## 4 Discussion
In the EBM/PBF preheat step the powder is heated by strategically scanning a relatively high power, high speed and defocused beam over the surface until sufficient temperature is reached to overcome the activation energy for sintering. The combination of parameters in the preheat step (e.g., line order, line offset, beam focus and beam speed) are tailored in such a way to add sufficient heat to the system to raise the temperature of the surface of the powder bed, while also allowing sufficient time for charge to dissipate between line scans until the powder is sintered. Only recently has work by [PERSON] et al. modeled these charge and heating/sintering effects for a mono layer of spherical metallic powder with an oxide layer during preheating [31]. However, the local variations in particle size distribution, morphology oxide thickness in typical AM powders significantly complicate the prediction of the onset of scattering and require further investigation. While not the initial focus of this study, it became apparent in the early stages of the development effort that the initial challenge with EBM/PBF of alnico powder was to identify a stable processing space for preheating of the powder bed of this magnetic material, while simultaneously avoiding \"over-sintering\" of the surrounding powder and volatilization of the lighter elements (e.g., Al). This was done heuristically, by slowly increasing the beam current from 0.1 mA and manipulating the parameters of beam speed and line order to effectively vary the duration between successive beam passes over a given region (allowing for charge dissipation) while increasing net heat input until sintering occurred. For the alnico powders this portion of the process development for EBM/PBF AM \"workable\" parameters were complicated by two key phenomena. First, the observed surface temperature for which sintering was achieved was close to 1100 \({}^{\circ}\)C, which is relatively close to the melting (solidus) temperature (\(\sim\)1257 \({}^{\circ}\)C). This resulted in a very narrow processing window for preheating: too little energy input and the powder would scatter, too much and the entire layer would over-sinter or begin melting and delaminate causing a spreading failure. Second, based on observations during preheating and melting, it was apparent that the magnetic properties of the powder likely caused a significant deviation in beam focus compared to the calibrated position, although this could not be verified with the available instrumentation. Such effects can have a profound influence on energy and charge transfer into the materials. It is also likely that the magnetic conditions of the powder bed were in constant flux as each new layer was spread and then heated from the as-received condition to above the Curie temperature during preheating, while the electron beam focus and location were also changing. Current predictive models do not account for variation in the magnetic properties of the powder bed during preheating or the potential influence on beam/powder interactions, and further studies will be required to map this process space to enable the stable processing of geometrically complex components. Here we have reported a case for which sintering of the powder bed was achieved without charge induced scattering (run #5, Table 3; Table 4). Although this is the first reported instance of EBM/PBF of alnico it is anticipated that additional magnetic materials will be developed and trialed within EBM/PBF systems beyond alnico magnets. The magnetic properties of these materials must be carefully tracked, along with potential magnetic field measurements with the AM system, in order to be able to move magnetic material AM technologies forward with appropriate scientific under-pinning. In particular, the Curie and melting temperatures of the magnetic alloys and the pre-heat and EBM temperatures must be correlated to allow optimal AM processing without affecting the AM system nor AM build quality.