Aluminium Welding Metallurgy: Hot Cracking, Porosity, HAZ Softening, and Filler Selection
Welding aluminium alloys presents a distinct set of metallurgical challenges that do not apply to steel: an oxide layer that melts at three times the alloy’s own melting point, hydrogen with a solubility ratio of nearly 20:1 between liquid and solid, wide solidification temperature ranges in high-strength alloys, and HAZ softening driven by precipitate dissolution rather than grain growth. This article examines each mechanism in depth and provides the quantitative framework for filler selection, process optimisation, and defect prevention.
Key Takeaways
- Hydrogen porosity is the dominant aluminium weld defect: liquid Al dissolves ~0.69 ml H/100 g; solid Al dissolves only ~0.036 ml/100 g, forcing gas into pores during solidification.
- Solidification cracking susceptibility is maximum at 1–2 wt% Cu or Mg dilution range; filler additions that shift composition outside this brittle range (e.g., adding Si via 4043) are the primary prevention strategy.
- HAZ softening in 6xxx-T6 alloys typically reduces yield strength by 30–50% due to dissolution and coarsening of beta’’ precipitates; minimum strength occurs 10–20 mm from the fusion line.
- ER4043 (Al-5Si) offers superior crack resistance and suits 6xxx base metals; ER5356 (Al-5Mg) offers higher weld metal strength and better colour match for anodising.
- The aluminium oxide layer (Al2O3, melting point ~2050 °C) must be cleaned mechanically and chemically before welding; it reforms within seconds and adsorbs moisture that drives hydrogen porosity.
- GTAW with AC current provides cathodic cleaning of the oxide in each positive-electrode half-cycle, making it the preferred precision process for thin aluminium sections.
Aluminium Filler Selection & Heat Input Calculator
Select base metal series and joint parameters to determine recommended filler metal and heat input range.
Step-by-Step Calculation
Physical Metallurgy of Aluminium: Properties Relevant to Welding
Aluminium melts at 660 °C and has a face-centred cubic (FCC) crystal structure, giving it excellent ductility and no ductile-to-brittle transition. However, several physical properties make aluminium distinctly different from steel in the welding context:
| Property | Aluminium | Low-Carbon Steel | Welding Implication |
|---|---|---|---|
| Melting point | 660 °C | 1520 °C | Lower energy input; rapid cooling rates in thin sections |
| Thermal conductivity | 237 W/m·K | 50 W/m·K | Heat dissipates rapidly; wider HAZ; preheat rarely needed but loss of heat input is faster |
| Thermal expansion coefficient | 23.6 µm/m·K | 12 µm/m·K | Nearly twice the distortion potential; high residual stress |
| Solidification shrinkage | ~6.5% | ~3% | Doubles the interdendritic strain during solidification; cracking risk elevated |
| H solubility (liquid) | ~0.69 ml/100 g | ~28 ml/100 g | Al has very low absolute solubility but the ratio liquid:solid is ~19:1, driving severe porosity |
| Surface oxide (Al2O3) | Melting point ~2054 °C | FeO, Fe3O4 (dissolve or flux) | Oxide does not melt; causes fusion defects and entraps hydrogen-bearing moisture |
| Colour change at melting | None (stays silver) | Visible red-orange glow | No visual warning of approaching melt; risk of burn-through in thin sections |
The Aluminium Oxide Layer
The native Al2O3 film on aluminium surfaces is thermodynamically stable and reforms within 1–3 seconds of mechanical removal in ambient air. In GTAW with alternating current, the positive electrode half-cycle provides cathodic cleaning: positive argon ions bombard the workpiece surface and sputter the oxide, removing it ahead of the molten pool. This is why AC GTAW is the preferred process for aluminium. DCEN (straight polarity) provides no cathodic cleaning and cannot be used with a conventional tungsten electrode on aluminium. DCEP (reverse polarity) provides cathodic cleaning but concentrates too much heat in the electrode.
