Aluminium casting alloys dominate structural and functional components across automotive, aerospace, and consumer sectors: engine blocks, cylinder heads, transmission housings, structural nodes, and heat exchangers are predominantly aluminium castings. The three most commercially significant alloys — A380 (Al-Si-Cu for die casting), A356 (Al-Si-Mg for structural castings), and A319 (Al-Si-Cu for engine components) — each represent a carefully balanced compromise between castability, mechanical properties, corrosion resistance, and machinability, all rooted in the Al-Si binary phase diagram.

The Al-Si Alloy System: Metallurgical Basis

Silicon is the most important alloying element in aluminium casting alloys, improving four critical casting attributes simultaneously: fluidity (lower surface tension and viscosity near the eutectic), reduced solidification shrinkage (from ~6.5% for pure Al to ~3.5% at the eutectic), lower melting temperature (577°C eutectic vs. 660°C for pure Al), and improved feeding characteristics. Silicon is essentially insoluble in the solid aluminium matrix at room temperature (less than 0.05 wt% Si at 20°C), so all silicon present above this solubility limit appears as silicon particles — either as eutectic silicon or, in hypereutectic alloys, as primary silicon crystals.

Hypoeutectic vs. Eutectic vs. Hypereutectic Alloys

All three commercially dominant casting alloys (A380, A356, A319) are hypoeutectic, containing less than the eutectic silicon content (12.6 wt%). On solidification they first form primary α-aluminium dendrites, then the remaining liquid transforms to Al-Si eutectic at 577°C, filling the interdendritic spaces. The volume fraction of eutectic increases with silicon content towards the eutectic composition. Hypereutectic alloys (>12.6 wt% Si, notably 390 with 17% Si) first precipitate primary silicon crystals — angular, hard particles that must be refined by phosphorus addition — before the eutectic reaction.

Volume fraction of eutectic (lever rule at 577°C for hypoeutectic alloys):
  f_eutectic = (C_alloy − C_α) / (C_eutectic − C_α)
  where C_α ≈ 1.65 wt% Si (max solubility at 577°C)
        C_eutectic = 12.6 wt% Si

A356 (7.0% Si):   f_eut = (7.0 − 1.65) / (12.6 − 1.65) = 5.35 / 10.95 ≈ 0.49 (49 vol%)
A380 (8.5% Si):   f_eut = (8.5 − 1.65) / 10.95           = 6.85 / 10.95 ≈ 0.63 (63 vol%)
A319 (6.0% Si):   f_eut = (6.0 − 1.65) / 10.95           = 4.35 / 10.95 ≈ 0.40 (40 vol%)

The eutectic fraction has a major influence on castability (higher eutectic = better fluidity and feeding) and on the response to modification treatment (the silicon particles within the eutectic are the target of Sr/Na modification).

The Three Principal Casting Alloys: Compositions and Properties

Why Iron Is Controlled Differently in Each Alloy

Iron is the most important impurity in aluminium casting alloys and is managed very differently depending on the casting process. In high-pressure die casting (A380), iron is deliberately maintained at 0.7–1.3 wt% because it reduces die soldering — the tendency of molten aluminium to weld to the steel die face — by reducing chemical activity at the die-melt interface. However, iron at these levels precipitates as the brittle β-Fe phase (Al₅FeSi), which forms coarse needle-shaped plates up to several millimetres long, severely reducing elongation and fatigue strength. Manganese additions convert β-Fe to the compact, less-harmful α-Fe phase (Al₁₅(Fe,Mn)₃Si₂) when Mn/Fe > 0.5 by weight.

In A356 and premium structural castings, iron is kept below 0.15–0.20 wt% to avoid β-Fe formation entirely. This is achievable in sand and permanent mould castings where die soldering is not a concern, but requires the use of primary (virgin) aluminium rather than cheaper secondary (recycled) metal which typically contains 0.5–1.0 wt% Fe.

