Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter.
Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 15
Additive Manufacturing in Aerospace:
Examples and Research Outlook
Brett lyonS
The Boeing Company
This paper presents an industrial view on the use of additive manufacturing
for production of aircraft components and provides research examples that show
the direction of related development. The advantages of additive manufacturing
are becoming broadly recognized, and the stringent requirements found in the
aerospace industry provide the context required to develop these complex pro-
cesses to the level of robust performance established by traditional manufacturing
methods.
INTRODUCTION TO AEROSPACE REQUIREMENTS
FOR ADDITIVE MANUFACTURING
Additive manufacturing (AM) processes are unique in their ability to form
the final part desired without any intermediate tooling. Additive processes such as
selective laser sintering (SLS) begin with a computer-generated three-dimensional
design of a given part. The part is then digitally segmented into very thin layers,
which are selectively solidified in the machine, layer by layer. This ability to
“grow” parts allows for designs with such complexity that they cannot viably be
built with other processes. This approach to manufacturing removes the need,
cost, and delay associated with tooling. Even with increasing rates of aircraft
production (The Boeing Company, 2011), aerospace companies have numerous
parts that are produced in very low quantities, making these tool-less processes
attractive from an economic perspective (Ruffo et al., 2006). The use of AM
provides a host of benefits, many of which are being recognized even in general
media (The Economist, 2011).
From an aerospace and defense (A&D) design perspective, the weight of the
15
OCR for page 16
16 FRONTIERS OF ENGINEERING
part is often the deciding design factor for choice of material and manufacturing
process use. Also, the uncompromised need for safety in air travel adds a long list
of complex requirements, even for the simplest part. To consistently produce parts
with identical and understood properties, the material and the process used to form
it must be understood to a very high level. This complicated aerospace manufac -
turing context, which blends low-volume economics with acute weight sensitivity
and the need for highly controlled materials and manufacturing processes, has led
to the development of knowledge within The Boeing Company required to safely
transition AM from the laboratory and model shop onto the factory floor.
To begin to understand the foundation of requirements placed on a commer-
cial aircraft part, one can look to the U.S. Federal Aviation Regulations, which
must be met before a Type Certification can be issued for a given aircraft series,
required for entry into service with an airline (Federal Aviation Administration,
2011). While this set of regulations is very extensive and detailed, the single most
pertinent language within the context of an AM review can be found in Title 14,
Section 25, Subpart D, Subsection 25.605: “The suitability and durability of mate-
rials used for parts, the failure of which could adversely affect safety, must (a) Be
established on the basis of experience or tests; (b) Conform to approved specifica -
tions (such as industry or military specifications, or Technical Standard Orders)
that ensure their having the strength and other properties assumed in the design
data; and (c) Take into account the effects of environmental conditions, such as
temperature and humidity, expected in service.” This brief but clear requirement is
one of many that leads to the incredible safety record of commercial air transpor-
tation and also provides the impetus to rigorously study new fabrication methods
such as SLS. Each A&D manufacturer will have internal specifications or will
look to established standards organizations for data that allow accurate design of
components from a given material, based on minimum allowable performance.
Examples of material performance factors that are considered for even the sim -
plest of components include specific strengths, fatigue, creep, use temperature,
survival temperature, several tests of flammability, smoke release and toxicity,
electric conductivity, multiple chemical sensitivities, radiation sensitivity, appear-
ance, processing sustainability, and cost.
USE OF ADDITIVE MANUFACTURING IN AEROSPACE
Within Boeing, both military (Hauge and Wooten, 2006) and commercial
(Lyons et al., 2009) programs use SLS to produce lightweight, highly integrated
systems and payload components, as seen in Figure 1, that eliminate non recurring
tooling costs and provide for life-cycle production flexibility. Since the first
implementations on Boeing aircraft, the use of SLS has grown organically within
a large number of programs. This is primarily due to its ability to produce thermo-
plastic parts that are lightweight, nonporous, thin-walled, and highly complex
OCR for page 17
17
ADDITIVE MANUFACTURING IN AEROSPACE
FIGURE 1 Photograph and illustration of laser-sintered air ducts.
geometrically and to do so in an economical fashion. These properties also led to
the frequent use of SLS within the burgeoning field of unmanned aerial vehicles.
With weight being a critical factor, the very thin walls and complex designs
possible in SLS are attractive for replacement of parts typically made through
established processes such as rotational, injection, or polymer matrix composite
molding. To take advantage of SLS, one must have a firm understanding of the
extreme four-dimensional energy input gradients that exist during processing. For
example, typical SLS machines use 75-watt CO2 lasers that have a 500-µm spot
size. That laser spot moves at up to 10 meters per second, over layers of nylon
powder only 100 µm thick, with each layer being completed in approximately
60 seconds. The thorough description and efforts to simulate the details of the
energies present in the SLS process can be found in the literature (Franco et al.,
2010). This unique manufacturing context requires that any aerospace company
develop in-depth knowledge of the materials and process used in order to draft
commercially efficient specifications.
