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Materials Science in Additive Manufacturing Defects in additively fabricated Al6061

stresses. 8,21,22 The literature indicates that addressing porosity 10 mm for the top surface area with a height of 10 mm.
and cracking in Al6061 without altering the alloy chemistry The process conditions were gathered from various studies
are challenging. However, utilizing modified alloy chemistry to identify optimal optical hatch spacing and comprehend
presents limitations in L-PBF applications. Manufacturing the influence of laser power, speed, and combined energy
powders with grain refiners, such as zirconium and Yttria density. Hence, there exists no precise experimental design
Stabilized Zirconia, incur higher costs than unmodified but rather a diverse array of data to explore the general
alloy powders. Yoon established a relationship between correlations among laser power, speed, and hatch spacing
23
aspect ratio and crack density in porous 3D structures with defect levels, encompassing porosity and cracks.
using experimental and optimization methods. Maamoun Overall, they can be separated into two sets. Set 1 (Table 1)
24
et al. investigated the impact of process parameters on the encompassed 26 conditions characterized by varying
microstructure and mechanical properties of L-PBF-printed volumetric energy density levels, where P is the laser
samples, employing a substrate preheating temperature of power, v is the laser speed, h is the hatch spacing, and th is
s
200°C. They observed a decrease in the cracks area fraction the layer thickness. The seven sets of laser power and scan
in samples printed with an energy density of 52.6 J/mm . velocity were random but covered a large range of linear
3
However, Uddin et al. have successfully produced crack- energy density (P/v ). Different hatch spacing, ranging
20
s
free samples by reducing thermal stresses through high- from 0.04 to 0.2 mm, were examined to varying volumetric
temperature substrate heating at 500°C. energy densities, ranging from E = 29.24 J/mm (P = 393
3
min
3
In this study, we aim to minimize defects, particularly W, v = 2830 mm/s, h = 0.095 mm) to E max = 159.39 J/mm
s
porosities and hot cracks, in Al6061 alloy fabricated using (P = 263 W, v = 550 mm/s, h = 0.06 mm). Set 2 (Table 2)
s
L-PBF technology. MATLAB modules were utilized to comprised 25 conditions, employing a fixed hatch spacing
optimize process parameters that reduce the occurrence of (h = 0.100 mm) and a combination of laser power and
both types of defects. In this study, hot cracks in Al6061 scan velocity. This set revolved around three nominal
parts are deemed as failures due to their critical impact conditions: P = 200 W, v = 100 mm/s; P = 275 W,
s
on engineering components. While Al6061 is used as a v = 4000 mm/s; and P = 450 W, v = 1870 mm/s. Prior
s
s
25
model material, the proposed optimization strategy is investigations into solidification conditions and material
adaptable to other materials processed through L-PBF. For properties informed us of the selected conditions. Each set
Al6061, the focus was on minimizing cracks to improve has a replica to ensure the consistency of the observations.
the effectiveness of subsequent post-build heat treatments, If large discrepancies are observed between two identical
such as HIP, which could potentially eliminate remaining conditions, a third sample is added to justify the results. It
defects. Hence, even under optimized process parameters should be noted that the experimental plan did not follow
identified in this research, printed parts are not intended any known experimental design format. However, some
for immediate end-use but serve as a foundation for factor and level combinations were excluded from the full
advancing toward defect-free production. factorial design to eliminate the influence of extraneous
factors and level combinations. In addition, within each
With this study, we aim to gain a deeper understanding group of conditions, parameters such as hatch spacing
of internal defects and flaws occurring during L-PBF under constant laser power and scan velocity, laser power
of aluminum alloy Al6061 through experiments, data under constant scan velocity and hatch spacing, and laser
analysis, and process parameter optimization. For this scan velocity under constant power and hatch spacing were
purpose, an experimental design for room-temperature varied, facilitating detailed investigations into individual
printing of aluminum alloy Al6061 cubes was conducted, factor contributions while minimizing non-linear
focusing on factors such as laser power, scan velocity, and correlation errors. However, this experimental design did
hatch distance, while keeping other parameters, such as not fully account for certain parameter interactions.
layer thickness and scan strategy rotation, constant. The
build cubes were then characterized for porosity and crack All the cubes were printed using the Aconity MIDI
density using 2D and 3D morphology methods at multiple L-PBF printer (Germany), maintaining oxygen levels
scales, and the total defect density, porosity, and crack below 100 ppm to prevent oxidation. The aluminum
density were reported accordingly. alloy Al6061 powder utilized was commercially procured
from Carpenter Technology (USA), featuring particle
2. Methods sizes with a D of 22 µm, a D of 36 µm, and a D of
50
10
90
54 µm. The powder layer thickness of 0.050 mm is fixed
2.1. Experiments for all samples during printing. A simple hatching strategy
This study employed the L-PBF process to fabricate was implemented, with the laser scanning back and forth
aluminum alloy Al6061 test cubes, each measuring 10 × across the entire sample with layer-to-layer rotation of 67°

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