## 3 Results and Discussion  
### Microstructure Analysis  
Spherical pores were observed in the MIM preforms (porosity 0.06%) and L-PBF section (porosity 0.01%) of as-built samples, whilst the heat treatment densified the L-PBF structure and led to further porosity free composition (porosity < 0.003%), Figure 2a,b. This is additional evidence that HT cycles are a necessary post-processing step to minimize any microstructural inhomogeneity of L-PBF components [52]. However, it had a negative impact on the MIM preforms as it introduced larger pores, and some damage was observed in Figure 2b,c, probably due to the expansion and release of entrapped gasses and/or residual binders from the bulk during the HT cycle. The pore sizes were assessed using a threshold method through the ImageJ software and they were approximately 40% larger when compared with those in the as-built hybrid sample. Such residues are retained in the MIM components even after the multiple stages of the sintering process, and they often occur due to the short dwell time, incorrect temperatures in the furnaces or ramp temperature [49]. At the same time, the HIP treatment led to almost pore and defect free microstructure (porosity < 0.004%) as can be seen in Figure 2d, which could be explained with the grain growth and some pore migration towards the grain boundaries of the MIM preterm. An example of pore segregation within the final consolidation after the heat treatment and HIP cycles is shown in Figure 2c,d. In particular, after the HT cycle the microstructure in the MIM preform did not demonstrate any grain growth, while, there is an increased number of voids, even along the grain boundaries as shown in Figure 2c. Conversely, the post HIP image depicts defects free microstructure with equiaxed grains and without any voids coalescence. This shows the combined effects of high pressure and temperature that have led to an improvement of the microstructure by increasing the material density through the induction of pore shrinkage and the accelerated material diffusion [39]. Regarding the L-PBF section, a blend of columnar and equiaxed grains can be seen in Figure 2e. The elongated columnar grains in the PBF section are common for the laser processed materials and are mainly due to the epitaxy growth between the deposited layers, resulting from the rapid solidification process of the melt pool in the--Z direction [34]. In addition, it can be stated that there were no porosity and signs of cracks near the interface in all three sets of samples. The analysis of the MIM/L-PBF interface in Figure 2f shows that there have been a microstructural assimilation and grain refinement in the deposited (L-PBF) section and also in the re-melt area (\(\sim\)100 \(\upmu\)m) of the MIM preforms. No grain growth occurred  
Figure 2: Cross sections of (**a**) as-built, (**b**,**c**) HT air-quenched, and (**d**,**f**) HIP samples. Arrows in (**a**,**b**) point at gas pores and keyholes. Higher magnification BSE-SEM images in (**c**,**d**) show the porosity state and grain growth, at the MIM preform in air-quenched and HIP samples, respectively. Higher magnification images (**e**,**f**) show the columnar and some equiaxed grains in the L-PBF section and the microstructural assimilation at the re-melt area/interface, respectively. at the fusion line (interface) or within the heat affected zone due to the thermodynamic stability of the prior coarser grains of the preform, in all three sets. Both, the deposited material and the heat affected zone below the interface have the same fine microstructure that can be explained with the rapid solidification rates [53]. The microstructure of HIP samples revealed to be austenitic and this was further supported from the location of nickel mass fraction within grains compared to that of air-quenched samples as shown in Table 1. This is further supported, as austenitic twins can be seen in Figure 2d. Such twinning is known to form as a result of recrystallisation during HIP.