What Makes Aluminum Alloys Difficult to Weld?

Weldability is the combined measure of a material’s resistance to cracking, the safety of the welded structure, and the economy of the product’s intended applications. A material is considered weldable when its chemical, metallurgical, and physical properties suit the welding process and the joint meets the required strength parameters.

Because aluminum and its alloys have distinctive physicochemical properties, welding them involves the following difficulties.

1. Rapid Oxidation

Aluminum oxidizes readily in air and during welding. The resulting aluminum oxide (Al₂O₃) is stable and difficult to remove. Its melting point is about 2,500°C, far above the approximately 660°C melting point of aluminum alloys. During welding, the oxide film obstructs melting and fusion of the base metal. Because its density is about 1.4 times that of aluminum, it does not readily float out of the weld pool and can cause incomplete fusion and slag inclusions. It also absorbs moisture, which can produce weld porosity.

Before welding, the oxide film must therefore be removed by strict chemical or mechanical surface cleaning, and renewed oxidation must be prevented during welding.

2. Susceptibility to Porosity

Potential pore-forming gases include hydrogen, carbon monoxide, and nitrogen. Nitrogen is insoluble in liquid aluminum, while aluminum alloys contain no carbon that would generate carbon-monoxide pores. Hydrogen is therefore the pore-forming gas of concern. Its solubility is 7 mL/kg in liquid aluminum but only 0.4 mL/kg at the 660°C solidification temperature. Much of the hydrogen originally dissolved in the liquid metal precipitates and forms bubbles as the weld solidifies. Pure aluminum and corrosion-resistant aluminum alloys are especially susceptible.

Hydrogen entry into the base and filler metals must be limited, and high-purity shielding gas should be used. Oxide, moisture, and oil must be thoroughly removed from workpieces, wire, and electrodes before welding. Interruptions should be minimized. Strong welding parameters are recommended: TIG welding uses a high current with a relatively high travel speed, while MIG welding uses a high current with a lower travel speed to keep the pool liquid longer and allow hydrogen to escape from the supersaturated solid solution.

3. Burn-Through Risk

Aluminum shows no conspicuous color change when it passes from solid to liquid, making pool temperature difficult to judge. Its mechanical strength also falls with temperature and is only 10 MPa at 370°C. The material may therefore fail to support the molten metal, causing poor bead shape, collapse, or burn-through.

A backing bar is commonly used. Heating must also be controlled carefully, flat-position welding should be used whenever possible, and the arc should be started and extinguished on run-on and run-off tabs.

4. Hot-Cracking Tendency

Aluminum’s coefficient of linear expansion is nearly twice that of steel, and its solidification shrinkage is also about twice as high, producing substantial welding stress. Alloy composition also has a major effect: impurities above the specified limits create more low-melting eutectic constituents in the pool. These effects together promote hot cracking.

Cracks commonly occur at arc starts, arc stops, sudden arc interruptions, tack welds, and repair welds. Prevention therefore requires reduced welding stress, adjustment of weld-metal composition, improved pool-solidification conditions, an appropriate process, and controlled parameters.

5. Joint Strength Below Base-Metal Strength

Welding heat softens the heat-affected zone, reducing strength and degrading mechanical properties. The joint consequently may not match the base metal’s strength. To obtain an equal-strength butt joint, the weld-zone metal thickness must be increased.

6. Reduced Corrosion Resistance After Welding

Joints in heat-treatable aluminum alloys, such as duralumin, can lose substantial corrosion resistance. Greater microstructural nonuniformity causes a greater reduction. Weld-metal purity and density also matter: abundant impurities, coarse grains, and precipitation of brittle phases such as FeAl₃ markedly reduce corrosion resistance and may cause both localized surface attack and intergranular corrosion. Welding stress is another important factor.

Corrosion resistance can be improved by:

1. Reducing compositional and microstructural nonuniformity by alloying the weld through suitable filler materials, refining grains, preventing defects, adjusting the process to limit the heat-affected zone and overheating, and applying post-weld heat treatment.
2. Relieving welding stress; localized surface tensile stress can, for example, be reduced by local hammering.
3. Applying protective treatments such as anodizing or coatings.