Aluminum AlF357: Composition, Properties, Temper Guide & Applications

Comprehensive Overview

AlF357 is a heat-treatable, silicon–magnesium aluminum casting alloy (typically grouped with the Al–Si–Mg family and often cross-referenced to A357/AlSi7Mg grades). It is most commonly produced as permanent-mold or pressure-cast components where high casting integrity, elevated strength, and good fatigue performance are required. The major alloying elements are silicon (Si) to provide castability and fluidity, magnesium (Mg) to permit precipitation hardening (Mg2Si), and controlled levels of iron (Fe) and copper (Cu) to tune strength and toughness. Strengthening is primarily through solution treatment followed by quench and artificial aging (T6/T5), with additional response available through modified chemistries and heat‑treatment schedules.

Key traits include relatively high static and fatigue strength for a casting alloy, good dimensional stability after heat treatment, and reasonable corrosion resistance in atmospheric environments. Weldability is workable but requires attention to filler selection and porosity control; formability in the wrought sense is limited because AlF357 is optimized as a casting alloy. Typical industries using AlF357 include automotive (structural castings, wheel components, suspension brackets), aerospace (non-critical structural castings and fittings), marine hardware, and industrial machinery housings. Engineers select AlF357 when a combination of castability, heat-treatable strength, and fatigue resistance provides a better cost-to-performance balance than wrought alloys or cheaper cast aluminum grades.

Temper Variants

Temper	Strength Level	Elongation	Formability	Weldability	Notes
O	Low	High	Poor (brittle vs wrought)	Good	As-cast annealed or stress-relieved; maximum ductility for casting alloys
T5	Medium-High	Moderate	Limited	Good	Cooled from casting and artificially aged; common for as-cast hardening
T6	High	Low-Moderate	Limited	Fair	Solution-treated, quenched and artificially aged; peak strength condition
T7	Medium	Moderate	Limited	Fair	Overaged for improved thermal stability and resistance to stress-corrosion
T651	High	Low-Moderate	Limited	Fair	Solution-treated, stress-relieved by stretching, then aged; dimensional control improved

The temper selection for AlF357 strongly affects performance trade-offs: T6 delivers the highest strength and best fatigue life at the cost of ductility and some machinability, whereas T5 is used when budget or process flow preclude solution treatment. T7 and stabilized tempers are chosen when components must retain properties after exposure to elevated service temperatures or when reduced susceptibility to stress corrosion and dimensional instability are priorities.

Chemical Composition

Element	% Range	Notes
Si	6.5 – 7.5	Primary alloying element for castability and strength; forms eutectic with Al
Fe	0.05 – 0.45	Controlled low iron reduces brittle intermetallics; higher Fe reduces ductility
Mn	0.05 – 0.25	Minor modifier; can refine intermetallic morphology
Mg	0.25 – 0.50	Provides precipitation hardening (Mg2Si) during aging
Cu	0.0 – 0.30	Increases strength and response to ageing but can reduce corrosion resistance
Zn	0.0 – 0.15	Typically residual; negligible strengthening effect here
Cr	0.0 – 0.10	Grain refiner / inhibitor for recrystallization in some melts
Ti	0.02 – 0.15	Added for grain refinement, especially in castings and ingots
Others	Balance Al	Trace elements controlled per casting practice; deliberately low impurity levels improve ductility and fatigue life

Silicon sets the casting characteristics and eutectic structure, while magnesium enables heat-treatable strengthening through Mg2Si precipitates. Low iron and controlled trace elements improve toughness and fatigue resistance by minimizing coarse intermetallic particles and promoting a fine microstructure during solidification and heat treatment.

Mechanical Properties

Tensile behavior of AlF357 is characterized by a marked increase in yield and ultimate strength after proper solution treatment and artificial aging, with T6 conditions producing the highest strengths for this alloy family. Elongation in T6 is reduced relative to the as-cast or annealed state but is still acceptable for many structural castings due to the alloy’s relatively fine eutectic and controlled impurity chemistry. Hardness follows the same trend as tensile properties and is commonly used as a shop-floor check for heat-treatment effectiveness.

Fatigue performance is an important driver for choosing AlF357; the combination of high tensile strength and sound casting practices (low porosity, proper gating/riser design) delivers superior fatigue life compared with standard Al–Si casting grades. Thickness and section size significantly affect both mechanical properties and heat-treatment response; thick sections may not fully homogenize during solution treatment and thus show lower developed strengths and reduced elongation compared with thin sections.

