The Aluminium Alloy Designation System: Weldability Guide

Aluminium is rarely used in its pure form. In almost every welding and fabrication application, you are working with an alloy — a combination of aluminium with one or more other elements that changes its strength, corrosion resistance, or other mechanical properties. With over 400 registered wrought aluminium alloys and more than 200 cast alloys, knowing how to read an alloy designation is a fundamental skill for any welder or fabricator.

This guide explains how the aluminium alloy designation system works for both wrought and cast alloys, how to read temper designations, and — critically — how the designation tells you something important about weldability. Not every aluminium alloy can be safely arc welded, and understanding why is just as important as knowing how to read the number.

1. The Wrought Aluminium Alloy Designation System

Wrought alloys — those that have been mechanically worked by rolling, extruding, or forging — use a four-digit numbering system. The digits encode the principal alloying element and identify the specific alloy within that series.

How to read the four digits

First digit (Xxxx) — Identifies the principal alloying element and the alloy series, from 1xxx through to 8xxx.

Second digit (xXxx) — If not zero, indicates a modification to the original alloy specification.

Third and fourth digits (xxXX) — Arbitrary numbers that identify the specific alloy within the series. Exception: in the 1xxx series, these digits indicate the minimum aluminium purity above 99%. Alloy 1350, for example, is 99.50% minimum aluminium.

Example: Alloy 5183 — the 5 indicates the magnesium (5xxx) series, the 1 indicates it is the first modification of alloy 5083, and 83 identifies it within the 5xxx series.

Table 1. Wrought aluminium alloy series

2. The Cast Aluminium Alloy Designation System

Cast alloys use a different system: a three-digit number followed by a decimal point and one further digit, written as xxx.x.

First digit (Xxx.x) — The principal alloying element series, similar to wrought alloys.

Second and third digits (xXX.x) — Arbitrary numbers identifying the specific alloy within that series.

Digit after the decimal (.x) — Indicates form: .0 = final casting shape, .1 or .2 = ingot.

Capital letter prefix — Indicates a modification to the original alloy. For example, A356.0: the A (Axxx.x) marks the modification, the 3 (A3xx.x) indicates the silicon plus copper and/or magnesium series, the 56 (Ax56.0) identifies the alloy within the 3xx.x series, and the .0 (Axxx.0) confirms it is a final casting, not an ingot.

Table 2. Cast aluminium alloy series

3. The Temper Designation System

The alloy number tells you what is in the material. The temper designation — added after a hyphen — tells you what has been done to it. Examples: 6061-T6, 6063-T4, 5052-H32, 5083-H112.

Table 3. Basic temper designations

Table 4. H temper subdivisions — strain hardened

The first digit after H indicates the operation:

• H1 — Strain hardened only
• H2 — Strain hardened and partially annealed
• H3 — Strain hardened and stabilised
• H4 — Strain hardened and lacquered or painted

The second digit indicates the degree of strain hardening:

• HX2 — Quarter hard
• HX4 — Half hard
• HX6 — Three-quarters hard
• HX8 — Full hard
• HX9 — Extra hard

Table 5. T temper subdivisions — thermally treated

• T1 — Naturally aged after cooling from an elevated temperature shaping process (e.g. extrusion)
• T2 — Cold worked after cooling from an elevated temperature shaping process, then naturally aged
• T3 — Solution heat treated, cold worked, and naturally aged
• T4 — Solution heat treated and naturally aged
• T5 — Artificially aged after cooling from an elevated temperature shaping process
• T6 — Solution heat treated and artificially aged
• T7 — Solution heat treated and stabilised (overaged)
• T8 — Solution heat treated, cold worked, and artificially aged
• T9 — Solution heat treated, artificially aged, and cold worked
• T10 — Cold worked after cooling from an elevated temperature shaping process, then artificially aged

Additional digits indicate stress relief: TX51 or TXX51 = stress relieved by stretching; TX52 or TXX52 = stress relieved by compressing.

4. Heat-Treatable vs Non-Heat-Treatable Alloys

One of the most practically important distinctions in the alloy system is whether an alloy is heat-treatable. This affects both how it gains its strength and how welding affects it.

Non-heat-treatable alloys (1xxx, 3xxx, 5xxx)

These alloys gain their strength through strain hardening — cold working the material. They cannot be strengthened by heat treatment. Because welding introduces heat, it anneals the work-hardened structure in the heat-affected zone (HAZ), causing some localised loss of strength near the weld. However, these alloys generally have good to excellent weldability. Strain hardening is not generally applied to castings.

