Aluminum AlSi7Mg: Composition, Properties, Temper Guide & Applications

Comprehensive Overview

AlSi7Mg is an aluminum-silicon-magnesium alloy that belongs to the Al–Si family of casting alloys and is typically encountered in foundry and pressure- or gravity-cast forms under the EN AC‑AlSi7Mg designation. It occupies the cast-Al alloy space rather than the wrought 2xxx–7xxx series, and is most often compared with A356/A357 grade materials in North American practice.

The principal alloying element is silicon (~6.5–7.5 wt%) with magnesium as a secondary alloying element (~0.2–0.5 wt%), and minor amounts of Fe, Cu, Mn, Ti and others as controlled impurities or microalloying additions. Strengthening arises primarily through solution heat treatment followed by precipitation hardening of Mg2Si intermetallics (heat-treatable); casting solidification structure and secondary dendrite arm spacing also play a major role in as-cast strength.

Key traits include excellent castability and fluidity for complex geometries, good combination of strength and ductility after T6-type treatments, reasonable corrosion resistance in atmospheric environments, and good thermal conductivity compared with many other aluminum alloys. Weldability and formability are moderate: casting alloys can be welded with appropriate procedures but are less ductile in the as-cast state than wrought alloys, limiting extensive cold forming.

Typical industries are automotive (structural castings, housings, wheel and suspension components), general machinery, pumps and valves, marine hardware, and some electronic enclosures or heat-dissipating cast parts. Engineers choose AlSi7Mg because it balances castability and post‑heat-treatment strength while remaining cost-effective relative to higher-alloyed or wrought alloys, and because it provides predictable, reproducible performance in foundry practice.

Temper Variants

Temper	Strength Level	Elongation	Formability	Weldability	Notes
O	Low	High	Excellent (relative for castings)	Excellent	Full anneal or stress-relieved as-cast condition; best ductility, lowest strength
T4	Low–Medium	Medium–High	Good	Good	Solution treated and naturally aged; intermediate strength with better ductility than T6
T5	Medium	Medium	Fair	Good	Cooled from casting and artificially aged; common for rapid production parts
T6	High	Medium	Fair–Poor	Moderate	Solution heat-treated, quenched and artificially aged; peak hardness and strength for design
T7	Medium–High	Medium	Fair	Moderate	Over-aged for improved thermal stability and reduced susceptibility to stress corrosion
F	Variable	Variable	Variable	Variable	As-fabricated without specific heat treatment control; properties depend on process

The temper selection controls microstructure: solution treatment dissolves soluble phases and homogenizes the matrix, while artificial aging precipitates fine Mg2Si particles to raise yield and tensile strength. As-cast (O/F) conditions provide the best ductility and formability for limited shaping, whereas T6 gives peak strength at the expense of some toughness and formability.

Chemical Composition

Element	% Range	Notes
Si	6.5–7.5	Main alloying element; improves fluidity and reduces shrinkage; forms eutectic phases
Fe	0.1–0.6	Impurity; higher Fe promotes brittle intermetallics (β‑Al5FeSi) that reduce ductility
Mn	0.05–0.35	Controls iron intermetallic morphology; small additions refine microstructure
Mg	0.2–0.5	Precipitation hardening element (Mg2Si); controls age-hardening response
Cu	0.05–0.2	Often limited; increases strength but can reduce corrosion resistance if elevated
Zn	≤0.2	Minor; typically limited to reduce undesirable effects
Cr	≤0.1	Grain/refinement control; limits recrystallization in some practices
Ti	≤0.2	Grain/refiner in castings (TiB additions often used in foundries)
Others	Balance Al	Trace elements controlled to specified limits in standards

Silicon establishes a eutectic structure that improves casting behavior and as-cast mechanical performance, while magnesium enables precipitation strengthening through Mg2Si when solution treated and aged. Controlled iron and manganese levels determine the morphology of brittle intermetallics and therefore critically influence ductility and fatigue performance. Minor elements such as Ti and Cr are used for grain refinement and to control solidification characteristics in production environments.

