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 2

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/dm 3 ) Al alloys AZ91D

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 4

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…* K 2 ZrF 6 decomposes following a peritectic reaction, and the liquid phase then reacts with Al 2 O 3 according to 3 « 2 KF.ZrF 4 » + 2 Al 2 O 3 = « 6 KF.4AlF 3 » + 3 ZrO 2 cryolithe Excess K 2 ZrF 6 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 K 2 ZrF 6 C yarns are dipped for 2 min. in an aqueous solution of K 2 ZrF 6 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 6

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%AlZnMnSiNiFeCuMg AZ91D <0.05<0.001<0.004<0.025Bal. 7

Results and Discussion (1) C fibre pretreated with K 2 ZrF 6 C yarn pre-infiltrated with Al Small white precipitates result from the decomposition of K 2 ZrF 6 8

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

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…) 10

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

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 K 2 ZrF 6 ) 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 K 2 ZrF 6 No detectable oxides layer (SEM) 12

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. 13

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 K 2 ZrF 6, 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 K 2 ZrF 6 14

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) 15