Metal parts can be fabricated by the SLS process either by partially melting the metal particles [ 41] or by melting the low-melting-point binder to bond the metal particles together [ 85, 86]. The binders used can be polymer, such as phenolic polymer, or low-melting-point metal, such as SnS. The metal parts fabricated using these processes require post-processing, including removal of the polymer binder, thermal sintering and liquid-metal infiltration (if needed), to achieve a fully dense part. For example, a nearly fully dense Ti6Al4V part can be fabricated using the SLS process and Hot Isostatic Pressing (HIP) process, in which the SLS laser beam fuses the boundaries of the metal particles followed up by the HIP process [ 40]. Metal parts also can be built indirectly using other non-melting methods, such as 3DP, SLA and LOM. In the 3DP process, a liquid binder is sprayed onto the surface of a metal powder bed and used to bond metal particles. SLA uses UV light to cure the suspension made by mixing small metal particles into a liquid photo-curable resin. Post processing has to be performed in order to achieve desired properties for these processes. For example, a 420 stainless steel tool was built using 3DP, and then the binder was removed and the tool infiltrated with Cu-10 Sn bronze [ 87]. Metal green parts in stainless steel 316L and 17-4 PH were fabricated via SLA by using a suspension with photo-curable resin and metal particles [ 88]. Metal parts also have been fabricated using the LOM process by joining metal sheets in a layer-by-layer fashion. As a critical step in the process, layer joining determines the strength of the part in the direction perpendicular to the layers, which can be joined by layer diffusion welding, soldering, and adhesives.

The direct method includes an AM process, such as SLM, EBM, or LMD/LENS, which uses a high-power laser/electron beam as the heating source. The bonding mechanism of these processes is full melting [ 41, 43]. A metal powder bed is used in SLM and EBM, and metal particles are continuously fed into the melt pool created by the high-power laser beam in LMD/LENS. Fully dense metal parts that have nearly the same or even better mechanical properties as the bulk metal can be produced. Table 3 lists the mechanical properties of the metal materials from these processes as well as the reference values for comparison. Stainless steels (304, 316, 410, 420, 17-4PH), tool steels (H13), nickel alloys (IN617, 625, 718), cobalt alloys (#6 Stellite, #21 Stellite), titanium alloys (Ti6Al4V, Ti-6-2-4-2), and a variety of hardfacing or cladding alloys have been processed successfully with LENS [ 89, 90] and SLM [ 91] by companies including Optomec, EOS, etc. and research institutes. Titanium alloys (e.g., Ti6Al4V, Ti6Al4V ELI) and the CoCr alloy have been qualified for use in the EBM process by Arcam [ 92]. Other materials, such as the nickel-based superalloys IN718 and 625, H13 steel, Stainless steels 316L and 17-4PH, and Aluminum alloys, have also been researched and developed. For example, the microstructure and mechanical properties of IN718 fabricated using EBM were investigated by Strondl et al. [ 93]. H13 steel parts were produced using EBM by Cormier et al. [ 50]. NiTi shape memory alloy was processed using EBM by Otubo and Antunes [ 94]. Figures 3 and 4 show some of the parts fabricated using these processes, including thin wall parts such as turbine blades and complex structures such as diamond lattice structure.

Tab.3

Tab.3

Tab.3 Mechanical properties of materials processed by laser or electron beam based full-melting processes Material Process Ultimate tensile strength/MPa Yield tensile strength/MPa Elongation/% Elastic modulus/GPa Source Ti6Al4V Reference (wrought) 951 883 14 110 - EBM 1020 950 14 120 Arcam LENS 1077 973 11 - Optomec LMD 1160 1060 6 115 [199] SLM ~1100 ~1000 ~8 ~120 EOS SLS+HIP 1116.9 - 5 - [40] 316SS Reference (wrought) 579 290 50 - - LENS 655 278 66.5 - Optomec LMD 579 296 41 - [58] IN718 Reference (rolled sheet) 1407 1172 21 - - LENS 1393 1117 15.8 - Optomec EBM 1238±22 1154±46 7 - [93] IN625 LENS 938 584 38 - Optomec LMD 745-800 480-520 31-48 - [199] 17-4SS SLM 1050±50 540±50 25±5 170±20 EOS Co-Cr alloy EBM 960 560 20 - Arcam Co-Cr-Mo alloy SLM 1400±50 960±50 9-13 210±10 EOS

Tab.3 Mechanical properties of materials processed by laser or electron beam based full-melting processes

Fig.3

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	Fig.3 (a) Example metal parts fabricated using LENS (Source: Optomec [ 133]); (b) fine grid structure for use in the medical field (material: Cobalt chrome alloy) fabricated using SLM (Source: Concept Laser [ 134])

