8.2 Aluminium & Aluminium Alloys

8.2 Aluminium & Aluminium Alloys

Quick Answer: Aluminium looks easy because it is light and melts at a relatively low temperature, but welding it is demanding. A high-melting oxide film, high thermal conductivity, hot-cracking susceptibility in many alloys, and hydrogen porosity dominate shop problems. ISO/TR 15608 places aluminium alloys mainly in groups 21–26. Inspectors verify designation/group, cleaning, process (often AC TIG or approved MIG), filler, and that HAZ softening on heat-treatable grades is accepted by design—not “fixed” by random extra heat.

Module WT2.17 expects IWI-S candidates to explain these aluminium-specific issues in inspection language and to know what must appear on procedures and work instructions.

Aluminium as a Base Metal

Pure aluminium and its alloys offer low density, good thermal and electrical conductivity, and useful corrosion resistance in many atmospheres because of a natural oxide. That same oxide, plus rapid heat extraction into the surrounding metal, makes fusion welding different from carbon steel practice:

• Melting point of aluminium metal is much lower than steel, yet heat sinks away quickly
• Oxide melting temperature is far higher than the metal underneath
• Coefficient of thermal expansion is high → distortion and residual stress
• No colour change like steel as temperature rises—welders cannot “read” heat by colour the same way

ISO/TR 15608 Groups 21–26 (Overview)

For procedure qualification ranges, aluminium alloys are grouped in ISO/TR 15608 roughly as follows (teaching overview; always confirm exact sub-limits in the report and project documents):

• Group 21 — pure aluminium and low-alloy aluminium
• Group 22 — non-heat-treatable alloys (e.g. Al–Mn, Al–Mg families in broad teaching terms)
• Group 23 — heat-treatable alloys (e.g. Al–Mg–Si type families)
• Groups 24–26 — further aluminium alloy categories including higher-strength and specialist compositions as defined in the grouping tables

The inspector’s job is not to recite every composition limit from memory under exam pressure, but to:

1. Read the group and alloy designation on the material certificate
2. Confirm they sit inside the WPQR/WPS range
3. Reject silent swaps (for example substituting a high-strength 7xxx plate for a 5xxx procedure without engineering control)

The Oxide Film Problem

Aluminium instantly forms Al₂O₃. The oxide is ceramic-like: hard, adherent, and with a melting point around 2000 °C, while aluminium metal melts near 660 °C. If oxide remains as continuous skin:

• Wetting and fusion fail → lack of fusion, black sooty oxide inclusions, poor bead wetting
• Arc behaviour becomes unstable
• Defects hide under a deceptively smooth face

Practical disruption methods

• Mechanical cleaning immediately before welding: stainless brushes dedicated to aluminium, scrapers, approved solvents for oil/grease—then weld promptly before thick re-oxidation and recontamination
• Chemical cleaning where specified (alkaline cleaners, deoxidisers) under controlled processes
• AC TIG cathodic cleaning action on the half-cycle that helps shatter and remove oxide on many aluminium applications
• MIG/MAG (GMAW) with appropriate polarity, spray or pulsed modes, and clean wire per WPS

Never grind aluminium with discs contaminated by steel dust, and never use the same brush that cleaned carbon steel. Contaminants seed porosity and inclusions.

Thermal Conductivity and Heat Management

Aluminium conducts heat several times better than carbon steel. Consequences:

• Higher welding current or preheat on thick sections may be needed to achieve fusion despite the lower melting point
• Heat spreads widely → larger distortion risk and wider softened HAZ on heat-treatable alloys
• Root openings and fit-up errors are less forgiving because the pool freezes fast if heat is stolen by cold mass

Preheat, when used, is typically modest compared with hardenable steels and is aimed at moisture removal and heat-sink management, not hydrogen diffusion control in the steel sense. Always follow the WPS numerical limits.

Hot Cracking Susceptibility

Many aluminium alloys have a wide freezing range and form low-melting eutectics. Combined with high shrinkage, this produces hot cracking / solidification cracking risk—especially in restrained joints, concave bead shapes, and incorrect filler choices.

Inspector-facing controls:

• Filler metal selected to tolerate solidification (often not “exact match” chemistry; e.g. common teaching pairs such as 4043 or 5356 families for certain 6xxx/5xxx bases—only as specified)
• Joint design and bead shape that avoid highly concave, crack-prone profiles
• Restraint reduction and sequencing
• Avoidance of excessive dilution into crack-sensitive base compositions when the procedure limits it

If the WPS names a filler, substituting a different AWS/EN designation because “it is also aluminium wire” is a nonconformance.

