How and Why Alloying Elements Are Added To Aluminium

Pure aluminium is not usually used for structural applications. To produce aluminium that is of adequate strength for the manufacture of structural components, it is necessary to add other elements to it. What elements can be added? Does adding these elements affect weldability? And in what applications are the resulting alloys used? This guide answers all three questions — and explains why correct alloy identification is one of the most critical steps before any aluminium welding operation.

Why Pure Aluminium Is Unsuitable for Structural Applications

It would be very unusual to find pure aluminium (1xxx series) chosen for structural fabrication. Although the 1xxx series are almost pure aluminium, they respond to strain hardening — particularly if they contain appreciable amounts of impurities such as iron and silicon. However, even in the strain-hardened condition, 1xxx series alloys have very low strength compared to other aluminium alloy series.

When 1xxx series alloys are chosen for a structural application, it is almost always for their superior corrosion resistance and/or high electrical conductivity rather than strength. The most common applications for 1xxx series alloys are aluminium foil, electrical bus bars, metallising wire, and chemical tanks and piping systems.

How and Why Alloying Elements Are Added to Aluminium

The addition of alloying elements to aluminium is the principal method used to produce the wide range of materials available for structural applications. Each of the seven designated wrought alloy series is defined by its primary alloying element:

Understanding what each alloying element does to aluminium — and what it does to weldability — is essential for selecting the correct welding procedure and filler alloy. For a full explanation of how to read alloy designations, temper codes, and weldability by series, see our Aluminium Alloy Designation System & Weldability Guide.

The Principal Effects of Alloying Elements in Aluminium

Copper (Cu) — 2xxx series

Aluminium-copper alloys typically contain between 2–10% copper, with smaller additions of other elements. Copper provides substantial increases in strength and facilitates precipitation hardening, but also reduces ductility and corrosion resistance and increases susceptibility to solidification cracking. Some 2xxx series alloys — including the widely used 2024 — are among the most challenging aluminium alloys to arc weld, and some are considered unweldable by conventional arc welding processes. These alloys include some of the highest-strength heat-treatable aluminium alloys. Common applications: aerospace structures, military vehicles, rocket fins.

Manganese (Mn) — 3xxx series

Manganese increases strength through solution strengthening and improves strain hardening without appreciably reducing ductility or corrosion resistance. These are moderate-strength non-heat-treatable materials that retain strength at elevated temperatures. They are seldom used for major structural applications. Common applications: cooking utensils, radiators, air conditioning condensers, evaporators, heat exchangers and associated piping.

Silicon (Si) — 4xxx series

Silicon additions to aluminium reduce melting temperature and improve fluidity. Silicon alone in aluminium produces a non-heat-treatable alloy; in combination with magnesium, it produces a precipitation-hardening heat-treatable alloy — so the 4xxx series contains both heat-treatable and non-heat-treatable alloys. Silicon additions are widely used in aluminium castings, and the 4xxx series is most commonly encountered as filler alloys for fusion welding and brazing. ESAB's OK Autrod 4043 is a silicon-based filler alloy (4.5–6.0% Si) widely used as a general-purpose aluminium welding and brazing alloy.

Magnesium (Mg) — 5xxx series

Magnesium increases strength through solid solution strengthening and improves strain hardening. The 5xxx series are the highest-strength non-heat-treatable aluminium alloys and are used extensively for structural applications. They are produced mainly as sheets and plates — rarely as extrusions, because these alloys strain-harden quickly and are expensive to extrude.

The 5xxx series have excellent weldability. ESAB's OK Autrod 5356 (5% Mg) is the most widely used aluminium welding alloy and the standard choice for welding 5xxx and 6xxx series base materials, particularly where post-weld anodising or higher shear strength is required. For high-magnesium 5xxx alloys such as 5083 where full procedure qualification is required, OK Autrod 5183 is the recommended choice. Common applications: truck and train bodies, buildings, armoured vehicles, shipbuilding, chemical tankers, pressure vessels and cryogenic tanks.

