Aluminum 4040: Composition, Properties, Temper Guide & Applications

Comprehensive Overview

4040 is an aluminum-silicon series alloy belonging to the 4xxx family, characterized by silicon as the principal alloying element. The 4xxx series is known for silicon-enhanced fluidity and weldability rather than for peak strength from precipitation hardening.

Major alloying elements in 4040 include silicon as the dominant addition, with controlled amounts of iron, manganese and trace additions of chromium and titanium to refine structure and control grain growth. The alloy is primarily strengthened by solid-solution effects from silicon and by strain hardening; it is considered non-heat-treatable in the classical precipitation-hardening sense.

Key traits of 4040 include moderate strength combined with good weldability, respectable thermal conductivity and improved fluidity for welding and brazing applications. Corrosion resistance in typical atmospheric environments is fair to good; formability in annealed tempers is excellent while work-hardened tempers offer increased strength at the expense of ductility.

Typical industries using 4040 include automotive (especially filler wire and structural extrusions), transportation, consumer goods, and fabricated assemblies requiring reliable weldability and good surface finish. Engineers select 4040 when a combination of welded-joint performance, thermal conductivity and moderate strength is required without needing the higher fill-strength of heat-treatable alloys.

Temper Variants

Temper	Strength Level	Elongation	Formability	Weldability	Notes
O	Low	High (20–35%)	Excellent	Excellent	Fully annealed condition for maximum ductility
H12	Medium-Low	Moderate (12–18%)	Good	Excellent	Quarter-hard, retains good formability for light drawing
H14	Medium	Low-Moderate (8–12%)	Good	Excellent	Half-hard trade-off between strength and ductility
H16	Medium-High	Low (6–10%)	Fair	Excellent	Three-quarter-hard for increased stiffness
H18	High	Low (4–8%)	Limited	Excellent	Full-hard condition for highest strength from cold work
H24	Medium	Moderate (10–15%)	Good	Excellent	Strain-hardened and partially annealed for form/fabrication balance

Tempers in 4040 are almost exclusively work-hardening (H‑series) or fully annealed (O‑series), because the alloy does not respond to classical T‑type precipitation hardening. Choosing a harder H‑temper raises yield and tensile strength but reduces elongation and cold formability; O‑temper is used where deep drawing and bending are required. Weldability is retained across most H‑tempers due to silicon’s favorable effect on melt fluidity and reduced hot-cracking tendency.

Chemical Composition

Element	% Range	Notes
Si	0.6 – 1.2	Primary alloying element; improves fluidity, reduces melting range and increases silicon solid-solution strengthening
Fe	0.3 – 0.9	Common impurity; forms intermetallics that can affect toughness and surface finish
Mn	0.2 – 0.8	Grain-refiner and strength contributor through dispersoids; improves corrosion resistance marginally
Mg	0.02 – 0.20	Low content; small effects on strength and work hardening behavior
Cu	≤ 0.20	Controlled low addition; higher amounts reduce corrosion resistance and are minimized
Zn	≤ 0.10	Kept low to avoid unwanted strengthening and reduced corrosion performance
Cr	0.02 – 0.20	Controls grain growth and improves elevated-temperature microstructure stability
Ti	0.01 – 0.10	Microalloy for grain refinement in cast and wrought products
Others (each)	≤ 0.05	Trace elements and residuals; remainder aluminum

Silicon is the dominant element shaping 4040 performance: it lowers the solidus-liquidus gap, improves castability and weld pool fluidity, and contributes modest solid-solution strengthening. Iron and manganese form intermetallic phases that influence strength, fatigue crack initiation and surface characteristics; careful control of these elements is critical for extrusion quality and forming performance.

Mechanical Properties

In annealed (O) condition 4040 exhibits relatively low yield and tensile strengths but high ductility, making it suitable for deep drawing and complex forming operations. When strain-hardened to H‑tempers, the alloy’s tensile and yield strengths rise substantially due to dislocation accumulation; however, the ductility and toughness decline, and susceptibility to localized thinning increases. Hardness correlates closely with temper: annealed material is soft and easy to machine or form, whereas H18 is substantially harder and provides improved stiffness but reduced formability.

Fatigue resistance in 4040 is moderate and highly dependent on surface finish, temper, and the presence of casting or extrusion-related defects; cold working to H‑tempers can improve high‑cycle fatigue strength but also raises sensitivity to stress concentrators. Thickness strongly affects mechanical performance: thin-gauge sheet tends to achieve higher apparent strength in H‑tempers due to strain distribution during cold work, while thicker sections may retain more ductility in comparable tempers.

