Aluminum 3B21: Composition, Properties, Temper Guide & Applications

Comprehensive Overview

3B21 is a member of the 3xxx series aluminum alloys, characterized primarily by manganese-based alloying and belonging to the non-heat-treatable family. It is formulated to leverage solid-solution strengthening from Mn and, in some variants, modest Mg additions; strengthening is achieved predominantly through cold work rather than precipitation heat treatment.

Key traits of 3B21 include moderate-to-good strength relative to pure aluminum, excellent formability in annealed condition, good resistance to general atmospheric corrosion, and straightforward weldability with standard Al welding methods. Typical industries using 3B21 range from transportation and automotive outer panels to consumer goods and some marine secondary structures where balance of formability and corrosion resistance is required.

Engineers choose 3B21 when a combination of ductility for forming operations and better mechanical performance than commercially pure aluminum is required without the complexity of heat treatment. Its competitiveness versus other alloys derives from low density, predictable cold-work response, and relatively low cost of production and fabrication.

Selection often favors 3B21 over higher-strength heat-treatable alloys when complex forming operations and good surface finish must be preserved, and over pure Al or softer alloys when additional structural capacity and dent resistance are desirable.

Temper Variants

Temper	Strength Level	Elongation	Formability	Weldability	Notes
O	Low	High (30–45%)	Excellent	Excellent	Fully annealed, best for deep drawing and forming
H12	Low-Medium	Medium-High (20–30%)	Very Good	Very Good	Light strain-hardened from partial mechanical working
H14	Medium	Medium (10–20%)	Good	Very Good	Common commercial cold-worked temper for sheet; higher yield
H16	Medium-High	Medium (8–15%)	Reduced	Good	Higher degree of work hardening for improved stiffness
H18	High	Low-Medium (6–12%)	Limited	Good	Heavier cold work, increased strength for structural panels
H24	Medium-High	Medium (10–18%)	Good	Very Good	Strain-hardened and stabilized; retains some formability
T3 (where applied)	N/A	N/A	N/A	N/A	Not a primary route — 3xxx alloys are not heat-treatable; T-designations used for stabilization after solution in some specs

Temper designation practice for 3B21 follows conventional 3xxx-series handling: soft O for maximal formability and a variety of H tempers for graduated increases in strength through strain hardening. Choosing a temper balances forming complexity, springback control, and required in-service stiffness; weld repair and post-forming joining must consider any local softening in welded H-tempered parts.

Chemical Composition

Element	% Range	Notes
Si	≤ 0.6	Typical impurity; excess reduces ductility and promotes intermetallics
Fe	≤ 0.7	Common impurity; influences grain structure and can form brittle phases
Mn	0.8–1.5	Primary alloying element for 3xxx family; enhances strength and inhibits recrystallization
Mg	0.1–0.6	Minor addition in some variants; increases solid-solution strengthening and improves strain hardening
Cu	≤ 0.2	Low levels may be present; improves strength slightly but reduces corrosion resistance
Zn	≤ 0.25	Usually low; higher contents not typical for 3xxx family
Cr	≤ 0.10	Trace amounts may be present to control grain structure and improve temper stability
Ti	≤ 0.15	Small additions used for grain refinement in cast or wrought products
Others (each)	≤ 0.05–0.15	Other residuals (Ni, Pb, Sn) kept low to avoid embrittlement or deleterious phases

The alloy chemistry of 3B21 is oriented toward a manganese-dominant balance that delivers stable cold-work strengthening and robust ductility. Mn reduces recrystallization and forms fine dispersoids that provide grain stability during forming and moderate thermal exposures. Mg, when present in modest amounts, augments strength through solid-solution effects and improves strain-hardening capacity but must be limited to avoid the susceptibility traits seen in higher-Mg 5xxx alloys.

Mechanical Properties

Tensile behavior of 3B21 follows classical 3xxx-series trends: the annealed (O) condition exhibits relatively low yield and tensile strengths with high uniform elongation enabling severe forming. Cold work (H tempers) produces significant increases in yield and tensile values at the expense of ductility and bendability, and springback rises with increasing work-hardening. Thickness and processing history strongly influence measured properties; thinner gauges can show higher apparent strength because of cold-roll work hardening during manufacture.

Hardness correlates closely with temper: Rockwell or Brinell hardness rises predictably with H-number. Fatigue performance in 3B21 is moderate — better than pure aluminum due to higher baseline strength but inferior to some heat-treatable alloys; surface finish, residual stresses from forming, and notches dominate life. Yield-to-tensile ratios are moderate; localized heat input (e.g., welding) can produce HAZ softening, especially in heavily strain-hardened tempers.

