2.3.4 Effect of layer thickness

The powder layer thickness is one of the important factors in in producing the pores formation. Figure 13 shows the topography of the second scan track formed by SLM using different powder layer thickness. It is clearly that a relatively continuous and stable scan track was generated at powder layer thickness of 35 μm. At low layer thickness of 25 μm or larger layer thickness of 50 μm, the scan track showed a severe irregular profile which will increase the possibility of the inter-layer pore

#### Figure 12.

Bonding between two adjacent tracks at laser power P = 180 W, scan speed v = 1000 mm/s, and hatch spacing of 50, 60, 70 μm.

#### Figure 13.

Influence of powder layer thickness on morphology of scan track in longitudinal view (left) and cross-sectional view (right). Laser power P = 180 W and scanning speed v = 1000 mm/s were fixed in simulations.

#### Figure 14.

Influence of powder layer thickness on the temperature of molten pool. Laser power P = 180 W and scanning speed v = 1000 mm/s were fixed in simulations.

formation. This is due to as the relatively larger powder layer thickness applied during SLM, the laser energy cannot melt previous layer, inducing inter-layer pores in the interface between two adjacent layers. However, decreasing the powder layer thickness increase the reflected radiation from the surface of the previous track, hence decreasing the peak temperature of molten pool, as shown in Figure 14. At powder layer thickness of 25 μm, the peak temperature of molten pool exhibits apparent fluctuation, and the following will produce the scan track with irregular surface. According to this numerical result, the thin powder layer thickness of 35 μm is recommended for AlSi10Mg in SLM process.

The pore and the balling defects lead to a decrease in the densification level of the SLM-processed samples. To evaluate the combined effect of the laser power P, scan speed v, hatch spacing H, and powder layer thickness d on the densification level of the as-built samples, a volumetric energy density (VED) is defined as:

$$\text{VED} = \frac{P}{vHd} \tag{12}$$

The relative density of the SLM-processed AlSi10Mg sample are shown in Figure 15 as a function of the VED.

Figure 15. Effect of the VED on the relative density of the SLM-processed samples.

Heat and Mass Transfer of Additive Manufacturing Processes for Metals DOI: http://dx.doi.org/10.5772/intechopen.84889

As the applied VED within the range of 75–105 J/mm3 during laser melting, the relative density of the as-built samples is larger than 97.5%. According to all the above research results, the VED of 102.86 J/mm3 (P = 180 W, v = 1000 mm/s, H = 50 μm, d = 35 μm) was proposed during AlSi10Mg SLM process.
