Welding of aluminium
Aluminium welding is one of the most challenging processes in the welding world. This material, popular for its low weight, strength, and corrosion resistance, places high demands on technique, preparation, and material selection. In this blog, we will discuss the positive and negative properties of aluminum and what to pay attention to when welding aluminum.
Aluminum comes in many types for many different applications, each with its own properties. They are divided into several main groups:
1xxx Series, Pure aluminium:
✅Good weldability.
Low strength, high corrosion resistance
2xxx Series, Cupper Alloys:
❌Not weldable or barely weldable.
Highly susceptible to heat cracking
3xxx Series, Manganese Alloys:
✅Good weldability
Used in tanks and pipes
4xxx Series, Silicon Alloys:
✅Good weldability
Low melting point and low viscosity
5xxx Series, Magnesium Alloys:
✅Excelent weldability
Well suited for shipbuilding and transportation
6xxx Series, Silicon - Magnesium Alloys:
✅Good weldability
Suitable for extrusion, widely used in automotive industriy and for profiles
7xxxx Series, Zinc Alloys
⚠️Some grades are reasonable to highly weldable. Other grades are not - or less weldable at all
Thermal conductivity: a matter of balance
Aluminum is known for its high thermal conductivity. This is one of the reasons why it is widely used in heat sinks, pans, heat exchangers, and aerospace applications, for example. This high thermal conductivity also has its disadvantages, which make the material unique but also challenging to weld. When starting the weld, the heat quickly flows away from the weld area. The result: slow heating, a difficult weld pool, and sometimes an unstable arc.
But once the workpiece reaches the right temperature, everything changes. The heat stays concentrated around the weld pool better, the weld pool flows more easily, and suddenly things can even go too fast. This can result in a weld that is too wide, sagging, or even burn-through.
Creating a beautiful and high-quality aluminum weld therefore involves finding the right balance: sufficient energy to create a molten pool, but not too much once the material has reached the right temperature. Amperage and travel speed are therefore essential parameters when welding aluminum. Modern welding machines with advanced settings can help with this, as can a foot pedal for TIG welding. Those who have good control over heat management will produce a perfect weld.
Aluminum oxide
Aluminum is highly reactive with oxygen. As soon as it comes into contact with oxygen, which is almost immediately after the production process, it reacts instantly and forms an oxide layer. The aluminum oxide forms a hard, thin layer on the surface. This oxide layer also presents the biggest challenge when welding aluminum. It melts at ± 2050 °C, while aluminum itself melts at ± 660 °C. Because the melting points are so far apart, this can cause problems during welding. The oxide layer is still intact while the underlying aluminum has long since melted. If you do not remove or “break” it, you also have the risk of porosity or inclusions in the weld.
For this reason it's important to:
- Remove the oxide layer by stainless steel brush, scraper or small milling cutter just before welding
- Remove grease and dirt using acetone.
GMAW Welding
GMAW welding is the welding process for high productivity with aluminum. It's ideal for sheet thicknesses from 3 mm and is widely used in series production, shipbuilding, transport and heavier constructions.
- Current and polarity: Welding is performed using direct current (DC+)
- Shielding gas: Usually 100% argon, but for thicker material, an argon/helium mixture (up to 75% He) can be used to increase the penetration depth. Helium increases the arc temperature, improves heat transfer, and influences the arc characteristics.
- Wire feed: Due to the softness of aluminum wire, push-pull systems are essential to prevent wire feed problems.
- Technique: Using a pulse arc reduces spatter, increases control and is useful for thin workpieces.
- Welding positions: Often performed horizontally/underhand for optimal quality.
GTAW Welding
GTAW welding is the welding process for high precision and aesthetics in aluminum. It's ideal for thin parts, visible work, repairs and prototyping, where accuracy takes precedence over speed.
- Current and polarity: Almost always alternating current (AC). The positive sine wave (DC+) breaks through the oxide layer and cleans the molten pool, while the negative sine wave (DC-) provides penetration and heat.
- Shielding gas: Similar to GMAW welding, 100% argon or an argon/helium mixture is used.
- Technology: Modern GTAW equipment offers advanced settings such as frequency control and balance control to optimize the ratio between cleaning and penetration.
- Electrode: Tungsten electrodes (pure or alloyed with lanthanum/zirconium/yttrium) are used for stability during alternating current welding.
- Welding positions: GTAW welding offers excellent control in all welding positions.
The strength and reliability of a welded joint are determined by several factors. These factors affect both the weld itself and the heat-affected zone (HAZ) around the weld.
• The welder’s skill
• The welding process used
• The as-delivered condition of the base material
• The preparation of the weld joint
• The type of filler material
• The thickness of the workpiece material.
During welding, the material adjacent to the weld joint is heated for a short or extended period of time, with temperatures ranging from room temperature to the melting point.
