Aluminum AlSiMg: Composition, Properties, Temper Guide & Applications

Comprehensive Overview

AlSiMg denotes the broad family of aluminum alloys alloyed primarily with silicon (Si) and magnesium (Mg). In wrought form this family overlaps strongly with the 6xxx series (Al-Mg-Si), which are precipitation‑hardening, heat-treatable alloys; in foundry practice the AlSiMg label also refers to cast Al‑Si alloys modified with Mg for improved strength and response to heat treatment. The defining metallurgical mechanism for the wrought Al‑Si‑Mg alloys is age hardening through formation of metastable Mg2Si precipitates following solution treatment and artificial aging; cast variants gain strength from a refined silicon morphology plus Mg-enhanced strength and limited precipitation hardening.

Key technical traits include a combination of moderate-to-high strength, good corrosion resistance in atmospheric environments, wide extrudability and formability, and reliable weldability when proper filler and post‑weld practices are used. Compared with high‑strength 2xxx or 7xxx series, AlSiMg grades trade maximum peak strength for improved corrosion performance and easier fabrication. Typical industries using AlSiMg alloys include automotive body and structural components, architectural extrusions, marine fittings, electronic housings and heat sinks, and certain aerospace fittings where a balance of strength, weight and corrosion resistance is required.

Engineers choose AlSiMg when a heat‑treatable alloy is needed that delivers good strength-to-weight, excellent extrudability, and the ability to achieve engineered strength levels through thermal processing. The family’s versatility—available as sheet, plate, extrusions and castings—plus its compatibility with anodizing and coating processes, keeps AlSiMg alloys favored for cost‑sensitive structures and medium‑duty structural parts where manufacturability and corrosion resistance are priorities.

Temper Variants

Temper	Strength Level	Elongation	Formability	Weldability	Notes
O	Low	High (20–35%)	Excellent	Excellent	Fully annealed; maximum ductility
H14	Low‑Medium	Moderate (10–20%)	Good	Excellent	Strain‑hardened; limited forming
T4	Medium	Moderate (12–18%)	Good	Good	Solution treated and naturally aged
T5	Medium	Moderate (10–16%)	Good	Good	Cooled from hot working and artificially aged
T6	High	Lower (8–14%)	Fair‑Good	Good	Solution treated and artificially aged; peak strength
T651	High	Lower (8–14%)	Fair‑Good	Good	T6 with stress relief by stretching
T7	Medium	Moderate (10–16%)	Good	Good	Overaged for improved stability and fracture toughness

Tempers control the microstructure and therefore the trade-offs between strength, ductility, and formability. Soft annealed (O) condition provides the best room‑temperature formability for deep draws and complex bending, while T6/T651 are used where maximum and stable strength is required after heat treatment.

Heat treatment path and any intermediate cold work significantly affect recrystallization, precipitate size/distribution and residual stress state; designers should select the temper based on required forming operations, final load case and corrosion environment.

Chemical Composition

Element	% Range	Notes
Si	0.2–1.6	Enables Mg2Si precipitation; higher Si refines casting microstructure
Fe	0.1–0.7	Impurity; forms intermetallics that reduce ductility and corrosion resistance
Mn	0–0.50	Controls grain structure and can form dispersoids that affect strength
Mg	0.3–1.2	Principal strengthening alloying element via Mg2Si precipitates
Cu	0–0.5	Raises strength but can reduce corrosion resistance and heat‑treat response
Zn	0–0.25	Generally low; excessive Zn may promote galvanic concerns
Cr	0–0.35	Controls grain boundary precipitates and improves toughness/stability
Ti	0–0.15	Grain refinement in cast and wrought products
Others	Balance Al	Trace additions and residuals; Zr/Sc can be present for high‑performance variants

The Si and Mg contents are the primary tuning knobs for strength: combined they produce Mg2Si precipitates during aging that dominate the yield and tensile properties. Minor elements and impurities such as Fe and Cu influence toughness, machinability and corrosion behavior; leaner Fe levels improve ductility and appearance while Cu increases strength at the expense of some corrosion resistance. Cast AlSiMg variants commonly have higher Si (up to ~12% in some casting alloys) and different impurity tolerances compared with wrought 6xxx alloys.

