Aluminum AlSi9Cu3: Composition, Properties, Temper Guide & Applications

Comprehensive Overview

AlSi9Cu3 is a cast aluminum alloy belonging to the 4xx or more precisely to the Al-Si-Cu family; it is commonly grouped with die- and gravity-cast hypoeutectic Al-Si alloys rather than wrought 6xxx or 5xxx series. The designation indicates nominal silicon near 9 wt.% and copper near 3 wt.%, making it a medium-silicon, copper-strengthened casting alloy optimized for combined strength and thermal stability.

The major alloying elements are silicon (Si) for castability and fluidity, and copper (Cu) for precipitation hardening and elevated temperature strength. Minor additions such as iron (Fe), manganese (Mn) and titanium (Ti) control intermetallic formation, grain structure and feedability during solidification. Strengthening is primarily heat-treatable via solution treatment and artificial ageing (T‑tempers) with a secondary contribution from the microstructure produced by solidification (eutectic Si morphology).

Key traits include good castability and dimensional stability, moderate-to-high static strength in aged tempers, reasonable fatigue resistance for cast parts, and acceptable corrosion resistance with proper post-treatment. Weldability is limited compared with pure aluminum but feasible with proper filler and pre/post treatments; formability is poor in the cast state relative to wrought alloys. Typical industries are automotive (engine and transmission castings, structural castings), industrial machinery, hydraulics, and some electronics enclosures where heat conduction and casting detail are required.

Engineers select AlSi9Cu3 when castability and a balance of strength plus thermal stability are more important than maximum ductility or electrical conductivity. It is preferred over higher-silicon alloys for toughness and over simpler Al-Si grades when elevated-temperature strength (from Cu) is required; it is chosen over wrought alloys when complex geometry or integrated cast features are necessary.

Temper Variants

Temper	Strength Level	Elongation	Formability	Weldability	Notes
O	Low	High (8–15%)	Limited (casting only)	Good (pre‑ and post‑heat controls)	As‑cast annealed or naturally cooled; softest condition.
T1	Low–Moderate	Moderate (6–12%)	Limited	Moderate	Cooled from casting and naturally aged; limited precipitation strengthening.
T5	Moderate	Low–Moderate (3–8%)	Poor	Moderate	Cooled from casting and artificially aged; common for castings requiring dimensional stability.
T6	High	Low (2–6%)	Poor	Challenging	Solution treated, quenched and artificially aged; peak strength for many applications.
T7	Moderate–High	Moderate (4–8%)	Poor	Moderate	Overaged condition for improved thermal stability and reduced stress sensitivity.

Temper exerts a strong influence on AlSi9Cu3 performance because the copper-rich phases formed during ageing dictate yield and tensile strength. T6 treatment (solution + artificial ageing) produces the highest strength and lowest ductility via precipitation of Cu-rich phases, while O and T1 states retain higher elongation but much lower static strength.

Chemical Composition

Element	% Range	Notes
Si	8.0–10.0	Primary alloying element; controls fluidity, shrinkage and eutectic microstructure.
Fe	0.3–1.3	Unavoidable impurity; forms intermetallics (β‑AlFeSi) that can embrittle if excessive.
Mn	0.05–0.5	Scavenges Fe to form less harmful intermetallics; improves toughness.
Mg	≤0.5	Usually low in this grade; can contribute to precipitation with Cu in complex phases.
Cu	2.5–3.5	Main strengthening element through precipitation; raises strength and hot hardness.
Zn	≤0.3	Minor; generally treated as impurity with little strengthening role.
Cr	≤0.2	Grain‑refining and control of recrystallization; small effect on strength.
Ti	≤0.2	Grain refiner to promote fine dendritic structure and improve mechanical properties.
Others (incl. Ni, Pb, Sn)	Balance/trace	Other elements kept minimal; can influence castability and machinability in small amounts.

Silicon sets the casting behavior and the morphology of the eutectic silicon plates/particles, which affects toughness and fatigue. Copper enables artificial ageing and higher elevated temperature strength, but increases susceptibility to certain corrosion modes and necessitates precise heat treatment control. Iron and manganese control brittle intermetallics formed during solidification; their balance is critical to avoid poor elongation and hot cracking.