EXAMPLES OF AEROSPACE-DRIVEN RESEARCH IN AM
To build parts with repeatable mechanical properties and dimensional con -
trol, the temperature distribution across the part-building platform must be held
at as even a temperature as possible. In order to accomplish this and reduce scrap
rates, The Boeing Company and its partners at the University of Louisville and at
Integra Services International (Belton, Texas) developed a patented method for
zonal control of the part bed temperature in SLS equipment (Huskamp, 2009).
The multizone, near-infrared (IR) wavelength heating elements, seen in Figure 2,
provide the fast response and spatial resolution required to maintain even part bed
temperature. This invention, when paired with real-time IR imaging, provides a
significant improvement in thermal control. This level of thermospatial control
OCR for page 18
18 FRONTIERS OF ENGINEERING
FIGURE 2 MZ heating (top), SLS part bed (middle), and same parts seen via infrared
thermography during laser scanning (bottom).
OCR for page 19
19
ADDITIVE MANUFACTURING IN AEROSPACE
has had to become more advanced than that found in most other thermoplastic
processing methods.
Another example of an aerospace-driven AM need being met by researchers
can be seen in the emergence of flame-retardant polyamides. When considering
that many polymers are derived from fossil fuel-based hydrocarbon feedstocks,
the concept of a related chemistry being self-extinguishing when exposed to flame
is impressive. That characteristic is required, to a greater or lesser degree, of all
polymer materials used on the interior of commercial aircraft. To gain the weight
and manufacturing benefits provided by SLS on its commercial aircraft, Boeing
collaborated with its suppliers to develop the first material that could be laser
processed and that passed the required flammability tests (Booth, 2010).
CURRENT AREA OF DEVELOPMENT
Progress has been made to increase the number of AM applications in aero -
space, which has identified three new performance challenges for SLS polymer
materials. The operating requirements of programs such as F-35 and 787 have
put requirements on the AM community to develop materials that can (a) oper-
ate at higher temperatures, (b) have significantly better flame resistance, and
(c) offer an adjustable degree of electrical conductivity (Shinbara, 2011). These
new physical performance targets must be met while maintaining as many of the
attributes already established by SLS polyamide materials as possible. Those
attributes include mechanical toughness, resistance to chemical attack, ultraviolet
radiation resistance, dimensional fidelity, and viable economics. Such a material,
if developed successfully, could have a wide range of applications within and
beyond aerospace. Two of the most notable non-aerospace applications for new
high-performance polymers in SLS are the potential use in medicine for implants
and devices (Schmidt et al., 2007) and low-volume automotive production.
As is historically the case when a new technology is enabled and near transi -
tion to useful service, numerous parties from many industrialized nations can be
seen working on the same technical problem simultaneously. Researchers in the
United States, Germany, Japan, and the United Kingdom have made the deepest
investigations into developing high-performance polymers for SLS (Hesse et
al., 2007; Kemmish, 2010). There are many high-performance polymers that are
attractive for development from a cost perspective; however, the cost of testing
required by aerospace makes multiple, simultaneous material development efforts
cost prohibitive. Because of the known performance of the polyaryletherketone
(PAEK) family of materials, they are viewed as the lowest-risk option for current
development. The PAEK family includes different chemistries such as poly -
etherketone, polyetheretherketone, and polyetherketoneketone. The choice of a
PAEK as the next material family to be developed is based on factors inherent
to its chemistry, including very good flammability and chemical resistance, low
moisture sorption, good mechanical performance, good resistance to creep and
OCR for page 20
20 FRONTIERS OF ENGINEERING
fatigue, compatibility with several methods of sterilization, and numerous material
grades and suppliers to choose from.
Owing to the comparatively small size of the SLS market and the cost
of developing new polymer chemistries, raw materials for SLS are typically
selected from commercial-off-the-shelf (COTS) grades. These COTS materials
have been designed for other applications such as coatings, films, or rotational
molding. While injection molding and other methods of polymer processing can
make use of both heat and pressure to form a part against the surface of a mold,
AM processes have to rely primarily on thermal energy input. Viable materials
for additive processes must have very specific viscosity and other properties to
be successfully processed. To begin generating material performance test data, a
processable material form must first be developed.
