Aluminum 356: Composition, Properties, Temper Guide & Applications

Comprehensive Overview

356 (commonly specified as A356 or 356.0) is an aluminum-silicon-magnesium casting alloy belonging to the family of Al-Si-Mg casting alloys. It is classed among the silicon-based casting alloys and is widely treated and specified as A356 under Aluminum Association nomenclature; designations in different standards reflect the same Al–Si–Mg chemistry optimized for casting performance.

Major alloying elements are silicon (Si, nominal ~7 wt%) and magnesium (Mg, typically ~0.2–0.5 wt%), with controlled levels of iron, copper, manganese and trace additions of titanium and chromium for grain refinement and control. The alloy is heat-treatable: strength is primarily derived from precipitation hardening (Mg2Si formation during artificial aging) after solution treatment and quench, and microstructure control through eutectic modification and grain refinement.

Key traits of 356 include excellent castability and fluidity, good dimensional stability, favorable strength-to-weight ratio after T6 aging, reasonable corrosion resistance in many environments, and acceptable thermal conductivity for heat-dissipating components. Weldability is workable with proper filler and pre/post treatments, and formability is limited relative to wrought alloys but manageable for thin-wall castings and local forming.

Typical industries using 356 include automotive (lightweight structural castings, wheels, suspension components), aerospace (non-critical cast fittings and housings), marine applications (corrosion-resistant cast parts), and electronics (thermal housings and heat-dissipating components). Engineers choose 356 when a balance of castability, thermal performance, good age-hardening response, and low-to-moderate weight is required versus alternatives that offer either higher peak strength or better wrought formability.

Temper Variants

Temper	Strength Level	Elongation	Formability	Weldability	Notes
F	Baseline as-fabricated	Moderate	Limited	Good	As-cast condition with no special treatment
O	Low	High	Best of cast tempers	Good	Annealed / fully soft condition after solution + slow cooling
T5	Moderate-High	Moderate	Limited	Good	Cooled from casting and artificially aged
T6	High	Low–Moderate	Limited	Acceptable with precautions	Solution heat-treated, quenched and artificially aged (peak strength)
T7	Moderate (stable)	Moderate	Limited	Good	Overaged or stabilized temper for improved resistance to thermal exposure
T4	Moderate	Moderate	Better than T6	Acceptable	Solution heat-treated and naturally aged; used for subsequent forming

Temper strongly controls the trade-off between strength and ductility in 356 castings. Solution treatment followed by quench and artificial aging (T6) produces the highest strength and hardness via precipitation of Mg2Si, but reduces elongation and makes local forming or machining burr-prone.

Lower tempers such as O or T4 are used when formability, dimensional stability during machining, or post-processing such as welding or brazing is prioritized; T7 is selected when thermal stability and stress relaxation resistance are required at some expense of peak strength.

Chemical Composition

Element	% Range	Notes
Si	6.5–7.5	Primary strengthening and casting agent; controls eutectic fraction and fluidity
Fe	≤ 0.20–0.8*	Impurity that forms intermetallics (β-AlFeSi); minimized to preserve ductility
Mn	≤ 0.10–0.35*	Helps modify Fe intermetallic morphology; small additions beneficial
Mg	0.20–0.45	Age-hardening element (forms Mg2Si precipitates during aging)
Cu	≤ 0.20	Can raise strength but reduces corrosion resistance if present in larger amounts
Zn	≤ 0.10	Typically very low in cast grades; limited effect
Cr	≤ 0.10–0.20*	Grain/phase modifier to improve thermal stability and control grain growth
Ti	≤ 0.15	Grain refiner added in small amounts to control solidification grain size
Others (including Sr, B, rare earths)	trace	Sr commonly used to modify Si eutectic morphology; B/Ti for nucleation control

*Note: Some specification ranges differ by standard and by casting practice; the ranges above are representative for commercially produced A356/356 alloys and may vary by source specification.