Mechanical Properties

Wrought AlSiMg (6xxx family) displays a characteristic age‑hardening tensile curve: starting from a relatively low yield in the annealed or T4 condition, yield and ultimate tensile strength rise substantially after artificial aging as fine Mg2Si precipitates form. Yield strength in the T6 condition typically reaches the practical design range for medium‑duty structural components, while elongation falls compared to annealed states; fracture modes are usually ductile fracture with some microvoid coalescence unless coarse intermetallics are present. Fatigue performance is favorable for properly surface‑finished parts and when metallurgical cleanliness is controlled; life is sensitive to surface defects, cold work and stress concentrators.

Thickness influences mechanical response because cooling rates after solution treatment and quench can vary; thicker sections quench more slowly, which can reduce supersaturation and subsequent age hardening, lowering achievable strength and increasing susceptibility to coarse precipitate formation. Hardness tracks tensile behavior and is commonly reported in Brinell or Vickers values; typical T6 hardnesses for common 6xxx alloys are in the range that supports machining and forming operations but require process control to avoid overaging.

Fracture toughness and notch sensitivity depend on alloy cleanliness and temper. Cast AlSiMg grades have a different mechanical profile: higher silicon content improves wear and machining in some cases but makes the alloy less ductile, with lower elongation and different fatigue crack initiation behavior than wrought alloys.

Property	O/Annealed	Key Temper (e.g., T6)	Notes
Tensile Strength	110–160 MPa	200–320 MPa	Range depends on specific alloy (e.g., 6061 vs 6063) and section thickness
Yield Strength	55–120 MPa	120–280 MPa	Yield increases substantially after T6; design allowables must consider temper
Elongation	20–35%	8–14%	Ductility reduced in peak‑aged tempers; higher in annealed and T4 states
Hardness	30–50 HB	70–130 HB	Hardness correlates with precipitate distribution and alloy chemistry

Physical Properties

Property	Value	Notes
Density	2.68–2.70 g/cm³	Typical aluminum density; varies negligibly with alloying
Melting Range	~555–650 °C	Solidus/liquidus vary with Si content and other alloying additions
Thermal Conductivity	130–160 W/m·K	Lower than pure Al; depends on alloy and temper
Electrical Conductivity	25–45 % IACS	Reduced from pure Al due to alloying; varies with temper and cold work
Specific Heat	~900 J/kg·K	Typical for aluminum alloys at ambient temperatures
Thermal Expansion	22–24 µm/m·K	Coefficient of thermal expansion for structural design

AlSiMg alloys retain much of aluminum’s favorable thermal and electrical performance, making them attractive for heat‑dissipation applications while still providing improved strength. Thermal conductivity reductions versus pure aluminum are modest and usually acceptable for structural parts that also serve as heat spreaders.

For thermal design, engineers must account for the coefficient of thermal expansion when mating AlSiMg to dissimilar materials; differential expansion can create thermal stresses in assemblies and joints.

Product Forms

Form	Typical Thickness/Size	Strength Behavior	Common Tempers	Notes
Sheet	0.3–6.0 mm	Uniform; thickness affects aging response	O, H14, T4, T5, T6	Widely used for body panels, architecture, facades
Plate	>6.0 mm up to 150 mm	Lower quenchability in thick sections	O, T6 (limited)	Thick‑section strength reduced by slow cooling
Extrusion	Profiles up to several meters	Excellent directional strength	T5, T6, T651	Extrudability is a key advantage of 6xxx alloys
Tube	0.5–20 mm wall	Standard structural/performance	O, T4, T6	Welded and seamless tubes common
Bar/Rod	Dia 3–150 mm	Isotropic in cross‑section	O, T6	Used for machined components and fasteners

Form affects microstructure: extruded profiles benefit from dynamic recrystallization and can be artificially aged for consistent properties, while plate/forgings require careful quench control to realize designed strengths. Sheet and thin extrusions quench rapidly and typically achieve closer to peak T6 properties, whereas thicker plate may need alternative design methods or overage tempers to ensure stability.