For GMAW, the cathodic cleaning effect is inherent in the DCEP polarity used with aluminium wire, providing oxide removal under the arc during transfer. This is a key reason why aluminium GMAW uses DCEP rather than the DCEN polarity sometimes used with steel.
Hydrogen Porosity: Mechanisms, Sources, and Prevention
Porosity is the most common weld defect in aluminium and is almost exclusively hydrogen-driven. The thermodynamic basis is the Sievert’s law relationship between hydrogen partial pressure and solubility:
[H] = K · √p(H₂)
where:
[H] = hydrogen concentration in the melt (ml/100 g or cm³/100 g)
K = temperature-dependent equilibrium constant
p(H₂) = partial pressure of hydrogen gas (atm)
Approximate values:
[H]ₗₗₑₖₙₑ at 750°C ≈ 0.69 ml/100 g
[H]ₘₙₗ₉ₖ at 660°C ≈ 0.036 ml/100 g
Solubility ratio (liquid/solid) ≈ 19 : 1
During weld pool solidification, the growing solid rejects hydrogen into the residual liquid. When the local hydrogen concentration exceeds the solubility limit in solid aluminium, nucleation of hydrogen gas bubbles occurs. Bubbles that form before dendrite coherency can escape to the surface; those formed after coherency is established are trapped as spherical pores.
Hydrogen Sources in Aluminium Welding
| Source | Mechanism | Prevention |
|---|---|---|
| Hydrated oxide on base metal | Al(OH)3 or AlOOH decomposes in arc; releases H at temperatures above 200 °C | Wire brush + degrease; weld within 4 hours |
| Moisture on filler wire | Adsorbed moisture on wire surface liberates H2O vapour in arc plasma | Store wire in sealed containers; bake in oven at 100°C for 2h before use; reject wire with tarnish or discolouration |
| Atmospheric humidity | H2O vapour dissociated by arc; H dissolves in melt | Weld in conditions below 70% relative humidity; use dry shielding gas (dew point <−40°C) |
| Lubricants and oils | Hydrocarbon residue from drawing lubricant on filler wire or machining oils on base metal | Degrease with acetone or IPA; never use chlorinated solvents near GTAW (toxic decomposition products) |
| Contaminated shielding gas | Moisture or air ingress through damaged hoses, loose connections, or cylinder impurity | Use certified industrial-purity argon (99.997% min); replace fittings and hoses regularly; test with electronic moisture analyser |
Porosity in aluminium welds appears in two morphologies: uniformly distributed fine porosity (below 1 mm diameter, scattered through the fusion zone) and gross porosity (pores 1–5 mm diameter, often in the weld root). Gross root porosity is typically associated with excessive travel speed in GMAW allowing collapse of the arc cavity, or inadequate back-purging in multi-pass joints. Fine distributed porosity often traces to atmospheric hydrogen pickup.
Solidification Cracking: The Brittle Temperature Range Concept
Solidification cracking (hot cracking) occurs when a partially solidified weld pool is subjected to tensile strains that exceed the capacity of the mushy zone to accommodate them. The key concept is the brittle temperature range (BTR) — the temperature interval in which the alloy has insufficient interdendritic liquid to heal forming cracks, yet insufficient solid fraction to resist fracture.
The Composition-Sensitivity Curve
For binary Al-X alloys, solidification cracking susceptibility follows a characteristic “lambda curve” when plotted against solute content: cracking rises sharply from pure aluminium, reaches a maximum at a critical composition (typically 1–3 wt% solute depending on system), then falls steeply as the composition approaches the eutectic. This behaviour occurs because:
- Near-pure aluminium: narrow solidification range, minimal BTR, high resistance.
- Intermediate compositions (the sensitive range): wide mushy zone, thin interdendritic films that cannot heal strains, maximum susceptibility.
