Aluminium Filler Alloy Selection Guide: 4043 vs 5356 vs 4047

Choosing the wrong filler alloy for an aluminium weld is one of the most common — and most avoidable — causes of weld failure, quality issues, and failed procedure qualifications. This guide covers the three most widely used aluminium filler alloys — 4043, 5356, and 4047 — what each one is, when to use each, when not to, and what happens when the wrong choice is made. It also explains why a common misconception about interchangeability can cause serious problems, using 5083 as a practical case study.

1. The Most Important Thing to Know First

There is a widespread misconception that 4043 and 5356 are interchangeable — that either can be used to weld any common aluminium base alloy. This is not the case. While many structural aluminium alloys can be welded with either filler, a number cannot, and using the wrong filler on these alloys can result in welds with low ductility, poor mechanical properties, or susceptibility to cracking.

Always start with a filler alloy selection chart. The guidance in this article helps you make the best choice once the chart confirms that multiple options are acceptable — and helps you understand the consequences when only one option is.

2. What Are 4043 and 5356?

4043 is an aluminium-silicon filler alloy containing 4.5–6.0% silicon. The silicon addition lowers the melting point and improves fluidity, making the weld pool easier to control and producing smooth, clean welds with less spatter and smut.

5356 is an aluminium-magnesium filler alloy containing approximately 5% magnesium. The magnesium addition increases strength, making 5356 the higher-strength option of the two and the preferred choice when mechanical performance is the primary consideration.

3. 4043 vs 5356: How to Choose

When the filler alloy selection chart permits the use of either 4043 or 5356 for a given base alloy, the following factors should guide your decision:

Colour match after anodising

If the welded component will be anodised after welding, do not use 4043. Because of its silicon content, 4043 typically turns dark grey after anodising and will not match the base material. 5356 provides a much closer colour match and is the correct choice for anodised finishes.

Elevated temperature service

4043 is suitable for service at elevated temperatures. 5356 is not recommended above approximately 65°C (150°F) due to its magnesium content, which can cause sensitisation and susceptibility to stress corrosion in sustained high-temperature environments. For heat exchangers, engine components, or other elevated temperature applications, 4043 is the safer choice.

Strength and shear strength

5356 has significantly higher shear strength than 4043. This is an important consideration when sizing fillet welds for structural applications. The table below shows typical shear strength values for common aluminium filler alloys:

Table 1. Typical filler alloy shear strengths

Ductility

4043 has lower ductility than 5356. If the welded assembly will be formed or bent after welding, 5356 is the better choice.

Weldability and cosmetic appearance

4043 typically offers higher weldability ratings and slightly lower crack sensitivity than 5356. It produces smoother weld surfaces with less spatter and smut, which can make it more appealing from a cosmetic standpoint and easier to work with in production environments.

Feedability in MIG/GMAW

4043 is a softer wire than 5356 in spooled form. In MIG welding (GMAW), this means feedability can require more attention with 4043, as the softer wire is more susceptible to birdnesting or buckling in the liner. 5356, being a more rigid wire, generally feeds more consistently. See our dedicated article on feedability and wire delivery in aluminium MIG welding for detailed guidance.

Quick reference summary

4. What Is 4047, and When Should You Use It?

4047 is a higher-silicon aluminium filler alloy containing 11.0–13.0% silicon — roughly double the silicon content of 4043 (4.5–6.0%). It was originally developed as a brazing alloy (BAlSi-4), taking advantage of its low melting point and very narrow freezing range of approximately 577–582°C (1070–1080°F).

In welding applications, 4047 is best understood as an enhanced version of 4043 for specific use cases, rather than a general-purpose replacement.

Improved fluidity

The higher silicon content gives 4047 exceptional fluidity (wetting action) during welding. This makes it particularly well suited to welding thin materials where leak-tight joints are required, such as heat exchangers, manifolds, and automotive cooling components. The improved wetting action reduces the risk of incomplete fusion on thin sections.

Reduced solidification cracking

4047 offers lower solidification cracking tendency than 4043, which can be an advantage on alloys where crack sensitivity is a concern within the range where 4xxx series fillers are appropriate.

Improved cosmetic appearance

The additional silicon in 4047 produces exceptionally smooth, cosmetically clean welds — an improvement over 4043 even on thin material applications.

Higher productivity in leak-critical applications

In the heat exchanger fabrication industry, switching from 4043 to 4047 has been shown to reduce weld leakage rates and improve production throughput, as fewer repairs are needed on leak-tested components.

AWS code acceptance

Under the AWS D1.2 Structural Welding Code for Aluminium, 4047 is acceptable as a direct replacement for 4043. Both alloys share the same F number (F23), meaning a procedure qualified with 4043 is also qualified for 4047.

Important limitations of 4047

4047 shares the same key limitation as 4043 when it comes to post-weld anodising: due to its silicon content, welds made with 4047 will also turn dark grey after anodising. Neither 4043 nor 4047 should be used where colour-matching after anodising is required.

5. When 4xxx Series Fillers Must Not Be Used: The 5083 Example

One of the most important rules in aluminium filler alloy selection is this: do not use a 4xxx series filler alloy on 5xxx series base materials with more than 2.5% magnesium content.

The reason is metallurgical. When a silicon-based filler (4xxx) is used to weld a high-magnesium base alloy (5xxx with >2.5% Mg), the resulting weld metal chemistry can produce a brittle microstructure with low ductility. The weld will be weak and prone to cracking under stress.

