Aluminum 4145: Composition, Properties, Temper Guide & Applications

Comprehensive Overview

Alloy 4145 is a member of the 4xxx series of aluminium alloys, a family characterized primarily by silicon as the principal alloying element. The 4xxx series is typically used where improved fluidity, reduced melting point, and enhanced wear resistance are desirable; 4145 aligns with this behavior and is often supplied as wrought product for structural and joining applications.

The major alloying element in 4145 is silicon; small controlled quantities of iron, manganese, and trace elements such as titanium and chromium are also present to tailor grain structure and mechanical response. Strength in 4145 is achieved predominantly through solid solution strengthening by silicon and by strain hardening (work hardening); it is not a conventionally heat-treatable aluminium alloy, so precipitation hardening routes like T6 produce limited benefit.

Key traits of 4145 include moderate to good strength for an Al-Si alloy, excellent resistance to softening in weld zones relative to some heat-treatable alloys, good thermal conductivity for heat-dissipating applications, and generally good formability in annealed tempers. Weldability is typically very good with appropriate filler metals, while corrosion resistance is serviceable in atmospheric and mildly corrosive marine environments but inferior to high-magnesium series alloys in aggressive seawater.

Typical industries that exploit 4145 include automotive (structural and joining components), welding and brazing consumables, consumer products where thermal performance matters, and light structural applications where economical strength and formability are required. Engineers select 4145 over other alloys when an Al-Si balance is wanted: it offers better high-temperature softening resistance in welded zones than many heat-treatable alloys while providing a compromise between formability and strength compared with pure aluminium or 5xxx series alloys.

Temper Variants

Temper	Strength Level	Elongation	Formability	Weldability	Notes
O	Low	High	Excellent	Excellent	Fully annealed, best for forming and brazing
H12	Moderate	Moderate	Good	Excellent	Partial strain-hardened, some increase in yield
H14	Moderate-High	Low-Moderate	Fair	Excellent	One-quarter hard; common for sheet applications
H18	High	Low	Poor	Good	Full hard, used where maximum strength from cold work is needed
T4*	Not applicable	Not applicable	Not applicable	Not applicable	Conventional solution-age treatment not effective for 4xxx family
T5*	Not applicable	Not applicable	Not applicable	Not applicable	Artificial aging after cooling from elevated temperature not typical

Temper categories shown reflect the practical tempers encountered in Al-Si wrought alloys like 4145. The 4xxx family does not respond to age-hardening routes the way 6xxx or 7xxx alloys do, so H-series strain-hardening tempers and O anneal are the primary production states. Selection of a harder H temper trades ductility and formability for higher yield and tensile strength but limits subsequent forming steps.

Chemical Composition

Element	% Range	Notes
Si	4.5–12.5 (typical)	Primary alloying element; controls melting behavior and solid solution strengthening
Fe	0.4–1.3	Impurity level that forms intermetallics; affects strength and ductility
Mn	0.05–0.6	Grain refiner and improves strength without large corrosion penalty
Mg	0.05–0.6	Small additions improve strength and work-hardening response
Cu	≤0.25	Usually kept low to avoid large reductions in corrosion resistance
Zn	≤0.25	Kept low as it provides minor strengthening but can compromise corrosion performance
Cr	≤0.25	Used for grain control and to reduce recrystallization during processing
Ti	≤0.15	Grain refiner in cast and wrought processing
Others (incl. residual Al)	Balance	Aluminium as the balance with trace residuals (e.g., Ni, V, Zr) controlled

The silicon content largely defines 4145’s behavior: with Si in the mid single-digit to low double-digit range, the alloy exhibits eutectic and near-eutectic solidification features that lower the melting onset and improve brazing/welding flow. Iron and manganese primarily influence the morphology of intermetallics and recrystallization; their control is important for toughness and formability. Minor additions of magnesium and chromium can be used to tweak cold work response and grain stability during thermal cycles.

Mechanical Properties

Tensile behavior of 4145 is dictated by the silicon level and the temper. Annealed (O) material typically displays moderate tensile strength with good elongation giving ductile behavior under quasi-static loading. Cold-worked (H-series) tempers elevate yield and tensile strength at the expense of ductility and can introduce anisotropy if heavily rolled or extruded.

Yield strength in annealed 4145 is modest, rising substantially with cold work; the alloy does not show significant precipitation hardening, so any post-fabrication gains rely on plastic deformation. Hardness follows the same trend; HB values increase with strain hardening but remain lower than peak-aged 6xxx alloys. Fatigue performance is influenced by surface finish and the presence of silicon-rich intermetallics; shot peening and polished surfaces markedly improve fatigue life.

Thickness has a notable effect on mechanical properties because cooling rates during processing influence silicon particle size and distribution; thin gauges produced by fast quenching or cold rolling tend to have finer silicon dispersions and somewhat higher strength. Welding and thermal exposure can locally soften H-tempers in heat-affected zones; careful design of process parameters and post-weld mechanical treatment can mitigate softening.

