## 5 Third Phase: Case Studies
### Case Study 2: Exhaust Manifold  
The second case study was an automotive exhaust manifold made of Inconel718. It conveys burnt gases from three engine cylinders into a single collector. The general design restrictions require the lowest amount of metal to occupy the least space and to have good efficiency. Usually, this component is made by casting, which does not allow thin pipes. Through SLM, a new efficient and lightweight component was built (Figure 15a). For the finishing of this component, the coupling flanges were machined before the overall surface conditioning in order to avoid defects over the already finished clamped zones. In order to shield the machined surfaces, a cover was designed and fabricated via fused deposition modeling in acrylonitrile butadiene styrene (Figure 15b); in this way, the BF action was avoided over these surfaces, and the finishing of the internal pipe surface was allowed.  
Six features were selected to understand how the BF operation affected the complex geometry. As shown in Figure 15a, zone A was convex and, thus, exposed to the conditioning. As demonstrated in Section 3, Inconel718 was efficiently processed by BF with the selected parameters. As expected, this feature showed a marked improvement of the surface; in only 12 h, the initial roughness of about 11 um was reduced by two-thirds; after 24 h, the Ra was about 1 um (Figure 16b). The internal surfaces of the pipes corresponding to features B, C, and D were characterized by reduced conditioning; only 30% was achieved after 24 h. As the working time was prolonged to 96 h, the obtained Ra was 2.8 um for features B and D, while the Ra was 3.5 um for feature C. This slight deviation between trends was attributed to the different inclinations of these pipes with respect to the BF active region flow direction. The opposite side of the header was measured by means of the exhaust internal surface E and the concave zone F (Figure 16c). The trend of feature E was very close to that of feature C, which had the same inclination, and an Ra of 3.4 um was obtained after 96 h. The convex zone F showed a trend close to that of feature A; however, a slight difference could be observed, highlighting a less performant BF action, as expected.  
The 3D maps of some features are reported in Figure 17 for a morphological analysis. Zone A (Figure 17a) was characterized by a very high peak-to-valley height. An St of 105 um was measured; this value was very close to the Rt value, highlighting a uniform fabricated surface. After 96 h, the surface was very flat, reaching an St of about 10 um. As shown in Figure 17b, some areas were characterized by small hollows. More defects were present in the maps related to features B and E, as shown in Figure 17c,d respectively. For these areas, the St was 18 um, and the hollows were smooth and uniformly distributed.  
Figure 15: Inconel718 automotive exhaust manifold (**a**) and covers used to shield machined zones (**b**).  
Figure 16: Investigated features (**a**,**c**) and average roughness trends as a function of BF working time (**b**,**d**).  
Figure 17: Three-dimensional (3D) map of feature A before finishing (**a**), feature A after 48 h (**b**), feature B after 48 h (**c**), and feature E after 48 h (**d**).