Manufacturing choices—rolling, extrusion, casting—also affect surface finish, internal cleanliness and residual stress, all of which influence downstream processes such as welding, anodizing and machining.

Equivalent Grades

Standard	Grade	Region	Notes
AA	6xxx series (e.g., 6061, 6063)	USA	Representative wrought Al‑Mg‑Si alloys used in structural/extrusion applications
EN AW	AlSiMg (casting) / EN AW‑6060 / EN AW‑6082 (wrought)	Europe	"AlSiMg" appears in casting grades; EN AW‑60xx are common wrought equivalents
JIS	A6061, A6063	Japan	JIS grades for typical Al‑Mg‑Si alloys used in extrusions and structures
GB/T	6061, AlSi9Mg (casting)	China	Chinese standards cover both wrought 6xxx and cast AlSiMg series

There is no single one‑to‑one equivalence for the AlSiMg label: it can denote both a family of wrought 6xxx alloys and a range of Al‑Si casting alloys modified with Mg. Wrought standards (e.g., 6061/6063/6082) have tightly specified compositions and mechanical properties, while cast AlSiMg grades are designated for foundry use and have different mechanical/corrosion profiles.

Engineers must review specific standard specifications and T‑temper designations for direct equivalency rather than relying solely on the AlSiMg family name during procurement.

Corrosion Resistance

AlSiMg alloys typically show good atmospheric corrosion resistance due to the naturally forming protective aluminum oxide layer, and they respond well to anodizing for enhanced surface protection and aesthetic finish. In mildly corrosive environments and industrial atmospheres they perform comparably to other 6xxx alloys, with resistance aided by lower copper levels and proper temper selection; pitting and crevice corrosion remain concerns in chloride‑rich environments if surface defects or coatings are compromised.

Marine performance is acceptable for many structural fittings and extrusions, but for long‑term exposure in seawater or splash zones designers often prefer higher‑Mg 5xxx alloys or apply sacrificial coatings and cathodic protection because localized corrosion rates and chlorides can accelerate attack. Stress corrosion cracking susceptibility for 6xxx family alloys is generally low compared with 2xxx or 7xxx alloys, but overaged tempers and high residual tensile stresses can increase SCC risk; therefore, appropriate temper selection and post‑weld heat treatment or stress relief are important.

Galvanic interactions must be considered when mating AlSiMg with more noble metals (e.g., stainless steel, copper alloys); insulating materials or coatings are commonly used to prevent accelerated corrosion. Compared with the 5xxx family, AlSiMg (6xxx) usually provides a better balance of anodizing appearance and dimensional stability but slightly lower ductility and lower absolute corrosion resistance in seawater.

Fabrication Properties

Weldability

AlSiMg wrought alloys weld well by common fusion methods (TIG, MIG/MAG) with predictable fusion zone microstructures; filler alloys such as ER4043 (Al‑Si) or ER5356 (Al‑Mg) are most commonly used depending on desired corrosion and strength balance. Hot‑cracking risk is low for properly prepared joints, though silicon segregation in cast AlSiMg grades can promote hot cracking and require preheat or modified joint design. The heat‑affected zone will typically soften relative to peak‑aged T6 base metal, so post‑weld aging or use of overaged tempers (T7) is often specified for structural applications.

Machinability

Machinability of AlSiMg alloys is rated moderate to good; free‑cutting behavior improves with higher Si content and with fine, homogeneous precipitate distributions. Carbide or coated carbide tooling is commonly used at high feed rates and moderate speeds; aluminum tends to produce long, sticky chips and built‑up edge, so tool geometry, adequate lubrication/coolant and chip breakers are important. Alloys with higher silicon or casting morphology will exhibit increased tool wear, particularly when Si appears as hard plates or eutectic particles.

Formability

Formability is excellent in annealed and naturally aged tempers and remains good in T4/T5 states for many stamping and extrusion forming operations. Minimum bend radii depend on temper, thickness and part geometry; typical guidelines for sheet in T4/T6 might recommend internal radii of 1.5–3× thickness for moderate forming to avoid cracking. Cold working (H‑tempers) increases strength via strain hardening but reduces elongation and springback control, so final temper and required dimensional tolerances must be planned together with forming steps.

