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Additive Manufacturing: An Overview
Additive Manufacturing: An Overview R Singh and S Singh, Guru Nanak Dev Engineering College, Ludhiana, India r 2017 Elsevier Inc. All rights reserved.
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Introduction to Additive Manufacturing Applications in Medicine Applications in Automobile/Aerospace Applications in Construction Conclusions
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Introduction to Additive Manufacturing
As per ASTM Standards (ISO/ASTM52900-15, 2015), additive manufacturing (AM) technologies can be best defined as techniques that work on the basis of the addition of material rather than subtraction. AM is a principle, not a technique, and the equipment based on this principle are called AM technologies. It is always difficult to distinguish between names such as freeform fabrication, rapid prototyping, AM, additive process, rapid manufacturing, layered manufacturing, and others. However, all these names are synonymous and the most widely used is AM. The AM technique has more than 25 years of history. Earlier technologies such as fused deposition modeling, laminated object manufacturing, stereolithography, jet printing, and others were limited to polymers only and the products thereof did not meet all the requirements of engineering materials (such as metals, ceramics, and composites) (Kruth et al., 2007). Previously, plastic-based prototypes were only used for inspection and communication of designs and product aspects (Santos et al., 2006). In this new era, with the invention of technologies such as selective laser sintering (SLS), direct metal laser sintering (DMLS), ZCast, three-dimensional (3D) printing, and others, the concept of rapid manufacturing has emerged these techniques have been used to produce end-use parts (Hague et al., 2004). ASTM later renamed AM techniques as AM technologies. Today, various types of AM technologies are commercial (refer Fig. 1); they are used in manufacturing and assembly principles for product design (Huang et al., 2013). Numerous types of AM technologies are widely discussed in the literature (Bourell et al., 2009; Wong and Hernandez, 2012). According to the Wohlers Report of 2013 (Wohlers Associates, Inc., 2013), the annual growth rate (compounded basis) of worldwide revenues from all AM products and services in the past 25 years is 25.4%. The growth rate from 2010 to 2012 has been recorded as 27.4% and reached $2.2 billion in 2012. The unit sales of industrial AM systems, which cost less than $5000, increased by 19.3% in 2012. However, the unit sales of 3D personal printers, which cost approximately $5000, increased by 46.3% in 2012 alone. AM technologies are known for the following various benefits (Holmström et al., 2010):
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No tooling is needed, thus significantly reducing production ramp-up time and expense. Small production batches are feasible and economical.
Fig. 1 Material categories for additive manufacturing (AM) technologies (Bikas et al., 2016).
Reference Module in Materials Science and Materials Engineering
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Additive Manufacturing: An Overview
Possibility to quickly change design. Allow product to be optimized for function (eg, optimized cooling channels). Allow economical custom products (batch of one). Possibility to reduce waste. Potential for simpler supply chains, shorter lead times, and lower inventories. Design customization.
AM is a digital technology used to produce physical objects layer-by-layer from a 3D CAD file. The process begins with generating a 3D CAD model of the object with all its details and dimensions. This 3D CAD file can also be generated from MRI/CT scan data by using sophisticated computer interface software. The 3D CAD file is sliced or sectioned into numerous, thin, twodimensional (2D) sections by a computer program (eg, Catalyst EX software for the Stratasys Inc. uPrint-FDM system). Then, the multiple slices of 2D sections are sent to the printing machine, which lays them on a platform, one after another (Kruth et al., 1998; Gibson et al., 2009). The process may take a few hours to a few days to produce an object, depending on the size, geometry, and precision. AM technologies are mainly dependent on the standard tessellation language (.STL) file because, for each part, the very first step is obtaining the required .STL file. The first .STL file was created in 1987 by 3D Systems Inc. and it was used for the stereolithography process. The .STL file creation process mainly converts the continuous geometry into a header, small triangles, or a triplet list of x, y, and z coordinates and the normal vector to the triangles. The interior and exterior surfaces are identified using the right-hand rule and additional edges are added once the figure is sliced. The slicing process also introduces inaccuracy to the file because the algorithm replaces the continuous contour with discrete stair steps (Iancu et al., 2010). To reduce this inaccuracy, the technique for a feature that has a small radius in relation to the dimension of the part involves creating .STL files separately and then combining them later. In addition to the .STl file, other types of files used are .SLC and .SLI from 3D Systems, command language interpreter (.CLI) from EOS, Hewlett-Packard graphics language (HPGL) from Hewlett-Packard, Stereolithography contour from Stratasys, and F&S from Fockele and Schwarze; initial graphics exchange specifications (IGES) are also used (Halloran et al., 2011). Due to the high cost of AM technologies, they have not been widely adopted by manufacturers; however, in the research and development sector of engineering, medicine, and construction, they are frequently being used (Chua et al., 1998; Flowers and Moniz, 2002). The research community has developed novel AM processes and has applied them to the aerospace, automotive, and biomedical fields, among others (Thomas et al., 1996; Song et al., 2002; Giannatsis and Dedoussis, 2009). Fig. 2 shows the applications of AM in various industrial sectors. Campbell et al. (2012) highlighted the development of AM technologies by three key industries for the following differing reasons:
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Automotive manufacturers exploited this technology because of the ability to help new products get to the market quickly and predictably. Small savings in time and development costs can result in significant overall savings in vehicle development. Aerospace companies are interested in these technologies because of the ability to realize highly complex and high-performance products and to integrate mechanical functionality, eliminate assembly features, and make it possible to create internal functionality (like cooling channels, internal honeycomb style structures, etc.). Medical industries are particularly interested in AM technologies because of the ease with which 3D medical imaging data can be converted into solid objects.
There are many AM techniques and numerous equipment available; therefore, proper selection of one that fulfills the required application criteria is necessary (Vayre et al., 2012). Accordingly, it is necessary to identify the specific manufacturing constraints
Fig. 2 Additive manufacturing in industrial sectors (Wohlers, 2011).
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and capabilities of available AM technologies. There are three main constraints for direct AM processes: the mechanism of material deposition (ie, nozzle must be parallel to the vertical axis), heat dissipation, and the part should be mounted on a rotating platter (which leads to accessibility constraints) to avoid collisions between the nozzle and the part. Additionally, the speed of material deposition depends mostly on the speed of the nozzle and the amount of material sprayed with the nozzle. The repetition of continuous acceleration and deceleration can cause the manufacturing to stop if the distance between the nozzle and the surface is too great, so that molten drops solidify before landing on the surface. To eliminate such problems, acceleration and deceleration stages must be minimized by avoiding sharp corners and replacing them with curves. Removal of powder from the part after its completion is another constraint of direct manufacturing-based AM technologies. Common engineering materials such as cast iron and aluminum alloys are not able to be processed with most AM technologies. This is a constraint of the manufacturing capabilities of direct manufacturing-based AM technologies. Cleaner production and sustainability are crucial for the manufacturing sector and direct manufacturing-based AM technologies have enabled the manufacture of functional/nonfunctional products. Bourhis et al. (2013) proposed a methodology to evaluate the environmental impact of a part from its CAD model by focusing on electrical, fluid, and material consumption. Significant progress has been made since 1980, resulting in various social benefits such as the following (Huang et al., 2013):
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Health care products customized to the needs of individual consumers. Reduced raw material usage and energy consumption, which are key contributions to environmental sustainability. On-demand manufacturing presented an opportunity to reconfigure the manufacturing supply chain to bring cheaper products to consumers.
Campbell et al. (2011) highlighted all these growing sectors of AM technologies. In their article, they surveyed the growth of AM in South Africa and highlighted some automotive, architectural, and medical applications, as shown in Fig. 3. AM is a promising approach for high-quality and efficient health care at economical prices and personalized care tailored to specific patient characteristics. Surgeons can build a patient-specific model to analyze when planning surgical procedures, those working in marketing and sales can perform surveys of new products, and so on (Wong and Hernandez, 2012). In 2001, approximately 20,700 Americans underwent chin augmentation surgery, 49% underwent lip augmentation, and 47% underwent cheek implantations (McCormick, 2011). The basic idea of making custom surgical implants relies on a computed tomography scan to obtain patient-specific data from which a solid model of the required implant is developed through reverse engineering. According to a recent publication, AM processes can be used to attain true 3D microproducts. These 3D micro-AM technologies were classified into three groups: scalable AM technologies for both macro scale and microscale; 3D direct writing technologies, which have been merely developed for microscale; and hybrid processes (Vaezi et al., 2013). Further, AM allows integration of functions from different parts for better performance even if there is a movability requirement for the part, like in the case of a ball and socket joint. The design freedom of AM enables simpler assembly, which becomes the focus and of higher importance (Lindemann et al., 2012). The fewer number of parts and fewer assembly steps may have a high impact on production costs. Fewer parts provide for advantages such as sourcing, labeling, and evaluating. Because there is no need for tooling for production of spare parts, it is unnecessary to keep legacy tooling in storage. Overall, AM has been proven to have extraordinary benefits in the existing manufacturing sector and could be applied to all its branches. However, there are some disappointments in terms of poor surface finish, lack of strength, and cost effectiveness. Higherend machines or mass production machines are costly and unaffordable for the medium-scale or small-scale manufacturing
Fig. 3 (a) Architectural model created using laser sintering, (b) titanium elbow implant and (c) turbine.
