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Processing of carbon fibers reinforced Mg matrix composites via pre-infiltration with Al

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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

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Outline

Introduction

Experimental Procedure

Results and Discussion

Conclusions

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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 K

2

ZrF

6

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 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

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Experimental procedure (1) –

Pre-infiltration

1) C yarns are treated with K

2

ZrF

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

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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

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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 yarn The 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

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Conclusions

Pre-treatment of C yarns with K

2

ZrF

6

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 betterassessed

 Cleaning of the surface of the molten Al bath might limit contamination with

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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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