**4.1. In situ synthesis of aluminium matrix composites**

**3.4. Others**

198 New Trends in 3D Printing

[79].

and CaF2 [83].

**4. Laser-induced in situ synthesis of MMCs**

a fine and uniform distribution of the reinforcing phases [29].

In view of their comparatively high cost, titanium matrix composites are somewhat less popular than Al-based or ferrous matrix composites. Again, carbides—particularly TiC [14, 15, 33, 71, 72], but also WC [73] or SiC [74]—have attracted a great interest as reinforcement in titanium matrix composites, with positive effect on their hardness and wear behaviour. TiB, TiB2 [20, 75] and TiN [18] have also been used as alternative or in combination with TiC.

Copper (Cu) alloys are rather difficult to process by laser additive processes, due to their high reflectivity to laser light [26]. Nevertheless, the DLS of Cu matrix composites reinforced with Co/WC particles has been studied extensively, with the aim of combining the excellent thermal and electrical conductivity of the Cu matrix with the high strength and hardness of the Co/WC reinforcement [76, 77]. Rare earth oxides nanoparticles (CeO2, La2O3) have been added in these Cu + Co/WC composites to favour microstructural refinement [78]. More recently, the direct addition of WC particles in Cu matrix composites processed by SLM has also been investigated

Researches on Co-based matrix composites have been largely focused on the LC of composite reinforced with WC or other cermet particles, in view of applications necessitating a very high abrasion resistance as, e.g. in cutting tools [5, 80–82]. WC present a greater reactivity with Co alloys than with Ni alloys, and the partial dissolution of WC accompanied by secondary precipitation of finely dispersed complex carbides is commonly reported. A recent publication also reports on the LC of self-lubricating Co-based composite coatings with additions of TiC

Reinforcement particles can also be synthesised in situ during the laser additive manufacturing of MMCs, either from a mixture of pure elemental powders [37, 84] or from ceramic particles that would decompose under the effect of laser irradiation and/or dissolve into the melt pool [2, 35, 38, 85]. The energy brought by the laser is used to fuse the metallic matrix powder and form new chemical compounds. Alternatively, the thermal energy brought by the laser may also trigger an exothermic chemical reaction that will not only produce new chemical com‐ pounds but also in itself generate enough thermal energy to propagate more chemical reactions [29, 86]. This process, known as self-propagating high-temperature synthesis, was originally used in combination with Selective Laser Sintering to produce Ni–Al intermetallics [39] and later extended to the synthesis of NiTi-matrix composites [40]. In this case, laser additive manufacturing can sometimes be pursued under a lower laser energy since the latent heat generated by the chemical reactions also contributes to increase the temperature of the melt pool [41, 86]. Besides, the in situ synthesis of MMCs has several other advantages: it allows for a better wetting and cohesion of ceramic particles with the metallic matrix, and for obtaining

The boundary between the ex situ fabrication and the in situ synthesis of MMCs is actually quite blurred. Indeed, in a few instances, the processing parameters for the fabrication of metalChang et al. [2] provides an interesting example of a careful and deliberate control of chemical reactions during the SLM of AlSi10Mg/SiC composites, making profit from the strong tendency of SiC to dissolve in Al and its alloys. As outlined previously (Section 3.1.1), the SLM processing parameters were adjusted in such a way as to favour the formation of mixed Al4SiC4 carbides over the potentially deleterious Al4C3. Moreover, the effect of the size of the SiC particles on the extent of secondary precipitation was investigated. It was found that the smallest average particle size of 5 μm gave way to the optimised distribution of refined Al4SiC4 precipitates and to the greatest improvement in hardness and wear behaviour.

Xu et al. [84] studied the in situ synthesis of Al-matrix composites reinforced with titanium diborides (TiB2) from pure elemental Al, Ti and B powders. B powder was coated with Fe in order to avoid its burning during interaction with the laser. The resulting Al-matrix composites were reinforced with TiB2 presenting a bimodal size distribution (i.e. with particles in size range 20–30 μm and smaller particles with a size < 500 nm). Besides, intermetallics Al3Ti and Al3Fe were also produced, and the wear resistance of the composites under low loads showed a marked improvement.

