ER4043 and ER5356 cover about 95% of aluminum MIG welding. Pick the wrong one and you’ll get cracking, poor color match after anodizing, or welds that fail under load. The short answer: ER4043 is the general-purpose wire for casting alloys and non-structural work; ER5356 is the go-to for structural applications and parts that get anodized.
The Core Difference
ER4043 is a silicon-alloyed wire (4.5-6.0% Si). The silicon lowers the melting point, improves fluidity, and reduces hot-crack sensitivity in certain alloy combinations. It flows like butter and produces a shiny, clean bead.
ER5356 is a magnesium-alloyed wire (4.5-5.5% Mg). Magnesium adds strength, hardness, and corrosion resistance. It produces a stiffer puddle with higher as-welded tensile strength. The bead is duller in appearance but mechanically superior.
Chemical Composition
| Element | ER4043 | ER5356 |
|---|---|---|
| Silicon (Si) | 4.5 - 6.0% | 0.25% max |
| Magnesium (Mg) | 0.05% max | 4.5 - 5.5% |
| Manganese (Mn) | 0.05% max | 0.05 - 0.20% |
| Chromium (Cr) | -- | 0.05 - 0.20% |
| Titanium (Ti) | 0.20% max | 0.06 - 0.20% |
| Iron (Fe) | 0.8% max | 0.40% max |
| Copper (Cu) | 0.30% max | 0.10% max |
These two wires solve different metallurgical problems. Silicon in ER4043 acts as a flux, improving flow and reducing the solidification temperature range. Magnesium in ER5356 forms Mg2Al3 precipitates that strengthen the weld. You can’t substitute one for the other on every alloy without consequences.
Mechanical Properties
| Property | ER4043 | ER5356 |
|---|---|---|
| Tensile Strength | 24,000 - 28,000 PSI | 38,000 - 42,000 PSI |
| Yield Strength | 10,000 - 14,000 PSI | 17,000 - 24,000 PSI |
| Elongation | 12 - 17% | 17 - 25% |
| Shear Strength | 15,000 - 18,000 PSI | 23,000 - 26,000 PSI |
ER5356 wins on strength across the board. Its tensile strength is roughly 50% higher than ER4043. For structural joints, load-bearing brackets, trailer frames, and any application where the weld carries stress, ER5356 is the correct choice.
ER4043 has lower strength but better crack resistance in certain alloy combinations. On casting alloys with high silicon content (356, 319, A356), ER4043 provides a compatible chemistry that solidifies without cracking. ER5356 on these same castings can produce hot cracks.
Alloy Compatibility Guide
This is the most critical table in this article. Using the wrong filler on the wrong base alloy causes cracking, corrosion, or both.
| Base Alloy | ER4043 | ER5356 | Preferred |
|---|---|---|---|
| 1100 to 1100 | Yes | Yes | Either |
| 3003 to 3003 | Yes | Yes | ER4043 |
| 5052 to 5052 | No | Yes | ER5356 |
| 5083 to 5083 | No | Yes | ER5356 |
| 6061 to 6061 | Yes | Yes | ER5356 (structural), ER4043 (cosmetic) |
| 6063 to 6063 | Yes | Yes | ER4043 |
| 356 casting | Yes | No | ER4043 |
| A356 casting | Yes | No | ER4043 |
| 5052 to 6061 | No | Yes | ER5356 |
| 3003 to 6061 | Yes | Yes | Either |
Key rule: Never use ER4043 on 5xxx-series alloys with more than 2.5% magnesium (5052, 5083, 5086, 5454). The silicon in 4043 combines with the magnesium in the base metal to form magnesium silicide (Mg2Si) at the grain boundaries. This creates brittle, crack-sensitive joints that fail under stress or thermal cycling.
For 6061: Both wires work, but with trade-offs. ER4043 on 6061 gives better flow, less crack sensitivity, and a cleaner bead. ER5356 on 6061 gives higher strength and a consistent anodized color. Pick based on whether the joint is structural (ER5356) or cosmetic (ER4043).
Anodizing Response
This is a deal-breaker for any parts that get anodized after welding.
ER4043 turns dark gray or black after anodizing. The silicon in the weld deposit reacts differently to the anodic oxidation process than the surrounding base metal. The result is a visible dark line along every weld. If appearance matters, this is unacceptable.
ER5356 anodizes to a color that closely matches the surrounding aluminum base metal. The weld line is still slightly visible but blends in well. For architectural, marine, and decorative applications where anodizing is part of the spec, ER5356 is mandatory.
