3D printing is also termed as digital manufacturing or additive manufacturing (AM) technology, used to fabricate model or part through variety of material by adding sequential thin layering (Chua et al., 2020; Gross et al., 2014). This disruptive technology first came into existence in 1980s as stereo lithography. Thereafter, with the advancement in technology, it started manufacturing prototype models to food products, houses etc., (Sakin & Kiroglu, 2017; Tibbits et al., 2014). 3D printing basically consists of three steps as presented in Figure 1.

Figure 1. General Steps of 3D Printing (Shahrubudin et al., 2019)

Modeling is first step of additive manufacturing, in which CAD model is created using distinct software and digital design is created by converting into STL format.

In second step of processing, the STL file is uploaded to the 3D printer to prepare an object using different materials. However, most of the 3D printers build objects by depositing material layer by layer onto a bed starting from the bottom most layer. To build distinct objects for different applications 3D printers may use distinct materials. But, among distinct material thermoplastic is the most widely used material in this process.

In the third step the supports are removed from the constructed object, and then surface finishing processes like polishing, painting, etc. are performed to complete the final object (Balla et al., 2007). 3D printing has been used in several countries for product development owing to its charming characteristics like flexibility in design, light weight, on demand printing, cost effective, less wastage, fast design, ease of access, customization, and eco-friendly. Although, there are some limitations in 3D printing such as limited material selection, restricted build size, high cost of 3D printer, additional cost in post processing, copyright issues, etc. However, a lot of research is being done to reduce printer cost (Balla et al., 2007). ASTM International developed a standard to classify 3D printing methods into seven distinct types. Additive manufacturing has lot of benefits over traditional manufacturing such as part customization, less wastage, design flexibility, good quality etc. (ISO/ASTM, 2015; Kohtala, 2015; Masood, 2014; Mehrpouya et al., 2019).

There are different types of 3D printing process. However, the most commonly used are explained below.

1.1 Material Extrusion Process

Material extrusion process incorporates fusion filament fabrication (FDM) method. In this method two type of materials (support material and model material) are used. First the filament material of unwound type is coming from the spool and heated in semi liquid state in the meltor and then extruded/fed from the nozzle onto a build platform. In this way, material is sprayed continuously in layer form and finally builds an object as desired. The typical materials used in this method are composites, metals, ABS, PC and PLA plastics. The low cost is the major advantage of this process (Ashish, 2019; Sachlos & Czernuszka, 2003).

1.2 Powder Bed Fusion Process

In this process electron beam or laser is used to fabricate the object by melting or fusing powder material. Powder bed fusion consists of following 3D printing techniques such as selective laser sintering (SLS); electron beam melting (EBM); and direct metal laser sintering (DMLS), etc. In case of EBM technique, it required vacuum to fabricate a functional part by using alloys and metals. Whereas, DMLS is similar as that of SLS, but plastic is not used. In SLS, the fabrication of a part is done powdered materials such as metals, ceramics, polymers and composite by heating and depositing it in layer form over the base by utilizing a leveling roller. The good mechanical properties, support free process and strong layer adhesion are the major characteristics whereas, internal porosity is the drawback of this technology (Gokuldoss et al., 2017; King et al., 2015; Salmori et al., 2009).

1.3 Directed Energy Deposition Process

This printing method is similar to material extrusion process but the only difference is that the nozzle move in multiple axis and use energy in the form of plasma arc, laser and electron beam. The major application of this method is to repair and maintain the structural components. The DED printer used laser beam, consist of a nozzle (multiple/single type) that sprayed melted material onto the surface, to fabricate an object in subsquential layers. The DED uses HA/PLA, metal alloy, HA/PCL direct metal deposition (DMD) and laser-engineered net shaping (LENS) are the major example of this method (Sing et al., 2020).

1.4 Sheet Lamination Process

This process is introduced by Helisys in 1998 which used distinct materials (PVC, Zirconia, HA) with localized energy medium in the form of laser/ultrasonic and laminated in a layer wise manner to develop a part. Laminated object manufacturing (LOM) is an example of this process method. The typical application of LOM mainly is the prototype fabrication as per the ASTM F-42 standard (Feygin et al., 1998; Rapid Park et al., 2000; Zhang, 2018). The schematic diagram of distinct AM process methods are illustrated in Figure 2 (a-d).