Property	O/Annealed	Key Temper (H14/H18)	Notes
Tensile Strength	120–170 MPa (typical)	200–270 MPa (typical)	Ranges depend strongly on Si content and cold work level
Yield Strength	60–110 MPa (typical)	140–220 MPa (typical)	H-series increases yield significantly via strain hardening
Elongation	18–30%	5–14%	Ductility drops with increasing cold work and Si intermetallics
Hardness (HB)	30–55	65–95	Hardness correlates with temper and silicon morphology

Physical Properties

Property	Value	Notes
Density	~2.68–2.72 g/cm³	Slightly variable with Si content but close to pure aluminium
Melting Range	~577–640 °C	Eutectic Al–Si at ~577 °C; solidus–liquidus band depends on Si% and minor elements
Thermal Conductivity	~120–180 W/m·K	Lower than pure Al as Si reduces conductivity; good for heat-dissipation components
Electrical Conductivity	~25–45 % IACS	Reduced from pure Al due to alloying elements
Specific Heat	~880–910 J/kg·K	Typical for aluminium alloys at ambient temperatures
Thermal Expansion	~22–24 µm/m·K (20–100 °C)	Comparable to other Al alloys; consider differential expansion with dissimilar metals

Physical properties show the trade-offs of silicon additions: conductivity and density remain favorable compared with many metals, yet thermal and electrical conductivities are reduced from pure aluminium. The lowered solidus temperature introduced by silicon improves castability and brazing characteristics but necessitates careful thermal control during welding and heat treatments to avoid localized melting or eutectic formation.

Product Forms

Form	Typical Thickness/Size	Strength Behavior	Common Tempers	Notes
Sheet	0.3–6.0 mm	Strength varies with temper and rolling reduction	O, H12, H14	Widely produced; used for formed panels and brazed assemblies
Plate	6–25 mm	Slightly lower work-hardening uniformity in thick sections	O, H18	Thicker sections may have coarser silicon particles affecting toughness
Extrusion	Cross-sections up to 200 mm	Strength depends on cooling and post-stretching	O, H12	Extruded profiles used for structural members and heat exchangers
Tube	OD 6–150 mm	Wall thickness affects mechanical stability	O, H14	Common for heat-exchange and fluid handling where brazing/welding required
Bar/Rod	Dia 3–60 mm	Cold-drawn rod improves strength and surface finish	O, H18	Used for machined components and fasteners where Si benefits wear resistance

Processing differences affect final properties: sheet and strip receive more uniform rolling and thinning leading to fine silicon dispersion, while plate and heavy extrusions can retain coarser microstructures that reduce ductility. Extrusions and tubes often undergo post-extrusion stretching to reduce residual stresses and improve dimensional stability. Selection of product form is driven by required section stiffness, forming steps, and post-fabrication joining processes.

Equivalent Grades

Standard	Grade	Region	Notes
AA	4145	USA	Designation within the Aluminum Association system for Al‑Si wrought alloy
EN AW	No direct equivalent	Europe	No single EN AW grade maps exactly; nearest Al–Si wrought families include AW‑4043/4047
JIS	No direct equivalent	Japan	Localized designations may exist for Al–Si compositions but not exact 4145 match
GB/T	No direct equivalent	China	Chinese standards include Al–Si wrought grades but 4145 may be supplied under proprietary spec

There is not always a one-to-one cross-reference for 4145 in international standards because compositional windows and application intents vary regionally. Where exact interchangeability is required, engineers should compare detailed chemical composition and mechanical property tables or request certification from suppliers. In many cases, Al–Si grades such as EN AW‑4043 or 4047 are functionally similar for welding/filler applications but differ in Si content and mechanical balance.

Corrosion Resistance

In atmospheric environments, 4145 exhibits good resistance due to the naturally forming aluminium oxide film; small silicon and alloying element percentages do not significantly degrade general atmospheric performance. Localized corrosion such as pitting is typically less pronounced than in high-silicon casting alloys, but it will not match the marine durability of high‑magnesium 5xxx series alloys.

Marine behavior is acceptable for components above the splash zone or for parts that are sacrificially protected and regularly inspected. In continuously wet, highly chloride-laden environments, 4145 is susceptible to localized attack and galvanic corrosion when coupled with cathodic metals; protective coatings or appropriate isolation are standard mitigation measures.

Stress corrosion cracking is not a primary failure mode for low-copper Al‑Si alloys like 4145; however, residual stresses from cold work or welding combined with aggressive environments can still produce SCC-like behavior in highly constrained geometries. Galvanic interactions should be considered when pairing 4145 with more noble materials such as stainless steels or copper alloys, especially in seawater, where aluminium will corrode preferentially unless insulated.

Compared with 6xxx or 7xxx heat-treatable alloys, 4145 offers better resistance to softening in weld HAZs but typically lower ultimate corrosion resistance than 5xxx series in marine exposure. The alloy is a practical compromise when corrosion resistance and weldability are both design criteria.

