Microwave is an electromagnetic radiation of wavelength from 1 mm to 1 m, while laser used in AM is of the order of μm. Microwave is used in sintering as well as in post-processing of AM parts because it saves time and energy for processing. While other radiation, such as laser, is absorbed on the surface of a powder, giving rise to surface absorption, microwave gives rise to volumetric absorption. Due to the non-linear effect of the absorption, the temperature of powders increases drastically after some time, giving rise to a high temperature at low input microwave energy. This high rise in the temperature, which is called the thermal-runaway effect, makes microwaves a potential thermal source for high-temperature process. Though it has unpredictable behavior at different conditions, which needs to be controlled.