Mechanical Properties

AlSi7Mg exhibits a wide range of mechanical behavior depending on casting method, section thickness, and temper. In the annealed or as-cast condition tensile strength is modest but ductility is relatively high for a casting alloy, with fracture behavior sensitive to porosity and intermetallic morphology. After solution treatment and artificial aging (T6), tensile and yield strengths increase markedly due to fine Mg2Si precipitates, trading some elongation for higher usable stress levels.

Yield strength in T6 condition commonly allows design using strength values comparable to medium-strength wrought alloys, but cast-in defects and section-size effects must be considered in fatigue and fracture-critical designs. Hardness correlates with temper: HBR or HBW increases significantly from O/T4 to T6, giving improved wear resistance in bearing or sliding applications. Fatigue performance is highly dependent on surface condition, porosity, and microstructural coarseness; shot-peening, refinement of solidification structure, and control of hydrogen porosity markedly improve S–N behavior.

Thickness and section geometry influence cooling rate and dendrite arm spacing, thereby affecting mechanical properties: thin-walled castings cool quickly, producing finer microstructures and better strength, while thick sections cool slowly and often require tailored solution treatment practices to avoid softer cores and inhomogeneous properties.

Property	O/Annealed	Key Temper (T6)	Notes
Tensile Strength	150–210 MPa	260–340 MPa	T6 values depend on casting quality and Mg content; typical ranges used for design
Yield Strength	70–140 MPa	200–260 MPa	Yield increases ~2×–3× from annealed to T6 in good castings
Elongation	6–18%	4–12%	Elongation drops with increasing strength and with casting defects
Hardness (HB)	40–70 HB	80–110 HB	Brinell hardness rises with aging; hardness influenced by section size and porosity

Physical Properties

Property	Value	Notes
Density	~2.68 g/cm³	Typical for Al–Si cast alloys; slight variation with alloying additions
Melting Range	~555–615 °C	Solidus and liquidus depend on Si content and microalloying; eutectic features near 577 °C
Thermal Conductivity	~100–140 W/m·K	Lower than pure Al but still good for heat-dissipating cast parts
Electrical Conductivity	~30–38 % IACS	Reduced from pure Al due to alloying; suitable for some conductive applications
Specific Heat	~870–910 J/kg·K	Similar to other aluminum alloys; temperature dependent
Thermal Expansion	22–24 ×10⁻⁶ /K	Typical linear thermal expansion at room temperature; important for joint design

AlSi7Mg combines relatively low density with reasonable thermal conductivity, making it suitable where weight reduction and heat transfer are required together with castability. The solidification range and eutectic behavior control porosity tendencies and feeding; understanding these thermal properties is essential for designing risers, chills, and heat-treatment schedules. Thermal expansion is moderate and must be accommodated when joining to steels or other metals to prevent thermal stresses in service.

Product Forms

Form	Typical Thickness/Size	Strength Behavior	Common Tempers	Notes
Sand/Gravity Castings	Variable, mm to hundreds mm	Strength varies with section size	O, T5, T6	Widely used for structural castings, good for medium-to-large components
Permanent Mold / Die Casting	Thin to moderate walls (2–20 mm)	Generally finer microstructure, higher as-cast strength	T5, T6	Excellent surface finish and dimensional control; common for automotive parts
Ingot / Billet	Up to several 100 mm	Homogenized for downstream processing	O, T4	Feedstock for remelting, forging of near-net shapes and secondary casting
Extrusion / Rolling (limited)	Limited feasibility	Not typical; wrought behavior inferior	—	AlSi7Mg is not widely used for standard extrusions or rolled sheet
Bar / Rod (chill/cold-finished)	Small cross-sections	Variable; often remelted and processed	O, T6	Supplied for machining blanks; mechanical properties depend on process

AlSi7Mg is primarily encountered as a casting alloy, and product forms reflect foundry practice: sand castings, gravity or permanent mold parts, and die-cast components dominate. Differences in processing (sand vs permanent mold vs die-cast) create distinct microstructures, porosity distributions and mechanical properties, so designers must select the form and heat treatment matching structural and surface-finish requirements. While limited wrought processing is possible by remelting and homogenization, traditional extrusions and heavy rolling are uncommon because the alloy chemistry and eutectic microstructure are optimized for casting.

