BACKGROUND
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 impacts associated with the metal powders that are used as input material. 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 on whether 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 are investigating a novel approach for establishing next generation forms of AM source materials. Specifically, we are examining engineered metal platelets produced by a vacuum roll coating process as an innovative source material for SLM-AM. High levels of customization make vacuum roll coating an attractive process. Some of the 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.
APPROACH
The overall objective of the proposed work is to investigate engineered platelets produced by vacuum roll coating as an innovative source material for SLM-AM process.
The specific aims are to 1) synthesize engineered platelets using a vacuum roll coating process (Ti-6Al-4V, 10 micron hexagonal shaped), 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.
We have established the facilities and expertise to prepare appreciable quantities of platelets with precisely controlled stoichiometry, morphology, and size through top-down physical vapor deposition of multilayer films on embossed substrates. This vacuum roll coating method of manufacture has advantages in controlling the morphology (size and shape) and preparing scalable quantities (g-kg amounts). Additionally, preparation on planar substrates is amenable to repetitive coatings and/or treatments in a layer-by-layer method allowing for manufacture of multilayer platelets and/or surface functionalized platelets that can either add functionality to the final AM part or assist with the laser melt process.
While the proposed study is intentionally focused on Ti-6Al-4V platelets as an exploratory feasibility study that can be compared with a spherical powder conventionally used for SLM-AM, the proposed process will allow for broader chemistries and customized layered platelets not otherwise accessible through current manufacturing processes of metal powders. For example, using this production process one could manufacture platelets that are sandwiched with thin layers of another material that can aid with laser melt reactions or form eutectics to strengthen interfacial bonding between layers.
ACCOMPLISHMENTS
We have just begun the third quarter of the project. We established the facilities for a vacuum roll coating method to prepare appreciable quantities of engineered platelets through top-down physical vapor deposition of multilayer films on embossed substrates. We developed processes for producing engineered titanium alloy platelets for SLM-AM that utilized a polyethylene terephthalate (PET) substrate material with an embossed 10-micron hexagonal pattern. To date, we conducted two vacuum roll coatings trials (TAV-1; TAV-2) on non-patterned (flat) PET and patterned PET, respectively. We characterized the structural and chemical properties of commercial Ti-6Al-4V AM powders along with the resultant prototype SwRI platelets using scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) spectroscopy, and laser light diffraction for particle size distribution. Commercial powders were predominately spherical with a mean size of 33.1 microns, and SwRI platelets were predominately hexagonal with a mean size of 10.9 microns. We conducted a control baseline SLM-AM build experiment to manufacture test coupons and tensile bars using an existing Renishaw AM250 SLM recommended process with commercial Ti-6Al-4V spherical powders. We characterized the structural properties of the SLM-AM parts made with commercial powders. Metallographic cross sections were prepared through selected as-built and hot isostatic pressed (HIP) samples to facilitate a comparative analysis of material structures using a combination of light and electron microscopy. The calculated porosity in microstructure of as-built commercial powder samples ranged from 0.43 percent to 4.78 percent in analyzed areas. The HIP/commercial powder sample did not exhibit significant porosity at the examined locations. The as-built sample microstructure consisted of acicular α phase which appears to be consistent with α-prime (martensite structure), whereas the HIP sample microstructure appeared to consist of α plates and intergranular/thin plates of β phase.