The welding of aluminium and its alloys

Alloying elements

The principal alloying elements are copper, silicon, manganese, magnesium, lithium and zinc. Elements such as nickel, chromium, titanium, zirconium and scandium may be added in small amounts to achieve specific proper­ties. Other elements may also be present in small amounts as unwanted impurities. These elements, known as tramp or residual elements, have no beneficial effects on mechanical properties and the aluminium producers attempt to eliminate these from their products. The main effects of the alloying elements are as follows:

• Magnesium (Mg) increases strength through solid solution strengthen­ing and improves work hardening ability.

• Manganese (Mn) increases strength through solid solution strengthen­ing and improves work hardening ability.

• Copper (Cu) gives substantial increases in strength, permits precipita­tion hardening, reduces corrosion resistance, ductility and weldability.

• Silicon (Si) increases strength and ductility, in combination with mag­nesium produces precipitation hardening.

• Zinc (Zn) substantially increases strength, permits precipitation hard­ening, can cause stress corrosion.

• Iron (Fe) increases strength of pure aluminium, generally residual element.

• Chromium (Cr) increases stress corrosion resistance.

• Nickel (Ni) improves elevated temperature strength.

• Titanium (Ti) used as a grain-refining element, particularly in filler metals.

• Zirconium (Zr) used as a grain-refining element, particularly in filler metals.

• Lithium (Li) substantially increases strength and Young’s modulus, provides precipitation hardening, decreases density.

• Scandium (Sc) substantially increases strength by age hardening, grain - refining element particularly in weld metal.

• Lead (Pb) and bismuth (Bi) assist chip formation in free machining alloys.