Fig.4

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	Fig.4 Titanium 3D-micro-framework-structure based on a diamond lattice fabricated using EBM (Source: [ 51])

Metal parts also can be produced using rapid casting by combining AM produced patterns, or casing shells and cores, and subsequently casting with molten metal, such as in investment casting and sand casting [ 84]. These molds (shells or cores), usually in ceramic/sand, can be fabricated using the same processes such as SLS, 3DP [ 95] and SLA, that are used to produce ceramic parts. For example, Si and Zr sands provided by 3D Systems and EOS can be used to produce molds and cores with the SLS process for metal casting. Ceramic molds for investment casting of turbine airfoils were fabricated by Das et al. [ 96, 97] via the Large Area Maskless Photopolymerization (LAMP) process, in which parts were built by curing the suspensions of ceramic powders in monomer solutions using UV light. Casting patterns built using AM processes were also applied to investment casting, such as polymer patterns via SLA, wax patterns via FDM, paper patterns via LOM, polymer patterns via 3DP and SLS. Figure 5 shows a metal cast and the corresponding shells and core made by Z Corporation using 3DP [ 98].

Fig.5

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	Fig.5 A cast metal part and the corresponding shells and core made by Z Corporation using 3DP (Source: [ 98])

Industrial ceramics (e.g., Si₃N₄, Al₂O₃, SiO₂, ZrB₂), advanced ceramics (e.g., lead zirconate titanate (PZT)) and biocompatible ceramics (e.g., hydroxyapatite) have been investigated to fabricate porous and dense parts using AM processes such as FDM, SLS, 3DP and SLA. These processes typically create a ceramic green body with a high content of organic or inorganic binders. Then, binder burnout and densification of the green body are conducted in a conventional sintering step. Several examples are discussed below.

Various functional components made of advanced ceramics [ 99] (i.e., alumina structures with photonic bandgap properties, bismuth titanate, and piezoelectric actuators) and structural parts in Si₃N₄, SiO₂ [ 100, 101] have been fabricated by using ceramic loaded polymer filaments in the FDM process. For the ceramic FDM ceramic process, also called fused deposition of ceramics (FDC), the green part is built by a hot extrusion process in which a ceramic particle loaded thermoplastic filament is extruded through a small nozzle and then subjected to conventional binder removal and sintering processes to produce fully dense components.

Fully dense parts in Ti₃SiC₂, a new class of ceramics with unique electrical and mechanical properties, were fabricated using 3DP by spraying a liquid binder onto the powder bed, followed by cold isostatic pressing and sintering [ 102]. A ZrB₂ part (fuel injector strut for aircraft engine) [ 103], alumina and silica cores and shells for investment casting (Fig. 6) [ 104], graphite bipolar plates for fuel cells [ 105], and bio-ceramic bone scaffolds [ 106] were fabricated using SLS by laser scanning the mixture of ceramic powder and binder and then removing the binder and sintering the parts in a furnace. Ceramic parts also have been produced by the SLA process, in which ceramic green bodies are created by laser scanning a ceramic suspension consisting of ceramic powder (i.e., silica, alumina, silicon nitride and PZT) dispersed within a photo-curable resin [ 107, 108, 109].

Fig.6

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	Fig.6 Alumina and silica ceramic cores produced using SLS for investment casting of turbine blades and other ceramic parts (Source: Phenix Systems [ 104])

Direct fabrication of ceramic parts using AM processes is much more challenging due to the high melting temperatures of ceramics such as Al₂O₃ (>2000℃) and SiO₂ (>1700℃), and also the large thermal gradients, thermal stresses and residual stresses associated with melting/re-solidifying in the laser based AM processes. Research attempts were carried out to directly manufacture fully dense ceramic components using AM processes. For example, the SLM process was investigated to fabricate ceramic parts from a mixture of zirconia and alumina by completely melting the ceramic powder [ 110]. The ceramic powder bed was preheated to a temperature higher than 1600℃ to reduce thermal stresses, and nearly fully dense, crack-free parts were obtained without any post-processing. Fully dense, net-shaped, alumina parts were produced using LENS by direct laser melting of the ceramic powder [ 111]. The as-processed structures show anisotropy in mechanical properties with a high compressive strength normal to the build direction and columnar grains along the build direction.

