Processing of c fibres 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. Euromat 2011, Montpellier. Outline. Introduction Experimental Procedure Results and Discussion Conclusions Prospects.

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Processing of C fibres Mg matrix composites via pre-infiltration with Al

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Processing of c fibres mg matrix composites via pre infiltration with al

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

Outline

  • Introduction

  • Experimental Procedure

  • Results and Discussion

  • Conclusions

  • Prospects


Introduction 1 aims of this research

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


Introduction 2 production of m m c by means of semi liquid state processes

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

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?

* [Rocher et al., 1989; Schamm et al., 1991; Margueritat-Regenet, 2002…]


Experimental procedure 1 pre infiltration

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


Experimental procedure 2 composites

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


Results and discussion 1

Results and Discussion (1)

C fibre pretreated with K2ZrF6

C yarn pre-infiltrated with Al

Small white precipitates result from the decomposition of K2ZrF6


Results and discussion 2

High variability in tensile tests

UTS ranging from 28 to 78 MPa

E~9 GPa

Results and Discussion (2)


Results and discussion 3

Results and Discussion (3)

Fibres pull out at the interface between AZ91D matrix and pre-infiltrated yarn

No extensive damage on the C fibre

The zone close to the interface is very disturbed (precipitates, porosities…)

AZ91D


Results and discussion 4

Results and discussion (4)

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

Reaction between that layer and Mg?


Results and discussion 5

Results and discussion (5)

A few small areas are contaminated by an excess of precipitates resulting from the decomposition of K2ZrF6

No detectable oxides layer (SEM)

  • 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


Conclusions

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.


Prospects

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 an excess K2ZrF6


Acknowledgements

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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