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Additive Manufacturing: An Overview
sectors. So far, there have been various reviews of AM technologies (Hopkinson et al., 2006; Gibson et al., 2010). The present article focuses on the recent innovations and applications of AM techniques in aerospace/automobile, biomedicine, and construction applications and is helpful for upcoming scholars working in the field of AM and exploring future possibilities.
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Applications in Medicine
Ample AM opportunities exist in the biomedical field regarding the fabrication of custom-shaped orthopedic prostheses and implants, medical devices, biological chips, tissue scaffolds, living constructs, drug-screening models, and surgical planning and training apparatuses (Huang et al., 2015). CAD/CAM-based AM technologies for metals have found applications in the near net shape fabrication of complex geometries with tailored mechanical properties for biomedical sectors. With the introduction of electron beam melting (EBM), DMLS, selective laser melting (SLM), and SLS processes allow direct and digitally enabled fabrication of structures with controlled mechanical properties and desired external and internal characteristics (Harrysson and Cormier, 2006; Lin et al., 2007). The range of materials used with these advanced manufacturing technologies has increased over time, thus broadening the spectrum of applications. Composites with lattice patterns created by using solid freeform fabrication have been used as a host for filler materials. Metallic materials currently used with AM technologies for metal include titanium alloy Ti–6Al–4V, commercially pure titanium, Co–Cr alloy, Inconel, stainless steel, tool steel, aluminum, hard metals, amorphous metals, copper, niobium, and beryllium (Parthasarathy et al., 2011). In medicine, patients might have individual needs based on specific anatomy and it may be possible to include autologous cells to enhance the treatment. The behavior of cells can be directed by tailoring their environment. Patterning technologies can control surface chemistry and topography at scales smaller than a single cell (Melchels et al., 2012). Furthermore, 2013 marked the 15th year of cell printing, which is an ambitious, developmental biology-enabled, scaffold-less technique for fabricate living tissues and organs by printing living cells (Mironov et al., 2009; Ringeisen et al., 2013). A typical cell printing process consists of three stages:
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Preprocessing: creating tissue or organ-specific CAD models for each patient using CT scan data. Processing: using AM processes to deposit living cells into 3D biological constructs. Postprocessing: incubating printed tissues or organs to encourage tissue fusion and maturation.
Dérand et al. (2012) described the workflow of craniomaxillofacial surgery, which includes imaging, virtual design, manufacturing of patient-specific titanium reconstruction plates, cutting guides, and mesh, and the utility of the workflow in connection with surgical treatment of acquired bone defects in the mandible. The major stage in the printing of a biomedical implant/limb is the conversion of the image into the product. Computers are used for filtering initial medical image scan data to remove the lowerdensity tissue of the bones. After this, the data are used for generating a 3D CAD file that is analyzed at the preliminary stage. Then, the modified design is converted to the .STL format as required by the printers. Ramosoeu et al. (2010) used the DMLS system for producing complex topology components of Ti–6Al–4V for medicine. The final prepared components were heat-treated at 1000 and 11001C and subsequently cooled. In the case of slow cooling, it has been observed through optical analysis that more of the alpha phase was present as compared to the laser-sintered components. In the case of rapid cooling, the martensitic transformation resulted in higher hardness. Cohen et al. (2010) used novel geometric feedback-based approaches and demonstrated the in situ repair of both chondral and osteochondral defects that mimic naturally occurring pathologies. A calf femur was mounted in a custom jig and held within a robo-casting-based AM system (see Fig. 4). Repair prints for both defects had mean surface errors less than 0.1 mm. Similarly, Kundu et al. (2012) investigated AM with a multi-head deposition system to fabricate 3D cell-printed scaffolds using layer-by-layer deposition of polycaprolactone (PCL) and chondrocyte cell-encapsulated alginate hydrogel. Appropriate cell dispensing conditions and optimum alginate concentrations for maintaining cell viability were determined. Further, an in vitro cell-based biochemical analysis was performed to determine glycosaminoglycans, DNA and total collagen contents from different PCL alginate gel constructs. Podshivalov et al. (2013) proposed a novel approach for generating microscale scaffolds based on processing actual micro-CT images and then reconstructed a highly accurate geometrical model. This model was manufactured using a state-of-the-art 3D AM process with biocompatible materials. At the microscale level, these scaffolds were similar to the original tissue, thus interfacing better with the surrounding tissue and facilitating more efficient rehabilitation for the patient. Moreover, by means of multi-resolution volumetric modeling methods and scaffolds, porosity can also be adapted according to specific mechanical requirements. A scaffold is the major pillar of most tissue engineering approaches; it mimics an extracellular matrix for cells. The scaffold is seeded with cells that should provide the appropriate biomechanical and biochemical conditions for cell proliferation and eventual tissue formation (Marga et al., 2012). Puppi et al. (2012) fabricated polymeric scaffolds based on wet-spinning of poly (e-caprolactone) (PCL) or PCL/hydroxyapatite (HA) solutions through AM technique (see Fig. 5). Two different scaffold architectures were designed and fabricated by altering the inter-fiber distance and fiber staggering. The scaffolds that developed showed good reproducibility of the internal architecture and were characterized by highly porous, aligned fibers with an average diameter ranging from 200 to 250 mm. Cell culture experiments using the MC3T3-E1 preosteoblast cell line showed good cell adhesion, proliferation, alkaline phosphatase activity, and bone mineralization on these scaffolds. Wang et al. (2015) reviewed the state-of-the-art topological design and manufacturing processes of various types of porous metals, particularly titanium alloys, biodegradable metals, and shape memory alloys. Khoda et al. (2013) proposed a novel
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Fig. 4 Femoral printing substrate.
Fig. 5 Scaffold structures.
technique using a controllable, heterogeneous architecture design suitable for AM processes. The proposed layer-based design uses a bi-layer pattern of consecutive radial and spiral layers to generate functionally gradient porosity, which follows the geometry of the scaffold. The proposed approach constructs the medial region from the medial axis of each corresponding layer, which is represented by the geometric internal feature or the spine. The radial layers of the scaffold are then generated by connecting the boundaries of the medial region and the layer’s outer contour. Gradient porosity changed between the medial region and the layer’s outer contour. The literature highlights that DMLS is one of the most useful technologies for preparing 3D porous bodies with complicated internal structures directly from titanium powders without any intermediate processing steps. These products are expected to be useful as a bone substitute or customized implant, as shown in Fig. 6 (Ciocca et al., 2010). Vaezi and Yang (2015) identified design, extrusion temperature, and ambient temperature of an extrusion-based AM process as the most important parameters for printing PEEK structures without warping, delamination, and polymer degradation. Compression and tensile tests were conducted to investigate mechanical properties of these new 3D printed PEEK structures. Murr et al. (2012) presented EBM for the fabrication of knee (see Fig. 7(a)) and hip implant (see Fig. 7(b)) components containing porous structures and demonstrated stiffness-compatible implants with optimal stress shielding for bones and bone cell in-growth. Systematic geometrical arrays of cellular reticulated mesh and open cell foams with interconnected porosities can be used to manufacture complex, functionally graded, monolithic structures as the next generation of biomedical implants. Cronskar et al. (2013) investigated the feasibility of EBM for biomedical applications. A case study of manufacturing hip stems for seven individuals was performed and compared with conventional machining process. Fatigue testing was performed to study the bone in-growth in the medial part of the stem and to determine how this surface influences fatigue properties. Hengsbach and Lantada (2014) explored the possibility of designing and manufacturing biomedical microdevices with multiple geometries with the help of AM technologies. The results of their study highlighted the versatility, accuracy, and manufacturing speed and allowed for the manufacturing of micro systems and implants with overall sizes up to several millimeters and with details of sub-micrometric structures.