Dadbakhsh and Hao [87] attempted to produce Al matrix/oxides by SLM of Fe2O3 particles mixed with pure Al powder or various other Al alloys. This resulted in a very fine dispersion of Al2O3 and Al–Fe intermetallics with a size < 400 nm. Unfortunately, the mechanical behav‐ iour of all the produced composites was dramatically compromised by an excessive volume fraction of porosities (i.e. at least 20 vol% or higher). Indeed, the oxygen introduced in the composite by the addition of Fe2O3 favoured the formation of an Al2O3 layer at the surface of the solidified material, thus preventing its proper wetting by the melt pool and a sufficient adhesion between successive layers which is a well-known issue in the SLM of Al alloys [88– 90].

#### **4.2. In situ synthesis of ferrous matrix composites**

Ferrous matrix composites reinforced with various types of carbides have already been synthesised successfully using laser additive manufacturing techniques. Fe/TiC composites have been produced by LC, resulting in a significant increase in the hardness of the composite coating when compared to the substrate [37, 91]. Finely dispersed NbC were obtained from blended elemental powders during the LC of FeCrBSi matrix composites, with the addition of CeO2 nanoparticles acting as heterogeneous nuclei enhancing the precipitation of NbC [57]. Song et al. [85] took advantage of the dissolution and reprecipitation of SiC micron-sized particles in pure Fe during SLM to produce Fe/SiC bulk nanocomposites. Similarly, Lo et al. [35] favoured the complete dissolution of WC particles and the reprecipitation of finely dispersed mixed carbides by selecting deliberately small (~1 μm) starting WC particles, with the aim of improving the cavitation erosion resistance of AISI316/WC composites.

As alternative to Fe/carbides composites, laser-cladded ferrous matrix composites reinforced with titanium diboride (TiB2), exhibiting excellent hardness and wear resistance, were successfully synthesised from Fe–B and Fe–Ti alloyed powders [92]. In a study by Tan et al. [38], FeO2 powder was added in a blend of pure elemental Fe, Ni, Cr, Al and graphite powders, with the aim of promoting the formation of finely distributed Al2O3 and various mixed carbides. A slight grain coarsening was observed in some specimens and interpreted as a consequence of the latent heat of reaction released by the exothermic formation of Al2O3.

## **4.3. In situ synthesis of MMCs in Ni- or Ti-based alloys**

Ni-based composites reinforced with Cr3C2 [93], TiC and WC [86] carbides have been synthes‐ ised in situ from elemental powders. Lou et al. [93] investigated more particularly the influence of the Cr/C ratio on the resulting microstructure of NiCr/chromium carbides composites, with the aim of favouring the formation of Cr3C2 over mixed M7C3 and M23C6 carbides that are characterised by lower hardness and melting point in comparison with Cr3C2. Man et al. [86] reported on the successful laser-induced self-propagating high-temperature synthesis of TiC and WC in a NiAl intermetallic alloy. The latent heat of reaction released by the strongly exothermic formation of TiC was found to play an important role in promoting the formation of WC and of finely dispersed Al3Ni2 and Al3Ni intermetallics, with a positive effect on the hardness of the composite clad layers.

Comparatively, the in situ synthesis of Ti-matrix composites reinforced with carbides has attracted less attention. Mixed (TiB+TiC)/Ti composites were synthesised in situ during the LC of blended boron carbide (B4C) with pure elemental Ti and Al powder, resulting in excellent wear resistance [94]. Mixed (TiN+TiB)/Ti composites were synthesised during the LC of hexagonal boron nitride (h-BN) with Ti powders on a Ti-3Al-2V substrate, with the aim of combining the high hardness and Young's modulus of TiB with the enhanced high-tempera‐ ture plastic behaviour of TiN [95]. High laser power should be preferred in order to ensure the complete dissociation of h-BN and an optimised formation of TiN and TiB, resulting in marked improvement of the wear resistance of the composite coating. In situ synthesised TiB/Ti composites have been the most popular so far among in situ Ti-based composites [96–99]. Earlier works [96, 97] focused on the obtention of a fine and uniform distribution of TiB precipitates in Ti-6Al-4V matrix composites. Recently, TiB particles have attracted a renewed interest in order to improve the wear resistance of Ti–Nb–Zr–Ta matrix composites for use in orthopaedic femoral implants, with the added advantage that additive manufacturing allows for the custom-designed fabrication of the implants [98, 99].

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 201
