A survey of additive manufacturing reviews

Nowadays, additive manufacturing (AM) technologies have been widely used in construction, medical, military, aerospace, fashion, etc. The advantages of AM (e.g., more design freedom, no restriction on the complexity of parts, and rapid prototyping) have attracted a growing number of researchers. Increasing number of papers are published each year. Until now, thousands of review papers have already been published in the field of AM. It is, therefore, perhaps timely to perform a survey on AM review papers so as to provide an overview and guidance for readers to choose their interested reviews on some specific topics. This survey gives detailed analysis on these reviews, divides these reviews into different groups based on the AM techniques and materials used, highlights some important reviews in this area, and provides some discussions and insights.


Introduction
Thirty years into its development, additive manufacturing (AM, also known as 3D printing) has become a mainstream manufacturing process. AM fabricates parts by adding materials layer-by-layer directly based on a 3D model. It is able to manufacture complex parts and allows more freedom of design optimization compared with traditional manufacturing techniques [1] . According to ISO/ASTM, AM can be divided into seven groups: vat photopolymerization, material jetting, binder jetting, powder bed fusion, material extrusion, directed energy deposition, and sheet lamination [2] . AM has its distinctive advantages over conventional manufacturing processes, for example, reduced product development time, lower cost, and ability to fabricate almost any complex shape. Therefore, AM has now been widely used in construction, medical, military, aerospace, fashion, etc. Until now, thousands of review papers have already been published in the field of AM, let alone the published research papers in this field. Figure 1 shows the number of published review papers in AM in each year. As can be seen, there are too many AM review papers published in recent years, with huge increasing rate. It is, therefore, perhaps timely to conduct a survey on AM review papers so as to provide an overview and guidance for readers to choose their interested reviews on some specific topics. This survey gives detailed analysis on these reviews, divides these reviews into different groups, and highlights some important reviews in this area along with discussions.

Material jetting
Material jetting is similar to inkjet printing. Inkjet printing deposits ink droplets onto a substrate drop by drop, while material jetting process directly deposits wax and/or photopolymer droplets onto the substrate by on-demand inkjet [24,25] . Light curing or heating is the driving force of the phase change of the sprayed droplets. A lot of research has been carried out on material jetting, including direct ink jetting of nanoink suspensions of ceramics [26,27] , semiconductor [28] , and metals [29] .

Binder jetting
In binder jetting, a liquid polymer is selectively deposited onto a bed of powder [30] . The jetted polymer droplet infiltrates the powder surface, leading to a printed powder agglomerate primitive. Powder spreading promotes recoating, as is done in powder bed fusion processes. The finished parts are composed of bound powder, which requires infiltration through post-processing to gain enough strength. Any powdered material that can be successfully spread and wet by the jetted binder can be used in this technique. Different materials have been studied using this technique, for example, foundry sand [31] , metal [32] , polymer materials [33] , and ceramic [34] . The binding mechanism of this technique is chemical and/or thermal reaction bonding. Depending on the bonding agent, chemical reaction is generally the source of activation. After completing the fabrication, post-processing may be necessary, including removal of loose powder and impregnation/infiltration of Survey of AM reviews Materials Science in Additive Manufacturing suitable liquid material depending on the powder material and intended application.

Directed energy deposition
In directed energy deposition (DED), metallic powder or wire is fed directly into the focal point of an energy beam to create a molten pool [35] . Laser Engineered Net Shaping (LENS), belonging to DED, was first developed at Sandia National Laboratories in 1995 and commercialized by Optomec [36] . Parts printed by LENS accommodate graded multi-materials [37] and allow microstructures with complex inner features [38] . DED systems with wire-fed methods have been achieved [39] , and DED of powder directly has also been successful [40,41] . Lasers and electron beams are the most commonly used energy source.