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Additive Manufacturing: An Overview
Fig. 6 Customized skull made of medical grade titanium.
Fig. 7 Knee joint (a) and hip joint (b) fabricated with electron beam melting.
Progress in AM technologies has led to the development of advanced and versatile systems for customized biomedical production. This is the new frontier of designing; with AM, fully integrated products and the automation necessary to fabricate ready-to-use tissue-engineered constructs on an industrial scale can be created. The rapid manufacturing of customized medial constructions allows fast production of large quantities of samples; this can enhance clinical routine procedures in terms of meeting daily surgical needs more effectively.
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Applications in Automobile/Aerospace
With AM technologies, it is now possible to fabricate large sections with light weights. Particularly in the automotive and aerospace industries, weight is always a major constraint because lighter parts are more efficient. AM technologies have enabled these industries to manufacture such parts along with precise, controlled, complex cross-sections (such as honeycomb cell) and desirable strengths (Bletzinger and Ramm, 2001). Further, AM technologies have made it possible to create structural parts for machines,
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thereby reducing the total weight (Williams, 2002). SLS and EBM are the two most widely used technologies in aircraft and aerospace industries. These technologies have created a new dimension of possible designs by using pre-alloyed metal powders (Liao et al., 2006). Regarding aerospace industries, the components often have complex geometries and are usually made from advanced materials such as titanium, nickel, special steels, and ultrahigh-temperature ceramics. Using conventional manufacturing processes, they are very difficult, costly, and tedious to manufacture (Huang et al., 2015). Further, production runs in aerospace industries are jobfocused or batch-focused; therefore, AM technologies are highly suitable for manufacturing these components. Kair and Sofos (2014) described the applications of AM technologies for passenger car engine components. Six significant components of the turbocharger (ie, compressor housing, compressor impeller, bearing system, center housing, assembly of turbine impeller, and shaft and turbine housing) were fabricated using AM. After conducting a critical analysis, it was noted that AM can help reduce the maintenance costs and extend the life of the turbocharger. According to an online source, BAE Systems approved a replacement part made using AM – a plastic window breather pipe for the BAE146 regional jet. Similarly, Optomec recently used the LENS process to fabricate complex metal components for satellites, helicopters, and jet engines. The US Navy hired Boeing Company to develop F/A-18E/F Super Hornet Fighter Jets. The preliminary demand was to minimize the cost of production and to shorten the manufacturing time to almost half. Improving product quality and adding six new avionics systems were also among the important objectives. Boeing has adopted SLS technology to manufacture air-cooling ducts to meet the functional requirements and limitations (see Fig. 8; Lyons, 2011). This AM technology enabled the engineering team to combine different ducts into single parts, to integrate the attachment mechanisms into them, and to reduce the overall number of parts (Hopkinson et al., 2006). Laser additive manufacturing (LAM) is a fusion-based process commercialized by AeroMet and developed by Johns Hopkins University and Pennsylvania State University. The resulting LAM process uses a high-wattage CO2 laser and a powder feed system to deposit wide, thick beads of Ti–6Al–4V onto a substrate. The foundation was developed for the fusion-based AM processes for the production of aerospace hardware. Unfortunately, AeroMet discounted this project in 2005. Advisory Council for Aviation Research, Innovation in the EU, and Flightpath 2050 worked for the reduction of fuel consumption and exhaust gases. To achieve this, SLM was used to manufacture lightweight engine components and structural parts from titanium alloy. Aurora Flight Sciences in collaboration with Stratasys Inc. fabricated and flew an aircraft with a 62-inch wingspan with a wing composed entirely of AM components. The wing was manufactured by FDM 3D printers. In a public–private venture, NASA and Pratt & Whitney Rocketdyne produced an AM-based rocket engine injector. This rocket engine injector is one of the most critical and expensive engine components in a launch vehicle, but with 70% less cost. AM-produced shipments for the US automotive industry were valued at $48 billion in 2011. As noted, approximately 19.5% of AM occurs within the automotive industry, with AM shipments estimated to be less than 0.05% of total US automotive shipments. Regarding automotive industries, AM technologies have been explored as an efficient tool in designing and developing components at low costs. Automotive companies are applying AM to an expanding range of parts, including engines and vehicle bodies. The automotive industry has used AM to make tool prototypes and custom parts for short production runs. It is increasingly applying the technology to metals, mainly Al alloys, to construct light-weight vehicles. AM processes have been used to make small quantities of structural and functional components (such as engine exhausts, drive shafts, gear box components, and braking systems for luxury vehicles). Various research institutes have successfully applied AM techniques to manufacture
Fig. 8 Design, production, testing, and implementation of additive manufactured parts (air-cooling ducts) deployed in the F-18E/F aircraft.
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Additive Manufacturing: An Overview
Fig. 9 (a) Urbee three-dimensional-printed vehicle and (b) Urbee body and inner structure.
Fig. 10 PUUNK frame showing areas where additive manufacturing can be used (Richardson and Haylock, 2012).
functional components for racing vehicles. Unlike passenger cars, vehicles for motorsports usually are made of lightweight alloys (eg, titanium) and have highly complex structures and low production volumes (Huang et al., 2015). Ford Motor Company has been using AM technologies to develop prototype parts for test vehicles since the 1980s. Ford engineers have produced prototypes of cylinder heads, brake rotors, and rear axles in less time than traditional manufacturing could. The company reports that use of AM technologies saves 1 month of production time for developing a casting. KOR EcoLogic’s Urbee car (see Fig. 9) is a wonderful example of AM. Urbee is a 3D printed vehicle designed by KOR Ecologic Inc. The intension of the Urbee project was to develop an energy-efficient infrastructure for a vehicle that could be powered entirely by renewable energy. To achieve this, the body of the vehicle was specifically designed to make the best use of materials for a lightweight construction. This project was assisted by Stratasys Inc., Tebis, and Autodesk (software) and by CD-Adapco (simulation). The body panels were designed to incorporate sophisticated honeycomb structures that were unable to be produced using traditional manufacturing methods. Similarly, the PUUNK velomobile (see Fig. 10) project aims to provide bicycles or tricycles with a shell to provide weather protection and aerodynamic gains. AM could help them in their work. The velomobile comprises a personalized, user-generated, up-cycled, and configurable kit that helps to develop the ethos of this vehicle. This point is a key aspect in enabling its openness in design, opportunities for personalization, and modular configuration.
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Applications in Construction
Creating an architectural/construction model can sometimes be a difficult task for architects, civil engineers, and contractors. In architecture, manual techniques are most often used to turn a concept or idea into a figure or rough drawing. However, when these models become complex, creating a physical model can be a difficult task. Thus, modeling is a very important aspect of construction for architects; they can study the models and their functionalities. These are also helpful when architects are explaining ideas to their customers and trying to convince them to fund their projects. AM technologies can provide architects with a very powerful tool for their business by enabling them to create a physical model faster without worrying about the complexity of the design. They also achieve better resolution than other processes used in
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architecture. Stereolithography is a very suitable process for architectural modeling because of the materials used and the printing resolution (Liao et al., 2006; Semetay, 2007; Xiong, 2009; Balla et al., 2008). Cement-based materials for AM were first introduced by Pegna (1997). There are three commercial AM processes targeted at construction and architecture applications (Khoshnevis et al., 2006). A research team at Loughborough University has developed some of the recent concrete printing technologies. All these AM technologies have allowed the successful manufacture of components of significant size and are suitable for construction and architectural applications (see Fig. 11). Contour crafting involves a crane-mounted device for on-site, in situ applications, whereas concrete printing is a gantry-based off-site manufacturing process. Although there is no specific reason why either process cannot be used on-site. Contour crafting uses two computer-controlled trowels to create surfaces on the object being fabricated. The layering approach enables creation of various surfaces. Counter crafting allows the design of structures with various architectural geometries that are difficult to realize using the current manual construction practices. Various materials may be used for outside surfaces and as fillers between surfaces (Tibaut et al., 2016). The D-shape process involves the use of powder that is selectively hardened using a binder in much the same way as the Z-Corp 3D printing process. Each layer of build material is laid to the desired thickness and compacted, and then the nozzles mounted on a gantry frame deposit the binder where the part is to be solid. This system has many advantages over traditional formative processes (use of formwork with concrete) as well as other additive building processes (eg, brick laying). It can use any sand-like material and produces little waste because the remaining sand can be reused (Tibaut et al., 2016). AM technologies like FDM, SLS, and stereolithography were later introduced into the construction industry to produce architectural models. The accuracy of these technologies was improved to 0.1–0.2 mm as compared to the accuracy of 0.2–0.4 mm in concept modeling (Ryder et al., 2002). WinSun, in 2014, built a two-story villa and a five-story apartment using AM technology and demonstrated the applicability of AM in large-scale buildings. One example of their work is provided in Fig. 12 (Wu et al., 2016). During their work they identified the following:
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Indirect process: the villa and apartment were not printed directly from electronic data. Brittleness: carbon fiber led to the brittleness of the printing material. Exclusion of building services: services such as electrical and plumbing were not integrated in the AM process.
