Figure 11 The Optical mirocgraph patterns (a) AlCoCrFeNiCu (b) AlTiCrFeCoNi HEA
Effect of Laser Parameters on Hardness
The mechanical strength of HEAs in the as-deposited states is greatly dependent on the solidification microstructure [91]. According to the Hall-Petch grain boundary strengthening relation, the equation where\(\sigma\) is the Stephan-Boltzmann constant plus the factor K, times the inverse of the square root of the grain size D, demonstrates that hardness is greatly improved by grain size reduction in laser deposition which is as a result of fast cooling explained in Eq. (4) [92].
\({\text{\ \ \ \ \ \ \ }\sigma}_{0=}\sigma_{i}+\frac{k}{\sqrt{D}}\)\(\left(4\right)\)
Consequently, the surface plots showing the correlation between the laser parameters and the hardness values for both alloys are seen in Figure 12 and 13. The samples with the highest scan speeds at 12 mm/s in both alloys showed the highest hardness properties shown in Figure 14. At high scan speeds, rapid solidification and fast cooling occurs, predicting the grain structure of the alloy. The increase in the hardness properties of the alloy is also attributed to the grain boundary strengthening due to the significant BCC phases observed [93]. This indicates that the BCC phase is stronger than the FCC phase because in the packing planes {110} of the BCC structure, a slip along this plane is more difficult than the FCC {111} plane. Furthermore, the denser and regular {111} plane contains lower lattice friction for dislocation motion and greater interplanar spacing compared to the {110} plane, thus giving the BCC structure excellent solution hardening mechanism [94]. At the first stage of experiments, at an increased laser power of 1000 W and low scan speed of 4 mm/s the hardness values were 560 HV. However, as the scan speed increased to 12 mm/s, the hardness values were the highest figures increasing by about 5% at 800 W, as observed in the surface response plots in Figures 12 and 13. At the second stage of experiments, a high laser power of 1600 W and low scan speed of 8 mm/s resulted in low hardness values of 389 HV. Thus, when the scan speed increases to 12 mm/s the hardness values increases by more than 100 %. In both cases, high hardness values were observed at an increase in scan speed for the AlCoCrFeNiCu HEA. On the other hand, AlTiCrFeCoNi HEA at stage one of the experiments showed hardness values of 667 HV at 1000 W and scan speed of 4 mm/s. And at 600 W and scan speed of 12 mm/s, the hardness increased by more than 20 %. At the second stage of the experiment, low hardness values of 380 HV were recorded at 1200 W and scan speed of 8 mm/s. However, as the scan speed increases to 12 mm/s, the hardness values increased by more than 30 % and this is attributed to the Ti content in AlTiCrFeCoNi. Titanium has a high melting temperature forming a strong BCC phase, which increases the strength of the metal at a lower laser power; however, titanium’s strength drops at elevated temperature or power input [95, 96].