14 Industrial Laser Solutions SEPTEMBER/OCTOBER 2014 www.industrial-lasers.com
SENSING AND DATA ANALYSIS
APPROACHES WORK TO MEET DEMAND
Within the past 18 months, additive manufactur- ing (AM) of metal parts has drawn an enor- mous surge of indus- trial interest. According to industry expert Terry
Wohlers (Wohlers Associates, Inc.; Ft. Collins, CO), while the
AM industry as a whole grew by 34. 9 percent in 2013, the metal
AM segment experienced growth of over 75 percent. Indeed,
Wohlers commented recently that metal AM “has come farther in 10 years than plastic [AM] did in 25 years” [ 1]. Industries
driving metal AM processes forward include the automotive
sector, medical technology, and particularly aerospace. GE
Aviation’s planned AM production of their LEAP engine’s fuel
nozzles (see “Additive manufacturing at GE Aviation” in the
November/December 2013 issue of ILS) as well as EADS’s
evaluation of AM structural components for Airbus aircraft
indicate that powder-bed metal AM is a technology on the
threshold of industrial acceptance.
Despite this, questions remain about process reliability
and the repeatability of finished parts’ material properties. As
noted in a recent interview with Dr. Florian Bechmann, head of
development at OEM equipment maker Concept Laser GmbH
(Lichtenfels, Germany), increasingly in metal AM machines,
“customers expect active process monitoring and series pro-
duction capability, i.e., reproducibility at an industrial level”
[ 2]. In situ, real-time monitoring of the selective laser melt-
ing (SLM) AM process promises to address these concerns,
but the monitoring technology is still in its early days. Here,
we review the state of the art of this highly active area of AM
research and equipment development.
Laser additive manufacturing (LAM) machines are of two types:
powder bed and powder-fed. The intense recent interest has
focused on the latter, and this discussion is confined to it.
FIGURE 1 shows a schematic of a generic powder bed system
where the bed is created by raking powder across the work
area, which rests on the build platform in an environmentally
controlled chamber. Laser energy is delivered to the surface of
the bed, locally melting and fusing the powder in areas where
solid metal is desired.
Typically, each pass of the laser melts down through and
re-solidifies several layers, which are commonly 20–150µm
thick. After each laser exposure, additional powder is raked
across the work area and the process is repeated to create a
solid, three-dimensional (3D) component. A single “build” can
contain thousands of layers, so each run can take tens to hundreds of hours. Dozens of identical or different parts may be
created in each build.
Due to the repeated layer-upon-layer melting and rapid
solidification of the metal AM process, parts experience a
complex thermal history involving directional heat transfer.
Some of the major alloys for aerospace and medical/dental
applications may also experience repeated solid-state phase
transformations. These factors complicate the analysis of the
finished part’s microstructural properties, relative to those
of parts made by conventional means. The directional heat
extraction frequently results in grain structures that are columnar in the z-direction (perpendicular to the build plane) and,
as a recent review of metal AM noted, “Microstructural and
mechanical property anisotropy is ubiquitous in AM with the
z-direction generally being the weakest.” [ 3] Typical defects
in the SLM process include micro-porosity and lack of fusion
between neighboring layers. Of particular concern for aerospace applications are fatigue cracks initiating at pores close
to the part surface, while surface roughness has also been
shown to affect fatigue life [ 4].
Taken together, particularly for structurally critical components, these issues mean that qualification and certifi-
cation are significant challenges to widespread adoption of
AM. Recently, AM technology reviews have repeatedly called
for real-time, closed-loop process controls and sensors to
ensure quality, consistency, and reproducibility across AM
machines [ 5]. The overall goal is robust layer-by-layer quality assessment with spatial resolution below 1 mm2 that will
eliminate today’s post-build inspection or destructive testing.
Leading aerospace manufacturers are enthusiastically supportive: Greg Morris, GE Aviation’s business development
leader for AM, says, “Today, post-build inspection proce-
dures account for as much as 25 percent of the time required
to produce an additively manufactured engine component.
By conducting those inspection procedures while the component is being built, [we] will expedite production rates for
GE’s additive-manufactured engine components like the
LEAP fuel nozzle” [ 6].