Fig. 11 Additive manufacturing techniques: (a) D-shape, (b) counter crafting, and (c) concrete printing (Lim et al., 2012).
Fig. 12 Connection details of the three-dimensional-printed villa by WinSun.
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Additive Manufacturing: An Overview
Fig. 13 Construction component fabricated with additive manufacturing.
Lim et al. (2009) presented some of the recent developments in scaling-up complex construction components, as shown in Fig. 13. In their work, they discussed the development of the approach and preliminary components manufactured with different nozzle diameters. It was found that concrete printing is a promising field that can further grow to provide high-quality construction parts. This digital fabrication is enabling the production of buildings with freeform surfaces. Clients of the construction industry are asking leading designers to build structures that cannot be built by any currently known methods. However, AM processes are capable of delivering components large enough for building structures but that are unlikely to be scaled-up version (Buswella et al., 2007).
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Conclusions
Currently, there are various types of AM technologies available to support numerous problems in different manufacturing sectors. Apart from having the same working principle, these technologies are distinctly different from one another, which has made these more suitable for satisfying universal problems. Three emerging areas of AM applications have been discussed in detail and the efficiency and effectiveness of these technologies have been highlighted. The AM-based medicine applications presented in this article indicate that this hybridization is mature due to the existence of realistic examples worldwide. However, in the case of the automotive and aerospace sectors, not enough has been investigated. Some of very interesting case studies are available, such as PUUNK, Urbee, and the F-18E/F aircraft. Further exploration is only possible through the collaborative efforts of industries and academia. The applications of AM technologies research to construction are ongoing. In the present article, it is believed that a new scenario for AM will be coming in the next 5 to 10 years and it will bring emerging applications, new developments, and business opportunities.
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Dérand, P., Rännar, L.E., Hirsch, J.M., 2012. Imaging, virtual planning, design, and production of patient-specific implants and clinical validation in craniomaxillofacial surgery. Craniomaxillofacial Trauma and Reconstruction 5, 37–44. Flowers, J., Moniz, M., 2002. Rapid prototyping in technology education. Technology Teacher 62, 7–10. Giannatsis, J., Dedoussis, D.V., 2009. Additive fabrication technologies applied to medicine and health care: A review. International Journal of Advanced Manufacturing Technology 40, 116–127. Gibson, I., Rosen, D.W., Stucker, B., 2009. Additive Manufacturing Technologies: Rapid Prototyping to Direct Digital Manufacturing. New York, NY: Springer. Gibson, I., Rosen, D.W., Stucker, B., 2010. Additive Manufacturing Technologies: Rapid Prototyping to Direct Digital Manufacturing. New York, NY: Springer. Hague, R., Mansour, S., Saleh, N., 2004. Material and design considerations for rapid manufacturing. 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Huang, S.H., Liu, P., Mokasdar, A., Hou, L., 2013. Additive manufacturing and its societal impact: A literature review. International Journal of Advanced Manufacturing Technology 67, 1191–1203. Huang, Y., Leu, M.C., Mazumder, J., Donmez, A., 2015. Additive manufacturing: Current state, future potential, gaps and needs, and recommendations. Journal of Manufacturing Science and Engineering 137 (1), 014001-1–014001-10. Iancu, C., Iancu, D., Stamcioiu, A., 2010. From Cad model to 3D print via “STL” file format. Available at: http://www.utgjiu.ro/rev_mec/mecanica/pdf/2010-01/13_Catalin% 20Iancu.pdf. ISO/ASTM52900-15, 2015. Standard Terminology for Additive Manufacturing – General Principles – Terminology. West Conshohocken, PA: ASTM International. Available at: www.astm.org Kair, A.B., Sofos, K., 2014. Additive manufacturing and production of metallic parts in automotive industry. Master Thesis in Production Engineering and Management, KTH Royal Institute of Technology. Khoda, A.K.M., Ozbolat, I.T., Koc, B., 2013. Designing heterogeneous porous tissue scaffolds for additive manufacturing processes. Computer-Aided Design 45, 1507–1523. Khoshnevis, B., Hwang, D., Yao, K., Yeh, Z., 2006. Mega-scale fabrication by contour crafting. International Journal of Industrial and System Engineering 1, 301–320. Kruth, J.P., Leu, M.C., Nakagawa, T., 1998. Progress in additive manufacturing and rapid prototyping. CIRP Annals-Manufacturing Technology 47, 525–540. Kruth, J.-P., Levy, G., Klocke, F., Childs, T.H.C., 2007. Consolidation phenomena in laser and powder-bed based layered manufacturing. CIRP Annals-Manufacturing Technology 56 (2), 730–759. Kundu, J., Shim, J.H., Jang, J., Kim, S.W., Cho, D.W., 2012. An additive manufacturing-based PCL–alginate–chondrocyte bioprinted scaffold for cartilage tissue engineering. Journal of Tissue Engineering and Regenerative Medicine 9, 1286–1297. Liao, Y.S., Li, H.C., Chiu, Y.Y., 2006. Study of laminated object manufacturing with separately applied heating and pressing. International Journal of Advanced Manufacturing Technology 27, 703–707. Lim, S., Buswell, R.A., Le, T.T., et al., 2012. Developments in construction-scale additive manufacturing processes. Automation in Construction 21, 262–268. Lim, S., Le, T., Webster, J., et al., 2009. Fabricating construction components using layered manufacturing technology. In: Proceedings of Global Innovation in Construction Conference, Loughborough University, pp. 512–520. Lin, C.Y., Wirtz, T., LaMarca, F., Hollister, S.J., 2007. Structural and mechanical evaluations of a topology optimized titanium interbody fusion cage fabricated by selective laser melting process. Journal of Biomedical Materials Research Part B 83 (2), 272–279. Lindemann, C., Jahnke, U., Moi, M., Koch, R., 2012. Analyzing product lifecycle costs for a better understanding of cost drivers in additive manufacturing. Solid Freeform Symposium, pp. 177–188. Lyons, B., 2011. Additive manufacturing in Aerospace; examples and research outlook, frontiers of engineering. Available at: http://www.naefrontiers.org/ File.aspx?id=31590 (accessed 15.08.12). Marga, F., Jakab, K., Khatiwala, C., et al., 2012. Toward engineering functional organ modules by additive manufacturing. Biofabrication 4. doi:10.1088/1758-5082/4/2/022001. McCormick, S., 2011. New stats: Chin surgery skyrockets among women, men, all age groups. Eurek Alert. Available at: http://www.eurekalert.org/ pub_releases/2012-044/ m-nsc041012.php. Melchels, F.P.W., Domingos, M.A.N., Kleina, T.J., et al., 2012. Additive manufacturing of tissues and organs. Progress in Polymer Science 37, 1079–1104. Mironov, V., Viconti, R.P., Kasyanov, V., et al., 2009. Organ printing: Tissue spheroids as building blocks. Biomaterials 30, 2164–2174. Murr, L.E., Gaytan, S.M., Martinez, E., Medina, F., Wicker, R.W., 2012. Next generation orthopaedic implants by additive manufacturing using electron beam melting. International journal of biomaterials 2012, 1–14. 245727. doi:10.1155/2012/245727. Parthasarathy, J., Starly, B., Raman, S., 2011. A design for the additive manufacture of functionally graded porous structures with tailored mechanical properties for biomedical applications. Journal of Manufacturing Processes 13, 160–170. Pegna, J., 1997. Exploratory investigation of solid freeform construction. Automation in Construction 5, 427–437. Podshivalov, L., Gomes, C.M., Zocca, A., et al., 2013. Design, analysis and additive manufacturing of porous structures for biocompatible micro-scale scaffolds. Procedia CIRP 5, 247–252. Puppi, D., Mota, C., Gazzarri, M., et al., 2012. Additive manufacturing of wet-spun polymeric scaffolds for bone tissue engineering. Biomed Microdevices 14, 1115–1127. Ramosoeu, M.E., Chikwanda, H.K., Bolokang, A.S., Booysen, G., Ngonda, T.N., 2010. Additive manufacturing: Characterization of Ti–6Al–4V alloy intended for biomedical application. In: Light Metals Conference, The Southern African Institute of Mining and Metallurgy, pp. 337–344. Richardson, M., Haylock, B., 2012. Designer/maker: The rise of additive manufacturing, domestic-scale production and the possible implications for the automotive industry. Computer-Aided Design & Applications: PACE 2, 33–48. Ringeisen, B.R., Pirlo, R.K., Wu, P.K., et al., 2013. Cell and organ printing turns 15: Diverse research to commercial transitions. MRS Bulletin 38, 834–843. Ryder, G., Bill, I., Graham, G., David, H., Bruce, W., 2002. Rapid design and manufacture tools in architecture. Automation in Construction 11, 279–290. Santos, E.C., Shiomi, M., Osakada, K., Laoui, T., 2006. Rapid manufacturing of metal components by laser forming. International Journal of Machine Tools and Manufacture 46 (12–13), 1459–1468. Semetay, C., 2007. Laser engineered net shaping (LENS) modeling using welding simulation concepts. ProQuest Dissertations and Theses, Lehigh University. Song, Y., Yan, Y., Zhang, R., Xu, D., Wang, F., 2002. Manufacturing of the die of an automobile deck part based on rapid prototyping and rapid tooling technology. Journal of Materials Processing Technology 120, 237–242. Thomas, C.L., Gaffney, T.M., Kaza, S., Lee, C.H., 1996. Rapid prototyping of large scale aerospace structures. In: Proceedings of Aerospace Applications Conference IEEE, Aspen, CO, vol. 4, pp. 219–230. Tibaut, A., Rebolj, D., Perc, M.N., 2016. Interoperability requirements for automated manufacturing systems in construction. Journal of Intelligent Manufacturing 27, 251–262. Vaezi, M., Seitz, H., Yang, S., 2013. A review on 3D micro-additive manufacturing technologies. International Journal of Advanced Manufacturing Technology 67, 1721–1754.