Uniform composites fabricated using AM processes are usually done by employing a pre-prepared mixture of proper materials, such as a mixed powder bed for SLS, SLM and 3DP, a filament in mixed materials for FDM, a composite laminate for LOM, or a mixture of liquid photo-curable resin with particulates for SLA. The composite materials that can be produced with AM technology include a polymer matrix, ceramic matrix, metal matrix, and fiber and particulate reinforced composites [ 112]. One of the most important uniform composite families for industrial application is fiber-reinforced composite. The AM processes can be used to produce fiber-based composites include FDM and LOM. AM processes, such as SLS and 3DP, are not ideal for working with fiber-based composites because making a smooth layer of the powder-fiber mixture is difficult in these processes. Incorporating the use of long or continuous fibers rather than short fibers into AM processes is difficult and has been limited to LOM [ 113]. FDM and LOM require the fabrication of respective fiber-reinforced composite filament tapes and laminates as a pre-step before AM processing. Several examples will be discussed later.

Metal-metal composites (e.g., Fe-Cu and stainless steel-Cu), metal-ceramic composites (e.g., WC-Cu, WC-Co, WC-CuFeCo, TiC-Ni/Co/Mo, ZeB₂-Cu, and TiB₂-Ni), and ceramic-ceramic composites (e.g., Si-SiC) have been processed by SLS/SLM [ 112]. These processed composites can be classified into two categories: those that aim to facilitate the process using a liquid-phase sintering mechanism, and those that combine various materials to achieve properties not possible with a single material. Examples of composites in the first category include Fe-Cu and stainless steel-Cu used in SLS, in which Cu acts as a binder to bond Fe or stainless steel particles rather than a reinforcement phase to enhance the mechanical or other properties of the final product. An example of the second category is the bio-composite poly-epsilon-caprolactone and hydroxyapatite (PCL/HA) bone scaffold fabricated using SLS, with the addition of HA to enhance the strength and biocompatibility of PCL [ 114, 115]. In terms of fabrication methods, SLS of composites can be achieved by varying the composition of the powder mixture, using in situ chemical reactions or conducting post-processing procedures (e.g., infiltration). Various polymer matrix composites, metal matrix composites, and short fiber-reinforced composites have been processed by using this method. Composites also have been manufactured through laser-induced chemical reactions that in situ create chemical compounds. One example is the fabrication of a Cu-based metal matrix composite reinforced with TiB₂ and TiC from a powder mixture of Cu, Ti and B₄C [ 116]. Conducting post-processing on laser-sintered materials is another way to manufacture composites (e.g., using a furnace for chemical reactions and infiltration). A common example of a composite produced using this method is the Si/SiC composite. A laser-sintered SiC preform is infiltrated at room temperature with phenolic resin, which converts to carbon in a furnace and reacts with the infiltrated Si to form SiC, thus producing the Si/SiC composite [ 117, 118].

In addition to SLS, 3DP also can be used to make composites, either by changing the component of the powder mixture or by infiltrating porous 3DP preforms with metal or alloy. An example of the former method is hyroxyapatite/apatite-wollastonite glass ceramic composite in situ fabricated using the proper powder mixture to improve the strength of bone scaffold [ 119]. An example of the latter method is dense TiC/Ti-Cu composites fabricated by infiltrating TiCu alloy into a porous carbon preform produced using 3DP [ 120].

By developing a feedstock filament with the proper composite, polymer-metal and polymer-ceramic composites could be produced with FDM. ABS-Iron composites have been made using FDM with a single-screw extruder by appropriately producing an iron particulate-filled polymeric filament [ 121]. Fibers, such as short glass fibers [ 122] and nanofibers (vapor-grown carbon fibers) [ 123], have been added into ABS filaments to improve the mechanical properties of the parts built using FDM. The fabrication of composites using LOM depends directly on the development of composite laminates, such as fiber or particulate reinforced sheets. Polymer matrix and ceramic matrix composites have been made with curved LOM by laying-up and shaping composite laminates from prepreg feedstocks, followed by vacuum bag/oven curing and consolidation [ 124, 125].

AM processes that can deliver different materials (usually through multiple feeding systems) to the building areas have the ability to build components with FGM, which is one of the primary advantages of AM technology that conventional methods cannot realize. This ability offers the flexibility to control the composition and optimize the properties of the built part. One example is a pulley that contains more carbide near the hub and rim to make it harder and more wear resistant, and less carbide in other areas to increase compliance [ 126]. Another example is a missile nose cone with an ultra-high temperature ceramic graded to a refractory metal from outside to inside in order to sustain extreme external temperatures while attaching easily to the metallic missile nose.

