Aluminum

ALUMINUM

Lightweight, Corrosion-Resistant & Highly Capable

Aluminum is one of the most useful materials in modern fabrication because it combines low density, corrosion resistance, good strength-to-weight ratio and excellent machinability. It is roughly one-third the density of steel, making it valuable where reducing weight matters without abandoning structural performance.

It is also one of the materials that most clearly separates an experienced fabricator from someone who simply knows how to run a welder.

Aluminum conducts heat rapidly, has a tenacious oxide layer, expands significantly when heated and can lose strength in the heat-affected zone depending on the alloy and temper. Those characteristics change how it must be prepared, welded, cleaned and finished.

THE FIRST THING TO UNDERSTAND: ALUMINUM IS NOT ONE MATERIAL

The alloy matters.

Common wrought alloys include:

• 5052 — strong corrosion resistance and excellent formability; common in sheet, tanks and fabricated components.

• 5083 / 5086 — higher-strength marine and structural alloys with excellent corrosion resistance.

• 6061 — extremely common in structural and general fabrication because of its availability and useful strength, but its welded heat-affected zone loses strength significantly from the original T6 temper.

• 6063 — common in extrusions and architectural applications where finish and formability are important.

• 2024 — high strength but poor corrosion resistance compared with common 5xxx and 6xxx alloys; generally not a general-purpose welding alloy.

• 7075 — very high strength, but generally considered difficult or unsuitable for conventional fusion welding because of cracking and loss of properties.

The important shortcut is simple:

Never specify “aluminum” when the alloy matters.

Two pieces of aluminum can look identical and behave very differently under welding, loading and corrosion.

WHY ALUMINUM WELDS DIFFERENTLY FROM STEEL

Aluminum's thermal and physical properties create several important differences.

It conducts heat rapidly, so heat spreads away from the weld much faster than it does in steel. At the same time, aluminum melts at a much lower temperature than steel.

The result is a material that can appear relatively cold and rigid one moment and suddenly collapse into a molten puddle once sufficient heat reaches it.

Aluminum also expands approximately twice as much as steel for the same temperature increase, increasing the potential for dimensional movement and distortion.

That combination makes heat management and fit-up particularly important.

THE OXIDE LAYER

Fresh aluminum is covered by a thin, extremely stable oxide layer.

The underlying aluminum melts at roughly 660°C, while aluminum oxide melts at well over 2,000°C.

That difference is fundamental to aluminum welding.

The oxide needs to be appropriately removed or disrupted so the welding arc can establish and maintain a clean connection with the base metal.

This is one reason aluminum preparation is so important.

The practical rule

Clean aluminum mechanically with dedicated stainless brushes and appropriate methods before welding.

Do not use the same abrasive tools that have previously been used on carbon steel unless contamination has been properly controlled.

Iron contamination can become embedded in the aluminum surface and create problems that have nothing to do with the underlying aluminum alloy itself.

TIG VS. MIG FOR ALUMINUM

Both GTAW/TIG and GMAW/MIG are widely used for aluminum, but they solve different problems.

TIG

TIG provides exceptional control of the weld pool and is particularly useful for thinner material, detailed fabrication, visible architectural work and applications where appearance and control matter.

For aluminum TIG, AC is normally used. The alternating current helps disrupt the oxide layer while maintaining an arc suitable for welding the base metal.

TIG is slower, but the operator has direct control over arc energy and filler addition.

MIG

MIG is generally much faster and better suited to productive fabrication, especially where larger amounts of aluminum need to be deposited.

Aluminum MIG commonly uses:

• Argon

• Larger-diameter wire where practical

• A spool gun or push-pull system to improve wire feeding

Because aluminum wire is soft, long conventional gun liners and feed paths can create friction and bird-nesting. Shorter, more controlled wire paths or specialized feeding systems can substantially improve reliability.

TIG prioritizes control. MIG prioritizes productivity.

SHIELDING GAS

For aluminum GMAW and GTAW, argon is the standard starting point for many applications because it provides the arc characteristics and shielding needed for the process.

Helium or argon-helium mixtures can be useful where greater heat input or penetration is desirable, particularly on thicker sections.

External shielding gas is essential for conventional TIG and MIG.

Aluminum also demonstrates an important distinction discussed elsewhere in the Hub:

Self-shielded FCAW is not a practical substitute for conventional aluminum MIG or TIG.

Aluminum welding requires a process and consumable system specifically suited to aluminum's chemistry and behaviour.

COMMON ALUMINUM FILLER WIRES

The filler alloy matters just as much as the base alloy.

