Page 26 - MSAM-3-3

Materials Science in Additive Manufacturing Defects in additively fabricated Al6061

crack density due to the different dominant mechanisms. the formation of large, irregularly shaped lack-of-fusion
In general, cracks have a higher probability of forming at porosities.
higher hatch spacing (at similar levels of laser powers and A comparison of the porosity level in each column, an
speeds) and higher laser speed (at similar levels of laser increasing hatch spacing resulted in decreased porosity,
power). This is probably due to the increased cooling rate notably reducing from 10.42% to 1.58% when hatch spacing
with increased hatch spacing and the faster delivery of increased from h = 0.06 to 0.1 mm, (Figure 3A and G).
laser energy, where smaller melt pools are expected. Since the process conditions share the same laser power

For aluminum alloys, when using a specific laser power, and speed, the heat dissipation rate during each track
a significant portion of the heat applied to the material can scanning remains similar. Another hypothesis to explain
dissipate rapidly due to the alloy’s high thermal conductivity. such observations is that excessive energy can result
However, at higher laser powers and faster scanning speeds, in significant vaporization, creating voids and a large
the heat is delivered to the material over a shorter duration, attenuation zone adjacent to the voids. Subsequent laser
even though the volumetric energy density may be lower. scanning may further enlarge the void due to the lack of
This rapid delivery of heat surpasses the rate at which the surrounding powder. With the addition of the next layer,
aluminum alloy can dissipate it, leading to potential issues uneven recoating, caused by the discontinuity created
such as increased thermal stress and the likelihood of by the voids, can further contribute to the instability of
defects such as hot cracks forming in the material. the melt pool. The repeated vaporization process across
multiple layers may further enlarge the pores (Figure 3A).
Three power/speed combinations covering a large The reduction in energy density prevents vaporization and
power range were selected to present the results, namely impacts from the denudation, thus reducing the porosity.
(i) P = 263 W, v = 550 mm/s, (ii) P = 315 W, v = 734 mm/s,
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and (iii) P = 490 W, v = 1581 mm/s, with varying hatch An elevation in crack percentage is observed for all three
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spacing. The optical images for microstructures of cubes conditions, with increasing hatch spacing. This could be
printed with small hatch spacing (<0.1 mm) are depicted due to larger hatch spacing accumulating less heat, leading
in Figure 3A‑C, while those with large hatch spacing to lower temperatures in finished layers, which results in
(>0.1 mm) are illustrated in Figure 3G-I. Corresponding faster cooling rates and more solidification cracks. The
defect-processed images for the respective process relationship between the experimentally obtained porosity
conditions (in Figure 3A-C) are displayed in Figure 3D-F, and crack density measurements for each experimental
and for conditions (in Figure 3G-I), the images are run is displayed in Figure 4. It is observed that porosity
displayed in Figure 3J-L. The laser power and speed remain and crack density vary at different processing conditions.
the same in each column whereas the hatch spacing varies. Minimizing porosity leads to increasing crack density
In each row, the decrease in the energy density leads to a whereas minimizing crack density results in high porosity.
reduction in porosity. The irregularly shaped pores and Literature suggests that porosities within materials can
discontinuities (Figure 3A-C) are typically characteristic serve as regions where thermal stresses, particularly
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indicators of lack of fusion defects. As extensively tensile stresses, are mitigated or relaxed, indicating that
documented in the literature, volumetric energy density higher porosity samples may yield lower crack densities.
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alone does not adequately represent the energy input, as it This effect occurs due to porosities enabling dissipation or
does not account for the duration over which heat is applied reduction of tensile stresses within the material. As a result,
to the material. One hypothesis argued here would be the the overall magnitude of tensile stresses in the material is
fact that Al6061 has indeed a high thermal conductivity. decreased, leading to a reduction in the density or severity
At high laser speed and power, heat can be delivered to the of cracks observed in the samples. This observation
powder bed in a short period, faster than heat dissipation complicates the task of simultaneously minimizing both
on the base materials. Therefore, cases with lower scanning porosity and crack density (the total defect density),
speeds may absorb less effective energy for melting as a highlighting the need for machine learning to analyze
significant portion of the energy is conducted away from the impact of each parameter and identify the optimum
the base. For instance, despite the sample processed at parameter settings.
P = 263 W, v = 550 mm/s, and h = 0.06 mm having a lower In summary, the irregularly shaped pores observed in
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volumetric energy density compared to the one processed Figure 3A-C suggest that their formation is more likely due
at P = 490 W, v = 1581 mm/s, and h = 0.14 mm, the lower to insufficient energy input rather than excessive energy
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laser power of the former may not be sufficient to fully melt input, highlighting the critical role of both laser power and
the powder layer and adequately fuse with the previously scanning speed in achieving proper fusion and minimizing
consolidated layers. This incomplete fusion can result in defects in AM processes.

Volume 3 Issue 3 (2024) 8 doi: 10.36922/msam.3652