Aluminum 6066: Composition, Properties, Temper Guide & Applications

Comprehensive Overview

The 6066 alloy belongs to the 6xxx series of aluminum–magnesium–silicon alloys, which are defined by a Mg-Si alloying system that forms Mg2Si precipitates during heat treatment. As a member of this series, 6066 is a heat-treatable aluminum alloy utilizing precipitation hardening to achieve elevated strength compared with pure aluminum or non-heat-treatable alloys.

Major alloying elements in 6066 are silicon and magnesium, often supplemented with controlled additions of copper, chromium, and trace titanium to refine microstructure and improve strength and toughness. The combination of elements is selected to balance peak-age strength, fracture toughness, weldability, and resistance to stress-corrosion cracking while maintaining reasonable machinability.

6066’s key traits include higher tensile and yield strength in heat-treated tempers, moderate corrosion resistance typical of Al-Mg-Si alloys, and good weldability with appropriate filler metals. Formability is intermediate: annealed tempers are highly formable while peak-aged tempers trade formability for strength.

Typical industries using 6066 include transport (automotive and railway), aerospace secondary structures and fittings, general engineering extrusions, and applications where a higher-strength 6xxx alloy is needed without sacrificing weldability. Engineers choose 6066 when they need a 6xxx-class combination of heat-treatable strength, good extrudability, and improved mechanical performance relative to baseline 6061/6063 in specific geometries.

Temper Variants

Temper	Strength Level	Elongation	Formability	Weldability	Notes
O	Low	High	Excellent	Excellent	Fully annealed condition; best for forming and machining
T4	Moderate	Good	Good	Good	Solution treated and naturally aged; balanced properties
T5	Moderate-High	Moderate	Fair	Good	Cooled from elevated temperature and artificially aged
T6	High	Moderate-Low	Reduced	Good	Solution heat-treated and artificially aged for peak strength
T61 / T651	High	Moderate	Reduced	Good	T6 with controlled stress relief (mechanical or thermal) for stability
H14	Moderate	Moderate	Limited	Good	Strain-hardened; limited cold work strengthening
H24	Moderate-High	Moderate	Limited	Good	Strain-hardened and partially annealed; compromise between form and strength

Temper selection strongly controls the trade-off between strength and ductility for 6066. Solution heat treatment followed by artificial aging (T6 family) produces the highest static strengths via fine Mg2Si precipitation, whereas O and T4 tempers favor forming and stretchability for complex shapes.

In welded assemblies, designers often specify T61/T651 or plan for post-weld heat treatments to stabilize dimensions and recover strength in heat-affected zones; cold-worked H-series tempers can avoid heat treatment but limit achievable peak strength.

Chemical Composition

Element	% Range	Notes
Si	0.40–1.00	Controls precipitation (Mg2Si) and fluidity during casting; influences strength and weldability
Fe	≤0.80	Impurity element; high Fe forms intermetallics that reduce ductility and toughness
Mn	≤0.50	Small additions refine grain structure and improve toughness; excessive Mn reduces conductivity
Mg	0.80–1.50	Principal strengthening element (forms Mg2Si); higher Mg increases strength and age hardenability
Cu	0.15–0.50	Raises strength and improves aging response; increases susceptibility to localized corrosion if high
Zn	≤0.25	Typically low; higher Zn can marginally increase strength but may affect SCC susceptibility
Cr	0.04–0.30	Controls recrystallization and grain structure; helps retain strength after thermo-mechanical processing
Ti	≤0.15	Grain refiner during casting and homogenization; improves toughness and extrudability
Others	Balance Al; residuals ≤0.05 each	Aluminum balance; trace elements controlled to limit deleterious phases

The alloy chemistry is tuned to produce a fine dispersion of Mg2Si precipitates upon aging and to control grain structure during casting and extrusion. Copper and chromium are purposeful additions to push peak yield and tensile strength while chromium and titanium counteract excessive recrystallization during thermal cycles.

