## 4 Conclusions and Future Research
### Future Directions  
A few considerations are mentioned below for possible future research on additive manufacturing potential in the spare parts SC. As mentioned in the SC process mentionedin Figure 2 and the content extracted in the review, the following are proposed for future directions. These parts are shown in Figure 3.
* Quantitative models lack complexity in terms of SC design instances to produce optimal solutions [38]. In addition, some parameters and factors are simplified for the modeling process. Modeling can be more representative if the impact of lead time on uncertainties related to post-processing is also considered [3]. Because of the complexity of SC for the adoption of hybrid manufacturing, focus can be turned towards developing a heuristics approach so that large larger demand scenarios can be analyzed [18; 38]. The availability of real data and practical numerical examples from real-world cases would enhance the reliability of the results and model solutions [29]. * Regarding the transition from a traditional SC to a digital SC configuration, intellectual property rights (IPR) could represent an obstacle because of the ownership rights of the spares CAD files. Therefore, further research in the area of the certification process and data management handling could help the industry to be assured of data exchange and facilitate the AM process [4; 20; 40; 52]. * Most AM research is focused on the aerospace industry, possibly due to high-value components and continuity of the spare parts needs. There are other industries, such as oil and gas, where heavy investments are made for capital assets and operational expenditures. Petroleum asset operations require heavy investment in spare parts acquisition and assessment, as high fulfilment rates and reliability levels have to be achieved [41]. Although the petroleum industry is also highly value-added and sensitive to quality, some AM capabilities may already be available for use in the petroleum industry. Future research could focus on specific spares, demand uncertainty, technology capability, and the development of optimal SC configurations for the petroleum industry. * Based on the content of the AM process shown in Figure 3, there are only a few studies discussing the inputs. The review shows that material availability and supply potential, demand, and the analysis of uncertainties are important, and they can vary from one industry to another. The availability of raw materials can avoid inventory and promote just-in-type AM, which will reduce the impact of demand changes and uncertainties related to other aspects. Therefore, input analysis for operating the AM process and its impact on AM SC processes could be considered in future research. The modeling process might require integration of the development and distribution processes and decision-making processes. * Researchers have recognized the value of AM in reducing emissions. The impact of the carbon cap, carbon tax policy, and carbon cap and trade policy can alter SC configurations. As SCs in AM can extend beyond a country, such research can help develop a greener and socially responsible AM process. Figure 3: Proposed areas of research directions are indicated in dashed lines. * The review could not obtain literature that focuses on the rules and regulations, manpower capability, and technology absorption. A higher level of technology absorption leads to adaption, innovation, and risk-taking in improving current technology. Technology absorption can also be increased through education and exposure [76], which can help in the adoption of new technologies [75; 76], such as additive manufacturing [77] in different application areas. These are management-related constraints, and therefore, they can be considered highly by the industry aiming to adopt AM. In addition, the concept of hybrid manufacturing as an upcoming technology] in the AM domain should also be studied for spare parts management, as this is a new area within AM. Conceptualization, F.T. and S.P. ; methodology, A.M. and S.P.