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Vaezi, M., Yang, S., 2015. Extrusion-based additive manufacturing of PEEK for biomedical applications. Virtual and Physical Prototyping. doi:10.1080/17452759.2015.1097053. Vayre, B., Vignat, F., Villeneuve, F., 2012. Designing for additive manufacturing. Procedia CIRP 3, 632–637. Wang, X., Xu, S., Zhou, S., et al., 2015. Topological design and additive manufacturing of porous metals for bone scaffolds and orthopaedic implants: A review. Biomaterials. doi:10.1016/j.biomaterials.2016.01.012. Williams, A., 2002. Architectural modelling as a form of research. Architectural Research Quarterly 6, 337–347. Wohlers, T., 2011. Wohlers Report 2011. Fort Collins, CO: Wohler Associates. Wohlers Associates, Inc., 2013. Wohlers Report 2013: Additive Manufacturing and 3D Printing State of the Industry. Fort Collins, CO: Wohlers Associates. Wong, K.V., Hernandez, A., 2012. A Review of Additive Manufacturing. ISRN Mechanical Engineering 2012. doi:10.5402/2012/208760. Wu, P., Wang, J., Wang, X., 2016. A critical review of the use of 3-D printing in the construction industry. Automation in Construction 68, 21–31. Xiong, Y., 2009. Investigation of the laser engineered net shaping process for nanostructured cermets. ProQuest Dissertations and Theses, University of California.
Further Reading Hopkinson, N., Dickens, P., 2003. Analysis of rapid manufacturing – Using layer manufacturing processes for production. Journal of Mechanical Engineering Science 217 (C1), 31–39. Kulkarni, P., Marsan, A., Dutta, D., 2000. A review of process planning techniques in layered manufacturing. Rapid Prototyping Journal 6 (1), 18–35. Riggs, B.C., Dias, A.D., Schiele, N.R., et al., 2011. Matrix-assisted pulsed laser methods for biofabrication. MRS Bulletin 36, 1043–1050. Xu, C., Chai, W., Huang, Y., Markwald, R.R., 2012. Scaffold-free inkjet printing of three-dimensional zigzag cellular tubes. Biotechnology and Bioengineering 109, 3152–3160.
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Introduction to Additive Manufacturing Applications in Medicine Applications in Automobile/Aerospace Applications in Construction Conclusions
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Introduction to Additive Manufacturing
As per ASTM Standards (ISO/ASTM52900-15, 2015), additive manufacturing (AM) technologies can be best defined as techniques that work on the basis of the addition of material rather than subtraction. AM is a principle, not a technique, and the equipment based on this principle are called AM technologies. It is always difficult to distinguish between names such as freeform fabrication, rapid prototyping, AM, additive process, rapid manufacturing, layered manufacturing, and others. However, all these names are synonymous and the most widely used is AM. The AM technique has more than 25 years of history. Earlier technologies such as fused deposition modeling, laminated object manufacturing, stereolithography, jet printing, and others were limited to polymers only and the products thereof did not meet all the requirements of engineering materials (such as metals, ceramics, and composites) (Kruth et al., 2007). Previously, plastic-based prototypes were only used for inspection and communication of designs and product aspects (Santos et al., 2006). In this new era, with the invention of technologies such as selective laser sintering (SLS), direct metal laser sintering (DMLS), ZCast, three-dimensional (3D) printing, and others, the concept of rapid manufacturing has emerged these techniques have been used to produce end-use parts (Hague et al., 2004). ASTM later renamed AM techniques as AM technologies. Today, various types of AM technologies are commercial (refer Fig. 1); they are used in manufacturing and assembly principles for product design (Huang et al., 2013). Numerous types of AM technologies are widely discussed in the literature (Bourell et al., 2009; Wong and Hernandez, 2012). According to the Wohlers Report of 2013 (Wohlers Associates, Inc., 2013), the annual growth rate (compounded basis) of worldwide revenues from all AM products and services in the past 25 years is 25.4%. The growth rate from 2010 to 2012 has been recorded as 27.4% and reached $2.2 billion in 2012. The unit sales of industrial AM systems, which cost less than $5000, increased by 19.3% in 2012. However, the unit sales of 3D personal printers, which cost approximately $5000, increased by 46.3% in 2012 alone. AM technologies are known for the following various benefits (Holmström et al., 2010):
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No tooling is needed, thus significantly reducing production ramp-up time and expense. Small production batches are feasible and economical.
Fig. 1 Material categories for additive manufacturing (AM) technologies (Bikas et al., 2016).
Reference Module in Materials Science and Materials Engineering
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Additive Manufacturing: An Overview
Possibility to quickly change design. Allow product to be optimized for function (eg, optimized cooling channels). Allow economical custom products (batch of one). Possibility to reduce waste. Potential for simpler supply chains, shorter lead times, and lower inventories. Design customization.
AM is a digital technology used to produce physical objects layer-by-layer from a 3D CAD file. The process begins with generating a 3D CAD model of the object with all its details and dimensions. This 3D CAD file can also be generated from MRI/CT scan data by using sophisticated computer interface software. The 3D CAD file is sliced or sectioned into numerous, thin, twodimensional (2D) sections by a computer program (eg, Catalyst EX software for the Stratasys Inc. uPrint-FDM system). Then, the multiple slices of 2D sections are sent to the printing machine, which lays them on a platform, one after another (Kruth et al., 1998; Gibson et al., 2009). The process may take a few hours to a few days to produce an object, depending on the size, geometry, and precision. AM technologies are mainly dependent on the standard tessellation language (.STL) file because, for each part, the very first step is obtaining the required .STL file. The first .STL file was created in 1987 by 3D Systems Inc. and it was used for the stereolithography process. The .STL file creation process mainly converts the continuous geometry into a header, small triangles, or a triplet list of x, y, and z coordinates and the normal vector to the triangles. The interior and exterior surfaces are identified using the right-hand rule and additional edges are added once the figure is sliced. The slicing process also introduces inaccuracy to the file because the algorithm replaces the continuous contour with discrete stair steps (Iancu et al., 2010). To reduce this inaccuracy, the technique for a feature that has a small radius in relation to the dimension of the part involves creating .STL files separately and then combining them later. In addition to the .STl file, other types of files used are .SLC and .SLI from 3D Systems, command language interpreter (.CLI) from EOS, Hewlett-Packard graphics language (HPGL) from Hewlett-Packard, Stereolithography contour from Stratasys, and F&S from Fockele and Schwarze; initial graphics exchange specifications (IGES) are also used (Halloran et al., 2011). Due to the high cost of AM technologies, they have not been widely adopted by manufacturers; however, in the research and development sector of engineering, medicine, and construction, they are frequently being used (Chua et al., 1998; Flowers and Moniz, 2002). The research community has developed novel AM processes and has applied them to the aerospace, automotive, and biomedical fields, among others (Thomas et al., 1996; Song et al., 2002; Giannatsis and Dedoussis, 2009). Fig. 2 shows the applications of AM in various industrial sectors. Campbell et al. (2012) highlighted the development of AM technologies by three key industries for the following differing reasons:
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Automotive manufacturers exploited this technology because of the ability to help new products get to the market quickly and predictably. Small savings in time and development costs can result in significant overall savings in vehicle development. Aerospace companies are interested in these technologies because of the ability to realize highly complex and high-performance products and to integrate mechanical functionality, eliminate assembly features, and make it possible to create internal functionality (like cooling channels, internal honeycomb style structures, etc.). Medical industries are particularly interested in AM technologies because of the ease with which 3D medical imaging data can be converted into solid objects.
