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].