The global advancements in additive manufacturing (AM) are far reaching. From a U.S. Department of Defense (DoD) perspective, all the Services (U.S. Air Force, Army, Navy, and Marine Corps) are working to advance AM materials, processes, and manufacturing technologies. The investment is substantial. Progress is being systematically made through a variety of means including, but not limited to, the following: (1) dedicated DoD Service AM implementation plans, (2) a Federally-funded national network of manufacturing institutes with an increasing number of research programs, and (3) successful AM efforts, including repairs and parts production for noncritical and flight/submarine-critical parts. An update on the status of AM in the DoD is presented, as opportunities are exploited and challenges overcome.
THE VALUE AND PROMISE OF ADDITIVE MANUFACTURING
Much has been written on the future value of AM, and several recent publications highlight the unique opportunity areas for the defense sector [1–4]. In 2014, Deloitte Consulting LLP  envisioned and imagined the real-world impact of AM in the DoD Maintenance Enterprise to deliver weapon systems faster and with improved platform designs. The following spring of 2015, the Defense Systems Information Analysis Center (DSIAC) covered the “Opportunities and Challenges of Additive Manufacturing in the DoD” to include major outcomes of enhancing Warfighter capability while reducing the current logistical footprint and total life cycle system costs . In the fall of 2016, Strategic Studies Quarterly published “Additive Manufacturing: From Form to Function” , which provided a perspective for future joint efforts by exploring the status and shaping of AM capabilities through the strategic framework contained within key U.S. Air Force (USAF) reports, planning documents, and other relevant resources within the DoD. The article explored the growth of AM within the military, the role of AM in logistics and sustainment, and its impact on the acquisition process and concluded with future opportunities and challenges.
The December 2016 issue of the Defense Acquisition, Technology and Logistics (AT&L) Magazine  was a special issue on additive manufacturing, with contributions from Deloitte Consulting LLP; the U.S. Navy; USAF; Lockheed Martin; Raytheon; Defense Acquisition University (DAU); Youngstown State University; Defense Logistics Agency (DLA); U.S. Army Armament Research, Development and Engineering Center (ARDEC); and several others. The 16 articles spanned topics ranging from “The Digital Thread as a Key Enabler” to “Challenges of Enterprise-wide AM for Air Force Sustainment.” The opening article addressed the trade-off between quality and time from the perspective of rapid defense acquisitions. For example, some AM products can be made quickly, inexpensively, and of low quality for form-and-fit checks or attributable assets. By comparison, other defense products demand high reliability, maintainability, and operation in a wide range of climates/terrains, modularity, the prospect of being upgradable, having well-designed user interfaces, and having built-in cybersecurity protection measures, etc. Therefore, the complexity of the threat will ultimately dictate our product requirements .
Some of the greatest benefits of AM can be categorized into time and cost, complexity and customization, and novelty compared to challenges in quality, workforce development, and trust in meeting the demanding requirements of many DoD applications (Figure 1). Although AM cannot answer all the toughest defense challenges, the status of AM in the DoD is on its way up the slope of enlightenment according to Gartner’s hype cycle (Figure 2; ). Some of the greatest momentum can be seen in the following selected AM defense efforts listed as subsections in this article: (1) DoD AM Implementation Plans Unique to Each Service, (2) DoD AM Research Ecosystem: Manufacturing USA, and (3) DoD AM Headline Accomplishments: From Repairs to Point Insertions.
DoD AM Implementation Plans Unique to Each Service
Strategic implementation plans for AM have been independently produced by the USAF  and the U.S. Department of the Navy (DON) . The U.S. Army has also developed a draft AM technology report .
In the USAF AM Strategic Implementation Plan, the development of AM is based upon a crawl, walk, run strategy described in now, near, and long terms . The following nine key challenges were identified to move from the current state of AM to the desired implementation of AM:
- Material standards and availability
- Part selection
- Skillset development
- Configuration control
- Part validation and qualification
- Process validation and qualification
- Reverse engineering
Currently, AM in the USAF is primarily decentralized and consists of polymer and metal-based technology (Figure 3 ). Therefore, as a near-term goal, selective AM capabilities will be developed in a centralized manner to qualify target, noncritical parts. This will allow standard AM equipment, training, processes, guidelines, tools, and post-processing procedures to be vetted, shared, and expanded across the DoD enterprise. Once these advancements have been made, the long-term vision is to place these established AM capabilities within the context of a global manufacturing network to enable on-demand printing. Other aspects of the future global AM network include a cybersecure parts library tied to the end user’s viewpoint or concept of operations. These goals are in the context of ensuring agility and flexibility for the Warfighter by improving readiness while reducing cost.