Heat-treatable alloys (2xxx, 6xxx, 7xxx)

These alloys gain their optimum mechanical properties through thermal treatment: solution heat treatment (heating to approximately 530°C / 990°F to dissolve alloying elements into solution), quenching, and then ageing — either natural ageing at room temperature or artificial ageing at approximately 160°C / 320°F to precipitate strengthening phases. The T temper designations describe exactly where in this process the material sits when supplied.

When these alloys are welded, the arc heat disrupts the carefully controlled microstructure in the HAZ, causing localised strength loss. More critically, some heat-treatable alloys — particularly in the 2xxx and 7xxx series — develop a dangerous susceptibility to cracking during and after welding.

For cast alloys, the 2xx.x, 3xx.x, 4xx.x, and 7xx.x series are heat treatable. The 4xxx series wrought alloys consist of both heat-treatable and non-heat-treatable alloys.

5. Alloy Series Weldability at a Glance

6. What Makes an Aluminium Alloy 'Unweldable'?

The term 'unweldable' is non-standard — it does not mean a weld physically cannot be made. It means the alloy is highly susceptible to serious problems during or after welding that make it unsuitable for structural or safety-critical applications. The two primary failure modes are hot cracking and stress corrosion cracking.

Hot cracking (solidification cracking)

Hot cracking occurs when high thermal stress and solidification shrinkage are present as the weld pool cools. The vulnerability of any alloy is closely linked to its coherence range — the temperature range between when the solidifying weld first develops mechanical strength and when it is fully solidified. A wide coherence range means the material is under stress for a longer period during solidification, increasing the likelihood of cracks forming at grain boundaries.

Hot cracking sensitivity in Al-Cu alloys increases as copper content rises to approximately 3% Cu, then decreases to a relatively low level at 4.5% Cu and above. Alloy 2219, with 6.3% Cu, has a relatively narrow coherence range and shows good resistance to hot cracking. This might suggest that alloy 2024, with approximately 4.5% Cu, would also be low-risk — but that overlooks the role of magnesium.

Free-machining alloys: a separate category to be aware of

Before discussing 2024 and 7075 in detail, it is worth noting a separate group of aluminium alloys that are poor candidates for welding: free-machining alloys such as 2011 and 6262. These contain small amounts of bismuth (Bi) and lead (Pb), added specifically to improve chip formation during machining. Because of their very low solidification temperatures, these elements can seriously impair weld soundness. If you are working with a component specified for machineability, always verify weldability before proceeding.

7. Why Alloy 2024 Is Generally Considered Unweldable

Alloy 2024 (Al-Cu-Mg) is one of the most widely used high-performance aluminium alloys, common in aircraft structures, sporting equipment, and precision components. Its high strength-to-weight ratio makes it extremely attractive. Its arc weldability makes it dangerous to repair or modify by welding.

Despite containing approximately 4.5% Cu — a level that alone would suggest lower crack sensitivity — 2024 also contains a small but critical amount of magnesium. The Mg depresses the solidus temperature without affecting the coherence temperature, which extends the coherence range and increases hot cracking tendency.

Beyond the immediate cracking risk, the heat of the welding operation causes segregation of alloying constituents at grain boundaries. This creates a metallurgical condition that can lead to intergranular micro-cracking and, critically, to stress corrosion cracking.

Important: Stress corrosion cracking in 2024 is a delayed failure. The completed weld may appear sound immediately after welding. The failure typically develops later, in service, when the component is subject to tensile stress and environmental exposure. The time to failure is unpredictable and dependent on the level of tensile stress applied, environmental conditions, and duration of exposure.

For components made from 2024 — particularly aircraft parts, structural elements, or any safety-critical application — arc welding is strongly discouraged. These components are typically joined by riveting or bolting for exactly this reason. If repair is being considered and there is any possibility that a weld failure could cause injury or property damage, arc welding should not be attempted.

8. Why Alloy 7075 Is Generally Considered Unweldable

Alloy 7075 (Al-Zn-Cu-Mg) is another high-performance alloy widely used in aerospace structures, bicycle frames, climbing hardware, and military equipment. Like 2024, its strength-to-weight ratio is exceptional. Like 2024, its arc weldability is poor.

The 7xxx series alloys use zinc as the primary alloying element, but alloys such as 7075 also contain small amounts of copper and magnesium. These additions extend the coherence range in the same way as in 2024, increasing hot cracking susceptibility. The metallurgical changes that occur adjacent to the weld establish conditions susceptible to stress corrosion cracking at a later date.

