Abstract
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 °C and measurements of tensile ductility at 800 °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.
Highlights
ADDITIVE manufacturing (AM) is a new process, which is promising for metals and alloys.[1]
Recent work on solidification cracking in AM has revealed that grain boundary misorientation is critical to susceptibility; hot cracks form only on high angle grain boundaries (HAGB)[8] and that grain boundary segregation and ensuing liquid film stability influence crack formation.[9,10]
The distribution of cracks across the XY plane is uniform in IN713 and ExpAM, whereas the others cracked to a greater extent at the edges, where the effective laser scan speed was reduced
Summary
ADDITIVE manufacturing (AM) is a new process, which is promising for metals and alloys.[1]. They are best known for their applications in aeronautics, hypersonics, rocketry and high temperature technologies more broadly.[5] They are an excellent test-case for the additive manufacturing process, for several reasons. Their compositional complexity requires as many as 15 different alloying elements to be present; this broadens the freezing range and exacerbates solidification-related cracking whereby the remaining liquid fails to fill the partially solidified microstructure which is pulled apart by shrinkage effects,[6,7] see Figure 1. Solid-state cracks are characterized by their morphologies which are long and sharp, in contrast to the more meandering nature of solidification
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