As a powder deposition process, LMD/LENS has the ability to vary the degree of material composition, leading to FGM by feeding different material powders from multiple nozzles. Porous Ti6Al4V implants with functionally graded Co-Cr-Mo coating have been produced using LENS by Bandyopadhyay et al. [ 127, 128]. These implants exhibited a high degree of hardness with an excellent interaction between the bone cell and other materials. The microstructure is shown in Fig. 7. The composition gradient was achieved by gradually increasing the feed rate of the Co-Cr-Mo alloy and accordingly decreasing the feed rate of the Ti6Al4V alloy powder. The graded structures exhibited good bonding between individual layers, avoiding the issue of cracking that will occur if 100% Co-Cr-Mo is transitioned from 100% Ti6Al4V. A functionally graded TiC/Ti composite [ 129] and a compositionally graded Ti-TiO₂ structure [ 57] were fabricated with LENS by employing different powders carried by non-reactive gases through different nozzles. Graded nickel-titanium components were built from Ti6Al4V to IN 718 using LENS by Domack and Baughman [ 130]. One process developed by Wang et al. [ 131] went beyond feeding different powders. They combined powder and wire for LMD, which allowed for multi-material fabrication without the mixing and waste of blown, unused feedstock powders.

Fig.7

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	Fig.7 Typical microstructure and Co distribution of LENS processed Co-Cr-Mo graded coating on porous Ti6Al4V alloy (Source: [ 127])

Several ceramic actuators and sensors with novel properties have been fabricated using a variant FDM process developed by Jafari et al. [ 30]. The modified system has multiple deposition units and the ability to deposit up to four different types of materials in any given layer. The Freeze-form Extrusion Fabrication (FEF) process can produce FGM parts by employing multiple extruders with different materials in different extruders. Material compositions are varied by controlling the extrusion speed of each extruder. Figure 8(a) shows the triple-extruder FEF equipment developed at Missouri S&T [ 132], and the fabricated FGM part with a gradient from 100% Al₂O₃ to 50% Al₂O₃ and 50% ZrO₂ is shown in Fig. 8(b).

Fig.8

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	Fig.8 (a) Triple-extruder FEF system; (b) FGM part with a gradient from 100% Al2O3 to 50% Al2O3 and 50% ZrO2 fabricated using the triple-extruder FEF process (Source: [ 132])

Optomec [ 89, 133] used the LENS process to fabricate complex components for satellites, helicopters and jet engines. An example is a 1/6 scale mixing nozzle for gas turbine exhaust for Bell helicopter, as shown in Fig. 9(a). Arcam [ 92] applied its EBM system to produce functional parts for end users. Some of these applications included commercial and military aircraft, space applications, missiles and various subsystems (e.g., engines and accessories) which use light-weight materials such as titanium alloys. For example, an EBM-produced compressor support case for a gas turbine engine using Ti6Al4V is shown in Fig. 9(b). The turbine blades, which are typical thin-wall parts with complex channels inside, were produced using SLM from Inconel 718 and cobalt chrome alloy by Concept Laser [ 134] and Morris Technologies [ 135], respectively, as shown in Figs. 9(c) and 9(d). A hollow static turbine blade in stainless steel (Fig. 9(e)) was cast using a ceramic mold and cores fabricated using 3DP by Prometal [ 136]. An engine housing (Fig. 9(f)) was produced using SLM by Concept Laser [ 134]. Also, AM built plastic parts, such as vents and ducts, have been used in aerospace industry. Meanwhile, polymers that are flame retardant, such as PEEK, have been developed for AM processes to meet the aerospace requirements.

Fig.9

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Fig.9 (a) Mixing nozzle for gas turbine exhaust produced by LENS (Source: Optomec [ 133]); (b) compressor support case for gas turbine engine produced by EBM (Source: Arcam [ 92]); (c) turbine blade with internal cooling channels produced by SLM (Source: Concept Laser [ 134]); (d) turbine blades fabricated by SLM (Source: Morris Technologies [ 135]); (e) hollow static turbine blade cast using the mold and cores fabricated by 3DP (Source: Prometal [ 136]); (f) engine housing produced by SLM (Source: Concept Laser [ 134])

In addition to directly manufacturing functional parts for aerospace applications, AM techniques also are used to repair aircraft engine parts in order to reduce the cost and extend the lifetime of such parts as compressors, turbine and combustor castings, housing parts, and blades [ 89]. Optomec has demonstrated that LENS can successfully repair parts used in gas turbine engines such as vanes, stators, seals and rotors, and even geometrically complex parts such as airfoils, blisks, ducts and diffusers [ 90, 133, 137]. Fraunhofer ILT (Germany) [ 56, 138] has been successfully certified by Rolls-Royce Deutschland for 15 different repair applications using LMD, including repair of high-pressure turbine case and compressor front drum. A blisk repaired by Optomec is shown in Fig. 10.

