Additive manufacturing began with organic materials but gradually gained applicability for the manufacture of metallic materials when the technique was able to meet the following requirements:
1. Desired level of performance
2. Efficiency necessary for the manufacturing process 3. Endurance or sustainability
4. Energy savings 5. Cost savings
Material Deposition Head Laser Beam
Z Motion Hermetically Sealed Chamber
Deposition Substrate X-Y Table
fIgure 1.9 Key LENS components and their roles in the process.75
A few of the commercially viable additive manufacturing techniques that have been developed and eventually consolidated as a direct consequence of the numerous benefits they offer were discussed in the preceding section. Some of these techniques have been developed or adapted specifically for metallic materials as extensions of the techniques used for polymeric materials. The additive manu- facturing system for metallic materials can be categorized or classified in terms of
1. Material feed stock 2. Source of energy used 3. Build volume
The most common metallic materials used in additive manufacturing processes are summarized in Table 1.3, and Table 1.4 provides an overview of existing manufacturers and their specific equip- ment. The manufacturing systems can be divided into three broad categories:
tAble 1.3
selected Metals and Alloys used Commercially in Additive Manufacturing
titanium Aluminum tool steels super Alloys stainless steel refractory
Ti-6Al-4V Al-Si-Mg H13 IN625 316 and 316L MoRe
ELI titanium 6061 Cermets IN718 420 Ta-W
CP titanium — — Stellite 347 Co-Cr
Gamma-titanium aluminide — — — PH 17-4 Alumina
Source: Frazier, W.E., J. Mater. Eng. Perform., 23, 1917–1928, 2014.
tAble 1.4
representative Additive Manufacturing equipment, processes, and sources of energy
system equipment process energy source
Powder bed ARCAM Electron beam melting Electron beam
EOS Direct metal laser sintering Yb-fiber laser
Concept laser cusing Selective laser melting Fiber laser
MTT Selective laser melting Fiber laser
Phoenix System Group Selective laser melting Fiber laser
Renishaw Selective laser melting Laser
Realizer Selective laser melting Laser
Matsuura Selective laser melting Fiber laser
Powder feed Optomec (LENS) Laser engineered net shaping Fiber laser
POM DMD Direct metal deposition Disk laser
Accufusion laser consolidation Laser cutting Nd:YAG laser
Irepa laser Laser deposition Laser cladding
Trumpf Laser deposition —
Huffman Laser deposition CO2 cladding
Wire feed Sciaky Electron beam deposition Welding
MER plasma transferred arc Plasma-transferred arc selected free-form fabrication
Plasma transferred arc using two 350A DC power supplies Source: Frazier, W.E., J. Mater. Eng. Perform., 23, 1917–1928, 2014.
1. Powder bed fusion system 2. Powder feed system 3. Wire feed system
Overall, the source of energy (arc, electron beam, or laser beam) for each feed system is the key to governing operation of the system.
1.3.1 powdeR bed FuSion
The powder bed fusion technology was initiated from selective laser sintering (SLS) and has gradu- ally evolved into various techniques that have similar working principles but use different mecha- nisms to bind the powders and the layers. Methods based on a combination of laser beams and powder beds include the original selective laser sintering and the subsequent and preferentially used direct metal laser sintering (DMLS). Replacing or substituting the laser beams with electron beams resulted in the technique of electron beam melting (EBM). A synergism of an inkjet head and a powder bed system resulted in the emergence of 3D printing. A schematic of the generic powder bed system used in additive manufacturing is shown in Figure 1.10.
The key ingredients of this process consist of a (1) laser scanning system, (2) powder delivery system, (3) roller, and (4) fabricated piston. Prior to start or initiation of fabrication, the powder delivery piston moves up and the fabrication piston moves down by one layer thickness. The powder is spread and subsequently lightly compressed by a roller over the surface of the fabrication piston.
A laser beam is then driven over the bed so as to selectively melt the powder under the guidance of a scanner system. Upon completion of a layer the fabrication piston moves down another layer thick- ness and a new layer of powder is spread over it. The process is repeated until the entire part or a solid three-dimensional component is constructed. Upon completion, the fabrication piston moves up and elevates the final object or part. The excess powder, if any, is brushed away and can be reused following proper treatment.75,76 A distinct advantage of the powder bed fusion system is that support structures are not required. Quite often supports are added to provide thermal pathways to facilitate rapid dissipation of the heat and to concurrently exercise better control over the geometry of the part being produced. An overview of the direct metal laser sintering technique is provided in Figure 1.11.