The DON has a developed structure for promoting and addressing AM. The Naval Additive Manufacturing Executive Committee, consisting of the Deputy Assistance Secretary for the Navy for research, development, test and evaluation; Deputy Chief of Naval Operations for Fleet Readiness and Logistics (OPNAV N4); and Deputy Commandant, Installations and Logistics, has developed and released multiple DON AM implementation plans (IPs). The first plan was developed in 2016 in accordance with the Secretary of the Navy’s 3 September 2015 memo “Additive Manufacturing/3-D Printing” and later revised in Version 2.0 (V2.0) released in 2017 . The DON AM
IP V2.0 identifies the following five objectives for the DON to move toward:
- Increase development and integration of AM systems.
- Develop the ability to qualify and certify AM parts.
- Standardize the digital AM framework and tools and enable end-to-end process integration
- Establish the DON’s advanced integrated digital manufacturing grid.
- Formalize access to AM education, training, and certifications for the DON workforce. Beyond these broad objectives, the DON AM IP V2.0 breaks down specific focus areas and implementation challenges to overcome while also showing progression milestones and demonstrations spanning 2017 through 2021+.
Although the Army AM implementation plan is currently under development by the U.S. Army Materiel Command, the Army AM Technology Roadmap (AAMTR)  comprises 15 technical objectives, 65 sequenced activities, and 241 requirements across four primary AM focus areas, or “swim lanes”— design, material, process, and value chain—based upon the America Makes DoD Roadmap framework [10, 11]. The Army plan calls out specific AM application areas, including (1) maintenance and sustainment, (2) new part/system production and acquisition, and (3) AM at the point of need and in expeditionary environments. Further, the Army plan elaborates on several other nontechnical key enablers that must be addressed for the successful implementation of AM, including cultural change, workforce development, data management, and policy change.
DoD AM Research Ecosystem: Manufacturing USA
Although each Service has its own unique ecosystem of AM research, all the services are members of the National Manufacturing Institutes, more recently renamed “Manufacturing USA” (Figure 4). Here, an update on the status of the manufacturing institutes most closely tied to AM will be discussed. These institutes are part of a larger and growing AM research ecosystem of federally-funded manufacturing efforts that include the Office of the Secretary of Defense (OSD) and Service-specific programs such as the Air Force Office of Scientific Research for basic (6.1) efforts, Commander’s Research and Development Fund for basic/applied (6.1/6.2) research, and the Small Business Innovative Research Program. As of 2018, the federal government committed over $1 billion, which has been matched by more than $2 billion in investment by industry, academia, and state and local governments. A variety of institutes have focused on technologies such as biofabrication, photonics, fibers and textiles, recycling, lightweight metals, digital manufacturing and design, process development, biopharmaceuticals, power, advanced composites, clean energy, and flexible hybrid electronics (Figure 4 ). The most recent institutes launched in 2017 were the Advanced Regenerative Medicine Institute, also known as ARMI’s BioFabUSA, and Advanced Robotics for Manufacturing Institute.
The primary center of AM institute activities at America Makes in Youngstown, OH, was founded in 2012 and has since funded over 60 projects [10–12]. Recently, America Makes worked with each DoD Service/agency (USAF, Army, Navy, and Defense Logistics Agency) and had over nine workshop sessions to generate a public-releasable AM roadmap for the systematic development of AM technology. The roadmap was divided into design, materials, processes, and value chain technical focus areas. The design area covers technological advancements in new design methods and tools. The materials area builds the body of knowledge for benchmark AM property characterization data and eliminates variability in “as-built” material properties. The process area drives technological advancements that enable faster, more accurate, and higher detail resolution AM machines. The value chain area encourages technological advancements that enable step change improvements in end-to-end value chain cost and time to market for AM-produced products. Other recent notable accomplishments for America Makes include coordinating and publishing a national roadmap for standards and specifications in collaboration with the American National Standards Institute and launching the digital storefront to digitally represent the live AM roadmap and index project data to the roadmap. Data was integrated from external sources to provide a fuller picture of the progress made against AM technical challenges as a community.
