Aluminum A356: Composition, Properties, Temper Guide & Applications

Comprehensive Overview

A356 is a cast Al-Si-Mg alloy in the 3xx.x family of aluminum casting alloys, typically referred to as AlSi7Mg in international nomenclature. It belongs to the Al–Si–Mg series where silicon is the principal alloying element (providing castability and fluidity) and magnesium enables precipitation strengthening through Mg2Si formation during heat treatment.

This alloy is heat-treatable and gains most of its strength via solution treatment, quenching and artificial aging (T5/T6 variants), though it may also be supplied in as-cast and stress-relieved conditions where ductility is prioritized. Key traits include good casting fluidity, moderate to high strength after aging, reasonable corrosion resistance for many environments, and fair weldability when properly prepared; however, formability is limited compared with wrought alloys and it is primarily used as a casting alloy.

Typical industries using A356 include automotive (wheels, structural castings), aerospace and defense (machined castings and fittings), consumer goods (compressor housings, pump bodies), and electronics (enclosures and heat-dissipating castings). Engineers choose A356 when a balance of lightweight, good castability, and age-hardenable mechanical properties is required and when complex shapes are more economically produced by casting than by wrought processing.

Temper Variants

Temper	Strength Level	Elongation	Formability	Weldability	Notes
O	Low	High	Excellent (for cast parts)	Excellent	Fully annealed / overaged; highest ductility and stress relief for machining.
T5	Moderate	Moderate	Fair	Good	Cooled from casting and artificially aged; practical for as-cast components.
T6	High	Low–Moderate	Limited	Good (with care)	Solution treated, quenched and artificially aged; highest strength for A356.
T651	High	Low–Moderate	Limited	Good (with care)	T6 plus stress-relief by stretching or vibration; reduces distortion in machining.
H14 (light strain-hardened)	Low–Moderate	Moderate	Moderate	Good	Cold worked slightly; uncommon for cast-only shapes but applicable to wrought forms.

Temper selection critically changes the strength–ductility balance and dimensional stability of A356 castings. O and overaged conditions maximize machinability and elongation at the expense of strength, while T5/T6/T651 transform the alloy microstructure by precipitating Mg2Si and clustering silicon morphology to raise yield and tensile strengths, often reducing elongation and increasing the risk of cracking under high constraint.

Chemical Composition

Element	% Range	Notes
Si	6.5–7.5	Principal alloying element; improves fluidity, reduces shrinkage, and modifies strength.
Fe	≤0.20–0.35 (typical spec-dependent)	Impurity element that forms brittle intermetallics; controlled to minimize porosity and hot tearing.
Mn	≤0.10	Limits iron intermetallic morphology; small additions improve toughness.
Mg	0.20–0.45	Provides age-hardening via Mg2Si precipitates; key for T6 response.
Cu	≤0.20	Small additions can raise strength but may reduce corrosion resistance and increase hot-cracking tendency.
Zn	≤0.10	Generally kept low; contributes little to strengthening here.
Cr	≤0.10	Controls grain structure and improves high-temperature stability marginally.
Ti	≤0.20	Grain refiner in castings; improves as-cast microstructure and feeding.
Others	≤0.05 each, ≤0.15 total	Trace elements and impurities; limits exist to ensure predictable casting and mechanical behavior.

The chemistry of A356 is optimized to balance castability and heat-treat response. Silicon sets the eutectic and controls solidification characteristics while magnesium level determines the volume fraction and distribution of Mg2Si precipitates that give A356 its age-hardening capability; tight control of iron and trace elements is essential to avoid deleterious intermetallics that impair ductility and fatigue performance.

Mechanical Properties

In the annealed (O) condition A356 displays relatively low tensile strength with high elongation due to a spheroidized silicon morphology and minimal precipitation hardening. After solution treatment and artificial aging (T6), tensile and yield strengths increase significantly because of finely dispersed Mg2Si precipitates and a refined silicon particle distribution, but ductility correspondingly decreases. Fatigue performance is sensitive to casting defects (porosity, shrinkage) and surface condition; shot-peening and hot isostatic pressing (HIP) are common methods to improve fatigue life in structural castings.

Thickness and cooling rate during casting influence the as-cast microstructure: thicker sections solidify more slowly, resulting in coarser silicon particles and reduced strength compared with thin-walled castings. Hardness correlates with temper state and is often used as a quick process control; typical Brinell hardness rises from low values in O to significantly higher values in T6. Thermal exposure near or above aging temperatures will change the precipitate state and can either overage (soften) or further age the alloy depending on time and temperature history.

