# An Overview of the Process Mechanisms in the Laser Powder Directed Energy Deposition
## 4 Solidification Mechanisms
### Microstructure
[128] analyzed the microstructure obtained from the deposition of  
Figure 28: OM images of the cross-section of melt pools of a near beta titanium single scan tracks using (a) 6 g/min, (b) 11 g/min, (c) 15 g/min, (d) 25 g/min, (e) 31 g/min, (f) 36 g/min, (g) 44 g/min, (h) 55 g/min and (i) 59 g/min, as mass deposition rate (reproduced with permission from [133], Copyright © 2015 Elsevier). H13 tool steel. They observed that each deposition pass was characterized by three regions: the interface region, the columnar grain, and the equiaxed grain region (Figure 29). The interface region corresponds to the remelted region and is characterized by a coarser mesh due to repeated thermal cycles. Above the interface region, the microstructures evolve into the columnar grain and then into the equiaxed grain, which has been attributed to a lower value of temperature gradient [128]. [PERSON], et al. [128] and [PERSON], et al. [134] demonstrated that the microstructures are highly influenced by the process parameters. In particular, increasing the laser power and/or reducing the travel speed result in coarser grains. The same behavior was observed by [PERSON], et al. [135] for Ti-6 Al-4V alloy and by [PERSON], et al. [136] for 316L stainless steel. Figure 30 shows the variation of grain structure dimensions obtained during the deposition of Ti-6 Al-4V thin walls by increasing the travel speed. [PERSON], et al. [137] deposited Inconel 718 samples on a substrate of the same material and observed that the microstructure was mainly characterized by elongated columnar dendrites along the building direction. They also observed that a finer grain structure was obtained in the regions of overlap between consecutive tracks, while a coarser grain structure was obtained in the other regions (Figure 31). This was attributed to the thermal cycles that occur during the process, which in turn cause the grain recrystallization and coarsening. Figure 30: SEM micrograph showing the effect of travel speed on grain dimensions. Travel speeds are (**a**) 300 mm/min, (**b**) 600 mm/min, and (**c**) 900 mm/min (reproduced with permission from [135]; Copyright © 2004 Elsevier). Figure 29: (**a**) Cross-section of H13 tool steel deposit and (**b**) close-up of deposition pass characterized by three-zone: interface region, columnar region, and equiaxed dendrite region (reproduced with permission from [128]; Copyright © 1999 Springer Nature). Several works have illustrated that in a metal part produced via the LP-DED process, different local thermal gradients and cooling rates can lead to the formation of various local microstructures [120; 138]. For instance, in a Ti-6 Al-4V component produced via LP-DED, the colonies of parallel and very fine lamellae are formed at the top of the sample (Figure 32); in contrast, at the bottom of the sample, the lamellae become thicker [130]. Figure 31: Cross-section of In718 deposit. Finer grain structure is observed in the overlapping regions, and coarser grain structure is observed elsewhere (reproduced with permission from [137]; Copyright © 2011 Elsevier). Figure 32: OM micrographs of a Ti-6 Al-4V sample produced via LP-DED process from the (**a**) top to the (**b**) bottom region [130].