Equivalent Grades

Standard	Grade	Region	Notes
AA / AMS	A356 / AlSi7Mg0.3	USA	A356 is a well-known commercial equivalent with tightly specified Mg content and impurity limits
EN	AC‑AlSi7Mg	Europe	Common European casting designation; variations exist between foundry specifications
JIS	ADC12 / A356 equivalents	Japan	ADC12 is a die-casting grade often higher in Cu; A356-equivalent cast alloys also used
GB/T	AlSi7Mg	China	Chinese standards list similar compositions under AlSi7Mg designations

Standards vary in allowable Mg and Fe limits and in the treatment definitions (T6 vs T61 etc.), so direct substitution requires checking of impurity limits and aging practices. For critical applications, designers should compare the specific standard’s compositional limits, cast method constraints, and defined heat-treatment practices to ensure interchangeability and to predict mechanical and corrosion performance.

Corrosion Resistance

AlSi7Mg exhibits good general atmospheric corrosion resistance due to the formation of a thin, protective aluminum oxide film and the absence of large amounts of copper, which promotes localized corrosion. In marine or chloride-rich environments the alloy can be susceptible to pitting and crevice corrosion, especially if porosity or intermetallic networks are present that accentuate local anodic sites.

Stress corrosion cracking susceptibility is lower than that of high-strength 2xxx and 7xxx series alloys, particularly when not over-aged and when porosity and hydrogen content are controlled; however, residual tensile stresses from casting or welding can reduce SCC margins. Galvanic interactions are a design consideration: when coupled to more noble metals (e.g., stainless steels) in a conductive electrolyte, AlSi7Mg will act anodically and corrode preferentially unless isolated or protected with coatings.

Compared with wrought 5xxx or 6xxx alloys, AlSi7Mg typically provides comparable or slightly lower localized corrosion resistance, but its cast microstructure and porosity sensitivity often make surface finish, post-casting sealing, or protective coatings decisive in long-term performance, especially in marine exposures.

Fabrication Properties

Weldability

AlSi7Mg castings can be welded using standard TIG (GTAW) and MIG (GMAW) techniques with attention to pre-cleaning and control of hydrogen pickup. Typical filler alloys are Al‑Si types such as ER4043 for silicon-rich castings to promote compatible solidification and reduce hot-cracking; Al‑Mg fillers (ER5356) may be used for improved ductility but can increase tendency for porosity or weld metal cracking if mismatched. Hot cracking risk exists in the weld and HAZ, and HAZ softening and dissolution of precipitates can locally reduce strength; post-weld solution treatment and aging may be required for critical parts.

Machinability

Machinability of AlSi7Mg is moderate and heavily influenced by casting quality and eutectic Si particle morphology. Carbide tools with TiN/TiAlN coatings or uncoated carbide are recommended for roughing and finishing; high-speed steel is acceptable for secondary operations. Cutting speeds are typically higher than steels but lower than free-machining wrought alloys; chip formation tends to be discontinuous with abrasive Si particles accelerating tool wear, so coolant use and tool geometry optimization are important.

Formability

As a cast alloy, AlSi7Mg offers limited cold formability compared with wrought alloys; bending and deep drawing are constrained by porosity and the brittle eutectic silicon network. Best forming results are achieved in annealed or solution-treated-and-drawn conditions, but even then the capacity for tight radii is limited and cracking can occur at sharp bends. Designers should favor net-shape casting for complex parts and limit post-casting forming to trimming, light bending, or machining where possible.

