Thixoforming of an Al-Si-Zn-Mg Alloy - Thermodynamic Characterization, Microstructural Evolution and Rheological Behavior

Abstract

Al-Si-Zn-Mg quaternary alloys represent a potential range of feedstock's for semisolid processing (SSM) that combine the advantages of the thixoforming process with the possibility of heat treatment to improve mechanical performance. In this work, the suitability of the Al-5wt%Si-5wt%Zn-0.5wt%Mg alloy for use in thixoforming was assessed by thermodynamic characterization and by an analysis of its microstructural evolution and rheological behavior. Thermodynamic analysis was performed by numerical simulation and by application of the differentiation method to Differential Scanning Calorimetry (DSC) curves for heating cycles up to 700 °C at 5 and 25 °C/min. Microstructural evolution during partial melting at 594 °C for 0, 30, 60 and 90 s and the corresponding rheological behavior were analyzed. The findings indicate good agreement between the thermodynamic behavior predicted by DSC and Calculation of Phase Diagrams (CALPHAD) and the data obtained from semisolid forging. Although the alloy has a coarse microstructure (G_S^max = 200 μm), its rheological behavior is appropriate for thixoforming (σ_max = 1.06 MPa and μ_max = 1.79x10⁴ Pa.s), indicating that the microstructure only plays a secondary role in determining viscosity when enough liquid (f_l = 0.63) is present in the slurry to enhance the flow. The alloy is suitable for use as raw material for semisolid processing.

Keywords:
Thixoforming; semisolid materials; Calphad; Al-Si-Zn-Mg alloy; microstructure; rheology