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Author: Brett Lyons
Additive manufacturing has the potential to revolutionize the production of aerospace and defense components.
The advantages of additive manufacturing are now widely recognized, even in the general media, and are predicted to revolutionize manufacturing processes for many industries (Economist, 2011). For aerospace, complex additive manufacturing processes must be developed to meet the industry’s stringent requirements and to ensure that products can achieve the robust performance levels established by traditional manufacturing methods. This article provides an overview of additive manufacturing technologies being used for the production of aircraft components and examples showing the direction of ongoing research and related developments.
Aerospace Requirements for Manufacturing
To meet the stringent conditions necessary to ensure safety in air travel, aerospace manufacturers must satisfy a long list of complex requirements for even the simplest part. The consistent production of parts with identical, well-understood properties requires that both materials and production processes be understood to a very high level. This complicated manufacturing context, which blends low-volume economics, acute weight sensitivity (weight is often the deciding factor in choosing materials and manufacturing processes for aerospace and defense components), and the need for highly controlled materials and manufacturing processes, has led Boeing to focus on transitioning additive manufacturing techniques from the laboratory and model shop to the factory floor.
Requirements for commercial aircraft parts are based on U.S. Federal Aviation Regulations, which must be met before a Type Certification can be issued for a given aircraft series; this certification is required for service with any airline (FAA, 2011). The regulations are extensive and detailed, but the single most pertinent language in the context of additive manufacturing can be found in Title 14, Section 25, Subpart D, Subsection 25.605:
The suitability and durability of materials 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 specifications (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 just one of many that have not only contributed to the incredible safety record of commercial air transportation, but have also provided the impetus for studying new fabrication methods. Every aerospace and defense manufacturer either has internal specifications or looks to established standards organizations for data that can support the accurate design of components from a given material, based on minimum allowable performance levels. Factors that must be considered for the material performance of even the simplest components include specific strengths, fatigue resistance, creep resistance, use temperature, survival temperature, several tests of flammability, smoke release and toxicity, electric conductivity, multiple chemical sensitivities, radiation sensitivity, appearance, processing sustainability, and cost.
Despite increased rates of production, the aerospace industry must still produce many parts in very small quantities (Boeing Company, 2011). Additive manufacturing processes, which can form finished parts without intermediate tooling, thus eliminating the associated costs and delays, are extremely attractive to the industry (Ruffo et al., 2006).
Selective Laser Sintering
One of these processes, selective laser sintering (SLS), begins with a computer-generated, three-dimensional design of a given part. The part is then digitally segmented into very thin layers that are selectively solidified in the machine, layer by layer. Essentially, the machine “grows” the part. With this capability, SLS can produce components with complex designs that would be extremely complicated and expensive to produce by other processes.
SLS can produce economical thermoplastic parts that are lightweight, nonporous, thin-walled, and highly complex geometrically. Since the first implementations of SLS on Boeing aircraft, this process has been adopted by a large number of programs, both military (Wooten, 2006) and commercial (Lyons et al., 2009), as well as by producers of unmanned aerial vehicles.
SLS produces lightweight, highly integrated systems and payload components (Figure 1) and, at the same time, eliminates non-recurring tooling costs and provides for life-cycle production flexibility. Because weight is often a critical factor, SLS, which can produce very thin walls and complex designs, is also attractive for replacing parts that are typically produced using established processes, such as rotational, injection, or polymer matrix composite molding.
Drafting commercially efficient specifications in the unique context of aerospace manufacturing requires that a company develop in-depth knowledge of materials and processes. For example, to take advantage of SLS, one must have a firm understanding of the extreme four-dimensional energy input gradients during processing. Typical SLS machines use 75-Watt carbon dioxide (CO2) lasers that have a 500-µm spot size. The laser spot moves at speeds of up to 10 meters per second over layers of nylon powder only 100 µm thick; each layer is completed in approximately 60 seconds. A thorough description of both the SLS process and efforts to simulate details of the energies present can be found in the literature (Franco et al., 2010).