- Near-eutectic compositions: abundant liquid at low temperatures heals incipient cracks by backfilling; susceptibility drops sharply.
For aluminium alloys, the approximate peak susceptibility compositions are: Al-Cu at ~1.7 wt% Cu, Al-Mg at ~1.5 wt% Mg, Al-Si at ~0.5 wt% Si (with rapid recovery above 1%). This is why ER4043 (Al-5Si) has excellent crack resistance — the weld pool composition is driven towards the Al-Si eutectic (12.6 wt% Si), far from the sensitive range.
CSI = t𝗦 / (t𝗦 + t𝗳)
where:
t𝗦 = time in susceptible temperature range (BTR, typically solid fraction 0.9–0.99)
t𝗳 = time in temperature range where liquid backfilling heals cracks (solid fraction 0.6–0.9)
Higher CSI → greater solidification cracking susceptibility
Al-Cu 2219 has CSI ≈ 0.7 (high); Al-Si 4043 has CSI ≈ 0.2 (low)
Liquation Cracking in the HAZ
Liquation cracking is distinct from solidification cracking and occurs in the heat-affected zone or partially melted zone (PMZ). In alloys containing low-melting-point grain boundary phases, these constituents melt at temperatures well below the alloy solidus:
| Eutectic / Grain Boundary Phase | Eutectic Temperature | Alloy Systems Affected |
|---|---|---|
| Al-Cu (Al + CuAl2) | 548 °C | 2xxx, overalloyed 7xxx |
| Al-Mg (Al + Mg2Al3) | 450 °C | 5xxx with Mg > 3.5 wt% |
| Al-Mg-Si (Al + Mg2Si) | 594 °C | 6xxx |
| Al-Zn-Mg (Al + MgZn2 + Al) | 489 °C | 7xxx |
When the weld thermal cycle heats these grain boundary phases above their local eutectic temperature, thin liquid films form at grain boundaries in the PMZ. These films cannot sustain tensile stresses from thermal contraction during cooling, resulting in intergranular cracks. Liquation cracking is particularly severe in 2024-T3 and 7075-T6 — a primary reason both alloys are generally considered unweldable by conventional fusion processes.
HAZ Softening in Precipitation-Hardened Alloys
For age-hardened aluminium alloys (2xxx, 6xxx, 7xxx), the mechanical strength of the base metal derives from coherent or semi-coherent metastable precipitate phases that impede dislocation motion. The welding thermal cycle systematically destroys this precipitation microstructure in a spatially graded manner across the HAZ.
Precipitation Sequences and Dissolution Temperatures
| Alloy Series | Key Strengthening Precipitate(s) | Peak Hardening Phase | Dissolution / Solvus Temp. | Typical YS Loss in HAZ |
|---|---|---|---|---|
| 2xxx (Al-Cu) | GP zones → θ′′ → θ′ → θ (CuAl2) | θ′′ (coherent) | ~480–505 °C | 20–40% |
| 6xxx (Al-Mg-Si) | GP zones → β′′ → β′ → β (Mg2Si) | β′′ (needle, coherent) | ~560–580 °C | 30–50% |
| 7xxx (Al-Zn-Mg) | GP zones → η′ → η (MgZn2) | η′ (semi-coherent) | ~460–490 °C | 25–45% |
In the near-weld dissolution zone (peak temperature above solvus), precipitates dissolve completely into solid solution. Unless full post-weld solution treatment and aging are applied, this region remains largely unstrengthened. In the outer overaging zone (peak temperature below solvus but above the aging temperature), precipitates coarsen — increasing spacing between obstacles, reducing yield strength by 10–25%. The combined effect is a HAZ hardness minimum typically observed at 10–20 mm from the weld centreline in 6082-T6 and similar alloys.
Strategies to Minimise HAZ Softening
- Minimise heat input: Lower heat input reduces the width of the dissolution zone. Pulsed GMAW or high travel speed GTAW reduces peak temperature gradients spatially.
