Zinc (7XXX series) added to aluminum with magnesium and copper produces the highest strength heat-treatable aluminum alloy. It is primarily used in the aircraft industry. The weldability of the 7XXX series is compromised in higher copper grades, as many of these grades are crack sensitive (due to wide melting ranges and low solidus melting temperatures.) Grades 7005 and 7039 are weldable with 5XXX fillers.
Other elements (8XXX series) that are alloyed with aluminum (i.e. lithium) all fall under this series. Most of these alloys are not commonly welded, though they offer very good rigidity and are principally used in the aerospace industry. Filler metal selection for these heat-treatable alloys include the 4XXX series.
Pure Aluminum (1XXX series), though not an alloying element, is considered nonheat-treatable and is used primarily in chemical tanks and piping because of its superior corrosion resistance. This series is also used in electrical bus conductors because of its excellent electrical conductivity. 1XXX series are easily welded with 1100 and 4043 alloys.
In addition to the primary aluminum alloying elements, there is a number of secondary elements, which include chromium, iron, zirconium, vanadium, bismuth, nickel and titanium. These elements combine with aluminum to provide improved corrosion resistance, increased strength and better heat treatability.
Physical Properties
Now that you have a basic background on aluminum metallurgy, we will move into the physical properties of base metal aluminum and how it compares to other metals, primarily steel.
The reason why aluminum is becoming specified for so many jobs is its physical properties. For instance, aluminum is three times lighter than steel and yet offers higher strength when alloyed with the right elements. It can conduct electricity six times better than steel and nearly 30 times better than stainless steel. This high electrical conductivity makes the effect of electrical stick-out in GMAW (Gas Metal Arc Welding) less significant when compared to steel (we will cover this concept in more detail later in this article.)
In addition, aluminum provides excellent corrosion resistance, is easy to shape and join, and also is non-toxic for food applications. Since it is non-magnetic, arc blow is not a problem during welding. With a thermal conductivity rate that is five times higher than steel and being less viscous, aluminum can easily be welded out-of-position. Aluminum does have its drawbacks, though, since its high thermal conductivity tends to act as a heat sink making fusion and penetration more difficult.
Since aluminum has a low melting point 1,200 degrees F (half that of steel) for the same wire size, the transition current for aluminum is much lower than it is for steel. Also, for the same welding current, the burn-off rate is about twice that of steel.
Chemical Properties
In terms of chemical composition, aluminum has a high maximum solubility for hydrogen atoms in the liquid form and a low solubility at the solidification point. This means that even a small amount of hydrogen dissolved in the liquid weld metal will tend to escape as the aluminum solidifies and porosity is likely to occur – a great cause of concern during the welding process.
Also, aluminum combines with oxygen to form an aluminum oxide layer instantaneously as it is machined. This layer is very porous and can easily trap moisture, oil, grease and other materials. The oxide provides excellent corrosion resistance, but must be taken off before welding as it prevents fusion due to its high melting point (3700 degrees F). Mechanical cleaning, solvents, chemical etching and purging are used to take off the oxide layer.
Mechanical Properties
Mechanical properties such as tensile strength, yield and elongation are affected by the choice of aluminum base and filler alloys. For groove welds, the Heat Affected Zone (HAZ) dictates the strength of the joint. In nonheat-treatable aluminum alloys, the HAZ will be completely annealed and the HAZ will be the weakest point. Heat-treatable alloys require much longer periods at annealing temperatures combined with slow cooling to completely anneal them so that weld strength is less affected. Such items as preheating, lack of interpass cooling, and excessive heat input from slow, weaving weld passes all increase peak temperature and time at temperature, which means minimum strength levels might not be met.
For fillet welds, strength is dependent on the composition of the filler alloy used to weld the joint. In structural applications, the selection of 5XXX instead of 4XXX series filler can provide twice the strength