Another institute focusing on AM is NextFlex, America’s Flexible Hybrid Electronics (FHE) Manufacturing Institute, based in San Jose, CA. To date, projects in the NextFlex portfolio range from next generation digital printing systems to flexible medical devices. NextFlex currently has 40 projects underway, 16 of which have been funded directly by government agencies, in addition to the core OSD funding. Part of the Institute’s long-term sustainability plan is a technology hub in San Jose, which will combine digital printing with traditional electronics manufacturing services tools to create a prototyping and low-volume manufacturing capability for FHE devices.
DoD AM Headline Accomplishments: From Repairs to Point Insertions
Recent AM successes in the USAF, Navy, and Army include repairs, flight and submarine critical parts, locally certified parts, printed armament, AM cast, and the addition of functionality through AM. The first set of AM repair examples involves a cold spray technology with a significant impact on tri-Service systems, including nonrepairable/nonprocurable components. This is important because new repair technologies are not systematically incorporated into the maintenance supply processes due to organizational barriers that prevent implementing new technology into aging weapon systems and lack of funding for developing repair processes on legacy weapon systems . For example, the B1 plane has undergone cold spray repair on forward equipment bay (FEB) panels (Figure 5 ). These parts were cracking due to chafing wear of the lightweight Al2024 composite bonded, stiffened skin panels upon repeated opening and closing for general maintenance. These external access panels are typically secured to the airframe with steel fasteners, which are designed to sit flush with the panel to enable laminar flow over the fastener head. However, after repeated access to the panels, the hole become enlarged around the fasteners and below the surface of the panel, resulting in turbulent airflow, vibration, and eventual pull-through and loss of the fastener in flight. The material developed to repair the FEB panels was Al6061, which was tested in 2014 for adhesion, tensile strength, and impact resistance, with additional tests in 2016 for wear, corrosion, and additional impact. Figure 6 shows that there is a strong interface between cold spray and original material .
Recent AM successes in the USAF, Navy, and Army include repairs, flight and submarine critical parts, locally certified parts, printed armament, AM cast, and the addition of functionality through AM.
There are currently three B-1’s flying with cold sprayed components, including an upper FEB panel and landing gear hydraulic brake lines. Most recently, the USAF 28th Bomb Wing Maintenance Group received authorization from the Change Evaluation Team in January 2018 for repairing nonsafety critical, nonstructural repair of aluminum (6061 Al) on magnesium (ZE41A-T5, AZ91C-T6, and EV31-T6) parts and is currently repairing FEB panels on additional B-1 planes.
Several additional success stories for cold spray repair include hydraulic aircraft tubing, a helicopter sump, and submarine actuator. For the titanium (Ti) hydraulic tubing in the B-1 that interfaces with the main and nose-landing gear, cold spray repair was used from 2009 to 2011 to prevent chafing wear. This was a notable achievement because the hydraulic Ti tubing is customized to each unique airframe, and when it goes out of tolerance, it must be individually addressed. Tests performed on the tubing included adhesion, hardness, wear, burst, hydraulic impulse, and pressurized rotating beam.
Cold spray AM dramatically improved readiness for DoD systems by decreasing repair time and cost for a variety of routinely damaged metal parts.
The UH-60 Blackhawk helicopter sump located at the bottom of the aircraft is traditionally cast out of a large piece of magnesium (Mg) and holds liquids such as hydraulic fluid. Subject to corrosion, its repairs have become backlogged due to the large number of damaged sumps. To address this, the Army instituted an aluminum (Al) cold spray process to coat the Mg, thereby returning the part’s integrity. Due to the huge scrap rate of large-cast Mg parts accounting for a greater economic impact to business, industry adopted the cold spray technique in the field and the factory rather than repairing previously damaged parts.
