Aluminum 357: Composition, Properties, Temper Guide & Applications

Comprehensive Overview

357 (commonly referenced as A357 or AlSi7Mg variants) is a 3xx series aluminum-silicon-magnesium alloy that sits in the family of heat-treatable casting alloys. Its principal alloying elements are silicon and magnesium, with silicon providing castability and wear resistance and magnesium enabling age-hardening through Mg2Si precipitation.

The alloy is principally strengthened by solution heat treatment followed by artificial aging (T6/T651), which produces fine Mg2Si precipitates; it also shows some strain hardening when cold worked in certain forms. Key traits include a favorable combination of moderate-to-high tensile strength, good ductility for a casting alloy, enhanced corrosion resistance relative to many copper-bearing alloys, and acceptable weldability when proper practices and filler metals are used.

Typical industries using 357 include automotive (structural castings, suspension components, wheels), aerospace (fittings and housings), motorsports, and high-performance marine applications where a balance of light weight, strength, and corrosion resistance is required. Engineers select 357 over other alloys when detailed cast geometries require the Si-enhanced fluidity and the alloy's heat-treatable peak strengths are needed without the cracking sensitivity of higher-copper alloys.

Temper Variants

Temper	Strength Level	Elongation	Formability	Weldability	Notes
O	Low	High	Excellent	Excellent	Fully annealed; used for stress relief and machining prior to heat treatment
H14	Medium	Medium	Fair	Good	Strain-hardened in wrought forms; limited for castings
T5	Medium-High	Medium	Fair	Good	Cooled from hot working and artificially aged; faster production route for castings
T6	High	Low–Medium	Fair	Good	Solution heat treated + artificial aging; peak-strength condition for many cast components
T651	High	Low–Medium	Fair	Good	T6 with stress-relief/stretch countering residual stresses; commonly specified for critical aerospace castings

The temper chosen for 357 strongly governs strength, ductility and residual stress state. T6/T651 conditions maximize tensile strength and hardness via Mg2Si precipitation, but reduce elongation and formability compared with annealed O condition.

For manufacturing, O and T5 allow easier machining and forming prior to final aging, while T6 and T651 are used for service components where dimensional stability and peak mechanical performance are required.

Chemical Composition

Element	% Range	Notes
Si	6.5–7.5	Primary alloying element; improves fluidity, reduces shrinkage, and increases wear resistance
Fe	0.2–0.6	Impurity from melting; high Fe leads to brittle intermetallics and reduced ductility
Mn	0.05–0.35	Controls Fe intermetallic morphology and can slightly improve strength
Mg	0.25–0.45	Age-hardening element forming Mg2Si precipitates; controls T6 response
Cu	0.15–0.6	Often limited in casting grades; increases strength but reduces corrosion resistance if high
Zn	0.05–0.2	Minor impurity; generally not a deliberate strengthening addition
Cr	0.02–0.2	Used in trace amounts to control grain structure and recrystallization in some variants
Ti	0.02–0.15	Grain refiner to improve casting structure and mechanical uniformity
Others	≤0.15 total	Trace elements and residuals; kept low to preserve corrosion and mechanical properties

Silicon and magnesium are the functional pair controlling castability and heat-treat response. Silicon forms the eutectic structures that set solidification behavior while magnesium dissolves in the Al matrix and precipitates as Mg2Si during artificial aging to raise tensile and yield strengths.

Mechanical Properties

As a casting alloy subjected to T6 heat treatment, 357 exhibits tensile strengths and yields substantially higher than typical wrought non-heat-treatable alloys, with moderate ductility for a cast component. The tensile curve is characterized by a distinct yield point followed by work hardening to UTS; elongation in T6 is typically constrained relative to annealed material but remains adequate for many structural cast parts. Hardness increases significantly with T6/T651 treatment due to fine dispersion of Mg2Si precipitates, and Brinell or Vickers hardness correlates well with tensile properties for specification purposes.

Fatigue behavior of 357 is influenced by casting defects (porosity, shrinkage) and microstructure; denser castings and proper gating minimize defect-related fatigue initiation. Thickness effects are pronounced because larger cross-sections cool slower, coarsening eutectic Si and increasing the risk of porosity, which reduces both static and fatigue strength.

For thin sections and rapidly cooled castings, T6 properties approach the higher end of the ranges; for thick sections and as-cast O condition, strength and hardness drop and ductility increases.

