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:
\(\text{Melt\ Pool\ Width\ of\ AlCoCrFeNiCu\ HEA}=5069.2\ \text{In}\left(E\right)-15633\)\(\left(7\right)\)