Mechanical Properties

AlSi9Cu3 exhibits tensile behavior strongly dependent on heat treatment and solidification cooling rate. In the as-cast or O condition tensile strength is moderate due to coarse eutectic silicon and soft matrix; after T6 ageing, precipitation of Cu-bearing phases increases tensile and yield strengths substantially while reducing elongation. Yield strength is typically a significant fraction of the ultimate in peak-aged conditions, reflecting the effectiveness of copper precipitates in impeding dislocation motion.

Elongation is limited in T‑tempers because the eutectic silicon particles act as crack initiation sites and intermetallic phases reduce ductility. Hardness (Brinell or Vickers) rises in the order O < T5 < T6, mirroring tensile properties; hardness is also sensitive to section thickness and cooling rate during casting. Fatigue performance is tied to casting defects, porosity and eutectic silicon morphology; optimized feeding and heat treatment improve endurance limit but cast alloys generally have lower fatigue strength than wrought counterparts.

Section thickness strongly influences mechanical properties because thicker sections cool slower, producing coarser microstructures and larger intermetallics that reduce strength and ductility. Post‑casting homogenization and controlled solution treatments mitigate gradients but cannot fully eliminate section‑dependent variability. Designers must account for casting-induced anisotropy and machining/removal of surface defects to reach expected fatigue and tensile performance.

Property	O/Annealed	Key Temper (T6)	Notes
Tensile Strength (UTS)	120–180 MPa	260–340 MPa	Wide range depends on casting method, section thickness and ageing cycle.
Yield Strength (0.2% offset)	60–110 MPa	200–270 MPa	Yield fraction increases with Cu-rich precipitates and finer microstructure.
Elongation (% in 50 mm)	8–15%	2–6%	Elongation drops sharply after peak ageing; thicker sections sometimes show higher local elongation.
Hardness (HB)	40–70 HB	90–130 HB	HB scales with tensile properties; hardness also affected by eutectic Si morphology.

Physical Properties

Property	Value	Notes
Density	~2.70 g/cm³	Typical of general aluminum alloys; beneficial strength‑to‑weight ratio.
Melting Range	Solidus ~520–570 °C; Liquidus ~580–650 °C	Al–Si alloys have a freezing range due to eutectic and primary phase solidification; precise values depend on composition.
Thermal Conductivity	~120–160 W/m·K (room temp)	Lower than pure Al because of Si and intermetallics; still good for heat dissipation in many applications.
Electrical Conductivity	~25–36 %IACS	Reduced relative to pure Al due to alloying; not recommended where high conductivity is critical.
Specific Heat	~880–910 J/kg·K	Comparable to other Al alloys; useful for thermal mass calculations.
Thermal Expansion	~21–24 µm/m·K (20–200 °C)	Coefficient influenced by silicon content and microstructure; important for thermal stress design.

Physical properties reflect mixed requirements for castings: thermal conduction and specific heat make AlSi9Cu3 useful for heat‑dissipating parts, while density keeps mass low. Melting and solidification behavior control casting defect formation and the need for tailored risering and chills. Electrical conductivity is significantly reduced compared with pure Al, so the alloy is seldom chosen primarily for electrical applications.

Product Forms

Form	Typical Thickness/Size	Strength Behavior	Common Tempers	Notes
Sand castings	Wall thicknesses 3–50 mm	Variable; coarser microstructure in thick sections	O, T1, T5, T6	Widely used for low‑volume and larger components; porosity control important.
Die castings	Thin walls 1–8 mm	Finer microstructure, higher strengths	T5, T6	High pressure die casting yields good surface finish and reproducible properties.
Gravity die	3–30 mm	Intermediate cooling and properties	O, T5, T6	Good for medium complexity parts with tighter tolerances than sand casting.
Cast bars/ingots	Variable	Homogenised behavior post‑process	O, T1	Stock for remelting and downstream casting; used to control chemistry.
Investment castings	Thin‑to‑moderate sections	Good dimensional control; moderate strength	T5, T6	Employed where intricate geometry and surface finish are required.

