## 5 Conclusions and Future Work  
Metal AM has some advantages over conventional manufacturing techniques in the fabrication of novel geometries, complex structures, and customised parts with minimum waste. This paper provided a comprehensive review of common metal AM processes, including recent advancements and the current state of industrial applications. Moreover, the main challenges and considerations of the current metal AM status were outlined and discussed. It should be noted that advancements in metallic materials have considerably published by researchers. Therefore, this study does not address metallic materials and their properties in detail.  
The leading industries in the metal AM market are the automotive, aerospace, medical, and dental industries. Over the recent years, metal AM has received attention from other industries such as oil and gas, electronics, construction, and railway to make significant inroads in the design and manufacturing of novel components. However, due to the drawbacks on printing quality, post processing, maintenance requirements, limited materials, geometry accuracy, part size limitations, standardisation, and costs, metal AM adoption has been slow. Continuous research and development in the metal AM field, including novel materials, design methods, and technologies have helped to overcome some of the discussed challenges and increase the industrial applications. Nevertheless, there are still some opportunities for improvement and further investigation. For example, the interrelations among metal AM processing parameters for each individual metal AM technology, mechanical properties, and geometry accuracy are still not fully understood. Accordingly, some theoretical models are required to be developed. These models may contain heat and mass transfer, porosity and residual stress prediction, distortion analysis, phase change, and so on. Sustainability issue for metal AM is another research gap to reduce wastage and energy consumption by using proper metal AM technologies and consequently employing just-in-time production. Additionally, fabricated metal AM part quality may be improved by using novel materials and/or processes, and topology optimisation techniques. Moreover, novel hybrid manufacturing may provide opportunities to improve a product's surface quality and repair and/or reshape existing products. In addition to the above potential research directions, optimisation analysis of process parameters and post-processing techniques are required for metal AM techniques to enhance the production efficiency. Moreover, further research is required to characterize the most important factors for metal AM adoption and develop a decision-making model to help industry sectors decide where metal AM would be best for them. Finally, establishing comprehensive widely accepted technical standards for metal AM is still an ongoing effort to assure the quality consistency of metal prints.  
Conceptualization, A.V. and F.G.; methodology, A.V.; validation, A.V., F.G. and A.R.; formal analysis, A.V.; investigation, A.V.; resources, A.V.; data curation, A.V.; writing--original draft preparation, A.V.; writing--review and editing, A.V., F.G., A.R. and K.H.; visualization, A.V.; supervision, A.R., F.G. and K.H.; funding acquisition, A.V. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the Edith Cowan University under Grant G1004423.  
The first author would like to thank [PERSON] from University of Bath, UK, for his insightful advice and information on metal additive manufacturing. The first author would also like to thank Edith Cowan University for the awarded Vice-Chancellor research fund.  
No potential conflict of interest was reported by the authors.