Design of binder jet additive manufactured co-continuous ceramic-reinforced metal matrix composites

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Abstract

Ceramic-reinforced metal matrix composites (MMCs) display beneficial properties owing to their combination of ceramic and metal phases. However, the properties are highly dependent on the reinforcing phase composition, volume fraction and morphology. Continuous fiber or network reinforcement morphologies are difficult and expensive to manufacture, and the often-used discontinuous particle or whisker reinforcement morphologies result in less effective properties. Here, we demonstrate the formation of a co-continuous ceramic-reinforced metal matrix composite using solid-state processing. Binder jet additive manufacturing (BJAM) was used to print a nickel superalloy part followed by post-processing via reactive sintering to form a continuous carbide reinforcing phase at the particle boundaries. The kinetics of reinforcement formation are investigated in order to develop a relationship between reactive sintering time, temperature and powder composition on the reinforcing phase thickness and volume fraction. To evaluate performance, the wear resistance of the reinforced BJAM alloy 625 MMC was compared to unreinforced BJAM alloy 625, demonstrating a 64 % decrease in the specific wear rate under abrasive wear conditions.

Introduction

Ceramic-reinforced metal matrix composites (MMCs) consist of a ductile metal matrix and a reinforcing ceramic phase, offering a combination of beneficial properties from both phases. The result is often a high specific strength/stiffness, low co-efficient of thermal expansion, improved wear resistance over the unreinforced metal matrix and good fatigue/fracture properties [1,2]. The ability of the reinforcing phase to improve the properties of the metal matrix is highly dependent on its composition, distribution, morphology and volume fraction. Discontinuous reinforcement morphologies – which are the most used and most affordable type of MMC – provide only modest improvements in properties and can suffer from inhomogeneous distributions. Agglomeration of the ceramic reinforcing particles or whiskers occurs more readily during manufacturing at high volume fractions, which is detrimental to mechanical properties and wear resistance. Continuous fiber reinforced MMCs are also possible, which typically contain aligned high aspect ratio fibers embedded in a metal matrix. This unidirectional reinforcement morphology is continuous along the fiber axis, and as such displays improved properties in the fiber direction. Although layers with various orientations can be combined in the form of laminates, this typically results in beneficial properties along only two axis and suffers from adhesion issues between layers of the laminate. For beneficial properties in all three axis, continuous network reinforcement morphologies can be used. In co-continuous MMCs, the reinforcing phase and metal matrix form an interconnected network throughout the entire composite [3]. However, these morphologies are difficult to manufacture.

Fabrication of co-continuous metal/ceramic composites are typically done via reactive metal infiltration of ceramic preforms, also known as reactive metal penetration. One example is the reaction and infiltration of a cordierite preform (Mg2Al4Si5O18) with excess aluminum demonstrated by Manfredi et al. [4], resulting in a composite containing a silicon-rich aluminum metal phase and ceramic phases composed of aluminum oxide and magnesium-aluminum oxide. This process requires elevated temperatures to obtain a kinetically favoured reaction, good wetting characteristics, and a volume reduction upon reaction of the metal penetrant and ceramic preform to prevent blockage of the infiltration pathways. However, these processes demonstrate slow infiltration rates that limit part sizes due to processing time and production costs. In general, co-continuous ceramics often use lower melting point metals such as Cu or Al alloys to facilitate infiltration, although liquid exchange processes have been used to replace these metals with ones more suited to high temperature applications. As shown by Caccia et al. [5], WC preforms infiltrated with molten Zr2Cu react to form a molten Cu phase that is then removed, leaving behind solid W metal and ZrC ceramic. To avoid challenges associated with infiltration using higher melting point alloys, incompatible metal and ceramic preform combinations, slow infiltration rates, and the potential for poor wetting that leads to porosities during infiltration processes, a solid-state processing technique for the formation of co-continuous ceramics is desired.

Binder jet additive manufacturing (BJAM) is a solid-state process that selectively applies a liquid binder to a bed of powder. This is followed by the spreading of a new layer of powder, and the process is repeated to construct 3D parts in a layer-wise fashion [6]. These parts are then post-processed by de-binding and sintering at elevated temperatures. BJAM has been previously adapted for the fabrication of metal/ceramic composites, although the available literature remains limited. One technique demonstrated by Kernan et al. [7] relies on the use of ceramic and metal oxide powders, which are then reduced during post-processing to form the metal phase. Composites have also been manufactured by the layer-wise addition of a ceramic-loaded resin to a metal powder bed [8]. However, most of the literature focuses on a hybrid BJAM and infiltration technique to make composites, in which a ceramic preform is printed using BJAM and infiltration is used to introduce the metal matrix. This was demonstrated by Cramer et al. [9] with the infiltration of a B4C preform using Al and by Levy et al. [10] with the infiltration of TiC preforms using steel.

A new approach is presented for the solid-state BJAM of co-continuous MMCs, which form an interconnected reinforcing phase within a metal matrix. An in situ reactive sintering process occurs between a binder and metal powder, in which alloying elements within the metal powder react with carbon from the decomposed binder. In this work, a Ni-based alloy 625 metal powder is used so that Cr alloying elements react to form a Cr3C2 phase at the metal particle boundaries that extend throughout the manufactured part.

Section snippets

Binder jet additive manufacturing

Spherical Inconel alloy 625 powder (24 μm average diameter) was used in a 3D SYSTEMS ZCorp Z 510/310 binder jetting powder-bed system to manufacture alloy 625 and alloy 625 MMC parts. As shown in a previous study [11] the alloy 625 parts are not reinforced and retain a nominal alloy 625 composition. Alloy 625 is a predominantly solid solution strengthened Ni superalloy, with notable quantities of Cr, Mo, Fe and Nb within the Ni matrix. After BJAM and reactive sintering, a previous study [12]

Microstructure

SEM images of alloy 625’s sintered surface after the use of a de-binding step shows a smooth profile (Fig. 2a), whereas the reactive sintered alloy 625 MMC surface consists of a Cr-rich phase (Fig. 2b). The phase does not fully envelop the particle, with some Ni detectible between the Cr-rich phase using EDX (Fig. 2c, d). A previous study [12] used XRD to identify the composition of the Cr-rich phase as Cr3C2, as well as identifying the presence of internal carbides (Mo2C and NbC) within the

Conclusions

The use of binder jetting and reactive sintering post-processing to form co-continuous ceramic-reinforced metal matrix composites was demonstrated. During reactive sintering of alloy 625, a reaction between the Cr alloying element in the alloy 625 particles and the binder forms an interconnected reinforcing Cr3C2 phase throughout the Ni matrix.

  • A measure of Cr3C2 growth between 1100 °C and 1200 °C suggests that the process is diffusion limited, with good fit demonstrated for parabolic growth

Acknowledgements

This work was performed with funding support from the Natural Sciences and Engineering Research Council of Canada (NSERC), the Canada Research Chairs (CRC) Program, Huys Industries and the CWB Welding Foundation, in collaboration with the Centre for Advanced Materials Joining and the Multi-Scale Additive Manufacturing Lab at the University of Waterloo.

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