A Guide to Welding Aluminum

Welding aluminum makes countless products possible. Like auto pistons, aircraft, marine valves, bikes, structural aluminum beams, and many others. Whether welding aluminum plate, sheet, bar, or tube to make these and other products, knowing the aluminum grade is what’s most critical.

Welding Aluminum Alloys

Aluminum is alloyed with various elements which accentuate certain characteristics, like conductivity and strength. Variability in chemical and mechanical properties means that aluminum alloys are not all created equal. They are not interchangeable when it comes to weldability. You should never weld aluminum if you aren’t certain of its specific alloy composition.

For starters, aluminum alloys fall into two categories, heat treatable and non-heat treatable. Non-heat treatable alloys are hardened by cold working and must not be tempered prior to. Heat treatable aluminum alloys, on the other hand, have been heated to about 400°F.

When it comes to welding these aluminum alloys, the alloys that have been previously heated are more susceptible to weakening and cracking.

Non-Heat Treatable Alloys and Weldability

• 1xxx – Good weldability, however, significant elongation occurs during welding. Though this does not negatively impact resistance to crack growth, 1xxx series aluminum is particularly sensitive to filler type.
• 3xxx – This manganese alloy series is considered very weldable. It is good for press brakes and truck boxes.
• 4xxx – This aluminum alloy series includes both non-heat treatable alloys and heat treatable alloys. Non-heat treatable 4xxx series alloys are mostly seen in filler rods. The silicon in these alloys slows the cooling rate, solidification rate, and shrinkage stresses.
• 5xxx – Generally, the magnesium alloy series is considered very weldable. It is often seen in marine and anodized tubing.

Heat Treatable Alloys and Weldability

• 2xxx – Generally, the copper alloy series is not considered very weldable, 2017 and 2024 are even considered non-weldable. However, with specific welding procedure, 2219 shows excellent strength-to-weight ratio and large service temperature range. You see it mostly in the aerospace industry.
• 6xxx – While this series is super weldable and strong, the heat affected zone (HAZ) is degraded during welding. This reduces the mechanical properties by 30 – 50%.
• 7xxx – Similar to the 2xxx series alloys, not much of the 7xxx series is weldable, because it will crack when welded. However, 7005 for example is considered weldable, because it does not have copper in its specific composition.

How to Weld Aluminum

Welding is a value added service that demands special knowledge, skill, and certification. When compared to steel and other metals, aluminum alloys are susceptible to poor and ineffective welds. This is especially true when the proper factors are not thoughtfully considered.

First, Understanding the Challenges

The Universal Technical Institute outlines 4 challenges to welding all aluminum:

• Oxidation — Aluminum shows a strong affinity for oxygen. Aluminum oxide, also known as alumina, forms as a hard whitish layer on aluminum alloys. Alumina has a higher melting point than aluminum alloys and must be considered throughout the welding processes.
• Impurities — Impurities can cause poor welds. In a molten state, aluminum is highly susceptible to impurities.
• Porousness — Porosity is caused during welding when shielding gas is trapped in the weld. Impurities can lead to porous welds, as the melting point of impure aluminum will be inconsistent.
• Thickness — Burn-through is a common issue for aluminum. Thinner aluminum burns more quickly, and thick aluminum needs to be penetrated deeply enough to produce a firm weld. Welders must be experienced enough to discern how much heat to use in response to thickness.

There are two additional factors to consider when welding aluminum:

• Thermal Conductivity — Aluminum gets hot quickly and easily distributes this heat, which can lead to quick welds and craters. Heat sinks can be particularly helpful.
• Filler Metal — To avoid weld defect, matching the best base material alloys, tempers, and filler materials is critical. Approximately 85% of weldments can be achieved using filler alloys 4043, 4943, and 5356. These are known to be the most readily available and cost effective. However, filler alloy type paired with post-weld processes, like anodization, can have a significant influence on weld appearance, as color can change from bright and clear to dark and gray. Please consult a filler metal and base alloy chart prior to welding.

Cleaning Aluminum Prior to Weld

One of the most important steps in how to weld aluminum is cleaning the aluminum base metal and filler rod thoroughly before welding.

To clean the aluminum use acetone and a stainless steel brush. Do not use the brush for anything else but aluminum, and if you can afford to, use a new brush each time you work with new material grades. Thoroughly dry all aluminum and weld within 8 hours. Clean the aluminum if more than 8 hours passes between the initial cleaning and beginning the welding process.