Important: As with 2024, stress corrosion cracking in 7075 is a delayed phenomenon — not detectable immediately after welding, and highly dependent on the tensile stress applied to the joint, environmental conditions, and time. It is strongly recommended that structural repairs to 7075 components are not performed by arc welding.

It should be noted that not all 7xxx alloys are unweldable. Alloy 7005, for example, is used in welded applications and is generally considered weldable with 5356 filler. The series designation alone is not sufficient — always verify the specific alloy.

9. Crack Sensitivity Is Not Only a Property of the Base Alloy

Elevated crack sensitivity is not exclusively an inherent property of a given base alloy. Even alloys with normally good weldability can become problematic when:

• Welding incompatible dissimilar base alloys — joining a 2xxx series alloy to a 5xxx series alloy, for example, can dramatically increase crack sensitivity in the weld pool even though both alloys weld satisfactorily when joined to themselves
• Using an incompatible filler alloy — selecting a 5xxx series filler to weld a 2xxx series base alloy, or vice versa, can push weld chemistry into a highly crack-sensitive range

This is why filler alloy selection is never optional — it directly affects the metallurgical compatibility of the weld and the resulting crack sensitivity. Always consult the appropriate filler alloy selection chart for every aluminium welding application.

9. ESAB Aluminium Filler Alloys

Once you have identified your base alloy series and confirmed weldability, selecting the right filler is the next critical step. ESAB's aluminium filler range is produced under the OK Autrod (MIG/GMAW) and OK Tigrod (TIG/GTAW) product families — part of the world's best-selling aluminium wire range — and covers the full spectrum of weldable alloy series.

4043 — General purpose silicon-based filler

The go-to general purpose filler for welding 6xxx series alloys and other AlMgSi and AlSi base materials. Lower crack sensitivity than 5xxx fillers, good fluidity, and suitable for elevated temperature service. Not recommended where post-weld anodising is required.

• OK Autrod 4043 — MIG/GMAW wire
• OK Tigrod 4043 — TIG/GTAW rod

5356 — High strength, anodising-compatible

The most widely used aluminium welding alloy. Higher shear strength than 4043 and the correct choice where post-weld anodising and colour matching are required. Widely used for 5xxx and 6xxx series base alloys in structural and marine applications. Not suitable for sustained service above 65°C.

• OK Autrod 5356 — MIG/GMAW wire

4047 — Enhanced fluidity for thin section and leak-critical applications

Higher silicon content than 4043 (11–13% vs 4.5–6%) delivers exceptional fluidity, reduced solidification cracking, and very smooth weld surfaces. Ideal for heat exchangers and thin-section work requiring leak-tight joints. Shares the same AWS F number (F23) as 4043. Not suitable for anodised finishes.

• OK Autrod 4047 — MIG/GMAW wire
• OK Tigrod 4047 — TIG/GTAW rod

5183 — For 5083 and high-magnesium 5xxx alloys

Developed specifically to meet the as-welded tensile requirements of 5083 and similar high-magnesium alloys that 5356 will not consistently achieve. The correct filler for shipbuilding, cryogenic, and structural fabrications on 5083, 5086, and 5456 base materials where groove weld procedure qualification is required. Never use 4043 or 4047 on these alloys.

• OK Autrod 5183 — MIG/GMAW wire

For full filler alloy selection guidance including a comparison of 4043, 5356, and 4047, see our Aluminium Filler Alloy Selection Guide.

10. Key Takeaways

• The first digit of a wrought alloy designation identifies the series and principal alloying element, giving an immediate first indication of weldability
• The temper designation (letter and digits after the hyphen) describes the condition of the material — critical for understanding HAZ behaviour and post-weld strength
• 1xxx, 3xxx, and 5xxx alloys are non-heat-treatable and generally weld well; 2xxx and 7xxx alloys are heat-treatable and frequently problematic or dangerous to arc weld
• 2024 and 7075 are generally considered unweldable by arc welding due to hot cracking susceptibility and the risk of delayed stress corrosion cracking in service
• Free-machining alloys (2011, 6262) should not be arc welded due to bismuth and lead additions
• Crack sensitivity also depends on the combination of alloys being joined and the filler alloy selected — always use a filler alloy selection chart
• When working with safety-critical components made from susceptible alloys: do not weld without first confirming the base material type and consulting the appropriate welding authority