THERMODYNAMIC AND MICROSTRUCTURAL INVESTIGATIONS OF THE
AL-MG-SI EXTRUSION ALLOYS:
CRITERIA FOR PHASE SELECTION AND FORMABILITY
K. Korgiopoulos1, *A.R. Farkoosh1, O. Rodop1, L.A. Villegas Armenta1, B. Cook1, H. Blanchette2 and M.
Pekguleryuz1
1
McGill University
3610 University Street
Montreal, Canada H3A 0C5
(*Corresponding author: amir.rezaeifarkoosh@mcgill.ca)
2
Raufoss Canada
4050 Rue Lavoisier
Boisbriand, Canada J7H 1R4
ABSTRACT
The excellent corrosion resistance and good formability of the Al-Mg-Si alloys make them a suitable
candidate material for automotive industry. The alloying additions, Si and Mg impart strength through agehardening, whereas Mn produces thermally stable dispersoids which prevent recrystallization and grain
growth during hot deformation. In this study the effects of varying levels of Si (0.5-1.5wt%), Mg (0.51.5wt%) and Mn (0.1-2.5wt%) on the phase selection have been studied through thermodynamic calculations
to understand the effects of the constituent particles, specifically, age-hardening precipitates, dispersoids,
and primary intermetallic phases on the grain structure, strength and formability. Thermodynamic
calculations, equilibrium and non-equilibrium, were conducted via FactSage™ software using the FTlite
database. Selected compositions have been cast, heat treated, extruded and characterized through scanning
electron microscopy (SEM) with energy dispersive spectroscopy (EDS) and electron backscatter diffraction
(EBSD) techniques. Calculations showed that at the hot deformation temperature (~540 oC) Mg and Si higher
that a specific amount lead to the presence the retained β-Mg2Si intermetallics and Si particles at the grain
boundaries in addition to α-Al-(Mn,Cr,Fe)-Si. It was suggested that, in order to avoid the undesirable
consequences of the retained β-Mg2Si particles, Si+2Mg should be less than 2.3wt% (Figure 1). According
to the calculations, Mn increase from 0.5wt% to 1wt% leads to a higher amount of dispersoids (increase from
1wt% to 2wt%). This increase will potentially slow down the recrystallization and grain growth during hot
deformation and lead to a better formability. Calculations at room temperature also shows that for an alloy
with 0.7wt% Mg, excess Si beyond 0.7 wt% promotes the precipitation of Si clusters in the Al matrix which
leads to an accelerated natural age-hardening. Results are confirmed by experimental investigations.
Si + 2Mg < 2.3
Figure 1- The amount of the retained Mg2Si at 540oC
KEYWORDS
FactSageTM, 6082 aluminum alloy, Age hardening, Primary intermetallics, Dispersoids, Automotive