ER4043 vs ER5356: Which Aluminum Filler Wire? Six Deciding Factors and One Hard Rule

“Should I weld 6061 with ER4043 or ER5356?” is the question we hear most on the shop floor. It rests on a common misconception: that any aluminum alloy can be welded with either one. Some base metals can only take one of them. The genuine “either works” cases are base metals like 6061 — and there, the right choice depends on what the part needs most. This guide from Mastership (sole authorized distributor of Canadian Indalco aluminum welding consumables in Asia) is based on AWS standards and industry filler selection data.

The short answer

Your situation	Choose	Why
Base metal is 5083, 5086 or 5456 (more than about 2.8% Mg)	ER5356 (or ER5183)	4043 is ruled out — see “The one hard rule” below
The part will be anodized and color must match	ER5356	4043 welds turn dark gray after anodizing
6xxx base metal in sustained service above 65 °C	ER4043	5356 contains about 5% Mg and sensitizes in long-term elevated-temperature service
Fillet welds carrying shear load	ER5356	Markedly higher shear strength (1.5 times that of 4043 or more)
The part will be bent or formed after welding	ER5356	Better ductility
Crack-prone joints, casting repair, cosmetic welds	ER4043	Better fluidity, lower crack sensitivity, less smut

The difference: silicon vs magnesium

	ER4043	ER5356
Alloy family	Aluminum-silicon (Al-Si)	Aluminum-magnesium (Al-Mg)
Main alloying element (AWS A5.10)	Si 4.5–6.0%	Mg 4.5–5.5%
Key traits	Fluid puddle, crack resistance, smooth beads	Higher strength, better ductility, good color match after anodizing

In one line: silicon makes it easier to weld; magnesium makes it stronger. Every trade-off below follows from that.

The one hard rule: no ER4043 on high-magnesium 5xxx base metal

This is the only rule with no room for judgment. When the base metal contains more than about 2.8% magnesium, the silicon from 4043 combines with that magnesium to form magnesium silicide (Mg₂Si), a very brittle compound. In quantity it lowers ductility and raises crack sensitivity in the weld. So:

• 5083, 5086, 5456 and other high-Mg 5xxx alloys: use ER5356 — or, for 5083 marine plate, preferably ER5183 (see ER5183 vs ER5356).
• 5052 (Mg 2.2–2.8%): below the threshold, so either works; choose by performance requirements.

When 6061 can take either: six deciding factors

6xxx base metals contain about 1% Mg₂Si. Welded autogenously, without filler, they crack, so they must be diluted with enough filler metal. Both 4043 and 5356 do that job; what decides it is what the part needs most.

1. Will it be anodized? → If yes, ER5356

Silicon turns the weld dark gray after anodizing, leaving a visible line against the base metal. 5356 stays close to the base-metal color. For bicycle frames, furniture, architectural trim and anything else anodized after welding, choose 5356.

2. Sustained service above 65 °C? → ER4043

When a filler with more than 3% Mg spends long periods above 65 °C (150 °F), magnesium segregates to the grain boundaries and the weld sensitizes, raising the risk of stress-corrosion cracking. At about 5% Mg, 5356 is unsuitable. 4043 is an Al-Si alloy and does not have this problem. (For 5xxx base metal in hot service, see ER5554.)

3. Fillet welds carrying shear? → ER5356

Fillet weld shear strengths listed in AWS D1.2:

Filler	Longitudinal shear	Transverse shear
ER4043	11.5 ksi (about 79 MPa)	15 ksi (about 103 MPa)
ER5356	17 ksi (about 117 MPa)	26 ksi (about 179 MPa)

For the same load, a 5356 fillet can be smaller; with 4043, size the fillet up at the design stage.

4. Bending or forming after welding? → ER5356

4043 has lower ductility than 5356. If the weldment will be bent, straightened or otherwise cold-worked, 5356 is less likely to crack at the weld.

5. Crack resistance and appearance first? → ER4043

Silicon makes the puddle more fluid. 4043 has slightly lower crack sensitivity, gives smoother beads with less spatter, and leaves less black smut beside the weld — most welders find it easier to run. For highly restrained, crack-prone joints and casting repairs, 4043 is the usual pick.

6. MIG feeding? → ER5356 is less trouble

4043 wire is softer and more likely to buckle and bird-nest when pushed over a long distance. 5356 is stiffer and feeds more reliably. When running 4043 with MIG, use U-groove drive rolls and a low-friction liner, and switch to a spool gun or push-pull system if needed (see MIG vs TIG for aluminum).

Groove welds: the HAZ sets the strength, not the filler

Many people choose 5356 because it is “stronger,” but on 6061-T6 groove welds that argument carries little weight. 6061-T6 is about 45 ksi (310 MPa) in tensile strength before welding. Welding heat destroys the heat-treated structure in the heat-affected zone, and as-welded strength typically drops to around 27 ksi (186 MPa); AWS D1.2 requires a minimum of 24 ksi (165 MPa) as welded. In a groove weld the weak link is usually the heat-affected zone, so changing filler does little for groove-weld strength. The strength difference between the two fillers shows up mainly in fillet-weld shear, above. Actual values are governed by your procedure qualification record (PQR).

Need post-weld heat treatment to restore strength?

Ordinary fillers, 4043 and 5356 included, barely respond to post-weld heat treatment unless heavily diluted with the base metal. For this the industry has ER4643: 4043 with reduced silicon and 0.10–0.30% magnesium added, developed for 6xxx weldments that are heat-treated after welding. If you need it, contact us first to confirm availability.

The three most common mistakes

• Welding 5083 marine plate with 4043: Mg₂Si makes the weld brittle. Use 5183 or 5356.
• Using 4043 on parts to be anodized: the weld becomes a dark gray line, and nothing fixes it after anodizing.
• Using 5356 on tanks in sustained service above 65 °C: sensitization brings stress-corrosion risk. Switch to 4043 on 6xxx base metal, or to 5554 on 5454.

Frequently asked questions

Can I put a 4043 root pass under a 5356 cap in the same weld?

Not recommended. Once the two weld metals melt together, the mix is high in both silicon and magnesium, which recreates the hard rule above (brittle Mg₂Si). Use one filler throughout a given weld.

Which one for 6063 extrusions?

The same as 6061: either works, and you decide the same way — 5356 if it will be anodized, 4043 for crack resistance and ease of welding.

Which one for aluminum castings such as A356?

Al-Si castings are usually welded with 4043, which is close in chemistry and flows well.

Stock and samples

Mastership is the sole authorized distributor of Canadian Indalco aluminum welding consumables in Asia. ER4043 and ER5356 are in stock as MIG layer-wound spools and TIG rods, shipped with the original datasheet and mill test certificate (MTC). Tell us your base alloy, joint type, whether the part will be anodized and its service temperature — we will help you choose and send a sample spool for trial. For an overview of all five mainstream grades, see our aluminum welding wire selection guide.