Comparing the Use of In-Situ Alloying and Pre-Alloyed Powder for the Manufacture of Novel Multicomponent Alloys; Example of Mo5nb35ti30v30

Abstract

Refractory high entropy alloys (RHEAs) generally show superior strength at high temperatures and consequently, due to the high melting point elements concerned, are difficult to manufacture via conventional methods. Additive manufacturing (AM) of RHEAs is gaining popularity as a method by which to produce homogeneous alloys, especially via use of blended powders and in-situ alloying (ISA). In this work, ISA of the Mo5Nb35Ti30V30 RHEA by the blending of off-the-shelf elemental powders is compared to the manufacture of the same alloy using pre-alloyed (PA) powder, via laser powder bed fusion. The resulting contamination, defects, elemental distribution, microstructural texture and grain morphology are compared for both sample types. The ISA powder resulted in higher interstitial content and gas porosity as well as un-alloyed elements at low input energies and crack-like spreading defects. The PA samples also showed un-alloyed Nb and Mo. While keyholes were prevalent in PA samples, the ISA powder suppressed keyhole formation and showed an almost homogeneous microstructure. The texture in the ISA samples is largely equiaxed due to high melting point elements acting as grain nucleation sites and grain refiners, compared to the epitaxial columnar grain growth seen when using PA powder. Based on these findings, a workflow diagram is therefore proposed, outlining the considerations involved in assessing an elemental powder for ISA. Overall it is shown that generally representative AM microstructures can be obtained using low flowability ISA powders, but that the defects, contamination, texture and optimum processing parameters will differ from the PA equivalent.

Keywords: Additive manufacturing, laser powder bed fusion, Refractory High Entropy Alloys, In-situ alloying