The development of a viable PAEK-SLS material is an area of competitive,
industrial research at this paper’s time of writing, so specific information from
any one party is generally not published. However, the comparison of established,
well-understood polyamides (PAs) to the PAEK family shows the problem space
of engineers working in this field. Table 1 provides comparative thermal proper-
ties of lower-temperature PA and PAEK materials, as found in the literature and
in manufacturer-published information (Kemmish, 2010; Kohan, 1995).
By understanding and comparing the bulk thermal properties of these two
material families, one can begin to understand how differently they will behave
within the SLS process. One such comparison can be seen when the amount of
energy required to heat the material is considered. To process a PA powder, the
lower melt temperature combined with the lower specific heat (the amount of
energy required to heat a given mass of material one degree Kelvin) indicates that
the effort required to achieve a given viscosity with heat input is much lower for
a PA than for a PAEK. The PAEK must be heated to twice the temperature just
to approach melt and will require almost twice the energy per degree of heating.
This is further complicated in SLS processing, as the transient heating retirements
of each layer must not change drastically.
A second comparison is the ratio of specific heat to the heat of fusion (the
energy flux exhibited in the transition from solid to liquid, and vice versa). In
the SLS process, the polymer powder is heated in stages from ambient condi -
TABLE 1 Comparison of PA and PAEK Family Thermal Properties
Heat of
Glass Fusion Thermal
Melt Trans. Specific (100% Thermal Expansion Specific
Temp Temp Heat crys.) Conductivity (ppm/Tg Gravity g/cc
Material °C °C J/g K J/g W/m K °C) (crystalline)
PA 180-186 42-55 1.26 226 0.19 85 1.03
PAEK 300-375 145-165 2.20 130 0.26 60 1.30
OCR for page 21
21
ADDITIVE MANUFACTURING IN AEROSPACE
tions to very near the melt point. PA has a lower ratio of specific heat to heat of
fusion than the PAEK family of materials (1.26/226 compared to 2.20/130). This
is important because, despite best efforts, there is some gradient of energy input
and temperature across the building area at any given time. If a material has a very
gradual transition into melt, such as fully amorphous polymers, it can be difficult
to feed smoothly onto the machine’s part-building area.
This ratio of specific heat to heat of fusion gives an indication of how easily
a given material can be heated to near the melt point, across the whole part bed,
without fusing particles together. The closer to the melt point the material can be
fed into the machine, the lower the energy input requirements are on the laser for
heat input that transitions the material into the melt region. The lower the require -
ment put on the laser for energy input, the lower the risk of polymer degradation.
This is because within the CO2 laser spot there is a roughly Gaussian distribution
of energy, the peak of which can cause degradation.
Also tied to this comparison is the speed at which the laser draws each layer
of the part. With a given energy input requirement put on the laser per the above
comparison, the layer can be drawn with faster or slower laser scan speeds. The
scan speed affects the overall per-layer time which, in addition to the proportion
of preheat to laser energy required, results in a variable temperature distribution
and cooling rate across a part’s cross section, per given layer. If too long a time
has passed between the start and stop of a given layer and too high of an energy
demand is put on the laser, sections of that layer will have cooled faster than others
and, in turn, will have shrunk nonuniformly. Dimensional distortion can result if
too high a cooling gradient exists relative to recrystallization temperature, thermal
conductivity, coefficient of thermal expansion, and a host of other material factors.
A thorough description of the interaction between just the properties shown
in Table 1 is beyond the scope of this paper, but the three examples give a window
into the problems currently being solved by AM researchers. Thankfully, despite
FIGURE 3 Examples of parts of PEAK materials processed via SLS.
OCR for page 22
22 FRONTIERS OF ENGINEERING
all of these complexities, multiple parties are reporting success with the process -
ing of PAEK materials via SLS, as can be seen in Figure 3.
MOVING FORWARD
Beyond new higher-performance polymers, numerous research frontiers exist
within AM. Separate from the specific material development identified earlier,
AM equipment must transition from comparable low-reliability laboratory-
grade equipment to hardened, cost-effective, high-temperature industrial-grade
machines. The AM industry can look to its predecessors in injection molding
and computer numerical control machining for examples of how to establish new
manufacturing technology and the supporting business case. The unique material
requirements posed by AM processing have, during the technology’s infancy
phase, tied machine manufacturers to material and even part sale activity. While
this has provided a good revenue source to support the new companies, it has also
impeded new applications by making new material development difficult for all
but the largest of users and material companies. This dependence on material and
part sales, and nonproductive patent litigation, has also distracted the machine
manufacturers from improving upon their equipment with an eye toward higher-
volume, economical industrial manufacturing. Equipment manufacturers such as
Toshiba, Haas Automation, MAG, Husky, and Arburg do not rely on material or
part sales to bolster their equipment business, and if the AM industry is to grow
successfully it might look to their business models and history for reference.
