Aluminum A357: Composition, Properties, Temper Guide & Applications

Comprehensive Overview

A357 is a heat-treatable aluminum-silicon-magnesium casting alloy commonly specified as AlSi7Mg in European notation and as AA A357 in ASTM/ASME listings. It belongs to the family of Al–Si–Mg casting alloys (often grouped conceptually with the 3xx/4xx series of wrought alloys for alloying character, but formally identified as a casting alloy), where silicon is the principal alloying element with magnesium added to enable precipitation strengthening.

Strengthening of A357 is achieved primarily through solution heat treatment followed by quenching and artificial aging (precipitation hardening) to produce Mg2Si precipitates; some property tailoring is also possible via modification (Sr, Na) and grain refinement (Ti, B). Key traits include a favorable strength-to-weight ratio in T6/T651 tempers, good castability and dimensional stability, moderate corrosion resistance in atmospheric environments, and generally acceptable weldability using appropriate filler metals; formability is limited relative to wrought alloys in peak-aged conditions.

Typical industries include automotive powertrain and structural castings, aerospace secondary structural components and brackets, general industrial castings, and some marine and consumer products where cast components are preferred. Engineers choose A357 for complex-shape castings that require a balance of high static strength, reasonable fatigue behavior, and good cast surface finish, and when heat-treatable responses (T6/T651) are needed without the increased copper or zinc of high-strength aerospace alloys.

Temper Variants

Temper	Strength Level	Elongation	Formability	Weldability	Notes
O	Low	High	Excellent	Excellent	Fully annealed, optimum for forming and machining.
T4	Medium	Medium-High	Good	Good	Solution heat-treated and naturally aged; intermediate properties.
T5	Medium-High	Medium	Fair	Good	Cooled from casting and artificially aged; used when solution treatment is not applied.
T6	High	Low-Medium	Limited	Good (with post-heat treat)	Solution treated, quenched and artificially aged for peak strength.
T651	High	Low-Medium	Limited	Good (with post-heat treat)	T6 with stress-relief by stretching; common for dimensionally stable castings.
F	Variable	Variable	Variable	Variable	As fabricated, properties depend on subsequent processing; not standardized.

Temper has a first-order influence on mechanical performance and processability because the solution/aging cycle precipitates Mg2Si to raise strength while reducing ductility. The fully annealed (O) condition optimizes ductility and ease of forming or machining, whereas T6/T651 conditions maximize tensile and yield strength at the expense of elongation and formability; welding typically requires local re-aging or post-weld heat treatment to recover properties.

Chemical Composition

Element	% Range	Notes
Si	6.5–7.5	Major alloying element for castability, fluidity and strength through eutectic silicon.
Fe	≤0.20–0.30	Impurity that forms intermetallics (Fe-rich phases) which may embrittle and reduce ductility.
Mn	≤0.10	Minor; helps modify Fe-intermetallic morphology when present.
Mg	0.35–0.60	Strengthenability via Mg2Si precipitation; controls response to heat treatment.
Cu	≤0.20	Usually low; raises strength but can reduce corrosion resistance and increase SCC risk.
Zn	≤0.10	Typically residual; limited strengthening effect at these levels.
Cr	≤0.10	Controls grain structure and can limit grain growth during processing.
Ti	0.02–0.10	Used for grain refinement during solidification (Ti-B systems common).
Others (each)	≤0.05–0.15	Residuals and intentional modifiers (Sr for silicon modification, Sr ~0.01).

The alloy chemistry is optimized to balance castability, heat-treat response and corrosion performance. Silicon sets the eutectic structure and promotes fluidity, magnesium provides the basis for precipitation strengthening, and low copper/iron keeps corrosion susceptibility and intermetallic embrittlement to manageable levels.

Mechanical Properties

A357 shows substantial variation in tensile and yield behavior driven by temper and casting method. In T6/T651 conditions the alloy attains relatively high ultimate tensile strength and yield for a cast Al–Si–Mg material due to a fine dispersion of Mg2Si precipitates and refined eutectic silicon, while the annealed condition exhibits much higher elongation and lower yield. Hardness tracks the tensile strength and rises markedly with aging; Brinell or Vickers hardness values increase from soft, easily machined levels in O to much higher levels in T6.

Fatigue behavior of A357 is generally superior to more brittle hypereutectic aluminum-silicon castings because the controlled silicon morphology and heat treatment reduce crack initiation sites; nevertheless, fatigue life is sensitive to casting defects, porosity and surface finish. Thickness and section size influence cooling rate during solidification which affects microstructure and hence mechanical properties; thicker sections cool slower, promoting coarser silicon and lower achievable strength after heat treatment.

Surface conditions, post-casting modification, and porosity mitigation techniques (vacuum-assisted casting, degassing and proper gating) directly improve mechanical consistency and fatigue performance in structural components.

