Prakt. Met. Sonderband 38 (2006) 67
SOLIDIFICATION STRUCTURES IN CAST Al-Mg-Si ALLOYS
in y-TiAl H.P.Degischer****** H. & J.Knoblich*, E.Maire**, L.Salvo***, M.Suery***
g im o-
amellare * Inst.of Materials Science & Technology, Vienna Univ.of Technology, Vienna/Austria
können * FEDERAMS, GEMPPM, INSA-Lyon, Villeurbanne/France
amellen- ** FEDERAMS, GPM2, INP-Grenoble, St.Martin d’Heres/France
tioneller
ıd dabei
1 ABSTRACT
Jen sich
t sind. Foundry Al-alloys containing 4.9-7.3 wt% Mg and 1.9-8.4 wt% Si are investigated after
ung der cooling at different solidification rates by light optical and scanning electron microscopy.
efunden The fractions of the phases solidified during 3 steps: 1) a-Al dendrites, 2) “chinese script”
Mg2Si in a-Al, and 3) a-Al / Mg2Si / Si triple eutectic are determined and correlated to the
equilibrium conditions. The three dimensional shapes of the chinese script Mg,Si as well
as impurity Fe containing phases are revealed by high resolution synchrotron tomography.
The fast solidification rate achieved by continuous bar and strip casting produces only
triple eutectic structures, whereas chinese script Mg,Si are formed during gravity casting.
ing und
WILEY-
ukunft‘, 1. INTRODUCTION
lurgy of Al-Mg-Si alloys with higher Mg and Si contents than in the wrought 6xxx series alloys are
591 gaining importance as foundry alloys by applying new casting technologies [1,2]. Alloys
6A1-9Nb with up to 7 wt% Mg and 4wt% Si are produced by continuous strip or bar casting at high
5 solidification rates. The ternary system is described in [3] as Al-Mg-silicide alloys, because
d State Mg2Si segregates from the melt. Mg2Si may solidify in complex shapes called “chinese
scripts” [4], according to their 2D metallographic images. The phase diagram for slowly
\l-based cooled Al-Mg-Si melts [5] contains an eutectic valley for o-Al and primary Mg,Si with a
temperature maximum of 594°C at 8.5 wt% Mg/5 wt% Si, from which it extends towards
Nelson the two ternary eutectics o-Al/Mg,Si/AlsMgs (in [5] Mg»Alz) at 450°C on the Mg-rich side
and o-Al/Mg,Si/Si at 551°C on the Si-rich side. The sequence of phases solidifying at
Y(TiAl)- equilibrium conditions can be calculated by Thermocalc™ [6] for different compositions.
_eyens), The primary phases o-Al, MgzSi, Si, and so-called AlFeSi (aluminides originating from the
Fe and Mn content) can be analysed by a combination of light optical microscopy (LOM)
shm, R. and scanning electron microscopy (SEM). Their three-dimensional extension can be
id State visualised by high resolution synchrotron tomography [7]. This study aims to interpret the
solidification path of different AIMgSi foundry alloys by characterising the solidified phases.
peratur-
eutsche
ber 05 2. MATERIAL DESCRIPTION
er Tab.1 presents the main characteristics of the investigated samples. The temperatures of
ages of complete melting (Tm) and complete solidification (Ts) are calculated for equilibrium by
3.2379 using [6]. Fig.1 indicates the positions of the alloys near the Al corner of the Al-Mg-Si
\g, F.-P. phase diagram and differentiates between the under and over stoechiometric alloys with
I-Alloys. respect to Mg,Si. The continuously cast industrial alloys are compared with gravity cast
samples of the same compositions in as cast and homogenised (550°C/30 min) conditions.
