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

Abstract

This study introduces and verifies a basic mechanism of surface topography evolution in electron beam additive manufacturing (E-PBF). A semi-analytical heat conduction model is used to examine the spatio-temporal evolution of the meltpool and segment the build surface according to the emerging persistent meltpool domains. Each persistent domain is directly compared with the corresponding melt surface, and exhibits a characteristic surface morphology and topography. The proposed underlying mechanism of topography evolution is based on different forms of material transport in each distinct persistent domain, driven by evaporation and thermocapillary convection along the temperature gradient of the emerging meltpool. This effect is shown to be responsible for the upper bound of the standard process window in E-PBF, where surface bulges form. Based on this mechanism, process strategies to prevent the formation of surface bulges for complex geometries are proposed.

Influence of Electron Beam Powder Bed Fusion Process Parameters at Constant Volumetric Energy Density on Surface Topography and Microstructural Homogeneity of a Titanium Aluminide Alloy

“…In order to understand the observed phenomena, it is essential to take a closer look at the influence of the process parameters on the melting conditions in PBF additive manufacturing. The interaction of beam power, scan velocity, and line offset can affect the melt pool dimensions, [32,34,55,56] melt pool dynamics, [23] and thermal boundary conditions [34,35,38,57] of the process.…”

“…Depending on the process parameters and the scanning strategy, different degrees of remelting and varying thermal boundary conditions may occur locally within each layer, which could be partly responsible for the emergence of bulges in distinct patterns. In a recent research paper, Breuning et al [57] proposed an additional explanation for the occurrence of pronounced bulges in certain areas of the surface: At high beam currents and scan velocities, the melt pool regime can change from a lens-shaped trailing melt pool to a persistent melt pool, that is, the melt pool is still active when the beam returns to the same position in the adjacent scan line. [56,57,61] If, for example, a rectangular area is melted with a snake-like scan strategy, a persistent melt pool regime may occur near the turning points, whereas the melt in the center of that area is already completely solidified within the beam return time.…”

“…In a recent research paper, Breuning et al [ 57 ] proposed an additional explanation for the occurrence of pronounced bulges in certain areas of the surface: At high beam currents and scan velocities, the melt pool regime can change from a lens‐shaped trailing melt pool to a persistent melt pool, that is, the melt pool is still active when the beam returns to the same position in the adjacent scan line. [ 56,57,61 ] If, for example, a rectangular area is melted with a snake‐like scan strategy, a persistent melt pool regime may occur near the turning points, whereas the melt in the center of that area is already completely solidified within the beam return time. Breuning et al [ 57 ] called this melt pool regime “partially persistent.” The occurrence and the extent of such persistent domains largely depend on the applied parameters, which determine the beam return time and the lateral velocity perpendicular to the scan direction (see Figure 1).…”

“…[ 56,57,61 ] If, for example, a rectangular area is melted with a snake‐like scan strategy, a persistent melt pool regime may occur near the turning points, whereas the melt in the center of that area is already completely solidified within the beam return time. Breuning et al [ 57 ] called this melt pool regime “partially persistent.” The occurrence and the extent of such persistent domains largely depend on the applied parameters, which determine the beam return time and the lateral velocity perpendicular to the scan direction (see Figure 1). Since the material transport governed by melt pool dynamics can significantly differ for trailing and persistent melt pools, material accumulations can form at the transition between the two melt pool regimes.…”

Feasibility of Melting NbC Using Electron Beam Powder Bed Fusion

“…The recommended melt depth for EB-PBF is between 1.5 and 4 times the layer thickness used in the printing process, but according to the literature as long as the melt depth is larger than the layer thickness it is deemed as sufficient. [39,40] Usually finding the optimum parameter sets for a powder requires some sacrifice in either surface roughness, amount of pores seen in the material, or printing time. As previously stated, the power was one parameter studied and evaluated for the different substrates.…”