Why Cleaning Aluminum Prior to Welding is So Important

Oxidation is one of the biggest challenges when it comes to welding aluminum. When the base metal and filler rod are not thoroughly cleaned prior to welding, three times the amount of heat will be required in order to break through the surface oxidation layer.

By the time the oxidation layer has been melted through, you will have a swimming pool of melted aluminum and clotty soot. In other words, the aluminum underneath the oxide layer had been melting the entire time. Cleaning the oxide layer equalizes the melting temperature leading to clean, strong welds.

Watch Out for These Signs

You will know if the base aluminum and filler rod weren’t cleaned properly if your arc wanders, the filler won’t blend, and an uncomfortable surface tension develops.

Welding Processes

There are many different kinds of welding processes. Some are outdated and have been replaced with new technology, and some are better for steel than aluminum.

• Laser Beam Welding
• Electron Beam Welding
• Resistance Welding
• Stick – Shielded Metal Arc Welding
• Flux-Cored Arc Welding
• Gas Metal Arc Welding
• Gas Tungsten Arc Welding

What is Arc Welding?

You’ll notice several of the welding processes listed above include “arc welding” in their name. So, what does it mean?

Arc welding is an extremely common welding process that requires electricity to heat the metal. As electricity flows from the filler metal—also known as the rod, or electrode—onto the base metal for joining you get an arc. The many different types of arc welding can vary in the type of current needed, or whether the welding is semi-automated, fully automated, or manual.

TIG Aluminum Welder

Probably the most popular technique to weld aluminum, is the Gas Tungsten Arc Welding, otherwise called “TIG” (tungsten inert gas) welding.

TIG welding is particularly great when working with lighter gauge aluminum. Plus, it is mechanically strong and visually appealing. For these reasons, it rose to a real golden era as the popularity of aluminum grew in the automotive industry. Most professional welders working in the automotive industry prefer the TIG welding process.

What to Expect During TIG Welding

TIG welding is a manual welding process and is sometimes criticized for being slow. However, when an operator is skilled, the difference in weld speed compared to other processes is negligible.

When you light up the arc, you’ll first notice a frosty area. This frosty area is also known as cleaning action. It’s the oxide layer melting.

Filler rod cannot be introduced until you see a shiny wet puddle, which indicates that the oxide layer has been melted through. Your travel speed should match the rate at which the aluminum is melting. As you wire feed manually, you dab along the line being welded. You’re looking for good taper and stable arc.

MIG Aluminum Welder

Sometimes viewed as the quicker way to weld aluminum is the Gas Metal Arc Welding (GMAW) or “MIG” welding process. MIG welding is semi-automated using a spool gun, and is considered better for welding thicker aluminum.

Argon is typically the gas used in MIG welding. You can also get an argon-helium blend. Helium increases the arc and is generally better for thicker metals.

Some Recommendations for MIG Welding

Spool guns can sometimes be less predictable than the manual dabbing process of TIG welding. Welding machines will note a wire speed recommendation, but you can scale that back, depending how well you can control the spool gun at that speed and temperature. Burning through contact tips during MIG welding is common, so plan to have more of them.

Another important consideration during MIG welding is the type of angle. Generally, push angles are preferred when using a spool gun for welding aluminum. The angle of the spool gun impacts the cleaning action and porosity of the weld.

Using a heat sink during MIG welding is also recommended. Heat sinks will make it possible to weld a little slower by distributing heat away from the spool gun. This is great when first learning how to MIG weld, and also helps to prevent against cracking.

What Equipment Do You Need?

• Acetone and stainless steel brush for cleaning the base aluminum and filler rod
• TIG/MIG welding machine with foot pedal
• An inverter — though not technically required, this makes welding machines much more efficient than normal transformers
• Shielding gas
• Weldable aluminum alloy sheet or plate
• Appropriate aluminum filler rod
• Aluminum fixture plate (jig) to prevent arc marks and secure working material throughout weld
• Heat sinks are recommended
• Personal protective equipment, including gloves, long sleeves, and welding helmet

Setting Up the Machine

Many welding machines include the option to TIG and MIG weld. Depending on your preferred welding processes, select TIG or MIG when it prompts you to select the tool.

Next, you’ll select the working metal. For example: 100% argon positive electrode, if MIG welding.

Then, you’ll type in the diameter, wire speed, and thickness. Note that the amps required depends directly on the thickness of the aluminum being welded.

Additional Considerations When Welding with Aluminum

Compared to welding steel, everything is more sensitive with aluminum welding. This makes aluminum welding a little more challenging.