Cast product form dominates the supply chain for AlSi9Cu3, and designers select the casting method to tune cooling rate, porosity and microstructure. Die casting yields the best mechanical repeatability and fine eutectic silicon, increasing tensile and fatigue properties versus sand casting. Machining allowances, heat treatment accessibility, and inspection for casting defects must be considered early in component design.

Equivalent Grades

Standard	Grade	Region	Notes
AA	AlSi9Cu3	International/USA	Common designation for cast alloy; compositions can vary between suppliers.
EN AW	AC‑AlSi9Cu3 (or AlSi9Cu3(Fe))	Europe	EN designations often append "(Fe)" to indicate controlled iron; mechanical data follow EN 1706 where applicable.
JIS	ADC10/ADC11 (similar)	Japan	ADC family alloys have similar Al–Si–Cu chemistries but differ in impurity limits and processing guidelines.
GB/T	AlSi9Cu3	China	Chinese standard uses same nominal composition but tolerances and testing requirements may differ.

Equivalency tables are approximate because each standard imposes different tolerances on impurities (Fe, Zn, Mn) and allows small compositional variations that shift casting characteristics and heat‑treat response. When substituting equivalents, verify mechanical data, recommended heat‑treat cycles and allowable defect levels, especially for critical fatigue or elevated‑temperature parts.

Corrosion Resistance

AlSi9Cu3 exhibits moderate atmospheric corrosion resistance typical of Al–Si casting alloys; naturally forming alumina provides a barrier but copper in the matrix can locally reduce corrosion performance. In industrial atmospheres it performs adequately if painted or coated, but exposed, untreated components may develop pitting or filiform corrosion where moisture and contaminants concentrate.

Marine environments are more aggressive: chloride‑induced pitting and crevice corrosion are primary concerns for AlSi9Cu3, especially in T‑tempers where galvanic couples with copper‑rich intermetallics and matrix differences accelerate localized attack. Protective coatings, sacrificial anodes, or corrosion‑resistant surface treatments are commonly applied for near‑shore applications.

Stress corrosion cracking is less common than in some high‑strength wrought alloys, but it can occur under tensile stress in chloride environments and in overaged conditions where intermetallic distributions create anodic sites. Galvanic interactions with dissimilar metals (steel, copper) should be managed by insulation or selection of compatible fasteners; AlSi9Cu3 is anodic relative to stainless steel and copper, so contact accelerates corrosion of the aluminum alloy. Compared with 5xxx and 6xxx wrought families, AlSi9Cu3 trades some natural corrosion resistance for higher casting performance and elevated‑temperature strength.

Fabrication Properties

Weldability

Welding cast AlSi9Cu3 is feasible with TIG and MIG processes but requires attention to porosity, hot cracking and filler selection. Use Al‑Si or Al‑Si‑Cu filler wires matched to the base alloy chemistry to minimize hot‑cracking and to reduce formation of low‑melting eutectics in the weld zone. Preheating and controlled interpass temperatures reduce thermal gradients and porosity; post‑weld solution treatment and ageing may be necessary to restore strength but can induce distortion.

Machinability

Machinability of AlSi9Cu3 is generally good for cast alloys but is influenced by eutectic silicon morphology and intermetallic particles that can harden tools. Carbide tooling with positive rake, high feed and moderate speeds is recommended; cutting fluids aid chip evacuation and heat control. Insert geometry that breaks chips and avoids long continuous swarf is beneficial; surface finish depends on silicon particle size and secondary finishing operations may be required.

Formability

As a casting alloy, AlSi9Cu3 has very limited cold formability and cannot be readily drawn or deep‑formed like wrought sheet alloys. Bend operations on cast thin sections are constrained by brittleness from eutectic silicon and intermetallics; minimum bend radii are typically large relative to thickness and depend on temper (O is more permissive than T6). If forming is required, design parts for near‑net shape casting and minimal post‑casting forming to reduce risk of cracking.