Silicon determines the eutectic content and casting characteristics, and magnesium provides the precipitation hardening response that enables T6 gains. Iron and manganese control the morphology of intermetallics that influence toughness and fatigue life, while trace elements and modifiers (Sr, Ti, B) are used by foundries to refine microstructure and improve mechanical consistency.

Mechanical Properties

356 alloys exhibit a broad range of tensile behavior depending strongly on temper, section thickness and casting method. In the solutionized and artificially aged T6 temper, A356 typically shows relatively high tensile strength and yield strength driven by fine Mg2Si precipitation; however, elongation is reduced compared to annealed conditions and is sensitive to porosity and coarse eutectic structure. Elastic modulus is close to that of other Al alloys (≈69 GPa) and does not vary significantly with temper.

Hardness correlates with temper and aging state: T6 hardness values are substantially higher than O or F conditions due to precipitate hardening. Fatigue performance is influenced by surface quality, porosity, and eutectic silicon morphology; properly modified and refined A356-T6 castings can achieve good high-cycle fatigue life for automotive and aerospace service. Thickness effects are pronounced: thicker sections require longer solution treatment and can retain coarser microstructure and segregated Mg/Si, reducing achievable strength compared to thin sections.

Property	O/Annealed	Key Temper (T6)	Notes
Tensile Strength (UTS)	~120–170 MPa	~240–320 MPa	Wide ranges reflect casting method, section size and quality; T6 peak strength due to Mg2Si precipitation
Yield Strength (0.2% proof)	~70–120 MPa	~170–260 MPa	Yield increases substantially after solution + aging; scatter from porosity and casting defects
Elongation (in 50–100 mm)	~8–18%	~2–8%	Ductility reduced in T6; affected strongly by porosity and casting microstructure
Hardness (HB)	~40–70 HB	~70–100 HB	Brinell hardness correlates with temper; T6 hardness typical for structural castings

Physical Properties

Property	Value	Notes
Density	2.66–2.68 g/cm³	Typical for Al–Si–Mg alloys, slightly less than steels and copper
Melting Range (solidus–liquidus)	~555–615 °C	Eutectic-rich alloy; solidus and liquidus depend on Si content and minor elements
Thermal Conductivity	~120–140 W/(m·K)	Lower than pure Al due to alloying and eutectic silicon; still good for heat dissipation
Electrical Conductivity	~28–36 %IACS	Reduced relative to pure Al because of alloying; conductivity depends on temper and composition
Specific Heat	~0.88–0.96 J/(g·K)	Comparable to other Al alloys; temperature-dependent
Coefficient of Thermal Expansion	~22–24 µm/(m·K)	Typical aluminum expansion; important for mating with dissimilar materials

356’s density and thermal properties make it attractive where a low mass-to-stiffness ratio and reasonable thermal conduction are required. Melting and solidification characteristics are central to casting practice; the alloy’s solidification range and silicon-rich eutectic aid mold filling and reduce shrinkage defects when properly processed.

Product Forms

Form	Typical Thickness/Size	Strength Behavior	Common Tempers	Notes
Sand Castings	Variable, from thin to very thick sections	Strength depends on section size and porosity	F, O, T5, T6	Widely used for larger, lower-volume parts; slower cooling impacts microstructure
Permanent Mold	2–50 mm typical wall thickness	Higher integrity than sand; improved mechanicals	T5, T6	Better surface finish and reduced porosity vs sand casting
Die Cast (where used)	Thin walls (<10 mm)	Higher cooling rates, fine microstructure	T5, T6	Pressure casting of A356 is used for some components; control of porosity essential
Investment Casting	Complex shapes, thin-to-moderate sections	Good dimensional accuracy	T5, T6	Less common but used for precision components
Ingot / Billet	Stock for secondary processing	Homogeneous chemistry	O, T6 after casting	Feedstock for remelting and secondary casting processes
Machined Components (from castings)	N/A	Localized strength dependent on temper and heat treatment	O, T6	Machining allowances and surface quality affect final properties

Processing route affects final properties strongly: permanent-mold and die-cast parts typically achieve finer microstructures and better mechanical performance than sand-cast equivalents. Post-cast heat treatment (solution + quench + age) is commonly applied to maximize strength for structural applications, but care with quench severity and distortion control is necessary to retain dimensional tolerances.

