Welding aluminum alloys

The question often comes up "Isn't there one filler metal I can use for all my aluminum welding?" The answer, unfortunately, is no. However, there are basic rules for filler metal selection.

First, know which aluminum alloys will be welded (Fig. 1). If all you know is that you have aluminum, that is not enough information to choose a filler metal. Once the aluminum alloys are known, that you will be joining alloy A to alloy B, there are a number of good tables for selecting filler metals (see table).

Filler metal selection can be simplified somewhat. Almost all of the aluminum filler metals fall into either the aluminum-silicon 4XXX series or the aluminum-magnesium 5XXX series.

Overall in North America, 80% of the aluminum filler metal sold is made from just two alloys, 4043 and 5356. Also, one of these two alloys is the right choice for 80% of applications.

Alloy 4043 works very well for joining any of the 6XXX extrusion/sheet/plate alloys, such as 6061 or 6063. It also works well for welding most casting alloys and for welding 6XXX alloys to castings. However, 4043 should not, with one exception, be used to weld the 5XXX alloys.

Alloy 4043 has a 5% silicon (Si) content, while the 5XXX alloys contain as much as 6% magnesium (Mg). If welding 5XXX alloys with 4043, a lot of the intermetallic compound Mg₂ Si is formed, which is very brittle and can adversely affect the toughness and ductility of the weld.

What about exceptions? The common alloy 5052 contains only 21/2% magnesium. Because of its low magnesium content, it is acceptable and common to weld it using 4043.

One time 4043 should not be used to weld 6XXX alloys is when the material will be anodized after welding. If 4043 is used, the weld will anodize with a very dark, unpleasant color. In this case 5356, should be used.

Alloy 5356 works very well for joining 5XXX alloys, 6XXX alloys, and 5XXX alloys to 6XXX alloys. However, it should never be used to weld any of the 2XXX alloys or other alloys containing copper, such as 6013 or 6111 because the weld will very likely crack.

Both 4043 and 5356 work well when welding 6XXX alloys. Which one should be chosen? 5356 will provide welds that have higher shear strength and better ductility, although the welds will show more black weld smut (Fig. 2). 4043 will be easier for the welder to use, will deposit a better looking weld, and will be less crack sensitive.

Filler alloy 2319 is specifically designed to weld alloys 2219 and 2319.

Filler alloy 4643 has been specifically designed to be heat treatable. When welding a component which will be completely heat treated (solution treated, quenched, and aged) after welding to re-establish T6 properties, consider using 4643. It will result in welds of 40 ksi after heat treating.

The 5XXX alloys with magnesium content above 3% (5356 falls into this category) can become susceptible to stress corrosion cracking if they are exposed to temperatures above 150 F for long periods of time. In this case, a filler alloy of lower magnesium content than 5356 should be used.

Filler alloy 5554 was developed specifically for this purpose.

Many welding codes require welds in higher strength 5XXX alloys, such as 5083 or 5456, to have a minimum tensile strength of 40 ksi. While welding these alloys using 5356 will sometimes meet this requirement, tensile samples will often fail to meet it, failing at 38 ksi or 39 ksi. If there are such tensile failures, consider changing to filler alloy 5556 or 5183.

When welding the 6XXX alloys and/or castings and there are cracking problems, including crater cracking, consider changing to 4047. Filler alloy 4047 is similar to 4043 but contains 12% silicon, has a narrower melting range than 4043, and is more resistant to cracking.