Effect of Nickel addition and solution treatment time on microstructure and hardness of Al-Si-Cu aged alloys

The effects of the addition < 2 wt.% of Nickel and solid solution treatment times on microstructure and hardness of Al-Si-Cu alloy (A319) are reported. The results obtained show that Ni additions and heat treatments influence changes in microstructure, hardness and precipitation kinetics. The dendrites size and number density of Al-Cu-Ni and Al-Ni intermetallic compounds in as-cast condition are affected with Ni additions; however, occurs a reduction of Al-Cu-Ni phases at longer solution times (7 h). The hardness values in alloyed samples with Ni are higher than those of reference samples, the maximum hardness peak are reached at shorter aging times; nonetheless, the decrease was slower during overaging step. The hardness values increase with Ni content ~ 4% HV where an increase of maximum hardness ~ 6% HV is reached with aging. After 10 h of aging treatment at 170 °C, the alloyed samples and not alloyed present different type of precipitates. In reference sample θ′-Al2Cu precipitates were identified, and in the alloyed samples, two types of coherent precipitates with different thickness and size were observed, in addition those precipitates present smaller size and higher number density than those observed in the reference alloy.