Property	O/Annealed	Key Temper (H14 / H18)	Notes
Tensile Strength	95 – 130 MPa	170 – 230 MPa	H14≈180–200 MPa typical; values vary with cold work and gauge
Yield Strength	30 – 55 MPa	120 – 170 MPa	H18 approaches upper range; yield increases rapidly with strain hardening
Elongation	25 – 35%	4 – 12%	Elongation drops markedly with higher H‑numbers
Hardness (HB)	20 – 35 HB	55 – 85 HB	Brinell hardness rises with H‑temper and correlates to tensile strength

Physical Properties

Property	Value	Notes
Density	2.70 g/cm³	Typical for wrought aluminum alloys; used for mass and stiffness calculations
Melting Range (solidus–liquidus)	~575 – 650 °C	Silicon lowers solidus compared with pure Al; exact range depends on Si and trace elements
Thermal Conductivity	130 – 170 W/m·K (25 °C)	Good conductor; slightly reduced compared with purer Al due to alloying
Electrical Conductivity	~40 – 50 % IACS	Lower than pure Al; alloying and cold work reduce conductivity
Specific Heat	~0.90 J/g·K (900 J/kg·K)	Useful for thermal transient and heat-sink calculations
Thermal Expansion	23 – 24 µm/m·K	Typical CTE for aluminium alloys; design for thermal mismatch is required with other materials

4040’s thermal conductivity and specific heat make it effective for heat‑dissipation components where welding is also required. The density and CTE are consistent with typical aluminum engineering practice and allow substitution into weight‑sensitive designs with predictable thermal expansion behavior.

Product Forms

Form	Typical Thickness/Size	Strength Behavior	Common Tempers	Notes
Sheet	0.3 – 6.0 mm	Thin gauges show higher apparent strength in H‑tempers	O, H12, H14	Used for body panels, heat sinks and welded assemblies
Plate	6 – 25 mm	Retains ductility in O; H‑tempers used for stiff panels	O, H14, H18	Thick sections require careful control of residual stresses
Extrusion	Sections up to several hundred mm	Strength varies with profile thickness and cooling	O, H24, H14	Good extrudability due to silicon, used for complex profiles
Tube	OD 6 – 200 mm	Strength governed by wall thickness and temper	O, H14	Common for welded tubing and structural sections
Bar/Rod	6 – 100 mm dia	Cold drawing increases strength; bars used for machined parts	O, H18	Solid sections for fittings and fasteners; surface finish critical for fatigue parts

Processing differences (rolling vs extrusion vs casting) create distinct microstructures that influence final properties: extrusions often have elongated grains and require tailored tempering, while rolled sheet is highly uniform and more predictable for stamping. Application choices reflect these differences; thin sheet in O‑temper is preferred for forming whereas extruded profiles in H‑tempers are chosen for structural stiffness and dimensional control.

Equivalent Grades

Standard	Grade	Region	Notes
AA	4040	USA	Industry designation for the alloy composition described here
EN AW	4xxx / AW-4040 (informal)	Europe	No single harmonized EN number; closely comparable 4xxx-series alloys used regionally
JIS	A4040 (informal)	Japan	Japanese standards may reference similar Al‑Si wrought alloys with regional composition tolerances
GB/T	Al‑4040 (informal)	China	Chinese standards have equivalent Al‑Si wrought alloys but direct one‑to‑one equivalents require checking chemical limits

Direct cross-standard equivalence is not always one-to-one because composition limits, impurity controls and temper definitions vary by standards body. Engineers must compare chemical composition ranges and mechanical property limits when substituting alloys across regions; processing history (extruded vs rolled) can further modify interchangeability even where nominal composition matches.

Corrosion Resistance

In typical atmospheric environments 4040 exhibits moderate corrosion resistance driven by its silicon content and low copper/Zn levels. The alloy forms a protective oxide that provides general protection, and anodizing further improves surface durability and aesthetics; localized pitting is usually minimal in non-chloride atmospheres.

In marine and chloride-rich environments 4040 performs acceptably but is generally outperformed by 5xxx magnesium-bearing alloys which offer superior seawater resistance. For exposed marine structural applications, designers often prefer 5xxx or clad alternatives; 4040 is still used for interior components, welded assemblies and where anodizing or coatings are applied.

Stress corrosion cracking susceptibility of 4040 is low compared with high‑strength 2xxx and certain 7xxx alloys; however, welded regions and cold-worked H‑tempers can exhibit localized embrittlement if combined with aggressive chemistries and tensile stress. Galvanic interactions must be considered: aluminum is anodic relative to copper, stainless steels and carbon steel in many environments so isolation or cathodic protection is required to prevent accelerated corrosion.

