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Materials Science in Additive Manufacturing Defects in additively fabricated Al6061

structure at hand may entail significant uncertainty about measurement techniques, such as vision-based 3D surface
the achievable properties, as many decision variables must topography monitoring and 3D contour monitoring 11-13
be considered. Such issues require new methodologies or X-ray computed tomography (XCT). 12,14,15 Vision
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in automatic defect and flaws detection and prevention sensing methods offer a non-destructive in situ approach
(in situ or ex situ) and rapid part qualification to reduce for monitoring geometric signatures, such as 3D surface
uncertainty in achievable properties and ensure robust topography and contour data of the fusion area in L-PBF.
PBF processes and defect-free production. 3 Özel et al. conducted predictive modeling on surfaces of
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For this reason, understanding and detecting process- additively fabricated nickel alloy 625 cubes to understand
induced defects and flaws in PBF processes has gained great the effects of process parameters, energy density, and scan
interest among researchers. The results of such research strategy in L-PBF on the areal surface texture parameters.
indicate that a number of defects or imperfections may occur A focus variation microscopy system (Alicona Infinite
during L-PBF processes while fabricating metal structures. Focus G5 XL200) was utilized in taking areal morphology
These can be classified as (i) internal porosity defects, measurements on the top and side surfaces of the as-built
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(ii) lack of fusion-related defects, (iii) internal cracks, test cubes produced with L-PBF. They used machine
(iv) balling phenomenon-related defects, (v) keyholing- learning methods to train their models by employing the
related defects, (vi) layer delamination, (vii) residual measured data and predicted resultant surface texture
stresses, (viii) geometrical distortions, and (ix) surface parameters for a given set of L-PBF process parameters.
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defects, as detailed in the literature. Among those residual Kim et al. presented a methodology to determine the
stresses, surface defects, and geometrical distortions can be probability of defect detection using XCT, which also
remedied with post-processing, and some of the internal included uncertainty from experimental measurements.
defects can be addressed with hot isostatic pressing (HIP) They proposed an XCT simulation model to better
to a certain degree. 2 address measurement uncertainty, defect locations, and
orientations. In addition, operando/in situ X-ray imaging
The morphology of additively fabricated parts with L-PBF,
along with the correlation between porosity and cracks, and techniques have been employed to observe the formation
7,10,18,19
the life-cycle performance of the built components have of pores and cracks during the printing process. Such
greatly interested the research community. Porosity defects techniques reveal the sources and propagation of pores
are mainly due to incomplete fusion, spattered material- and cracks but are typically limited to special setups with
related roughness on the surface, and bubble entrapment thin domains with different solidification environments
during turbulent flow and gasification within the melt from the actual fabrication process. In addition, analyzing
pool. Their random occurrence creates undesirable quality large data sets is time-consuming and computationally
issues, affecting mechanical properties and reducing the expensive, making them impractical for quick industrial
overall density of the fabricated parts. 4-6 uses.
Solidification cracking is a predominant issue in The key issues of utilizing L-PBF for fabricating
L-PBF of Al6061 alloy, attributed to a wide solidification aluminum builds are associated with defects such as cracks
7-9
range of 70°C and insufficient liquid feeding during the and porosity. The main goal is to reduce crack density
terminal stages of solidification. The cracks observed to maintain structural integrity. Minimizing the mean
in the L-PBF of Al6061 alloy have been confirmed as porosity and crack density, along with their variances,
solidification cracks, evidenced by dendritic features in is essential for optimizing tensile strength and fatigue
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the cracked regions. The columnar grain structure along life. L-PBF of aluminum alloy Al6061 is usually deemed
the build direction, resulting from epitaxial grain growth, improper and suffers from poor printability due to
exacerbates solidification cracking across multiple layers. solidification cracking and/or excessive porosity, resulting
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These cracks primarily form due to large thermal gradients in excessive defects and flaws. Zhang et al. investigated
during laser scanning, where heating and cooling cause a the impact of scan velocity, while maintaining constant
mismatch in thermal expansion and shrinkage behavior, power, on defect formation, noting a reduction in porosity
leading to crack formation on or below the surface of the in conditions characterized by high energy density.
powder bed. Solidification cracking occurs in the final Research has mainly focused on either minimizing the
stage of solidification, driven by the entrapment of solute- thermal gradient associated with L-PBF to suppress the
rich liquid between solid interfaces and the tensile residual cracking associated with epitaxial grain growth or modifying
stresses that pull these interfaces apart. 10 the alloy composition by the minor addition of grain refiners
The primary challenge is accurately capturing the to heterogeneously nucleate grains for better resistance
3D morphology of the AM build structure using known against L-PBF-associated thermal and/or mechanical

Volume 3 Issue 3 (2024) 2 doi: 10.36922/msam.3652