Heat and fluid flow in additive manufacturing - Part II: Powder bed fusion of stainless steel, and titanium, nickel and aluminum base alloys

The most important metallurgical variables that affect the structure and properties of components produced by powder bed fusion (PBF) are examined using a model, proposed and validated in part-I of this paper. These variables include the temperature and velocity fields, build shape and size, cooling rates, solidification parameters, dendrite arm spacing, hardness, distortion and lack of fusion defects for four common alloys used in additive manufacturing (AM), stainless steel 316 (SS 316), Ti-6Al-4V, Inconel 718 and AlSi10Mg. The process parameters examined include laser power, scanning speed, powder layer thickness, packing efficiency and hatch spacing. Among the four alloys, the largest molten pool of AlSi10Mg ensures good fusional bonding among layers and hatches but exhibits high solidification shrinkage. Therefore, AlSi10Mg is the most susceptible to distortion among the four alloys. SS 316 exhibits the opposite trend because of its smallest molten pool among the four alloys. For a particular alloy, lack of fusion and distortion can be minimized by careful selection of hatch spacing and scanning speed. For the dendritic growth of SS 316 and AlSi10Mg, refinement of the solidification microstructure through close spacing of the dendrite arms can be achieved using thinner layers and faster scanning. Asymmetry in liquid pool geometry because of the difference in the thermal properties of powder bed and solidified build can be minimized by reducing the scanning speed.