Aluminium is a widely used engineering material prized for its excellent strength-to-weight ratio, corrosion resistance, and workability. However, welding aluminium – particularly using the MIG (GMAW) process – requires careful control of variables not encountered when welding steel.
With a melting point of 660.3°C, and a thermal conductivity of 235 W/m·K, aluminium dissipates heat rapidly and requires a well-controlled arc and stable heat input to achieve successful fusion. Its naturally forming oxide layer, with a melting point of 2072°C, must be removed prior to welding to avoid fusion defects.
Aluminium also expands significantly when heated, with a thermal expansion coefficient of 23.1 µm/m·K, making it prone to distortion and residual stresses. Its rapid oxidation rate, up to 2.0 µg/cm² per hour, adds further complexity to welding operations.
At NECIT, we specialise in welding qualification and certification services, helping clients meet the following internationally recognised standards:
Pulse Transfer
Pulse MIG welding is ideal for thin to medium-thickness aluminium, typically between 1.5mm and 6mm. This method alternates between a high-energy peak current, which transfers a single droplet of metal, and a lower background current, which sustains the arc without adding excess heat.
This controlled waveform allows for stable arc characteristics with lower overall heat input, making it highly suitable for aluminium applications where distortion, grain coarsening, and HAZ softening must be avoided. It is particularly effective for positional welding and materials with variable thickness.
Typical parameters include:
By using pulse transfer, operators can achieve excellent fusion with controlled penetration while protecting the mechanical properties of precipitation-hardened alloys.
Spray transfer is the preferred process for welding thicker aluminium sections (3mm and above). It delivers a continuous, fine stream of molten droplets into the weld pool at high amperage and voltage, creating a stable, low-spatter arc and high-quality fusion.
Spray transfer requires a minimum of around 180A to initiate and maintain spray mode. Falling below this threshold can lead to globular transfer, which produces more spatter and less consistent penetration.
Typical settings include:
Spray transfer is best suited for flat or horizontal welding, as the weld pool remains very fluid. It is widely used in automotive, structural, and marine sectors where high deposition and deep penetration are critical.
Porosity is one of the most common defects encountered in aluminium welding. It results from gas entrapment in the weld metal during solidification, with hydrogen being the primary cause.
Hydrogen can originate from moisture in the base material, filler wire, or the atmosphere – and aluminium’s high solubility for hydrogen at molten temperatures makes it especially vulnerable.
One of the biggest contributors is the aluminium oxide layer, which has a strong affinity for hydrogen. If this layer is not properly removed, it can trap contaminants and prevent complete fusion, leading to extensive porosity.
To prevent porosity:
Effective control of these variables is essential for delivering sound, defect-free welds.
Heat-affected zone (HAZ) softening is a metallurgical concern primarily seen in 6xxx and 7xxx series aluminium alloys, which rely on precipitation strengthening. Exposure to high welding temperatures causes these fine precipitates to coarsen or dissolve, reducing mechanical strength, hardness, and fatigue resistance in the HAZ.
This softening is not reversible without post-weld heat treatment (solution heat treatment and artificial ageing). As this is often impractical in field applications, preventative measures must be used.
To mitigate HAZ softening:
Where mechanical strength cannot be restored, joint design must consider reduced strength in the HAZ using conservative design allowances.
Liquation cracking occurs when low-melting constituents within the base material partially melt during welding and fail to re-solidify properly, leaving behind cracks in the HAZ.
This is most common in high-magnesium and high-zinc alloys, such as those in the 7xxx series. It is aggravated by steep thermal gradients, rapid heating/cooling, and excessive weld restraint.
Prevention strategies include:
Careful procedure development and operator skill are key to minimising cracking risk.
Solidification cracking, also called hot cracking, forms in the centre of the weld bead during the final stages of cooling. It occurs when the weld metal is partially solidified but under tensile stress, and there is insufficient liquid metal available to feed the shrinking centreline.
Factors that contribute to this include:
To prevent it:
Correct process control, good arc stability, and joint design optimisation are all required to avoid solidification cracking, especially in fully restrained joints.
NECIT is a trusted provider of independent third-party welding qualification services, offering expertise across multiple sectors and international codes.
We provide certification to any international standard such as:
We support both initial qualification and renewals, and offer full technical guidance, including:
Whether you need a single welder qualified or a complete production procedure certified for an aerospace or rail contract, NECIT can deliver with confidence, independence, and technical authority.