Two of the most common aluminum filler alloys are:

ER4043

A silicon-containing filler that generally offers good fluidity and attractive weld appearance. It is widely used with compatible 6xxx-series aluminum and can reduce hot-cracking sensitivity in some applications.

ER5356

A magnesium-containing filler commonly used with 5xxx-series materials and many compatible 6xxx applications. It can provide higher weld strength than 4043 in certain combinations and is often selected where appearance, anodizing compatibility and mechanical requirements support it.

There are additional fillers for specific alloys and applications.

Important shortcut

Do not choose 4043 vs. 5356 based only on which spool is already in the shop.

The base alloy, service environment, required strength, finishing method and applicable specification should drive the selection.

6061 IS A GOOD EXAMPLE OF WHY ALLOY KNOWLEDGE MATTERS

6061-T6 is extremely common because it combines useful strength, machinability and availability.

But welding changes it.

The T6 temper comes from heat treatment. Welding introduces enough heat to alter the local microstructure and substantially reduce strength in the heat-affected zone.

So a 6061-T6 part is not simply:

“6061-T6 everywhere after welding.”

The welded region must be considered according to the actual design and service requirements.

That distinction matters when fabricating:

• Frames

• Brackets

• Structural components

• Equipment

• Trailer components

• Machinery parts

• Custom assemblies

The alloy designation and temper are not trivia—they are part of the engineering information.

5xxx VS. 6xxx ALUMINUM

A useful customer shortcut:

5xxx Series

Often selected for:

• Corrosion resistance

• Sheet fabrication

• Marine environments

• Tanks

• Formed components

• Welded structures

6xxx Series

Often selected for:

• Extrusions

• Frames

• Structural shapes

• General fabrication

• Machined components

• Architectural applications

There are exceptions, and the exact alloy still matters.

One especially important distinction is that some 5xxx alloys are much more naturally suited to welding than many precipitation-hardened high-strength alloys.

HEAT, DISTORTION & FIT-UP

Aluminum's high thermal conductivity and expansion rate make distortion control important.

Long welds, thin material and poorly planned sequences can move an assembly substantially.

Techniques may include:

• Accurate fit-up

• Appropriate tack sequence

• Controlled weld sequence

• Fixturing

• Alternating weld locations

• Minimizing unnecessary heat input

• Managing interpass conditions where applicable

The welder also has to account for a practical phenomenon known as heat sink.

A large aluminum casting or heavy section can pull heat away from the weld rapidly. A thin edge on the same assembly can become excessively hot far sooner.

That means a single machine setting may not behave identically across the entire part.

POROSITY & CONTAMINATION

Aluminum is particularly sensitive to hydrogen contamination, and moisture is one of the major sources of hydrogen.

Common contributors to porosity include:

• Dirty base material

• Oil or grease

• Moisture

• Contaminated filler wire

• Poor shielding

• Excessive torch distance

• Drafts

• Contaminated surfaces

This is why aluminum welding often looks deceptively simple from the outside.

The actual challenge is controlling the conditions surrounding the puddle.

A useful shortcut

Clean metal + clean filler + proper gas coverage = fewer problems.

If a supposedly sound aluminum weld contains significant porosity, increasing machine settings blindly is rarely the first answer.

WHY ALUMINUM CAN LOOK CLEAN BUT STILL BE WRONG

Aluminum can produce visually attractive welds that conceal underlying problems.

Possible defects include:

• Porosity

• Lack of fusion

• Cracking

• Inadequate penetration

• Oxide contamination

• Excessive heat-affected softening

• Distortion

A good-looking bead is evidence of appearance.

It is not automatically evidence of structural adequacy.

For critical work, the acceptance criteria come from the applicable drawing, specification, welding procedure and governing standard—not visual preference.

WELDING THIN ALUMINUM

Thin aluminum is particularly demanding because there is little margin for error.

Heat builds quickly in the local area while the material can suddenly reach the point where it collapses or burns through.

TIG is often favoured because it provides greater control, but properly configured MIG can be extremely productive on thin-to-moderate material.

The important variables are:

Heat input

Travel speed

Material thickness

Joint preparation

Fit-up

Filler selection

Torch movement

There is no universal thickness where one process automatically becomes “the right one.”

THICK ALUMINUM

Thicker aluminum creates a different challenge.

The large heat sink can prevent the joint from reaching the temperature necessary for proper fusion quickly enough.

Depending on alloy, thickness, joint design and procedure, this may require:

• Greater heat input

• Larger-capacity equipment

• Appropriate preheating or temperature control where permitted

• Multiple passes

• Better joint preparation

• Careful sequencing

The exact procedure has to follow the material and applicable specification.