Fig.10

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	Fig.10 Damaged blisk repaired using LENS (Source: Optomec [ 133])

Xue and Islam (National Research Council Canada) [ 139] investigated LMD with various materials, such as IN 625, IN 738, Ti6Al4V, and Fe-based tool steel, showing that LMD-processed materials have mechanical properties comparable to and sometimes better than conventionally cast or wrought materials. Some example parts (e.g., airfoils, shown in Fig. 11) for aerospace application were given. Besides fabricating parts, researchers also have applied LMD/LENS techniques to repair expensive and complex parts for aerospace applications. Xue and Islam [ 139] demonstrated repair of the tips and seal of a damaged IN 738 blade. Richter et al. [ 140] applied LMD to repair a Ti6-2-4-2 blade and studied the microstructure, hardness, residual stress, mechanical properties, and fatigue of the repaired parts. A geometry-based adaptive toolpath LDM process was developed by Qi et al. [ 141] to improve the geometric accuracy of the repaired part such as an airfoil, which has a wall thickness usually varied from sub-millimeters at the edges to several millimeters in the middle of the cross-section.

Fig.11

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	Fig.11 (a) Airfoil (material: IN 738) produced by LMD on cast IN 738 substrate; (b) airfoil with embedded cooling channels (material: Ti6Al4V) produced by LMD (Source: [ 139])

A hybrid process combining multi-axis laser deposition and CNC machining was developed by Liou et al. [ 142, 143]. By rotating the building part with a multi-axis system, the process can create overhanging features without support structures. The hybrid process provides greater build capabilities, accuracy and surface quality and has been applied successfully to the building and repair of functional metallic parts. An example of a repaired die core is shown in Fig. 12 [ 144]. Other AM processes, such as SLA, FDM and 3DP, can fabricate metal parts (e.g., turbine blades) for aerospace applications by building casting patterns for investment casting. Integrally cored ceramic molds for investment casting of turbine blades have been fabricated using ceramic stereolithography [ 145], and as well as by gelcasting ceramic slurry into plastic molds made from SLA patterns [ 146, 147].

Fig.12

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	Fig.12 A die core repaired using an LDM based hybrid rapid manufacturing system: (a) before the repair, showing the top of the core damaged and the surrounding surface worn; (b) after deposition, showing the portion requiring repair covered with new material; (c) after surfacing machining, showing the repaired core. (Source: [ 144])

Aside from the direct benefits of AM processes, special structures such as porous mesh arrays and open cellular foams can be produced by varying their density and stiffness to provide unique energy efficiency and excellent corrosion resistance and to impact on the absorption features, thermal management, and stiffness and strength of the sandwich cores. Numerous potential applications in aerospace, aeronautics and automotive systems can benefit from this density-compensated strength and stiffness. Figure 13 shows a Ti6Al4V open cellular foam fabricated using EBM from a CAD model based on CT scans of common aluminum alloy foam [ 148, 149]. The strength of these foams can be as much as 40% higher than that of fully dense EBM-fabricated components.

Fig.13

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	Fig.13 Example of an enlarged Ti6Al4V open cellular foam prototype fabricated using EBM (Source: [ 148])

Another aerospace application of AM is the building of wind tunnel testing models for aircraft, missiles, airfoils, etc. to study the aerodynamic characteristics of the designs. AM techniques reduce the time and cost associated with manufacturing these models, which usually have complicated geometries. For example, Daneshmand et al. [ 150] used SLS to build a wing-body-tail launch vehicle configuration model with glass-reinforced Nylon (Fig. 14). The aerodynamic data obtained from the SLS built model agreed well with the data from the physical model produced by CNC machining.

Fig.14

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	Fig.14 Wing-body-tail launch vehicle configuration model for wind tunnel testing produced by SLS using glass-reinforced Nylon (Source: [ 150])

In addition to metallic components, the aerospace industry also places great demands on ceramic parts, especially those made of ultra high temperature ceramics (UHTC) such as ZrB₂ and ZrC, which have an excellent ability to withstand extremely high thermal (>2000℃) and tough chemical environments. Such parts can be applied in, for example, hypersonic flight systems and rocket propulsion systems. It is difficult to fabricate geometrically complex UHTC parts using traditional manufacturing techniques such as drilling and milling operations because of the extremely brittle nature of ceramics. AM technology provides a promising way to make 3D UHTC parts that are difficult to make by conventional means. For example, a fuel injector strut with crossing channels inside that potentially could be used for a hypersonic aircraft engine was fabricated by SLS using ZrB₂ to provide resistance to extremely high temperatures [ 103]. Also, scaled-down versions of missile nose cones made of Al₂O₃ and ZrB₂ were fabricated using FEF [ 36, 37, 38], which extrudes aqueous ceramic pastes layer by layer below the water freezing temperature (Fig. 15(a)). Green parts fabricated using the FEF process first undergo freeze-drying followed by binder burnout and finally sintering to produce dense ceramic components, as shown in Figs. 15(b) and 15(c). The FEF process also can be extended beyond the fabrication of monolithic ceramics to the production of FGM parts by employing a triple-extruder mechanism, as shown in Fig. 8. Possible applications of FGM parts include fabrication of UHTC-refractory metal parts consisting of a UHTC graded to a refractory metal for the leading edges of hypersonic vehicles, nose cones of missiles, and nozzle throats of spacecraft propulsion engines, which are required to sustain extremely high temperatures and thermal gradients for future aerospace systems.