Fabrication Properties

Weldability

Weldability of 4145 is generally very good with both TIG and MIG processes when using appropriate shielding and filler. The alloy’s silicon content promotes wetting and flow in fusion welding and brazing operations, reducing the incidence of lack-of-fusion defects. Recommended filler metals are Al‑Si type rods/wires (e.g., AlSi alloys) to maintain silicon content and avoid hot‑cracking; copper-rich fillers should be avoided. Heat‑affected zone softening is less severe than in age‑hardening alloys, but over‑heating can still cause localized melting or eutectic segregation, so thermal control is essential.

Machinability

Machinability of 4145 is moderate; the presence of silicon increases tool wear compared to pure aluminium but improves chip control and machinability stability. Carbide tooling with polished rake faces and positive rake geometries is recommended; cutting speeds can be similar to other Al alloys but feeds should be managed to avoid built-up edge. Lubrication is often required for higher Si contents to protect tool life and maintain surface finish.

Formability

Formability in the annealed O temper is excellent; 4145 can be deep-drawn, bent, and formed using conventional tooling with small bend radii relative to thicker aluminium grades. Cold-working to H tempers reduces ductility significantly and raises spring-back, so forming operations are typically performed in soft tempers followed by light strain-hardening. For critical bends, minimum internal bend radii of 1–2× thickness are practical in O tempers, but actual allowances depend on gauge and tooling.

Heat Treatment Behavior

As a predominantly Al–Si alloy, 4145 is not effectively strengthened by conventional precipitation heat treatments; solution heat treatment and artificial aging deliver marginal increases in strength. Attempts to apply T6-style treatments will produce limited response because silicon does not precipitate in the same strengthening manner as Mg2Si in 6xxx alloys.

Heat-treatment practice therefore focuses on softening (annealing) and on controlling grain structure via thermal processing. Full anneal (O) is accomplished through extended soaking above recrystallization temperatures followed by controlled cooling to produce a ductile microstructure. Work-hardening remains the primary route to increase strength, and temper transitions are accomplished through mechanical strain followed by stabilization (e.g., H14 from O via controlled cold work).

High-Temperature Performance

Strength loss in 4145 becomes significant above roughly 150–200 °C as recovery and recrystallization processes lead to softening and coarsening of silicon-rich phases. Continuous service above about 200 °C is generally avoided for load-bearing applications, although transient exposure during brazing and welding is accommodated by the alloy’s favorable melting characteristics.

Oxidation at elevated temperatures is limited by the aluminium oxide scale, but silicon-rich phases can alter oxide adherence locally; long-term high-temperature exposure can lead to embrittlement and scale spallation in cyclic thermal conditions. Heat-affected zones from welding can show different high-temperature responses due to microstructural changes; these areas require consideration if components operate near the alloy’s temperature limits.

Applications

Industry	Example Component	Why 4145 Is Used
Automotive	Heat exchanger fins and brackets	Good thermal conductivity and formability; brazing/welding friendly
Marine	Brackets and non-critical structural fittings	Adequate corrosion resistance with economical strength
Aerospace	Secondary structural fittings, clips	Favorable strength-to-weight and weldability for non-primary structures
Electronics	Heat sinks and chassis	Thermal conductivity and ease of forming into fins and assemblies
Consumer Appliances	Cooking appliance housings and heat exchangers	Good combination of formability and thermal behavior

The combination of silicon-driven thermal behavior, weldability and reasonable mechanical properties makes 4145 a practical choice for components requiring thermal management combined with economical manufacture. Its balance of properties enables designers to minimize machining and use formed or brazed assemblies.

Selection Insights

Choose 4145 when you need an Al‑Si alloy that brazes and welds well, offers good thermal conductivity, and provides a middle ground between formability and strength. It is particularly appropriate for heat-exchange components, formed housings, and welded assemblies where T‑temper age-hardening is not required.

Compared with commercially pure aluminium (1100), 4145 trades slightly reduced electrical and thermal conductivity for higher strength and better wear/brazing behavior. Compared with work‑hardened alloys such as 3003 or 5052, 4145 typically provides higher thermal performance and similar or slightly lower corrosion resistance while giving competitive strength after cold work. Compared with heat‑treatable alloys such as 6061 or 6063, 4145 is preferred when superior weld/H A Z stability and brazability are more important than achieving the highest possible peak strength.

In procurement, weigh availability and cost against required temper states; because 4145 relies on cold work rather than age-hardening, inventorying O and one H temper covers most design needs, potentially simplifying supply chain logistics.

Closing Summary

Aluminium alloy 4145 remains a practical engineering choice where silicon’s benefits—improved brazability, good thermal conductivity, and robust weld zone behavior—are needed alongside reasonable mechanical properties and formability. Its niche is defined by applications that require a durable Al–Si compromise rather than maximum age-hardened strength, and it continues to be relevant in industries that demand economical, weldable, and thermally conductive aluminium solutions.