Equivalent Grades

Standard	Grade	Region	Notes
AA	A356 / 356.0	USA	Common Aluminum Association designation for cast Al–Si–Mg alloy
EN	EN AC-AlSi7Mg / AlSi7Mg	Europe	European foundry designation widely equivalent to A356 chemistry
JIS	ADC12 (not direct) / AlSi7Mg	Japan	ADC12 is a different Al–Si–Cu casting alloy; AlSi7Mg is the closer equivalent
GB/T	AlSi7Mg / ZL104	China	Multiple national designations map to similar chemistries; ZL104 is often used for similar castings

Subtle differences between standards can include tighter limits on iron or copper, required Sr modification, or different allowable ranges for Mg, which impact final mechanical performance and castability. Buyers should reference the specific standard and lot certificates, as foundry practices (e.g., Sr modification, grain refining) and impurity controls significantly influence properties even when nominal chemistries match.

Corrosion Resistance

356 exhibits generally good atmospheric corrosion resistance typical of Al–Si alloys because a protective Al2O3 film forms rapidly and the Si-rich eutectic is relatively inert. In neutral or mildly corrosive atmospheres the alloy performs well with limited pitting; however, in chloride-rich marine environments, localized pitting of cast surfaces and intermetallic sites can occur if the protective film is compromised or if porosity traps corrosive agents.

Stress corrosion cracking is not a major failure mode for A356 compared with certain high-strength wrought Al-Zn alloys, but susceptibility can increase with high local tensile stresses, defects or aggressive environments. Galvanic interactions make 356 anodic when mated to more noble metals (stainless steel, copper); designers should insulate interfaces or avoid direct coupling in wet conditions to prevent galvanic-driven corrosion.

Compared with 5xxx magnesium-containing wrought alloys, 356 typically offers similar or slightly better generalized corrosion resistance but less ability to withstand severe marine immersion without sacrificial protection. Versus high-strength 6xxx wrought alloys, the cast A356 has comparable resistance in many service conditions, though specific alloying and heat-treatment differences drive the final corrosion performance.

Fabrication Properties

Weldability

356 can be welded using TIG (GTAW) and MIG (GMAW) techniques; preheating and weld sequence control reduce thermal gradients and hydrogen porosity risk. Common filler alloys for repair welding are Al-Si fillers such as 4043 (Al-Si) to match fluidity and reduce hot-cracking susceptibility; 5356 (Al-Mg) may be used but can increase risk of galvanic corrosion and differ in mechanical response. Heat-affected zones (HAZ) experience local overaging or softening in previously T6-treated castings; post-weld aging or re-solutionizing is often required to recover strength.

Machinability

356 is regarded as machinable among cast aluminum alloys due to its free-machining eutectic silicon phase, but silicon particles accelerate tool wear and can cause abrasive action on cutting edges. Carbide tooling with high positive rake, appropriate chip breakers and coolant application is recommended; moderate to high spindle speeds with conservative feed rates maximize tool life. Surface finish depends on microstructure and porosity; attention to casting quality and proper cut-off of porous skin is essential for consistent results.

Formability

Forming is limited compared with wrought alloys because castings contain a brittle eutectic Si phase and have lower ductility, especially after T6 treatment. For localized bending or stamping, use solution-annealed (O/T4) conditions and keep bend radii large (typical minimum inside radius 2–4× thickness for thin sections, larger for thicker castings) to avoid crack initiation at Si-rich regions. Incremental forming, warm forming and localized machining-to-form strategies are commonly used to achieve final geometries without inducing cracks.