Fig.15

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	Fig.15 (a) FEF system developed at Missouri S&T. Sintered ceramic parts fabricated using the FEF process: (b) Al2O3 nose cones; (c) ZrB2 nose cones (Source: [ 36, 37])

CRP Technology (Italy) [ 151] has successfully applied AM techniques including SLS, SLM and EBM to develop and produce various components for motorsports. The produced parts include F1 gearboxes (titanium), MotoGP 250R air boxes, motorbike dashboards and supports, camshaft covers for MotoGP engines, reed valves, F1 suspension systems, etc. Significant advantages have been realized by applying AM technology. For example, the F1 gearbox produced using these new design and fabrication techniques saves 20%-25% weight and approximately 20% volume, and it has twice in torsion stiffness, less gear wear, and less power absorption. Figure 16(a) shows a titanium upright (which transmits the engine’s rotational power to wheels) made for the Minardi F1 team by CRP Technology via rapid casing based on polystyrene patterns made by SLS. Optomec [ 133] produced Ti6Al4V components including suspension mounting brackets (Fig. 16(b)) and drive shaft spiders for the Red Bull Racing car using LENS, resulting in a>90% material reduction, as well as significantly reduced time and cost. Arcam [ 92] applied EBM using Ti6Al4V to produce parts such as gearboxes (Fig. 16(c)), suspension parts and engine parts with lattice structures for race cars. Using SLM, Concept Laser [ 134] produced many steel and aluminum components for cars, including wheel suspensions, oil pump housings, engine blocks, exhaust manifolds and valve blocks; two examples are shown in Figs. 16(d) and 16(e). Prometal [ 136] successfully applied its Prometal process (a rapid casting technique based on 3DP) to manufacture engine components, such as cylinder heads, intake manifolds, and engine blocks, for the development of passenger car engines and production of race engines. This process significantly reduces the development time for car engines; for example, an intricate shape engine block that includes cooling passages and oil recirculation lines (Fig. 16(f)) can be produced completely in only one week.

Fig.16

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Fig.16 (a) F1 upright (right) cast via rapid casting process using polystyrene patterns produced by SLS (left) (Source: CRP Technology [ 151]); (b) suspension mounting bracket for Red Bull Racing produced by LENS (Source: Optomec [ 133]); (c) race car gear box produced by EBM (Source: Arcam [ 92]); (d) exhaust manifold produced by SLM (Source: Concept Laser [ 134]); (e) oil pump housing produced by SLM (Source: Concept Laser [ 134]); (f) engine block cast using the mold and cores fabricated by 3DP (Source: Prometal [ 136])

Universities and research institutes also have investigated AM technology for automotive applications. For example, Vilaro et al. [ 152] fabricated a water pump for motorsports cars (Fig. 17(a)) by SLM using aluminum alloy (AlSi10Mg). Their experimental results showed that the produced parts have mechanical properties equivalent to conventional heat-treated AlSi10Mg.

In addition to the direct manufacturing of functional parts, rapid tooling [ 153] has been studied widely and applied in automotive industries for quite a few years. Much research has demonstrated the flexibility of AM to fabricate complicated components for the automobile industry by using casting patterns, or molds and cores built by SLS, FDM, SLA, 3DP and LOM. For example, one part used in the automotive industry (Fig. 17(b)) was produced by investment casting starting from 3D-printed starch patterns and 3D-printed molds [ 154].

Fig.17

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	Fig.17 (a) Water pump for a motorsports car produced by SLM (Source: [ 152]); (b) automotive part produced by investment casting with 3D-printed starch patterns and molds (Source: [ 154])

AM technology also offers a fast way to make parts with previously unattainable properties in order to improve fuel efficiency and engine life. For example, an engine part with a lattice structure may reduce the engine’s weight while enhance its stiffness (Fig. 18), and metal matrix composites may provide extraordinarily durable, low-inertia valves. A titanium aluminide alloy with low density and high specific strength (ratio of elastic modulus vs. density) and stiffness (ratio of yield strength vs. density) was investigated using EBM for its potential to fabricate automotive engine components (e.g., engine exhaust valves and pistons) [ 155]. Ti6Al4V open cellular foams fabricated using EBM demonstrated high potential for novel applications in automotive systems due to their light weight and exceptional mechanical properties [ 148]. An intake system for a 600cc formula automotive engine was designed to minimize pressure losses and maintain an equal charge for each cylinder supply; it was manufactured using a combination of FDM and subsequent lamination of a carbon-fiber composite material [ 156], as shown in Fig. 19.

