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

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

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 metalceramic composites and the size of the ceramic particles have been deliberately adjusted in such a way as to favour the partial dissolution of the ceramic and obtain reaction products with a well-tailored, fine and uniform distribution, thus taking advantage of the combined strengthening effect of the initial ceramic particles and of the secondary precipitates [2]. As a line of demarcation, the present section focuses on reviewing systematically works that involve a *conscious and deliberate* usage of chemical reactions in view of tailoring the microstructure and properties of MMCs.
