An Overview of Direct Laser
Deposition for AM: Part 1
Abstract
Laser-based AM (LBAM) processes
can be used to produce functional parts
from the ground-up via layer-wise cladding
Providing an opportunity to generate
complex-shaped, FGM or custom-tailored parts
that can be used in a variety of eng. applications
Direct Energy Deposition(DED), utilizes a concentrated heat source, which may be a laser or electron beam, with in situ delivery of powder or wire-shaped material, for subsequent melting to accomplish
layer-by-layer part fabrication
or single-to-multi layer cladding/repair
Direct laser Deposition (DLD), a form
of DED, provides the potential to:
- rapidly prototype metallic parts
- produce complex and customized parts
- clad/repair precious metallic components
- manufacture/repair in remote
or logistically weak locations
DLD and Powder Bed Fusion-Laser (PBF-L)
are 2 common LBAM processes for
AM metal fabrication and are currently:
demonstrating their ability to revolutionize
the manufacturing industry
breaking barriers imposed via traditional,
subtracting metalworking processes
The thermal/fluid phenomena inherent to DLD directly influence the solidification heat transfer
which thus impacts the part's microstructure
and associated thermo-mechanical props.
A thorough understanding of these phenomena
is vital for optimizing the DLD process
and unsure consistent, high-quality parts
1.1. Preface
The mechanical behaviour, and thus the trustworthiness/durability of materials fabricated via LBAM is currently not well understood.
Since the ultimate mechanical behaviour of metallic parts is realted to its thermal-history-dependent microstructure, AAM parts will have different, more anisotropic props. than their wrought (forjadas) counterparts
In order to control their microstructure and resultant mechanical props., it is vital to understand and predict during the manufacturing process:
Thermal gradients
localized solidification phenomena
residual stresses
If these events can be effectively modelled as a function of relevant process parameters (e.g., laser power, scan rate, etc.), LBAM can ultimately become a "hands-off" operation
governed by insitu diagnostics and feedback control
providing for application-optimized parts with tailored mechanical props.
The success of DLD lies within effective
layer-wise bonding of material which is
accomplished by thermal energy transfer
From "deposition-to-part", there are many impactful
thermal and fluidic phenomena at work, including:
melt port initiation
(powder melting)
melt pool superheating
& solidification
melt pool fluid mechanics
& wetting behaviour
boundary heat-transfer
(part-to-environment & vice-versa)
convection
thermal radiation
internal heat
transfer
conduction
(melt pool -> substrate)
heat generation/destruction
(related to solid state transformations)
Based on the importance of heat transfer during DLD, real-time non-destructive evaluation (NDE) via thermal diagnostics/monitoring continues to aid in predicting post-fabrication results
Control of thermal behaviour thus provides a means to ensure product reproducibility
The measurable thermal signature at the melt pool, as well as temperature distribution along the part, can be correlated w/ final part props. so that closed-loops control algorithms can tailor a part for optimal functionality
Residual stresses within the part can also be controlled by monitoring and controlling the inherent DLD temperatures
This article discusses:
- The laser and powder delivery of DLD
- melt pool thermal/fluidic behaviour
- solification heat transfer
- sensible heat transfer to/from part
- thermal monitoring
- ongoing challenges and future outlook of DLD
Detailed topics:
laser/powder/deposition
heat transfer to/from build surface
melt pool formation and dynamics
solidification
cooling rates
thermal cycling
heat affected zone (HAZ)
effects of bottom substrate
pyrometry/thermography, etc.
