  The susceptibility of nickel-based superalloys to processing-induced crack
formation during laser powder-bed additive manufacturing is studied. Twelve
different alloys -- some of existing (heritage) type but also other
newly-designed ones -- are considered. A strong inter-dependence of alloy
composition and processability is demonstrated. Stereological procedures are
developed to enable the two dominant defect types found -- solidification
cracks and solid-state ductility dip cracks -- to be distinguished and
quantified. Differential scanning calorimetry, creep stress relaxation tests at
1000$^\circ$C and measurements of tensile ductility at 800$^\circ$C are used to
interpret the effects of alloy composition. A model for solid-state cracking is
proposed, based on an incapacity to relax the thermal stress arising from
constrained differential thermal contraction; its development is supported by
experimental measurements using a constrained bar cooling test. A modified
solidification cracking criterion is proposed based upon solidification range
but including also a contribution from the stress relaxation effect. This work
provides fundamental insights into the role of composition on the additive
manufacturability of these materials.
