Effects of recycling on defects and microstructure in Ti-6Al-4V powder particles and samples fabricated by Electron Beam Melting process

Electron Beam Melting (EBM) is an additive manufacturing process that forms part of the Powder Bed Fusion (PBF) category. All the PBF processes share the same basic printing steps and use as material feedstock the metal powder which is previously atomized according to several existing methods. After the printing cycle, the powder in excess may be reused following a suitable sieving procedure to lower the production costs, otherwise, a large percentage of unused powder would be lost. The waste powder may differ in several respects compared to the starting virgin ones, depending on how many times it has been reused. Moreover, degradations in powder properties and its chemical composition can lower the quality and the mechanical performances of the components fabricated by Electron Beam Melting. The aim of this work is to investigate the microstructure variations in two batches of Ti-6Al-4V powder: virgin powders produced using a plasma atomization process and powders recycled more than one hundred times. Printed with the two batches of powder feedstock, 6 EBMed cylindrical bars were analyzed both from the point of view of microstructure and internal defects using an Optical Microscope. The bars were firstly cut and then embedded in resin, polished, and etched with a Hydrofluoric acid (HF) solution. The experimental results show several types of internal imperfections including macropores, micro voids, Lack of Fusion (LOF) defects, which are directly dependent on process parameters and quality of the material feedstock. While for the microstructure few changes were found both in powder particles and in their printed bars.