Hot Cracking Behavior
Hot cracking (solidification cracking) happens when the weld metal contracts during solidification and tears at weak grain boundary films. Different alloy-filler combinations have different susceptibilities.
ER4043 reduces hot cracking in most situations because silicon widens the solidification range and promotes a more eutectic (fluid) solidification pattern. The liquid film between solidifying grains stays mobile longer, allowing it to heal small tears.
ER5356 is more crack-sensitive on certain base alloys, particularly casting alloys with high silicon content. Mixing high-magnesium filler with high-silicon base metal creates a crack-prone composition.
The most dangerous zone for cracking is a diluted weld on 6061 where the combined magnesium and silicon land in the crack-sensitive range (roughly 0.5-2.0% Mg2Si). ER4043 pushes the composition out of this range by adding excess silicon. ER5356 pushes it out by adding excess magnesium. Both strategies work for the right reason.
Feeding Aluminum MIG Wire
Aluminum wire is soft. Standard MIG gun setups designed for steel won’t feed it reliably. You need specific hardware changes.
Drive rolls: Switch to U-groove (concave) rolls. V-groove rolls dig into soft aluminum wire and shave off material that clogs the liner. Set tension as light as possible while maintaining consistent feed.
Liner: Replace the steel liner with a Teflon (PTFE) or nylon liner. Steel liners create too much friction and cause bird-nesting. Cut the liner slightly long per the gun manufacturer’s specs so it meets the contact tip without a gap.
Contact tips: Use tips one size larger than the wire diameter. Aluminum expands when heated by current flowing through the contact tip. A 0.030" wire needs a 0.035" tip. A 0.035" wire needs a 0.040" or even 0.045" tip.
Spool guns: For wire runs longer than 10-12 feet, a spool gun is the most reliable feeding solution. The 1 lb spool mounts right at the gun, eliminating the long push through a conduit. This is the standard approach for most small-shop aluminum MIG work.
Push-pull guns: Production aluminum shops use push-pull systems with a motor at both the wire feeder and the gun. This handles long conduit runs without the limitations of a spool gun’s small spool size.
Shielding Gas
Both ER4043 and ER5356 use the same shielding gas: 100% argon. Pure argon provides a stable, clean arc on aluminum without reacting with the base metal or filler.
For thicker aluminum (over 1/4"), adding helium to the argon improves penetration. Common blends include 75% argon / 25% helium for moderate improvement and 50/50 Ar/He for heavy plate. Helium increases arc voltage, heat input, and travel speed. It also costs significantly more than straight argon.
Flow rate for aluminum MIG: 25-35 CFH. Aluminum’s high thermal conductivity means the puddle solidifies quickly, and adequate gas coverage is essential during that brief solidification window.
Never use CO2 or CO2-containing mixes on aluminum. CO2 reacts with aluminum at welding temperatures, producing aluminum oxide contamination and porosity.
Welding Parameters
| Wire Size | Material Thickness | Voltage | Wire Feed (IPM) | Transfer Mode |
|---|---|---|---|---|
| 0.030" | 1/16" - 1/8" | 17-20 | 350-450 | Short circuit / pulsed |
| 0.035" | 3/32" - 3/16" | 19-23 | 350-500 | Spray |
| 3/64" | 1/8" - 1/4" | 22-27 | 300-450 | Spray |
Aluminum MIG runs at higher wire feed speeds than steel. The lower density and melting point of aluminum require faster wire delivery to maintain a stable arc. Spray transfer is the preferred mode for aluminum MIG because short-circuit transfer often produces cold lap defects due to aluminum’s rapid solidification.
Common Mistakes
Using steel drive rolls and liner for aluminum. This causes immediate feeding problems. Dedicate a set of U-groove rolls and a Teflon liner for aluminum, or use a spool gun.
Wrong gas. CO2-containing blends destroy aluminum welds. Pure argon only.
Insufficient preheat on thick sections. Aluminum above 1/4" thick benefits from preheating to 200-300F (93-150C) to overcome the heat sink effect. Don’t exceed 300F on heat-treatable alloys (6061, 2024) or you’ll damage the temper.
Storing wire improperly. Aluminum wire oxidizes on the surface, increasing feeding friction and degrading arc quality. Keep spools sealed. In humid climates, store in a climate-controlled area.
Skipping post-weld cleaning. The black soot (aluminum oxide and magnesium oxide smoke) that forms around aluminum MIG welds traps moisture and promotes corrosion. Wire brush with a stainless brush after each pass.