Examples of Aerospace-Driven Research
Controlling Temperature in the Part-Building Platform
To build parts with repeatable mechanical properties and dimensional control, the temperature distribution across the part-building platform must be as even as possible. To accomplish this and reduce scrap rates, Boeing and its partners at the University of Louisville and Integra Services International (Belton, Texas) developed a patented method for zonal control of the temperature of the part bed in SLS equipment (Huskamp, 2009). Multi-zone, near-infrared (IR) wavelength heating elements (Figure 2) provide the rapid response and spatial resolution necessary to maintain a steady, even temperature. This invention, when paired with real-time IR imaging, has improved thermal control to a more advanced level than is required for most other thermoplastic processing methods.
The Development of Flame-Retardant Polyamides
Another aerospace-driven need that researchers have addressed is for flame-retardant polyamides. Considering that many polymers are derived from fossil fuel-based hydrocarbon feedstocks, the requirement that a related chemistry be self-extinguishing when exposed to flame poses a serious challenge. Nevertheless, flame-retardance is necessary to a greater or lesser degree for all polymer materials used in the interiors of commercial aircraft. To take advantage of the weight and manufacturing benefits of SLS on its commercial aircraft, Boeing collaborated with its suppliers to develop the first laser-processed material that could pass the required flammability tests (Booth et al., 2010).
New Performance Challenges
As the number of potential additive manufacturing applications in aerospace has increased, three new performance challenges have arisen for SLS polymer materials. Operating requirements for the F-35 and B787 have challenged researchers to develop materials that (1) can operate at higher temperatures, (2) have significantly better flame resistance, and (3) offer adjustable electrical conductivity (Shinbara et al., 2010).
To add to these challenges, the new physical performance targets must be met while maintaining as many of the attributes already established for SLS polyamide materials as possible. These attributes include mechanical toughness, resistance to chemical attack, resistance to ultraviolet radiation, dimensional fidelity, and viable economics.
Historically, when a new technology is enabled and nears transition to useful service, researchers in many industrialized nations tend to work on the same technical problems simultaneously. In this case, researchers in the United States, Germany, Japan, and the United Kingdom have conducted in-depth investigations into high-performance polymers for SLS (Hesse et al., 2007; Kemmish, 2010).
If such a material were developed successfully, it could have a wide range of applications in and beyond aerospace. Two notable non-aerospace applications for new high-performance, SLS-produced polymers are for medical implants and devices (Schmidt et al., 2007) and the production of low-volume automotive parts.
The Focus in Aerospace Research
Although many high-performance polymers are attractive from a cost perspective, the high cost of testing for aerospace applications makes focusing on multiple materials simultaneously cost prohibitive. Unlike injection molding and other methods of polymer processing that rely on both heat and pressure to form a part against the surface of a mold, additive manufacturing processes rely primarily on the input of thermal energy. Thus, viable materials for additive processes must have very specific viscosity and other properties to be processed successfully.
Polyaryletherketone (PAEK) Materials vs. Polyamides
Because of their known performance, the polyaryl-etherketone (PAEK) family of materials is considered the lowest risk option for current development for aerospace applications. The PAEK family includes different chemistries (e.g., polyetherketone, polyetherether-ketone, and polyetherketoneketone). The choice of a PAEK as the next material to be developed is based on inherent factors, including very good flammability and chemical resistance, low moisture sorption, good mechanical performance, good resistance to creep and fatigue, compatibility with several methods of sterilization, and numerous material grades and suppliers to choose from. In light of 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 grades of PAEKs, which have been designed for coatings, films, rotational molding, and other applications.
At the time of this writing, the development of a viable PAEK-SLS material is an area of competitive industrial research, so specific information from any one party is generally not published. However, a comparison of established, well-understood polyamides and the PAEK family of materials shows the problem space for engineers working in this field.
Table 1 shows comparative thermal properties of lower temperature polyamides and PAEK materials, as found in the literature and in manufacturer-published information (Kemmish, 2010; Kohan, 1995). By comparing the bulk thermal properties of these two material families, one can begin to understand how differently they will behave in the SLS process.
