**2. Additive manufacturing techniques for the production of metal matrix composites**

Following standard ASTM F2792-10, additive manufacturing is defined as "*the processing of joining materials to make objects from 3D model data, usually layer upon layer, as opposed to subtractive manufacturing technologies"*. This definition encompasses a large variety of manufacturing processes suitable for the fabrication of MMCs (**Table 1**). 3D printing that relies on a binding agent to consolidate a powder has been used successfully to produce polymer matrix compo‐ sites with particulate reinforcements [29]. This technique can also be applied to the manufac‐ turing of metallic materials, but due to the intrinsic nature of its consolidation process [29, 30], it does not allow for making as dense products as can be obtained from polymers. In this case, the advantage of additive manufacturing lies in the excellent dimensional accuracy of the part [30] and in the possibility to design and fabricate directional 3D preforms with optimised out-of-plane mechanical properties [31]. Fully dense parts can only be obtained by further infiltrating the porous preform [29, 30]. This process is thus very similar to the conventional processing of MMCs by infiltration or squeeze casting, and it will not be discussed in more details in this chapter.

corrosive environments and/or increased mechanical loads. At the same time, demands for metallic materials with enhanced specific properties are also increasing in view of the trends for lightweighting in portable applications. In this context, metal matrix composites (MMCs), combining the advantages of the metallic matrix with the beneficial contribution of a wellselected second phase, appear as materials of choice that can be designed and tailor-made in

The vast choice of potential second phases opens unlimited possibilities in terms of the usage properties that can be attained. Indeed, the reinforcements may take on different morphology (i.e. long fibres, short fibres or particles) and size (i.e. in the micro- or nano-size range) [1]. Various reinforcements may even be combined to make a hybrid composite [1, 2]. Among the most popular types of reinforcements, carbides such as tungsten (WC) [3–7], chromium (Cr3C2) [3, 8], silicon (SiC) [2, 9–12] or titanium carbides (TiC) [13–15] have often been used in view of their high hardness to enhance the wear resistance of the composites. Oxides [16, 17], nitrides [18] or borides [19, 20] also proved of interest as reinforcement, as did intermetallics [9, 15]. Alternatively, the second phase may also be selected in order to fulfil a specific function, such as, self-cleaning [21], self-healing [22, 23] or as solid lubricant in self-lubricating MMCs that are currently attracting a growing interest for applications where classical lubrication

A number of different methods can be used for the fabrication of MMCs. Melting metallurgical processes include infiltration of a preform by squeeze casting [1, 27], reaction infiltration or stir casting [1]. Powder metallurgy processes involve sintering, pressing or forging of a mixture of powders or of composite powders [1, 24], while severe plastic deformation processes such as friction stir processing rely solely on solid-state material flow [27, 28]. These conventional processes for the elaboration of MMCs share a common limitation. Indeed, it is very difficult to fabricate MMC components with complex shapes by these methods [2]. On the other hand, additive manufacturing and particularly powder-based additive techniques offer the possi‐ bility to fabricate any complex geometry directly from the powders [2, 29]. General features of additive manufacturing processes suitable for the fabrication of MMCs will be reviewed in more details in the second section of this chapter, while Sections 3 and 4 will focus on some specific examples of MMCs processed by additive manufacturing, along with their properties

**2. Additive manufacturing techniques for the production of metal matrix**

Following standard ASTM F2792-10, additive manufacturing is defined as "*the processing of joining materials to make objects from 3D model data, usually layer upon layer, as opposed to subtractive manufacturing technologies"*. This definition encompasses a large variety of manufacturing processes suitable for the fabrication of MMCs (**Table 1**). 3D printing that relies on a binding agent to consolidate a powder has been used successfully to produce polymer matrix compo‐ sites with particulate reinforcements [29]. This technique can also be applied to the manufac‐

view of a specific application, in bulk or as coating [1].

methods do not work [23–26].

188 New Trends in 3D Printing

and envisioned applications.

**composites**


**Table 1.** Classification of additive manufacturing processes for the production of metal matrix composites (MMCs) [12, 29, 32].

