Aluminum 319: Composition, Properties, Temper Guide & Applications

Comprehensive Overview

319 is a cast aluminum alloy belonging to the 3xx series of Al-Si-Cu casting alloys. It is primarily designed as a heat-treatable, silicon-enriched aluminum casting alloy where copper is added to raise strength and improve mechanical stability at elevated temperatures.

The major alloying elements are silicon and copper, with controlled levels of iron, manganese, magnesium, chromium and trace titanium and others. Strengthening derives predominantly from solution treatment and artificial aging (precipitation hardening of Cu-rich phases) combined with the microstructural refinement of eutectic silicon and intermetallic dispersions.

Key traits of 319 include relatively high as-cast and age-hardened strength, good thermal stability, reasonable corrosion resistance for automotive environments, and good castability for complex thin-wall components. Weldability and machinability are good with appropriate consumables and practices, while formability is limited compared with wrought alloys; this makes 319 ideal for cast components rather than formed sheet or extrusions.

Typical industries include automotive powertrain and structural castings, engine and transmission components, pump housings, and some marine fittings. Engineers choose 319 when complex geometries and moderate-to-high strength after heat treatment are required and when cast processing and dimensional integration outweigh the benefits of wrought products.

Temper Variants

Temper	Strength Level	Elongation	Formability	Weldability	Notes
O (annealed/as-cast)	Low	Moderate	Limited	Good with repair procedures	As-cast or stress-relieved condition; highest ductility among cast tempers
T5	Medium-High	Moderate-Low	Limited	Good with preheat	Cooled from casting and artificially aged; improves strength without solutionizing
T6	High	Low-Moderate	Limited	Repairable; HAZ softening risk	Solution heat-treated and artificially aged; common production temper for 319
T7	Medium	Moderate	Limited	Good with proper filler	Overaging stabilization for improved thermal stability and dimensional stability
Hxxxx (local cold work)	Variable	Variable	Poor	Often requires special procedures	Local cold work used rarely; most 319 applications rely on heat treatment rather than extensive cold forming

Temper significantly controls the balance between strength and ductility for 319 castings. T6 yields the highest practical strength for many components but reduces ductility and increases the risk of HAZ softening around weld repairs, while T7 or T5 are used where thermal stability or as-cast strength without full solution treatment is desired.

Chemical Composition

Element	% Range	Notes
Si	5.5–7.5	Primary casting alloying element; improves fluidity and reduces shrinkage
Fe	≤1.3	Impurity element; forms intermetallics that can embrittle and affect fatigue
Mn	0.2–0.6	Controls Fe intermetallic morphology and improves toughness
Mg	0.05–0.45	Minor contributor to age-hardening in some tempers; often controlled low
Cu	2.5–4.0	Principal strengthening alloying element via precipitation of Cu-rich phases
Zn	≤0.2	Minor element; generally limited to control corrosion effects
Cr	0.04–0.25	Refines grain structure and stabilizes microstructure against overaging
Ti	0.02–0.12	Grain refiner for casting microstructure control
Others	≤0.15	Includes Ni, Pb, Sn, Bi and residuals; kept low to maintain castability and mechanical behavior

The composition ranges above are representative of common A319 specifications; actual limits depend on the supplying standard and foundry practice. Silicon sets the casting behavior and eutectic morphology, while copper provides precipitate-strengthening after solution treatment and aging; iron and manganese control intermetallic phase morphology that influences ductility and fatigue.

Mechanical Properties

Tensile behavior for 319 shows a distinct dependence on temper and section thickness. In the as-cast or minimally treated condition the alloy displays moderate ultimate tensile strength with reasonable elongation, while solution-treated and artificially aged conditions (T6) produce a marked increase in yield and ultimate strengths at some cost to ductility.

Yield strength is improved substantially by precipitation of Cu-rich phases in the T6 condition; typical yield-to-ultimate ratios indicate a relatively narrow elastic-plastic transition compared with more ductile wrought alloys. Elongation is often limited in heavy cast sections due to coarser silicon eutectic and intermetallic networks, so design should account for low ductility in thick-wall components.

Hardness correlates with temper and microstructure, rising noticeably after solutionizing and aging; Brinell values reflect this with T6 significantly harder than as-cast. Fatigue performance is moderate for 319 and strongly influenced by casting defects, surface finish, and the presence of intermetallics; shot-peening, surface machining and proper heat treatment are common fatigue-improvement strategies.

