How to Choose Aluminum Welding Wire (ER4043 vs ER5356)

Choosing between ER4043 and ER5356 depends on the aluminum alloy you are welding. There is a correct answer for every common alloy, and getting it wrong causes cracking. Here is the decision framework.

The Short Version

Welding 6061 or cast aluminum? Use ER4043. Welding 5052, 5083, or 5086? Use ER5356. Welding 6061 that will be anodized? Use ER5356. Not sure what alloy it is? Use ER4043, it is more forgiving.

That covers 90% of decisions. The details below explain why.

Why the Base Metal Matters

ER4043 is aluminum with 5% silicon. ER5356 is aluminum with 5% magnesium. When you melt filler into a base metal, the chemistries mix in the weld zone.

The problem: silicon and magnesium together form a low-melting eutectic compound (Mg2Si). If both are present in the wrong proportions in the weld, hot cracks form as the weld solidifies.

ER4043 on 5xxx base metal: the silicon from the filler meets the magnesium in the base metal at the fusion line, creating the cracking eutectic. That is why ER4043 should not be used on 5xxx alloys.

ER5356 on 6xxx base metal: this works but is more crack-sensitive because 6xxx already contains both silicon and magnesium. ER4043 dilutes the magnesium content and avoids the eutectic, which is why it is preferred for 6xxx.

Anodizing and Color Match

ER4043 welds turn dark gray or black when anodized. The silicon content reacts with the anodizing process differently than the base metal. If the finished part will be anodized and the weld needs to match, ER5356 is required.

For parts that will be painted, powder-coated, or left bare, the color difference does not matter.

Strength Comparison

ER5356 produces stronger welds: 38,000 psi tensile vs 26,000 psi for ER4043. For structural applications, marine frames, and load-bearing joints, ER5356 provides a meaningful strength advantage.

For general fabrication, brackets, enclosures, and non-structural work, the lower strength of ER4043 is adequate and the better crack resistance is more valuable.

Classification system defined by .