Study On Heat Treatment Of B96 Aluminum Alloy

The influence of homogenization, solution and aging treatment on the microstructure and mechanical properties of an Al-Zn-Mg-Cu alloy was investigated systematically using optical microscope,scanning electron microscope, differential scanning calorimetry, transmission electron microscope, X-ray diffraction analysis,hardness, tensile test,conductivity tests etc. The optimum processing of the heat treatment was obtained, which provided an experimental basis for the industrial production.The research showed that the as-cast microstructure of the alloy exhibited apparently dendrite segregation phenomena,non equilibrium eutectic structure was on the grain boundary, with the occurrence of network. Most of these non equilibrium phase was MgZn₂(η)and Al₂Mg₃Zn₃(T)phase, MgZn₂(η)phase was schistose, and Al₂Mg₃Zn₃(T) was massive phase.During the course of one-step homogenization treatment, the dendrite structure and coarse equilibrium phase disappeared gradually as the homogenization temperature increased and the homogenization time prolonged. MgZn₂(η) phase was almost disappeared by the homogenization treatment of 470℃/24h, but the massive phase Al₂Mg₃Zn₃(T) still keep remanent, the alloy was overburnt slightly when the homogenization temperature was over 480℃.Two-step homogenization treatment was 470℃/24h+480℃for different holding time. The remanent second phase dissolve gradually, the grains growed up when the second stage homogenization time exceed 12h, which can effect properties of the alloy. The optimum processing of the two-step homogenization treatment was 470℃/24h+480℃/12h. The tensile strength and yield strength was increased about 20 Mpa, and the elongation was increased 6%.Studies on the solution treatment and aging showed that the remanent second phase decreased and the supersaturation increased as the homogenization temperature increase, the tensile strength, yield strength and elongation after aging was increased, but the over-high temperature resulted in overburnt and the grains grew up, which would affect the property of the alloy.Both aging temperature and aging time affeted the microstructure and mechanical property of the alloy, at under-aged state, ageing precipitates were finer and more dispersive, the volume fraction was lower and precipitates on the grain boundary were continuous. The microstructure of the peak-aged alloy still consists of fine dispersed precipitates and precipitates on the grain boundary are continuous, and the volume fraction was increased. At over-aged state, precipitates was rod-like, and the density decreased, the distance between precipitated phases became larger, precipitates phases was rod-like,which intermittent scattered on the grain boundary.