## 4 Challenges and Considerations
### Post Processing  
Depending on the metal AM technology, post processing operations are needed after each print. These operations may include powder removal, stress relief, part and support structure removal, machining operations, and the quality of parts by reducing or eliminating print-induced defects such as lack of fusion and porosity, but increases the cost of the process [135]. Among these operations, a stress relieving operation requires further attention and is generally needed for fabricated metal parts using most metal AM technologies, since residual stresses are caused by the rapid thermal cycles, and concurrent melting of the new layer and re-melting of the former solidified layers. This includes the following operations, which are used to reduce the interior defects, improve the microstructure and mechanical properties of AM parts, and are necessary for nearly all metal AM parts [136]: Hot isostatic pressing (HIP) is commonly used to release stresses, improve mechanical properties, and decrease any existing porosity for metal AM fabricated parts after the build [137]. In this process, high temperature and pressure were applied to the product in an inert environment [138]. In the most recent studies, the effect of the HIP method on microstructure and mechanical properties of different materials such as Ti6 Al4V [139, 140], Ni-base single crystal [141], ALSi10 Mg [142] and copper [143] were investigated. In general, the results of these studies showed that in the following HIP treatment, the internal pores were reduced, and the tensile strength was reduced due to the microstructure changes. In contrast, the ductility was considerably improved for different materials. However, the results are dependent on the applied metal AM process, initial material properties and the selected metal AM process parameters. Therefore, it is recommended to perform a heat treatment operation after the HIP to enhance the effectiveness of the HIP process [144, 145]. Heat treatment is used to improve surface quality and decrease residual stress gradients that can generate part distortion such as wraps and cracks that deteriorate the functionality of the end-use metal AM parts [146, 147]. It should be noted that this process is best performed under a protective environment using an inert gas or a vacuum to minimise the chemical reactions [137]. Moreover, to improve dimensional accuracy, it is better to heat treat parts before finishing operations [148]. Laser processing is mainly used to improve the surface roughness, wear resistance, porosity, and microstructure of metal AM fabricated parts [149, 150]. As shown in Figure 13a, during the process a melted thin layer is added to the surface using short laser pulses in a vacuum environment and at a low power density to make a smooth surface [151]. Cold rolling is commonly used for improving microstructure and geometry and reducing porosity of DED fabricated parts [152, 153]. This is due to the fact that DED processes are used for the fabrication of large products and clearly the post-processing approaches are not the same as for other metal AM categories. To investigate the effect of the cold rolling operation on the large metal AM fabricated parts, [PERSON] et al. [154] used different common rolling methods, which are schematically shown in Figure 13b. In this study Titanium and Aluminium were used. The results showed that rolling methods improved both the microstructure and mechanical properties of Titanium made parts significantly. The cold rolling approaches improved the mechanical properties of Aluminium made parts; however, the reduction of the porosity was considerable. It should be note that the post processing operations may affect the dimensions of parts. To ensure dimensional accuracy of metal AM parts, surface finishing operations such as machining, chemical etching, and vibrating might be required [148].