Fig.18

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	Fig.18 Engine part with lattice structure fabricated by EBM using Ti6A14V to reduce engine weight while enhance stiffness (Source: Arcam [ 92])

Fig.19

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	Fig.19 (a) Final assembly of an intake manifold fabricated by FDM; (b) completed intake system after a composite layup process and final assembly of sensors and mounts (Source: [ 156])

Many companies, research institutes and universities are exploring ways to apply AM processes to manufacturing of medical implants. The application of AM in orthopedic and dental implants benefits significantly from the ability of AM technology to manufacture complex geometries and structures, to make rough, engineered surface for more effective bone integration, and to allow implants to be personalized to match each patient’s individual needs. Arcam [ 92] has applied EBM to manufacture a wide range of implant types such as acetabular cups (Fig. 20(a)), hips, knees, shoulders and spinal implants, and a number of implants have been certified on the market. For example, using Arcam EBM technology, Adler Ortho Group [ 158] launched the CE-certified Fixa Ti-Por acetabular cup in the European market in 2007, and more than 2000 of these cups have been implanted. Another fast-growing area for AM applications is the dentistry business [ 159]. Several companies including Concept Laser and MTT Technologies are using SLM to produce copings for crowns and bridges. Figures 20(b) and 20(c) show examples of a dental prosthesis and a dental bridge, respectively.

Fig.20

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	Fig.20 (a) Acetabular cups with designed porosity (material: Ti6Al4V) produced using EBM (Source: Arcam [ 92]); (b) dental prosthesis (material: Ti6Al4V) produced using SLM (Source: Concept Laser [ 134]); (c) 3-unit dental bridge (material: CL111 CoCr) produced using SLM (Source: Concept Laser [ 134])

Research institutes and universities also have explored the application of AM to biomedical implants. Ti6Al4V implants (Fig. 21(a)) with tailored mechanical properties that mimic the stiffness of bone in order to reduce stress shielding have been fabricated by EBM [ 53], and functional hip stems (Fig. 21(b)) with designed porosity have been made from titanium by LENS [ 127]. Cellular Ti6Al4V structures with interconnected pores (see Fig. 4) for bone implants were fabricated using EBM by Heinl et al. [ 51, 55], demonstrating the suitability of these implants for tissue ingrowth and vascularization. SLM also has been used to fabricate implant parts in biocompatible metal alloys (i.e., Ti6Al4V and CoCrMo) [ 160].

Fig.21

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	Fig.21 (a) Hip stems with mesh, hole and solid configurations fabricated using EBM (Source: [ 53]); (b) functional hip stems with designed porosity (no porosity,<2 vol% porosity, and 20 vol% porosity) fabricated using LENS (Source: [ 127])

In tissue engineering, three-dimensional scaffolds play a vital role as extra-cellular matrices onto which cells can attach, grow and form new tissues. The conventional fabrication of tissue scaffolds has relied on techniques such as solvent casting, melt molding, freeze drying, and foam replication to create the shape and architecture of a scaffold. These methods have limitations in the areas of manual interaction requirements, difficulty in the control of complicated internal architectures, and reproducibility. In contrast, fabrication of tissue scaffolds using AM technology allows versatility in the use of biomaterials and the fabrication of scaffolds with complex geometries and designed internal architectures [ 12, 161, 162, 163, 164]. AM processes have been used both directly to manufacture scaffolds (in biodegradable polymer, bioactive ceramic or glass [ 165]) and indirectly as a ‘manufacturing tool’ for the molds required to cast scaffolds [ 11].

The methods employed to fabricate tissue scaffolds directly can be using SLA [ 166], 3DP [ 67, 79], FDM [ 167, 168] or SLS [ 163, 169]. Researchers have demonstrated the ability to make tissue scaffolds from biopolymers such as PCL and PEEK, bioceramics such as hydroxyapatite (HA) and β-tricalcium-phosphate ( β-TCP), and bioglasses such as the 13-93 glass using 3DP [ 67, 79, 170] and SLS [ 171, 172]. Multiple materials also can be used to fabricate scaffolds. For example, HA reinforced PCL scaffolds can be fabricated using the FDM process to improve the mechanical strength of the biopolymer [ 167]. The fabrication of apatite-mullite glass-ceramic bone scaffolds using SLS was investigated both with binder (for bonding glass-ceramic particles) and without binder (by directly melting glass-ceramic particles) [ 106, 169]. Photopolymerizable biomaterials, such as polyethylene glycol (PEG) and polyethylene oxide, were used in the SLA process to fabricate bioactive scaffolds with living cells encapsulated inside, for a variety of tissue engineering applications [ 173, 174].

