The Effect of Silver Content on the Precipitation of the Al-4.6Cu-0.3Mg Alloy

The effect of Ag concentrations of up to 0.9 mass% on the precipitation of the strengthening phases in Al-4.6Cu-0.3Mg casting alloy were investigated. Microstructural features were elucidated by optical microscopy, electron probe X-ray microanalysis, measurement of electrical conductivity and differential scanning calorimetry. The microstructure was correlated with Rockwell hardness. The results revealed that the addition of Ag at a concentration of under 0.6 mass% promoted the precipitation of the Ω and θ′ phases and the augmentation of hardness of T7 tempered alloys. 0.9 mass% Ag caused the extensive precipitation of the Ω phase, but only mildly suppressed the precipitation of the θ′ phase, slightly increasing the hardness. The high density of the precipitates of the Ω phase is responsible for the excellent thermal stability and mechanical properties of the microstructure. The Ω precipitates out before the θ′ phase, and does so more quickly, during the aging process.