Fractal and Conventional Analysis of Cu Content Effect on the Microstructure of Al-Si-Cu-Mg Alloys

This work analyzes the effect of Cu additions on the microstructure of quaternary alloys Al-6Si-7Mg-xCu (x= 3, 5 and 7 wt.%) produced by conventional metallurgy. Microstructural modifications were studied using Optical Microscopy (OM), X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM), focusing on second phases identification and quantification. Conventional image analyses were developed measuring the total percentage of second phases, and individually for each phase determining its equivalent diameter, shape factor, roundness and aspect ratio; while fractal analyses were carried out complementarily through fractal dimension measurements for the whole microstructure and for individual phases. Compression tests were also carried out. SEM and XRD results revealed that second phases were eutectic Si, Mg₂Si, Al₂Cu and Al₅Cu₂Mg₈Si₆ (Q). The increase in Cu content led to higher quantities of these phases, mainly Al₂Cu, which was barely present for the alloy with the lowest Cu content. Morphologies of second phases depended on Cu content, being present as eutectic, block like, finely dispersed or primary particles. The combination of conventional and fractal parameters provided a useful tool for comparatively analyze microstructural modifications and second phases features. It was also found that the compressive behavior of the alloys depended on the resulting microstructures.

Keywords:
Al alloys; microstructure; mechanical properties; SEM; fractal; second phases