## 2 Microstructure
Laser power, laser scanning speed, powder feeding rate, and scanning strategy are the main important process and design parameters that can affect the thermal history, microstructure and level of residual stresses within the laser deposited component. Indeed, these parameters affect the cooling rate and local temperature gradient by changing the melt pool geometry and incident energy [42; 47]. Figure 10: Microstructure of a direct laser-deposited Ti-6 Al-4V part in Y–Z plane at (**a**) the top region and (**b**) the bottom region. A low energy density which is a combination of high scanning speed and low laser power results in a fine microstructure whereas, at low laser scan speed and high laser power (high energy density) a coarse microstructure consisting of columnar grains is the predominant microstructure [47; 48]. [PERSON] et al studied the effect of process parameters of DED on the microstructure of thin wall components of Ti-6 Al-4V alloy produced by a LENS\({}^{\text{TM}}\) system. They found that by increasing the energy density the size of DEDed components increased. Moreover, they showed that the effect of powder feeding rates at higher laser powers is less so that at higher powder feeding rates the microstructure turns into a columnar microstructure. On the other hand, by increasing the laser scan speed the size of \(\alpha\) and \(\beta\) laths decreases and the level of porosity increases [48]. [PERSON] et al. studied the correlation between the powder feeding rate and powder density in the melt pool. A laser rapid forming system with a 5 kW continuous wave CO\({}_{2}\) laser, a 4-axis numerical working table, and a powder feeder with latheral nozzle were chosen to deposit the components. They showed that the powder feeding rate does not have a serious effect on the distribution of powder density in the melt pool [99]. They reported that the layer height increases linearly on increasing the powder feeding rate. For a given powder feeding rate, the amount of powder which is delivered into the melt pool varies in different directions of laser scanning. This discrepancy is likely related to the different distance between the powder feeding stream and the laser spot. By changing the direction of laser scanning the point of powder delivery changes and it can be ahead, in line or behind the laser spot (Figure 11). This variation will affect the geometry of the melt pool, boundary, and solidification heat transfer and consequently the height of deposition. As can be seen in Figure 11, when the powder delivery point (A) is in front of laser spot (O) the amount of delivered powder into the melt pool is less while, this amount is slightly higher in the case when the powder stream is behind the laser spot. Thus, in order to have a constant mass flow rate of deposition the powder feeding rate and laser scan speed should be set according to the laser scan direction and the distance between the laser spot and nozzle. The amount of powder that is delivered into the melt pool is also varied according to the flow rate of the shielding gas so that by increasing the gas flow rate the layer height increases significantly. The high flow rate of shielding gas results in a higher level of porosity and also higher spreading out of the particles and thus affects adversely the deposition. Generally speaking, raster, bi-directional, offset and fractal patterns are the main four deposition strategies (Figure 12) [100]. In addition to these scan strategies, some sub-strategies can be used to deposit the component. For instance, in the case of the offset strategy, offset-out and offset are considered as other deposition strategies. In general, as discussed earlier various scanning strategies remarkably affect the final properties and amount of residual stress within the component after the deposition [93; 95].