Power Source

One main difference when welding aluminum is the required power source. Aluminum can only be welded with alternating current. AC breaks through the oxide film layer, which melts at a higher temperature than the aluminum.

DCEN welding current flows negative to positive. This is used when welding steel, nickel alloys, and titanium. But it will not break through the aluminum oxide. If you attempt to weld with a DCEN current you’ll get a sooty weld, which, after a little polishing, will reveal holes throughout the weld. It doesn’t work.

DCEP welding current flows with reverse-polarity. You get the desired cleaning action as the current flows from the workpiece to the electrode. However, the electrode will overheat, leading to an uneven arc. So, even though cleaning action is good, the electrode will get so hot that your filler rod will form a ball at the tip and become unable to join to the base metal.

With AC current you get a mix of EP and EN. Good heat input and good cleaning action make welding aluminum viable.

Thermal Conductivity

Aluminum also has a greater thermal conductivity than steel. The heat created when welding aluminum is dispersed more rapidly than when welding an iron-based alloy. This increases the potential for cold starts, where penetration of the weld is too shallow.

Potential for Craters

Increased thermal conductivity can also increase the potential for large craters, or concave dips in a weld. This happens because by the time you reach the end of a weld there is more heat than was present at the start, as the heat has been dispersed throughout the metal. Craters should be filled.

Other Factors That Will Influence Weld

• Arc length – Arc length influences weld penetration and weld surface shape. Much depends on the number of amps used
• Torch Angle
• Type of Gas Cup
• Shielding Gas — Both the flow and the type
• Electrode — Both the type and the diameter
• Thickness of Aluminum
• Who’s Welding? Welder’s comfort, knowledge and skill set should not be overlooked.

Common Weld Types

• Outside Corner Joint / Edge Joint — The outer edge of two plates butted up 90 degrees parallel to each other. It’s considered one of the easier joints. Recommended to hold a longer arc compared to other joints and to use a drip pan or tray while welding.
• Inside Corner Joint —The edges of two plates butt up to each other at a 90 degree angle. It usually provides a groove to fill providing good penetration. Recommended to keep a tight arc during weld.
• Butt Joint — Two pieces butted up against each other. Only the top and bottom surface can be welded. Without good penetration, this weld does not have the strength of a multi-pass fillet weld, or beveled joint.
• Lap Joint — Most often used to joint two pieces with differing thicknesses together. Pieces are placed over lapping one on top of the other. It is considered a fillet type, and the weld can be made on one or both sides.
• Tee-Joint —Tee welding joints are formed when two pieces intersect at a 90° angle. This results in the edges coming together in the center of a plate or component in a ‘T’. Tee joints are considered a type of fillet weld, and can also be formed when a tube or pipe is welded onto a base plate. Recommended to keep arc small and tight, minimum torch angle shape.
• Cast Aluminum Repair — Castings have more oxidation, so it is recommended to let the cleaning action run for longer than normal.

Signs of a Good Weld

• The weld is even and consistent. It looks like a stack of dimes.
• There is no porosity or cracking. You can check this by polishing the weld for better visibility.
• Appropriate depth of fusion to metal thickness. A cross-section of a weld can be cut to reveal whether the depth of fusion matches the metal thickness and will be durable.
• The weld doesn’t bend. This means that the appropriate heat was applied throughout the weld.
• The weld is in alignment with jig. This indicates that pieces are held tightly together without any gaps.

Common Beginner Mistakes

• Not cleaning the aluminum before welding. Even when it looks clean or has a protective sheen on it, you must clean it. Make sure to clean both the base metal plate/sheet as well as the filler rod.
• Too long an arc. A long arc length causes the rod to melt before it gets into the shiny aluminum puddle. If you can’t control the arc you risk porosity, not getting into the root of the join, and overall lack of fusion.
• Too much heat in a weld. This will soften up the weld joint, making it bendable.
• If you don’t know what grade it is, don’t use it. Again, not all aluminum is created equal when it comes to weldability. Scrap metal should not be used for anything critical.

We hope this helps you take into account the major steps and considerations when welding aluminum. The most important takeaway is that the composition of the alloy will dictate how you weld, followed by the particular welding process selected. Choose an alloy that is weldable, and that also fits the desired end product’s mechanical requirements. Select a welder who is familiar with aluminum alloys, and a welding process that is comfortable to them, and that is also suitable for the thickness of the aluminum being welded.