Laminated object manufacturing allows the fabrication of MMCs reinforced with either particulates or fibres (**Table 1**) [29]. However, this process requires the production of composite sheets prior to making the composite 3D part, as is also the case for ultrasonic consolidation in which metallic—or MMC—foils are joined together by ultrasonic welding (**Table 1**) [29]. Both these methods offer the possibility to control precisely the placement of the reinforcement inside the final component, but they are not particularly well adapted to the fabrication of MMCs parts with complex geometries.

When it comes to making MMCs components with complicated shapes, powder-based additive manufacturing techniques demonstrate the greatest potential, in spite of being generally limited to particulate reinforcements (**Table 1**) [12, 29]. Powder-based additive manufacturing techniques can be classified into two broad categories, i.e. (1) beam deposition processes in which a high-energy beam, typically a laser, is used to fuse a powdered material as it is projected onto a substrate and (2) powder-bed fusion processes that use a focused energy beam to melt locally selected zones of a powder bed. The first category encompasses the processes known as direct metal deposition (DMD), laser deposition (LD), laser engineering net shaping (LENS) or laser cladding (LC)—the latter denomination can also be used to designate the laser surface treatment of pre-deposited powdered layer. Direct metal laser sintering (DMLS), laser beam melting (LBM) and selective laser melting (SLM) are examples of the second category [12, 29, 32].

From a practical viewpoint, powder-bed fusion processes require that the two constituents of the composite, i.e. the matrix and reinforcing powders, should be pre-mixed prior to their spreading in the powder bed [2]. In beam deposition processes, on the other hand, it is also possible to feed the two powders into the beam from two separate hoppers, without premixing. This latter set-up is favourable in order to avoid any segregation of the constituents due to difference in the densities of the powders that could impair a precise compositional control of the composite material [15]. Moreover, it opens the possibility of gradually varying the ratio of the matrix and reinforcement powders during processing so as to make functionally graded materials (FGMs) [10, 11, 15, 18]. LC of pre-deposited pre-mixed powders may still be preferred in some occasions, e.g. for depositing a composite layer on a substrate characterised by a very high reflectivity [26]. Finally, it is worth noting that while the vast majority of the research on the production of MMCs by beam deposition processes has been focused on powder-feed techniques, few investigations have also been carried out into the fabrication of composites by a mixed wire- and powder-feed process [6, 33].

Practical differences set aside, all powder-based laser additive manufacturing processes are faced with similar issues and challenges during the fabrication of MMCs reinforced by ceramic particles. These challenges are related

**1.** to interactions between the ceramic particles and the laser beam: In a few instances, the reinforcing particles have been reported to melt or decompose when exposed to the action of the laser [25, 34]. Besides, absorptivity of the laser beam by ceramic particles may differ significantly from the absorptivity by metallic powders, thus affecting the transfer of energy from the laser to the built [2, 5, 14, 15, 35]. As a consequence, it may prove necessary to adjust continuously the processing parameters as a function of the volume fraction of ceramic particles during the fabrication of FGMs [15]. Alternatively, Przybylowicz and Kusinski [5] suggested to coat WC or TiC particles with nickel to lower down their absorption coefficient to values comparable to the absorption coefficient of the metallic powders.

On the Role of Interfacial Reactions, Dissolution and Secondary Precipitation During the Laser Additive Manufacturing of Metal Matrix Composites: A Review http://dx.doi.org/10.5772/63045 191

inside the final component, but they are not particularly well adapted to the fabrication of

When it comes to making MMCs components with complicated shapes, powder-based additive manufacturing techniques demonstrate the greatest potential, in spite of being generally limited to particulate reinforcements (**Table 1**) [12, 29]. Powder-based additive manufacturing techniques can be classified into two broad categories, i.e. (1) beam deposition processes in which a high-energy beam, typically a laser, is used to fuse a powdered material as it is projected onto a substrate and (2) powder-bed fusion processes that use a focused energy beam to melt locally selected zones of a powder bed. The first category encompasses the processes known as direct metal deposition (DMD), laser deposition (LD), laser engineering net shaping (LENS) or laser cladding (LC)—the latter denomination can also be used to designate the laser surface treatment of pre-deposited powdered layer. Direct metal laser sintering (DMLS), laser beam melting (LBM) and selective laser melting (SLM) are examples

