Aluminum AlSi12: Composition, Properties, Temper Guide & Applications

Comprehensive Overview

AlSi12 is part of the aluminium-silicon family and is classified among the cast Al-Si alloys rather than the wrought 1xxx–7xxx series; it is commonly referenced under casting designations rather than the wrought series numbers. The alloy nominally contains approximately 11–13% silicon with residual levels of iron, manganese, copper and trace elements such as titanium and chromium used for grain refinement and property control.

The dominant strengthening mechanism for AlSi12 is microstructural: a near-eutectic silicon phase distributed in an aluminium matrix provides inherent stiffness and wear resistance. AlSi12 is not a classical precipitation-hardenable (heat-treatable) wrought alloy; improvements in mechanical properties are obtained by control of silicon morphology (modification, spheroidization) and by limited thermal treatments rather than by Mg/Cu-driven aging processes.

Key traits include excellent casting fluidity, low solidification shrinkage, good dimensional stability in cast form, and reasonable corrosion resistance due to the protective aluminium oxide film. The alloy exhibits moderate tensile strength, modest elongation in as-cast condition, and good thermal conductivity relative to other casting alloys. Typical industries using AlSi12 are automotive (die-cast engine components, housings), industrial machinery (pump bodies, valve housings), marine hardware, and certain thermal-management applications.

Engineers choose AlSi12 when castability, low shrinkage, and a balance of strength and thermal conductivity are prioritized over the highest possible tensile peak strengths. Its silicon content makes it attractive for complex thin-walled castings and components requiring good wear characteristics and stability during thermal cycling.

Temper Variants

Temper	Strength Level	Elongation	Formability	Weldability	Notes
F (As-fabricated)	Low–Medium	Low–Medium	Limited	Moderate	Typical as-cast condition from die or sand casting
O (Annealed)	Low	Medium–Higher	Improved	Good	Stress-relief/anneal to improve ductility and reduce residual stresses
T5	Medium	Low	Limited	Moderate	Cooled from casting and artificially aged for some property stabilization
T6	Medium–High	Low	Limited	Moderate	Solution treatment + artificial aging can spheroidize Si and increase strength modestly when alloy chemistry permits
T4 (solutionized)	Medium	Low–Medium	Limited	Moderate	Solution treatment with natural aging; used selectively after modification
H14 (strain-hardened)	Not typical	Not typical	Not typical	Not typical	Strain hardening is generally not applicable to cast AlSi12; included for reference

Temper has a strong influence on AlSi12 performance because silicon morphology and casting defects are the primary property drivers. Heat treatments such as solutionizing and artificial aging (T6/T5) can refine silicon particle distribution and reduce microsegregation, giving modest strength or ductility changes relative to as-cast condition.

Practical selection of temper depends on casting route and part function; annealing and controlled aging are used to improve machinability and reduce internal stresses, while aggressive thermomechanical approaches used on wrought alloys are not generally applicable to AlSi12 castings.

Chemical Composition

Element	% Range	Notes
Si	11.0–13.0	Principal alloying element; controls eutectic structure, fluidity and wear resistance
Fe	0.3–1.3	Common impurity; higher Fe promotes intermetallics that embrittle and reduce ductility
Mn	0.1–0.6	Controls Fe-intermetallic morphology; improves castability and mechanical properties
Mg	0.05–0.5	May be present in trace to small amounts; enables some precipitation hardening if present in sufficient quantity
Cu	0.05–0.5	Generally low; increased Cu can raise strength but reduces corrosion resistance
Zn	0.05–0.5	Trace; typically residual and controlled to avoid hot-cracking
Cr	0.05–0.25	Minor addition for grain structure control and to tie up Fe in benign phases
Ti	0.02–0.20	Grain refiner used to control primary aluminium grain structure
Others	Balance Al with trace impurities	Residual elements (Ni, Pb, Sr from modification) are controlled to meet casting performance

Silicon is the primary performance lever in AlSi12: it lowers the melting range, increases fluidity and reduces shrinkage, and provides wear resistance due to hard Si particles. Iron and manganese control detrimental intermetallic phases formed during solidification. Trace elements such as Ti and Sr are used to refine grains and modify silicon shape from plate-like to fibrous or spheroidal, improving toughness and machinability.

Mechanical Properties

AlSi12 castings exhibit tensile behavior dominated by the eutectic silicon morphology and casting quality (porosity, shrinkage, and solidification rate). As-cast strength is moderate and elongation is limited; directional properties in castings may vary with section thickness due to solidification gradients. Strength can be modestly increased by solution treatment and aging when the alloy chemistry and microstructure are properly modified.

Yield behavior is generally lower than high-strength heat-treatable wrought alloys; however, the alloy offers stable performance across different casting techniques when shrinkage and porosity are well controlled. Hardness correlates with silicon particle distribution: fine, spheroidized Si yields higher toughness and slightly lower surface hardness than coarse, flake-like silicon.