AC TIG — Why Alternating Current Appears So Often

TIG (GTAW) on aluminium frequently uses alternating current (AC):

• One half-cycle provides cathodic cleaning (oxide disruption) when electrode positive portion acts on the work
• The other supports penetration and electrode cooling balance depending on AC balance settings
• Modern inverter AC TIG allows balance and frequency adjustment; the WPS or manufacturer data define acceptable windows

DCEN TIG can be used with helium-rich gases for some deep-penetration applications, but oxide cleaning must then rely more on mechanical preparation. Do not change polarity or gas outside the qualified procedure.

Argon is the common shielding gas; helium or Ar–He mixes appear for thicker sections and hotter arcs. Gas purity and flow, torch condition, and draft protection matter because aluminium is unforgiving of turbulence and contamination.

Porosity from Hydrogen

Hydrogen is the principal gas causing porosity in aluminium welds. Solubility drops sharply on solidification, so dissolved hydrogen nucleates pores. Sources include:

• Moisture on plate, wire, or in the gas system
• Hydrated oxide films and corrosion products
• Oil, grease, dye penetrant residues, and shop dirt
• Contaminated or improperly stored filler wire
• Leaking water-cooled torches

Unlike steel hydrogen cold cracking, the aluminium story for inspectors is primarily porosity and cleanliness, not delayed HAZ hydrogen cracks of the carbon-steel type. Prevention is preparation and process hygiene:

• Dry storage of wire; end caps; no bare wire dragged across dirty floors
• Fresh mechanical cleaning; remove thick oxide and hydrated films
• Approved degreasing; complete evaporation before arc start
• Sound gas lines, adequate post-flow, correct cup size

Porosity acceptance still follows the applicable quality level (e.g. ISO 5817 thinking in later chapters)—the inspector records size, distribution, and whether repair is required.

Heat-Treatable vs Non-Heat-Treatable Series (Inspector Basics)

Aluminium alloys are commonly discussed by series (AA/EN designation families):

Non-heat-treatable (strengthen mainly by cold work / solid solution)

Teaching examples:

• 1xxx — commercially pure Al
• 3xxx — Al–Mn
• 5xxx — Al–Mg (structural sheet and plate common)

Welding anneals cold work in the HAZ → softening relative to highly work-hardened tempers. Designers often assume annealed HAZ strength. Filler selection for 5xxx must also respect service (e.g. corrosion, anodising, crack sensitivity).

Heat-treatable (precipitation hardening)

Teaching examples:

• 2xxx — Al–Cu (aerospace-type; many are fusion-weld sensitive)
• 6xxx — Al–Mg–Si (extrusions, structural)
• 7xxx — Al–Zn–Mg (high strength; many restricted for fusion welding)

Solution treatment and ageing create strength. Welding overages or solutionises local regions → HAZ strength loss. Post-weld heat treatment can restore properties only when the entire assembly and code allow it—rarely possible on large structures. Inspectors must not assume the weldment recovers parent temper strength just because the bead looks good.

Temper designations (T4, T6, etc.) on certificates matter for both design and any PWHT claims on the procedure.

MIG Welding Notes for Inspectors

Gas-metal-arc welding of aluminium uses push technique more often, spool guns or specialised feeders to avoid wire bird-nesting (soft wire), and clean contact tips. Parameter windows for pulsed MIG are tight. Verify:

• Wire alloy and diameter vs WPS
• Drive-roll type (U-groove typically) and feed condition
• Shielding gas and flow
• Absence of black smut beyond what the procedure and cleaning plan allow

Common Nonconformances

1. Welding through water stains, cutting fluid, or marker ink → porosity clusters
2. Wrong filler on 6xxx extrusions → hot cracks at crater or centreline
3. Steel tooling contamination → inclusions and corrosion cells
4. Ignoring group/temper on mixed multi-alloy fabrications → unqualified joints
5. Excessive weave and heat on thin heat-treatable sheet → severe HAZ softening and distortion

Exam and Site Mindset

When a question or site problem mentions aluminium, run the checklist: oxide, heat sink, hydrogen porosity, hot crack + filler, series/temper/group, AC cleaning or approved process. Link findings to the WPS and material certificates. Aluminium rewards preparation and punishes improvisation more consistently than mild steel.