**5. Synoptic table of laser additive manufacturing of MMCs and concluding remarks**

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

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.

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

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

the aim of improving the cavitation erosion resistance of AISI316/WC composites.

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

for the custom-designed fabrication of the implants [98, 99].

hardness of the composite clad layers.

200 New Trends in 3D Printing

The major characteristics of the different MMCs reviewed in Sections 3 and 4 are summarised in **Table 2**. In the following concluding remarks, special care is also taken to identify current trends and important fundamental issues in the laser additive manufacturing of MMCs.



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 203

**Composite Resulting microstructure Remarks** 

of SiC, formation of

Cr3C2, Cr7C3 [11, 51–53]

WC, precipitation of

of TiC [51, 55]

M23C6 [3, 51]

[3, 34]

CrB, Cr2B and Fe2B [51]

FeCrBSi + NbC and nano-CeO2 CeO2 favours a fine dispersion NbC \_

Fe-36Ni "invar" + TiC by DMLS Partial dissolution of TiC [56] Loss of "invar"

reprecipitation to form finely dispersed nano-SiC [85]

M6C, M23C6, M7C3 [3, 35, 51, 54]

Fe + TiC by in situ synthesis Fine dispersion of TiC [37, 91] Increase in hardness [37, 91]

Partial dissolution of Cr3C2 and precipitation of mixed M7C3 and

Al2O3 decomposes under laser irradiation

Finely dispersed Al2O3 and mixed carbides [92]

Partial dissolution of CrB2 and precipitation of finely dispersed

Fe2Si [10]

**Fe-matrix with carbides**

202 New Trends in 3D Printing

Stainless steel + Cr3C2 by laser cladding

**Fe-matrix with oxides** Stainless steel + Al2O3 by laser cladding

Stainless steel + Cr2O3 by laser cladding

Fe-Ni-Cr-Al + Al2O3 by in situ synthesis

Stainless steel + CrB2 by laser cladding

**Fe-matrix with other particles**

Mild steel + SiC by laser cladding Partial dissolution

Stainless steel + SiC by laser cladding Partial dissolution of SiC,

Stainless steel + WC by laser cladding Partial dissolution of

Stainless steel + TiC by laser cladding Very little dissolution

Fe + nano-SiC by DMLS Dissolution of micron-size SiC and

volume fraction of porosity [87]

precipitation of Fe2Si, Fe3Si, Fe3Si5,

Enhanced hardness, wear and corrosion resistance [10]

Enhanced hardness and wear resistance [11, 51–53]

Enhanced hardness and wear

resistance [3, 35, 51]

properties due to the enrichment of the FeNi matrix with Ti [56]

Corrosion resistance is less compromised than with WC [3,

Enhanced hardness and cavitation erosion resistance

\_

\_

51]

\_

\_

[51]

\_ Extensive cracking [51]



**Table 2.** Synoptic table of representative MMCs fabricated by laser additive manufacturing.

Data from **Table 2** highlight the importance of carefully selecting the reinforcing phase of the composites in view of not only the desired properties and application but also considering the compatibility of the reinforcement with the metallic matrix:


should be prepared by carefully mixing the nanoparticles with the metallic powder used for the matrix. Laser-induced in situ synthesis of MMCs is a very interesting alternative, allowing for a refined and uniform distribution of nanoparticles.