Fatigue performance in AlSi12 is sensitive to casting defects such as gas porosity and oxide inclusions, which serve as initiation sites; sound castings with minimal defects can achieve respectable fatigue life for non-critical rotating components. Section thickness affects cooling rate and thus silicon morphology: thin sections cool rapidly producing a finer microstructure and higher strength than thick sections that cool slowly and form coarser Si.

Property	O/Annealed	Key Temper (T6/T5)	Notes
Tensile Strength (UTS)	90–140 MPa (typical)	150–240 MPa (typical, alloy- and process-dependent)	Wide ranges due to casting method, porosity control and microstructure modification
Yield Strength (0.2% offset)	40–90 MPa	100–170 MPa	Yield improvement after solution/agings is modest compared with high-Mg/Cu alloys
Elongation	2–10%	1–6%	Ductility decreases after aging; thin-walled castings show higher elongation
Hardness (Brinell)	35–70 HB	60–110 HB	Hardness increases with fine Si morphology and heat treatment; values depend on section and processing

Physical Properties

Property	Value	Notes
Density	2.68 g/cm³	Typical density for Al-Si cast alloys; slightly higher than pure aluminium due to silicon content
Melting Range	~577–600 °C	Near-eutectic alloy has a low melting point relative to pure Al; eutectic reaction near ~577 °C
Thermal Conductivity	~110–140 W/m·K	Lower than pure aluminium but still good for heat dissipation in cast components
Electrical Conductivity	~30–40% IACS	Reduced relative to pure aluminium due to silicon and intermetallic scattering
Specific Heat	~0.88–0.92 kJ/kg·K	Close to that of pure aluminium; useful for thermal mass calculations
Thermal Expansion	~21–24 µm/m·K	Slightly lower coefficient than pure Al in some heat-treated states due to Si content

AlSi12’s physical properties make it attractive for components where thermal management or dimensional stability during heating/cooling cycles is important. The lower melting point and good fluidity assist in producing thin-walled, complex geometries with good mold filling.

Electrical conductivity is reduced versus pure aluminium and should be considered when electrical performance is required. Density and specific heat are close to other aluminium cast alloys, which is beneficial for lightweight structural and thermal applications.

Product Forms

Form	Typical Thickness/Size	Strength Behavior	Common Tempers	Notes
Sand-cast parts	Wall thicknesses 3–50 mm	Variable; coarser microstructure in thick sections	F, O, T5	Used for large, complex components; microstructure controlled by cooling rate
Die-cast (permanent mold)	Thin walls 1–10 mm	Higher strength due to faster cooling; fine Si	F, T5, T6	Common for automotive housings and precision parts
Gravity die / Permanent mold	2–20 mm	Intermediate strength and surface finish	F, T5, T6	Good repeatability and mechanical properties for medium production runs
Sand cores/ingots	Varies	Baseline feedstock	F	Raw feedstock or re-melted material for casting shops
Bar / Cast rod	Diameters up to 200 mm	Similar to castings; limited forging or rolling	F, O	Used for machining blanks and some small structural parts

Different product forms arise from the intended application and required mechanical/geomteric tolerances. Die casting yields the finest microstructure and best dimensional control, making it suitable for thin-wall, high-volume automotive parts. Sand casting tolerates larger sections and complex internal geometries but requires careful defect control.

Form of supply affects secondary processing: die-castings typically require less machining allowance and have better surface finish, while sand-cast parts may require more machining and heat treatment to achieve desired tolerances.

Equivalent Grades

Standard	Grade	Region	Notes
AA	AlSi12 (casting)	USA	Generic designation for Al-Si casting with ~12% Si
EN	EN AC-AlSi12 / AlSi12	Europe	Standardized cast designation (formerly AlSi12); EN grade often includes “F” for foundry condition
JIS	ADC12 (similar)	Japan	ADC12 is widely used in die casting and is compositionally close but often contains higher Cu and Zn levels
GB/T	AlSi12	China	Chinese standard for Al-Si casting alloys broadly equivalent; composition tolerances can differ

Subtle differences between regional grades are mostly in allowable levels of Cu, Zn and trace elements, and in the permitted levels of iron and manganese. ADC12 (JIS) often has higher copper content for improved mechanical properties in die-castings but sacrifices some corrosion resistance. EN AC-AlSi12 tends to be controlled for low Fe and is widely specified for high-quality castings in Europe.

Corrosion Resistance

AlSi12 exhibits generally good atmospheric corrosion resistance due to formation of a protective aluminium oxide film. In neutral and mildly aggressive environments the alloy performs well, but localized anodic dissolution can occur at casting defects, porosity or where intermetallic phases are present. Surface finishing and sealing paint systems substantially improve long-term performance in exposed environments.

In marine and chloride-containing environments the alloy can suffer pitting and crevice corrosion, especially where defects or rough surfaces trap corrosive media. The relatively low copper content of typical AlSi12 improves resistance to stress-corrosion compared to Cu-rich alloys, but designers should still consider cathodic/anodic interactions when joining to more noble metals.