Figure 2. Schematic Diagram of Distinct Additive Manufacturing Methods: (a) Fusion Deposition Modeling (b) Electron Beam with Wire Feedstock (c) Direct Energy Deposition (d) Laminated Object Manufacturing (Liu et al., 2019)

The additive manufacturing used wide range of materials for the fabrication of model, implant, etc. These materials includes metals, ceramics, concrete, plastics, polymers, composite, smart material (shape memory polymer and alloy), special materials (food, textile), etc., for distinct applications. The basic features of 3D printing materials with respect to different technologies are described in Table 1.

Table 1. Comparison of Distinct 3D Printing Materials, Technology Used and Field of Applications

The major applications of AM technology are in the manufacturing of functional components, metal casting model, visual aids, etc. It is a commercial technology, and widely used in medical industries, dentistry, design, manufacturing, engineering, research and development, reverse engineering, aerospace industry, automotive industries, architecture, food and agriculture, etc. (Azari & Nikzad, 2009; Borille et al., 2017; Buswell et al., 2007; Dwivedi et al., 2017; Javaid & Haleem, 2019; Liu et al., 2017; McMillan et al., 2017; Minetola & Iuliano, 2014; Salmi et al., 2013; Soe et al., 2012; Strong et al., 2017; Vaughan & Crawford, 2013). The sector wise primary applications of AM are presented in Figure 3.

Figure 3. AM Applications in Different Sectors

3.1 Automotive Industry

The 3D printing technology in automobile industry has increased very rapidly over the past decade. Although, recently, automobile industry faces lot of challenges such as development of new tools, light weight models with low cost parts, etc. AM technologies have radically developed and manufactured new product allowing for manufacturing highly complicated structures with light weight and reasonable cost. The major applications in the automobile industry are the fabrication of light weight components, complicated shape, product optimization, design flexibility, manufacturing of small batch sizes having low cost, tooling, end-part production, etc. The common uses of AM in automotive applications are to produce complicated ducting, bellow for assembly, aesthetic prototypes, lightweight bracket, etc. Nowadays, the another 3D printing applications in automotive field includes valves and pump fabrication in fluid handling system, seat frame, wheels, tyres, engine parts, etc. However, the major challenges faced by automakers is the limited fabrication size of several 3D printing systems, high adoption in the part production volume, etc. (Biamino et al., 2014; Cooper et al., 2015; Cotteleer et al., 2014). The AM is expected to be utilized in the near future for the manufacturing of corrosion resistant super alloys. AM techniques can reduce the material wastage, reduce fabrication time as well as cost. The use of AM technology in automotive field is shown in Figure 4 (Kumar et al., 2018; Pessoa & Guimarães, 2020; Sreehitha, 2017).

Figure 4. Automotive Application: Toyota's i-ROAD Vehicle

3.2 Medical Industry

The application of 3D printing in medical sector has rapidly increased and is being used the development of individual patient models, implants, vascular structures, etc. Apart from these applications, 3D printing technology in medical industry is used for development of skull models of human, skull defects and prosthetic leg, bioprinting tissues and organs, surgical model, etc. for pre-surgical planning, and customization solution for every patient’s need. Some of the distinct medical applications (Do et al., 2015; Domínguez- Rodríguez et al., 2018; Jiménez et al., 2019; Parthasarathy et al., 2011; Tukuru et al., 2008; Tuomi et al., 2014; Ventola, 2014) are presented in Figure 5.

Figure 5. (a) 3D Digital Reconstruction of Skull Defect. (b) Skull Model. (c) Pre-Surgical Planning (d) Prosthetic Leg

3.3 Architecture Applications

Additive manufacturing is highly suitable to construct building parts as they are ecofriendly. There are distinct types of AM processes used in developing an architectural model, but SLA (streolithography) printing technique is highly suitable because printing materials are easily available. The Amsterdam (Canal House) and the Printed House in Russia built by Apis Cor are the major examples that applied this manufacturing technology. In addition, it has many advantages such as to develop a building in very short time which helps to save the time, seamless and economic integration, etc. (Bogue, 2013; Hager et al., 2016; Paolini et al., 2019; Sakin & Kiroglu, 2017; Tay et al., 2017; Wong & Hernandez, 2012).

3.4 Fabric and Fashion Industry

Additive manufacturing is highly important in fabric and fashion industry, especially in clothing and jewelry, owing to the advantages such as to manufacturing distinct shapes in short time, on-demand custom fit and deliver items in limited quantity. There are distinct types of AM methods, but among all these Fused Disposition Modeling (FDM) method is most widely used because of light weight materials and low cost of the printer. In addition, AM helps the designers to develop designs that are highly complicated and innovative (Attaran, 2017; Gaget, 2018). The specifications of distinct printers used in fashion industry are presented in Table 2.

