Underbody threats are an age-old problem for all military ground vehicles. During the Vietnam War it was estimated that 73 percent of all vehicle losses were suffered because of anti-personnel and anti-tank mines. While developments in maneuverability and protection systems have aided the warfighter to avoid such threats, the rapid replenishment of vehicle underbody inventory remains critical.
Former Army ManTech programs for underbody hulls focused on technologies such as forging and forming for consolidated hull structures and high-energy buried arc weld to minimize weld porosity. The focus was on fabrication methods for thicker underbody hulls of vehicles that required fewer welded joints during manufacturing, directly increasing the durability of the hull against underbody attacks. They were able to demonstrate successful fabrication and material quality, but restricted geometrical design freedom and agility in manufacturing. A truly versatile technology enables exploration of new design concepts, rapid implementation, and flexibility to accommodate multiple vehicle platforms. Forging, forming, and welding were not well matched for vehicle program needs.
Additive manufacturing (AM) technologies present new opportunities in material processing and flexible fabrication that provide a compelling avenue for producing new vehicle hull concepts.
An additive manufacturing method can be generally described as a freeform fabrication system that creates a part by digitally controlling a material processing unit to build the part one layer at a time in the designed shape. An additive manufacturing process does not require additional tooling such as molds and dies. In contrast to additive manufacturing, subtractive manufacturing starts with a bulk of material (block, plate, bar, etc.) and selectively removes the unneeded material until the shape of the designed part remains.
AM system capabilities for creating large metallic components—such as whole vehicle hulls--are still in their infancy. Some metal AM capabilities have been demonstrated that have dimensional restrictions, large in one plane or axis. Commercially available off the shelf systems for metal AM technologies generally have maximum build volumes on the order of 1 x 1 x 1 m and can enable many application opportunities. Large area additive manufacturing (AM) systems (>1 m3) have been demonstrated in recent years, most prominently for processing polymeric materials and therefore limited mainly to tooling applications. Scaling the available technology and machine capabilities to larger platforms to enable a versatile capability to access a broad range of large-scale applications, especially in metal, is needed.
ASTRO America, ALMII, and US Army DEVCOM-GVSC collaborated in a sponsored program to produce a large-scale metal AM system for the US Army’s Rock Island Arsenal to make large-scale metallic parts common in Army ground vehicles. The vision of the program is to combine large format machine tools, historically proven in various industries, and additive manufacturing technologies to reduce production lead times. The resulting capability will enable metal additive manufacturing on a scale that has not been widely demonstrated or available, not only for ground vehicle systems but across all large-scale applications. Enabling the printing of such large metal parts will open new opportunities for applications as well as expanding the uses for additive manufacturing processes.
The current project will manufacture and deliver two separate machines. The first machine will have the capability to print and machine a part up to 1 x 1 x 1 m in size and as a key milestone demonstrator, parts of that build volume (Figure 1a and 2a). This machine will be used to conduct required process development, print strategies, and print path planning needed for building parts up to its size limit, as well as for parts that will be built on the second larger machine. The second larger machine will have the capability to print and machine parts within its build volume of 10 x 6.5 x 4 m (length x width x height) (Figure 1b and 2b). Figure 1b shows a three-dimensional rendering of the second, full-scale system that will be produced at the end of this program. To date, the system will be the largest metal AM system that has been publicly released.
The team selected for the project to design, construct, and develop the machines is a collaborative effort consisting of a system integrator, industrial hardware and software developer, and metal AM process developer. The combination of this expertise allows this team to create a capability not seen before.
The team members collaborating on this project include:
Ingersoll Machine Tools, Rockford, Ill.: Machine tool builder that has extensive experience in building robotics systems for the composites processing in the aerospace industry (i.e. “extra-large” components). They use off the shelf (namely Siemens) components and software but build gantry systems and do system integration. The have recently adapted products for printing large scale, 23' long (7 m), composites tooling.
Siemens Industry, Inc., Elk Grove Village, Ill.: The Siemens team selected to participate in this program focuses on multi-axis, CNC-based applications and process simulations, therein. They intent is for the large-scale AM system to utilize commercially available hardware and software products and begin building process planning routines that are unique to the system.
MELD Manufacturing Corp., Christiansburg, Va.: The developer of the MELD process (also referred to as additive friction stir deposition), MELD Manufacturing Corp. is producing and delivering the deposition systems that will enable the additive component of these large metal AM machines. Their patented processing technology utilizes wrought feedstock material in a solid-state process that relies on severe plastic deformation to deposit the material at the point of deposition.