Property	O/Annealed	Key Temper (T6 / T651)	Notes
Tensile Strength	90–160 MPa (typ.)	230–320 MPa (typ.)	Wide ranges reflect section thickness, casting method and porosity levels.
Yield Strength	35–80 MPa (typ.)	140–240 MPa (typ.)	Yield rises substantially after solution and aging; T651 improves dimensional stability.
Elongation	10–30% (typ.)	2–10% (typ.)	Ductility decreases with increasing strength; elongation depends on defect population.
Hardness (HB)	30–50 HB	70–100 HB	Hardness used for QA; correlates with age-hardening and microstructural scale.

Physical Properties

Property	Value	Notes
Density	2.68 g/cm³	Typical for Al–Si casting alloys; lighter than steel and many other metals.
Melting Range	~557–640 °C	Eutectic/mushy solidification range influenced by Si content and casting cooling rate.
Thermal Conductivity	~120–150 W/(m·K)	Lower than pure Al due to silicon and intermetallics; still good for heat-dissipation parts.
Electrical Conductivity	~30–40 % IACS	Reduced compared with purer aluminum alloys because of Si and other solutes.
Specific Heat	~0.88–0.90 J/(g·K)	Typical of aluminium alloys; useful for thermal design in electronics and heat sink components.
Thermal Expansion	21–24 µm/(m·K)	Moderate coefficient; important for mating to steels or composites in assemblies.

A356 offers a favorable combination of low density and decent thermal conductivity, making it attractive for lightweight structural and thermal management applications. The presence of silicon lowers overall electrical and thermal conductivity compared with pure aluminum but retains sufficient conductivity for many heat-sinking and electronic enclosure uses while offering superior castability.

Product Forms

Form	Typical Thickness/Size	Strength Behavior	Common Tempers	Notes
Casting (Sand)	Sections from a few mm up to several hundred mm	Strength varies strongly with section size	O, T5, T6, T651	Widely used for prototype and low-volume parts; slower cooling → coarser microstructure.
Casting (Permanent Mold / Die)	Thin to moderate sections (≤100 mm)	Higher as-cast strength due to faster cooling	T5, T6	Better surface finish and dimensional control; common for wheels, housings.
Extrusion	Limited / not typical	N/A for standard extrusion practice	H-temper variants if produced	A356 is not a primary extrusion alloy; extruded AlSi alloys exist but are less common.
Tube	Cast and fabricated tubes	Variable; depends on forming/processing	O, T5	Specialized near-net-shape cast tubes or flow-formed components possible.
Bar/Rod/Billet	Forged or cast billets for machining	Machinable; properties from subsequent heat treatment	O, T6 (after solution/age)	Used as feedstock for CNC-machined components from cast billets or forged blanks.

A356 is primarily a casting alloy; the production route (sand, permanent mold, high-pressure die) strongly influences microstructure and resultant mechanical behavior. Post-casting heat treatment and stress-relief procedures further control the final properties and dimensional stability, and choices between casting processes rest on volume, tolerances, surface finish, and thermal history considerations.

Equivalent Grades

Standard	Grade	Region	Notes
AA	A356 / A356.0	USA	Common Aluminum Association designation for cast AlSi7Mg-type alloy.
EN AW	AlSi7Mg0.3 (≈ EN AW-226)	Europe	Standardized European designation approximating A356 chemistry and performance.
JIS	AC-AlSi7Mg (approx.)	Japan	Japanese casting alloy equivalents share similar Si–Mg levels but differ in allowed impurities.
GB/T	AlSi7Mg or A356 (approx.)	China	Chinese standards use similar composition windows; foundry practice and impurity limits may vary.

Equivalent grades across standards are broadly interchangeable for many applications, but subtle differences in allowable impurity levels (notably iron and copper) and the precise Mg range influence aging response and casting behavior. Buyers should compare certified chemical and mechanical data sheets and, if critical, require sample testing or processing trials because casting techniques and quality control can create larger variability than nominal grade differences.

Corrosion Resistance

A356 displays generally good atmospheric corrosion resistance due to the naturally forming Al2O3 film, and it performs satisfactorily in urban and industrial atmospheres provided the surface is maintained and chloride exposure is limited. In marine and chloride-rich environments, pitting and crevice corrosion can occur preferentially at silicon-rich phases or casting defects, so protective coatings, anodizing or cathodic protection are often employed for long-term service.

Stress corrosion cracking is less common in A356 than in high-strength Al–Cu alloys, but susceptibility increases with higher strength tempers, elevated residual tensile stresses, and the presence of microstructural defects; designers should avoid tensile over-stressing and consider post-heat-treatment stress relief (T651). Galvanic interactions with more noble materials (stainless steels, copper) will drive aluminum corrosion in the anodic role; insulating layers or sacrificial anodes are typical mitigation strategies.

Compared with 5xxx (Al–Mg) alloys, A356 has similar overall corrosion resistance in many environments but will generally perform worse than highly alloyed, anodizable 6xxx series in aggressive chloride environments; selection should be driven by required mechanical properties, exposure conditions, and availability of coatings or post-processing.

