Aluminum is a popular material in manufacturing because it’s lightweight, strong, and corrosion resistant. However, welding aluminum is different and often more difficult than welding steel. This article will explain these characteristics and methods used to join aluminum, ensuring the parts are built to last.
Properties of Aluminum in Welding
Aluminum and its alloys have several characteristics when welding:
Refractory Oxide Film
Aluminum has a strong affinity for oxygen and readily combines with ambient oxygen to form a dense, refractory Al2O3 oxide film. This oxide film hinders weld fusion and formation and easily leads to defects such as slag inclusions and porosity in the deposited metal. Therefore, before welding, the oxide film on the surface of aluminum and aluminum alloy parts must be cleaned, and an inert gas shield must be used during the welding process to prevent oxidation of the weld pool.
High Thermal Conductivity
Aluminum’s high thermal conductivity means that during welding, a significant amount of heat is rapidly drawn into the base metal. Coupled with a large specific heat capacity, welding aluminum alloys of the same thickness as steel requires a considerably higher heat input. Consequently, welding must employ highly concentrated, high-power heat sources, sometimes supplemented by process measures like preheating.
Hot Cracking
Aluminum has a relatively large coefficient of linear expansion and a solidification volume shrinkage of about 6.6%, which leads to high internal stresses, distortion, and a tendency for cracking within the weld joint. Hot cracking is often mitigated by adjusting the filler wire composition and controlling the liquid metal fluidity.
Weld Porosity
Moisture is commonly adsorbed on the surface of aluminum alloys and filler wires. This water decomposes in the high-temperature arc, and the resulting hydrogen dissolves into the liquid metal. During the rapid cooling and solidification of the weld pool, hydrogen does not have sufficient time to escape and becomes trapped, forming porosity in the weld.
Low Strength and Ductility at High Temperature
Aluminum has very low strength and ductility at high temperatures, which can prevent it from adequately supporting the liquid weld pool. This can result in poor weld bead formation or even defects like collapse and burn-through. For this reason, jigs and backing plates are typically required.
Loss of Alloying Elements
Aluminum alloys containing low-boiling-point elements such as magnesium and zinc are prone to significant element vaporization and burn-off during the welding process. This changes the chemical composition of the weld metal, degrading the performance of the welded joint.
Difficulty in Temperature Assessment
Aluminum has high reflectivity to radiant energy and has no distinct color change from low to high temperatures, or during the transition from solid to liquid. This makes it difficult to judge the heating status of the weld based on visual color change, complicating the welding operation.
Types of Aluminum Welding Methods
There are numerous methods for welding aluminum and its alloys, each with specific applications. The selection of a welding method must be based on factors such as the alloy composition, part thickness, joint configuration, performance requirements, and economic viability.
With ongoing research, aluminum alloy welding technology has advanced significantly. Major welding methods include gas welding, TIG, MIG, resistance welding, laser beam welding, and friction stir welding.
Oxy Gas Welding
Oxy gas welding uses a low thermal power flame, which results in dispersed heat. This method leads to large workpiece distortion and low productivity. Preheating is required for welding thicker aluminum parts. Post-weld, the weld metal often has a coarse grain structure, is porous, and is prone to defects like aluminum oxide inclusions, porosity, and cracking. This method is generally limited to non-critical aluminum structures and casting repairs in the thickness range of 0.5 to 10 mm.
Gas welding has low thermal efficiency and non-concentrated heat input. It requires the use of a flux when welding aluminum and aluminum alloys, and the residue must be cleaned afterward. The quality and properties of the joint are relatively low. However, due to the simple equipment, lack of need for a power source, and flexible operation, it is often used for aluminum alloy components where quality requirements are low, such as thin sheets and small parts, as well as for repairing aluminum alloy components and castings.
Tungsten Inert Gas Welding
Tungsten Inert Gas (TIG) welding is a classic inert gas shielded process and the most used method. It uses a non-consumable tungsten electrode, with helium or argon supplied as a shielding gas to protect the arc. High voltage discharge instantaneously melts the filler wire and the base metal to form the weld. TIG welding is used for the fabrication of aluminum alloy components and the repair of casting defects.
TIG welding is best suited for welding thin sheets less than 3 mm thick. Workpiece deformation is noticeably less than with gas welding and Shielded Metal Arc Welding (SMAW). The alternating current in TIG provides a “cathodic cleaning” action that helps remove the oxide film, eliminating the need for flux and thus avoiding post-weld corrosion caused by residual flux/slag. Joint configurations are not restricted, and the weld bead formation is good with a bright surface finish.
