Additive Manufacturing— The Advantages and the Challenges

John Schmelzle, P.E.

Additive manufacturing (AM) provides engineers with unprecedented design freedom. This technology enables designers to rapidly fabricate complex components that previously could not be manufactured. And it enables designers to consolidate assemblies, reduce mass, and create intricate geometries for enhanced fluid flow or heat transfer performance, among its other benefits.

AM also offers the capability to reduce development times and to create parts on demand. Taking advantage of the design space enabled by AM does necessitate an understanding of the limitations and capabilities of the specific AM processes used for production, the system-level design intent, and the post-processing and inspection/qualification implications. This article compares the advantages and challenges of AM versus traditional (subtractive) manufacturing.
[See also Department of Energy video on AM]

The Advantages

The reduced cost of complexity. In traditional (subtractive) manufacturing, added complexity to a part’s design usually significantly increases the manufacturing cost of the part. This creates a trade-off in the design process between the need to minimize complexity to reduce cost at the expanse of a part’s performance. A significant advantage of AM is that this trade-off is unnecessary.

Added complexity when using AM has a much lower impact on cost. In fact, since the cost of using the AM process depends on the amount of material used, added complexity incorporated to reduce weight often reduces overall manufacturing cost. This removes a significant constraint on the designer, allowing a design that minimizes weight, consolidates parts, and incorporates other functions that could not be added when using a subtractive process.

Weight and material savings. New software technology exists in computer-aided design to reduce a part’s weight using topology optimization (TO). This mathematical method can optimize material layout for a design based on defined boundary/load constraints. It is used to improve performance at the part, assembly, and/or system level. TO is usually an iterative process based on a Finite Element Analysis to eliminate unnecessary material for a smaller or lighter part. The AM Manifold in the photo below shows a part where significant material has been removed using TO. This software can remove unnecessary material from a part’s design and can create elaborate internal lattice structures. This added complexity would make a part very expensive and perhaps even impossible to manufacture using a subtractive manufacturing method; however, it has little impact when making the part using AM technology.

Multiple materials and color—and functional grading. Among the more recent developments in AM is the use of multi-material processes. These involve using several materials to manufacture a part. In addition to multiple materials, similar capabilities exist for multi-color AM, achievable by combining materials with different colors during the jetting process when the feedstock is extruded. AM equipment uses several extruders in multi-material deposition. With AM, it is possible to create a part that gradually changes from one material to another in order to meet specific part performance requirements. These products qualify as Functionally Graded Materials (FGMs).

As part of a multi-material additive research project, the Naval Research Lab (NRL) printed a metal part made of two alloys. The part was a heat exchanger manufactured using stainless steel powder (SS 316) and nickel alloy (Inconel 718). In the transition from Inconel 718 to SS 316, layers of the one and the other material were alternately printed. A gradual transition from Inconel 718 to SS 316 occurred as the laser melted the powder as well as the underlying layers. Other FGM projects at NRL have involved parts consisting of metal alloys with ceramic particles to increase rigidity, temperature resistance, and abrasion resistance. Multi-alloy parts have added complexity due to differences in mechanical properties and microstructure between the different alloys.

Part consolidation. AM provides opportunities for consolidating multipart assemblies. Consolidation may reduce the size and weight of the assembly while improving performance, minimizing failure/leak points, and reducing an item’s logistic footprint. A generalized design approach for consolidating parts is presented in “Designing for Part Consolidation: Understanding the Challenges of Metal Additive Manufacturing” in the Journal of Mechanical Design, November 2015, Vol. 137, and can be reviewed to help designers realize the freedoms that metal AM provides. That article documents the process used to combine 17 parts of a hydraulic manifold assembly (Stack Manifold Assembly photo) into a single AM component (AM Manifold photo).

Rapid manufacturing. AM technology has significantly shorter set-up times and correspondingly lower costs compared to traditional manufacturing processes. This provides the capability to manufacture small amounts rapidly and at reasonable cost (Figure 1). This has resulted in extensive use of AM in manufacturing rapid prototypes. This rapid capability has extended its use beyond prototypes to addressing immediate supply shortages that cannot be satisfied by other manufacturing technologies.

In addition to its technical advantages, AM’s enabling of rapid on-site manufacturing with little set-up costs creates new logistical opportunities.

The low set-up cost of AM also makes it an ideal technology to support just-in-time inventory, by which parts are manufactured when needed, reducing inventory costs involved in storing spare parts.

On-site manufacturing, customization. AM offers the ability to manufacture parts where they are needed rather than at a distant manufacturing facility. This eliminates the cost and time involved in transporting these parts. The U.S. Navy uses long supply lines to support its aircraft carriers and has been looking for ways to maximize the AM advantage by deploying AM equipment to its facilities and ships.

Because significantly shorter set times and the reduced need to produce part-specific tooling, AM provides unique opportunities for creating parts customized for particular uses. The photograph below shows custom earplugs made through AM by the Naval Air Systems Command. Custom-molded earplugs are believed to provide the highest and most repeatable levels of real-world noise attenuation. However, the complex logistics associated with creating a physical ear impression, shipping the impression to the manufacturer, and receiving and distributing such earplugs prevent widespread use. Utilizing AM minimizes these logistical costs.

