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

Euromat 2011, Montpellier

Outline

Introduction Experimental Procedure Results and Discussion Conclusions Prospects

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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

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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

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Introduction (3) – Why not use a pre-infiltrated reinforcement?

Treating the C fibres in an aqueous solution of K2ZrF6 has 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 to3 « 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* [Rocher et al., 1989; Schamm et al., 1991; Margueritat-Regenet, 2002…]

Experimental procedure (1) – Pre-infiltration

1) C yarns are treated with K2ZrF6

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 processing of Mg matrix composites by squeeze casting

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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.

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Results and Discussion (1)

C fibre pretreated with K2ZrF6

C yarn pre-infiltrated with Al

Small white precipitates result from the decomposition of K2ZrF6

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Results and Discussion (2)

High variability in tensile tests

UTS ranging from 28 to 78 MPa

E~9 GPa

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Results and Discussion (3)

AZ91D

No extensive damage on the C fibre

Fibres pull out at the interface between AZ91D matrix and pre-infiltrated yarnThe zone close to the interface is very disturbed (precipitates, porosities…)

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Results and discussion (4)

Presence of an oxide layer on the pre-infiltrated yarn?

Reaction between that layer and Mg?

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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

No detectable oxides layer (SEM)

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Conclusions

Pre-treatment of C yarns with K2ZrF6 and 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.

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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 better assessed

Cleaning of the surface of the molten Al bath might limit contamination with an excess K2ZrF6

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Acknowledgements

The Walloon Region for financial support through the Winnomat program

H.-M. Montrieux, J. Lecomte-Beckers (Université de Liège)

J. Halleux (Sirris) F. Delannay (UCL)

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