Minor elements and impurity limits (Fe, Si balance) are critical in thin-walled sections because intermetallic particles act as crack initiation sites under fatigue and reduce elongation in cold-formed parts. Design of heat treatment schedules must account for these composition-driven kinetics.

Mechanical Properties

Tensile behavior in 6066 reflects classical precipitation-hardening response: annealed material exhibits low yield and high ductility, while solution-treated and artificially aged conditions show substantial increases in yield and ultimate tensile strength. Yield-to-tensile ratios in T6-type tempers are typically favorable for structural applications, and strain-to-failure decreases as peak hardness is approached.

Hardness correlates with aging state and grain size; peak-aged T6 will show the highest Brinell/Vickers values and improved resistance to local indentation. Fatigue performance is strongly affected by surface finish, heat treatment, and the presence of stress concentrators; extruded and wrought products with fine precipitate distributions perform well under alternating loading but remain sensitive to surface defects and corrosion pits.

Thickness and section geometry influence achievable strength due to quench sensitivity and precipitation kinetics; thinner sections are easier to fully solution-treat and quench, yielding higher homogeneity and peak properties, whereas thick plate may show overaged cores and lower effective strength.

Property	O/Annealed	Key Temper (T6 / T651)	Notes
Tensile Strength	160–220 MPa	320–380 MPa	T6 peak range depends on section thickness and exact tempering schedule
Yield Strength	80–140 MPa	260–340 MPa	Yield increases markedly with age; yield/tensile ratio typically 0.75–0.90 in T6
Elongation	15–25%	6–12%	Elongation falls as alloy is aged; values depend on specimen geometry and temper
Hardness (HB)	40–70 HB	85–130 HB	Hardness correlates with tensile strength and aging condition

Physical Properties

Property	Value	Notes
Density	2.70 g/cm³	Typical for Al-Mg-Si wrought alloys; useful for strength-to-weight calculations
Melting Range	~555–650 °C	Solidus–liquidus span influenced by Si and other alloying elements
Thermal Conductivity	140–170 W/m·K	Slightly lower than pure Al due to alloying; still good for heat-sinking applications
Electrical Conductivity	~30–45 %IACS	Lower than high-purity aluminum; conductivity reduces with higher alloy content and cold work
Specific Heat	~880 J/kg·K	Typical for aluminum alloys at room temperature; varies slightly with temperature
Thermal Expansion	23–25 µm/m·K (20–100 °C)	Coefficient comparable to other 6xxx alloys; important for thermal cycling and assembly design

6066’s thermal properties make it attractive where moderate thermal conductivity and low density are advantageous, such as heat spreaders and lightweight structural members. Engineers must account for coefficient of thermal expansion when joining dissimilar materials to avoid thermal stresses and fatigue at interfaces.

Electrical conductivity is adequate for certain conductor roles but is traded off for mechanical strength; if high conductivity is critical, lower-alloyed aluminum grades should be considered.

Product Forms

Form	Typical Thickness/Size	Strength Behavior	Common Tempers	Notes
Sheet	0.4–6.0 mm	Uniform across thin gauges; good formability in O/T4	O, T4, T5, T6	Used for panels, heat exchangers, and formed components
Plate	>6 mm up to 150+ mm	Strength may be reduced in thick sections due to quench sensitivity	T6, T651	Thick plate requires controlled quenching and can show gradient aging
Extrusion	Complex cross-sections, up to large profiles	Excellent directional strength; precipitate distribution influenced by extrusion speed	T4, T5, T6	Widely used for structural profiles and rails
Tube	Ø small to large	Properties dependent on forming method (extruded vs welded)	O, T6	Seamless or welded tubes for structural and fluid-handling roles
Bar/Rod	Ø small to 200 mm	Machinability varies with temper; good dimensional stability in T651	O, T6, H14	Bars used for machined fittings, studs, and components

Wrought products of 6066 are typically supplied in tempers matched to intended processing: annealed for deep drawing, T4/T5 for subsequent forming and aging, and T6/T651 for finished structural parts. Extrusion benefits from 6066’s good hot-workability and its ability to accept high levels of cold work or precipitation strengthening post-extrusion.

