Grain refinement of Fe--Ti alloys fabricated by laser powder bed fusion

Grain structure control is essential in metal additive manufacturing. It is used to avoid anisotropic mechanical properties, hot cracking and to increase the strength via the Hall-Petch effect. Here we demonstrate the use of grain refining particles for grain control in ferritic alloys, specifically Fe-Ti model alloys. We introduce the particles in three different ways, namely (i) by using oxygen-rich raw powders, (ii) by reaction of the molten material with the process gas atmosphere and (iii) by direct addition of the refining ceramic phase in powder form. Samples are produced by Laser Powder Bed Fusion with various concentrations of Ti and under Ar and N2 atmosphere. The resulting microstructures are analyzed using optical and electron microscopy. We demonstrate a transition from a microstructure containing columnar grains >100 μm in length to a strongly grain refined microstructure with equiaxed grains of approx. 1 μm in size. The refining sub-micron-sized particles in all cases are cubic Ti(O,N). We discuss the findings in the light of thermodynamic calculations as well as established grain refinement models.