Fabrication Properties

Weldability

4040 is highly weldable with excellent fusion characteristics due to silicon-enhanced fluidity; it is commonly used as filler for TIG and MIG welding of aluminum assemblies. Standard filler alloys for dissimilar welds include 4043 (higher Si filler) or compatible 4xxx filler wires to match metallurgical and mechanical behavior. Hot-cracking risk is low, but HAZ softening from welding can reduce local strength in H‑tempers; control of heat input and post-weld mechanical design is important to maintain performance.

Machinability

Machinability of 4040 is moderate to good compared with commercially pure aluminum; it machines cleanly with carbide tooling and produces long, continuous chips that require active chip management. Recommended tools are sharp carbide inserts with positive rake and moderate feed rates; cutting speeds for turning typically range from 150–350 m/min depending on tool and coolant, with lower speeds for interrupted cuts. Surface finish and dimensional control are excellent in O‑temper, while H‑tempers may require higher forces and tooling rigidity.

Formability

Formability is excellent in the annealed O condition and suitable for deep drawing, bending and stretching operations; minimum bend radius can often be as low as 1–1.5× thickness in O‑temper for sheet applications. Cold-working to H‑tempers reduces ductility and increases springback and required forming forces, so H‑tempers are used for applications where final geometry is close to net shape and minimal forming is required. Warm forming or preheating can extend formability limits for complex shapes without resorting to annealing.

Heat Treatment Behavior

4040 is effectively non-heat-treatable for precipitation strengthening and therefore does not show the T‑temper response characteristic of 2xxx or 6xxx families. Attempts at solution treatment and aging produce only minor changes because silicon in the 0.6–1.2% range does not form strengthening precipitates comparable to Mg2Si.

The practical metallurgical lever for altering properties in 4040 is mechanical (cold) work and thermal annealing. Full anneal (O) is achieved by heating to temperatures in the typical aluminum anneal range (~350–415 °C depending on gauge and product) followed by controlled cooling to restore ductility. Partial anneal or stress-relief cycles are used to reduce residual stresses in thick sections or heavily worked components.

High-Temperature Performance

4040 retains useful mechanical properties up to moderately elevated temperatures, but strength and stiffness decline progressively with temperature increase above ~100 °C. Creep resistance is limited compared with specialty high-temperature alloys; long‑term static loads at temperatures above 150 °C can produce measurable creep and should be avoided for structural applications. Oxidation is minimal because aluminum forms a stable oxide, but HAZ and thermally cycled zones can show grain coarsening and localized softening that reduce fatigue resistance.

Designers should therefore limit continuous service temperatures for load-bearing components to the 120–150 °C range and evaluate creep and fatigue for components exposed to both elevated temperature and cyclic loading. For short-term exposure or thermal cycling with sufficient design margins, 4040 performs reliably especially when surface coatings or anodizing are applied to protect from environmental attack.

Applications

Industry	Example Component	Why 4040 Is Used
Automotive	Welded brackets, filler wire for body assembly	Excellent weldability and good thermal conductivity for welding and heat management
Marine	Interior structural fittings and welded assemblies	Moderate corrosion resistance and good weldability for fabricated parts
Aerospace	Non-critical fittings, thermal management brackets	Favorable strength-to-weight and ease of fabrication for secondary structures
Electronics	Heat sinks and housings	Good thermal conductivity combined with formability and weldability
Consumer Goods	Appliance panels and extruded profiles	Surface finish, anodizing capability and low distortion during welding

4040 is chosen where the combination of weldability, thermal performance and moderate strength delivers a cost-effective solution. Its balance of properties supports both welded assemblies and formed components where peak precipitation-strengthening is not required.

Selection Insights

Choose 4040 when your design requires excellent weldability, good thermal conductivity and moderate strength with good formability in annealed condition. It is particularly suitable for welded fabrications, filler-wire applications and components where heat dissipation matters as well as joining performance.

Compared with commercially pure aluminum (e.g., 1100), 4040 trades some electrical conductivity and formability for higher strength and improved weld pool behavior; choose 1100 when maximum ductility or conductivity is mandatory. Compared with common work-hardened alloys (e.g., 3003 / 5052), 4040 typically offers better weldability and melt fluidity but slightly lower seawater corrosion resistance than 5xxx alloys; prefer 5052 for critical marine skins. Compared with heat-treatable alloys (e.g., 6061 / 6063), 4040 provides easier welding and better filler compatibility but lower peak tensile strength; select 4040 where welding ease and thermal performance outweigh the need for maximum strength.

Closing Summary

4040 remains a practical choice for engineering applications that prioritize weldability, good thermal conductivity and balanced mechanical properties without the complexity of heat treatment. Its solid-solution and work‑hardening behavior, combined with predictable fabrication performance, make it a reliable alloy for welded structures, thermal management components and formed parts in multiple industries.