Key Features:
- Easy to operate, flexible, and controllable, adapting to various working environments, and relatively low cost.
- Narrow Heat Affected Zone with adequate filler metal feed, weld joint deformation is minimal, leading to higher overall joint performance.
- Good and stable welding process performance, welds are dense and aesthetically pleasing.
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Metal Inert Gas Welding
Metal Inert Gas (MIG) welding is also an inert gas shielded process. The key difference from TIG is that MIG welding uses the consumable filler wire itself as the electrode, whereas TIG uses a fixed tungsten electrode.
In aluminum alloy MIG welding, voltage and current are applied to the tip of the filler wire electrode, creating an instantaneous high-voltage arc with the base metal. This melts the base metal and joint groove. Molten droplets detach from the wire tip and transfer vertically to the base metal weld pool, forming the weld zone.
The application of aluminum alloy MIG welding is somewhat restricted because the softness of the aluminum wire can lead to poor wire feeding stability, and molten aluminum is prone to spatter during welding.
MIG welding offers higher welding speeds than TIG, requires less movement for large workpieces, and can achieve welding efficiencies of several meters per minute by adjusting the wire feed speed.
Laser Beam Welding
Laser Beam Welding (LBW) utilizes high-energy laser pulses to locally heat a small area of the material. The laser energy spreads inward through thermal conduction, melting the material to form a specific weld pool. Upon solidification, the materials are joined together.
LBW offers the advantages of a small welding spot, a highly concentrated high-power heat source capable of welding thick plates, a narrow HAZ, and minimal welding distortion. However, LBW requires high precision in weld positioning, the equipment is expensive, and overall welding costs are high. Furthermore, metals like aluminum and magnesium have high laser reflectivity, making direct welding challenging.
When the power density on the workpiece reaches 107 W/cm2 or higher, the metal in the heated zone rapidly vaporizes. The resulting gas congregates in the melt pool, forming a cavity known as a “keyhole.” Heat transfer centers around this keyhole, forming the surrounding weld pool. This is the Keyhole Effect of deep-penetration laser welding. To prevent issues of weld pool non-uniformity caused by this phenomenon, methods include reducing laser energy, increasing welding speed, or controlling the remelting of the fusion zone to eliminate bubbles and minimize porosity.
Resistance Welding
This method is suitable for welding thin aluminum alloy sheets with a thickness of up to 4 mm. For products requiring higher quality, direct current impulse spot welding or seam welding machines may be used. This process requires complex equipment, but involves high welding currents and offers high productivity, making it particularly suited for high-volume production of parts.
Friction Stir Welding
Friction Stir Welding (FSW) is a novel solid-state joining technique derived from traditional friction welding. The principle involves a non-consumable, specially shaped tool (composed of a rotating probe and shoulder) plunging into the interface to be welded. As the tool traverses the joint line, the welding material heats up. The plasticized metal undergoes intense plastic deformation under the combined effects of mechanical stirring and forging pressure. After diffusion and recrystallization, a dense, solid-state bond is formed.
Compared to traditional fusion welding methods, FSW offers the following advantages:
- Low welding temperature, resulting in minimal welding distortion.
- Superior mechanical properties in the weld joint.
- Simple, economical, and environmentally friendly welding process.
The following table summarizes the suitable applications of each welding process：
| Methods | Applications |
| Oxy Gas Welding | Non-critical aluminum structural components and castings with thicknesses ranging from 0.5 to 10 mm |
| Tungsten Inert Gas Welding | Aluminum and aluminum alloys with a thickness of 1 to 20 mm |
| Metal Inert Gas Welding | Pure aluminum and aluminum alloy plates with a thickness of 50 mm or less |
| Laser Beam Welding | Aluminum alloy sheets with a thickness of 3 to 75 mm |
| Resistance Welding | Aluminum alloy sheet with a thickness of 4 mm or less |
| Friction Stir Welding | Aluminum workpiece with a circular cross-section and a maximum diameter of 100 mm |
Practices for Aluminum Welding
Cleaning. Before welding, the aluminum alloy surface must be meticulously cleaned to remove oil, dirt, and contaminants. The weld area can be cleaned with acetone. For thicker aluminum plates, initial cleaning should involve a stainless steel wire brush, followed by a final cleaning with acetone.
Preheating. If the plate material is relatively thick, preheating may be necessary. This prevents a lack of fusion/penetration due to insufficient heat.
Crater filling. When terminating the weld, use a small current for crater filling to prevent the formation of end-of-weld craters and associated cracking.
Welding gun cable length. The welding gun cable should not be excessively long, as this can negatively affect the stability of the wire feeding system.
