Figure 14: Microhardness Chart Showing Stage 1 and Stage 2 of
Experiments (a) AlTiCrFeCoNi HEA (b) AlCoCrFeNiCu HEA
Effect of Laser Parameters on Melt Pool Geometry
The laser parameters played a significant role in the variation of the
laser absorptivity and the melt pool geometry [98, 99]. The
thickness of the metal powder was made uniform throughout during
deposition due to the smooth surface of the substrate after sandblasting
[100, 101]. Single tracks are generated by the melting powder
deposited on the base plate, which solidifies together and forms a
uniform melt pool [102, 103]. The start and end of some tracks are
different due to the difference in scan speed while shifting from one
track to another. At low power from 600-1200 W and high scan speeds from
10 mm/s-12 mm/s, there were defects in the microstructure of the
geometries indicating that the parameters were not at optimum range.
This is attributed to the inadequate heat generated in the melt pool. At
a high scan speed and low power, the base plates were slightly slanted
and this uneven surface led to an increase in layer thickness at that
area attributed to inconsistencies in the melt pool [104, 105]. The
laser parameters also had an effect on the geometry of the melt pool.
The width, height and depth were measured obtaining the average
dimensions using an optical microscope to know the influence of the
laser power and scan speed on the melt pool.
The energy density is also an important factor to be considered for the
melt pool geometry. The various laser energy density input led to
different variations of the melt pool geometry along the build
direction. The deepest molten pool geometry was the sample with the
highest energy density (\(\alpha_{\lambda}\frac{Q}{v})\)and lowest scan
velocity. Conversely, as the scanning speed increased, the depth of the
molten pool geometry and the thermal gradient reduced [103, 106].
The reduction resulted in the defect of some samples that is attributed
to the small amount of powder fed into the melt pool per unit length of
deposit and the weak bonding on the deposited layers. These occurrence
was reported during the laser deposition of Ag alloy by Xiong et
al. [107]. As the energy density increases, the melt pool
width became wider in both alloys. A curve fit is used to represent the
variation in the melt pool geometry with the energy density as shown in
Figures 15, 16 and 17.
For example, considering the curve fit of Figure 15 (a) for the melt
pool width measurement of AlCoCrFeNiCu HEA Eq. (5), (6) and (7) shows
the curve fit values obtained:
\(y=5069.2\ \text{In}\left(x\right)-15633\) \(\left(5\right)\)
It can also be written in terms of the laser power (P) and scan speed
(mm/s):
\begin{equation}
y=5069.2\ \text{In}\left(\frac{P}{v}\right)-15633\text{\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ }\left(6\right)\nonumber \\
\end{equation}In terms of the energy density E:
\(\text{Melt\ Pool\ Width\ of\ AlCoCrFeNiCu\ HEA}=5069.2\ \text{In}\left(E\right)-15633\)\(\left(7\right)\)