Property	O/Annealed	Key Temper (T6)	Notes
Tensile Strength	180–240 MPa (approx)	260–350 MPa (approx)	Wide variation with section thickness and casting method
Yield Strength	90–140 MPa (approx)	170–240 MPa (approx)	Cu precipitation raises yield markedly in T6
Elongation	2–10% (varies with section)	1–6% (varies with section)	Elongation falls in T6 and with increasing section thickness
Hardness	60–90 HB (approx)	90–130 HB (approx)	Hardness corresponds to precipitation state and silicon morphology

Physical Properties

Property	Value	Notes
Density	2.68 g/cm³	Typical for Al–Si casting alloys; good strength-to-weight ratio
Melting Range	~520–640 °C	Solidus-liquidus range depends on Si and Cu content; eutectic features present
Thermal Conductivity	~120 W/m·K (approx)	Lower than pure Al due to alloying; high enough for many thermal applications
Electrical Conductivity	~30–40 % IACS (approx)	Reduced relative to pure Al due to alloy content and intermetallics
Specific Heat	~900 J/kg·K	Typical for aluminum alloys in room-temperature range
Thermal Expansion	~22–24 µm/m·K	Coefficient of thermal expansion similar to other Al–Si casting alloys

The physical property set supports the selection of 319 for thermally loaded castings where weight saving and reasonable thermal conductivity are important. Melting and solidification behavior are crucial for mold design and porosity control because the alloy has a broad freezing range and forms complex intermetallics.

Product Forms

Form	Typical Thickness/Size	Strength Behavior	Common Tempers	Notes
Casting (sand, permanent mold, die)	Thin walls to heavy sections (1 mm to >100 mm)	Varies widely with section thickness and cooling rate	O, T5, T6, T7	Primary product form; excellent for integrated complex geometries
Plate / Casting-Plate	Up to several tens of mm (as cast or homogenized)	Similar to cast behavior; rolling uncommon	O, T6 after heat treatment	Rare as rolled plate; usually cast-to-size and finish-machined
Extrusion	Not typical	Not applicable	—	319 is not produced as standard extrusion stock; composition not optimized for extrusion
Tube	Limited (cast tube or fabricated)	Varies	O, T6	Cast tubing or machined from cast blanks used for specialized parts
Bar / Rod	Limited (cast bar)	Varies	O, T6	Available as cast billets or ingots for machining; not common as wrought rod

319 is primarily a casting alloy and most product forms are cast components produced by sand, permanent-mold or pressure-die methods. Wrought forms and traditional sheet/plate/extrusion products are uncommon or nonstandard because the alloying balance is optimized for castability and precipitation hardening rather than extensive cold work.

Equivalent Grades

Standard	Grade	Region	Notes
AA	319	USA	Aluminum Association casting alloy designation; common baseline specification
EN	AlSi9Cu (approx)	Europe	Approximate match in composition and intended use; exact mechanicals differ by spec
JIS	AC9x (approx)	Japan	Japanese casting classes with comparable Si–Cu families; verify specific JIS number
GB/T	AlSi9Cu3 (approx)	China	Common Chinese casting alloy with similar balance of Si and Cu; check local tolerances

There is no single one-to-one global standard that exactly matches A319 because casting alloy families vary regionally and standards group alloys differently. Equivalent grades listed above are approximate composition or function matches and engineers must compare specific chemical and mechanical limits in each standard before substitution.

Corrosion Resistance

Atmospheric corrosion resistance of 319 is generally good for automotive and industrial environments because the silicon-rich matrix and controlled copper content provide reasonable passive behavior. Copper, however, reduces corrosion resistance relative to very low-alloy aluminum grades and can increase susceptibility to localized attack in aggressive chloride environments.

In marine or high-chloride exposure, 319 performs moderately well but will not match specialized marine alloys such as 5xxx-series Al–Mg alloys or certain stainless materials; sacrificial coatings, anodizing or protective paints are commonly applied for prolonged service. Pitting resistance is affected by casting porosity, surface finish and heat treatment, so post-casting sealing or machining often improves long-term performance.

Stress corrosion cracking (SCC) is not a dominant failure mode for 319 under normal service temperatures, but the presence of copper and tensile residual stresses (for example from welding) can increase SCC risk in highly aggressive environments. Galvanic interactions with more noble metals (e.g., stainless steel, copper) can accelerate localized corrosion at contact points, so isolation or coatings are recommended where dissimilar-metal contact occurs.

Compared with other alloy families, 319 offers better corrosion resistance than high-copper wrought alloys but inferior resistance to 5xxx-series Al–Mg alloys; designers select 319 where castability and thermal stability are prioritized while accepting moderate corrosion protection measures.

Fabrication Properties

Weldability

Welding of 319 castings is feasible using TIG, MIG or brazing techniques when proper preheat and filler selection are used. Aluminum-silicon filler alloys such as ER4043 or ER4047 are commonly recommended to reduce hot-cracking tendencies and to accommodate differences in thermal expansion and melting behavior.