- Post-weld artificial aging (T5 or T6 temper): Partial re-precipitation in the HAZ; cannot fully recover dissolution zone but significantly improves overaging zone strength.
- Full post-weld solution treatment + aging (T6): Maximises strength recovery but causes distortion and requires fixturing of complex assemblies.
- Friction stir welding (FSW): Peak temperatures remain below the solidus (typically 400–500 °C for 6xxx), greatly reducing dissolution zone width; post-weld aging then allows near-base-metal strength.
Filler Metal Selection: Engineering Principles and AWS Classifications
Filler metal selection for aluminium welding is governed by four criteria: (1) cracking resistance in the fusion zone, (2) mechanical properties of the weld deposit, (3) service conditions (corrosion environment, elevated temperature), and (4) post-weld processing requirements (anodising colour match, PWHT compatibility). The two dominant fillers are ER4043 and ER5356, with ER5183, ER5554, ER5556, ER4047, and ER2319 covering specialised applications.
ER4043 (Al-5Si)
ER4043 contains approximately 4.5–6.0 wt% Si with small additions of Fe, Cu, Mn, and Ti. The high silicon content drives weld pool composition towards the Al-Si eutectic at 12.6 wt% Si — far from the peak cracking susceptibility range — and produces a weld pool with excellent fluidity and wetting characteristics. Key characteristics:
- Excellent crack resistance; suitable for 6xxx alloys where dilution brings Si into weld pool from base metal
- As-deposited weld metal UTS: 165–195 MPa; yield strength 55–75 MPa; elongation 8–12%
- Lower melting point than base metal improves gap-filling
- Anodising produces a dark grey or black colour (Si particles do not anodise)
- Not recommended for service above 65 °C in certain environments (Si segregation can sensitise to corrosion)
- Cannot be post-weld heat treated to recover strength in most applications
ER5356 (Al-5Mg)
ER5356 contains 4.5–5.5 wt% Mg with additions of Mn and Cr for corrosion resistance. It is the highest-volume aluminium filler and the default choice for structural 5xxx and 6xxx joints requiring higher strength:
- As-deposited weld metal UTS: 260–290 MPa; yield strength 120–140 MPa; elongation 17–22%
- Good colour match for clear anodising (close to natural aluminium colour)
- Not recommended for 2xxx or 7xxx base metals (cracking risk; inadequate composition compatibility)
- Not recommended for service temperatures above 65 °C in sustained stress conditions (Mg content can sensitise to stress corrosion cracking over time)
- Excellent corrosion resistance in marine and industrial environments
Filler Selection Matrix
| Base Metal | Primary Filler | Alternate Filler | Notes |
|---|---|---|---|
| 1xxx (pure Al) | ER1100 | ER4043 | Match composition for conductivity; 4043 improves crack resistance |
| 2xxx (Al-Cu) | ER2319 | ER4043 | 2xxx alloys are generally difficult to weld; 2219 most weldable of series; avoid 5xxx fillers |
| 3xxx (Al-Mn) | ER4043 | ER5356 | 3003 readily weldable; 4043 preferred for crack resistance |
| 5xxx (<2.5% Mg) | ER5356 | ER4043 | 5052, 5086; 5356 gives strength match; 4043 acceptable for crack-critical joints |
| 5xxx (>2.5% Mg) | ER5356 | ER5183 | 5083, 5086: 5356 standard; 5183 for maximum strength and corrosion resistance |
| 6xxx | ER4043 | ER5356 | 6061, 6082: 4043 superior crack resistance; 5356 for higher strength or anodising colour match |
| 7xxx weldable (7020, 7005) | ER5356 | ER5183 | Age-hardenable in HAZ post-weld; avoid 4xxx fillers |
| 7xxx high-strength (7075) | Not recommended for fusion welding | FSW if joining required | Severe solidification and liquation cracking risk; no filler gives acceptable properties |
Welding Process Selection for Aluminium
GTAW (TIG Welding) of Aluminium
GTAW with AC current (typically 60 Hz, square wave) provides two essential functions: the electrode-positive half-cycle delivers cathodic cleaning of the oxide layer; the electrode-negative half-cycle concentrates heat in the workpiece for penetration. Modern inverter-based power sources allow adjustment of the AC frequency (from 20 to 250 Hz) and the balance (the ratio of positive to negative half-cycles). Higher frequency narrows the arc cone (improving access in fillet welds) and reduces tungsten tip erosion. Increasing the electrode-positive balance improves cleaning action but heats the tungsten more.
