Processing of C fibres Mg matrix
composites via pre-infiltration with Al
A. Mertens, H.-M. Montrieux, J. Halleux, J. Lecomte-Beckers and F. Delannay
Outline
Introduction
Experimental Procedure
Results and Discussion
Conclusions
Introduction (1) – Aims of this research
Mg is much lighter than Al alloys, but with lower resistance and
stiffness
C fibres as reinforcement to improve mechanical properties Mg
matrix composites
Good composites
‘Good’ properties of the interface No porosities
Material y (MPa) E(GPa) (kg/dm3)
Al alloys 95-610 68-80 2.5-2.9
AZ91D 83-124 45-47.3 1.8-1.81
Introduction (2) - Production of M.M.C. by
means of
(semi-)liquid state
processes
Squeeze Casting,
Thixomolding…
Issues:
Control of Solidification Wetting: poor wetting
can be overcome thanks to the external pressure
Stiffness of the preform
C-Mg M.M.C.
Squeeze Casting
Introduction (3) – Why not use a
pre-infiltrated reinforcement?
Treating the C fibres in an aqueous solution of K
2ZrF
6has
been shown to favour the spontaneous wetting of C fibres
by Al…*
K2ZrF6 decomposes following a peritectic reaction, and the liquid
phase then reacts with Al2O3 according to
3 « 2 KF.ZrF4 » + 2 Al2O3 = « 6 KF.4AlF3 » + 3 ZrO2 cryolithe
Excess K2ZrF6 reacts with Al to produce more cryolithe and free Zr
…but it does not work with Mg-based alloys
C yarns pre-infiltrated with Al as reinforcing phase?
5
Experimental procedure (1) –
Pre-infiltration
1) C yarns are treated with K
2ZrF
6 C yarns are dipped for 2 min. in an aqueous solution of K2ZrF6 at
95°C
C yarns are dried in an oven at 110°C for 2 hours
2) Pre-treated yarns are dipped for 30 s in a bath of Al
molten at 750°C
3) These pre-infiltrated yarns are used as preforms for the
Experimental Procedure (2) - Composites
Squeeze casting with alloy AZ91D (good castability, good mechanical
properties)
Applied pressure: 25 MPa m Mg = 380g
T cast = 780°C
T die = T punch = 350°C
Preforms preheated at 400°C, under Ar flow
Characterisation
Microstructure: SEM, EDX
Mechanical properties: uniaxial tension
Wt% Al Zn Mn Si Ni Fe Cu Mg
AZ91D 8.5-9.5
0.45-0.9 0.17-0.4 <0.05 <0.001 <0.004 <0.025 Bal.
Results and Discussion (1)
C fibre pretreated
with K2ZrF6 C yarn pre-infiltrated with Al
Small white precipitates result from the
Results and Discussion (2)
High variability in tensile
tests
UTS ranging from 28 to
78 MPa
E~9 GPa
Results and Discussion (3)
AZ91D No extensive damage on the C fibre Fibres pull out
at the interface between AZ91D matrix and pre-infiltrated yarn The zone close to the interface is very disturbed (precipitates, porosities…)
Results and discussion (4)
Presence of an
oxide layer on the pre-infiltrated yarn?
Reaction between
that layer and Mg?
Results and discussion (5)
Where does this fairly thick (~30 µm) oxygen-rich layer come from?
The pre-infiltrated yarns can pick up some dirt (including an excess of
K2ZrF6) and oxides when they are pulled out of the molten Al bath
The pre-infiltrated yarns can also become oxidised during storage
between pre-infiltration and squeeze casting
And finally, they can become oxidised upon pre-heating before squeeze
casting: 30 min at 400°C, under Ar flow
A few small areas are contaminated by an excess of precipitates resulting from the decomposition of K2ZrF6
Conclusions
Pre-treatment of C yarns with K
2ZrF
6and pre-infiltration
with Al do not cause extensive damage to the C fibres
Composites can be produced from pre-infiltrated preforms
by means of squeeze casting
Interface between the pre-infiltrated C yarn and the AZ91D matrix is
the weak point of the composite. This is possibly due to the
formation of oxides layers at the surface of the pre-infiltrated yarn and to further reaction(s) between these layers and Mg
These interfacial layer(s) should be avoided, or their
thickness should be strongly decreased in order to improve
the mechanical properties of the composite.
Prospects
In a very near future: take a deeper look at the evolution of the surface
of the pre-infiltrated yarns upon pre-heating
Pre-heating conditions could be adapted, or the pre-heating step could be
suppressed
Some way of mechanically controlling the shape and size of the
pre-infiltrated yarns might also help in order to get a cleaner surface
The effect of the precipitates resulting from the decomposition of
K2ZrF6, and of their inhomogeneous distribution in the microstructure, should be betterassessed
Cleaning of the surface of the molten Al bath might limit contamination with
Acknowledgements
The Walloon Region for financial support through
the Winnomat program
H.-M. Montrieux, J. Lecomte-Beckers (Université
de Liège)