Removal of iron from a secondary Al-Si die-casting alloy by metal melt filtration in a laboratory filtration apparatusWednesday (07.10.2020) 18:00 - 18:20
The influence of the chemical composition and cooling rate was investigated on a secondary die-casting alloy aiming at the reduction of iron using molten metal filtration in a specially developed laboratory filtration apparatus. Based on stable thermal boundary conditions of the device, DSC cooling curves were determined to obtain formation temperatures of the primary iron-containing intermetallic phases (also called sludge phases) for an AlSi9Cu3 alloy (VAR 226 D) with different contents of Si, Cu, Fe, Mn, and Cr. The DSC samples were then metallographically analyzed. After that, the sludge formation temperatures of the alloys were adjusted based on furnace operating curves to confirm the presence of iron-containing intermetallic phases by sedimentation tests. Finally, filtration experiments were performed with the defined thermal parameters using the laboratory filtration apparatus, whereby the iron reduction was measured by an optical emission spectrometer (OES). The samples from filters and crucibles were investigated by using light microscopy and image analysis to determine particle size and distribution. Furthermore, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and electron backscatter diffraction (EBSD) were used to measure local chemical compositions and for phase identification. The results were also compared with a phase field simulation in the software JMatPro®.
From the DSC cooling curves, the formation temperature of sludge phase tends to shift to higher temperatures with increasing the content of the elements Fe, Mn, and Cr. An increasing percentage of the area of iron-rich intermetallic phases was detected by image analysis as a function of higher initial concentrations of the elements. Due to the filtration, a morphological transformation of the iron-containing sludge phases from a primary coarse-polyhedral and star-like morphology to a finely branched and/or Chinese script-like shape could be demonstrated. The filtration thus led to a complete separation of intentionally created, primary sludge particles.