Cu-induced phase transformations in Al-7Si-0.75Mg-0.15Sc-0.2Zr-xCu alloys and its effects on microstructure and mechanical properties

To investigate the influence of Cu content on the evolution of solidification phases and precipitates (β", Q" and Si-dispersoids), as well as the mechanical properties of Al-7Si-0.75 Mg-0.15Sc-0.2Zr-xCu, alloys with Cu contents ranging from 0.1 to 1.0 wt% were prepared using a copper mold. The results demonstrate that Cu addition does not refine α-Al grains, with EBSD confirming grain sizes of 100–120 μm. Increasing Cu content leads to the gradual disappearance of the primary β-Mg 2 Si phase, accompanied by the formation of Cu-containing intermetallic compounds (θ-Al 2 Cu and Q-Al 5 Cu 2 Mg 8 Si 6 ). After heat treatment, the precipitates primarily consist of precursor phases of β, Q, and θ, along with Si dispersoids. At 0.1 wt% Cu, the Al matrix contains β" precipitates, which coarsen as Cu increases to 0.25 wt.%. A phase transformation occurs at 0.5 wt.% Cu, where β" precipitates disappear, replaced by finely dispersed Q" precipitates. At 0.75 wt.% Cu, granular Q' precipitates dominate, while at 1.0 wt.% Cu, a coexistence of Q', θ", and θ’ precipitates is observed. The size of Si dispersoids exhibits a non-monotonic trend with increasing Cu content: initial growth, followed by reduction, and subsequent re-growth. Furthermore, the precipitation mechanism of AlSi 2 (Sc,Zr,Ti) 2 phases was elucidated. The AlSi 2 (Sc,Zr,Ti) 2 phase lacks crystallographic orientation relationships with θ” or θ’ precipitates. In this work, the alloy with 0.5 wt% Cu achieves optimal mechanical properties, exhibiting a yield strength of 368 MPa, ultimate tensile strength of 419 MPa, and elongation of 5.3%. • Revealing the phase transformation mechanism from β" to Q" precipitates. • Clarifying formation and evolution of multi-scale precipitates (submicron AlSi 2 (Sc,Zr,Ti) 2 , nano Si-dispersoids, and nano β"/Q"/Q'). • Tailoring Cu content with multi-step solution and dual-stage aging achieves superior mechanical properties.