What Are the Precautions For Aluminum Bronze Heat Treatment?

Aluminum bronze with an aluminum mass fraction of less than 7.4% is a single-phase α solid solution at all temperatures, and is often used for intermediate recrystallization annealing heat treatment after pressure processing. When the mass fraction of aluminum is 7.4%~9.4%, it is a dual-phase aluminum alloy. When the annealing cooling rate is too fast, the incomplete transformation of β→α will occur in the alloy, and part of the B phase will undergo eutectoid transformation during the subsequent cooling. β→α+γ2. γ2 is the phase of hard and brittle intermetallic compounds Cu-Al and Cu-Al-Fe, which increases the strength and hardness of the metal, but decreases the plasticity. The aluminum bronze with an aluminum mass fraction of 9.4%-15.6% undergoes a β→α+γ2 eutectoid transformation during annealing heat treatment and slow cooling, forming a chip-like eutectoid structure, which reduces the plasticity and toughness.

Dual-phase aluminum bronze (mass fraction of aluminum is 9.4%-15.6%) is often quenched for heat treatment strengthening. When heated to the 5-phase zone, β-β′ phase transition will occur during rapid cooling. This phase transition is a non-diffusible phase transition, forming metastable acicular martensite. During the transformation of β-β’, it passes through an intermediate stage, that is, an ordered solid solution β1 is formed first, and then it is transformed into martensite β’or γ’. That is, β-ordered β1→non-diffusion phase becomes β′ or γ′, β′ and γ′ are martensite with different aluminum content. β’martensite is a kind of metastable structure, and then heating it will form β’→α+γ2 eutectoid transformation, which improves the strength and hardness.

The quenching range of aluminum alloy is small, and it cannot be overheated when it is close to the melting point temperature. The strength of the alloy decreases as the cooling rate of the quenching heat treatment decreases. When the cooling rate is low, the α+γ2 phases of Cu-Al and Cu-Al-Fe alloys appear, while Cu-Al-Ni, Cu-Al-Ni-Fe appear in α+KⅢ phases, and both γ2 and KⅢ phases appear. It is a hard phase, which increases the brittleness of the alloy while decreasing its toughness.