Solidification Metallurgy and Microstructure

Primary α-Aluminium Dendrites and SDAS

Solidification of hypoeutectic Al-Si alloys begins with nucleation and growth of primary α-aluminium dendrites. The dendrite morphology follows the well-established cubic FCC pattern with primary arms growing along <100> directions. The secondary dendrite arm spacing (SDAS) is the most important microstructural length scale in casting and is directly determined by the local solidification time t_f:

SDAS relationship (Kurz and Fisher, modified):
  SDAS = a × t_f^n     (units: SDAS in µm, t_f in seconds)

For Al-Si alloys:
  a ≈ 39,  n ≈ 0.33   (A356, A380 range)

Typical SDAS values by process:
  High-pressure die casting (HPDC):     5–15 µm   (t_f ~ 1–5 s)
  Permanent mould (gravity):            15–30 µm   (t_f ~ 30–120 s)
  Sand casting (greensand):             30–60 µm   (t_f ~ 300–3000 s)
  Large sand casting sections:          60–120 µm  (t_f ~ 3000–30000 s)

Effect on UTS (Caceres et al.):
  σ_UTS ≈ 384 − 1.47 × SDAS  (MPa, A356-T6, approximate)
  → SDAS 15 µm: UTS ≈ 362 MPa
  → SDAS 60 µm: UTS ≈ 296 MPa

Eutectic Silicon Morphology and Modification

In unmodified Al-Si alloys cooled at typical casting rates, eutectic silicon solidifies as coarse, interconnected acicular (needle-like) plates or flakes with aspect ratios of 10:1 to 100:1. These plates act as stress raisers with an estimated stress concentration factor k_t of 3–10, causing brittle fracture at low strains. Elongation in unmodified alloys is typically below 2%, even in A356.

Chemical modification by sodium (150–300 ppm Na, added as AlSi5Na master alloy) or strontium (100–300 ppm Sr, added as Al-10Sr master alloy) changes the silicon growth mechanism from faceted to non-faceted (twin-plane re-entrant edge, or TPRE, growth is suppressed), producing a fine, fibrous or coral-like silicon morphology with aspect ratios typically below 3:1. The physical mechanism involves adsorption of modifier atoms on silicon growth steps, locally poisoning faceted growth.

Effect of Sr modification on A356 properties (typical values):

Property            Unmodified      Sr-modified (120 ppm)
Eutectic Si form    Coarse plates   Fine fibrous/coral
Average Si length   80–200 µm       5–15 µm
Elongation (T6)     2–4%            8–12%
UTS (T6)            280 MPa         310 MPa
YS (T6)             240 MPa         262 MPa
Fatigue limit       80 MPa          110 MPa (approx. R=-1)

Strontium is strongly preferred over sodium in modern industrial practice because: (1) Na is highly reactive and produces hydrogen pickup from moisture; (2) Na fades (evaporates) rapidly from the melt at holding temperatures, requiring re-addition; (3) Sr has a substantially longer fading time (4–8 hours vs. 20–40 minutes for Na), compatible with industrial ladle practice.

Heat Treatment: T6 and T7 Processing

Only alloys with a sufficient concentration of age-hardening alloying elements — principally Mg (A356) and Cu (A319) — respond to T6 artificial ageing treatment. A380 contains both Mg and Cu but HPDC parts are typically not T6-treated in production because the high dissolved gas content in HPDC castings causes blistering at solution treatment temperature (535°C). Vacuum-assisted HPDC and rheocasting/thixocasting processes are specifically designed to reduce this internal porosity and permit heat treatment of die castings.