Property	O/Annealed	Key Temper (T6/T651)	Notes
Tensile Strength (UTS)	130–220 MPa	300–360 MPa	T6 values depend on section thickness and solidification rate
Yield Strength (0.2%YS)	60–150 MPa	240–300 MPa	Yield rises sharply after solution + aging
Elongation (El%)	10–18%	4–10%	Elongation drops in T6; geometry and porosity affect values
Hardness (HB)	50–90 HB	90–130 HB	Hardness tracks aging response and section cooling history

Physical Properties

Property	Value	Notes
Density	2.67–2.68 g/cm³	Typical for Al-Si-Mg casting alloys; good strength-to-weight ratio
Melting Range (solidus–liquidus)	~520–615 °C	Eutectic and primary silicon shift solidification range; values depend on exact composition
Thermal Conductivity	110–140 W/m·K	Lower than pure Al but high compared with many engineering alloys
Electrical Conductivity	~30–40 % IACS (~17–23 MS/m)	Reduced by alloying elements; acceptable for thermal/electrical components with design allowances
Specific Heat	~0.90 J/g·K (900 J/kg·K)	Typical of aluminum alloys at room temperature
Thermal Expansion	21–24 µm/m·K	Relatively high thermal expansion requires careful mating with dissimilar materials

The combination of relatively high thermal conductivity and low density makes 357 suitable for components where heat dissipation and light weight are required, although its conductivity is reduced compared with pure aluminum. Thermal expansion and dimensional stability under temperature cycles must be addressed in assemblies with dissimilar metals to avoid galvanic or mechanical stress.

Section thickness and porosity will affect thermal response; denser, fine-grained castings provide more consistent thermal performance and better fatigue life.

Product Forms

Form	Typical Thickness/Size	Strength Behavior	Common Tempers	Notes
Sand Castings	varied (5–200 mm+)	Lower due to slower cooling; coarser microstructure	O, T5, T6	Used for large, complex shapes; may require HIP to reduce porosity
Permanent Mold / Gravity Die	2–60 mm	Better mechanical properties from faster cooling	T5, T6, T651	Preferred for structural parts with tighter tolerances
Investment/Precision Castings	thin sections to moderate	High integrity, good surface finish	T6	Used for aerospace and high-performance components
Forged/Hot-worked (limited)	varies	Not common for 357; properties derive from work + age	H variants	Rare; composition tailored more to cast processing
Ingot / Billet	barges/ingots	Feedstock for downstream casting or extrusion	O prior to processing	Used to produce low-porosity feedstock for specialized castings

Cast product form dominates 357 usage; permanent mold and investment casting produce the best mechanical and fatigue performance due to faster cooling and reduced porosity. Sand castings are economical for large parts but require process controls or secondary treatments (e.g., HIP) to close internal defects. Choice of form and cooling rate directly influences achievable temper response and final microstructure.

Equivalent Grades

Standard	Grade	Region	Notes
AA	357	USA	ASTM/AA designation for Al-Si-Mg casting alloy commonly used in industry
EN AW	AlSi7Mg0.6	Europe	European alloy closest in chemistry and properties; often used as direct equivalent
JIS	AlSi7Mg	Japan	Japanese casting classification for similar Al-Si-Mg compositions
GB/T	AlSi7Mg	China	Chinese standard grade for Al-Si-Mg casting alloys, commonly matched to A357 chemistry

While chemical and mechanical targets are similar across standards, subtle differences in allowable impurity limits (Fe, Cu, Ti) and required mechanical tests can produce variations in performance. European EN grades may specify slightly different Mg or Si minima to meet mechanical property boxes for specific casting processes. Buyers should request specific spec sheets and heat-treatment qualification certificates to ensure interchangeability for safety-critical applications.

Corrosion Resistance

357 provides generally good atmospheric corrosion resistance due to its relatively low copper content and the protective aluminum oxide film. In industrial and rural atmospheres it performs comparably to other Al-Si-Mg casting alloys, and it resists pitting better than high-copper aluminum alloys.

In marine environments, 357 shows satisfactory performance for splash and atmospheric marine exposure, but prolonged immersion in seawater accelerates galvanic and pitting corrosion, especially in areas with deposits or crevices. Metallurgical cleanliness, porosity control, and surface finishes significantly affect marine lifetime; protective coatings and anodizing are common mitigations.

Stress corrosion cracking (SCC) susceptibility in 357 is lower than in certain high-strength Al-Cu alloys, but care is necessary for highly stressed, corrosive-service components. When mated with more noble metals, 357 can suffer galvanic corrosion; electrical isolation or sacrificial anodes are recommended for mixed-metal assemblies.