There are many AM techniques and numerous equipment available; therefore, proper selection of one that fulfills the required application criteria is necessary (Vayre et al., 2012). Accordingly, it is necessary to identify the specific manufacturing constraints
Fig. 2 Additive manufacturing in industrial sectors (Wohlers, 2011).
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and capabilities of available AM technologies. There are three main constraints for direct AM processes: the mechanism of material deposition (ie, nozzle must be parallel to the vertical axis), heat dissipation, and the part should be mounted on a rotating platter (which leads to accessibility constraints) to avoid collisions between the nozzle and the part. Additionally, the speed of material deposition depends mostly on the speed of the nozzle and the amount of material sprayed with the nozzle. The repetition of continuous acceleration and deceleration can cause the manufacturing to stop if the distance between the nozzle and the surface is too great, so that molten drops solidify before landing on the surface. To eliminate such problems, acceleration and deceleration stages must be minimized by avoiding sharp corners and replacing them with curves. Removal of powder from the part after its completion is another constraint of direct manufacturing-based AM technologies. Common engineering materials such as cast iron and aluminum alloys are not able to be processed with most AM technologies. This is a constraint of the manufacturing capabilities of direct manufacturing-based AM technologies. Cleaner production and sustainability are crucial for the manufacturing sector and direct manufacturing-based AM technologies have enabled the manufacture of functional/nonfunctional products. Bourhis et al. (2013) proposed a methodology to evaluate the environmental impact of a part from its CAD model by focusing on electrical, fluid, and material consumption. Significant progress has been made since 1980, resulting in various social benefits such as the following (Huang et al., 2013):
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Health care products customized to the needs of individual consumers. Reduced raw material usage and energy consumption, which are key contributions to environmental sustainability. On-demand manufacturing presented an opportunity to reconfigure the manufacturing supply chain to bring cheaper products to consumers.
Campbell et al. (2011) highlighted all these growing sectors of AM technologies. In their article, they surveyed the growth of AM in South Africa and highlighted some automotive, architectural, and medical applications, as shown in Fig. 3. AM is a promising approach for high-quality and efficient health care at economical prices and personalized care tailored to specific patient characteristics. Surgeons can build a patient-specific model to analyze when planning surgical procedures, those working in marketing and sales can perform surveys of new products, and so on (Wong and Hernandez, 2012). In 2001, approximately 20,700 Americans underwent chin augmentation surgery, 49% underwent lip augmentation, and 47% underwent cheek implantations (McCormick, 2011). The basic idea of making custom surgical implants relies on a computed tomography scan to obtain patient-specific data from which a solid model of the required implant is developed through reverse engineering. According to a recent publication, AM processes can be used to attain true 3D microproducts. These 3D micro-AM technologies were classified into three groups: scalable AM technologies for both macro scale and microscale; 3D direct writing technologies, which have been merely developed for microscale; and hybrid processes (Vaezi et al., 2013). Further, AM allows integration of functions from different parts for better performance even if there is a movability requirement for the part, like in the case of a ball and socket joint. The design freedom of AM enables simpler assembly, which becomes the focus and of higher importance (Lindemann et al., 2012). The fewer number of parts and fewer assembly steps may have a high impact on production costs. Fewer parts provide for advantages such as sourcing, labeling, and evaluating. Because there is no need for tooling for production of spare parts, it is unnecessary to keep legacy tooling in storage. Overall, AM has been proven to have extraordinary benefits in the existing manufacturing sector and could be applied to all its branches. However, there are some disappointments in terms of poor surface finish, lack of strength, and cost effectiveness. Higherend machines or mass production machines are costly and unaffordable for the medium-scale or small-scale manufacturing
Fig. 3 (a) Architectural model created using laser sintering, (b) titanium elbow implant and (c) turbine.
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sectors. So far, there have been various reviews of AM technologies (Hopkinson et al., 2006; Gibson et al., 2010). The present article focuses on the recent innovations and applications of AM techniques in aerospace/automobile, biomedicine, and construction applications and is helpful for upcoming scholars working in the field of AM and exploring future possibilities.
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Applications in Medicine
Ample AM opportunities exist in the biomedical field regarding the fabrication of custom-shaped orthopedic prostheses and implants, medical devices, biological chips, tissue scaffolds, living constructs, drug-screening models, and surgical planning and training apparatuses (Huang et al., 2015). CAD/CAM-based AM technologies for metals have found applications in the near net shape fabrication of complex geometries with tailored mechanical properties for biomedical sectors. With the introduction of electron beam melting (EBM), DMLS, selective laser melting (SLM), and SLS processes allow direct and digitally enabled fabrication of structures with controlled mechanical properties and desired external and internal characteristics (Harrysson and Cormier, 2006; Lin et al., 2007). The range of materials used with these advanced manufacturing technologies has increased over time, thus broadening the spectrum of applications. Composites with lattice patterns created by using solid freeform fabrication have been used as a host for filler materials. Metallic materials currently used with AM technologies for metal include titanium alloy Ti–6Al–4V, commercially pure titanium, Co–Cr alloy, Inconel, stainless steel, tool steel, aluminum, hard metals, amorphous metals, copper, niobium, and beryllium (Parthasarathy et al., 2011). In medicine, patients might have individual needs based on specific anatomy and it may be possible to include autologous cells to enhance the treatment. The behavior of cells can be directed by tailoring their environment. Patterning technologies can control surface chemistry and topography at scales smaller than a single cell (Melchels et al., 2012). Furthermore, 2013 marked the 15th year of cell printing, which is an ambitious, developmental biology-enabled, scaffold-less technique for fabricate living tissues and organs by printing living cells (Mironov et al., 2009; Ringeisen et al., 2013). A typical cell printing process consists of three stages:
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Preprocessing: creating tissue or organ-specific CAD models for each patient using CT scan data. Processing: using AM processes to deposit living cells into 3D biological constructs. Postprocessing: incubating printed tissues or organs to encourage tissue fusion and maturation.