T6 Heat Treatment of A356

A356-T6 heat treatment cycle:

Step 1 — Solution treatment:
  Temperature:  535–545°C (must not exceed 560°C — risk of incipient melting
                of Cu-Al eutectic and grain boundary films)
  Time:         6–12 hours (depending on section thickness and SDAS)
  Purpose:      Dissolve Mg and Si into α-Al matrix; spheroidise eutectic Si

Step 2 — Quench:
  Medium:       Water at 65–80°C (hot water quench reduces distortion and
                residual stress vs. cold water, with minor property penalty)
  Time to quench: < 30 seconds (minimise precipitation on quench)

Step 3 — Artificial ageing:
  Temperature:  155–165°C
  Time:         3–6 hours
  Precipitate:  β'' (Mg₅Si₆) → β' → β (Mg₂Si) sequence

Target properties (A356.0-T6):
  Yield strength:  ≥ 200 MPa (min), typically 220–262 MPa
  UTS:             ≥ 285 MPa, typically 295–320 MPa
  Elongation:      ≥ 6%, typically 8–12% (Sr-modified)
  Hardness:        70–85 HRB

Precipitation Hardening Mechanism in A356

The strengthening in A356-T6 derives from the Mg-Si precipitation sequence in the α-aluminium matrix. On quenching, Mg and Si are retained in supersaturated solid solution. During ageing at 155–165°C, the precipitation sequence proceeds:

A356 ageing precipitation sequence (Mg-Si system):
  SSSS → GP zones (Mg-Si clusters) → β'' (Mg₅Si₆, monoclinic, coherent)
       → β' (Mg₉Si₅, hexagonal, semi-coherent) → β (Mg₂Si, cubic, incoherent)

Maximum hardness at peak age (T6): β'' dominant — coherency hardening + Orowan bypass

Over-ageing (T7, 200–230°C / 6h):
  β particles coarsen and lose coherency
  Δσ decreases ≈ 20–30 MPa vs. T6
  Elongation improves ≈ 2–3% vs. T6
  Used where dimensional stability and resistance to SCC are critical

The Orowan bypass stress increment from fine coherent β'' precipitates is the dominant strengthening mechanism at peak ageing. This can be estimated as:

Orowan-Ashby equation for precipitation strengthening:
  Δσ = M × 0.4 × G × b / (π × L) × ln(r̄ / b)

Where:
  M  = Taylor factor ≈ 3.06
  G  = shear modulus ≈ 26 GPa (Al matrix)
  b  = Burgers vector ≈ 0.286 nm
  L  = inter-precipitate spacing (nm)
  r̄  = mean precipitate radius (nm)

For A356-T6 (β'' needles, mean radius ≈ 2 nm, L ≈ 25–40 nm):
  Δσ ≈ 80–120 MPa above the solution-treated condition

Casting Processes and Their Microstructural Consequences

High-Pressure Die Casting (HPDC) Metallurgy

HPDC injects molten metal at high velocity (20–60 m/s gate velocity) and pressure (700–1500 bar intensification pressure) into a water-cooled steel die. The extremely rapid heat extraction (solidification times of 0.5–5 seconds) produces a very fine SDAS (5–15 µm) in the casting interior, but also causes two characteristic microstructural features unique to HPDC: (1) a surface skin of approximately 0.5–1.5 mm depth which is segregation-free and highly refined due to contact with the cold die face; and (2) an interior zone that contains entrapped gas (both dissolved hydrogen and air/steam entrained during die fill) that appears as spherical or irregular pores typically 50–500 µm diameter.

Sand Casting and Permanent Mould Metallurgy

Sand and permanent mould processes solidify much more slowly, producing coarser SDAS but lower dissolved gas content because the melt has time to degas during pouring. Porosity is dominated by solidification shrinkage rather than gas — angular interdendritic voids in the last-to-solidify regions. These are controlled by proper gating, risering design, and solidification simulation. For A356 structural sand castings, per ASTM A356/A356M (Note: aluminium casting alloys are specified under ASTM B26 for sand and B108 for permanent mould), porosity acceptance criteria are specified using radiographic standards (ASTM E155) or through density measurements.