Property	O/Annealed	Key Temper (T6)	Notes
Tensile Strength	150 – 240 MPa	300 – 380 MPa	T6 values depend on section thickness and solution treatment efficacy
Yield Strength	70 – 130 MPa	230 – 300 MPa	Offset yield; pronounced increase after aging
Elongation	8 – 18%	4 – 10%	Elongation decreases with increased strength and section restraint
Hardness	40 – 70 HB	90 – 120 HB	Hardness correlates with the precipitation state and silicon morphology

Physical Properties

Property	Value	Notes
Density	~2.65 g/cm³	Typical for Al–Si cast alloys; good strength-to-weight ratio
Melting Range	~555 – 595 °C	Eutectic and solidification range influenced by Si content
Thermal Conductivity	~120 – 150 W/m·K	Lower than pure Al but suitable for many heat-dissipation applications
Electrical Conductivity	~30 – 45 %IACS	Reduced compared with pure Al due to alloying and silicon content
Specific Heat	~0.90 J/g·K (900 J/kg·K)	Typical aluminum alloy specific heat
Thermal Expansion	~20 – 23 µm/m·K	Coefficient influenced by Si content; important for dimensional design

The physical properties make AlF357 attractive where a combination of moderate thermal conductivity and low density are required. The alloy’s melting and solidification characteristics allow high-quality castings with predictable shrinkage and soundness when proper foundry practices are used.

Product Forms

Form	Typical Thickness/Size	Strength Behavior	Common Tempers	Notes
Sheet	Limited / Thin sections only	Inconsistent — not typical	T5 / As-cast	Not a primary form; wrought processing is uncommon
Plate	Limited	Section-dependent	T5/T6	Thick plate castings can be heat-treated but require long solution cycles
Extrusion	Rare	Not standard for cast chemistry	N/A	AlF357 is not intended for conventional extrusion processes
Tube	Limited (cast or semi-solid)	Dependent on wall thickness	T5/T6	Usually produced as cast-sleeve or machined from billet; not drawn tubing
Bar/Rod	Cast billets / forgings	Can be heat treated to T6	T5/T6	Machinable cast billets and forged shapes available for CNC parts

AlF357 is primarily a casting alloy, and the most common product forms are permanent-mold, die-cast, and sand-cast components or billets for machining. Differences in processing routes (permanent-mold vs. die-cast vs. sand-cast) strongly affect microstructure, porosity levels, and achievable mechanical properties; designers must account for section size, cooling rate, and subsequent heat treatment when specifying component geometry and expected performance.

Equivalent Grades

Standard	Grade	Region	Notes
AA	A357 / AlSi7Mg	USA	Common ASTM/AA casting designation; AlF357 often cross-referenced to this chemistry
EN AW	EN AC-AlSi7Mg	Europe	European casting equivalent under EN 1706 nomenclature
JIS	ADC12 (not exact) / AlSi7Mg	Japan	ADC12 is higher Cu and not a direct match; check JIS specifics for low-Cu variants
GB/T	AlSi7Mg	China	Chinese standard casting grade closely matching A357 chemistry

Subtle differences between standards relate mainly to impurity limits (Fe, Cu, Zn) and tightness of Mg and Si ranges; these affect heat treatment response and long-term properties such as corrosion resistance and fatigue life. When cross-referencing standards, verify the exact composition and any additional quality controls (e.g., iron max, hydrogen porosity limits) applied by suppliers.

Corrosion Resistance

AlF357 offers good general atmospheric corrosion resistance, typical of aluminum–silicon–magnesium cast alloys. The naturally forming oxide layer provides baseline protection, and the alloy's low copper content (when controlled) helps maintain resistance in urban and mildly industrial environments. In marine or chloride-rich environments the alloy is moderately susceptible to pitting and localized attack; surface treatments, anodizing, or proper coatings are recommended for long-term exposure.

Stress corrosion cracking (SCC) is less severe for Al–Si–Mg casting alloys than for some high-strength wrought aluminum–copper alloys, but susceptibility increases with higher tensile stresses and chloride presence. Galvanic interactions are typical of aluminum: AlF357 is anodic relative to stainless steels and copper-based alloys, so electrical isolation or sacrificial anodes should be considered in mixed-metal assemblies. Compared with 5xxx and 6xxx wrought families, AlF357 trades slightly lower corrosion resistance for improved as-cast strength and fatigue life, but it does not match the marine performance of carefully optimized 5xxx alloys.

Fabrication Properties

Weldability

AlF357 can be welded using TIG (GTAW) and MIG (GMAW) processes, but cast porosity, hydrogen pickup, and hot-cracking require careful control. Aluminium-silicon filler alloys such as ER4043 (Al–Si) are commonly used to match wetting characteristics and reduce cracking tendency; ER5356 (Al–Mg) may be used with caution where stronger weld metal is required. Post-weld heat treatment can restore strength for some components but will not eliminate casting-related defects; preheating and degassing of the molten pool are important to minimize porosity.