The heat-affected zone can experience local softening due to dissolution or coarsening of precipitates in heat-treated components, and repair welds should be followed by appropriate thermal treatments where dimensional stability and mechanical properties are critical. Preheating, controlled interpass temperatures, and minimization of restraint will reduce cracking and distortion during welding.

Machinability

319 is regarded as a good candidate for machining due to the presence of eutectic silicon which promotes chip breaking and abrasion resistance of the tool does not degrade excessively. Carbide tooling with positive rake and adequate coolant control is recommended for higher material removal rates and predictable surface finish.

Cutting speeds for aluminum casting alloys are generally high compared with steels, but should be adjusted for silicon content and section hardness; tool life benefits from adequate chip evacuation and avoiding prolonged rubbing. Surface finish and dimensional accuracy are strongly influenced by casting porosity and microstructural heterogeneity; finish machining often follows a stress-relief or solution-treatment step.

Formability

Forming 319 is limited because it is a casting alloy not intended for large plastic deformation processes; bending, stretching or deep drawing are typically done only on thin or specially prepared sections. Best practices favor designing cast features into the part geometry to avoid post-cast forming and to exploit casting’s capability for complex shapes.

Local cold forming or mechanical bending may be used for minor adjustments, but the low ductility in age-hardened tempers and the brittle nature of certain intermetallics limit extensive forming. Where formed features are required, consider using alternative wrought alloys or designing the feature into the casting to eliminate forming steps.

Heat Treatment Behavior

319 is a heat-treatable casting alloy that responds well to solution treatment and artificial aging cycles to develop precipitation-strengthened microstructures. Solution treatment typically occurs at temperatures in the range of ~505–545 °C for several hours depending on section thickness, with a rapid quench to retain solute in solid solution.

Artificial aging (T6) is commonly performed at temperatures between ~150–200 °C for several hours to precipitate Cu-rich phases (theta prime and related intermetallics) that increase strength and hardness. T5 (direct aging) is used when full solutionizing is not practical; it provides improvement but typically less peak strength than full T6.

T7 or overaged conditions are used when thermal stability and resistance to property changes in service are required; overaging coarsens precipitates, lowers peak strength but improves dimensional stability and resistance to thermal softening. Control of quench rates, aging profiles and section-specific soak times is crucial to achieve consistent properties and to limit quench-induced distortion or residual stresses.

High-Temperature Performance

319 retains useful mechanical strength up to moderate elevated temperatures, but precipitation-strengthened phases begin to coarsen above service temperatures typically exceeding ~150–200 °C. For continuous exposure above these temperatures, designers should expect progressive strength loss and consider overaged tempers or alloys specifically designed for high-temperature stability.

Oxidation of aluminum at elevated temperatures is modest and forms a protective oxide layer, but environment and chloride presence can alter oxidation behavior and accelerate degradation. Weld heat-affected zones and local reheated regions can experience softening and loss of peak properties when exposed to high temperatures or repeated thermal cycling.

For components subject to thermal fatigue or cyclic thermal loading, careful selection of temper (T7 or stabilized tempers), control of residual stress and the use of coatings or anodic protection can improve life. For sustained high-temperature loads, alloys specifically intended for elevated temperatures or using different material classes may be required.

Applications

Industry	Example Component	Why 319 Is Used
Automotive	Transmission housings and engine components	Good castability, dimensional integration and age-hardened strength
Marine	Pump housings and structural cast fittings	Reasonable corrosion resistance with good cast detail capability
Aerospace	Non-critical fittings and brackets	High strength-to-weight for cast complex parts and thermal stability
Electronics	Enclosures and heatsink housings	Adequate thermal conductivity and ability to cast integrated features

319 is often chosen where complex geometry, integral mounting bosses, thin-wall castings and post-machining consolidation reduce assembly and weight. The alloy balances castability, strength after heat treatment, and cost for many mass-produced components in automotive and transport sectors.

Selection Insights

When selecting 319, favor it for complex cast geometries requiring moderate-to-high strength after heat treatment and where integrated features reduce assembly operations. Its castability and T6 mechanical performance make it a pragmatic choice for automotive and industrial housings where wrought alternatives are impractical.

Compared with commercially pure aluminum (1100), 319 trades electrical and thermal conductivity and formability for markedly higher strength and better thermal/mechanical stability. Compared with work-hardened alloys such as 3003 or 5052, 319 provides higher age-hardened strength but generally less corrosion resistance in some environments; design compensation via coatings or corrosion allowances may be needed.

Compared with common heat-treatable wrought alloys such as 6061/6063, 319 may have lower peak performance