Casting Defects: Types, Causes, and Prevention

Hydrogen Porosity: Theory and Control

Hydrogen is the only gas with significant solubility in liquid aluminium, and its solubility drops dramatically at the liquidus:

Hydrogen solubility in aluminium (Sievert's law):
  [H]_liquid  ≈ 0.65 mL H₂/100g Al at 660°C in equilibrium with H₂O at 1 atm
  [H]_solid   ≈ 0.034 mL H₂/100g Al at 660°C  (ratio ≈ 19:1)

Sievert's law:  [H] = K × √(P_H₂)    (K temperature-dependent)

Sources of hydrogen:
  • Moisture in furnace atmosphere, charge materials, fluxes, tools, refractories
  • Hydrated Al₂O₃ on scrap surfaces
  • Hydrocarbon combustion products (burner-heated furnaces)

Degassing control:
  Rotary impeller degassing: Ar or N₂ at 30–60 min, 720–740°C
  Target: density index (DI) < 1%

  Density index:  DI (%) = (ρ_atm − ρ_vac) / ρ_atm × 100
  where ρ_atm = density of solidified sample at atmospheric pressure
        ρ_vac = density of sample solidified under vacuum (~100 mbar)
  DI < 0.5%: premium quality
  DI 0.5–1%: acceptable for most structural castings
  DI > 2%:   reject — excessive porosity risk

Hot Tearing: Mechanism and Susceptibility

Hot tearing forms when the cumulative shrinkage strain in the semi-solid mushy zone exceeds the limited plastic deformation capacity of the partially solidified structure. The critical condition occurs at solid fractions between 0.70 and 0.95, where liquid feeding is insufficient to accommodate shrinkage but the solid skeleton is too weak to sustain the thermal stresses. Alloys with wide freezing ranges spend more time in this vulnerable state.

Approximate freezing range comparison:
  A356  (Al-7Si-0.3Mg):      577°C − 615°C → ΔT ≈ 38°C   (narrow; good HT resistance)
  A380  (Al-8.5Si-3.5Cu):    477°C − 595°C → ΔT ≈ 118°C  (wider due to Cu)
  A319  (Al-6Si-3.5Cu):      507°C − 605°C → ΔT ≈ 98°C   (intermediate)
  206   (Al-4.5Cu):           507°C − 649°C → ΔT ≈ 142°C  (widest; highest HT susceptibility)

Prevention: proper taper on cores, avoid abrupt section changes, grain refinement,
            correct pouring temperature (minimise superheat), heated die/mould
            in critical areas to slow local cooling.

Industrial Applications and Alloy Selection

The three alloys occupy distinct market niches determined by the combination of casting process, required mechanical properties, and post-processing requirements:

A357 and 357: The Premium A356 Variants

A357 (Al-7Si-0.55Mg) is a premium variant of A356 with higher Mg content (0.45–0.60 wt% vs. 0.25–0.45 wt% for A356) and a tighter iron limit of 0.12 wt% maximum. The higher Mg provides increased Mg₂Si precipitation hardening response, giving T6 properties of approximately 310 MPa YS and 325 MPa UTS with 5% elongation. A357 is used in aerospace castings per AMS 4219 where the full AMS specification imposes tighter chemical, microstructural, and radiographic acceptance criteria than commercial ASTM grades.

Melt Quality Assessment

For structural castings, melt quality is assessed by the reduced pressure test (RPT) and calculated as the density index (DI). Complementary methods include:

• Prefil Footprinter: Pressure filtration of melt through a fine ceramic filter; the filtration curve distinguishes bifilm content from oxide content.
• Telegas / Alscan hydrogen probe: In-line electrochemical or thermal conductivity measurement of dissolved hydrogen content in the melt before casting.
• Optical emission spectrometry (OES): Verification of alloy composition, Sr modification level, Mn/Fe ratio, and trace element content before casting.
• ASTM E155 radiographic reference standards: Acceptance criteria for porosity level, type, and location in finished castings, typically applied to safety-critical zones.

Further coverage of corrosion mechanisms relevant to aluminium castings, hardness testing methods for verifying T6 condition, and impact testing of aluminium alloys can be found elsewhere on MetallurgyZone. For the underpinning solidification theory, see the grain boundaries and nucleation articles.