PART 2: WELDABILITY OF ALUMINUM ALLOYS
The various series of aluminum alloys are all weldable, but some variants of the 7XXX series are more difficult to weld due to their high tendency to cracking. Generally, the operational weldability of aluminum is good; however, certain precautions are necessary when welding aluminum.
The family of aluminum alloys that are easiest to weld are the 1XXX, 3XXX and 5XXX series in which no heat treatment has been applied. The 6XXX series can be welded readily, but welding reduces their mechanical properties.
The high-strength 4XXX and 2XXX series are weldable, but special precautions must be taken. In the 7XXX family, only the 7039 and 7005 alloys are weldable. In addition, welds age naturally, and after 30 to 90 days, only 70 to 90% of their previous mechanical strength remains.
Overall weldability
The heat required to melt the base metal when welding lowers the mechanical strength within the heat-affected zone through annealing. With alloys that have been cold worked to obtain their mechanical properties, the properties in this zone cannot be regenerated through heat treatment.
Operational weldability
The alumina (Al2O2) layer covering aluminum and its alloys is about 0.0004 inch (0.01 mm) thick and tends to increase with temperature. This coating acts as a thermal and electrical insulation and is insoluble in the molten metal. It is also less dense, which explains why it remains on the surface of the weld metal.
To weld aluminum, you must therefore eliminate this layer which hampers welding, either with a mechanical device like a grinder or stainless steel brush, or through chemical methods such as strongly alkaline solutions.
When welding with gas (Sodel 480) or coated electrodes (Sodel 118), the flux contained in the rod itself, or in the slag from its melted coating, prevents the oxide layer from re-forming. In a similar manner, in the GTAW (TIG) or GMAW (MIG) weld processes, the gas shield protects the weld metal from air contamination and helps to minimize re-formation of the oxide layer. In addition, the use of alternating current for GTAW (TIG) welding breaks the oxide layer that forms at high temperatures.
Another welding problem with aluminum is its high thermal conductivity. To counter heat losses due to thermal diffusion, the piece to be welded must often be preheated to 400 to 500°F (200 to 260°C). Preheating stabilizes the welding arc by making it less erratic and promotes good penetration. When working with aluminum pieces that are to be heat treated or may age, it is important to understand the consequences of preheating on the mechanical properties of the alloy in question. Sometimes an excessively high temperature causes embrittlement or a reduction in certain mechanical properties.
Metallurgical weldability
Welding aluminum alloys can result in a lowering of the mechanical strength of the base metal if the latter is cold worked, whenever the part’s temperature rises above 660°F (350°C) during welding. It is impossible to regenerate the mechanical properties obtained through cold working by heat treating after welding. Depending on the application, it may be important to take into account this loss of strength in the HAZ.
It is important to remember that if an alloy has undergone one or more heat treatments prior to welding, welding will undo all the effects of the previous heat treatment. The larger the heat affected zone, the more this reduction of mechanical properties will lower the part’s performance in service. However, small zones have limited impacts in terms of reducing mechanical properties. This is why it is always important to limit the HAZ as much as possible.
For precipitation-hardened aluminum alloys, it is usually preferable (although sometimes difficult in practice) to follow one of the two sequences shown below, in the order given :
-homogenizing / welding / solution annealing / aging
or
-welding an alloy that has already been solution annealed (T4) / aging
Filler metal selection
The choice of a filler metal primarily depends upon weldability, desired mechanical properties, or corrosion resistance.
To minimize hot-cracking, Sodel 118 can be used for shield metal arc welding (SMAW); Sodel 480 can be used for gas welding. The high silicon content of these three products provides excellent de-oxidizing power and lowers the melting point of the filler metal. Their low magnesium content also reduces the likelihood of hot cracking.
PRACTICAL TIPS FOR WELDING ALUMINUM
1- For heat-treated alloys :
– Select the alloy that will be least affected by the weld thermal cycle.
– Design assemblies so that weld joints are located in places less subject to stress.
– Use as short a weld cycle as possible
2- Aluminum-copper alloys (2XXX series) are highly susceptible to burn through. To minimize the risks, pass through the solidification range quickly.
3- If the base metal temperature exceeds 660°F (350°C), recrystallization of the cold worked zone occurs causing a lowering of mechanical strength.
4- When welding, annealing may occur, thereby undoing all the effects of previous heat treatments.
5- Tendency to hot cracking is greater when the weld metal contains from 0.5 to 1.5% silicon and/or 0.5 to 2.5% magnesium.
6- Aluminum-magnesium alloys (6XXX series) have the highest resistance to atmospheric corrosion. However, they are anodic with respect to several non heat-treated alloys and can corrode when welded to the latter.
7- Using filler metal containing silicon (4043) when welding an alloy of a type other than aluminum-silicon produces blackening of the weld during the post-weld anodizing treatment.
8- Careful joint preparation ensures good penetration and strong mechanical properties (see Operational weldability section)