# An Overview of the Process Mechanisms in the Laser Powder Directed Energy Deposition
## 4 Solidification Mechanisms
### Microstructure  
In the LP-DED process, morphology and grain size as the main microstructural features can be significantly influenced by the thermal history of the component during the building process. In general, it has been reported that the cooling rate and thermal gradient can substantially define the thermal history of a part. However, it is well documented that there are lots of process parameters/variables that significantly affect the thermal history of the LP-DED parts. Therefore, this complex correlation between the process variables/parameters and thermal history makes predicting the microstructural features of LP-DED components very challenging. Nonetheless, in order to produce LP-DED parts with superior mechanical properties, it is very important to overcome this challenge and establish effective control mechanisms. In the literature, several works have studied the role of specific parameters on the microstructure and performance of the LP-DED parts with specific shapes [121; 122; 123]. However, in most of these works, the investigations have been carried out on samples with simple geometries, such as cubes and blocks. In contrast, in the case of complex shape components that can experience various thermal histories in different parts, it is unclear how to apply the outcomes of these investigations. In the literature, it is reported that the as-built microstructure depends mainly on the solidification rate within the melt pool (\(R\)) and the thermal gradient at the solidification front (\(G\)) [124; 125]. \(G/R\) and \(G\times R\), which is also known as cooling rate, are the two critical solidification parameters that influence the shape of the solidification front and the dimensions of the microstructure, respectively [126; 127]. Columnar (elongated grain morphology), columnar-equiaxed, and equiaxed are the main structure morphology that can be achieved at different \(G\) and \(R\) values. Figure 24 shows the effects of \(G\) and \(R\) values on the solidified microstructure. As can be seen, the columnar to equiaxed transition can be achieved at very high solidification rates, whereas, at high cooling rates, a finer microstructure can be obtained in the as-built LP-DED parts. It should be emphasized that the optimum \(G\) and \(R\) values can be determined by different factors such as material properties, machine conditions, part geometries, and process parameters. In general, in the LP-DED process, the cooling rate value ranges from \(5\times 10^{2}\) to \(5\times 10^{5}\) K/s [20; 22; 128]. The high cooling rate values, as discussed earlier, could lead to several advantages, such as suppression of solid-state transformation, formation of non-equilibrium phases, and very fine microstructures [20]. The \(G\times R\) value at the bottom of the deposited track is very low, then increases rapidly with the track height until it reaches its maximum value, which is approximately equal to the travel speed near the free surface of the melt pool. On the other hand, the maximum value of \(G/R\) is obtained at the bottom of the melt pool and decreases as it approaches the free surface of the melt pool. As a result, a coarse, planar microstructure is frequently obtained at the bottom of the deposited track, which evolves into cellular, dendritic or equiaxed structures in the upper regions. Thedistribution of the microstructure is visible in Figure 25, which shows the microstructural evolution that occurs during the deposition of a single track of Co-based alloy on steel. Previous works have reported that epitaxial grain growth in the direction of maximum thermal gradient is the common mechanism in most metallic materials, such as stainless steel and titanium alloys [130; 131], etc. However, various prior beta grain morphologies have been revealed in different LP-DED titanium components.