Tungsten electrodes for AC aluminium welding: use pure tungsten (EWP, AWS A5.12) or zirconiated tungsten (EWZr) which forms a stable hemispherical ball end under AC. Ceriated or lanthanated tungstens (common in DC steel welding) are sometimes used on modern square-wave inverters but require careful balance setting. Electrode diameter and amperage for AC GTAW aluminium:
| Tungsten Dia. (mm) | AC Amperage Range (A) | Typical Plate Thickness | Argon Flow (L/min) |
|---|---|---|---|
| 1.6 | 40–100 | 1–3 mm | 6–8 |
| 2.4 | 80–180 | 3–8 mm | 8–12 |
| 3.2 | 150–280 | 6–15 mm | 10–14 |
| 4.0 | 230–380 | 12–25 mm | 12–16 |
GMAW (MIG Welding) of Aluminium
GMAW with DCEP is the production process of choice for aluminium sections above approximately 4–5 mm. Key considerations distinct from steel GMAW include the use of a push-only torch (or push-pull torch for wire diameters below 1.2 mm), the critical importance of wire cleanliness, and the use of softer liner materials (Teflon or nylon rather than steel). Aluminium wire is softer than steel and birdnests in standard steel liners. Spray transfer is the normal mode with argon shielding at currents above approximately 80–130 A depending on wire diameter; short-circuit transfer is not used for structural aluminium due to poor fusion and porosity.
Pulsed GMAW is increasingly standard for aluminium: pulsing allows spray transfer at lower average currents, reducing heat input and distortion on thin sections whilst maintaining complete fusion. It also improves control of the arc length and reduces spatter.
Heat Input Calculation and Its Significance
Qₙₑₜ = η · (U · I) / v
where:
Qₙₑₜ = net heat input (kJ/mm or J/mm)
η = thermal efficiency factor (dimensionless)
GTAW: 0.60–0.80
GMAW: 0.80–0.90
PAW: 0.60–0.75
U = arc voltage (V)
I = welding current (A)
v = travel speed (mm/s or mm/min; convert to mm/s for kJ/mm)
Example: GMAW, η=0.85, U=22V, I=180A, v=300mm/min (5mm/s)
Q = 0.85 · (22 · 180) / 5 = 0.85 · 3960/5 = 0.85 · 792 = 673 J/mm = 0.67 kJ/mm
For aluminium, heat input governs HAZ width (and therefore the extent of HAZ softening), the solidification cooling rate (which affects grain size and cracking tendency), and distortion. Lower heat input is generally preferred for precipitation-hardened alloys to minimise HAZ softening; however, excessively low heat input increases travel speed and can cause lack-of-fusion defects, as aluminium’s high thermal conductivity rapidly conducts heat away from the weld pool.
Industrial Applications and Weldability Classification
Understanding aluminium HAZ metallurgy and weld process selection has direct industrial implications across multiple sectors:
Marine and Offshore Structures
5083-H111 and 5086-H116 are the standard hull plate alloys for aluminium vessels (ABS, DNV-GL, Lloyd’s Register structural rules). ER5183 filler provides maximum strength and saltwater corrosion resistance. Weld joint efficiency in AWS D1.2 structural design is typically 0.55–0.75 depending on joint configuration and post-weld treatment, reflecting HAZ softening.