Fabrication Properties

Weldability

Welding of 357 castings is practicable with TIG and MIG processes using Al-Si filler alloys such as ER4043 or low-silicon ER4047 for minimizing hot-cracking and improving fluidity of the weld pool. Preheating may be used to reduce thermal gradients and minimize porosity; however, the heat-affected zone can suffer partial softening due to overaging or loss of solutionized condition. Proper post-weld heat treatment is often required to restore mechanical properties in critical areas.

Machinability

Machinability of 357 is moderate and better than many high-strength wrought alloys due to its silicon content which promotes chip breaking and dimensional stability. Carbide tooling with positive rake angles and controlled feeds speeds provide best results; high-speed steels struggle with abrasive silicon phases. Machining speeds should be adjusted for section thickness and potential porosity to avoid chatter and surface tearing.

Formability

As a casting alloy, cold formability of 357 is limited compared with wrought aluminum sheet alloys; thin-walled precision castings can accept limited bending and stamping if supplied in O temper. For complex forming, machining or casting with integrated features is typically preferred over post-cast forming. When forming is necessary, anneal (O) or partial solution treatment followed by controlled deformation and final aging can be used in specialized workflows.

Heat Treatment Behavior

Solution treatment for 357 is typically conducted by heating to approximately 500–540 °C (depending on section thickness and exact alloy variant) to dissolve Mg and Si into a solid solution before quenching. Rapid quenching from solution temperature preserves a supersaturated solid solution that is the precursor for artificial aging; quench rate and section thickness control retained solute levels and subsequent precipitation behavior.

Artificial aging is commonly performed at 155–190 °C for times ranging from 4 to 12 hours depending on desired strength vs. ductility trade-offs; T6 conditions aim for the balance of peak strength and acceptable toughness. Overaging or extended high-temperature exposure will coarsen Mg2Si precipitates and reduce strength; T7 conditions may be used when stability at elevated temperature or reduced distortion is required.

Non-heat-treatable strengthening is limited for casting forms; however, targeted cold work in wrought or forged derivatives can increase strength modestly. Annealing to O condition is used to relieve stresses and improve machinability prior to final aging cycles.

High-Temperature Performance

At elevated temperatures above approximately 150–200 °C, 357 begins to lose a significant fraction of its artificial-age strength as Mg2Si precipitates coarsen and dissolve; sustained structural loads above this range are not recommended without alloy-specific qualification. Oxidation of aluminum is self-limiting at typical service temperatures, but prolonged exposure at higher temperatures accelerates degradation and can increase surface roughness and oxide scale formation.

In welded components, the heat-affected zone is especially vulnerable to strength reductions and microstructural changes when exposed to elevated temperatures; post-weld heat treatments help but cannot fully restore properties if service temperatures cause overaging. Design margins and periodic inspection are prudent for components operating near the alloy's temperature limits.

Applications

Industry	Example Component	Why 357 Is Used
Automotive	Structural castings, suspension housings	Good castability, T6 strength, lightweight
Marine	Rudder and strut castings, housings	Corrosion resistance with acceptable strength
Aerospace	Gearbox housings, fittings	High strength-to-weight and dimensional stability (T651)
Electronics	Heat-dissipating housings	Thermal conductivity and light weight

357 is selected where a combination of castability, heat-treatable strength, and corrosion resistance are essential. Its use in load-bearing cast components is predicated on process control to minimize porosity and maximize solution/aging response for consistent mechanical performance.

Selection Insights

Choose 357 when cast geometries and the need for heat-treatable peak strength outweigh the higher conductivity and superior formability of pure aluminum grades. Compared with commercially pure aluminum (1100), 357 trades higher strength and better dimensional stability for reduced electrical conductivity and somewhat lower formability, making it better for structural castings but worse for electrical conductor applications.

Against common work-hardened alloys (3003 / 5052), 357 provides substantially higher achievable strength after T6 aging but is less ductile and less straightforward to cold-form. Corrosion resistance is comparable or slightly better than copper-bearing alloys, but 5052 may be preferred for severe marine sheet applications where forming is required.

Compared with heat-treatable wrought alloys such as 6061/6063, 357 often offers better castability and more complex geometry capability with respectable strength; it is preferred when casting economics and near-final shape production outweigh the slightly higher peak strength and broader fabrication versatility of wrought 6061/6063.

Closing Summary

357 remains relevant because it combines the castability advantages of Al-Si systems with a robust T6 age-hardening response to deliver a high strength-to-weight option for structural castings. When process controls limit porosity and correct heat treatments are applied, 357 provides a cost-effective compromise of strength, corrosion resistance, and manufacturability for automotive, aerospace, marine, and high-performance industrial components.