Heat Treatment Behavior

AlSi9Cu3 is heat‑treatable: the classical sequence is solution treatment, quenching and artificial ageing to develop Cu‑based precipitates and elevated strength. Typical solution temperatures range around 500–540 °C to dissolve copper and silicon phases; soak times depend on section thickness but commonly 2–6 hours for cast components. Rapid quenching (water) preserves a supersaturated solid solution and is followed by artificial ageing at ~160–200 °C for several hours to precipitate strengthening phases and reach T6 properties.

Overaging (T7) trades some peak strength for improved thermal stability and reduced susceptibility to cold cracking; this is used when parts operate at elevated temperatures or require dimensional stability. Incomplete solution or poor quench leads to heterogeneous properties and reduced peak strength. For parts that require only modest strength and higher ductility, natural ageing or T1 will be used, but the full potential of Cu strengthening is achieved only with controlled solution and artificial ageing.

For scenarios where heat treatment is impractical, some benefit can be gained by controlled work hardening of thin cast sections, although cast alloys do not respond as well to cold work as wrought alloys. Homogenization anneals can reduce segregation and dissolve some coarse intermetallics before final machining or heat treatment.

High-Temperature Performance

AlSi9Cu3 maintains better mechanical strength at elevated temperatures than many non‑Cu Al–Si casting grades because copper precipitates provide improved hot hardness. However, above roughly 150–200 °C the strength advantage diminishes as precipitates coarsen and the matrix softens; long‑term exposure above 200–250 °C will significantly reduce yield and fatigue life. Designers must therefore limit continuous service temperature or select overaged tempers that provide more stable but lower strength.

Oxidation is modest due to the protective alumina film, but high temperatures accelerate scale formation and can alter surface chemistry; protective coatings or paints are often used in high‑temperature environments. The heat‑affected zone (HAZ) around welds is susceptible to softening and precipitate dissolution, which reduces localized strength and may create stress concentrators; post‑weld heat treatment is recommended for critical components to restore uniform properties.

Applications

Industry	Example Component	Why AlSi9Cu3 Is Used
Automotive	Engine blocks, cylinder heads, gearbox housings	Good castability, thermal stability and elevated‑temperature strength with Cu ageing.
Marine	Pump housings, valve bodies (protected)	Castability for complex shapes and acceptable corrosion resistance with coatings.
Aerospace	Secondary structural fittings, housings	Favorable strength‑to‑weight and ability to cast complex geometries.
Electronics	Heat sinks, enclosures	Thermal conductivity and ease of casting detailed geometries for heat management.
Industrial Machinery	Hydraulic bodies, compressor parts	Dimensional stability, wear resistance (with surface treatments) and machinability.

AlSi9Cu3 excels where functional complexity, moderate-to-high static strength and thermal performance are required from cast parts. The alloy’s capacity for reliable T6 ageing makes it suitable for components that must retain properties after thermal cycles and machining.

Selection Insights

AlSi9Cu3 is a practical choice when a cast component requires a combination of good castability, elevated‑temperature strength and dimensional stability. Select it when near‑net shape casting avoids costly assemblies and when T6 heat treatment can be applied to achieve required strength.

Compared with commercially pure aluminum (1100), AlSi9Cu3 sacrifices electrical conductivity and formability but delivers substantially higher static and elevated‑temperature strength, making it appropriate for structural castings. Compared with common work‑hardened alloys such as 3003 or 5052, AlSi9Cu3 offers higher strength and better high‑temperature performance at the cost of lower ductility and potentially less uniform corrosion resistance. Compared with heat‑treatable wrought alloys like 6061, AlSi9Cu3 typically has lower peak specific strength in thin sections but is preferred where complex cast geometry and integrated features outweigh the maximum achievable strength of wrought extrusions and forgings.

Use a short procurement checklist: confirm casting method and section sizes, specify temper and heat‑treat schedule, require porosity and NDT limits for fatigue parts, and verify equivalent standard tolerances (EN, JIS, GB/T) if cross‑sourcing material.

Closing Summary

AlSi9Cu3 remains relevant because it fills a niche where castability, thermal performance and precipitation‑hardenable strength are needed in a single material system. Its balanced Si–Cu chemistry allows designers to produce complex, durable parts with controlled heat treatment, making it a mainstay for automotive, industrial and thermal management components where near‑net shape manufacture and service stability are priorities.