Table 2. Specifications 3D Printers used in Fashion Industry

3.5 Food Industry

Healthy food ingredients are very important to each and every human being for their well being. Recently, different shaped food products (chocolate, pizza, etc.) are produced using distinct AM techniques (binder jetting, ink jet printing extrusion based printing) which fulfill the individual demand of the customer. Hence, 3D printing plays a vital role in food industry also (Frazier, 2010; Liu et al., 2019; Mami et al., 2017).

3.6 Aerospace Industry

AM in aerospace applications plays an effective role using RP (Rapid Prototyping) technology. It saves time and cost and is much helpful in part assembly and repair work in the aerospace field. As per the report of 'NASA' the application of 3D printing indicates that there is a reduction of 8.3% NOx emission (Haller, 2015). Apart from these part consolidations, waste reduction, repair and maintenance, function are the major advantages of AM technology in aerospace industry. The example of the LENS repair system includes; low heat input, good mechanical properties, more repair access and low HAZ (heat affected zone). In addition 3D printing technologies have revealed great capabilities and the potential to reduce cost, turnaround time and weight of the part used in aerospace applications (Baron, 2011; Cooley, 2005; Gausemeier et al., 2011; Trivedi, 2014).

The results of various researchers from the 3D printing techniques are discussed below.

Lu et al. (2020) used laser shock peening as an effective surface treatment technique in order to improve the microstructure and mechanical properties of selected laser melted Ti6Al4V parts. The results showed that the surface micro-hardness of the vertical and horizontal selective laser melted parts is about 324 HV and 319 HV. However, under the effect of laser shock wave (LSW) it is increased to 420 HV and 405 HV. The ultimate tensile strength and elongation of both horizontal samples were larger as compared to vertical samples. Laser shock wave (LSW) produced a good combination of ductility and ultimate tensile strength of both selective laser melted samples.

Onuike et al. (2018) used two different materials (IN718+Cu alloy and GRCop-84 Cu alloy) for fabrication of biomedical product using laser engineering net shaping (LENS) technique. Results showed that there is an enhancement in thermal diffusivity in case of bimetallic = 1.33 mm²/s, which approximately increased by 250% compared to pure Inconel 718 alloy at 3.20 mm²/s. The conductivity is also improved by 300% than Inconel 718.

Bartolomeu et al. (2017) adopted 316L Stainless Steel for fabrication of implant through 3D printing and conventional processes (HP, casting and SLM). The result showed that selective laser melting (SLM) process produced superior mechanical properties and tribological performance for 316L stainless steel than hot pressing and casting method. This may be attributed to the finer micro-structure produced by selective laser melting. Thus SLM is an effective method to develop a customized 316L stainless steel implants with enhanced wear and mechanical performance.

Taniguchi et al. (2016) investigated the influence of pore size on bone tissue growth in a rabbit using vivo analysis. For this investigation, Ti samples having distinct pore size (300 μm, 600 μm and 900 μm) in 3D shapes were developed using SLM technique. Results showed that the porous implant having pore size of 600 μm is best for orthopedic applications due to its proper bone ingrowth and highest bone-material fixation ability.

Hwang et al. (2015) used ABS+Fe/Cu material for biomedical product application through Fusion Filament Fabrication (FFF) process. Results indicate that as we increase the value of loading metal particles the composites tensile strength is reduced. However, at the time of printing, the improvement in thermal conductivity of ABS thermoplastic filament will decrease the distortion of the final part.

Li et al. (2012) used Ti-6Al-4V for analysis of biological characteristics through EBM technique. The study revealed that the electron beam porous (Ti6Al4V implant) decreases the stress shielding as well as exerted good osteoconductive properties.

Parthasarathy et al. (2011) utilized functionally graded porous structures for biomedical product through EBM technique. Results reveled that there is an improvement in tensile strength (which is approximately 115%), modulus of elasticity (700%) and moderate porosity. Implants designed with this new strategy and fabricated by using 3D printing technology would satisfy the long-felt requirements for light weight implants for aesthetic needs of the surgical community.

Facchini et al. (2010) used Ti6Al4V alloy for fabrication of biomedical components by selective laser melting technique to check the part strength. Result showed that there is an improvement in tensile properties of 3D printing part than hot worked components, but ductility is found to be less.

