**2.1 Powder-based additive manufacturing technology**

SLS/SLM technology is a solid powder-based AM technology that uses a laser to selectively scan the powder bed and utilizes the interaction of the laser with the solid powder to sinter/melt loose powders together as shown in **Figure 4a** [21]. The basic bonding mechanisms of SLS/SLM include solid-state sintering, chemically induced bonding, liquid-phase sintering-partial melting, and full melting [22]. The essence of solid-state sintering is the thermal diffusion of the melting temperature of the material between the particles through heat transfer, which can realize the processing and combination of a variety of materials. When the laser-powder interaction time is short and no binder is present, a chemically induced bonding mechanism occurs, forming a new binder phase that helps facilitate the post-curing process. Liquid-phase sintering-partial melting occurs when insufficient laser heat is provided and only partial melting is achieved to obtain the bonding of structural particles. Full melting is mainly used in metal materials, which can directly produce nearly full dense materials without post-processing.

Compared with other AM technologies, the main advantage of SLS/SLM is the flexibility of material selection. Single-component powder particles, composite powder particles, mixtures of different powder particles, and different binder materials are all suitable for SLS/SLM. The size and shape of the particles directly determine the shrinkage, precision, and density of the constructed object [23, 24]. There are van der Waals forces between particles of smaller size, and it is easy to form agglomerates, which may directly lead to uneven dispersion of powder particles; while larger-sized particles directly affect the porosity, showing poor surface roughness, even significant cracking or delamination effects may form [25]. The quality of the constructed object is highly dependent on the correct choice of the processing parameter setting, such as laser power, layer thickness, scan speed, as well as hatch spacing [26]. Uniform particle size distribution and optimized processing parameters can effectively reduce the occurrence of "step effect" and shrinkage deformation and finally obtain a satisfactory SLS/SLM construction object [27].