Stress corrosion cracking is not a major failure mode for AlSi12 because of its limited solute content for intergranular embrittlement, but fatigue-corrosion combined with casting defects can lead to premature failures in cyclic marine or industrial environments. Galvanic interactions with steels and copper alloys should be mitigated with insulating barriers or appropriate corrosion allowances to avoid accelerated attack.

Compared with 5xxx-series marine-grade wrought alloys, AlSi12 has lower intrinsic toughness and is more sensitive to casting defects, but it provides advantages in castability and dimensional stability for complex shapes. Designers should evaluate environmental exposure and detail cast integrity when selecting AlSi12 for corrosive-service applications.

Fabrication Properties

Weldability

Welding of AlSi12 castings is feasible with TIG and MIG processes when appropriate filler alloys and pre-weld preparation are used. Aluminum-silicon fillers such as ER4043 (Al-5Si) and ER4047 (Al-12Si) are commonly recommended to match base metal silicon content and reduce hot-cracking risk. Preheating and degassing are frequently used to reduce hydrogen-related porosity; however, welding can locally change silicon morphology in the HAZ and generate shrinkage/stress concentrations that require post-weld heat treatment or grinding to remove defects.

Machinability

Machinability of AlSi12 is generally good for cast aluminium but the hard silicon particles accelerate abrasive tool wear compared with pure aluminium. Carbide tooling with titanium nitride or similar coatings and positive rake geometry are recommended for high-speed machining; chip control is typically acceptable but tool life must be monitored for long runs. Machining parameters should consider section thickness and local hardness variations caused by silicon morphology and heat treatment history.

Formability

Forming of AlSi12 is limited because the alloy is typically used in cast form and has low ductility relative to wrought aluminium alloys. Cold bending or deep drawing is not practical for typical AlSi12 cast components; instead designers rely on mold design, cores and inserts to achieve the required geometry. For improved formability, semi-solid processing or modification to a spheroidized silicon microstructure through heat treatment can improve local ductility but will not match the formability of 5xxx or 6xxx wrought alloys.

Heat Treatment Behavior

AlSi12 is not a primary precipitation-hardenable aluminium alloy because it lacks significant magnesium and copper for classical T6 strengthening. Nevertheless, thermal treatments affect silicon morphology and residual stresses. Solution treatment at temperatures near 520–540 °C followed by rapid quench can partially homogenize the microstructure and spheroidize silicon particles; subsequent artificial aging (T5/T6) can stabilize the microstructure and yield small strength improvements.

For many AlSi12 castings, the most valuable heat treatments are annealing and homogenization to relieve casting stresses and reduce microsegregation. These treatments improve machinability and reduce the incidence of hot-cracking during secondary operations. Because work hardening is not practical for cast components, designers rely on microstructural control, heat treatment and alloy modification (e.g., Sr treatment) to tune properties.

Process control during heat treatment is critical: overaging or improper solutionizing can coarsen silicon and reduce ductility, while insufficient degassing or slow quench rates lead to retained porosity and poor mechanical performance. For alloys containing slight Mg or Cu additions, controlled solution and aging schedules provide the greatest benefit; otherwise heat treatments focus on stress relief and silicon morphology optimization.

High-Temperature Performance

At elevated temperatures AlSi12 experiences progressive strength loss as thermal activation allows dislocation motion and coarsening of microstructural features. Practical service temperatures are often limited to below ~150–200 °C for structural applications to avoid significant creep and loss of stiffness. Short-term exposures up to 250 °C are possible for non-loadbearing thermal components, but long-term mechanical reliability will degrade.

Oxidation behavior is typical of aluminium: a stable oxide forms that protects the bulk, but oxide-scale formation and growth can degrade thermal contact resistance in heat-transfer applications. The HAZ generated by welding or local heat treatments may have altered silicon distributions that reduce high-temperature performance locally and accelerate creep or oxidation-driven cracking. For components intended for elevated temperature service, careful design to minimize stress concentrations and to avoid thin sections in load-bearing areas is essential.

Applications

Industry	Example Component	Why AlSi12 Is Used
Automotive	Transmission housings, gearbox casings, valve bodies	Excellent die-castability and dimensional stability for complex thin-walled components
Marine	Pump housings, non-critical structural castings	Corrosion resistance and castability for complex geometries
Aerospace & Defence	Brackets, housings for non-primary structures	Good strength-to-weight for cast components and good thermal stability
Industrial Machinery	Gear housings, bearing housings, valve bodies	Low shrinkage, good wear resistance and adequate mechanical strength
Electronics / Thermal Management	Heat spreader housings and thermal-mass components	Reasonable thermal conductivity combined with complex-casting capability

AlSi12 performs well where the manufacturing route benefits of casting (complex shapes, integrated ribs, thin walls) outweigh the need for the highest tensile performance or extreme formability. Its combination of dimensional accuracy, thermal conductivity and adequate strength makes it widely used in high-volume die-cast components and economical medium-sized castings.

Selection Insights