Magnesium and Silicon (Mg₂Si) — 6xxx series

The combination of magnesium and silicon produces the compound magnesium silicide (Mg₂Si), which gives the 6xxx series their heat-treatability. These alloys are easily and economically extruded and are most often found in a wide range of extruded shapes. They form an important complementary system with the 5xxx series — 5xxx plate is frequently joined to 6xxx extrusions in structural fabrications. Common applications: handrails, driveshafts, automotive frame sections, bicycle frames, scaffolding, tubular garden furniture, stiffeners and braces used on trucks, boats and many other structural applications. 6061 and 6063 are among the most commonly welded alloys in fabrication; both are typically welded with OK Autrod 4043 or OK Autrod 5356 depending on application requirements.

Zinc (Zn) — 7xxx series

Zinc additions to aluminium — combined with magnesium and/or copper — produce the highest-strength heat-treatable aluminium alloys available. Zinc substantially increases strength and enables precipitation hardening. However, some 7xxx alloys — including 7075 — are susceptible to stress corrosion cracking after arc welding and are generally not fusion welded for structural applications. Other alloys within the series, such as 7005, are regularly fusion welded with excellent results. Common applications: aerospace structures, armoured vehicles, baseball bats, bicycle frames.

Additional alloying elements

Iron (Fe) — The most common impurity in aluminium; intentionally added to some 1xxx series alloys for a slight strength increase.

Chromium (Cr) — Added to control grain structure and prevent grain growth in Al-Mg alloys. Also helps prevent recrystallisation in Al-Mg-Si and Al-Mg-Zn alloys during heat treatment, reduces stress corrosion susceptibility, and improves toughness.

Nickel (Ni) — Added to Al-Cu and Al-Si alloys to improve hardness and strength at elevated temperatures and reduce the coefficient of thermal expansion.

Titanium (Ti) — Added primarily as a grain refiner. The effect is enhanced when boron is present or when added as a master alloy containing TiB₂. Titanium is a common addition to aluminium weld filler wire as it refines the weld structure and helps prevent weld cracking.

Zirconium (Zr) — Added to form a fine precipitate of intermetallic particles that inhibit recrystallisation.

Lithium (Li) — Substantially increases strength and Young's modulus, enables precipitation hardening, and reduces density — making Al-Li alloys particularly attractive for aerospace applications.

Lead (Pb) and Bismuth (Bi) — Added to improve machinability and chip formation. These free-machining alloys (such as 2011 and 6262) are generally not weldable because lead and bismuth produce low-melting constituents that result in poor mechanical properties and high crack sensitivity on solidification. See our Weldability Guide for more detail on free-machining alloys and other unweldable aluminium types.

Summary: Why Alloy Identification Matters Before Welding

There are currently over 400 wrought alloys and over 200 casting alloys registered with the Aluminium Association. One of the most important steps in any aluminium welding operation is positive identification of the base alloy type. If the base material type is unknown, it is extremely difficult to select a suitable welding procedure or the correct filler alloy.

General guidelines exist for the most probable alloy type in a given application — most extruded shapes are 6xxx series, most marine plate is 5xxx series, most aerospace high-performance components are 2xxx or 7xxx series — but these are guides only. Incorrect assumptions about alloy chemistry can have serious consequences for weld performance, particularly where crack sensitivity or stress corrosion cracking susceptibility is involved.

It is strongly recommended that positive identification of the aluminium type is made before welding, and that welding procedures are developed and qualification-tested to verify weld performance.

ESAB Aluminium Filler Alloys

Once the base alloy is identified, selecting the correct filler is the next critical step. ESAB's aluminium filler range covers all common weldable alloy series:

• OK Autrod 4043 — general purpose AlSi filler for 6xxx and AlSi base alloys; good fluidity, lower crack sensitivity, suitable for elevated temperature service
• OK Autrod 5356 — the most widely used aluminium welding alloy; higher shear strength, anodising-compatible, standard choice for 5xxx and 6xxx series structural work
• OK Autrod 4047 — higher silicon than 4043 (11–13%); exceptional fluidity for thin section and leak-critical applications such as heat exchangers
• OK Autrod 5183 — developed specifically for 5083 and other high-magnesium alloys where 5356 will not consistently meet tensile requirements

For full guidance on choosing between these alloys — including a comparison of properties, shear strengths, and application guidance — see our Aluminium Filler Alloy Selection Guide.