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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. 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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  1. 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

  2. Outline • Introduction • Experimental Procedure • Results and Discussion • Conclusions • Prospects

  3. 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

  4. 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

  5. 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…]

  6. 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

  7. 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

  8. Results and Discussion (1) C fibre pretreated with K2ZrF6 C yarn pre-infiltrated with Al Small white precipitates result from the decomposition of K2ZrF6

  9. High variability in tensile tests UTS ranging from 28 to 78 MPa E~9 GPa Results and Discussion (2)

  10. 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

  11. Results and discussion (4) Presence of an oxide layer on the pre-infiltrated yarn? Reaction between that layer and Mg?

  12. 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

  13. 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.

  14. 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

  15. 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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