New Grain Formation Mechanisms during Powder Bed Fusion

Abstract

Tailoring the mechanical properties of parts by influencing the solidification conditions is a key topic of powder bed fusion. Depending on the application, single crystalline, columnar, or equiaxed microstructures are desirable. To produce single crystals or equiaxed microstructures, the control of nucleation is of outstanding importance. Either it should be avoided or provoked. There are also applications, such as turbine blades, where both microstructures at different locations are required. Here, we investigate nucleation at the melt-pool border during the remelting of CMSX-4® samples built using powder bed fusion. We studied the difference between remelting as-built and homogenized microstructures. We identified two new mechanisms that led to grain formation at the beginning of solidification. Both mechanisms involved a change in the solidification microstructure from the former remelted and newly forming material. For the as-built samples, a discrepancy between the former and new dendrite arm spacing led to increased interdentritic undercooling at the beginning of solidification. For the heat-treated samples, the collapse of a planar front led to new grains. To identify these mechanisms, we conducted experimental and numerical investigations. The identification of such mechanisms during powder bed fusion is a fundamental prerequisite to controlling the solidification conditions to produce single crystalline and equiaxed microstructures.

Basic Mechanism of Surface Topography Evolution in Electron Beam Based Additive Manufacturing

“…Due to severe heat loss of the small specimens, an additional heating step to reach the target temperature was added after remelting of each individual specimen. For more information about the remelting procedure the reader is referred to the recent publication by Rausch et al [ 26 ]. To investigate the influence of the 90 hatch rotation, 15 × 15 × 25 samples were fabricated from the powder bed.…”

“…Material properties are assumed to be constant and uniform. The estimated material parameters adapted from Rausch et al [ 26 ] are summarized in Table 1 .…”