## 3 Results and Discussion
### Mechanical Properties  
The tensile tests were performed to investigate the mechanical properties of the hybrid MIM/L-PBF samples in the three sets, i.e., as-built and after air-quenching and HIP. The results for the as-built and air-quenched samples were used as a reference to compare them with those obtained after the HIP treatment, and thus to determine the effects on the strength and ductility of the hybrid MIM/L-PBF components. Furthermore, fracture surfaces and locations were also investigated to determine the mode of failure and to consider the point of weakness within the hybrid samples. The fracture of the hybrid tensile specimens occurred as a result of the reduced cross section of the tensile samples at the PBF sections, for all sets of samples. This is attributed to the vertical build orientation of the PBF section and to the high UTS of the preforms, and conforms to the reported results [19; 55]. In particular, the microstructure of the MIM was optimised after the HIP treatment, as a result of the pore shrinkage and consequently led to an enhanced performance. The PBF sections, were built vertically and thus are more susceptible to failures due to the build orientation which is parallel to the applied tensile loads, compared to the MIM preforms that were built horizontally, therefore any defects at the weld successions were parallel to cracks and thus prone to crack propagations. The HIP samples failed in a reduced cross section manner which developed to the typical slant fracture. The fractographic analysis of the HIP sample is provided in Figure 3a-c. The fracture topography shows a disjoint surface texture, while magnified SEM images in Figure 3b,c, revealed a cone and cup topography with some shear fracture planes together with some dimples and/or micro voids. In particular, Figure 3b indicates a large number of voids coalescence and thus their coalescence demonstrates void-covered intergranular fracture (J), together with microductilities (K) which are attributed to be formed due to tensile deformation. Notably, there is no evidence of growth of transgranular micro cracks and their nucleation on the facets that could eventually develop into macroracks and ultimately lead to a brittle fracture. The fractography in Figure 3d,e revealed pores and micro voids coalescence on the fracture surface for both air-quench and as-built conditions. Entrapped gas pores were visible on the fractured topography for the as-built sample, whilst residues due to the expansion and release of entrapped gases and/or heterogeneities from the bulk during the HT cycle were visible on the fracture surface for the air-quench condition. Figure 3f,g shows bar charts of ultimate strain and 0.2% offset yield strength (R\({}_{\text{p0.2}}\))/ ultimate tensile strength (R\({}_{\text{m}}\)), respectively obtained for the three investigated sets of samples. R\({}_{\text{p0.2}}\), R\({}_{\text{m}}\), and elongation (A) of as-built samples were 178 \(\pm\) 26 MPa, 506 \(\pm\) 25 MPa, and 66 \(\pm\) 7%, respectively. The air-quenching led to an increase of R\({}_{\text{p0.2}}\) by 13%, whilst R\({}_{\text{m}}\) and A decreased by 11 and 23% (205 \(\pm\) 20 MPa, 449 \(\pm\) 6 MPa, and 51 \(\pm\) 1%), respectively. This could be due to the reduction of dislocations after the HT and the significantly high porosity in the material [56,57]. Conversely, the HIP treatment led to a bigger R\({}_{\text{p0.2}}\) increase of 68% (299 \(\pm\) 20 MPa), while R\({}_{\text{m}}\) increased by 11% (560 \(\pm\) 6 MPa). At the same time, a decrease from 66 to -53%. The A values were lower compared to the as-built samples and this can be explained with the possible formation of brittle phases during the densification. Particularly, the air trapped inside the closed pores is released to the pore walls during the pore shrinkage due to the high pressure over the HIP process and thus oxides or other phases can be initiated [58].