Cold forming limits are governed by temper and grain size; O temper permits deep draw radii down to panel-sensitive tolerances, whereas H18 may require larger bend radii and progressive forming steps. Typical values below are representative ranges for typical sheet gauges and common tempers.

Property	O/Annealed	Key Temper (e.g., H14)	Notes
Tensile Strength	90–130 MPa	150–220 MPa	Values vary with gauge and exact alloy batch; H14 typical for moderate structural panels
Yield Strength	30–70 MPa	100–160 MPa	Yield increases substantially with cold work; O gain low
Elongation	30–45%	8–20%	Elongation depends on temper and strain-path during forming
Hardness (HB)	25–45 HB	50–85 HB	Hardness increases with H temper; conversion to HRC/HRB varies

Physical Properties

Property	Value	Notes
Density	~2.70 g/cm³	Typical of wrought aluminum alloys; beneficial for strength-to-weight
Melting Range	~640–655 °C	Alloying depresses solidus slightly vs pure Al (660 °C)
Thermal Conductivity	~120–150 W/m·K	Lower than pure Al; still high for heat dissipation applications
Electrical Conductivity	~28–40% IACS	Reduced relative to 1xxx-series due to alloying; depends on Mn/Mg content
Specific Heat	~0.88–0.91 J/g·K	Typical for aluminum alloys around room temperature
Thermal Expansion	~23–24 ×10⁻⁶ /K (20–100 °C)	Similar to other Al alloys; important for thermal cycling design

3B21 maintains the thermal and electrical advantages of aluminum while accepting modest reductions from alloying. Thermal conductivity remains high enough for heat-spreading components and consumer thermal management parts. The coefficient of thermal expansion is comparable to other Al alloys and must be accommodated in multi-material joints to control thermal stresses.

Product Forms

Form	Typical Thickness/Size	Strength Behavior	Common Tempers	Notes
Sheet	0.2–6.0 mm	Strength varies with temper; thin gauges often slightly harder	O, H14, H24	Widely used for panels, housings, and formed parts
Plate	>6 mm up to 25 mm	Lower cold-work contribution in thicker plate	O, H18	Used where larger cross-sections are needed; machining/plate forming considered
Extrusion	Profiles up to several meters	Strength depends on cross-section cooling; moderate	O, H112	Extruded shapes leverage Mn to stabilize grain during extrusion
Tube	Diameters from small to large	Wall thickness and temper determine stiffness	O, H16	Drawn or extruded tubes for lightweight framing
Bar/Rod	Diameters and flats	Typically softer in annealed condition; can be cold-drawn	O, H12	Production for machined components and fasteners where appropriate

Forming and processing routes dictate product form choice: sheet rolling provides superior surface finish and tight thickness control for visible panels, while extrusion offers complex cross-sections but requires attention to age/strain effects. Plate and thicker sections often require different forming methods (incremental forming, hot forming) to achieve comparable shapes. Welded assemblies must consider local temper softening and potential distortion in thin components.

Equivalent Grades

Standard	Grade	Region	Notes
AA	3B21	USA	Designation used in some supplier catalogs; not universally standardized across all AA lists
EN AW	3003 / 3xxx family	Europe	Nearest wrought European equivalents are in the 3xxx AW series; direct one-to-one equivalency requires composition check
JIS	A3003 / A3xxx	Japan	Japanese 3xxx-series grades exhibit similar Mn-based chemistry and properties
GB/T	3B21	China	Chinese designation 3B21 aligns with local alloy numbering and composition controls

Direct cross-referencing between standards must be performed using composition limits and mechanical requirements rather than name alone. Slight differences in allowable impurities, Mg content, and tempering practice can produce measurable property differences; therefore, for qualification-critical applications, request mill certificates and run comparative tests rather than relying solely on nominal equivalency tables.

Corrosion Resistance

3B21 offers good general atmospheric corrosion resistance typical of Mn-bearing 3xxx alloys; an oxide film forms rapidly that protects the matrix in most non-aggressive environments. In urban and industrial atmospheres the alloy performs well, and in many cases it is chosen over pure Al when a small increment in mechanical strength is required without compromising ambient corrosion behavior.

In marine environments, 3B21 provides reasonable resistance to uniform corrosion but is more vulnerable to localized attack (pitting) and exfoliation in highly chloride-rich conditions than some Al-Mg (5xxx) or specially coated alloys. Surface finish, cladding, and alloy cleanliness (reduced Fe and Cu) significantly influence performance in marine exposures.

Stress corrosion cracking susceptibility for 3xxx Mn alloys is generally low compared with Cu-containing or high-Mg alloys; however, galvanic interactions with more noble materials (stainless steel, copper) will promote accelerated corrosion of the aluminum when in electrolyte contact. Designers must consider sacrificial protection and electrical isolation to prevent galvanic degradation in mixed-metal assemblies.

