Aluminum A390: Composition, Properties, Temper Guide & Applications

Comprehensive Overview

A390 is a hypereutectic aluminum-silicon casting alloy in the 3xx/4xx-style family of cast Al‑Si‑Cu‑Mg materials rather than a wrought 6xxx or 7xxx series. Its chemistry is dominated by very high silicon (typically ~17–19 wt%) with copper and magnesium as secondary strengthening elements, combined with small levels of iron, manganese and trace titanium for grain control and modification.

The primary strengthening mechanism is precipitation hardening of the aluminium matrix by Cu/Mg intermetallics after solution treatment and artificial aging, together with microstructural strengthening from hard, primary silicon particles distributed through the matrix. This makes A390 a heat-treatable casting alloy with a microstructure and mechanical response distinct from work‑hardened wrought alloys.

Key traits include high wear resistance and compressive contact strength due to the large hard Si particles, good dimensional stability after heat treatment, moderate corrosion resistance that is degraded by copper additions, and limited ductility and formability compared with common wrought alloys. Typical industries using A390 are automotive (pistons, cylinder liners, wear inserts), hydraulic and pneumatic components, pumps, and some heavy‑duty engine components where wear resistance and castability are critical.

Engineers choose A390 when a combination of hypereutectic wear resistance, castability into complex shapes, and the ability to reach strengthened T6‑like conditions is required; it is selected over lower‑Si casting alloys when surface stability under sliding or abrasive contact and tight thermal dimensional control are priorities.

Temper Variants

Temper	Strength Level	Elongation	Formability	Weldability	Notes
As‑Cast (F)	Low–Moderate	Low (1–4%)	Poor	Limited	As‑cast microstructure; primary Si present; minimal ductility
O / Annealed	Low	Moderate	Improved relative to F	Limited	Softened matrix by solutionizing/anneal to relieve stresses
T5 (Artificially aged as‑cast)	Moderate	Low (1–3%)	Poor	Limited	Rapid artificial aging from cooled casting without prior solutionizing
T6 (Solution treated + artificial aging)	High	Low (0.5–3%)	Poor	Limited	Peak strength for A390; typical for pistons and wear parts
T7 / Overaged	Moderate	Low (1–4%)	Poor	Limited	Stabilized, improved thermal stability at expense of peak strength

Temper has a pronounced effect on A390 because silicon morphology and the distribution of Cu/Mg precipitates directly control strength and toughness. Solution treatment followed by quench and artificial aging (T6) maximizes matrix strength but does little to improve elongation because the large primary Si particles remain a limiting factor.

In practical terms, designers trade ductility for hardness and wear resistance: as‑cast and T5 conditions are used where minimal thermal processing is preferred, while T6 is specified where higher tensile and yield strengths and improved fatigue resistance are needed.

Chemical Composition

Element	% Range	Notes
Si	17.0–19.0	Primary strengthening and wear phase; hypereutectic content produces primary Si particles.
Fe	0.6–1.2	Intermetallic forming element; too much causes brittle Fe‑phases and lowers fatigue.
Mn	0.2–0.6	Helps modify Fe intermetallics; improves toughness slightly.
Mg	0.3–0.6	Contributes to precipitation hardening with Cu as Mg2Si and complex precipitates.
Cu	3.5–4.5	Major precipitation hardener; improves strength but reduces corrosion resistance.
Zn	≤0.25	Minor; usually impurity level, little strengthening effect.
Cr	≤0.2	Scavenges Fe and stabilizes microstructure in some melts.
Ti	0.02–0.12	Grain refiner for castings, controls nucleation of Al matrix.
Others (Ni, Sr, Sr modifiers)	≤0.5 cumul.	Ni may be added for high‑temperature stability; Sr used for Si modification in some melts.