Thomsen et al. (2009) prepared Ti6Al4V implants by FFF and were utilized either as produced or after machining and compared with wrought machined Ti6Al4V. The bulk chemical as well as mechanical characteristics of the reference material and EBM material were within the ASTM F136 specifications. In addition fabricated EBM Ti6Al4V implants have enhanced surface roughness, more iron content and thicker surface oxide, and almost same surface chemical composition than with machined EBM Ti6Al4V and machined wrought Ti6Al4V implants.

Balla et al. (2007) used Ti6Al4V+CoCrMo material for implant fabrication in biomedical sector through LENS 3D printing process technique. Results showed that Co–Cr–Mo alloy (50%) in general provide resistance against wear and best combination of bio-compatibility. The hardness of such alloy is also superior to the average value of hardness in case of Ti6Al4V alloy (laser sprayed) which has approximately 333 ± 16 HV.

Li et al. (2005) produced a porous Ti6Al4V sample using a polymeric sponge replication technique for orthopedic application. Result showed that porous Ti6Al4V sample is most promising biomaterial for orthopedic implant, with total porosity (90%) compressive strength (10.3±3.3 MPa) comprehensive strength (10.3±3.3 MPa) and elastic constant (0.8±0.3 GPa).

The mechanical properties and characteristics of materials used for 3D printing is summarized in Figure 6.

Figure 6. Distinct Mechanical Properties Normally Generated for Structural Materials (Lewandowski & Seifi, 2016)

However, the distinct mechanical property of distinct AM methods and their particular applications are illustrated in Table 3.

Table 3. AM Methods, Application, Materials, Mechanical Property and their Results

Nanomaterials such as carbon nanotube, graphite, graphene, ceramic and metal nanoparticle possess unique thermal, mechanical and electrical properties. However, the high-performance functional composites can be produced by the addition of nanomaterials with polymers for printing. In contrast, nanomaterials are used for enhancing the mechanical properties of printed composite parts. Thus, the addition of nanomaterials leads to increase in tensile strength of printed composite parts than with unfilled polymer parts.

The results are summarized in Table 4. Here, FDM-Fusion Deposition Modeling, SLA-Stereo lithography, DLP-Digital Light Processing, SLS-Selective Laser Sintering, ABS-Acrylonitrile Butadiene Styrene.

Table 4. Effect of the Addition of Reinforcing Materials on the Mechanical Properties of 3D Printed Polymer Parts

Finally, the summary of distinct 3D printing methods along with their applications is given in Table 5 (Chu et al., 2014; Fotovvati et al., 2018a, 2018b; Fotovvati et al., 2019; Miyanaji et al., 2018; Mostafaei et al., 2018; Shim et al., 2016; Shishkovsky et al., 2018; Tsang & Bhatia, 2004; Turner & Gold, 2015; Wang et al., 2019)

Table 5. Summary of Distinct 3D Printing Methods and their Field of Applications

This paper aimed to figure out the scenario of 3D printing in different sectors including, automobile, medical, military, government and aerospace, etc. Among these sectors, 3D printing in medical field has great potential. This technology helps the doctors to treat more patients with high accuracy and quality. Medical sector is an emerging sector owing to customization, design flexibility, high accuracy and ability to produce intricate geometries. 3D printing is also mostly used in tissue engineering, where the production of micro-scale lattice structures is an intrinsic requirement. The enhancement in 3D printing materials and product quality are expanding the usefulness of 3D printing in medical sector. A lot of research in medical sector to explore its new applications has been done. Some advance medical application such as organ printing is under progress. A compensative literature review has been performed to study the mechanical properties of distinct 3D printing processes. In addition, literature revealed that material reinforcement help to improve mechanical properties of 3D printed part.

Future Scope and Challenges

In the future, researchers can explore the investigation to identify optimum parameter settings for specific 3D printing process and materials to obtain desired properties in the product. In addition, some research on lead time, product cost, and process capability of distinct type of 3D printing techniques can be performed for comparison and economic benefits.

The main challenges that 3D printing faces are:

• Intellectual property right of 3D printer user (for example pistol manufacturing).
• Nearly anything can be printed by 3D printer and this is a troubling prospect, if criminals used 3D printer to create illegal product.
• Production speed as well as cost.
• Regular changing set of competitors.
• Automation of 3D printing process and system planning to enhance the manufacturing efficiency.