Methods discussed
Analytical
Numerical
Experimental
Monitoring
Controlling
1.2. Background
Manufacturing methods
Traditional
(subtractive)
Removes material from
an initial (larger) volume
machining
Relies on
pre-fabricated dies
forging
stamping
casting
AM
Allows for the generation of a part from
the ground-up (contrarily to traditional)
AM applications
Unmanned Aerial Vehicles (UAVs) [4]
Fuel nozzles [5]
Houses [6]
tooling [7,8]
biomedical
implants [9]
AM premise
Means of creating a par by effectively joining materials
(either like or dissimilar), typically in a layer-by-layer
fashion via CNC (Computer Numeric Control)
displacement, from imported 3D model data
Through AM, a machine can "assemble" a 3D part by bonding materials, with each new layer of material being a manifestation of 3D model cross-sectional data
These models are typically in the CAD form in STL file format and are numerically sliced into many 'fictitious' layers/cross-sectional areas which dictate the CNC displacement
Typical ASTM-recognized
AM methods
material extrusion
material jetting
sheet lamination
vat polymerization
binder jetting
Direct energy deposition (DED)
Powder bed fusion (PBF)
The AM process is
traditionally open-loop
However, real-time feedback control can be integrated into the AM machine for ensuring better part quality (see Part II)
Feedback control is accomplished by having a NDE
system installed that indirectly measures part quality
via
infrared thermal imaging
pyrometers
determining
localized hot regions that manifest
residual stress
part morphing
The data obtained by monitoring can be used to adjust the AM process parameters in real-time as to rectify the manufacturing process automatically - hands-off operation
To ensure correct part dimensions/shape and integrity,
post-AM procedures are typically performed
excess material removal
In PBF; a "de-powdering" is required to remove excess
metallic powders adjoined to the fabricated part
heat treatment
vat photopolymerization parts
may require excess curing time
For a target volume,
a part built via AM
is at risk of being
over-dense
under-dense
due to the
presence of
contaminants
voids
cavities
1.3. AM of Metals
Most feasible processes:
DED and PBF
Both processes involve the:
deposition of
of powder metal
(or less common preforms such as wire)
and their simultaneous or subsequent melting
via a focused thermal energy source
Require an electrom beam or laser beam to
accomplish layer-to-layer metallurgical bonding
to overcome the relatively high enthalpy of
fusion and melting temperature of metals
When a laser is used
DED becomes Direct Laser Deposition (DLD)
PBF is refered as a form of LBAM
Powder Bed Fusion-Laser (PBF-L)
(aka Selective Laser Melting (SLM)
Used to generate metallic parts via the incremental
height-wise movement of a table consisting of a compact, uniformely distributed layer of metallic powder that is selectively melted by a focused laser beam.
The melting pattern, or laser
scanning pattern, can be
continuous lines
near-random pulses
Upon the completion of a singfle layer, the powder bed is lowered by the heght of the deposited layer, a new bed of powder is deposited with a roller, and the process is repeated
This repetituous process results
in excess metal powder which
can help in supporting the
part during the build
Supported by the unmelted
powder bed
reduces
residual stress formation
potential of part collapse
during the build
can also result in powder remaining in the part
if it consists of passages/channels in the design
In MY case the powder
has to be removed
Typically occurs in an enclosed,
inert-gas atmosphere
to reduce oxidation rate of
the part during the build
The part is built upon a base plate
(i.e. build plate, substrate, platen)
the finished part must be sheared
off from the plate after AM process
typically accomplished by Electrical
Discharge Machining (EDM)
Direct Laser
Deposition (DLD)
Instead of a separate material and
selective energy delivery process
It combines the material/energy delivery for
simultaneous deposition and part forming
within a similar region (fig. 2)
The metal preform can be:
wire
provides for better control
on the deposition efficiency
powder
(more abbundant)
typically blown through nozzles
can result in non-used powder
accumulation in the DLD machine
current machines
may have up to 4
(or more) nozzles
use inert gas as to minimize high oxidation rates
inherent for elevated temperature metal processing [13]
PBF-L vs DLD
PBF
Can provide for finished
parts with finer quality
but a depowdering
procedure is required
The lasers typically have lower power
(dur to finer powder size)
DLD
may require a post-AM
machining procedure
but the surface quality may not
be as good as a PBF-L part
Since it does not rely on a pre-deposited
layer of metallic powder
it may be used as means to repair
or coat parts via cladding [14-16]
Due to the combined material/energy
delivery method
can be readily used for creating FGMs parts
with varying material/alloys concentrations
Preform mixing can
be accomplished [17-18]
Coaxial powder delivery
Lateral wire feeding
1.4. Direct Laser
Deposition (DLD)
Coupled with a moving substrate, DLD provides for a pool of molten metal that travels in space/time
effectively creating a 3D
part from zero medium
It contains a deposition head, consisting of
either a single or multiple powder nozzles,
in line with a focused beam (fig. 2)
Thermal monitoring
may be implemented
using [13, 31-38]
infrared cameras
pyrometers
can be used for
DLD feedback
data collection
Can be used for AM of a variety
of metals and ceramics
Prominent controllable
operating parameters
laser/substrate relative velocity
(traverse speed)
laser scanning
pattern
laser power
laser beam diameter
hatch spacing
particle/powder
feed rate
interlayer idle time
Particles with spherical shape can reduce any entrapment of inert gas within the melt-pool
thus leading to a final part
with less porosity
In the mid-to-late 1990s, researchers at Sandia
National Laboratories innovated a process:
Laser Engineered Net Shaping (LENS)
which was another form
of powder-based DLD
and included multiple nozzles for
more effective powder delivery
The LENS process is now the most common means to accomplish powder-based DLD within research & industry
2. Laser &
powder delivery
6. Conclusions