Fabrication Properties

Weldability

A356 can be welded using conventional processes such as TIG and MIG though welding cast A356 entails attention to porosity and hot-cracking risks. Preheating and the use of matching filler alloys (e.g., Al-Si fillers such as 4043 or Al-Mg-Si fillers like 5356 in specific cases) reduce hydrogen porosity and thermal mismatch; post-weld heat treatment is often required to restore age-hardening. Heat-affected zones (HAZ) experience localized softening and require process controls to avoid distortion and property degradation in critical sections.

Machinability

As-cast A356 has good machinability for a casting alloy, particularly in the O or semi-annealed conditions; carbide tooling and moderate feeds/speeds are recommended to handle hard silicon particles. Tool wear is driven by abrasive silicon and intermetallics, so tool geometry emphasizing positive rake and coolant use is beneficial; interrupted cuts should be minimized where possible and chip evacuation managed to prevent surface damage. Machined surfaces and tolerances are improved when using permanent-mold or die-cast feedstock due to finer microstructures.

Formability

Cold formability of A356 is limited compared with wrought aluminum alloys; bending and stamping are rarely applied to as-cast parts except for thin permanent-mold castings. Best forming behavior is achieved in overaged or O conditions, but designers generally prefer to create geometry via casting rather than post-casting forming. When some forming is required, local heating or solutionizing prior to forming, followed by appropriate aging, can enable limited shaping while preserving strength after re-aging.

Heat Treatment Behavior

A356 is a heat-treatable alloy that responds to solution treatment followed by quenching and artificial aging to achieve the T6 condition. Typical solution treatment occurs near 525–540 °C to dissolve Mg and create a supersaturated solid solution; rapid quenching minimizes precipitate formation during cool-down, and subsequent artificial aging at ~150–180 °C for several hours precipitates fine Mg2Si particles to raise strength. T5 is a shorter artificial aging applied to castings not solution treated; it provides moderate strength improvements without full solutionizing.

Overaging, prolonged exposure to elevated temperatures, or inadequate quench rates will coarsen precipitates and lower strength, so process control is critical. For non-heat-treatable behavior (relevant to non-standard batches or certain wrought modifications), strengthening occurs via work hardening and cold deformation, while annealing or full solution treatments are used to recover ductility and relieve stresses before final machining.

High-Temperature Performance

A356 experiences progressive loss of strength above typical aging temperatures; service temperatures above ~150 °C will reduce the efficacy of the Mg2Si precipitate structure and cause softening over time. Elevated-temperature creep is limited compared with high-temperature alloys, so A356 is generally restricted to moderate-temperature applications or intermittent thermal exposure; design factors must account for long-term exposure and potential overaging. Oxidation is minimal at normal operating temperatures because of the protective alumina film, but prolonged exposure at high temperatures accelerates microstructural coarsening and may promote embrittlement of intermetallic phases in the casting.

Applications

Industry	Example Component	Why A356 Is Used
Automotive	Wheels, transmission housings, engine crankcase covers	Good castability, weight savings, and age-hardenable strength for structural castings.
Aerospace	Structural castings, brackets, fittings	Favorable strength-to-weight ratio and ability to produce complex near-net-shape parts.
Marine	Hull fittings, pump housings, outboard components	Reasonable corrosion resistance and capability to cast complex shapes resistant to seawater with protective finishes.
Electronics	Enclosures, heat-sink housings	Combination of thermal conductivity, castability, and machinability for thermal-management components.

A356 is commonly selected when designers need to produce complex geometries with good mechanical performance after heat treatment, while minimizing post-machining and assembly work. Its balance of castability, machinability and age-hardening response allows cost-effective production of medium-strength cast components across multiple industries.

Selection Insights

Choose A356 when complex or thin-walled cast shapes are required with post-processable mechanical performance, and when weight reduction is important but the highest wrought alloy strengths are not needed. It is particularly attractive for components that benefit from good castability and moderate age-hardening (wheels, housings, fittings).

Compared with commercially pure aluminum (1100), A356 trades electrical and thermal conductivity and inherent formability for much higher post-heat-treatment strength and dimensional stability in cast forms. Compared with common work-hardened alloys (3003, 5052), A356 provides higher age-hardened strength but typically lower ductility and similar or slightly reduced corrosion resistance in chloride-bearing environments. Compared with commonly used heat-treatable wrought alloys (6061, 6063), A356 can be preferred when complex cast geometries and superior casting economics outweigh the higher peak strength and better weldability of those wrought alloys.

Closing Summary

A356 remains a workhorse casting alloy for engineers who require a practical combination of castability, low density, and an effective age-hardening response, making it especially valuable in automotive, aerospace, marine and thermal applications where complex shapes and moderate-to-high strength are needed at reasonable cost.