Laser Beam Melting (LBM), also known as Selective Laser Sintering / Melting (SLS / SLM) is an Additive Manufacturing (AM) process using a layer by layer fabrication procedure in which the laser beam energy is used to build up a 3D part from a powder bed. The basic principle of LBM comprises five steps (see Figure 1):
1. A 3D-CAD (Computer Aided Design) file is sliced in two dimensional elements corresponding to laser scan vector data for each layer.
2. A layer of powder (with a controlled thickness) is spread over the building platform.
3. The powder bed is sintered or molten selectively by the laser beam deflected by a galvanometer scanner.
4. The building platform is lowered by a height corresponding to one layer thickness defined at the step (1).
5. Steps (2) to (4) are repeated until completion of the object building up.
The powder not irradiated by laser is brushed away from the as result 3D part and can be re-used later to build up another part.
Nowadays, SLM is considered as a mature technique to process metals and polymers . On the contrary, manufacturing of ceramics through SLM is not usual. There are specific issues with ceramics which are still investigated in the laboratories (cracking, poor cohesion of the manufactured parts, phase changes during the laser treatment…). Nevertheless, recent developments show some progresses.
As reported in literature, ceramics oxides exhibit a weak optical coupling with light at near infrared wavelengths. Alumina absorbs less than 10% of laser energy for wavelength around 1000 nm [2, 3]. Most of the LBM industrial machines are equipped with Nd:YAG like lasers operating at 1.06 µm. As a consequence, oxides can hardly be processed using commercial machines. To overcome this issue, two approaches have been studied in literature:
· The first one is called “indirect”. In this approach, ceramic grains are mixed with a polymeric binder which is molten during the AM step to ensure cohesion of the part. The ceramic itself remains unaffected by the laser treatment. After building-up, the part is debinded and sintered in a classical manner. The main drawback of this procedure is the presence of inter-agglomerate porosities in the final parts. The indirect process is also time consuming: a complete cycle of debinding, sintering, post-infiltration and /or post-isostatic pressing (to improve the relative density of 3D parts) may require several days. Nevertheless, it is a functional approach and for instance, J. Deckers has achieved alumina items with relative density up to 87.6% . This approach is also compatible with different ceramics independently of their absorptivity at the laser wavelength.
· The other approach is still a direct one (i.e the ceramic is sintered or molten during the process ) but an additive is added to the powder to enhance the optical coupling with the laser. This approach is clearly the less advanced so far because of specific issues but it remains promising for obvious reasons (reduction of the fabrication time, no post-treatment required, …)
Concerning this second approach, several different materials have been tested during the last years. Bertrand et al. used a fibre laser to manufacture zirconia 3D parts. Complex shapes with a relative density of 56% were achieved . Hagedorn et al, have studied the eutectic ratio of alumina and zirconia from dense particles of both oxides. An experimental setup of the LBM process equiped with a CO2 and a Nd:YAG lasers was used. The CO2 laser is used to preheat the powder up to 1600°C then the Nd:YAG laser brings selectively a limited amount of energy to melt the ceramics. Parts with a relative density near to 100% without cracks were achieved. Nevertheless, the obtained surface quality exhibit a high roughness .
On the contrary, Juste et al , as well as Ferrage and al.  have used a commercial LBM equipment (dedicated to metals) to process ceramics. The results show that in order to process ceramics, the powder need to have some very stringent features: suitable particle size distribution, morphology and flowability. It also has to comprise an additive to enhance the optical coupling of the ceramics with the laser wavelength (see Figure 2). Mixing alumina with a limited amount of colloidal carbon proved very efficient to improve the apparent absorptivity of the powder bed. With such an approach, alumina parts were built up with final relative densities up to 97.5% and 96% with yttria-stabilized zirconia [7, 8].
Despite those results, the microstructures of the manufactured parts still exhibit cracks and pores (Figure 3). If structural applications are prohibited for such parts, the intrinsic damages can be turned as an advantage to create molds for high temperature investment casting. It has been demonstrated that such alumina molds processed in LBM exhibit a thermal shock resistance up to 600°C .
Example of printing parts
Made by: BCRC
Material : Alumina
Made by : BCRC
Material : Alumina
Description : (a) Mold for investment casting, (b) Radiography X exhibiting hollow part of mold
 L. Pawlowski, Thick laser coatings: A review, J. Therm. Spray Technol. 8(2), 279 (1999).
 D. Hellrung, et al., High-accuracy micromachining of ceramics by frequency-tripled Nd:YAG-lasers, Proc. of the SPIE Conference on Laser Applications in Microelectronic and Optoelectronic Manufacturing IV, Vol. 3618, 348 (1999).
 J. Deckers, Selective laser sintering and melting as additive manufacturing methods to produce alumina parts, 2013
 E. Juste et al., Shaping of ceramic parts by selective laser melting of powder bed, J. Mater. Res., 2014, vol. 9-(17), 2086-2094
 L. Ferrage, Dense yttria-stabilized zirconia obtained by direct selective laser sintering, Additive Manufacturing 21 (2018) 472–478
 G. Bister et al., Functional refractory molds for metal casting built by additive manufacturing, ECerS meeting (T01/105) Budapest, July 10, 2017