# An Overview of the Process Mechanisms in the Laser Powder Directed Energy Deposition
## 4 Solidification Mechanisms
### Microstructure
For example, in a near beta titanium alloy (Ti-5 Al-5 Mo-4V-1 Cr-1 Fe), a \"bamboo-like\" grain morphology was detected  
Figure 24: Effect of the temperature gradient and the solidification rate on the solidification microstructures and grain dimension [127]. Figure 25: Experimental analysis of microstructure formation during the deposition of a single scan of Co-based alloy on steel. At the bottom of the deposited track (point A), a coarse planar front is observed. In the centre of the track (point B), columnar and coarse dendrite structures are observed. At the top of the track (point C), equiaxed and finer dendrites are obtained (reproduced with permission from [129]; Copyright © 2017 Elsevier). in the building direction, which is a mixture of small columnar and equiaxed grains, as shown in Figure 26[132]. [PERSON], et al. [132] studied the correlation between the microstructural evolution of a near beta titanium alloy during the LP-DED process and the part geometry (Figure 27). As seen in Figure 27a, during the deposition of a single track, due to the high-temperature gradient and rapid cooling at the bottom of the melt pool, the morphology of the grains is columnar. Thereafter, as the height of the melt pool increases, the thermal gradient and cooling rate decrease, resulting in a transition from columnar to equiaxed at the top of the melt pool. Instead, in the case of multi-track deposition (Figure 27b), in the overlapping zones, all the equiaxed grains remelt and subsequently solidified with a columnar morphology. This transition in these areas is due to the shallower melt pool depth in these zones, which changes the local thermal history in those locations. This different thermal history, particularly higher thermal gradients and cooling rates, leads to  
Figure 27: Schematic illustration of the microstructural evolution during the LP-DED process, (**a**) One-track, (**b**) Multi-track, and (**c**) Multi-layers (reproduced with permission from [132]; Copyright © 2013 Elsevier). Figure 26: The “bamboo-like” grain morphology in an LP-DED Ti-5 Al-5 Mo-5V-1 Cr-1 Fe alloy (reproduced with permission from [132]; Copyright © 2013 Elsevier). the formation of columnar grain. In the case of multi-layer deposition, shown in Figure 27c, it is revealed that partially melted equiaxed gains of the last layer solidify and act as nuclei for the epitaxial growth of the columnar grains, which have an identical morphology to that of the previous layer. The grain morphology in the overlapped zones, in this case, is also the same as in multi-track deposition. However, they revealed that a variation in the LP-DED process parameters could change grain morphologies, so further studies on the effect of process parameters on microstructural evolution are needed. In another work, [PERSON], et al. [133] studied the effect of mass deposition rate (\(m\)) on the nucleation and growth mechanisms during the solidification of the melt pool in the LP-DED process. They reported that by increasing the mass deposition rate, the area fraction of the equiaxed grains increases significantly (Figure 28). In fact, by increasing the \(m\) value, the epitaxial growth of the parent grains is more prevented, and nucleation of the new equiaxed grains increases significantly. Besides, it was revealed that higher mass deposition rates result in a lower melting pool depth, and after a critical point, no penetration occurs, which is undesirable. Therefore, they concluded that through the mass deposition rate, it would be possible to change the morphology of the microstructure, but there is a critical threshold that should be respected. [PERSON], et al. [93] analyzed the microstructure of 316L stainless steel thin walls. By varying the laser power, they found that a columnar microstructure was predominant near the interface, whereas a cellular microstructure was observed in the remaining regions. They observed that the dimensions of the microstructures were smaller using a low value of laser power. [PERSON], et al.