From a practical viewpoint, powder-bed fusion processes require that the two constituents of the composite, i.e. the matrix and reinforcing powders, should be pre-mixed prior to their spreading in the powder bed [2]. In beam deposition processes, on the other hand, it is also possible to feed the two powders into the beam from two separate hoppers, without premixing. This latter set-up is favourable in order to avoid any segregation of the constituents due to difference in the densities of the powders that could impair a precise compositional control of the composite material [15]. Moreover, it opens the possibility of gradually varying the ratio of the matrix and reinforcement powders during processing so as to make functionally graded materials (FGMs) [10, 11, 15, 18]. LC of pre-deposited pre-mixed powders may still be preferred in some occasions, e.g. for depositing a composite layer on a substrate characterised by a very high reflectivity [26]. Finally, it is worth noting that while the vast majority of the research on the production of MMCs by beam deposition processes has been focused on powder-feed techniques, few investigations have also been carried out into the fabrication of

Practical differences set aside, all powder-based laser additive manufacturing processes are faced with similar issues and challenges during the fabrication of MMCs reinforced by ceramic

**1.** to interactions between the ceramic particles and the laser beam: In a few instances, the reinforcing particles have been reported to melt or decompose when exposed to the action of the laser [25, 34]. Besides, absorptivity of the laser beam by ceramic particles may differ significantly from the absorptivity by metallic powders, thus affecting the transfer of energy from the laser to the built [2, 5, 14, 15, 35]. As a consequence, it may prove necessary to adjust continuously the processing parameters as a function of the volume fraction of ceramic particles during the fabrication of FGMs [15]. Alternatively, Przybylowicz and Kusinski [5] suggested to coat WC or TiC particles with nickel to lower down their absorption coefficient to values comparable to the absorption coefficient of the metallic

MMCs parts with complex geometries.

190 New Trends in 3D Printing

of the second category [12, 29, 32].

particles. These challenges are related

powders.

composites by a mixed wire- and powder-feed process [6, 33].

**2.** to interactions between the ceramic particles and the metallic melt pool: while a few examples of matrix-reinforcement pairs exist for which the dissolution of the reinforcing particles in the melt pool remains very limited [19], most often ceramic particles tend to dissolve at least partially in the melt pool [8, 11–13], as illustrated in **Figure 1** in the case of a stainless steel 316L-WC composite [36]. Consequently, new phases may form as a result of secondary precipitation inside the metallic matrix. Due to the ultra-fast thermal cycles typically imposed by laser additive manufacturing processes, these new phases generally exhibit ultra-fine structures often resulting in enhanced hardness and wear properties [2, 11, 35, 36]. Interfacial reactions, dissolution and secondary precipitation phenomena have even been used to synthesise the reinforcing phase of MMCs by in situ reactions. In this case, the energy of the laser is not only used to fuse the powdered materials but also to overcome the activation energy barrier for the reaction and form new chemical compounds [2, 29, 37–41].

**Figure 1.** SEM micrograph of a stainless steel 316L composite coating with 16 vol% of tungsten carbides deposited by laser cladding [36].

Either way, in both the ex situ and the in situ processing of MMCs, a careful control of the reactions taking place during laser additive manufacturing is needed in order to produce MMCs with enhanced properties. The following sections of this chapter thus aim at reviewing systematically pairs of metallic matrix and reinforcing phase that have already been produced by additive manufacturing, challenges that were met during the production of these compo‐ sites and the properties that could be attained. Section 3 focuses on the fabrication of ex situ MMCs, and Section 4 on the in situ synthesis of MMCs. A synoptic table summarising the major characteristics of these MMCs is proposed in Section 5. Special care is also taken to identify current fundamental issues that should more particularly be the object of future work.
