Common Classifications of Copper

Common Classifications of Copper and Copper Alloys

Copper is not a single material: it is a family of pure coppers and alloys with different compositions, properties and designations. Understanding the classification helps buyers and engineers select the right material and interpret international and Chinese designations. This article covers the four main groups - pure copper, brass, bronze and cupronickel.

Pure copper is a rose-red metal that develops a purple oxide film on its surface, which is why industrial pure copper is often called "purple copper" or electrolytic copper in Chinese practice. Density is about 8.96 g/cm³ and melting point about 1084°C. It has excellent electrical and thermal conductivity (second only to silver), good plasticity for hot and cold working, and good corrosion resistance in the atmosphere, seawater and certain non-oxidizing acids (hydrochloric, dilute sulphuric), alkalis, salt solutions and organic acids (acetic, citric). In the Chinese system, pure copper is classified as: ordinary copper (T1–T4), oxygen-free copper (TU1, TU2 and high-purity vacuum grades), deoxidized copper (TUP, TUMn), and special coppers with small alloying additions (arsenic copper, tellurium copper, silver copper).

Metallurgical notes: trace impurities strongly affect conductivity - titanium, phosphorus, iron and silicon reduce it markedly, while cadmium and zinc have little effect. Oxygen, sulphur, selenium and tellurium have low solubility in copper and form brittle compounds that reduce processing plasticity. When ordinary copper is heated in a reducing atmosphere containing hydrogen or carbon monoxide, the gas reacts with grain-boundary cuprous oxide (Cu₂O) to form high-pressure water vapour or CO₂, which can crack the copper - the well-known "hydrogen disease". This is why oxygen-free copper is preferred for brazing in reducing atmospheres. Bismuth or lead form low-melting eutectics that cause hot brittleness, and brittle bismuth films at grain boundaries also cause cold brittleness. Phosphorus sharply reduces conductivity but improves fluidity and solderability; moderate additions of lead, tellurium or sulphur improve machinability.

Brass is an alloy of copper and zinc. In simple (binary) brass, increasing zinc raises strength but lowers plasticity; industrial brass generally contains not more than about 45% zinc, above which brittleness degrades performance. Brass is produced as cast products and wrought products.

Single-phase (α) brass: below about 39% zinc, zinc dissolves in copper to form the single α phase - good plasticity, suitable for both cold and hot working (e.g. H68).

Two-phase (α+β) brass: above about 39% zinc, a β phase appears - reduced plasticity but higher tensile strength, mainly hot-worked (e.g. H59).

Chinese designation: "H + number", where the number is the copper mass fraction (H68 = 68% Cu); cast brass is prefixed with "Z" (e.g. ZH62). H90 and H80 are single-phase and golden-yellow, used for plating, decoration and medals; H68 and H59 are widely used for structural parts such as bolts, nuts, washers and springs. Special brasses add lead, tin or aluminium (e.g. HPb59-1 = 59% Cu, 1% Pb, balance zinc).

Bronze was originally the name for copper-tin alloys, named for their greenish-grey colour. Modern bronze includes tin bronze and special (tin-free) bronzes - aluminium bronze, silicon bronze, beryllium bronze and lead bronze - which are cheaper than tin bronze and offer specific properties. Chinese designation: "Q + main element symbol and mass fraction" (e.g. QAl7 aluminium bronze; cast products prefixed with "Z", e.g. ZCuSn10P1 tin bronze with about 10% Sn and 1% P).

Copper-nickel alloys appear silver-white and are called white copper (cupronickel). Simple cupronickel is a binary copper-nickel alloy; complex cupronickel adds manganese, iron, zinc or aluminium. Nickel significantly improves strength, corrosion resistance and electrical (thermoelectric) properties. Industrial cupronickel is divided into structural grades and electrochemical (electrical) grades; the classic marine grade is 90-10 (BFe10-1-1) and 70-30 (BFe30-1-1) copper-nickel.

Copper and copper alloys are supplied as rods, wires, plates, strips, tubes, foils and profiles. Forming methods: rolling (hot and cold for plate and strip; strip and foil are cold-rolled), extrusion and drawing (tubes and bars are extruded or drawn products; wire is drawn).

Copper materials are classified into pure copper, brass, bronze and cupronickel, each with its own composition, properties and designation system. The metallurgical behaviour of impurities (conductivity reduction, hydrogen disease, hot/cold brittleness) determines which grade suits which application. Confirm the designation against the applicable standard (GB, ASTM, EN or JIS) before ordering.