In addition, several new AM variations have been developed for biomedical applications. HA scaffolds have been made using robocasting [ 175], in which a syringe deposits highly concentrated colloidal suspensions in layers to form a scaffold, followed by drying and sintering. Another FDM-based extruding deposition method, called precision extruding deposition (PED), was applied by Shor et al. [ 176] to fabricate PCL tissue engineering scaffolds. In contrast to the conventional FDM process that requires the use of precursor filaments, the PED process directly extruded scaffold materials in a granulated form, thereby avoiding the need for filament preparation.

SLS and FEF have been applied to fabricate bone tissue scaffolds in bioactive glass (13-93 glass), a new generation of biomaterial that not only bonds with the surrounding tissue but also actively aids in tissue regeneration [ 177, 178]. The bone scaffolds fabricated using SLS and FEF are shown in Figs. 22(a) and 22(b), respectively. Figure 23 shows the SEM images of the bio-test results with MLO-A5 cells seeded on the commercial scaffold (BD CaP) and the SLS scaffold after two days of incubation. The higher-magnification SEM images in Figs. 23(c) and 23(d) show that the cells visible on both the BD CaP scaffold and the SLS scaffold appear anchored well by lamellipodia and filopodial extensions. The optical image of cell-seeded SLS scaffolds incubated after 2, 4, and 6 days is shown in Fig. 24. The relative intensity of purple formazan staining on these scaffolds increased dramatically with the duration of incubation, indicating that metabolically active cells undergoing vigorous growth on the scaffolds and the fabricated bone scaffolds promote cell growth.

Fig.22

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	Fig.22 (a) Bone scaffolds fabricated using SLS; (b) bone scaffolds with 600 µm pores fabricated using FEF (Source: [ 177])

Fig.23

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	Fig.23 SEM images of MLO-A5 cells on control BD CaP (a, c) and SLS-1 scaffolds (b, d) after 2 days of incubation (Source: [ 178])

Fig.24

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	Fig.24 MTT labeling of MLO-A5 cells on porous 13-93 SLS scaffolds after culture intervals of 2, 4, and 6 days (Source: [ 178])

Indirect AM of tissue scaffolds are normally obtained by building a mold (in polymer) using an AM process and then casting with biocompatible materials into the mold cavity to form a scaffold. Lin et al. [ 179] fabricated porous β-TCP scaffolds with a polygradient controllable structure of both macro and micro pores by combining FDM and freeze drying. An artificial bone with a porous internal structure was fabricated by injecting calcium phosphate cement (CPC) into an SLA mold [ 180]. Scaffolds with villi features were produced by solvent casting into 3D printed plaster molds, followed by particulate leaching [ 65].

Biofabrication [ 181] using living cells, a new paradigm of AM application in the biomedical industry, has evolved through the convergence of engineering and life sciences. Biologics or biomaterials are used as building blocks to fabricate biologic and bio-application oriented substances, devices, and therapeutic products through a broad range of engineering, physical, chemical and biologic processes. Biofabrication encompasses an extremely wide range of applications in tissue engineering, disease pathogenesis and drug studies, biochips and biosensors, drug delivery, in-cell printing, patterning, assembly, and organ printing.

AM techniques for cell manipulation have been developed, e.g., syringe-based cell deposition for tissue constructs [ 157]; inkjet-based cell printing [ 182, 183]; micro-contact printing of cells and bacteria; cell manipulation by mechanical, optical, electrical, magnetic, and ultrasound methods for micro-fluidics; and cell patterning by photo- or electro-etching and soft lithography. A syringe-based, layered, direct cell writing, bioprinting process with a multi-nozzle was developed at Drexel University [ 157]. The system has the ability to deposit multiple cell types and bioactive factors in controlled amounts at a precise spatial position for the freeform fabrication of biopolymer-based, three-dimensional, liver cell-embedded tissue constructs. Boland et al. [ 184, 185, 186] developed a process (Fig. 25) by which a printer can print gels and cells, and cell aggregates into a 3D gel. Their process consists of the following three stages: 1) pre-processing (creating CAD models, which can be generated from CT scanned data), 2) processing (printing) and 3) post-processing (perfusion of printed organs and their biomechanical conditioning).

Fig.25

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Fig.25 A bioprinter and images of printed cells and tissue constructs. (a) Schematic representation of the bioprinter model; (b) bovine aortic endothelial cells printed in 50 µm drops in a line; (c) cross-section of the p(NIPA-co-DMAEA) gel showing the thickness of each sequentially placed layer; (d) actual bioprinter; (e) print head with nine nozzles; Endothelial cell aggregates ‘printed’ on collagen (f) before and (g) after their fusion (Source: [ 184, 185, 186])