Figure 5 Crack Free Samples of AlCoCrFeNiCu and AlTiCrFeCoNi
HEAs with Optimized Parameters of 1200 W-1600 W and 8-12 mm/s after
Preheating Temperature.
However, solidification cracks were observed in some samples at a high
scan speed and low power input, and at a low scan speed with high laser
power. Previous studies have shown the effect of laser scan speed on the
microstructure of laser deposited samples [50, 51]. Solidification
usually follows the heat flow direction resulting in different crystal
orientations. Thus, the cracks are attributed to the partial
solidification that occurs at a high scan speed, low laser power and
energy density during deposition. At low energy densities, rapid
solidification occurs due to insufficient laser power without completely
filling the voids within the material at a small melt pool. This implies
that the energy required is insufficient to obtain fully dense
structures [52].
The high thermal gradient produced by high scan speed and the low laser
energy densities is represented as \(q(vr)\) . Where q is the power of
the laser, v is the scanning speed and r is the beam diameter. The
increment in scan speed during dendrite formation prevented the flow of
liquid, which became crack initiation sites and is influenced by
stresses during solidification [53]. Thus, the cracks observed at a
low scan speed, high laser power and energy density are attributed to
thermal shrinkage during deposition. At a low scan speed, the alloy
powder becomes fully molten and shrinkage occur rapidly and generating
residual stresses that result in microcracks [54]. Nonetheless, the
samples with moderate scan speed and laser power had the best dilution
rates and defect free structures at 1200-1600 W, 8-12 mm/s and
preheating temperature of 400 °C in both alloys. The grain sizes of each
alloy were compared through SEM images and for each alloy, grain size
reductions were observed in samples at higher scan speeds. At a high
scan speed, fast cooling occurs during solidification of the molten
alloy. Nucleation and grain growth also occurs during solidification.
Therefore, when the rapid solidification occurred, aggregates of the
alloys within the melt pool had insufficient time for the grains to
grow, resulting in the development of finer grain sizes [55, 56].