The high silicon fraction produces a two‑phase microstructure of Al matrix and hard Si particles that dominates wear and stiffness. Copper and magnesium together form precipitates after heat treatment that significantly raise strength and hardness, while iron and manganese control brittle intermetallics that influence fatigue and fracture. Small additions of Ti or Sr are used during foundry processing to refine grain structure and modify silicon particle morphology to improve casting properties.

Mechanical Properties

A390 exhibits a combination of relatively high compressive and wear strength with limited tensile ductility because of its hypereutectic silicon phase. In T6 condition the aluminium matrix contributes significant yield and tensile strength through Cu/Mg precipitates, but elongation remains low and fracture tends to be controlled by the brittle silicon particles and intermetallics. Fatigue performance is strongly dependent on casting quality, porosity, and the size/distribution of primary Si particles; smooth surface finishes and heat treatment can improve fatigue life but do not eliminate the Si‑controlled crack initiation behavior.

Thickness and section size have strong effects because cooling rate during solidification determines primary Si size and distribution and eutectic spacing; thicker sections cool slower, producing coarser Si and lower mechanical properties. Hardness correlates with temper and microstructure: as‑cast hardness is moderate and rises substantially after solutionizing and artificial aging to T6 conditions, where HB numbers approach those required for wear‑resistant components.

Property	As‑Cast / Annealed (F/O)	Typical Key Temper (T6)	Notes
Tensile Strength (UTS)	140–220 MPa	280–360 MPa	T6 values depend on heat‑treat time/temperature; wide scatter due to porosity and Si morphology.
Yield Strength (0.2% offset)	70–140 MPa	220–320 MPa	Yield rises sharply with T6; as‑cast yield is low and variable.
Elongation (A%)	1–6%	0.5–3%	Low ductility typical; higher elongation possible in thin, refined castings.
Hardness (HB)	70–110 HB	110–160 HB	Hardness increases with aging; high HB correlates with wear resistance.

Physical Properties

Property	Value	Notes
Density	~2.75 g/cm³	Slightly higher than pure Al due to Cu; high Si lowers density marginally.
Melting Range (solidus–liquidus)	~520–585 °C	Hypereutectic alloy with broad solidification range; primary Si crystallizes early.
Thermal Conductivity	~90–120 W/m·K	Lower than pure Al and low‑Si alloys; conductivity reduced by Cu and Si particles.
Electrical Conductivity	~25–35 %IACS	Alloying and intermetallics reduce conductivity compared with commercial pure Al.
Specific Heat	~0.88–0.95 kJ/kg·K	Typical for Al alloys; varies slightly with temp and composition.
Thermal Expansion (20–200 °C)	~21–23 µm/m·K	Coefficient influenced by high Si; overall CTE modestly reduced vs wrought Al.

A390’s composite‑like microstructure (Al matrix with hard Si particles) reduces thermal conductivity and electrical conductivity relative to pure aluminium but improves wear stability and thermal dimensional stability in sliding contacts. The melting and solidification behavior is important for casting process design, since primary Si crystallization can impact feeding, shrinkage behavior and tool wear during die casting and permanent mold casting.

Product Forms

Form	Typical Thickness/Size	Strength Behavior	Common Tempers	Notes
Sheet	Not typical	N/A	N/A	A390 is not produced as thin wrought sheet; unsuitable for rolling/forming.
Plate	Limited / Thick castings (≥10 mm)	Variable with section	F, T5, T6	Thick cast plates can be produced by gravity or permanent mold casting for heavy parts.
Extrusion	Not applicable	N/A	N/A	A390 is a casting alloy and is not used for extrusion.
Tube	Rare as cast tubes	Variable	F, T6	Cast tubes possible for specialized hydraulic components; not common.
Bar/Rod	Ingot bars/forging blanks	Variable	F, T6	Typically supplied as castings or ingots for further machining; wrought bar is uncommon.

