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Process Control (5.2. Deposition Control (Many sensing systems have been…
Direct Laser Deposition (DLD) was born
as an open-loop control manufacturing
process, w/ an end-user providing,
and responding to, process params.
As mentioned, these
process params. are often
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Traditionally,
process params. (such as the traverse speed,
laser power, etc.) are held constant w/ time
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Feedback Control
as a result of such observations, an element of
DLD control has emerged over the years and has
been researched extensively (feedback control)
W/ feedback (or closed-loop) control, DLD machines are able to monitor and automatically respond to faulty or low quality tracks/builds, to achieve parts w/:
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Controlled process params. are selected based on their responsiveness to changes and primarily include
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process time intervals, etc.
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5.1. Melt Pool Control
Like many laser-based/welding
manufacturing processes[157], thermal
monitoring is employed (see part I)
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such thermal control is established by
means of using this thermal data as input
for an employed feedback control system
As shown in fig. 19, PID control (via LabVIEW)
on the DLD process can be accomplished
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Based on thermal feedback during the DLD process,
there are 2 means as to alter temperature behaviour
during the DLD process (and thus microstructural formation)
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:+1: Closed-loop control during DLD is of importance, because it can lead to hands-off, unsupervised manufacturing that produces high-quality, repeatable parts
:-1: However, the wide-scale application
of control is a challenge since
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and thermal control is limited by the current state-of-the-art
in thermography and diagnostics[157,159]
:star: The melt pool morphology, while in its
liquid phase, is paramount to the integrity
and shape of each solidified track/layer
As described in Part I, due to bulk
heating effects and other variables
the melt pool can elongate, shrink, splash and/or
become excessively superheated and unstable.
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Raghavan et al. [160]
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and the heat transfer and liquid metal flow was
modelled to calculate <..> during laser processing
of a Ti-6Al-4V alloy [160]
:warning: Results indicate that feedback control based solely on maintaining a target top surgace geometry can be limited
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Despite similar top surface contours, the overall
pool geometry can vary considerably (fig. 20)
Furthermore, since a clear correlation between the peak temperature and melt pool geometry is not readily apparent
it may be difficult to accurately implement process
control based solely on thermal imaging of the temperature profiles on the top surface of the melt pool [160].
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Tang and Landers [154] investigated closed-loop thermal con- trol for single-nozzle DLD of H13 tool steel.
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An empirical thermal controller was designed
based on a first-order melt pool temperature
transfer function with the form (eq. 3.)
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where the thermal time constant, Tau , was experimentally measured to be approximately 30 ms.
The parameters V, Q and M correspond to the traverse speed, laser power and powder feed rate, respectively.
Experimental data was fitted to determine the unknown parameters: Kt , alpha, and ß
Experiments demonstrated that the thermal control method could track time-varying and constant reference temperature for both constant and transient operating parameters.
Using the thermal control, bulk heating effects along the traverse and build-height direction were reduced.
:-1: However, when tested
for multi-track deposition,
it was
found that
a wavy, non-uniform
morphology was produced
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More robust control is required for multi-track
DLD, and higher-order modeling terms are
needed to describe more complex melt pool
morphologies with regard to net heat input.
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5.2. Deposition Control
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Many sensing systems have been developed to solve the powder flow rate measurement problem including:
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compact pressure
sensor [166], etc.
:-1: the compact pressure sensor relies on using a screw feed mechanism with compressed air as the carrier gas.
Closed loop optical feedback may have first been integrated into DLD by Mazumder et al. [66,77,167] using a ‘reflective topography’ technique.
The optical feedback assembly consisted of three evenly spaced photodetectors within the laser focal plane and aligned to observe the melt pool region.
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This height controller demonstrated the ability to sense
when the layer is building up higher in the overlapping region
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Currently, most DLD research employs
a constant powder flow rate
e.g. Xing et al. [172]
Used
infrared photodetector
while the powder feed rate was also monitored, and held constant, by employing a customized optoelectronic sensor.
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:+1: This approach to feedback control of DLD was shown to effectively improve clad tolerances and build features.
:-1: However, using the constant powder flow rate
may result in non-uniform layer thickness
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Tang et al. [166] developed a variable powder flow rate control to maintain uniform/consistent track morphology even as the DLD system decelerates and accelerates.
The authors demonstrated that by adjusting the powder flow at the corners and on the edges of the part, layer height variation can be reduced substantially compared to the parts fabricated with a constant powder flow rate.