Heat Treatment Behavior

A356 is heat-treatable and responds predictably to solution heat-treatment and artificial aging. Typical solution treatment is performed around 525–545 °C for times scaled by section thickness (commonly 2–4 hours for thin sections, longer for thick sections) to dissolve Mg and Si into solid solution and spheroidize the eutectic silicon. Rapid quenching to room temperature is required to retain solute in supersaturated solution and enable subsequent precipitation hardening.

Artificial aging for T6 is typically performed at ~150–175 °C for several hours (e.g., 6–12 hours) to precipitate fine Mg2Si particles and develop peak hardness and strength. T5 is achieved by direct artificial aging after casting cooling without a full solution treatment; it gives moderate strength and is useful when distortion control is critical. T7 or overaging treatments at higher temperatures reduce peak strength but improve dimensional and thermal stability and increase resistance to thermal embrittlement. Quench sensitivity, section size effects and porosity all modify achievable hardness and mechanical response.

High-Temperature Performance

356 loses significant strength above approximately 150–200 °C as precipitates coarsen and the Mg2Si precipitate structure dissolves or spheroidizes; long-term use above ~150 °C will cause softening and dimensional change for T6 temper. Oxidation in air at typical service temperatures is minor due to protective Al2O3 formation, but elevated temperatures accelerate diffusion processes that degrade precipitate structure. The HAZ of welded components experiences localized softening and coarsening of microstructure; thermal cycling can exacerbate fatigue initiation at HAZ and porosity sites.

For high-temperature or thermally cycled applications, select T7 or stabilized tempers, use coatings or thermal barriers where oxidation or galvanic effects are problematic, and design to limit sustained exposure above recommended service temperatures to preserve mechanical integrity.

Applications

Industry	Example Component	Why 356 Is Used
Automotive	Brake calipers, wheel components, transmission housings	Good castability, thermal stability and acceptable strength after T6
Marine	Pump housings, gear housings	Corrosion resistance in atmospheric/mild saltwater, ease of casting complex shapes
Aerospace	Non-critical fittings, fairings, housings	Weight saving and castability for complex geometries with good mechanical properties
Electronics	Heat sinks and housings	Thermal conductivity and ability to cast complex cooling geometries
Industrial Machinery	Pump and compressor casings	Dimensional stability, wear resistance and fatigue performance in cast form

356 is chosen for components where the combination of good fluidity, dimensional accuracy, age-hardenable strength and corrosion resistance outweighs the limitations in wrought formability. Its ability to be cast into complex shapes with relatively low defect rates and to accept subsequent heat treatment makes it versatile for many mid-to-high-volume applications.

Selection Insights

Use 356 when castability, age-hardened strength and thermal performance are primary requirements and when complex geometries are best produced in a single cast operation. Choose T6 for maximum strength and stiffness when post-treatment distortion is controllable, and choose T5/T7/O when formability, dimensional stability or thermal stability is more important.

Compared with commercially pure aluminum (e.g., 1100), 356 trades electrical conductivity and superior forming for much higher strength and better casting behavior; choose 1100 when forming and conductivity dominate the design. Compared with common work-hardened alloys (e.g., 3003 / 5052), 356 delivers higher age-hardened strength at the expense of room-temperature formability and generally similar or slightly better corrosion resistance in many environments. Compared with common heat-treatable wrought alloys (e.g., 6061 / 6063), 356 provides superior castability and often better dimensional accuracy for complex cast shapes while offering competitive strength for cast components; select 6061 when wrought fabrication or higher fatigue-critical strength in drawn/extruded forms is required.

Closing Summary

A356 (356) remains a staple aluminum casting alloy because it balances excellent castability, predictable age-hardening response, good corrosion resistance and favorable thermal properties, making it a pragmatic choice for automotive, aerospace, marine and thermal-management components where complex shapes and reasonable structural performance are required.