Materials aspects encountered with the selective laser melting of metallic powders.

Selective laser melting (SLM) is an additive manufacturing technique by which structural geometrical complex parts  an be created directly by selectively melting consecutive layers of powder. The high energy density applied by a laser beam results in high and directional thermal gradients. In combination with the additive character of the process, this results in the formation of a unique microstructure in SLM parts which can be altered by varying the process parameters or the constitution of the alloy. 

In this work, the microstructure and texture of pure Ta, Ti-6Al-4V, AlSi10Mg and maraging steel 18Ni(300) SLM parts are characterised by microstructural analysis (LOM, SEM and EBSD) and X-ray diffraction. The influence of the unique microstructure on the mechanical properties are analysed as well. The experimental work is supported by using a pragmatic model for SLM to calculate the temperature distribution and by using a model to estimate the plastic deformation behaviour based on the measured texture.

For most alloys, the high thermal gradients result in very fine submicron-sized cellular-dendrites growing toward the centre of the melt pool along the easygrowth direction (AlSi10Mg and maraging steel 18Ni(300)). Due to the partial remelting or previously consolidated layers, most grains solidify epitaxially and grow across the layers. For pure metals (Ta) and Ti-6Al-4V, the solidification front remains stable, resulting in large elongated grains more or less along the building direction.

The competition between the epitaxial solidification and the orientation of the easy-growth of the parent grain with respect to the local heat flow direction is found to be the main determining factor to determine the morphological as well as crystallographic texture during SLM.