Characterization of Ti-5553 parts printed by Selective Laser Melting (SLM)
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Date
2018-12-20
Authors
Bakhshivash, Soheil
Journal Title
Journal ISSN
Volume Title
Publisher
University of Waterloo
Abstract
Nowadays, titanium alloys are widely used in a variety of industries like automotive
and aerospace due to their high strength to weight ratio. Ti-5%Al-5%V-5%Mo-3%Cr is
a metastable near beta titanium alloy with excellent fatigue performance and corrosion
resistance. Hence, it is mainly used in the airframe structure and the landing gear components.
As the additive manufacturing (AM) industry grows everyday, so does the interest in
printing of strategic Titanium alloys. Selective Laser Melting (SLM) is a powder-bed fusion
process utilizing the laser power as the heat source which scans over a compact powder layer
along a pre-de fined path. In this study, the printability of Ti-5553 by pulsed-laser SLM is
investigated, thoroughly. To this end, a batch of powder was first characterized in terms of
size distribution, porosity level, chemical composition and microstructure. Afterwards, the
effect of Volumetric Energy Density (VED), sample geometry and scanning strategy was
evaluated on the microstructure and properties of the printed parts. Various tests such
as Archimedes method, Nano-CT scanning, surface roughness, Optical Microscopy (OM),
Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), X-ray
diffraction (XRD), Electron backscattered diffraction (EBSD) and hardness mapping were
applied. Results showed that the virgin plasma-atomized powder had a high quality with a
mean sphericity of 0.95 and porosity of less than 0.01%. In addition, it was observed that
there was an optimized VED range leading to a nearly fully dense part, i.e. 99.7%, with
a smooth surface. Low and High VEDs resulted in a lack of fusion and spattering, respectively,
both undermining the density of the printed parts. Moreover, high VEDs provided
the energy required for the beta to alpha transformation and caused in-situ precipitation
hardening. This in-situ heat treatment is not desired due to the lack of homogeneity. Furthermore,
the sample geometry, i.e. cubic or cylindrical, was determined to have negligible
effects on the achieved properties while the choice of scanning strategy directly affected
the texture and density of the printed parts. It was seen that the chessboard, stripes and
total ll strategies led to the optimized, medium and poor properties, respectively. Finally,
the best density of 99.94% was achieved by using the VED of 112 J/mm3 and the chessboard
scanning strategy. The sample showed a homogeneous hardness distribution with
an average of 295 ±10 HV. The TEM analysis determined that the sample was in the beta
to omega phase transformation stage. Also, it had a columnar grain structure elongated
parallel to the building direction indicating a highly crystallographically textured part.
The EBSD analysis supported these fi ndings by suggesting a preferred growth direction
with crystal texture seven times higher than the random intensity. The melt pool shape
of the optimized sample was seen to be goblet-like with a lower penetration compared to
the sample printed by the stripes strategy.
Description
Keywords
Metal additive manufacturing, Near-beta titanium alloys, Omega phase, Selective laser melting (SLM)