Vat photopolymerization
The definition of Vat photopolymerization is an "additive manufacturing process in which liquid photopolymer in a vat is selectively cured by light-activated polymerization" [2] . Vat photopolymerization uses a (liquid) photopolymer resin which is able to cure (solidify) under a light source [42,43] . Stereolithography (SLA) and digital light processing (DLP) are the most used techniques which belong to Vat photopolymerization. The scanning speed of vat photopolymerization is relatively high and minimum layer thickness is adjustable depending on the curing depth [44] . Once finishing the printing, post-processing may be needed, for example, support material removal and/or post-curing by further UV exposure.

Sheet lamination
Sheet lamination is an AM process in which sheets of material are bonded to form a part [45] . The process works by scrubbing each layer together with pressure and/or binders continuously. In this technique, the raw material typically is paper, metal foil, polymers or composite sheets predominately formed of metal, or ceramic powder material. Thermal reaction, chemical reaction bonding, or ultrasound can be used for binding. The source of activation includes localized or large-scale heating, chemical reaction, and ultrasonic transducers.

Analysis and discussion of AM review papers
This section gives the detailed analysis of review papers published within the field of AM. Top authors, source journals, affiliations of authors, and countries, are discussed. Then, the review papers are analyzed and discussed based on their different focuses, for example, different AM techniques (as briefly introduced in the previous section) and materials used. The database used is Scopus. Scopus is one of the most used databases, and it includes more papers than the Web of science.

Top 10 authors
As shown in Figure 2, Ramakrishna Seeram from National University of Singapore has the most review papers (21) published within AM field, followed by Chua Chee Kai from Singapore University of Technology and Design, and Yeong Wai Yee from Nanyang Technological University. It is interesting that all the top three authors are from Singapore. Researchers may refer their publications to catch up the up-to-date research in the AM field.

Top ten journals
Looking at the sources of these review papers (Figure 3), most of these AM review papers are published in journal Additive Manufacturing, followed by Materials and International Journal of Advanced Manufacturing Technology. Researchers may check these journals' websites to see the state-of-theart developments of AM technologies.

Top 10 affiliations
As shown in Figure 4, most of the review papers in additive manufacturing are from Nanyang Technological University, followed by Singapore Centre for 3D Printing.

Top ten countries
Looking at the countries of the authors from, United States has the most review papers in AM, with 756 review papers published, followed by China with 617 publications ( Figure 5).

Review papers in the seven AM techniques
Dividing these review papers into the seven AM techniques as introduced in section 2, it can be found that most review papers are about powder bed fusion, and no review paper is found in sheet lamination ( Figure 6). This is probably because powder bed fusion is the most focused research area within AM, due to its application potential in aerospace, engineering, and biomedicine. While, sheet lamination seems a little bit out of focus at this moment. Note that, the review papers collected in this subsection only consider the broad review in these seven AM techniques, excluding the review papers focused on a specific topic (e.g., process parameters' influence, fatigue analysis, and path planning). For the broad reviews in these seven AM techniques, the most cited papers are listed in Table 1. Readers can check these papers based on their interests. Table 2 gives more review papers focusing on the specific topics in each AM technique. For example, Nohut and Schwentenwein [46] focuses on functionally graded materials in vat photopolymerization, while Xu et al. [47] focuses on drug https://doi.org/10.18063/msam.v1i4.21 Survey of AM reviews Materials Science in Additive Manufacturing delivery and medical device in Vat photopolymerization. In terms of powder bed fusion, Luo and Zhao [48] focuses on thermal stress, while McCann et al. [49] focuses on process monitoring and machine control. More details on the topics these review papers focus on are shown in Table 2.