The Challenges

Although AM technologies offer many significant advantages, some unique challenges need to be overcome. These challenges have been reduced as the technology has matured. However, consideration should be given to the following factors when selecting AM:

Geometry limitations. Considerations for part geometry include both the geometric requirements of the part and the ability of the AM machine to produce parts that meet those requirements. Geometry considerations include, but are not limited to, the following:

1. Building volume limitations: Designers must consider the build envelope when selecting the AM process. Depending on the AM process and the available equipment, there may be significant restrictions on the overall part size.
2. Minimizing part size, feature size, wall thickness, and details that the AM process can produce (e.g., ribs, bosses, holes, sharp corners) and developing methods to remove supports for these parts and features.
3. Minimizing feature spacing to ensure that parts and features are not fused together during the manufacturing process.
4. Maximizing aspect ratio (height to width), build orientation, and supports for thin parts and features.
5. Maximizing unsupported features (e.g., angle, overhang, holes).

Although am technologies offer many significant advantages, some unique challenges need to be overcome.

Material properties. The effects of AM on material properties, such as fatigue and strength or strain to failure, are not well understood. This is due to a lack of historical testing and standardization of AM materials and the poorly understood influence of fabrication and post-processing on the variability of material properties. Design for AM needs to account for impacts on the material properties, such as input material characteristics, build orientation (anisotropic effects or variable internal strength), build layout/order, processing variables, and post-processing variables, including heat treatments and finishing. AM also tends to produce defects such as voids. The impacts of these defects on material properties are not well characterized. Thus, any design using AM materials should consider the need for testing to characterize the performance and variability of AM material properties.

Required supports. Supporting structures or materials, removed prior to end use, often are required for the following reasons:

1. They provide a surface and structure upon which to build features. If an item has holes, slots, or overhanging geometry, supports may be required.
2. They provide an interface to anchor the part to the build surface.
3. They reduce distortion. Because many AM processes (particularly metallic processes) operate at high temperatures, distortion may occur due to residual thermal stresses as the part cools.
4. They facilitate heat dissipation. Since many AM processes (particularly metallic processes) operate at high temperatures, a method to dissipate heat may be required.

Regardless of why supports are needed, consideration must be given to their placement and removal. Furthermore, if internal features exist, any required support could be challenging to remove.

Surface roughness. Current AM technology may not achieve a part’s required surface finish directly from the AM machine, which could prohibit printing the part “to-size.” This can be of particular concern when a part is life-limited such as a Fatigue/Fracture Critical or Fatigue Sensitive (FS) Component. One approach is to design the part oversized and machine it to the required size. Although this is an acceptable solution for some features on a part, machining 100 percent of all surfaces would most likely prove cost prohibitive. Technology is advancing to overcome some of these barriers. Experiments may be required to verify that key properties are adequately addressed and not degraded by the finishing process. Final surface finishing typically occurs after separation of parts from the build platform as applicable (e.g., if a specific surface roughness, surface profile, or specialized coating is required).

Dimensional accuracy. AM processes cannot achieve the same precision and accuracy possible with machining. The tolerances possible for AM builds vary by the process selected for manufacturing and are a result of the method used to deposit and join material. The thickness of deposited material layers can range from 100 to 10,000 micro inches (μin), which limits the achievable tolerances. The relative accuracies of seven AM processes are shown in Table 1.

The dimensional accuracy of AM can vary based on AM technology, vendor manufacturer, AM equipment model, AM equipment serial number, and AM build parameters. Variation can exist even in identical builds from the same printer. This was demonstrated by Pennsylvania State University’s Applied Research Laboratory in the study, “Dimensional Accuracy of Titanium Direct Metal Laser Sintered Parts.”. This variation must be considered when manufacturing parts.

The accuracy of some AM equipment can be improved by calibrating for the geometry of the specific part being manufactured. By doing so, an adjustment to the AM equipment can increase its accuracy. This should be done only when a small increase in accuracy will create a significant benefit. After the part’s manufacture, a second calibration to readjust the AM equipment will likely be required.

Conclusion

AM is a rapidly maturing technology that can, at times, offer significant advantages over other manufacturing technologies. It provides engineers with unprecedented design freedom and can help to streamline supply changes. We still need to overcome challenges that may restrict the use of this technology. However, we expect that the maturation of AM technology will reduce the effects of these challenges and offer further opportunities for wider use of its capabilities.

Read the full issue of
Defense Acquisition magazine

SCHMELZLE is the Additive Manufacturing and Model Based Definition Lead at the Naval Air Warfare Center Aircraft Division, in Lakehurst, N.J. He previously headed NAVAIR’s Design and Analysis Branch of the Support Equipment Engineering Division. He received a bachelor’s degree in mechanical engineering from the State University of New York at Stony Brook and a master of science degree in Mechanical Engineering from Drexel University.

The author can be contacted at [email protected].

The views expressed in this article are those of the author alone and not the Department of Defense. Reproduction or reposting of articles from Defense Acquisition magazine should credit the authors and the magazine.