Heat Treatment Behavior

Solution treatment for AlSiMg (wrought 6xxx) is performed near the solvus of Mg2Si, commonly in the range of 510–550 °C for typical alloys, held long enough to dissolve phase particles and homogenize the solid solution. Rapid quenching to room temperature is essential to retain Mg and Si in supersaturated solid solution and enable subsequent precipitation during artificial aging; quench sensitivity increases with section thickness. Artificial aging (T6) is typically performed at 160–185 °C for several hours, producing fine, coherent precipitates that raise yield and tensile strengths; aging parameters are tuned to the alloy to balance peak strength against toughness and stress relief.

T‑temper transitions include T5 (cooled from hot working then aged), T6 (solution treated and artificially aged), T651 (T6 with straightening/stretching), and T7 (overaged for improved stability and SCC resistance). Cast AlSiMg alloys often rely more on Mg modification and heat‑treat paths adapted for cast microstructures; solution and aging steps may be adjusted for reduced solubility and slower diffusion in large castings.

For non‑heat‑treatable or overaged variants, work hardening and annealing remain the primary methods to tailor properties; full anneal (O) at ~350–420 °C followed by slow cool restores ductility but removes age hardening.

High-Temperature Performance

AlSiMg alloys lose strength progressively with increasing temperature as precipitate stability falls and dislocation interactions weaken; practical long‑term service temperature limits for structural performance are commonly set below 150 °C to avoid significant softening and loss of mechanical properties. Above ~150–200 °C, coarsening of Mg2Si precipitates leads to overaging and irreversible reductions in yield strength and hardness, making these alloys unsuitable for sustained high‑temperature load bearing.

Oxidation is limited compared with steels, but elevated temperature exposure can alter surface oxide thickness and color and may affect adhesion of paints and coatings; protective coatings or anodizing must be selected for elevated temperature stability. In welded assemblies, the HAZ can experience local softening and lower creep resistance; designers should avoid high service temperatures in critical welded regions or specify appropriate post‑weld heat treatment and overaged tempers for stability.

Applications

Industry	Example Component	Why AlSiMg Is Used
Automotive	Body panels, bumpers, structural extrusions	Balance of formability, extrudability and age‑hardening strength
Marine	Deck fittings, frames	Good atmospheric corrosion resistance and light weight
Aerospace	Secondary structural fittings, interior frames	Favorable strength‑to‑weight and compatibility with anodizing
Electronics	Heat sinks, housings	Thermal conductivity combined with ease of machining/extrusion

AlSiMg alloys are selected where a combination of manufacturability and service performance is required; their adaptability across sheet, extrusion and casting forms enables multi‑disciplinary use across vehicle, marine and industrial equipment sectors.

Selection Insights

AlSiMg is an engineering choice when designers need a heat‑treatable aluminum with good extrudability and balanced corrosion resistance. Compared with commercially pure aluminum (1100), AlSiMg trades some electrical conductivity and formability for much higher yield and tensile strength, making it better for structural parts where some formability remains necessary.

Compared with work‑hardened alloys such as 3003 or 5052, AlSiMg typically provides higher achievable strength after aging with similar or slightly lower corrosion resistance in aggressive chloride environments; choose AlSiMg when higher structural strength and better anodizing appearance are priorities. Compared with higher‑strength heat‑treatable alloys (e.g., 2xxx or 7xxx series) and common 6xxx variants like 6061/6063, AlSiMg grades are often preferred when manufacturability, extrudability and corrosion performance are more important than absolute peak strength; for very high strength requirements, other alloy families may be necessary.

When selecting a specific grade and temper, balance required tensile/yield values, expected service environment (especially chloride exposure), fabrication route (wrought vs cast) and availability in the desired product form; always verify standard specifications and supplier certification for critical applications.

Closing Summary

AlSiMg alloys remain a versatile and widely used class of aluminum materials because they combine precipitation‑hardening strength, good fabrication characteristics and respectable corrosion resistance across a broad set of product forms, making them a pragmatic choice for many automotive, marine, architectural and electronic applications where balanced performance and manufacturability are required.