Machinability

As a hypoeutectic Al–Si casting alloy, AlF357 machines well: silicon particles provide a chip-break effect and dimensional stability, but they also increase tool wear compared with softer wrought alloys. Carbide tooling with positive rake and high coolant flow is recommended to control heat and evacuate chips; typical cutting speeds are higher than steel but depend on section and heat-treatment condition. Surface finish and dimensional tolerance are readily achievable with stable fixtures and appropriate feeds.

Formability

Cold forming of AlF357 is limited due to its casting-oriented microstructure and moderate to low ductility in T6 condition; bending radii must be conservative and often lead to cracking. Hot forming or forging of near-net-shape cast billets is a more realistic route when geometry demands significant deformation. Best formability is seen in the annealed or as-cast condition, but such states sacrifice strength and are rarely used for structural parts.

Heat Treatment Behavior

AlF357 is heat-treatable via solution treatment and artificial aging. Typical solution treatment temperatures are in the range of 510–540 °C, held long enough to homogenize and dissolve soluble phases in thin sections, followed by rapid quenching to retain a supersaturated solid solution. Artificial aging (precipitation) commonly occurs at 155–185 °C to develop Mg2Si precipitates and achieve T6-level properties; aging time is a function of part thickness and desired property balance.

T5 is achieved by cooling from casting and performing artificial aging without an intervening solution step, providing increased strength with lower processing cost but reduced maximum achievable properties. T7 or overaged tempers use higher or extended aging to improve thermal stability and reduce susceptibility to stress-corrosion cracking, at the cost of some peak strength. If the alloy is used in non-heat-treatable conditions, work hardening is not an effective strengthening method due to the cast microstructure; annealing provides relief of residual stresses and improved ductility.

High-Temperature Performance

AlF357 begins to lose a significant portion of its T6 strength above approximately 150 °C, with progressive softening and precipitate coarsening as temperature increases; long-term service is generally limited to temperatures below this range. Oxidation is not a primary failure mode at these temperatures, but microstructural overaging and coarsening reduce fatigue strength, yield, and hardness. Weld heat-affected zones can suffer localized softening and reduced fatigue resistance; designers must account for these gradients when parts are welded post‑heat treatment.

For intermittent or short-duration exposures up to 200 °C, some properties can be retained if an appropriate T7 overaged condition is specified, but sustained high-temperature service is better met by specialty alloys engineered for elevated temperatures.

Applications

Industry	Example Component	Why AlF357 Is Used
Automotive	Structural castings, transmission housings	Good castability, high T6 strength, fatigue resistance
Marine	Pump housings, non-critical structural brackets	Reasonable corrosion resistance and cost-effective casting
Aerospace	Fittings, small structural castings	High strength-to-weight ratio for medium-duty components
Electronics	Housings and heat spreader castings	Dimensional stability and moderate thermal conductivity

AlF357 is chosen where the production economics of casting combined with a heat-treatable chemistry yield components that meet structural and fatigue demands without the expense of wrought fabrication. It occupies a practical niche between lower-strength casting alloys and more expensive, higher-strength wrought materials for medium-duty structural parts.

Selection Insights

AlF357 is an attractive choice when designers require cast geometry and T6-level performance from a cost-effective aluminum alloy. Compared with commercially pure aluminum (1100), AlF357 trades electrical conductivity and superior ductility for substantially higher strength and improved fatigue resistance; it is less suitable when high conductivity is a priority. Compared with common work‑hardened alloys such as 3003 or 5052, AlF357 typically provides higher as-treated strength and better fatigue life but less cold formability and sometimes reduced corrosion resistance in aggressive chloride environments. Compared with heat‑treatable wrought alloys such as 6061/6063, AlF357 may offer simpler production for complex cast geometries and competitive strength for certain sections, despite generally lower peak-strength per weight and differing forging/extrusion capabilities.

Use AlF357 when casting is the preferred manufacturing route, when T6-style mechanical properties are required in cast form, and when designers can control section thickness and heat treatment to realize the alloy’s potential. Avoid AlF357 where deep, ductile cold forming, maximum electrical conductivity, or highest-temperature sustained service are the primary requirements.

Closing Summary

AlF357 remains relevant because it combines predictable casting behavior with heat-treatable strengthening, delivering high fatigue and static strength for complex cast components at a relatively low cost. When chosen with attention to casting practice, temper selection, and corrosion protection, AlF357 offers a robust solution for many automotive, aerospace, marine, and industrial applications where cast geometry and mechanical performance must be balanced.