A390 is primarily supplied and used in cast forms — high‑pressure die cast, gravity/permanent mold, and precision sand castings are the normal production routes. The alloy’s high silicon content promotes low thermal expansion and reduced shrinkage but increases tool and mold wear, so foundry practice and tooling materials are significant considerations. Designers should plan near‑net shapes to minimize post‑machining and select casting processes consistent with required section thicknesses to control silicon morphology and porosity.

Equivalent Grades

Standard	Grade	Region	Notes
AA	A390	USA	Aluminium Association casting designation for hypereutectic Al‑Si‑Cu‑Mg alloy.
EN AW / EN AC	AlSi17Cu4 / EN AC‑43400 (approx.)	Europe	Approximate EN simplifications exist; verify chemical and mechanical spec per specific standard.
JIS	ADCxx (approx.)	Japan	No exact one‑to‑one JIS equivalent; some ADC alloys are similar but differ in Cu/Si balance.
GB/T	A390 (or AlSi17Cu4)	China	Chinese standards may use analogous designations; check local spec for exact limits.

Cross‑references to international standards are approximate because casting standards use different tolerances, impurity caps and mechanical testing requirements. Engineers must compare full chemical composition tables and mechanical‑property testing conditions (casting method, heat treatment, porosity limits) when substituting grades across regions.

Corrosion Resistance

A390 has moderate atmospheric corrosion resistance typical of Al‑Si casting alloys, but the relatively high copper content reduces its resistance compared with low‑copper alloys. In industrial or mildly corrosive rural atmospheres the alloy forms a protective oxide film, but copper and intermetallic particles can act as local cathodes and increase the risk of pitting and localized corrosion, particularly if the matrix is not well passivated.

Marine exposure is more challenging: chloride environments accelerate pitting and crevice corrosion and the presence of copper exacerbates localized attack. For marine or aggressive chloride service, protective coatings, anodizing (where applicable) or sacrificial design measures are usually required.

Stress corrosion cracking (SCC) is less common in heavily Si‑reinforced casting alloys than in highly stressed, high‑strength wrought Al‑Cu alloys, but residual stresses from casting and heat treatment combined with corrosive environments can promote cracking at defect sites such as porosity or large Si particles. Galvanic interactions are important to consider; A390 is anodic to many stainless steels and nickel alloys and will corrode preferentially, so isolation or appropriate coating systems are recommended. Compared with 5xxx and 6xxx wrought alloys, A390 trades some corrosion resistance for wear and strength in cast parts.

Fabrication Properties

Weldability

Welding A390 is difficult and typically discouraged because primary Si and intermetallics cause hot cracking and poor fusion zones. Local melting during TIG or MIG can produce brittle weld metal and significant HAZ softening; filler alloys must be selected to balance ductility and corrosion resistance, and pre‑ and post‑weld heat treatments are limited in effectiveness. For repair welding, specially formulated Al‑Si‑Cu fillers and strict control of weld heat input, interpass temperature and cleanliness are required, but machined and bolted repairs are often preferred.

Machinability

Machining behaviour is good in many cases because the hard Si phase acts as an in‑service cutting abrasion that increases tool wear but provides high metal removal rates; carbide tooling is recommended and cutting fluids should be used to manage heat and chip evacuation. Typical machinability indices exceed many wrought alloys because of the fractureable matrix and brittle Si particles, but tool life is largely influenced by silicon particle morphology and casting porosity. High speed machining with rigid setups, PCBN or coated carbide inserts, and interrupted cutting strategies work well for A390 components.

Formability

Cold forming and conventional bending are very limited because of low ductility and the presence of large primary Si phases that promote cracking. Small, local deformations are possible in annealed or specially treated castings, but typical forming should be engineered by casting near final geometry. Hot and semi‑solid forming techniques exist for Al‑Si cast alloys, but these require dedicated processes and are not common for standard A390 cast components.