Property	O/Annealed	Key Temper (T6/T651)	Notes
Tensile Strength (MPa)	140–190	260–320	Wide range depending on casting method and section thickness.
Yield Strength (MPa)	60–110	200–260	Yield increases substantially after solution treatment and aging.
Elongation (%)	10–18	4–8	Ductility reduced in peak-aged conditions; fracture mode usually ductile-brittle mix.
Hardness (HB)	40–70	85–120	Brinell hardness increases with aging and finer eutectic silicon morphology.

Physical Properties

Property	Value	Notes
Density	2.65–2.68 g/cm³	Typical for Al–Si casting alloys; useful for mass/weight calculations.
Melting Range	~560–635 °C	Solidus and liquidus depend on exact Si and modifier levels; eutectic influences freezing range.
Thermal Conductivity	120–150 W/m·K	Lower than pure Al due to alloying and silicon; still good for heat-sinking compared with steels.
Electrical Conductivity	~30–40 % IACS	Reduced compared with pure aluminum; conductivity declines with cold work and alloying.
Specific Heat	~0.89 kJ/kg·K	Typical for aluminum alloys, used in thermal calculations.
Thermal Expansion	22–24 µm/m·K	Coefficient of linear thermal expansion similar to other Al–Si alloys; important for mating dissimilar materials.

The physical properties make A357 attractive where a low-density, thermally conductive metal is required but full electrical conductivity of pure aluminum is not essential. Thermal expansion and conductivity data are critical when designing assemblies with steels, composites or coatings, since differential expansion can lead to stress or seal failure.

Product Forms

Form	Typical Thickness/Size	Strength Behavior	Common Tempers	Notes
Sheet	Rare; thin castings 1–6 mm	Variable; typically lower than wrought sheet	O, T5	Limited availability; used for specialized cast sheet processes.
Plate	6–100 mm (cast plate segments)	Section-dependent properties	O, T6/T651	Plate-like castings show reduced properties in thick sections due to coarsening.
Extrusion	Not common	Not applicable	—	A357 is not typically used for extrusion; composition and casting focus make it unsuitable.
Tube	Cast or machined from billets; sizes variable	Dependent on casting and heat treat	O, T6	Cast tubes are less common than wrought tubing; used for complex cross-sections.
Bar/Rod	Cast billets and forgings	Variable; heat treatable	O, T6	Often produced as ingots or billets for subsequent machining into components.

A357 is primarily a casting alloy, and most commercial forms are sand, permanent mold, or investment castings and ingots/billets. Differences in processing (e.g., permanent mold versus sand casting) change cooling rates and thus microstructure and final mechanical properties; designers must match casting method, section thickness and temper to the intended load and fatigue environment.

Equivalent Grades

Standard	Grade	Region	Notes
AA	A357	USA	Common ASTM designation for castings; used in aerospace and automotive specs.
EN AW	AlSi7Mg	Europe	General European equivalent; composition tolerances and heat-treatment procedures may differ.
JIS	ADC10/ADC12 (approx)	Japan	ADC series are die-casting alloys similar in Si content; ADC12 has higher Cu and different properties.
GB/T	AlSi7Mg (or A357 analog)	China	Local standards mirror EN/ASTM but chemical and mechanical tolerances can vary by producer.

Equivalency is approximate because casting practices, impurity limits and heat-treatment protocols differ by region and standard body. Engineers should cross-check mechanical property data and heat-treatment instructions when substituting between standards to ensure functional equivalence for critical components.

Corrosion Resistance

A357 generally exhibits good atmospheric corrosion resistance for an aluminum casting alloy because the silicon-rich matrix and low copper content reduce galvanic tendencies in typical environments. The naturally forming aluminum oxide film provides baseline protection, but localized corrosion can initiate at intermetallic particles or casting defects where the passive film is disrupted.

In marine or chloride-containing environments A357 performs moderately well but is not as robust as specialized marine alloys (e.g., Al–Mg 5xxx series); prolonged exposure to salt spray or splash zones requires protective coatings, anodizing or cathodic isolation to avoid pitting. Stress corrosion cracking susceptibility is relatively low due to the low copper content and moderate Mg levels, although high tensile stresses combined with aggressive environments can still produce SCC in critical applications.

Galvanic interactions require attention: when mated to more noble metals (e.g., stainless steel, copper alloys), A357 will be anodic and corrode preferentially if exposed to an electrolyte; insulating materials or protective coatings are commonly applied to avoid accelerated corrosion. Compared with wrought 6xxx series alloys, A357 offers similar corrosion resistance but with the caveat that casting-related porosity or intermetallic distributions can localize attack.

Fabrication Properties

Weldability

A357 can be welded using common fusion techniques such as TIG (GTAW) and MIG (GMAW) with appropriate filler metals; for Al–Si casting alloys ER4043 (Al–5Si) is a widely used filler because it promotes silicon compatibility and reduces hot-cracking. Hot-cracking risk is moderate in cast alloys due to constrained solidification and coarse eutectic structures, so pre-weld cleaning, good joint design, and control of heat input are essential. Heat-affected zone (HAZ) softening is expected in T6 material since local heating dissolves precipitates; post-weld solutionizing and aging or local post-weld artificial aging is recommended to recover mechanical properties as required.

