## 3 Discussion
### Discussions on AM Applicability and Analysis Methods  
This review aims at content analysis to address four research questions. The first research question (RQ1) concerned the 'opportunities for AM in spare parts SC'. The review shows that multiple aspects of opportunities were considered by researchers, such as materials, technology, processes, and SC configurations. This review shows that research on AM is increasing significantly. In the past five years, AM was considered a manufacturing technology rather than a prototyping tool. The review shows that research focuses mostly on the process part rather than on the analysis of inputs and constraints. Studies also focused on resource allocation and decision-making. However, no research on the details of the rules and regulations, supply situation, manpower capability, and technology absorption could be found in the AM literature. A few authors considered emissions from AM SC configurations which can support the industry in making an environmentally friendly decision by correctly locating the AM facilities in the SC. The review also shows that the sourcing of spare parts was also a point of analysis, and most studies recommend having dual sourcing or mixed sourcing rather than single sourcing based on the current technological option. While research shows more confidence in conventional manufacturing for long-term spare parts management, AM can provide a short-term supply that can be replaced immediately upon receipt of conventionally manufactured parts. The second research question (RQ2) concerned the models available to analyze AM in SC. The review shows that such models can be, in terms of technology, analytical models, SC configurations, and carbon analysis. This review shows different SC configurations and their advantages in adopting AM. The configurations were analyzed both qualitatively and quantitatively. The literature shows that a distributed SC design may be more appropriatefor spare parts management based on currently available technologies. With a distributed SC, AM hubs can be developed at different service locations to reduce lead time and transportation costs. The review also shows that AM SCs are neither totally centralized nor totally decentralized. The combination can be made to make strategic decisions on the spare parts model at a centralized location because of the availability of expertise and panning ability to assess the market situation and operational decisions at decentralized locations for faster replenishment of demand [74]. From the input perspective, only a few frameworks or models can be found in the literature. Although the availability of raw material, supply potential, capacity, and demand is crucial for AM-based systems, it seems that researchers focus more on processes and technologies rather than the inputs for driving the AM process. The raw material must be supplied at the right time in the appropriate quantities. Suppliers of AM materials are not widespread, and the raw materials used for AM are costly. Therefore, alternative materials for both short- and long-term use and the recuperation of the materials from the AM parts or the conventional parts do not seem to be in the purview of AM SC researchers. Industry-wise, most research on AM is related to the aviation sector. This may be a point of attraction because of the high cost and stringent specifications of spare parts. Researchers mainly focus on making the total SC efficient in terms of lead time, inventory holding, and demand management. The reviewed authors used scenario comparisons and quantitative methods for analysis. Based on the review, it can be perceived that researchers may be focused on technological improvement on one side (based on the product management concept) and the strategic management of spare parts management on the other. As service SC is the focus of this study, many of the models focus on inventory minimization and service level as a way to promote AM.