Effects of ultrasonic treatment on the microstructures and hardening of Al-Cu-Mg-Ag alloys

Abstract

The influence of ultrasonic treatment (UT) on the microstructure morphology and hardness of Al-Cu-Mg-Ag alloy is systematically investigated by a combination of electron backscatter diffraction (EBSD), x-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM) analysis. Our results show that the hardness of as-cast and T6-tempered alloys becomes higher by UT. TEM analysis indicates that UT greatly accelerates the precipitation of θ precipitates and prohibits its rapid coarsening during exposing at 225°C. This positive effect is ascribed to the combined results of the increasing Cu content in matrix, reduced dendritic segregation and more uniform distribution of Cu solutes by UT. Besides, UT is found to affect the precipitation kinetics of Ω phase because the higher number density of Ω plates is detected in UT alloys. This feature ensures a greater precipitation strengthening contribution to hardening in T6-termperd alloy with UT. Although the grain sizes of as-cast and T6-tempered alloys are reduced by UT, the grain boundary strengthening effect is very limited in UT alloys. This work also reveals that the solute content within the matrix increases by UT and therefore favors a stronger solid solution strengthening effect, leading to the higher hardness of as-cast UT alloys. After exposing at 225°C for 100 h, all the T6-tempered alloys with and without UT exhibit a similar hardness despite the exposed microstructures exhibit clear differences. The absence of the enhanced hardening by UT in exposed alloys is arising from the fact that Ω and θ precipitates significantly coarsen during exposing.