Dérand et al. (2012) described the workflow of craniomaxillofacial surgery, which includes imaging, virtual design, manufacturing of patient-specific titanium reconstruction plates, cutting guides, and mesh, and the utility of the workflow in connection with surgical treatment of acquired bone defects in the mandible. The major stage in the printing of a biomedical implant/limb is the conversion of the image into the product. Computers are used for filtering initial medical image scan data to remove the lowerdensity tissue of the bones. After this, the data are used for generating a 3D CAD file that is analyzed at the preliminary stage. Then, the modified design is converted to the .STL format as required by the printers. Ramosoeu et al. (2010) used the DMLS system for producing complex topology components of Ti–6Al–4V for medicine. The final prepared components were heat-treated at 1000 and 11001C and subsequently cooled. In the case of slow cooling, it has been observed through optical analysis that more of the alpha phase was present as compared to the laser-sintered components. In the case of rapid cooling, the martensitic transformation resulted in higher hardness. Cohen et al. (2010) used novel geometric feedback-based approaches and demonstrated the in situ repair of both chondral and osteochondral defects that mimic naturally occurring pathologies. A calf femur was mounted in a custom jig and held within a robo-casting-based AM system (see Fig. 4). Repair prints for both defects had mean surface errors less than 0.1 mm. Similarly, Kundu et al. (2012) investigated AM with a multi-head deposition system to fabricate 3D cell-printed scaffolds using layer-by-layer deposition of polycaprolactone (PCL) and chondrocyte cell-encapsulated alginate hydrogel. Appropriate cell dispensing conditions and optimum alginate concentrations for maintaining cell viability were determined. Further, an in vitro cell-based biochemical analysis was performed to determine glycosaminoglycans, DNA and total collagen contents from different PCL alginate gel constructs. Podshivalov et al. (2013) proposed a novel approach for generating microscale scaffolds based on processing actual micro-CT images and then reconstructed a highly accurate geometrical model. This model was manufactured using a state-of-the-art 3D AM process with biocompatible materials. At the microscale level, these scaffolds were similar to the original tissue, thus interfacing better with the surrounding tissue and facilitating more efficient rehabilitation for the patient. Moreover, by means of multi-resolution volumetric modeling methods and scaffolds, porosity can also be adapted according to specific mechanical requirements. A scaffold is the major pillar of most tissue engineering approaches; it mimics an extracellular matrix for cells. The scaffold is seeded with cells that should provide the appropriate biomechanical and biochemical conditions for cell proliferation and eventual tissue formation (Marga et al., 2012). Puppi et al. (2012) fabricated polymeric scaffolds based on wet-spinning of poly (e-caprolactone) (PCL) or PCL/hydroxyapatite (HA) solutions through AM technique (see Fig. 5). Two different scaffold architectures were designed and fabricated by altering the inter-fiber distance and fiber staggering. The scaffolds that developed showed good reproducibility of the internal architecture and were characterized by highly porous, aligned fibers with an average diameter ranging from 200 to 250 mm. Cell culture experiments using the MC3T3-E1 preosteoblast cell line showed good cell adhesion, proliferation, alkaline phosphatase activity, and bone mineralization on these scaffolds. Wang et al. (2015) reviewed the state-of-the-art topological design and manufacturing processes of various types of porous metals, particularly titanium alloys, biodegradable metals, and shape memory alloys. Khoda et al. (2013) proposed a novel
Additive Manufacturing: An Overview
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Fig. 4 Femoral printing substrate.
Fig. 5 Scaffold structures.
technique using a controllable, heterogeneous architecture design suitable for AM processes. The proposed layer-based design uses a bi-layer pattern of consecutive radial and spiral layers to generate functionally gradient porosity, which follows the geometry of the scaffold. The proposed approach constructs the medial region from the medial axis of each corresponding layer, which is represented by the geometric internal feature or the spine. The radial layers of the scaffold are then generated by connecting the boundaries of the medial region and the layer’s outer contour. Gradient porosity changed between the medial region and the layer’s outer contour. The literature highlights that DMLS is one of the most useful technologies for preparing 3D porous bodies with complicated internal structures directly from titanium powders without any intermediate processing steps. These products are expected to be useful as a bone substitute or customized implant, as shown in Fig. 6 (Ciocca et al., 2010). Vaezi and Yang (2015) identified design, extrusion temperature, and ambient temperature of an extrusion-based AM process as the most important parameters for printing PEEK structures without warping, delamination, and polymer degradation. Compression and tensile tests were conducted to investigate mechanical properties of these new 3D printed PEEK structures. Murr et al. (2012) presented EBM for the fabrication of knee (see Fig. 7(a)) and hip implant (see Fig. 7(b)) components containing porous structures and demonstrated stiffness-compatible implants with optimal stress shielding for bones and bone cell in-growth. Systematic geometrical arrays of cellular reticulated mesh and open cell foams with interconnected porosities can be used to manufacture complex, functionally graded, monolithic structures as the next generation of biomedical implants. Cronskar et al. (2013) investigated the feasibility of EBM for biomedical applications. A case study of manufacturing hip stems for seven individuals was performed and compared with conventional machining process. Fatigue testing was performed to study the bone in-growth in the medial part of the stem and to determine how this surface influences fatigue properties. Hengsbach and Lantada (2014) explored the possibility of designing and manufacturing biomedical microdevices with multiple geometries with the help of AM technologies. The results of their study highlighted the versatility, accuracy, and manufacturing speed and allowed for the manufacturing of micro systems and implants with overall sizes up to several millimeters and with details of sub-micrometric structures.
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Fig. 6 Customized skull made of medical grade titanium.
Fig. 7 Knee joint (a) and hip joint (b) fabricated with electron beam melting.
Progress in AM technologies has led to the development of advanced and versatile systems for customized biomedical production. This is the new frontier of designing; with AM, fully integrated products and the automation necessary to fabricate ready-to-use tissue-engineered constructs on an industrial scale can be created. The rapid manufacturing of customized medial constructions allows fast production of large quantities of samples; this can enhance clinical routine procedures in terms of meeting daily surgical needs more effectively.
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Applications in Automobile/Aerospace
With AM technologies, it is now possible to fabricate large sections with light weights. Particularly in the automotive and aerospace industries, weight is always a major constraint because lighter parts are more efficient. AM technologies have enabled these industries to manufacture such parts along with precise, controlled, complex cross-sections (such as honeycomb cell) and desirable strengths (Bletzinger and Ramm, 2001). Further, AM technologies have made it possible to create structural parts for machines,
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thereby reducing the total weight (Williams, 2002). SLS and EBM are the two most widely used technologies in aircraft and aerospace industries. These technologies have created a new dimension of possible designs by using pre-alloyed metal powders (Liao et al., 2006). Regarding aerospace industries, the components often have complex geometries and are usually made from advanced materials such as titanium, nickel, special steels, and ultrahigh-temperature ceramics. Using conventional manufacturing processes, they are very difficult, costly, and tedious to manufacture (Huang et al., 2015). Further, production runs in aerospace industries are jobfocused or batch-focused; therefore, AM technologies are highly suitable for manufacturing these components. Kair and Sofos (2014) described the applications of AM technologies for passenger car engine components. Six significant components of the turbocharger (ie, compressor housing, compressor impeller, bearing system, center housing, assembly of turbine impeller, and shaft and turbine housing) were fabricated using AM. After conducting a critical analysis, it was noted that AM can help reduce the maintenance costs and extend the life of the turbocharger. According to an online source, BAE Systems approved a replacement part made using AM – a plastic window breather pipe for the BAE146 regional jet. Similarly, Optomec recently used the LENS process to fabricate complex metal components for satellites, helicopters, and jet engines. The US Navy hired Boeing Company to develop F/A-18E/F Super Hornet Fighter Jets. The preliminary demand was to minimize the cost of production and to shorten the manufacturing time to almost half. Improving product quality and adding six new avionics systems were also among the important objectives. Boeing has adopted SLS technology to manufacture air-cooling ducts to meet the functional requirements and limitations (see Fig. 8; Lyons, 2011). This AM technology enabled the engineering team to combine different ducts into single parts, to integrate the attachment mechanisms into them, and to reduce the overall number of parts (Hopkinson et al., 2006). Laser additive manufacturing (LAM) is a fusion-based process commercialized by AeroMet and developed by Johns Hopkins University and Pennsylvania State University. The resulting LAM process uses a high-wattage CO2 laser and a powder feed system to deposit wide, thick beads of Ti–6Al–4V onto a substrate. The foundation was developed for the fusion-based AM processes for the production of aerospace hardware. Unfortunately, AeroMet discounted this project in 2005. Advisory Council for Aviation Research, Innovation in the EU, and Flightpath 2050 worked for the reduction of fuel consumption and exhaust gases. To achieve this, SLM was used to manufacture lightweight engine components and structural parts from titanium alloy. Aurora Flight Sciences in collaboration with Stratasys Inc. fabricated and flew an aircraft with a 62-inch wingspan with a wing composed entirely of AM components. The wing was manufactured by FDM 3D printers. In a public–private venture, NASA and Pratt & Whitney Rocketdyne produced an AM-based rocket engine injector. This rocket engine injector is one of the most critical and expensive engine components in a launch vehicle, but with 70% less cost. AM-produced shipments for the US automotive industry were valued at $48 billion in 2011. As noted, approximately 19.5% of AM occurs within the automotive industry, with AM shipments estimated to be less than 0.05% of total US automotive shipments. Regarding automotive industries, AM technologies have been explored as an efficient tool in designing and developing components at low costs. Automotive companies are applying AM to an expanding range of parts, including engines and vehicle bodies. The automotive industry has used AM to make tool prototypes and custom parts for short production runs. It is increasingly applying the technology to metals, mainly Al alloys, to construct light-weight vehicles. AM processes have been used to make small quantities of structural and functional components (such as engine exhausts, drive shafts, gear box components, and braking systems for luxury vehicles). Various research institutes have successfully applied AM techniques to manufacture
Fig. 8 Design, production, testing, and implementation of additive manufactured parts (air-cooling ducts) deployed in the F-18E/F aircraft.