Alloy 5083 — one of the most widely used structural aluminium alloys in shipbuilding, cryogenic tanks, military vehicles, and pressure vessels — is a clear example. 5083 contains approximately 4.5% magnesium and must not be welded with 4043 or 4047.

Correct filler alloys for 5083

5083 should be welded with one of the following: 5356, 5183, or 5556. The choice between them depends on the application and strength requirements:

The same rule applies across all high-magnesium 5xxx alloys, not just 5083. Any 5xxx series base material with more than 2.5% magnesium — including 5086, 5456, and others — should not be welded with 4xxx series filler.

6. Why Wrong Filler Choice Causes Guided Bend Test Failures

The guided bend test is a standard quality control procedure used to evaluate the ductility of a weld joint. A test specimen is bent through 180° — face bend tests place the weld face in tension; root bend tests place the weld root in tension. Failures indicate either weld discontinuities or insufficient ductility in the weld metal.

When a 4xxx series filler is incorrectly used on 5083 or similar high-magnesium base materials, the resulting weld metal has low ductility and will typically fail the guided bend test even when the weld appears visually sound. This is a filler selection problem, not a welding technique problem — no amount of improved technique will fix a metallurgically incompatible filler choice.

However, filler incompatibility is not the only cause of guided bend test failures. If the correct filler alloy is confirmed but failures are still occurring, the following should be investigated:

• Weld discontinuities — the most common cause is lack of fusion, which creates a stress concentration point that fractures during bending. Inspect the failed sample carefully for signs of incomplete fusion at the weld root or sidewalls
• Incorrect sample preparation — samples that have not been prepared in accordance with the relevant specification (incorrect thickness, surface condition, or geometry) can fail even on sound welds
• Incorrect bend radius — bending over a mandrel of the wrong diameter for the base material and thickness will produce incorrect results. Verify the bend radius against the applicable code requirements
• Test procedure errors — the type of bend test used (face, root, or side) must match the specification. Review the full test procedure against the qualifying standard

7. ESAB Aluminium Filler Alloys

ESAB's aluminium filler alloy range is produced under the OK Autrod (MIG/GMAW) and OK Tigrod (TIG/GTAW) product families, part of the world's best-selling aluminium wire range. All alloys are available in multiple diameters and are manufactured with ESAB's proprietary surface treatment technology for consistent, tangle-free feeding and optimised weld appearance.

4043 — General purpose, lower crack sensitivity

OK Autrod 4043 and OK Tigrod 4043 are among the most widely used aluminium welding and brazing alloys. The silicon additions improve fluidity and wetting action, producing bright, near smut-free welds with lower crack sensitivity than 5xxx series fillers. Suitable for elevated temperature service. Not recommended for components that will be anodised.

• OK Autrod 4043 — MIG/GMAW wire
• OK Tigrod 4043 — TIG/GTAW rod

5356 — Higher strength, anodising-compatible

OK Autrod 5356 is the most widely used aluminium welding alloy overall, chosen primarily for its higher shear strength and its ability to colour-match after anodising. The preferred choice for structural applications, marine fabrication, and any work requiring a close colour match after surface treatment. Not suitable for sustained service above 65°C.

• OK Autrod 5356 — MIG/GMAW wire

4047 — Enhanced fluidity for thin section and leak-critical work

OK Autrod 4047 and OK Tigrod 4047 offer higher silicon content (11–13%) than 4043, delivering exceptional fluidity, reduced solidification cracking, and exceptionally smooth weld surfaces. The first choice for heat exchangers, manifolds, and thin-section work where leak-tight joints are required. Shares the same AWS F number (F23) as 4043, so procedures qualified with 4043 cover 4047. Not recommended for anodised finishes.

• OK Autrod 4047 — MIG/GMAW wire
• OK Tigrod 4047 — TIG/GTAW rod

5183 — High strength for 5083 and high-magnesium alloys

OK Autrod 5183 was developed specifically to meet the as-welded tensile requirements of alloy 5083 and other high-magnesium alloys — requirements that 5356 will typically fail to meet consistently. The standard choice for shipbuilding, cryogenic tanks, military vehicles, and structural fabrications where full groove weld procedure qualification is required. Not recommended for elevated temperature applications.

• OK Autrod 5183 — MIG/GMAW wire

8. Key Takeaways

• Always start with a filler alloy selection chart — not all aluminium base alloys can be welded with 4043 or 5356
• Use 5356 when post-weld anodising is required; 4043 turns dark grey and will not colour-match
• Use 4043 (not 5356) for elevated temperature service above approximately 65°C
• 5356 has significantly higher shear strength than 4043 — factor this into fillet weld sizing calculations
• 4047 is an enhanced alternative to 4043 for thin-section, leak-critical work; it has the same F number (F23) under AWS D1.2 and shares 4043's limitation regarding post-weld anodising
• Never use 4043 or 4047 on 5xxx series base alloys with more than 2.5% magnesium — including 5083, 5086, and 5456
• For 5083, use 5183 or 5556 when groove weld procedure qualification is required; 5356 may not consistently meet the 40 ksi minimum tensile strength requirement
• Guided bend test failures on 5083 are often caused by using the wrong filler alloy — check filler selection before investigating technique