(a) briefly describe, aided with schematic drawings, the operation principles of the two 6. powder bed (selective laser melting and selective electron beam melting) metallic additive manufacturing systems. (b) for the two powder bed additive manufacturing processes, describe the differences in powder size and chamber gas that are used and explain why there are differences. (c) explain why planer growth (layer), which is normally observed in welding, is not observed in selective laser melting of metallic alloys that are commonly used.