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Fig. 9 (a) Urbee three-dimensional-printed vehicle and (b) Urbee body and inner structure.
Fig. 10 PUUNK frame showing areas where additive manufacturing can be used (Richardson and Haylock, 2012).
functional components for racing vehicles. Unlike passenger cars, vehicles for motorsports usually are made of lightweight alloys (eg, titanium) and have highly complex structures and low production volumes (Huang et al., 2015). Ford Motor Company has been using AM technologies to develop prototype parts for test vehicles since the 1980s. Ford engineers have produced prototypes of cylinder heads, brake rotors, and rear axles in less time than traditional manufacturing could. The company reports that use of AM technologies saves 1 month of production time for developing a casting. KOR EcoLogic’s Urbee car (see Fig. 9) is a wonderful example of AM. Urbee is a 3D printed vehicle designed by KOR Ecologic Inc. The intension of the Urbee project was to develop an energy-efficient infrastructure for a vehicle that could be powered entirely by renewable energy. To achieve this, the body of the vehicle was specifically designed to make the best use of materials for a lightweight construction. This project was assisted by Stratasys Inc., Tebis, and Autodesk (software) and by CD-Adapco (simulation). The body panels were designed to incorporate sophisticated honeycomb structures that were unable to be produced using traditional manufacturing methods. Similarly, the PUUNK velomobile (see Fig. 10) project aims to provide bicycles or tricycles with a shell to provide weather protection and aerodynamic gains. AM could help them in their work. The velomobile comprises a personalized, user-generated, up-cycled, and configurable kit that helps to develop the ethos of this vehicle. This point is a key aspect in enabling its openness in design, opportunities for personalization, and modular configuration.
4
Applications in Construction
Creating an architectural/construction model can sometimes be a difficult task for architects, civil engineers, and contractors. In architecture, manual techniques are most often used to turn a concept or idea into a figure or rough drawing. However, when these models become complex, creating a physical model can be a difficult task. Thus, modeling is a very important aspect of construction for architects; they can study the models and their functionalities. These are also helpful when architects are explaining ideas to their customers and trying to convince them to fund their projects. AM technologies can provide architects with a very powerful tool for their business by enabling them to create a physical model faster without worrying about the complexity of the design. They also achieve better resolution than other processes used in
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architecture. Stereolithography is a very suitable process for architectural modeling because of the materials used and the printing resolution (Liao et al., 2006; Semetay, 2007; Xiong, 2009; Balla et al., 2008). Cement-based materials for AM were first introduced by Pegna (1997). There are three commercial AM processes targeted at construction and architecture applications (Khoshnevis et al., 2006). A research team at Loughborough University has developed some of the recent concrete printing technologies. All these AM technologies have allowed the successful manufacture of components of significant size and are suitable for construction and architectural applications (see Fig. 11). Contour crafting involves a crane-mounted device for on-site, in situ applications, whereas concrete printing is a gantry-based off-site manufacturing process. Although there is no specific reason why either process cannot be used on-site. Contour crafting uses two computer-controlled trowels to create surfaces on the object being fabricated. The layering approach enables creation of various surfaces. Counter crafting allows the design of structures with various architectural geometries that are difficult to realize using the current manual construction practices. Various materials may be used for outside surfaces and as fillers between surfaces (Tibaut et al., 2016). The D-shape process involves the use of powder that is selectively hardened using a binder in much the same way as the Z-Corp 3D printing process. Each layer of build material is laid to the desired thickness and compacted, and then the nozzles mounted on a gantry frame deposit the binder where the part is to be solid. This system has many advantages over traditional formative processes (use of formwork with concrete) as well as other additive building processes (eg, brick laying). It can use any sand-like material and produces little waste because the remaining sand can be reused (Tibaut et al., 2016). AM technologies like FDM, SLS, and stereolithography were later introduced into the construction industry to produce architectural models. The accuracy of these technologies was improved to 0.1–0.2 mm as compared to the accuracy of 0.2–0.4 mm in concept modeling (Ryder et al., 2002). WinSun, in 2014, built a two-story villa and a five-story apartment using AM technology and demonstrated the applicability of AM in large-scale buildings. One example of their work is provided in Fig. 12 (Wu et al., 2016). During their work they identified the following:
• • •
Indirect process: the villa and apartment were not printed directly from electronic data. Brittleness: carbon fiber led to the brittleness of the printing material. Exclusion of building services: services such as electrical and plumbing were not integrated in the AM process.
Fig. 11 Additive manufacturing techniques: (a) D-shape, (b) counter crafting, and (c) concrete printing (Lim et al., 2012).
Fig. 12 Connection details of the three-dimensional-printed villa by WinSun.
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Fig. 13 Construction component fabricated with additive manufacturing.
Lim et al. (2009) presented some of the recent developments in scaling-up complex construction components, as shown in Fig. 13. In their work, they discussed the development of the approach and preliminary components manufactured with different nozzle diameters. It was found that concrete printing is a promising field that can further grow to provide high-quality construction parts. This digital fabrication is enabling the production of buildings with freeform surfaces. Clients of the construction industry are asking leading designers to build structures that cannot be built by any currently known methods. However, AM processes are capable of delivering components large enough for building structures but that are unlikely to be scaled-up version (Buswella et al., 2007).
5
Conclusions
Currently, there are various types of AM technologies available to support numerous problems in different manufacturing sectors. Apart from having the same working principle, these technologies are distinctly different from one another, which has made these more suitable for satisfying universal problems. Three emerging areas of AM applications have been discussed in detail and the efficiency and effectiveness of these technologies have been highlighted. The AM-based medicine applications presented in this article indicate that this hybridization is mature due to the existence of realistic examples worldwide. However, in the case of the automotive and aerospace sectors, not enough has been investigated. Some of very interesting case studies are available, such as PUUNK, Urbee, and the F-18E/F aircraft. Further exploration is only possible through the collaborative efforts of industries and academia. The applications of AM technologies research to construction are ongoing. In the present article, it is believed that a new scenario for AM will be coming in the next 5 to 10 years and it will bring emerging applications, new developments, and business opportunities.
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Vaezi, M., Yang, S., 2015. Extrusion-based additive manufacturing of PEEK for biomedical applications. Virtual and Physical Prototyping. doi:10.1080/17452759.2015.1097053. Vayre, B., Vignat, F., Villeneuve, F., 2012. Designing for additive manufacturing. Procedia CIRP 3, 632–637. Wang, X., Xu, S., Zhou, S., et al., 2015. Topological design and additive manufacturing of porous metals for bone scaffolds and orthopaedic implants: A review. Biomaterials. doi:10.1016/j.biomaterials.2016.01.012. Williams, A., 2002. Architectural modelling as a form of research. Architectural Research Quarterly 6, 337–347. Wohlers, T., 2011. Wohlers Report 2011. Fort Collins, CO: Wohler Associates. Wohlers Associates, Inc., 2013. Wohlers Report 2013: Additive Manufacturing and 3D Printing State of the Industry. Fort Collins, CO: Wohlers Associates. Wong, K.V., Hernandez, A., 2012. A Review of Additive Manufacturing. ISRN Mechanical Engineering 2012. doi:10.5402/2012/208760. Wu, P., Wang, J., Wang, X., 2016. A critical review of the use of 3-D printing in the construction industry. Automation in Construction 68, 21–31. Xiong, Y., 2009. Investigation of the laser engineered net shaping process for nanostructured cermets. ProQuest Dissertations and Theses, University of California.
Further Reading Hopkinson, N., Dickens, P., 2003. Analysis of rapid manufacturing – Using layer manufacturing processes for production. Journal of Mechanical Engineering Science 217 (C1), 31–39. Kulkarni, P., Marsan, A., Dutta, D., 2000. A review of process planning techniques in layered manufacturing. Rapid Prototyping Journal 6 (1), 18–35. Riggs, B.C., Dias, A.D., Schiele, N.R., et al., 2011. Matrix-assisted pulsed laser methods for biofabrication. MRS Bulletin 36, 1043–1050. Xu, C., Chai, W., Huang, Y., Markwald, R.R., 2012. Scaffold-free inkjet printing of three-dimensional zigzag cellular tubes. Biotechnology and Bioengineering 109, 3152–3160.