One important difference is the amount of energy required to heat the material. To process a polyamide 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 polyamide than for a PAEK. The PAEK must be heated to twice the temperature just to approach melt and requires almost twice the energy per degree of heating. This is further complicated in SLS processing because the transient heating requirements of each layer must not change drastically.
A second difference 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 conditions to very near the melt point. Polyamide material has a lower ratio of specific heat to heat of fusion than PAEK materials (1.26/226 compared to 2.20/130). This is important because, despite our best efforts, there is still some gradient of energy input and temperature across the building area at any given time. If a material very gradually transitions into melt, as fully amorphous polymers do, it can be difficult to feed the material smoothly onto the machine’s part-building area.
The 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 is when fed into the machine, the lower the energy input requirements on the laser for heat input to transition the material into the melt region. The lower the requirement on the laser for energy input, the lower the risk of polymer degradation. This is because in the CO2 laser spot there is a roughly Gaussian distribution of energy, the peak of which can cause degradation.
With a given energy input requirement on the laser (per the comparison above), 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 for a cross section of the part, per given layer.
If too much time passes between the start and stop of a given layer and the energy demand on the laser is too high, some sections of that layer will cool faster than others and the layer will not shrink uniformly. Dimensional distortion can result if the cooling gradient is too high relative to the recrystallization temperature, thermal conductivity, coefficient of thermal expansion, and a host of other factors.
A thorough description of the interaction, just for the properties shown in Table 1, is beyond the scope of this paper, but these examples provide a window into the problems currently being addressed by researchers. Thankfully, despite the complexities, several research teams are reporting success with the processing of PAEK materials via SLS (Figure 3).
Transitioning from the Laboratory to the Factory
Numerous research frontiers lie beyond the development of higher performing polymers. For one thing, additive manufacturing equipment must transition from comparable low-reliability laboratory-grade equipment to hardened, cost-effective, high-temperature, industrial-grade machines. The additive manufacturing industry can look to its predecessors in injection molding and computer numerical-control machining for examples of establishing new manufacturing technology and developing a supporting business case.
So far, in the infancy phase, the unique material requirements of additive manufacturing processing have limited machine manufacturers to selling materials and some parts. Although this has provided a good source of revenue to support new companies, it has also impeded new applications by making the development of new materials difficult for all but the largest users and material companies.
Dependence on the sale of materials and parts, as well as nonproductive patent litigation, have also distracted machine manufacturers from improving upon their equipment with an eye toward higher volume, economical, industrial manufacturing. Equipment manufacturers (e.g., Toshiba, Haas Automation, MAG, Husky, and Arburg) cannot rely on the sales of materials or parts to bolster their businesses. Thus for the additive manufacturing industry to grow, it might look to the business models and history of such machine manufacturers for reference.
Manufacturing Metal Parts
The use of metals in additive manufacturing for aerospace is as complex and exciting as the development of polymers. So far, engine manufacturers have shown leadership in the direct manufacturing of metal parts, which is a highly dynamic field. However, leveraging the capabilities of additive manufacturing to create components with highly complex shapes also lends itself to tool production, for both metal and composite components. In fact, new tooling-focused machines and materials are being actively developed to leverage the process benefits of additive manufacturing for faster production of metal castings (Halloran et al., 2010) and long-fiber-reinforced composites (Wallen et al., 2011).
Analysis of Complex Geometries
Independent of material and processing conditions, the analysis of complex geometries that can be built only with additive methods is another important area of active research. Even with material test data, the predictive behavior of the types of structures additive manufacturing can build, such as trussed airfoils (Figure 4), is difficult to analyze. This field of study is focusing on developing new software tools for the generation and predictive analysis of complex structures, such as three-dimensional trusses (Engelbrecht et al., 2009).
The descriptions of current research, along with examples, such as Boeing’s use of SLS for producing parts for commercial and military aircraft, show that the aerospace industry has the opportunity to lead, and the responsibility to contribute to, this revolutionary field of manufacturing technology. Technologies that provide this level of manufacturing flexibility will greatly contribute to meeting burgeoning global demands for energy, transportation, defense, and medical technologies here on earth and may even prove to be enabling technologies for exploring horizons beyond our planet.
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