Aerospace Structures
High-strength 2xxx and 7xxx alloys dominate aerospace primary structure. The near-unweldability of 2024-T3 and 7075-T6 by fusion welding drove the development of friction stir welding (FSW) at TWI in 1991, which has since become the joining process of choice for aerospace panel construction (Boeing 747 and 777 fuselage panels, Space Shuttle external tank). Hydrogen-related defects are subject to zero tolerance in flight-critical joints.
Automotive and Rail
6xxx extrusions and sheets (6061-T6, 6082-T6) are standard in automotive body-in-white structures. Laser welding is increasingly used as it provides extremely low heat input, narrow HAZ, and high travel speeds compatible with automated production. Hardness mapping of the HAZ is a standard qualification test for automotive body joints per DVS 2933.
Pressure Vessels and Cryogenic Equipment
5083-O (ASME Section VIII, UNS A95083) is the dominant material for cryogenic pressure vessels and LNG storage. It retains adequate toughness to −196 °C and is not precipitation-hardened, so HAZ softening is not a concern. Weld procedure qualification per ASME Section IX requires impact testing at the applicable minimum design metal temperature (MDMT).
Quality Control and Inspection of Aluminium Welds
Post-weld inspection of aluminium follows similar principles to steel but with process-specific considerations:
- Radiographic testing (RT): Highly effective for porosity detection; minimum 2% sensitivity per AWS D1.2. Porosity appears as dark rounded spots. Elongated or linear indications suggest solidification cracking.
- Penetrant testing (PT): Effective for surface-breaking cracks (solidification cracks, liquation cracks). Must use dedicated aluminium-compatible penetrant systems; fluorescent PT preferred for sensitivity.
- Ultrasonic testing (UT): Challenging in aluminium due to high attenuation and large grain size in cast zones; phased array UT (PAUT) provides improved discrimination over conventional UT.
- Hardness testing: Vickers microhardness traverses across the weld and HAZ (typically HV0.5 or HV1) map the softening profile. Minimum HAZ hardness requirements are specified in some codes (e.g., EN 15085 for rail vehicle welding).
- Bend testing: Transverse face and root bend tests per AWS D1.2 Clause 4.8 assess fusion quality and ductility. Aluminium requires a smaller mandrel diameter relative to thickness than steel due to its lower ductility in the HAZ.
Frequently Asked Questions
Why is hydrogen porosity such a severe problem in aluminium welds?
Hydrogen has extremely high solubility in liquid aluminium (approximately 0.69 ml/100 g at the melting point) but near-zero solubility in solid aluminium (approximately 0.036 ml/100 g). During solidification, dissolved hydrogen is rejected from the growing solid and, if unable to escape, forms spherical gas pores. Sources include moisture on the base metal surface, hydrated oxide layers, contaminated filler wire, and atmospheric humidity. Effective prevention requires wire brushing and degreasing immediately before welding, storing filler wire in controlled environments, and using dry shielding gas.
What is solidification cracking and which aluminium alloys are most susceptible?
Solidification cracking (hot cracking) occurs in the weld fusion zone during the final stages of solidification when tensile stresses from thermal contraction cannot be accommodated by the partially solidified mushy zone. Alloys with wide solidification temperature ranges are most susceptible. The 2xxx (Al-Cu) and 7xxx (Al-Zn-Mg-Cu) series are the most crack-susceptible. The 5xxx (Al-Mg) and 6xxx (Al-Mg-Si) series show moderate susceptibility. Pure aluminium and Al-Si alloys (4xxx) have narrow solidification ranges and good resistance to cracking.
What causes HAZ softening in precipitation-hardened aluminium alloys?