Processing differences (plate vs. extrusion) impart anisotropy based on thermo-mechanical history; designers should reference directional tensile and fatigue data for critical structural elements and use stress-relief steps to minimize warpage when machining thick sections.

Equivalent Grades

Standard	Grade	Region	Notes
AA	6066	USA	Wrought Al-Mg-Si alloy; recognized within some supplier catalogs and AMS specs
EN AW	6066	Europe	Often referenced similarly; EN standards may list alloy under comparable chemical ranges
JIS	A6066	Japan	Japanese designation may be used for comparable chemistries in domestic specifications
GB/T	6066	China	Chinese standard may list a 6066 alloy with similar composition but local control limits differ

Equivalency between standards is approximate and depends on exact chemical limits and temper definitions; there is not always a direct 1:1 mapping for mechanical properties because heat-treatment practices and temper codes can vary by region. Engineers must validate supplier mill certifications and mechanical test data when specifying cross-standard equivalents for critical applications.

Where trace elements or maximum impurity levels differ, mechanical performance (especially fatigue and fracture toughness) can diverge; callouts for specific process routes (e.g., solution treatment time, quench medium, artificial aging curve) help ensure comparable performance across standards.

Corrosion Resistance

In atmospheric environments 6066 exhibits corrosion resistance typical of Al-Mg-Si alloys, forming a protective aluminum oxide layer that limits uniform attack. Sensible alloying and tempered conditions reduce susceptibility to pitting and intergranular corrosion, but copper additions—if elevated—can locally decrease corrosion resistance and increase risk in aggressive environments.

Marine behavior is generally acceptable for offshore or shipboard secondary structures when surface protection (anodizing or painting) is used; chloride environments increase the risk of pitting and crevice corrosion, so attention to design details, coatings, and cathodic protection is required for long-term durability. Stress corrosion cracking (SCC) susceptibility is lower than high-copper 2xxx alloys but can still occur under tensile stress and corrosive media; temper and residual stresses influence SCC risk.

Galvanic interactions with dissimilar metals should be designed to minimize exposure of 6066 as a cathode/anode depending on the mating material; with stainless steels and titanium, aluminum is anodic and will corrode preferentially unless isolated. Compared to 5xxx series (Al-Mg) alloys, 6066 trades a bit of pure corrosion resistance for improved strength and heat-treatability but remains superior in general corrosion resistance to many high-strength 2xxx alloys.

Fabrication Properties

Weldability

6066 welds readily using common fusion processes (TIG/MIG) with appropriate filler alloys such as ER4043 or ER5356 depending on required strength and corrosion performance. Welds will locally soften in the heat-affected zone due to dissolution and coarsening of precipitates; post-weld heat treatment or local cold-working may be required to restore strength in critical joints.

Hot-cracking risk in 6xxx alloys is generally low compared with high-copper alloys, but weld metal composition and joint design must control solidification range and exogenous contamination. Pre-weld cleaning and controlling heat input minimize porosity and ensure repeatable weld quality.

Machinability

Machinability of 6066 is fair to good in annealed and T4 conditions, with tool life and surface finish improving in softer tempers. Carbide tooling with positive rake and high-feed strategies are recommended for higher productivity; cutting speeds for aluminum commonly range from 200–600 m/min depending on tool material and rigidity.

Chip control is favorable due to ductile behavior; lubricant/coolant choices affect surface finish and swarf evacuation. High-strength T6 tempers will increase cutting forces and can accelerate tool wear, so process planning should consider temper before machining.

Formability

Forming in O and T4 tempers is excellent; minimum bend radii can be small relative to sheet thickness for deep drawing and complex stamping operations. Cold working in H-series tempers increases yield and reduces ductility, so designers should specify forming tempers and consider age-hardening post-forming for final strength.

