirt, etc. The latter products may be present either on the base metal or the filler metal, or on the tools reserved for cleaning and preparation. Naturally
- Fabrication conditions and welding parameters
- The products or parts produced by die-casting has naturally a much higher porosity level as compared with those obtained by static casting or extrusion. As a result, the welded parts become unavoidably more susceptible to porosity in the weld joint or even in heat affected zone.
- Welding parameters:
- Welding near air draughts (open doors, fans, operating fans, etc.);
- Excessive electrical stick-out or CTWD(Contact To Work Distance);
- Insufficient gas shielding (low flow rate);
- Turbulence in the molten pool due to a high shielding gas flow rate;
- Short-Circuit (S) or Globular (G) GMAW transfer modes produce spatters and, eventually, more porosity than the other transfer modes.
- Erratic wire feeding in semi-automatic welding process;
- Splashes deposited inside the nozzle prevents normal flow conditions of shielding gas;
- Too tilted electrode work angle to the normal;
- Low heat input;
- Number of welding layers: multipass welding (ex. thick joints) is more subjected to the phenomenon of porosity than mono-pass welding (thin joints).
PRÉVENTION OF POROSITY
Porosity may be controlled by first making a diagnosis of the main incriminated factors of its occurrence prior to bringing the necessary corrective actions. This involves the control of the storing conditions of filler materials, the use of appropriate and clean tools and also the application of proper welding procedures. This is described below.
- Filler materials
Filler metals must be stored in a dry environment or under a relative humidity of 35% maximum with a temperature of 15°C minimum. They also must be conserved in their original packagings until their use
Covered electrodes having exposed to moisture can be reconditioned by a redrying at 100-120°C for 1 to 2 hours.
Filler wires and rods must be free from traces of solid lubricant that have been used during drawing to reduce friction.
- Tools, cleaning and preparation:
- Use clean tools which are only reserved to aluminum such as stainless steel wire brushes or ceramic grinding discs to remove the oxide surface;
- Select moisture-resistant gas-hoses, ex. From metal or neoprene;
- Check the tightness of torch (joints, fittings);
- Degrease and/or wipe the solid filler rods using solvents like acetone;
- Wear clean gloves when handling metal filler materials and metal parts;
- Place filler materials as well as base metal in the same location a hour at least prior welding;
- Keep an interval time of 6h maximum between the cleaning and the start of welding operation. Otherwise, cleaning must be done again prior to welding;
- Shielding and purging gases have to conform to the minimal purity degree as required by codes;
- Welding method
- preheat the base metal (>1/4″ thick.) and the backing plate, when used, at 200°C (400°F);
- use high-frequency alternating current HF-AC for TIG welding in order to get a better cleaning action on the base metal surface;
- increase the heat input in order to produce molten pool with a good fluidity;
- use a mixture gas Ar-He (He: 25-75%) instead of straight argon to get a hot arc while inducing more heat into the weld pool and the base metal;
- avoid having turbulences in the molten pool by ensuring a constant wire feeding (GMAW);
- reduce the cooling rate by reducing the travel speed;
- keep the shortest possible distance between the nozzle and the surface to weld;
- Weld with an angle of about 20° relevant to the vertical axis.
As mentioned above, residual hydrogen in liquid aluminum results in porosity during the cooling of the weld metal. Then, in order to get a sound and porosity-free weld metal, one should eliminate the different sources generating hydrogen in the weld pool such as moisture and contamination on one side, and adjust the proper welding parameters regarding the welding application on the other side.
It is concluded that ensuring a good control on the hydrogen sources leads to a considerable increase in the chance of getting a sound and porosity-free weld metals.