## 4 Challenges and Considerations
### Standards Compliance  
The production environments of end-use metal AM require much higher precision than those for prototyping applications. Accordingly, qualification plays an important role in a production environment. This covers qualification of AM equipment, materials, and staff, and quality control techniques and instruments. The accuracy and repeatability of the manufactured products are important over the entire print, between prints, and across different makes of printer [120]. Currently, the incapability of metal AM to guarantee dimensional accuracy and material properties for a given product is an issue identified with respect to AM adoption in industry. One of the main reasons for this problem is the lack of widely accepted technical standards within metal AM [121]. These cause the following problems: datasets reported by different users are not comparable; little repetition of results exists between printer providers and service branches; different process parameters are employed to operate the same makes of equipment according to users' own requirements; few requirements exist to assure that a product is printed as specified; and consequently, the continued industrial adoption of metal AM is hindered. Figure 11 shows that the standardisation of AM has been undertaken mainly by the International Organisation for Standardisation (ISO) and the American Society for Testing and Materials (ASTM) [122]. The ISO working group, TC261, in collaboration with the ASTM F42 committee are developing AM standards, focusing processes, terms and definitions, process chains, test techniques, and quality controls. The European Committee for Standardisation (CEN) has also undertaken some activities with an AM scope on a regional level. Furthermore, a number of national standard organisations such as Deutsches Institut fur Normung (DIN) and British Standards Institution (BSI) have published technical guidelines. Parallel to these activities, European Union projects are under development such as SASAM, Support Action for Standardisation in Additive Manufacturing, and STAIR AM, Standardisation, Innovation and Research. Although, a broad range of standards have been published in the AM field, some researchers believe that available standards require further improvement and development. For instance, [PERSON] et al. [121] highlighted that the available standards do not respond to AM's characteristics properly. This researcher conducted a survey in which 22 companies responded; approximately 50% of AM users mentioned that existing standards were not appropriately applicable and they proposed the development of standards for mechanical properties testing, printing parameters, and tolerance. The lack of these makes the qualification process difficult, particularly when it is mandatory for various industries such as medical, oil and gas, aerospace and automotive. However, it should be noted that the level of qualification is also important. A qualified part may not be sufficient for certain components such as fatigue critical aircraft components due to the presence of residual porosity and stress in some metal AM processes. Today, metal AM is moving towards industrial production. Therefore, the importance of technical standards including all views of metal AM technologies becomes more significant. Figure 12 shows the number of published and ongoing standards, which provides a clear understanding about the standardisation works within the scope of metal AM. These results were taken from 40 identified ISO and ASTM standards. The list of these standards is presented in the Appendix A for interested readers. Although 8% of the published identified standards is dedicated to the design area, 5% of standards in this area are under development, which highlights the importance of having new design guidelines for new technologies. Additionally, it can be seen that currently 45%, 35%, and 8% of the metal standards are related to the materials and processes, testing methods and qualifications, and environment, respectively.