ALUMINUM & CORROSION

Aluminum does not behave like carbon steel.

It naturally forms a protective oxide layer, which is why it generally performs well in many atmospheric environments.

But aluminum can still corrode.

Potential concerns include:

• Pitting

• Galvanic corrosion

• Chemical attack

• Salt exposure

• Crevice conditions

• Contamination

The most important issue in many fabricated assemblies is galvanic corrosion.

When aluminum is electrically connected to a dissimilar metal in the presence of an electrolyte such as water, a galvanic cell can form and accelerate corrosion of the less noble material.

That means an aluminum component attached directly to certain steel or stainless components may require appropriate isolation or design consideration.

Material compatibility matters at the connection, not just in the main component.

ALUMINUM AND FINISHING

Aluminum can be left with its natural finish or finished using processes such as:

• Anodizing

• Powder coating

• Painting

• Mechanical polishing

• Brushing

The intended finish should be considered before fabrication.

For example, anodizing can produce an attractive and durable finish, but welded areas may not match the original extruded appearance because welding changes the local material and surface.

Powder coating can provide a durable architectural finish, but contamination, preparation and masking still matter.

A polished aluminum component demands a completely different level of surface preparation from a heavy industrial component that will simply be coated.

REPAIRING ALUMINUM

Aluminum repair is often more difficult than steel repair because the welder has to manage:

• Unknown alloy

• Existing contamination

• Oxide layers

• Heat-affected properties

• Previous repairs

• Oil or moisture contamination

• Distortion

• Cracking

A damaged aluminum casting is a particularly good example.

A crack may be obvious, but the real question is whether the surrounding material can be reliably rewelded and whether the repaired component will retain the required mechanical performance.

Cleaning and identifying the material can be more important than immediately putting the torch on it.

A REAL-WORLD EXAMPLE

Fabricating an Aluminum Equipment Frame

Suppose a customer needs a lightweight frame to support equipment in an environment where corrosion is a concern.

Aluminum appears to be an obvious choice—but a responsible fabrication decision still asks:

Which alloy?

A 6061 extrusion might provide an excellent starting point, but the welded heat-affected zones need to be considered.

What wall thickness?

The frame must have enough section strength and stiffness without adding unnecessary weight.

Which welding process?

MIG may be preferable for production speed; TIG may make more sense for smaller, highly visible or intricate components.

Which filler?

4043 and 5356 can behave differently and produce different final properties.

How will it be finished?

An unfinished component, powder-coated frame and anodized architectural component each have different requirements.

How will dissimilar metals be connected?

If aluminum interfaces with steel, galvanic corrosion needs to be considered.

The final frame is therefore the result of several linked decisions—not simply “an aluminum weld.”

WHERE ALUMINUM MAKES SENSE

Aluminum is particularly useful when a project benefits from:

• Low weight

• Corrosion resistance

• Good strength-to-weight ratio

• Ease of machining

• Architectural appearance

• Reduced handling weight

Applications can include:

• Equipment frames

• Architectural metalwork

• Marine-related fabrication

• Transportation components

• Guards

• Platforms

• Enclosures

• Custom brackets

• Lightweight structures

• Specialized repairs

WHERE ALUMINUM MAY NOT BE THE BEST CHOICE

Aluminum is not automatically the superior material because it is lighter or more corrosion-resistant.

Carbon steel may provide better value where:

• Weight is not important

• High stiffness is required

• Cost is a dominant factor

• Heavy structural sections are needed

• The environment is already well controlled

Stainless may be more appropriate where a combination of corrosion resistance, appearance and specific mechanical requirements makes it the better fit.

The correct comparison is always based on the entire application, not one material characteristic.

ALUMINUM: THE IMPORTANT SHORTCUTS

Know the alloy.
“Aluminum” is not a complete material specification.

Clean it properly.
Oxide, oil, moisture and contamination can create problems before the weld begins.

Choose filler based on the base metal and application.
4043 and 5356 are not interchangeable in every situation.

Remember that welding changes heat-treated aluminum.
A 6061-T6 component does not retain the original T6 properties through the heat-affected zone.

Control heat and distortion.
Aluminum moves more readily than steel and can lose strength locally.

Protect the connection, not just the aluminum.
Dissimilar-metal connections can create galvanic corrosion.

A clean-looking aluminum weld is not automatically a sound weld.
Appearance, fusion, penetration, porosity and the applicable acceptance criteria are separate questions.