These temperature variations cause:
• Structural changes in the base material
• A decrease in strength and toughness in the heat-affected zone
• Stresses and deformation
Aluminum has a number of unique properties that significantly affect the heat-affected zone (HAZ) and the weld:
Aluminum alloys (especially 6xxx and 7xxx series) derive their strength from strengthening particles (precipitates). During welding, these particles dissolve due to the high temperature, causing the heat-affected zone to lose strength without this being visible on the surface. This is the main reason why an aluminum weld joint is often weaker than the base material, even if the weld itself is perfect.
Unlike steel, aluminum does not form hard phases (such as martensite) during cooling. This means that rapid cooling does not cause brittleness, but it also does not result in any additional strength gain. In fact, the strength will decrease.
In many cases, the strength of the welded joint is therefore lower than that of the original base material. However, in practice, a strength comparable to that of the base material can often be achieved through the weld overlay and sufficient penetration. The overall strength of the joint can also be increased by carefully selecting a filler metal with higher strength.
For example: For 5xxx aluminum alloys, a filler material with a high Mg content may be more advantageous for strength.
Suitable material series
1xxx/3xxx/4xxx/6xxx cast and wrought alloys
Properties
Reduces hot cracking, good fluidity
Typical applications
Automotive, mechanical engineering, repairs
Suitable material series
High Silicon Casting alloys
Properties
Very high fluidity, ideal for casting
Typical applications
Engine blocks, transmission parts, high-pressure castings
Suitable material series
5xxx alloys
Properties
Good corrosion resistance in salt water
Typical applications
Tanks, chemical industry, transportation sector
Suitable material series
5xxx/7xxx alloys
Properties
High toughness and strength
Typical applications
Shipbuilding, Offshore
Suitable material series
5xxx alloys
Properties
Highest corrosion resistance, pressure vessels
Typical applications
Offshore, transportation, storage tanks
You can determine which welding wire is the best choice with DissiSelect, Simply select your base materials and you will immediately see the best CEWELD option. At a quick glance, you will gain insight into strength, weldability, corrosion resistance, and the possibility of anodizing. This tool also helps you choose the right welding wire for dissimilar aluminum joints.
In the production of aluminum welding wire, the surface condition of the wire is crucial for the final weld quality. This is because aluminum oxidizes rapidly as soon as it comes into contact with oxygen. This oxide layer (Al₂O₃) can lead to porosity, inclusions, and bad weld quality. That is why a high attention is paid to cleaning the welding filler material during the manufacturing process.
- Continuous casting of bars – aluminum is cast into bars measuring approximately 9.5 mm.
- Scraping (peeling) – immediately after casting, the outer layer of the rod is removed. This removes casting defects, oxides and inclusions.
- Cold drawing to final diameter – the wire is drawn to the desired diameter (e.g., 1.2 mm or 1.6 mm) in several steps.
- Surface treatment – after drawing, the wire is further treated to obtain a clean and homogeneous surface. This can be done by:
- Chemical cleaning – where acids or alkaline baths dissolve the oxide layer.
- Electrolytic cleaning – using electric current to remove oxides and contamination from the wire surface.
- Rinsing and conditioning – the yarn is cut to length or wound onto spools and packaged under controlled conditions.
By default, the wire is scraped once, immediately after continuous casting of the rod. This is the most important step in removing coarse oxides and casting defects. During drawing, a thin oxide skin forms again. This is much thinner and more homogeneous than the original layer and is usually not scraped again. Instead, chemical or electrolytic cleaning is used. Double scraping is rare because it is not economically attractive. CEWELD aluminum wires are scraped at least twice and undergo a special drawing process to ensure the cleanest possible weld metal.
The production method of the wire makes a big difference to the use of the welding filler material:
- Less porosity: a clean and oxide-free surface significantly reduces the risk of pores in the weld.
- Improved wire feed: a smooth surface reduces friction in the torch and cables, resulting in more stable wire feed.
- Less downtime: because fewer peelings or particles come loose, there is less chance of the wire jamming or the liner becoming blocked.
- Consistent weld quality: a wire that is pure and clean produces a more consistent arc and better weld quality.
| Fault | Cause | Prevention / Correction |
|---|---|---|
| Porosity | Contamination, moisture, insufficient gas protection | Deep cleaning, dry welding environment, proper gas flow |
| Hot cracks | Incorrect welding filler material, too much heat | Selecting the right filler material, controlled heat input |
| Oxide-inclusions | Insufficient AC balance or cleaning | Mechanical brushing, correct AC setting |
| Distortion | High thermal expansion, incorrect clamping | Tack welding, clamping, symmetrical welding |
| Lack of fusion | Insufficient current, excessive welding speed, oxide layer | Increased amperage, oxide layer removal |
| Spatters (GMAW) | Incorrect voltage/wire speed ratio | Optimize welding parameters |
- Work in a draft-free, dry environment.
- Use short hose packages with wide bends or push-pull systems.
- Keep filler materials dry and clean, and avoid contact with bare hands (grease).
- Check gas flow, gas cup for contaminants, and gas cup distance.
- Preheating thicker materials can reduce susceptibility to cracking, but avoid overheating.