Fabrication Properties

Weldability

Welds in 3B21 are readily made with common processes such as TIG and MIG using argon-based shielding and standard preparation practices. Filler selection typically favors Al-Mn or Al-Si type fillers to match mechanical behavior and minimize hot-cracking; 4043 (Al-Si) and 5356 (Al-Mg) are commonly used depending on desired ductility and strength balance. Heavily strain-hardened tempers will experience HAZ softening and potential reduction of local properties; pre- and post-weld mechanical conditions should be planned.

Machinability

Machinability is moderate and poorer than free-machining Cu alloys or steels, but workable with appropriate tooling. Carbide-tipped tools with positive rake, high-feed strategies, and flood cooling provide the best balance between tool life and surface finish. Chips tend to be continuous and ductile; control of built-up edge and adequate chip-breaking geometry is advisable for production machining.

Formability

Formability is excellent in O temper and remains good in low-H tempers for most common stamping and drawing operations. Typical minimum bend radii in O temper can be as low as 0.5–1.0T for simple bends, but allowances must be made for springback and thinning during deep drawing. Cold-worked tempers (H16–H18) require larger radii and multi-stage forming to avoid cracking; annealing can restore formability when required.

Heat Treatment Behavior

As a principally non-heat-treatable alloy, 3B21 does not respond to solution treatment and precipitation aging to produce significant strengthening. Attempts at conventional solution-and-age cycles will not yield the same gains achievable in 6xxx/7xxx families. Instead, control of mechanical properties is achieved through cold work (strain hardening) and controlled annealing.

Annealing to recover ductility is performed in typical aluminum ranges, commonly between ~300–420 °C depending on sheet thickness and desired grain structure, followed by controlled cooling. Over-annealing can coarsen grains and reduce formability in some operations; stabilized H-temper (e.g., H24) practices use modest thermal stabilization or low-level stress relief to minimize property drift.

For designs requiring higher strength than achievable through cold work, engineers should evaluate heat-treatable alternatives; otherwise cold-forming sequences, progressive die work, and work-hardening schedules are the standard approach for 3B21.

High-Temperature Performance

3B21 retains useful properties at modest elevated temperatures but experiences progressive strength loss above approximately 100–150 °C, with significant reductions in yield strength and creep resistance at higher temperatures. For continuous high-temperature service or where creep is a concern, high-temperature alloys or stainless steels are typically favored.

Oxidation is not a limiting concern in air for brief exposures owing to the protective aluminum oxide film; however, prolonged exposure in aggressive oxidizing or chloride-laden atmospheres at elevated temperatures will degrade protective scales and accelerate attack. Weld heat-affected zones can exhibit diminished mechanical capacity and local softening, which must be considered in thermal cycling or high-temperature load cases.

Applications

Industry	Example Component	Why 3B21 Is Used
Automotive	Inner/outer body panels, trim	Good formability for complex stamping and moderate strength for dent resistance
Marine	Secondary structures, trim	Adequate corrosion resistance and ease of fabrication; lightweight
Aerospace	Non-critical fittings, fairings	Favorable strength-to-weight and formability for shaped panels
Electronics	Enclosures, heat spreaders	High thermal conductivity and easy surface finishing

3B21 is commonly selected where a balance of formability, corrosion resistance, and moderate structural performance is required at low cost. Its versatility across sheet and extruded product forms makes it a practical material for visible panels, formed housings, and secondary structural members where weight savings and fabrication economy are priorities.

Selection Insights

When selecting 3B21, prioritize applications that require excellent formability and good corrosion performance with moderate strength. Choose O temper for deep drawing and complex shapes; use H-tempers for components needing improved stiffness or dent resistance without moving to heat treatment complexity.

Compared with commercially pure aluminum (1100), 3B21 trades somewhat lower electrical and thermal conductivity for materially higher strength and better work-hardening capability. Compared with common work-hardened alloys such as 3003 or 5052, 3B21 typically sits near the mid-point: stronger than pure Al but with similar or slightly improved corrosion resistance versus higher-Mg 5xxx alloys. Compared with heat-treatable alloys like 6061 or 6063, 3B21 offers superior formability in annealed condition and easier fabrication, but lower peak strength; prefer 3B21 when forming and corrosion resistance outweigh the need for maximum strength.

Closing Summary

3B21 endures as a practical Mn-based wrought aluminum that combines excellent formability, reliable corrosion resistance, and predictable cold-work strengthening for a wide range of lightweight structural and formed applications. Its balance of properties and straightforward fabrication make it a cost-effective choice where moderate strength and high manufacturability are the primary design drivers.