2.2. Efects of Process
Design and Parameters
Localized heat transfer,
especially in the HAZ
can result in
layer re-melting
numerous reheating
cycles
both of which can change
the microstructure by
tempering
aging
There are many DLD process parameters that affect the thermal history (and thus microstructure,
residual stress, etc.) of the part, such as:
powder feed rate
laser power
laser substrate relative
traverse speed
laser scanning
strategy
As discussed in Part 1,
these parameters
affect
melt pool shape
incident energy
and consequently
affect
cooling rate
local thermal
gradients
2.2.1. Process
Parameters
The combination of
higher traverse velocity
lower laser power
results in
lower incident energy
at the top of the part
typically resulting in finer
microstructures due to
higher cooling
rates
In contrast,
results in
higher incident energy
at the top of the part
typically resulting in coarser
microstructures due to
lower cooling
rates
decreains traverse speed
increasing laser power
During the DLD process, the majority of
sensible incident energy is transferred via
conduction through the deposited structure [32]
Heat is quickly conducted away by the
substrate at the bottom of the sample
whereas convection and radiation become
more prevalent at the top [57]
Since relatively high cooling rates occur near the
substrate or through previously deposited layers
forming highly directional
columnar structures
Powder feed rate
Has an immediate impact on the distribution of powder density in the melt pool (deposited mass flow rate) [90] and thus layer height and microstructures during DLD.
For a fixed powder feed rate, the amount of powder that is injected into the melt pool varies for different laser scanning directions because of the distance between the powder stream and laser spot
Depending on the scanning direction, the powder
injection point may be (OR) the laser spot (fig 3)
ahead
behind
To maintain a constant
deposited mass flow rate
the powder feed rate and
laser scanning velocity
can be adjusted based on
scan direction and
the distance between laser beam and powder
nozzle (other methods described in section 4)
Adding to this complexity is the
dynamics of the DLD process
change in the relative positions of the
laser beam and powder injection point as
parts become taller
due to deposition
⭐ Therefore, any process optimization
and control scheme should be designed
in a time-varying fashion that accounts for
the height of the
deposited layer
Shielding gas
flow rate
also affects the amount of powder
injected into the melt pool
Experimental studies demonstrate that
layer height increases with gas flow rate
as more powder can be
carried into the melt pool
However, w/ higher gas flow rates, partciles become
more prone to reflecting off the melt pool surface
⭐ Hence, an optimum
gas flow rate may exist [6]
2.2.2. Deposition
Patterns
4 common
deposition patterns
raster
bidirectional
offset
fractal
Affect significantly the
depending on the starting
point of deposition
offset-in
offset-out
geometry
mechanical props.
Choosing proper scanning
patterns reduces [84]
production of
residual stresses
thermal distorion
most commonly used
mainly due to its ease
of implementation
independent of the shape
of the fabricated part
thus, can be implemented
to a variety of parts [92]
Additional research is needed
for offset or fractal patterns
This is challenging for a part w/ complex geometry
Nevertheless, the
patterns have advantages:
geometric accuracy
less energy consumption
Nickel et al. [93] found that fractal and offset-out
generate the smallest substrate deformations
Square corners can lead to these
patterns causing interior defects
⭐ Therefore, adopting these patterns w/
arc paths can produce parts w/ higher quality
2.2.3. Layer Slicing
Strategy
In conventional DLD processes, the part
CAD is sliced into parallel layers and
produced in a layer-wise manner.
Parts are built to completion layer by layer, from bottom to top
Due to the thickness of a deposition layer, surfaces, whose normal vectores do not make an angle of either 0º or 90º w/ the build direction can only be deposited approximately.
The designed part shape (e.g. curved surface) is typically approximated by a series of parallel layers (fig. 5)
Also, the material shrinkage
of a single layer during DLD
creates slanted walls
around the layer itself
while causing previously deposited
layers to shrink (via dragging)
These combined effect of geometric approximation
and layer dragging generates a surface w/
step-like features aka "Staircase effect" [94]
Depends on
layer thickness
the angle made by the surface
normal w/ the build direction
Inconvenients
Results in poor surface quality
Requires post-manufacturing
process to form the desired shape
e.g. machining
increases overall
process time
Research on
slicing procedure
however, these
approaches
are degined for fixed direction deposition processes
do not completely eliminate the stair-case effect
such as
controlling layer
thickness [95]
controlling vol. difference
between layers [96]
Multi-axis processing
Is another alternative to
mitigate the staircase effect
Instead of adopting the traditional
parallel slicing approach
it rotates the sicing direction 90º when
an overhang structure occurs [97,98]
⚠ Collision may occur when
rotating the part orientation by 90º
For example, collision between the powder nozzle and the deposited part occurs at the deposition of part (4), when the deposited part is rotated 90º (fig. 5 (a) & (c))
To address the aforementioned challenges in the layer slicing strategy, Ruan et al. presented a method of non-parallel layer slicing for more precise deposition along the part geometry
In this method, the slicing direction
is rotated only as needed
instead of
turning the slicing
direction 90º
resulting in
layers w/ non-uniform thickness
for better fitting shapes
in other words
the thickness varies
at different locations
👍
reduces the staircase effect
decreases manufacturing times
👎
Imposes unprecedent
challengs on implementation
because process control params.
must be delicately controlled
so that the target layer
thickness can be achieved
No analytical models are available to characterize the relation and effect between process params. and layer thickness
Rely on empirical models to
predict process params. in advance
in order to achieve varying layer height
Linear empirical models
have been used to
correlate layer height &
laser scanning speed
while fixing the laser power
and the powder feed rate [99]
usually under specific
experimental conditions
e.g. single track deposition
near the substrate
the resulting prediction of layer height may not be
accurate when the experimental conditions vary