Solidification behavior during directed light fabrication Page: 3 of 9
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SOLIDIFICATION BEHAVIOR DURING
DIRECTED LIGHT FABRICATION
D.J. Thoma, G.K. Lewis, and R.B. Nemec
Los Alamos National Laboratory, Materials Science and Technology Division,
Mail Stop G770, Los Alamos, NM 87545 USAAbstract
Directed light fabrication (DLF) is a process that fuses gas
delivered metal powders within a focal zone of a laser beam
to produce fully dense, 3-dimensional metal components. A
variety of materials have been processed with DLF, ranging
from steels to tungsten, and including intermetallics such as
NiAl and MoSi2. To evaluate the processing parameters and
resulting microstructures, solidification studies have been
performed on defined alloy systems. For example,
solidification cooling rates have been determined based upon
secondary dendrite arm spacings in Fe-based alloys. In
addition, eutectic spacings have been used to define growth
velocities during solidification. Cooling rates vary from 10'-
105 K s' and growth rates vary between 1-50 mm s-. As a
result, process definition has been developed based upon the
microstructural development during solidification.
DIRECTED LIGHT FABRICATION (DLF) is a rapid
prototyping process that fuses gas delivered metal powders
within a focal zone of a laser beam to produce 3-dimensional
metal components [1]. The focal zone of the laser beam is
programmed to move along or across a part cross-section,
and coupled with a multi-axis sample stage, complex metal
geometries can be produced. The DLF technique offers
unique advantages over conventional thermomechanical
processes in that many labor and equipment intensive steps
can be avoided. For example, typical processing of metals
into desired shapes and assemblies involves casting and metal
forming (rolling, stamping, forging extrusion) followed by
maching and joining operations. The DLF process yields a
final geometry from a single piece of equipment and the
appropriate software control.
Development of the plastics rapid prototyping
processes since the early 1980's demonstrates the feasibility
of producing parts from three dimensional solid modeldesigns by a single process and a single piece of equipment
[2]. The processing of parts are made by additive deposition
of planar layers of plastic material until the complete part is
formed. However, this processing has been restricted
primarily to plastics and not successfully extended to produce
fully dense metal parts. Potential metal rapid prototyping
processes include liquid metal spraying, plasma spraying,
electron beam vapor deposition, and investment casting
processes. These metal processing techniques are non-
directional deposition processes that require mold patterns or
masks to gain the detail for complex parts and assemblies.
Since DLF processing offers unique capabilities and
advantages for rapid prototyping of complex metal
components, an examination of the microstructural
development is required to define and optimize the processed
materials. The intent of this study is to address the
solidification behavior during DLF processing of simple
geometries to characterize the technique.
Experimental Procedure
The DLF process consists of generating tool paths
from computer generated 3-dimensional solid models. The
tool paths continuously move the focal zone of the laser
systematically along areas of the part to fuse metal powder
particles that are gas delivered to the focal zone. A schematic
diagram of the process is shown in Figure 1. Three Nd-YAG
pulsed lasers (1 KW) that are connected in series to simulate
a continuous wave (CW) laser are delivered via fiber optics
to a sealed boom that holds the laser focusing head and is
attached to the "z" (vertical) axis. The focused laser beam
enters the chamber through a quartz window in a nozzle that
also delivers the metal powder to the focal zone. The entire
process takes place in an inert gas box connected to a dry
train that reduces the oxygen content to 5 ppm or less. In the
upper right of the schematic diagram is a chamber that can be
evacuated and back-filled with an inert gas that contains the
powder feeder. The powder feeder entrains the powder in an
Dan J. Thoma
Page 1
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Thoma, D.J.; Lewis, G.K. & Nemec, R.B. Solidification behavior during directed light fabrication, report, October 1, 1995; New Mexico. (https://digital.library.unt.edu/ark:/67531/metadc622872/m1/3/: accessed March 28, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.