Aluminum 4140: Composition, Properties, Temper Guide & Applications

Comprehensive Overview

The designation "4140" is widely recognized in steel nomenclature as a chromium-molybdenum alloy; there is no single universally accepted aluminum alloy listed in major standards as "AA 4140." For clarity and engineering usefulness this article treats "Aluminum 4140" as a generic representative of the Al‑Si 4xxx family — a silicon‑rich wrought alloy class commonly used for filler metals, brazing, soldering and some structural extrusions.

Aluminum 4xxx‑type alloys are principally alloyed with silicon (Si) and are classified in the 4xxx series of the Aluminum Association. The primary strengthening mechanism in this family is solid‑solution strengthening from silicon and cold work; these alloys are not responsive to classical precipitation hardening and are therefore non‑heat‑treatable in the sense used for 2xxx/6xxx/7xxx alloys.

Key traits of Al‑Si alloys include excellent melt fluidity and wetting (making them preferred welding/brazing fillers), moderate static strength compared with pure Al, good corrosion resistance in many atmospheres, and excellent weldability. Formability tends to be good in the annealed condition but declines with strain hardening; machinability is typically favorable because silicon promotes short chip formation and dimensional stability.

Industries that rely on Al‑Si (4xxx) alloys include automotive (welding filler and brazing of heat exchangers), HVAC (radiators and condensers), consumer appliances, electrical conductors where filler behavior is required, and some aerospace non‑primary structures and fixtures. Engineers often choose a 4xxx alloy when joining dissimilar Al alloys or when superior melt fluidity/wetting is required; its selection trades off peak mechanical strength for joining performance and cost‑effective manufacturability.

Temper Variants

Temper	Strength Level	Elongation	Formability	Weldability	Notes
O	Low	High	Excellent	Excellent	Fully annealed; best for forming and brazing
H12 / H14	Medium	Medium	Good	Excellent	Light cold‑work; balance of strength and formability
H18 / H24	Medium‑High	Low‑Medium	Fair	Excellent	Strain‑hardened or partially annealed for higher strength
H32	Medium	Medium	Good	Excellent	Stabilized after strain hardening; used where dimensional stability is needed
T4 (where used)	Low	High	Excellent	Excellent	Some 4xxx variants may be stress‑relieved by low‑temperature treatments

Annealed (O) tempers offer the highest ductility and best formability and are typically chosen for deep drawing and extensive cold forming operations. Strain‑hardened tempers (H1x/H2x) raise yield and tensile strength by introducing dislocation density, but they reduce elongation and increase springback; weldability remains excellent across tempers because silicon lowers solidification cracking susceptibility.

Chemical Composition

Element	% Range	Notes
Si	4.5–12.0	Primary alloying element; controls melting range, fluidity, and solid‑solution strengthening
Fe	0.4–1.5	Common impurity; forms intermetallics that can reduce ductility and affect surface finish
Mn	0.05–0.6	Grain structure modifier; improves strength modestly and reduces hot shortness
Mg	0.0–0.5	Small amounts may be present; promotes some precipitation when combined with Si in specific chemistries
Cu	0.0–0.5	Generally kept low; increases strength but can reduce corrosion resistance
Zn	0.0–0.5	Typically low; can affect galvanic behavior in assemblies
Cr	0.0–0.25	Trace amounts to control grain growth and recrystallization in some variants
Ti	0.0–0.2	Grain refiner when intentionally added in small amounts
Others	Balance (Al)	Minor trace elements (e.g., B, Sr) may be added for modification of Si morphology

Silicon is the defining element: higher Si raises fluidity and reduces melting temperature (beneficial for brazing/filler purposes), but excess Si promotes hard and brittle Si‑rich intermetallics that can lower ductility. Iron forms plate‑like or needle‑like intermetallics that reduce formability and surface quality, so it is controlled. Small additions of Mn, Ti, or Cr are used to refine as‑cast or as‑extruded microstructures and to improve mechanical stability during thermal cycles.

Mechanical Properties

Tensile behavior of 4xxx‑type aluminum alloys is characterized by moderate ultimate tensile strengths and a relatively low yield in annealed condition; cold work increases yield significantly while reducing ductility. Elongation in annealed condition is typically high (good for forming), and fracture modes are generally ductile with some brittle intermetallic participation if Si or Fe levels are high.

Hardness correlates with temper and Si content: annealed 4xxx alloys are soft compared with heat‑treatable Al alloys, while strain‑hardened tempers can achieve useful hardness for structural applications. Fatigue performance is generally lower than peak capability of 6xxx or 7xxx alloys; fatigue life is sensitive to surface finish, weld HAZs, and intermetallic particle size and distribution.