Warm forming and controlled pre-aging operations can expand formability windows for certain complex geometries. Springback is more pronounced in higher-strength tempers and should be compensated for in die design and finite-element process simulation.

Heat Treatment Behavior

6066 is a heat-treatable alloy that responds to solution heat treatment followed by quenching and artificial aging to develop peak strengths through Mg2Si precipitation. Typical solution treatment temperatures fall in the 520–545 °C range, held long enough to homogenize the solute, then quenched rapidly to retain a supersaturated solid solution.

Artificial aging (T6) is performed at temperatures commonly between 150–190 °C for times tuned to section thickness and desired strength–ductility balance; overaging reduces strength but can improve toughness and stress-corrosion resistance. T temper transitions (T4 → T6) are used to provide formability during shaping followed by final aging to achieve design strength.

If not heat-treated, 6066 can be strengthened by cold working; however, work hardening cannot reach the peak levels available by precipitation hardening. Annealing (O) is employed to restore ductility and relieve residual stresses prior to forming or welding.

High-Temperature Performance

6066 experiences progressive strength loss with increasing temperature as precipitates coarsen and dissolve; useful strength retention is typically maintained up to ~120–150 °C, while above ~200 °C significant softening occurs. Creep resistance is limited compared with specialty high-temperature alloys; for sustained loads at elevated temperature, alternative materials should be considered.

Oxidation is minimal in atmospheric temperatures due to the protective alumina layer, but prolonged exposure to elevated temperatures can alter surface oxide characteristics and affect subsequent coating adhesion. Heat-affected zones from welding can be sites of reduced high-temperature performance due to overaging and microstructural coarsening.

Designers should apply conservative derating factors for long-term service above aging temperatures and consider post-weld or post-forming stabilization treatments for components exposed to thermal cycles.

Applications

Industry	Example Component	Why 6066 Is Used
Automotive	Structural extrusions, cross-members	Higher strength than 6000-baselines for lightweight structural parts
Marine	Superstructure fittings, rails	Good corrosion resistance and weldability for marine environments
Aerospace	Secondary fittings, mounting brackets	High strength-to-weight and heat-treatable control of properties
Electronics	Heat sinks, chassis	Good thermal conductivity combined with manufacturability

6066 is often selected for components where a higher-strength 6xxx alloy provides a favorable balance between formability, weldability, and mechanical performance. Its ability to be extruded into complex profiles and subsequently aged to high strength makes it a practical choice for lightweight structural and thermal-management components.

End users benefit from the alloy's adaptability to common manufacturing routes and the ability to tune properties through temper selection and heat treatment.

Selection Insights

Use 6066 when you need a heat-treatable aluminum with higher achievable strength than 6061/6063 in specific geometries while retaining good weldability and reasonable formability. It is a logical choice when extrusion or plate sections require better mechanical performance without moving to higher-cost, higher-corrosion-risk alloys.

Compared with commercially pure aluminum (1100), 6066 trades conductivity and easy formability for significantly higher strength and dimensional stability; choose 1100 only when electrical or extreme formability is the overriding requirement. Compared with work-hardened alloys such as 3003 or 5052, 6066 offers superior strength and age-hardening capability but may be slightly more susceptible to certain localized corrosion modes depending on copper content; pick 3xxx/5xxx for marine sheet where maximum ductility and corrosion resistance are needed.

Compared with common heat-treatable alloys like 6061 and 6063, 6066 is selected when a higher as-aged strength, particular extrusion performance, or specific temper stability is required despite potentially lower availability and slightly greater cost. Validate supplier data and temper controls when specifying 6066 as a direct substitute.

Closing Summary

6066 remains relevant as a higher-performance member of the 6xxx family, offering designers an opportunity to push structural strength while maintaining many of the fabrication advantages of Al-Mg-Si alloys. Its balanced chemistry, favorable heat-treatment response, and adaptability to extrusion and wrought products make it a practical choice for a range of transport, aerospace, and engineering applications where an optimized strength-to-weight ratio and good weldability are required.