aluminium bronze

Aluminium bronze is a copper-based alloy containing between 5% and 10% aluminium by weight, often with smaller additions of iron, nickel, or manganese. Unlike brass (copper-zinc) or tin bronze (copper-tin), the aluminium addition creates a fundamentally different metallurgical structure. The resulting material is significantly harder and stronger than pure copper, with tensile strength typically in the range of 500 to 700 MPa depending on composition and heat treatment.

The alloy derives its corrosion resistance from a thin, self-healing aluminium oxide layer that forms on the surface. This makes it invaluable in marine environments, where it outperforms both copper and tin bronze in saltwater. It resists seawater corrosion far better than brasses and does not suffer from dezincification, a failure mode that can plague brass in certain waters. Naval applications include propellers, seawater pump impellers, and valve bodies. The material also performs well in chemical plants and food processing where copper leaching is a concern.

Aluminium bronze is notoriously difficult to weld and cast. The high affinity between molten aluminium and oxygen means that proper inert gas shielding is essential, and solidification is prone to gas porosity and shrinkage cavities. Sand casting is common for larger components, but grain refinement and degassing procedures are critical. Machining is feasible but requires careful tool selection and speeds lower than those used for brass, as the material tends to work-harden rapidly and generate heat.

Common variants and applications

Standard aluminium bronze (5% to 8% aluminium) is used for marine hardware and general corrosion resistance. Nickel-aluminium bronzes add 4% to 6% nickel and are specified for even more demanding saltwater service, particularly in propeller shafts and submarine components. Iron is sometimes added to improve casting fluidity and strength. The material cannot be hardened by quenching like some brasses; instead, strength depends on composition, cold work, and careful annealing. A fully annealed rod or bar will be softer and more ductile; cold-worked material is harder but less formable.

Aluminium bronze sits in the gap between pure copper (soft, highly conductive, weak) and stainless steel (expensive, poor machinability in some grades, less forgiving in casting). Its cost falls between brass and stainless. The name reflects the addition that dominates its behaviour: aluminium provides the corrosion resistance and much of the strength gain, even though copper remains the base element. It has been used in industrial service for over a century and remains the default choice for seawater-wetted bearings, bushings, and pump components where a non-ferrous alloy is required.