Short laser/powder interaction times characterise selective laser melting on powder bed. These localised and short-lived high temperature variations significantly affect the microstructure at the local (melt pool) scale. In turn, the additive character of the process and unique solidification conditions generate distinctive morphological and crystallographic textures at the component scale.
This means the SLM products can have a high hardness even without the application of a precipitation hardening treatment (such as AlSi10Mg  or Ti64 ).
Generally, rapid solidification gives rise to the occurrence of segregation phenomena (enrichment of a materials constituent at a free surface or interface) and the presence of non-equilibrium phases . Segregation, in addition to heat build up generated in the component during scanning and partial remelting of the subsequent layers, leads to the precipitation of intermetallic phases. This is what makes the melt pool boundaries visible upon etching (in as-built condition). Melt pools’ cross-sections are typically half-cylindrical in shape: due to the moving heat source, the melt pool is not circular, but elongated. Their size is dependent of the laser processing parameters (power, scan speed, layer thickness, etc) . High-energy density of the laser gives rise to a directional heat transfer and as a result can also give rise to a directional solidification . As a result, the scanning strategy is expected to have an influence on directionality of the solidification.
The directional solidification in the melt pool causes not only a morphological texture but a crystallographic texture as well. It is seen that grains are growing perpendicular to the melt pool border towards the centre of the melt pool.
In Ti6Al4V  and AlSi10  for instance, the growth direction of the grains in the substrate plane is perpendicular to the isotherms of the melt pool. In other words, elongated grains are observed to be parallel and aligned with the local conductive heat transfer .
Since a relatively small melt pool is created on top of a large already consolidated block of material, the heat flows away radially and grains grow towards the centre of the melt pool. This creates a morphological (dendrites, cells, etc…) as well as crystallographic (grains) texture in the parts.
The macrostructure is determined by the way in which the different individual tracks are combined. In other words, it is defined by the scanning strategy across a certain layer of the product.
Across few layers, and due to partial remelting of the previous layers, elongated grains of several can grow hundred micrometers across successive layers , , . It is interesting to note that the extent of the morphological and crystal texture could be adjusted by varying the hatch spacing and/or the layer thickness since these parameters directly determine the amount of partial remelting of the neighbouring tracks.
To sum up...
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