Powder Bed Component
Chamber Scanner
Laser
Roller/Rake
Powder Delivery System
fIgure 1.10 The key components and their role in the powder bed system used in additive manufacturing of metallic materials.20
1.3.2 powdeR Feed SySteM
The build volume of this system is generally large. The powder feed system generally lends itself to building a larger scale volume compared to powder bed units. In this system, the metal powder particles are conveyed through a nozzle onto the build surface. A laser is then used to melt a mono- layer or more of the powder into the shape desired. The process is repeated to create a solid three- dimensional component. There are two variations of this system:
1. The work piece remains stationary while the deposition head moves.
2. The deposition head remains stationary while the work piece is moved.
The two most noticeable advantages of using this system are the following:
1. Large build volume
2. The ability to refurbish both worn and damaged components
An illustration of the powder feed system is provided in Figure 1.12. The use of lasers allows this technique to process a wide range of metallic alloys, including titanium, nickel-based superal- loys, stainless steels, and tool steels. These materials are commonly available in the powder form required by the process.76,77 Sustained research and development efforts have culminated in two major well-developed techniques:
1. Laser Engineered Net Shaping (LENS), which was initially developed at Sandia National Laboratories (Albuquerque, NM) in the 1990s
2. Direct metal deposition (DMD), which was developed by the POM Group, Inc. (Auburn Hills, MI)
The key difference between the two techniques lies in the details specific to machine control and implementation.78 Direct metal deposition allows for processing in an open atmosphere with local shielding provided to the molten metal. The metal powders used in this technique are both delivered and distributed using an inert gas carrier to shield the pool of molten metal from oxidation while allowing layer-to-layer adhesion to ensure a better overall wetting of the surface.79
Sealed Chamber Sealed Chamber
Powder Delivery Piston
Fabrication Piston Part
fIgure 1.11 Visualization of the various parts involved in direct metal laser sintering (DMLS).75
1.3.3 diRected eneRgy depoSition: wiRe-baSed Method
Directed energy deposition (DED) is another class of additive manufacturing technique. This tech- nique makes use of a solid wire feed stock instead of metal powders. A schematic of the wire feed system is shown in Figure 1.13. A key technique belonging to this class that has gained in both significance and approval for use is electron beam freeform fabrication (EBF3). The technique of electron beam freeform fabrication was developed in 1999 by Lockheed Martin (Bethesda, MD) and disclosed to the public in 2002.81 The technology was subsequently studied and improved upon at the NASA Langley Research Center (Hampton, VA) for the production of unitized structures using such aerospace materials as81
1. Alloys of aluminum 2. Alloys of titanium 3. Nickel-based superalloys 4. Titanium–aluminides 5. High-strength steel 6. Metal–matrix composites
Deposition
Head Additive
Manufacturing Deposit
Powder Supply Carrier Gas Beam Guidance System
Laser
fIgure 1.12 Key components of the powder feed system used for additive manufacturing of metallic materials.20
Electron Beam Gun
Electron Beam Wire Feed
Deposition Layers Substrate
fIgure 1.13 Simple line diagram illustrates the principle behind the wire feed system used for additive manufacturing of metallic materials.20
The technique of EBF3 is similar to laser engineered net shaping except that electron beams are used as the heat source. This technique is often performed in a vacuum environment (10–4 torr or lower) and incorporates a metal wire feed system to deliver feedstock to the molten pool.
The electron beam can be controlled and deflected very precisely and can synchronize well with highly reflective materials. Sustained research and development efforts have fine-tuned the pro- cess to give efficiencies as high as 100% in the consumption of wire feed stock and at least 95%
in the use of power.81 A schematic of the electron beam freeform fabrication technique is shown in Figure 1.14.
The National Aeronautics and Space Administration (NASA) has two types of EBF3 systems:
1. Ground-based—The ground-based system has a dual-wire feed system that can be loaded with either fine or coarse wire to achieve various feature definitions. Alternatively, two different alloys can be used to produce compositional gradients or components made from multiple materials.
2. Portable—The portable system has a single wire feeder and can be used for finer metal wire; it has high precision in positioning compared to the ground-based system, making it ideal for the fabrication of smaller parts having intricate detail.
NASA also has two portable EBF3 machines. The first machine has been flown on a microgravity research plane, and the second machine is being used for a spectrum of activities related to develop- ing potentially viable in-space manufacturing applications.