All commercially available metal additive technologies were considered for the project. However, there is a small subset that are well suited and capable to be scaled to produce components within the scope of the project’s size requirements. The configuration chosen for the large-scale machine is a traversing gantry machine with the metal printing and machining tool directly mounted on one axis (Z-direction) of the machine. This configuration is conducive to handling large, (heavy) metallic components and allows for flexible manipulation of manufacturing methods, including integration of multiple manufacturing technologies, in this case, the addition of a machining capability.
Printing large parts requires a large amount of material and handling of the feedstock for the process is another major consideration. Many popular metal additive manufacturing technologies make use of metallic powders as the feedstock. While powder-fed (or blown powder) directed energy deposition systems have demonstrated the ability to scale to some larger sizes, technologies based on powder beds have not. While development of these technologies for larger scale systems is ongoing, the inherent challenges related to handling and processing metal powders will remain and will be magnified as systems scale to larger sizes.
In additive manufacturing the joining of material from one layer to the next is a critical aspect of the process and technology. The joining of material is one of the fundamental differences between additive and subtractive manufacturing processes. In many metal additive manufacturing processes the joining of material is a melt-based process, requiring the feedstock material and material from the previous layer to be heated beyond the melt temperature to enable joining via melting and subsequent rapid solidification of the deposited metal. With careful control of the energy imparted onto the material during the process it is possible to accurately (with lasers or electron beams) or quickly (with wire or powder fed) print material and build parts.
However, the process of melt-based joining can present challenges as size scale increases. When fabricating large parts, issues with residual stress resulting from the repetitive melting and solidification of material can present challenges with maintaining geometrical tolerances in printing a part. As the size scale of parts increases the residual stress issue is exacerbated because of the cumulative effects as the part becomes larger and larger.
Lower temperature AM methods were considered to help reduce stress and stress build-up created in the process. MELD Manufacturing’s unique solid-state friction stir based additive process was selected as the most promising to meet the size and material needs of our large-scale system. The technique consists of a hollow, rotating shoulder in which material is fed through and deposited onto a substrate. The rotation and pressure from the tool head onto the feedstock results in plastic deformation and flow of material that deposits it onto a substrate or previously deposited layer.
Like other deposition-based AM techniques, the deposition head is traversed around the build area of the machine, typically depositing a single track of material on the order of 1 mm thick and 38 mm wide. As the MELD process is a solid-state process, the material is maintained below its melting temperature, avoiding repetitive melting and solidification of material, and minimizing extreme thermal gradients and excessive residual stress. MELD’s solid-state process enables depositing material while maintaining temperatures at 60-90 percent of the material melt temperature. Additionally, the solid-state process avoids material cracking resulting from stress accumulation and can reduce (or prevent) the formation of voids (i.e. porosity) that can be observed with melt-based AM processes. Because the MELD fabricated parts are printed fully dense, they don’t require secondary processing to remove any volumetric defects such as voids or porosity. Furthermore, it has been reported that the MELD process produces a fine equiaxed grain structure resulting in better material performance in the “as-printed” state, as opposed to melt-based additive technologies or cast components which exhibit larger directionally solidified grains and lower material performance in the “as-printed/as-cast” state.
The lower material processing temperature of MELD’s technology has highlighted some advantages over more traditional manufacturing methods and other large scale AM systems. For example, since the material is printed at temperatures below the material’s melting temperature, the system itself does not require a specialized chamber (i.e. vacuum chamber, inert gas environment, advanced air handling). The machine operates and prints parts in an open, warehouse-type environments typical in manufacturing. Not being restricted or confined by a machine chamber and the related operation requirements also allows for flexbility and reconfigurability of the process, and also requires less time for production preparation needed for a given component. The MELD process is material agnostic, which means we can investigate future materials beyond what may be the current state of the art. Next generation materials not yet developed could offer even better performance.
To enable metal additive manufacturing, at scales large enough to sustain Army ground vehicles, many challenges and approaches must be considered and addressed by the system being developed in this program. The goal is to create a system that can rapidly fabricate large metallic structures for a wide range of applications requiring the properties of metallics materials. After considering challenges and limitations of metal AM processes with potential to scale to larger sizes, it was determined that a solid state process was best to enable to the intended size scale. As part of the project’s machine development, manufacture materials samples will be tested and analyzed to optimize the system’s processing conditions and characterize end use properties. Ultimately, the goal will be on producing a flexible manufacturing system that extends the manufacturing capability to address many needs including sustainment, readiness, and advances in design and performance of vehicle scale parts.
Note: The authors acknowledge The American Lightweight Materials Manufacturing Innovation Institute (ALMII), Detroit, MI, for their project management contributions to the project.
DISTRIBUTION A. Approved for public release; distribution unlimited. OPSEC#6283
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