Beyond polymers the use of metals in AM for aerospace is equally as com -
plex and exciting. Leading the way in direct metal part manufacturing have been
engine manufacturers. While direct part manufacturing is a highly dynamic field,
the leveraging of AM’s ability to create highly complex shapes is very applicable
to tooling for both metal and composites components. New tooling-focused
machines, processes, and materials are being actively developed that leverage the
process benefits of AM while delivering the performance of cast metals (Halloran
et al., 2010) or long-fiber-reinforced composites (Wallen et al., 2011).
Independent of material or processing conditions, the analysis of complex
geometries that can be built only with additive methods is also an active and
important area of research. Even with material test data generated, the types of
structures that AM can build, such as the trussed airfoils seen in Figure 4, are
difficult to analyze for predictive behavior. This field of study is generating new
software tools for the generation and predictive analysis of complex structures,
such as three-dimensional trusses (Engelbrecht et al., 2009).
These descriptions of current research areas, along with examples such as
Boeing’s use of SLS on commercial and military aircraft, show that the aerospace
industry has the opportunity to lead, and responsibility to contribute to, this revo -
lutionary field of manufacturing technology.
OCR for page 23
23
ADDITIVE MANUFACTURING IN AEROSPACE
FIGURE 4 Two complex truss examples that indicate the difficulty of predictive analysis.
REFERENCES
The Boeing Company Press Release. 2011. Available at http://boeing.mediaroom.com/index.
php?s=43&item=1426.
Booth, R., October 7, 2010. Methods and systems for fabricating fire retardant materials. U.S. Patent
Application 2010/0255327.
The Economist. “Print me a Stradivarius.” February 12, 2011. p. 11.
Engelbrecht S., L. Folgar, D. Rosen, G. Schulberger, and J. Williams. 2009. Cellular structures for op-
op-
timal performance. Proceedings of the University of Texas at Austin Solid Freeform Fabrication
Symposium, Austin, Texas, 2009.
Federal Aviation Administration. 2011. Available at http://www.faa.gov/regulations_policies/
faa_regulations/.
Franco, A., M. Lanzetta, L. Romoli. 2010. Experimental analysis of selective laser sintering of poly-
poly-
amide powder: An energy perspective. Journal of Cleaner Production 18:1722–1730.
Halloran, J.W., V. Tomeckova, S.P. Gentry, S. Das, P. Cilino, D. Yuan, R. Guo, A. Rudraraju, P. Shao,
T. Wu, T. Alabi, W. Baker, D. Legdzina, D. Wolski, W. Zimbeck, and D. Long. 2010. Photo-
polymerization of powder suspensions for shaping ceramics. Journal of the European Ceramic
Society 31(14): 2613–2619.
Hauge, R., and J. Wooten. 2006. Rapid Manufacturing: An Industrial Revolution for the Digital Age,
Chap. 15, p. 233. Chichester, U.K.: John Wiley & Sons Ltd.
Hesse P., P. Tillmann, R. Weiss. November 22, 2007. Powder for rapid prototyping and associated
production method. U.S. Patent Application 2007/0267766.
Huskamp, C. April 7, 2009. Methods and systems for controlling and adjusting heat distribution over
a part bed. U.S. Patent 7,515,986.
Kemmish, D. 2010. Update on the Technology and Applications of Polyaryletherketones, Shawbury,
U.K.: iSmithers.
Kohan, M. 1995. Nylon Plastics Handbook. Cincinnati, Ohio: Hanser/Gardner Publications.
Lyons, B., E. Deck, and A. Bartel. 2009. Commercial Aircraft Applications for Laser Sintered Poly -
amides. Society of Automotive Engineers Technical Paper 09ATC-0387.
OCR for page 24
24 FRONTIERS OF ENGINEERING
Ruffo, M., C. J. Tuck, and R. J. M. Hague. 2006. Cost estimation for rapid manufacturing–laser
sintering production for low to medium volumes. Proceedings of the Institution of Mechanical
Engineers, Part B: Journal of Engineering Manufacture 220(9):1417–1427.
Schmidt, M., D. Pohle, and T. Rechtenwald. 2007. Selective laser sintering of PEEK. CIRP Annals–
Manufacturing Technology 56(1):205–209.
Shinbara, T. 2011. High temperature laser sintered components. Presented at the Society of Manu -
facturing Engineer’s AeroDef Manufacturing 2011 Event, Anaheim, California, April 7, 2011.
Wallen, M., J. Rossfeldt, C. Aune, and Z. Wing. 2011. Washout Tooling for Hollow Composites:
Toward Rapid, Low Cost Production Technology. Society of Manufacturing Engineers: Com -
posite Manufacturing, Talk #6793, Dayton, Ohio.
