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We have seen from previous articles that aluminum alloys, with their inherent lightweight, their overall versatility and corrosion resistance, can offer benefits that make the material appealing for a variety of industries. The metal, however, is not without its challenges when it comes to welding. We have noted that there are challenges due to its low melting temperature and high thermal conductivity and that care must be taken to prevent the formation of hydrogen porosity. Similarly, care must be taken to prevent burn through on thin material and to ensure proper fusion on thicker material.Ìý

We have also seen that the preferred welding processes are GMAW and GTAW both in their conventional and pulsed form.Ìý Using the right process for the job in hand is important but so too is selecting the right filler metal and understanding the weld characteristics each type provides.

There are many filler metals available. These range from pure aluminum to alloyed varieties with added copper, silicon or magnesium to match the various alloys we have discussed in previous articles. Suppliers of aluminum welding consumables have produced extensive charts showing what fillers are available for the various alloys and these charts are readily available from the literature or directly from these consumable suppliers.

If we were to look at these available charts, we will see the variety of fillers available** to join most of the weldable alloys. When choosing the optimum filler alloy, the end use of the weldment and its ultimate performance will be of prime consideration. Only one may be optimal for a specific application. Nominally matching filler metals are often employed for non-heat-treatable alloys. However, for the lower alloy (alloy lean) and heat-treatable alloys, non-matching fillers are used to prevent cracking occurring on solidification.

** Filler metal families fall into either 1xxx, 2xxx, 4xxx and 5xxx families

Extracting two common alloys from the available charts, 6061 and 5083, shows the following, as shown in the Table 1 below

Table 1. Filler Alloys available for Joining 6061 and 5083 Alloy

The primary factors when consider the final choice of filler from those listed in Table 1, will be governed by the end product properties as given in Table 2, below:

Table 2. Identification of Properties Required from Joint

In each one of these categories, most consumable suppliers’ charts will give a rating from A....good through D ...which is less than optimal.Ìý

Regarding W in Table 2, solidification cracking can occur in aluminium alloys because of high stresses generated across the weld due to the high thermal expansion (twice that of steel) and the substantial contraction that occurs on solidification.

Solidification cracks form in the centre of the weld, usually extending along the centreline during final solidification. They can also occur in the weld crater at the end of the welding operation. The cracking risk can be reduced by using a non-matching, crack-resistant filler (usually from the 4xxx and 5xxx series alloys).Ìý

It is not the purpose of this article to expand on all the wire combinations for any particular alloy system but just to illustrate what is available and how they may be defined. To this end, we will take a look at the properties of two of the most common filler wires, 4043 and 5356. Together, those filler metals are used for welding about 80 percent of the time and are available in wires for both GMAW welding or filler rods for GTAW welding.

Both 4043 and 5356 filler metals operate with 100 percent argon shielding gas, as it provides good arc initiation and stability. Thicker aluminum applications may require the addition of helium, which improves the heat transfer to the base metal and helps increase weld penetration.

Knowing how to choose between the two alloys, as well as the characteristics each provide is important to producing good weld quality.

Providing the base metal is compatible, a 4043-aluminum filler metal will be preferable for the following:

•ÌýÌý ÌýApplications subject to long-term elevated temperature exposure above 65 Deg C
•ÌýÌý ÌýWhere it is necessary to reduce the risk of solidification cracking.Ìý
•ÌýÌý ÌýWhere aesthetics are important, this filler provides clean looking welds

A 5356-aluminum filler metal would be selected for:

• Joining 5XXX or 6XXX series of aluminum when the tensile strength is most important.Ìý.

• Applications requiring higher ductility and toughness.Ìý

• Applications requiring higher shear strength

• Good anodized colour matching on 5XXX and 6XXX base metals

It is important to keep the filler metals in a clean and dry area that is of a similar ambient temperature as the welding location. Moving them from a cold area to a warmer area can cause condensation to form on the surface of filler metals and lead to porosity in the final weld.

If a spool of aluminum wire isn’t going to be used, then the spool should be covered in a plastic bag or with another protective cover. GTAW rods are best stored in their original box to protect against dirt and debris.

In conclusion, there are many filler wires available for the welding of aluminum and its alloys. There is a wealth of information in the literature and on line. However, when necessary, look for further information within your welding engineering, welding technology and filler metal manufacturing community to provide additional help and advice.