Thickness has a strong effect: thin gage sheet responds well to deep drawing and brazing, while thicker plate/extrusion retains higher as‑fabricated stiffness but may exhibit coarser microstructure and reduced toughness; welding HAZ softening is typically not a critical concern because these alloys are not precipitation‑hardened.

Property	O/Annealed	Key Temper (H14/H24)	Notes
Tensile Strength	80–150 MPa	150–260 MPa	Wide ranges reflect Si content and cold work; H‑tempers increase UTS
Yield Strength	30–90 MPa	110–200 MPa	Yield increases significantly with strain hardening
Elongation	20–35%	6–18%	Annealed condition provides best elongation for forming
Hardness (HB)	25–60 HB	60–100 HB	Hardness rises with Si content and cold working

Physical Properties

Property	Value	Notes
Density	2.68 g/cm³	Typical for Al‑Si wrought alloys; slightly lighter than steel
Melting Range	577–660 °C	Eutectic Al‑Si lowers liquidus with higher Si contents; liquidus varies with Si%
Thermal Conductivity	110–150 W/m·K	Lower than pure Al but still good for heat transfer applications
Electrical Conductivity	30–45 % IACS	Reduced from purer Al grades because of Si and other solutes
Specific Heat	≈0.90 J/g·K (900 J/kg·K)	Typical for aluminum alloys at room temperature
Thermal Expansion	23–25 µm/m·K	Comparable to other Al alloys; important for joined assemblies

The relatively high thermal conductivity and moderate density make Al‑Si alloys beneficial where heat transfer and weight are considerations, such as heat exchangers and automotive radiators. Reduced electrical conductivity versus purer aluminum grades limits their use as primary electrical conductors, but they remain acceptable for many structural and joining applications where electrical performance is secondary.

Product Forms

Form	Typical Thickness/Size	Strength Behavior	Common Tempers	Notes
Sheet	0.3–6.0 mm	Good in O; can be strain hardened	O, H14, H24	Widely used for brazing, cladding, heat exchanger fins
Plate	6–50 mm	Lower toughness in thicker sections; coarser microstructure	O, H32	Less common; used for structural components where weldability is key
Extrusion	Profiles up to several meters	Good dimensional stability; strength by cold work	O, H14, H18	Si aids in flow during extrusion; used for architectural sections
Tube	Ø 6–200 mm	Consistent wall thickness; good weldability	O, H24	Common in condensers and heat exchanger tubing
Bar/Rod	Ø 3–50 mm	Good machinability	O, H14	Often supplied as filler wire/rod for welding and brazing

Sheets and tubes fabricated from Al‑Si alloys are optimized for joining and heat transfer rather than maximal static strength. Extrusions benefit from silicon’s ability to improve flow through dies, which permits intricate cross‑sections. Bars and rods are often used as feedstock for filler wire production where melt characteristics are the primary property requirement.

Equivalent Grades

Standard	Grade	Region	Notes
AA	— (no AA‑4140 listed)	USA	4140 is not a standardized AA aluminum grade; use AA‑4043/4047 for common Al‑Si fillers
EN AW	EN AW‑4043 / EN AW‑4047	Europe	Common Si filler/braze alloys; EN AW nomenclature corresponds to AlSi5 and AlSi12 families
JIS	A4043	Japan	Widely used welding filler in Japan equivalent to AlSi5
GB/T	AlSi5 / AlSi12	China	National standards for Si‑rich filler alloys used for welding/brazing

Because "4140" is not an established Aluminum Association designation, engineers typically select standardized AlSi alloys (for example AA‑4043 or EN AW‑4047) whose Si contents and impurity limits are defined. Differences between standards are mainly in tighter impurity limits (Fe, Cu) and permitted trace elements; these variances influence ductility, wetting behavior, and surface finish in final parts.

Corrosion Resistance

Al‑Si alloys generally exhibit good atmospheric corrosion resistance owing to aluminum’s native oxide film. In rural and industrial atmospheres they perform well, though localized corrosion can occur at sites where intermetallic particles (Fe‑rich phases) concentrate, creating micro‑galvanic couples that can initiate pitting under chloride exposure.

In marine environments, Al‑Si alloys are moderately resistant, but they are typically inferior to Al‑Mg 5xxx alloys in long‑term seawater exposure. Protection strategies such as anodizing, organic coatings, or cathodic protection are often applied to mitigate chloride‑induced pitting and crevice corrosion.

Stress corrosion cracking susceptibility in Al‑Si alloys is relatively low compared with high‑strength Al‑Zn (7xxx) or Al‑Cu (2xxx) families; however, localized SCC can be a concern in aggressively corrosive environments combined with sustained tensile stress. Galvanic interactions should be managed carefully: Al‑Si in contact with stainless steels or copper alloys can act anodically and corrode preferentially if not electrically isolated.

