dc.description.abstract |
Additive manufacturing (AM) is a unique manufacturing technology that aims at
fabricating parts that were previously regarded as impossible by other manufacturing
techniques. One of the AM techniques that has established itself in the aerospace
industry is the laser powder bed fusion technique. However, poor surface roughness
and porosity of the as-built components are the limiting factor for this technique. This
study carried out an experiment to address these limitations by fabricating
overhangs, i.e. parts without any support structures.
Parallelepipeds samples were manufactured with inclination angles from 25° to 90°
with 5° increment between angles. Samples were built with optimal processparameters
developed by the unique Aeroswift high-speed laser powder bed fusion
system. Two directions of scanning were used: parallel-to-powder deposition
direction and perpendicularly.
Subsequent to the building process, samples were cut from the base plate and
prepared for characterisation. Two surfaces were examined, i.e. downward-facing
surface (downskin) and the upward-facing surface (upskin). The surface morphology
for the as-built samples were analysed using contact-type surface roughness meter
Mitutoyo Surftest SJ-210a, image processing by digital microscope Zeiss
Smartzoom 5 and microCT scans by a General Electric VTomeX L240 system.
Surface morphology was validated by gathering images and 2D profiles. Statistical
analysis includes calculation of average values, standard deviations and Student’s ttest
for corresponding groups. MicroCT data was visualised and analysed in Volume
Graphics VGStudioMax 3.0 to characterise the 3D aerial surface roughness
parameters, such as: Sa, Sz, Sq, Ssk, etc. MicroCT scans and optical microscope
images of polished cross-sections were used to quantify porosity for different
inclination angles.
From the measured surface roughness results, it was determined that roughness is
affected by the inclination angle. The downward-facing surface was identified as
having the worst surface roughness at low inclination angles (25°–45°). However,
surface roughness improved as the inclination angle increased. From the findings it
was determined that this was attributed to powder sticking to the surface and stair-stepping effect. Powder sticking to the surface occurred as a result of the melt pool
solidifying on top of loose powder during processing of overhangs. The particles on
the surface were clearly visible on the 3D images and SEM images. The level of
peaks and valleys was also improved as the angle increased which agreed to the
measured results. The cross-sectioning obtained using an optical microscope
showed a more rugged profile on downskin surface at low inclination angles.
The reverse occurred on the upskin surfaces: an increased in inclination angle did
not yield any improvement in the level of surface roughness. Surface roughness for
the upskin surfaces was found to be influenced mostly by stair-stepping effect at low
angles and by partially sintered powder particles sticking to the surface at higher
inclination angles. This phenomenon resulted in a zigzag curve (roughness data
versus inclination angle from 25° to 90°). The qualitative images validated this
observation as there was no improvement in the amount of roughness on the upskin
surface of the samples.
The microCT results and cross-sectional analysis showed that there was no
significance difference in the level of near-surface area porosity in the comparison
between the inclination angle for the upskin and the downskin surfaces. All the
manufactured samples for different inclinations and orientations exhibited porosity
levels less than 0.1%.
The scanning direction was found to have an influence on the level of surface
roughness. The results showed that surfaces that were created by start-end parts of
the tracks with contouring (XZ-orientated samples) achieved better quality than the
ones that were created by lateral sides of the tracks with contouring (YZ-orientation).
Furthermore, the scanning direction was also observed to affect the level of
deformation to the samples. The samples that were scanned along the long side, i.e.
YZ-orientated samples, experienced more deformation compared to the ones
scanned in the perpendicular direction.
This study concluded that parts can be successfully produced using higher
processing parameters. However, lower inclination angles pose a challenge with
regard to surface quality of samples. To achieve a better surface quality during
production the parts need to be XZ-orientated as described above and inclination angles less than 45 should be avoided where possible. In addition, it was found that
neither the orientation of samples or inclination angle has an effect on the level of
porosity when process parameters are properly optimised.
Some promising directions for further investigation of surface characterisation in
high-speed LPBF parts based on the study’s findings are discussed. |
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