Heat Treatment Behavior

A390 is a heat‑treatable hypereutectic casting alloy where controlled solution treatment and artificial aging generate the desired precipitate structure in the aluminium matrix. Typical solution treatment temperatures are in the range of 500–540 °C for times dependent on section thickness to dissolve soluble Cu and Mg constituents, followed by rapid quenching to retain a supersaturated solid solution. Artificial aging is commonly performed at 150–200 °C for periods that might range from 2 to 10 hours to achieve peak T6 strength; times and temperatures are optimized for component size and desired properties.

Because primary Si particles are stable at heat‑treatment temperatures, heat treatment modifies matrix properties without appreciably changing the brittle Si content; this is why elongation improvements are limited. Overaging (T7) produces coarsening of precipitates for improved thermal stability and stress relaxation at a cost in peak strength, which can be a useful trade for high‑temperature dimensional stability. For castings, controlling quench severity and minimizing quench‑induced distortion and residual stress are practical concerns; some castings require solution‑treatment fixtures or modified quench media to manage distortion.

High-Temperature Performance

A390’s mechanical properties degrade with increasing temperature due to precipitate coarsening and reduced matrix strength; usable structural strength typically falls off above ~150–200 °C. For continuous service at elevated temperatures the T7 or overaged conditions offer better stability albeit at lower room‑temperature strength, while short exposure to higher temperatures can cause partial reversion of aging and strength loss. Oxidation of aluminium is minimal compared to ferrous metals, but the presence of copper-rich intermetallics can affect high‑temperature corrosion behavior in oxidizing or chloride environments.

The heat‑affected zone during localized heating (welding, friction) can experience softening and embrittlement; design must account for creep, stress relaxation and dimensional drift in high‑temperature applications. For cyclic thermal exposure, differential thermal expansion between Al matrix and hard Si particles can produce microcracking over time, so component geometry and supports should mitigate thermal strain concentrations.

Applications

Industry	Example Component	Why A390 Is Used
Automotive	Pistons and piston skirts	Hypereutectic Si provides wear resistance and reduced thermal expansion; good dimensional stability.
Automotive / Powertrain	Cylinder liners, wear rings	High surface hardness and lubricated wear properties for sliding contacts.
Hydraulic / Pneumatic	Valve bodies, pump housings	Castability into complex geometries and good strength after T6.
Industrial Machinery	Bearings and bushings	Wear resistance and compressive strength for repeated contact loading.
Electronics / Thermal	Heat‑resistant housings (limited)	Good thermal stability and machinability for precision parts.

A390 is chosen when components demand high wear resistance, dimensional stability under thermal cycling, and the ability to cast complex near‑net shapes. Its combination of hypereutectic silicon microstructure and precipitation‑hardening matrix is particularly well suited to reciprocating and sliding components where service life under contact loading is critical.

Selection Insights

A390 is appropriate when wear resistance and castability are prioritized over ductility and electrical conductivity; choose A390 for pistons, liners and wear inserts where hypereutectic Si provides durable sliding surfaces. Compared with commercially pure aluminium (1100), A390 trades substantially lower electrical conductivity and formability for much higher hardness, wear resistance and compressive strength under contact loads.

Against work‑hardened alloys like 3003 or 5052, A390 offers far greater wear resistance and higher achievable strength after T6, but typically worse corrosion resistance and much lower formability; those wrought alloys are better where forming and corrosion are primary concerns. Compared with common heat‑treatable wrought alloys such as 6061/6063, A390 provides superior abrasive and seizure resistance and better thermal dimensional stability in cast components, and is preferred where cast near‑net complexity and wear trump the higher peak tensile ductility of wrought 6xxx series alloys.

Closing Summary

A390 remains an important engineering casting alloy where hypereutectic silicon morphology, castability into complex geometries, and precipitation‑strengthened matrix properties coincide to deliver high wear resistance and dimensional stability. Its specific strengths make it a frequent choice for high‑duty sliding and reciprocating components in automotive and industrial applications, provided designers account for its limited ductility and corrosion trade‑offs.