The quality and the properties of parts manufactured by the Selective Laser Melting (SLM) Additive Manufacturing (AM) process are strongly influenced by many factors. Key among these factors is the impact that composition and form of input metal powders have on the part build quality and strength. The importance of the metal powder characteristics (i.e., particle shape, size distribution, grain size, impurities, density, surface chemistry, and roughness) in SLM-AM processes has become increasingly recognized and identified as common themes for technical challenges. Beyond these current material-related technical issues, there are concerns if the existing materials sources are advanced enough to meet future needs. As users find new applications for metal components made via AM processes, the demand for customized forms of metal source materials increases.
In this project, we investigated a novel approach for establishing next generation forms of AM source materials. Specifically, we examined engineered metal platelets produced by a vacuum roll coating process as an innovative source material for SLM-AM. Platelets are small planar particles of uniform geometry (i.e., hexagons, diamonds) and high aspect ratios. High levels of customization make vacuum roll coating an attractive process. Some of advantages to vacuum roll coating engineered platelets include precisely controlled morphology (size and shape), increased packing density for the powder bed, ability to manufacture multilayer and/or coated particles, and tailorable material properties including surface textures and chemistries.
This study was intentionally focused on Ti-6Al-4V platelets as an exploratory feasibility study that could be compared with a spherical powder conventionally used for SLM-AM. The specific aims were to 1) synthesize engineered platelets using a vacuum roll coating process (Ti-6Al-4V, 10 micron hexagonal shaped based on existing patterned materials), 2) manufacture SLM-AM test parts using the engineered platelets and control SLM-AM test parts made with commercially available spherical powder (Ti-6Al-4V), and 3) examine and compare the structural, corrosion, and mechanical properties of SLM-AM parts made with engineered platelets with those made with traditional metal powders. While the proposed study was intentionally focused on Ti-6Al-4V platelets, the proposed process will allow for broader chemistries and customized layered platelets not otherwise accessible through current manufacturing processes of metal powders.
Altogether, the results of the project met the main objective to investigate a new powder feedstock technology capable of producing customized, engineered platelets for future AM applications. Engineered Ti-6Al-4V platelets were successfully prepared by a vacuum roll coating process. Microscopy and particle size analysis of the resulting platelets revealed control of the powder morphology (size and shape) where platelets were predominately hexagonal with mean particle size of 10.9 microns. SLM-AM line scan experiments and builds with the engineered Ti-6Al-4V platelets using manual powder distribution and standard Renishaw AM250 Ti-6Al-4V build parameters revealed successful platelet powder fusion. However, poor flowability, due to the wetness of produced platelets, inhibited manufacture SLM-AM test parts.
A control build using commercial Ti-6Al-4V powders was designed and executed to produce specimens from which to obtain “baseline” structure and properties. An auxiliary benefit to this project was the knowledge gained and the accompanying data set on the structural, mechanical, and corrosion properties of as-built versus hot isostatic pressed Ti-6Al-4V SLM-AM parts. The project took the concept (technology readiness level [TRL] 1) to demonstration in a laboratory environment (TRL 3). Further development work is required to increase the technology readiness level and identify specific applications. Lessons learned defined next development steps in 1) optimization of the platelet design for AM and 2) improvement of flowability through process modification from an aqueous to an organic release layer and/or development of improved post-processing steps for drying the powders.