Machinability

Machinability of A357 is generally good compared with many high-silicon casting alloys because its silicon content and modified eutectic morphology reduce tool wear relative to hypereutectic alloys. Carbide tooling with positive rake geometry and high-speed machining parameters yield good productivity; flood coolant or mist lubrication improves chip evacuation and tool life. Machining large sections of T6 material will require consideration of hardness and chip control; pockets and thin webs should be designed to avoid chatter and distortion.

Formability

Cold formability is limited in peak-aged tempers (T6/T651) because of reduced ductility, while the annealed (O) or solution-treated (T4) conditions offer substantially better bendability and stretch formability. Typical recommended minimum bend radii depend on thickness and temper but are generally larger than those for ductile wrought alloys; designers commonly specify O-temper forming followed by final heat treatment to achieve geometry and mechanical requirements. Incremental forming processes for cast parts are possible but require careful control of heat and residual stress.

Heat Treatment Behavior

A357 is a heat-treatable cast alloy and responds to conventional Al–Si–Mg thermal cycles used to produce T6/T651 tempers. Solution treatment is typically performed at temperatures around 520–540 °C for sufficient time to dissolve Mg and partially modify silicon networks; time depends on section thickness and must balance homogenization against incipient melting of low-melting constituents. Rapid quenching to room temperature traps solute in supersaturated solid solution and sets the stage for artificial aging at 150–200 °C to precipitate fine Mg2Si particles and develop target strength levels.

T5 temper is obtained by artificial aging after cooling from casting; it is used when full solution treatment is impractical. T651 adds a stress-relief stretch after quenching to minimize residual stresses and improve dimensional stability, which is important for die-cast or high-precision castings. Overaging at higher temperatures or excessive aging times coarsens precipitates and reduces peak strength while improving ductility.

High-Temperature Performance

A357’s mechanical properties degrade progressively with increasing temperature due to precipitate coarsening and reduced solute-strengthening effectiveness; useful static strength is typically maintained up to about 125–150 °C, with significant softening above this range. Creep resistance at elevated temperatures is modest and inferior to specialized high-temperature aluminum or nickel alloys, so A357 is not recommended for long-term load-bearing applications above ~150 °C.

At elevated temperatures oxidation is limited to formation of a stable alumina film, but surface scaling and interactions with aggressive atmospheres can be an issue for long durations. Welding or localized thermal cycles can further alter the microstructure in the HAZ and adjacent regions, producing zones with reduced strength and increased susceptibility to creep or fatigue at elevated service temperatures.

Applications

Industry	Example Component	Why A357 Is Used
Automotive	Transmission housings, pump housings, structural brackets	Good castability, high strength after T6, and cost-effective production for complex shapes.
Marine	Gearbox housings, pump components	Reasonable corrosion resistance and good strength-to-weight for wet environments with coatings.
Aerospace	Fittings, brackets, non-critical structural housings	Heat-treatable strength and dimensional stability in T651 for less-critical structural elements.
Electronics	Heat sinks and enclosures	Thermal conductivity and cast-form geometry for integrated thermal management.

A357 is favored where the geometry of the part benefits from casting, where T6-level static properties are required, and where weight savings and thermal performance contribute to system-level advantages. Proper design for casting quality and post-cast processing ensures consistent performance across these industries.

Selection Insights

When selecting A357, consider it primarily when a cast component requires heat-treatable strength combined with reasonable corrosion resistance and good castability; it is a good choice for medium-strength, complex-shape parts that benefit from T6/T651 aging and dimensional stability. For applications prioritizing ductility and forming, specify O or T4 tempers or choose a wrought alloy instead; for long-term elevated-temperature service or extreme fatigue regimes, consider alternatives.

Compared with commercially pure aluminum (1100), A357 trades off electrical and thermal conductivity and superior formability for a much higher strength and better dimensional stability after heat treatment. Compared with common work-hardened alloys such as 3003 or 5052, A357 provides substantially higher peak strength when aged but may have comparable or slightly lower corrosion resistance in chloride environments; use A357 when casting complexity and strength are more important than extensive cold forming. Compared with ubiquitous heat-treatable wrought alloys like 6061/6063, A357 offers better castability and comparable precipitate-strengthening mechanisms; A357 is preferred when complex cast geometry and lower density are required despite slightly lower peak strength than some wrought 6xxx alloys.

Closing Summary

A357 remains a relevant and widely used cast aluminum alloy because it combines excellent castability with a robust heat-treatable response that yields high static strength, reasonable fatigue performance and acceptable corrosion behavior for many structural and mechanical components. Proper selection of casting method, temper and post-process treatments enables designers to exploit its strengths while managing limitations in formability and elevated-temperature performance.