Heat Treatment Behavior

AlSi7Mg is heat-treatable by solutionizing and artificial aging to obtain T6-type properties. Typical solution treatment temperatures range from ~525–545 °C for several hours depending on section thickness to dissolve Mg-bearing phases and homogenize the matrix, followed by rapid quenching to retain a supersaturated solid solution. Artificial aging is commonly performed at 155–185 °C for several hours to precipitate fine Mg2Si dispersoids that raise strength and hardness.

T5 (cooling from casting + artificial aging) provides a practical compromise for production where full solutionizing is impractical, giving reasonable strength with less thermal processing. T7 overaging cycles are used to improve thermal stability and reduce susceptibility to stress corrosion in components exposed to elevated temperatures. Careful control of soak times, quench rates and aging profiles is essential to avoid incipient melting in low-melting eutectic regions or coarse precipitate formation that degrades mechanical properties.

High-Temperature Performance

AlSi7Mg experiences progressive loss of strength at elevated temperatures: significant reductions in yield and tensile strength occur above approximately 150 °C, with design practice typically limiting continuous service temperatures well below this threshold. Creep becomes a concern for sustained loads at elevated temperatures, particularly in coarse-grained or over-aged castings with continuous eutectic networks. Oxidation resistance is similar to other aluminum alloys, with the native oxide providing protection; however, scaling is negligible compared with ferrous alloys and oxidation is not typically a limiting factor.

Welding and local heat input create HAZ softening and potential microstructural coarsening, which reduce local high-temperature capability; therefore, thermal design and post-heat-treatment strategies are critical for components subjected to cyclic or high-temperature service.

Applications

Industry	Example Component	Why AlSi7Mg Is Used
Automotive	Transmission housings, brake components, wheel hubs	Excellent castability, good strength after T6, dimensional stability
Marine	Pump housings, propeller hubs, small hull fittings	Reasonable corrosion resistance and castability for complex shapes
Aerospace	Light structural cast fittings, non-critical brackets	Good strength-to-weight ratio for cast components and manageable heat treatment
Electronics	Enclosures, heat‑dissipating housings	Thermal conductivity and ease of forming complex cast shapes for heat management

AlSi7Mg is chosen for applications where near-net-shape casting efficiency and moderate-to-high post-heat-treatment strength are required together with reasonable corrosion resistance. In many cases the alloy enables lower-cost manufacturing of complex components that would be expensive or impossible to produce from wrought materials.

Selection Insights

AlSi7Mg is a strong candidate when castability, low-cost near-net-shape production, and a heat-treatable strength upgrade are priorities. Compared with commercially pure aluminum (1100), AlSi7Mg trades higher strength and better castability for lower electrical conductivity and somewhat reduced formability, making it unsuitable where maximum conductivity or extensive cold forming is required.

Against work‑hardened alloys such as 3003 or 5052, AlSi7Mg typically provides higher peak strength after T6 but may offer slightly lower corrosion resistance in aggressive chloride environments; choose AlSi7Mg when the design needs casting complexity and higher strength rather than superior ductility or the excellent marine corrosion performance of 5xxx wrought grades.

When compared with common heat-treatable wrought alloys like 6061, AlSi7Mg can be preferred for complex cast geometries and where casting economics outweigh the higher peak strength and better surface finish of wrought 6061; use AlSi7Mg for integrated cast housings, then select 6xxx alloys when large-scale extrusion, tight dimensional tolerances or higher fatigue performance are required.

Closing Summary

AlSi7Mg remains a widely used engineering cast alloy because it combines excellent castability with a heat-treatable route to useful strength levels, acceptable corrosion performance, and favorable thermal properties; this balance of attributes makes it a pragmatic choice for many automotive, marine, and industrial cast components where near‑net shaping and cost control are decisive.