In precipitation-hardened alloys (2xxx, 6xxx, 7xxx), strength derives from coherent or semi-coherent precipitate phases (GP zones, metastable theta-prime, beta-prime, eta-prime). The welding thermal cycle dissolves these precipitates in the near-weld HAZ region where peak temperatures exceed the solvus. In the outer HAZ, lower peak temperatures cause precipitate coarsening (overaging). Both mechanisms reduce yield strength. In 6082-T6, the HAZ minimum can represent 30–50% of base metal yield strength, occurring approximately 10–20 mm from the weld centreline.
When should ER4043 filler be chosen over ER5356 for aluminium welding?
ER4043 (Al-5Si) is preferred when cracking resistance is the primary concern, when welding 6xxx base metals, when anodising after welding (produces grey/dark colour), or when good gap-filling is needed. ER5356 (Al-5Mg) is preferred when higher weld metal strength is required, when the joint will be colour-anodised (produces a closer natural aluminium colour match), or when welding 5xxx base metals for marine or structural applications. Neither filler is suitable for 7075 or 2024 — these alloys are generally not welded by conventional fusion processes.
What is liquation cracking and how does it differ from solidification cracking?
Liquation cracking occurs in the partially melted zone (PMZ) or heat-affected zone, not in the fusion zone. Low-melting-point constituents (Al-Cu eutectic at 548 °C, Al-Mg eutectic at 450 °C, grain boundary segregates) melt at temperatures below the alloy solidus, creating thin liquid films at grain boundaries that cannot support tensile stresses. Solidification cracking, by contrast, occurs within the fusion zone as the last liquid solidifies between dendrite arms. Both are “hot cracking” phenomena but occur in different zones and by different mechanisms.
Why must aluminium be cleaned differently from steel before welding?
Aluminium forms a tenacious Al2O3 oxide layer approximately 2–10 nm thick that reforms within seconds of removal. This oxide melts at approximately 2054 °C — far above aluminium’s 660 °C melting point — so it does not melt into the weld pool and instead causes lack of fusion or entraps as inclusions. Additionally, the oxide adsorbs moisture that liberates hydrogen in the arc. Cleaning requires mechanical removal with a dedicated stainless steel brush followed by chemical degreasing. Welding must commence within 2–4 hours.
What shielding gas should be used for TIG versus MIG welding of aluminium?
For GTAW (TIG), pure argon is standard for most aluminium sections up to approximately 12 mm. Argon-helium mixtures (up to 75% He) increase arc energy density, improving penetration in thick sections. For GMAW (MIG), pure argon is standard; argon-helium mixtures (25–75% He) improve wetting and penetration in thick plate. Pure helium is not recommended for GTAW because AC arc stability is poor. Nitrogen and CO2 are not used with aluminium — they cause oxide formation and severe porosity.
How does welding process selection affect aluminium weld grain structure?
The ratio of temperature gradient (G) to solidification growth rate (R) determines solidification mode. High G/R at the fusion line promotes cellular or planar growth; lower G/R further into the weld promotes equiaxed-dendritic solidification. Higher heat input promotes columnar dendritic growth that can extend over centimetres. Lower heat input and higher travel speed promote a finer, more equiaxed microstructure. Equiaxed grains improve crack resistance by providing more grain boundaries to distribute solidification strain. Inoculants such as Al-Ti-B in the filler wire can also promote equiaxed solidification.
Can precipitation-hardened aluminium alloys recover properties after welding through post-weld heat treatment?
Yes, in principle. Post-weld solution treatment followed by artificial aging (T6 temper) can restore close to base metal strength in the HAZ, provided distortion from the quench can be managed. For 6xxx alloys, natural aging (T4) after welding provides partial recovery over several weeks. For 7xxx weldable alloys (7020, 7005), post-weld aging alone can significantly improve HAZ strength without solution treatment. For 7075, re-aging is insufficient without prior solution treatment, and the process is rarely practical for fabricated assemblies.
Recommended References
Core references for deepening your understanding of aluminium metallurgy and welding engineering:
Further Reading & Related Topics