Review paper categories based on materials
From the point view of materials, there are also various review papers in additive manufacturing focusing on different materials. In this survey, the materials are categorized into ten groups, including metal, ceramic, polymer, biomaterial, concrete, fiber, food, smart material, glass, and wood for AM. As shown in Figure 7, most review papers revolve around polymer and metal. This is probably because both polymer and metal are the most commonly used materials and have already been studied a lot. Table 3 lists the most cited review papers in each type of material. Table 4 presents more review papers in

Review paper categories based on research area
In this section, AM review papers that focus on key/hottest areas (e.g., aerospace, tissue engineering) will be discussed. Nowadays, AM is widely used in different fields, including aerospace, tissue engineering, construction, drug delivery, topology optimization, etc. The most cited review papers focused on these areas are provided in this subsection, as shown in   Galante et al. [132] Bone tissue 3D printing of ceramic-based scaffolds for bone tissue engineering: An overview Du et al. [133] SiC ceramic Progress and challenges toward additive manufacturing of SiC ceramic He et al. [134] Graphene Direct ink writing technology (3d printing) of graphene-based ceramic nanocomposites: A review Pinargote et al. [135] Ceramic membrane A comprehensive review of recent developments in 3D printing technique for ceramic membrane fabrication for water purification Dommati et al. [136] Cellular ceramic Cellular ceramic architectures produced by hybrid additive manufacturing: A review on the evolution of their design Pelanconi et al. [137] Polymer Mechanical Mechanical characterization of 3D-printed polymers Dizon et al. [138] Polymer-fiber A review on additive manufacturing of polymer-fiber composites Parandoush and Lin [95] Nanocomposites High performance polymer nanocomposites for additive manufacturing applications De Leon et al. [139] 3D printing of polymer nanocomposites via stereolithography Manapat et al. [140] Natural fiber Additive manufacturing of natural fiber reinforced polymer composites: Processing and prospects Balla et al. [141] Gradient scaffolds 3D printing for the design and fabrication of polymer-based gradient scaffolds Bracaglia et al. [142] Stainless  Ti-based alloys Mechanical Additive manufacturing and post-processing of Ti-6Al-4V for superior mechanical properties Qian et al. [155] Fatigue A review of the as-built SLM Ti-6Al-4V mechanical properties towards achieving fatigue resistant designs Agius et al. [156] Biomedical A review of powdered additive manufacturing techniques for Ti-6al-4v biomedical applications Harun et al. [157] Chemical polishing Chemical polishing of scaffolds made of Ti-6Al-7Nb alloy by additive manufacturing Lyczkowska et al. [158] Mechanical Mechanical properties of titanium-based Ti-6Al-4V alloys manufactured by powder bed additive manufacture Tong et al. [159] Process parameters Selective laser manufacturing of Ti-based alloys and composites: impact of process parameters, application trends, and future prospects Singh et al. [82] Heat treatment A review of heat treatments on improving the quality and residual stresses of the Ti-6Al-4V parts produced by additive manufacturing Teixeira et al. [83] Surface roughness A review on the influence of process variables on the surface roughness of Ti-6Al-4V by electron beam powder bed fusion de Campos Carolo and Ordoñez [160] Biomaterial Bioink Bioink properties before, during and after 3D bioprinting Hölzl et al. [161] Biomedical; tissue 3D bioprinting for biomedical devices and tissue engineering: A review of recent trends and advances Derakhshanfar et al. [162] Printability Printability and Shape Fidelity of Bioinks in 3D Bioprinting Schwab et al. [163] Cell-Hydrogels Design and printing strategies in 3D bioprinting of cell-hydrogels: A review Lee et al. [164] Skin 3D bioprinting of skin: A state-of-the-art review on modeling, materials, and processes Vijayavenkataraman et al. [165] Hydrogel 3D bioprinting of photo crosslinkable hydrogel constructs Pereira et al. [166] Cardiac tissue; cell 3D Bioprinting of cardiac tissue and cardiac stem cell therapy Alonzo et al. [167] Machine learning A perspective on using machine learning in 3D bioprinting Yu et al. [168] Organ The emergence of 3D bioprinting in organ-on-chip systems Fetah et al. [169] Liver transplantation Bioprinting for liver transplantation Kryou et al. [170] Process parameters Effects of processing parameters of 3D bioprinting on the cellular activity of bioinks Adhikari et al. [171] Concrete Simulation Numerical simulations of concrete processing: From standard formative casting to additive manufacturing Roussel et al. [172] Extrusion-based Extrusion-based additive manufacturing of concrete products: Revolutionizing and remodeling the construction industry Valente et al. [173] Biomimicry Biomimicry for 3D concrete printing: A review and perspective du Plessis et al. [174] Functionally graded concrete  Bone repair Recent progress on 3D-printed polylactic acid and its applications in bone repair Chen et al. [187] 4D printing 4D printing of shape memory polylactic acid (PLA) Mehrpouya et al. [188] Process Parameter; Mechanical The influence of the process parameters on the mechanical properties of PLA specimens produced by fused filament fabrication-A review Cojocaru et al. [189] PEEK Process parameter An overview on the influence of process parameters through the characteristic of 3D-printed PEEK and PEI parts El Magri et al. [190] FDM Applications of 3D-printed peek via fused filament fabrication: A systematic review Dua et al. [191] Aluminum alloys