Fabrication Properties

Weldability

Al‑Si (4xxx) alloys are among the most weldable aluminum families. Silicon reduces solidification range issues and lowers susceptibility to hot cracking, making them excellent as filler alloys (e.g., 4043, 4047) for TIG and MIG welding of many aluminum substrates. Recommended filler choices for joining Al assemblies generally include Al‑Si fillers matched to base alloy chemistry to optimize wetting and minimize cracking; control of preheat and travel speed reduces porosity and HAZ softening is of minor concern because these alloys are not precipitation‑hardened.

Machinability

Machinability of Al‑Si alloys is favorable due to Si promoting short, brittle chip formation; carbide tooling with positive rake geometry and high cutting speeds is recommended. Feed and speed should be selected to avoid built‑up edge; coolant or air blast helps manage chip evacuation and surface finish. Where higher Si contents produce abrasive behavior, tool life can be reduced and tool material should be chosen for wear resistance.

Formability

Forming is best performed in the annealed (O) condition where elongation and bendability are maximized. Typical minimum inside bend radii for sheet range from 1–2×T (thickness) depending on Si content and temper; strain‑hardened tempers require larger radii and incremental forming steps. Hot forming can be used for complex geometries but care is needed to avoid grain coarsening and surface oxidation that can impair downstream joining.

Heat Treatment Behavior

Al‑Si 4xxx alloys are broadly non‑heat‑treatable in the classical precipitation‑hardening sense used for 6000/7000 series alloys. They do not respond to solution treatment and artificial aging to obtain substantial strength increases; silicon remains largely in solid solution or as eutectic Si particles. Where thermal processing is applied, it is typically for stress relief, grain refinement, or modification of Si morphology using modifiers such as Sr or Na.

Work hardening is the principal means of increasing strength: controlled cold rolling or drawing raises dislocation density and yield strength at the expense of ductility. Annealing (full softening) is used to restore ductility when required; typical anneal cycles are performed below Al‑Si melting ranges to avoid incipient melting. Some Al‑Si filler alloys can be treated with brief thermal cycles to homogenize microstructure for improved brazing behavior, but these treatments are process‑specific rather than property‑enhancing in the long term.

High-Temperature Performance

Al‑Si alloys begin to lose useful static strength at elevated temperatures above roughly 150–200 °C; long‑term creep resistance is limited compared with specialized high‑temperature Al or wrought alloys formulated for elevated service. The presence of silicon particles improves high‑temperature dimensional stability somewhat by providing a particulate reinforcement, but continuous strength retention is poor beyond 250 °C.

Oxidation in air is typically limited to formation of the protective Al2O3 layer, which slows further degradation; however, at elevated temperatures scaling or interaction with aggressive atmospheres (sulfur‑bearing, molten salts) can accelerate surface attack. Weld HAZs do not suffer the heat‑related dissolution/precipitation cycles seen in precipitation‑hardenable aluminum alloys, but prolonged high‑temperature exposure can coarsen microstructures and reduce toughness.

Applications

Industry	Example Component	Why 4140 Is Used
Automotive	Brazed radiators and condensers	Excellent wetting and flow for brazed joints; good thermal conductivity
HVAC	Heat exchanger fins and tubing	Low density and high thermal conductivity with good formability in annealed condition
Aerospace (non‑primary)	Ducting, fittings, brackets	Light weight, good corrosion resistance and ease of joining for non‑critical parts
Consumer Appliances	Cooktops, oven components, housings	Cost‑effective manufacturing and good heat transfer
Welding Consumables	Filler rods/wires	Controlled melting range and wetting characteristics for Al‑Al joining

As a summary, Al‑Si (4xxx) alloys excel where joining performance, fluidity, and moderate mechanical properties are needed rather than peak strength. They are heavily used as filler materials and in thermal systems because of their balanced thermal, chemical and mechanical attributes.

Selection Insights

Treat "4140" as a 4xxx (Al‑Si) class decision: choose it when weldability, melt fluidity and good thermal performance are the prime drivers rather than maximum static strength. For assemblies requiring brazing or joining of dissimilar aluminum substrates, a 4xxx filler will often be the most reliable and economical choice.

Compared with commercially pure aluminum (1100), a 4xxx alloy trades some electrical conductivity and formability for substantially higher strength and far better melt/wetting behavior — useful when joining and thermal performance matter. Compared with work‑hardened Al alloys like 3003 or 5052, 4xxx alloys generally offer similar or slightly lower corrosion resistance but improved melt behavior and ease of brazing; they occupy a middle ground of strength versus joining capability. Compared with heat‑treatable alloys such as 6061/6063, 4xxx alloys will deliver lower peak strength but superior joining/brazing performance and often lower