The U.S. Navy Seawolf TD-63 actuator was also repaired with an AI-based cold spray AM technology. The TD-63, a valve actuator body for the periscope, experienced corrosion and required sealing the surface to prevent water leakage. Damage to the original material included corrosion pits in the box structure of the actuator; however, the lead time for replacement parts was approximately a year. As a result, cold spray AM dramatically improved readiness for DoD systems by decreasing repair time and cost for a variety of routinely damaged metal parts.
Naval Air Systems Command’s successful flight critical AM part, the MV-22B Osprey nacelle link (Figure 7 ), was flight tested in July 2016 at the Naval Air Station in Patuxent River, MD. The nacelle link was chosen due to its history as a legacy part, its incorporation of redundancy (configured with three other nacelles to ensure that if the AM part broke, backups would keep the engine fastened to the wing), and the suitability of printing in a technically mature material—Ti-6Al-4V . The development process took 18 months and included developing four different production designs and fitting the printed link with instrumentation to ensure its safety and performance. (Done in a traditional way, this would have taken years.) Multiple V-22 components, built by Naval Air Warfare Center Aircraft Division (NAWCAD) in Lakehurst, NJ, and Pennsylvania State University Applied Research Laboratory, were tested at Patuxent River to validate performance. The final part was printed at the NAWCAD. Another Navy example is the NAVSEA AM cast for the PL8/9 tail piece on a Seawolf class submarine, which showcases the ability of AM to quickly produce casts for a submarine critical part.
The Army has printed functional armament such as an M-4 rifle, M203 grenade launcher, and 40-mm rounds [15, 16] (Figure 8 [16, 17]). The Army’s grenade launcher called “Rapid Additively Manufactured Ballistics Ordnance” (RAMBO) includes a standalone kit with printed adjustable buttstock, mounts, grips, and other modifications. More than 90% of the components in the prototype grenade launcher (Figure 8, top right) were printed with AM in just 35 hours and on a single build plate. The M781 components were 3-D-printed during a 6-month collaborative effort that involved the Research, Development and Engineering Command (RDECOM), ManTech, and America Makes. The Army also fielded a portable manufacturing lab dubbed the “Rapid Fabrication via Additive Manufacturing on the Battlefield” (R-FAB), which is linked to a database for 3-D printable files called the Repository of Additive Parts for Tactical and Operational Readiness . Additionally, some parts locally certified for experimental flights by the Army included a sensor fairing on the front of the Bell 407 helicopter and cooling ducts for the UH-60 Blackhawk.
With added functionality, the USAF demonstrated a direct-write Cu plasma antenna on an MQ-9 Reaper remotely piloted aircraft (Figure 9, top left [18–20]). Mesoscribe, established in 2002 as a spin-off company at Stony Brook University and the Long Island High Technology Incubator, commercialized the technology. In this case, by structurally integrating the electronics onto a servo cover, they demonstrated improvements in aerodynamic efficiency and reductions in susceptibility to damage compared with conventional blade/pod approaches. An additional benefit to the direct-write technology was relocating antennas to enable greater navigational precision. The U.S. Air Force Research Laboratory (AFRL) continues to look for ways to retrofit servo cover caps with conformal antennas to use Link 16, a military tactical data exchange network used by fourth-generation fighter jets such as F-15 Eagles and F-16 Fighting Falcons .
The future of AM materials for defense continues to grow and includes high-temperature polymer composites made with carbon fiber-infused polymer resin and selective laser sintering.
The future of AM materials for defense continues to grow and includes high-temperature polymer composites made with carbon fiber-infused polymer resin and selective laser sintering. These materials for extreme environments have potential use in engine components and on the leading and tail edges of next generation fighter jets (Figure 9, top right). Other materials considered for resilient hybrid electronics include silver inks (Figure 9, bottom) and high-temperature/chemically-resistant polymers such as poly ether ether ketone (PEEK) .
Additive manufacturing has gained a lot of attention for improving defense systems. In the DoD, AM is gaining momentum, as witnessed by the Service-specific AM implementation plans, growth in National Manufacturing Institutes, and an increasing number of parts and technologies that substantiate and raise the anticipation of revolutionary outcomes.