Microstructure; mechanical
Microstructure and mechanical property considerations in additive manufacturing of aluminum alloys Ding et al. [192] Mechanical Mechanical properties of SLM-printed aluminium alloys: A review Ponnusamy et al. [193] Heat treatment Heat treatment of aluminium alloys produced by laser powder bed fusion: A review Fiocchi et al. [194] WAAM Challenges associated with the wire arc additive manufacturing (WAAM) of aluminium alloys Thapliyal [195] Corrosion Corrosion and corrosion protection of additively manufactured aluminium alloys-a critical review Revilla et al. [196] Copper Pure copper A review on additive manufacturing of pure copper Jiang et al. [197] Food Functional Toward the design of functional foods and biobased products by 3D printing: A review Portanguen et al. [198] Plant-based 3D food printing: Applications of plant-based materials in extrusion-based food printing Wang et al. [199] Food material A review on 3D printable food materials: types and development trends  Haleem et al. [202] Wearable application Potentials of additive manufacturing with smart materials for chemical biomarkers in wearable applications Kwon et al. [203] Glass Crystallization Crystallization in additive manufacturing of metallic glasses: A review Liu et al. [204] Silica Glass Overview of 3D-printed silica glass Zhang et al. [205] Wood Wood powders A review on wood powders in 3D printing: processes, properties and potential applications Das et al. [206] Statistics from Scopus database; access date: October 19, 2022

Conclusions
In this work, we conducted a survey on published review papers in AM. Analysis and discussion on reviews in https://doi.org/10.18063/msam.v1i4.21 Survey of AM reviews Materials Science in Additive Manufacturing seven AM techniques are given (i.e., material extrusion, powder bed fusion, material jetting, binder jetting, directed energy deposition, vat photopolymerization, and sheet lamination). As can be seen, most of the review papers are in the categories of powder bed fusion and directed energy deposition. No review papers in sheet lamination were found. In the future, it is necessary to carry out a review on sheet lamination, although it is not a famous AM technique. In addition, typical review papers are categorized into different groups based on the materials these review papers focused on (e.g., metal, ceramic, polymer, biomaterial, concrete, fiber, food, smart material, glass, and wood). The specific objectives of each review paper are listed, as shown in Table 4. For example, He et al. [134] focuses on SiC ceramic in AM, and readers can refer accordingly based on their interests. The aim of this survey paper is to provide a guidance to the development of AM review papers, give a comprehensive analysis on the current available review papers in this field, and hopefully, provide some insights and inspire more ideas. As the review papers published in AM are increasing; nowadays, the selected review papers in this survey are based on the Scopus database, which might have some limitations. In addition, this survey only considers the most cited papers in each category based on the number of citations, while the published time of the review papers is not considered.