## 1 Introduction  
Additive manufacturing (AM) or three-dimensional (3D) printing is a manufacturing process that enables fabrication of objects by successively printing layer upon layer guided by a digital 3D model [1, 2]. This unique feature allows the manufacture of complex geometries that are almost impossible to fabricate using conventional systems. Therefore, AM is a tool resource that enables designers to create custom or intricate models in one-step without conventional manufacturing limitations such as high material waste, difficulty to manufacture complex shapes, and the need for specialised tooling. Accordingly, through the use of AM, engineers have increased 'design freedom' [3]. Furthermore, part counts can be reduced and consequently, assembly time and cost are reduced or eliminated [4]. In addition, parts can be manufactured on demand, which improves response time, shortens supply chain(s), reduces storage needs, eliminates delivery costs, and decreases lead-time for critical replacement parts [3]. [PERSON] et al. [5] concluded that it is expected that the AM industry growth continues over the next few years, and the sale of AM is predicted to exceed AU$22.9 (US$15.8) billion worldwide by industry sectors. According to the data published by [PERSON] et al. [5], Figure 1 represents the distribution of AM revenues for the end-market in 2018, and represents the diverse industrial adoption of AM. AM technologies can use various materials including polymers, ceramics, and metals. Among these materials, metallic materials are gaining an increased interest from researchers and industries. In addition to the above mentioned benefits, metal AM may provide some environmental advantages such as less wastage, quality improvement, less pollutant emissions, and making parts on demand [6,7]. While metal AM provides these benefits, the extent of utilisation of metal AM technologies in industry is currently limited to a few industry sectors such as dental [8], construction [9], and aerospace [10]. In addition, researchers have generally focused on the investigation of a narrow set of metal AM processes. For instance, [PERSON] [11] reviewed different direct metal AM processes and addressed constraints related to applications of them. [PERSON] et al. [12] introduced arc welding AM processes and reviewed mechanical properties of metallic materials such as Titanium alloy. [PERSON] and [PERSON] [13] presented a review of binder jetting processes, materials, and current technologies. Some researchers focused only on the material properties of metal AM products. For instance, [PERSON] et al. [14] introduced microstructures, defects, and mechanical properties of metallic printed materials. [PERSON] et al. [15] reviewed the effects of key printing parameters on the mechanical properties and microstructure of several metallic materials such as Titanium, Aluminium, and Nickel-based alloys. Additionally, [PERSON] et al. [16] reviewed metal AM materials, particularly refractory alloys, precious metals, and compositionally graded alloys. From the above, it can be seen that the metal AM has been widely published from a material development point of view by different researchers; however, the application and suitability of different metal AM require further investigation. Clearly, an overview that introduces all of the most recent common metal AM processes and provides a wider exploration of industry applications, limitations, and challenges is required. The aim of this study is to present a comprehensive review of the current state of metal AM, in particular introducing the most common processes, examining industrial applications across different industry sectors, exploring recent advancements, and outlining current challenges and limitations. To achieve these goals, the research and industrial data that was mainly published between 2010 and 2020 was used.