2.3.2 Effect of laser power and scanning speed

Figure 7 shows the powder melting and solidification evolution process in SLM. First, laser beam is turned off, as shown in Figure 7a. When the laser is turned on, the beam energy is absorbed by the powder layer. With the heat accumulation of powder, powder layer melts. In Figure 7b, a depression area is made by the work of recoil force. Because the apply of laser beam in Gaussian distribution, the temperature gradient is created between the molten pool center and the molten pool edge, with the induced surface tension gradient and the Marangoni effect. Lastly, the depression area was filled with fluid by the Marangoni effect, as shown in Figure 7c.

Figure 8 shows the characteristic microstructure on the polished cross section of the SLM-processed AlSi10Mg samples. The cross-section of the samples produced

#### Figure 7.

Powder melting and solidification process evolution at the laser power of 180 W and scan speed of 1000 mm/s: (a) initial state (b) heating process (c) solidification.

Figure 8. Influence of laser power and scanning speed on the microstructure of SLM-processed samples.

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

by using the laser power of 180 W and scan speed of 1000 mm/s showed a fine microstructure without any apparent pores. At the low laser power of 150 W or scan speed of 600 mm/s, the cross-section of the as-built samples consisted of irregularshaped pores were visible. However, at relatively high laser scan speed of 1400 mm/s, a large amount of the balling formation was present on the cross-section. It can be concluded that the balling phenomena was one of the typical metallurgical defects at high laser scan speed. In order to understand how the laser power P combined with the laser scan speed v affects the pores and balling defects, the line energy density (LED) is defined as [24]:

$$LED = \frac{P}{v} \tag{11}$$

Furthermore, the numerical studies were used to describe the forming mechanism of the metallurgical defects during laser melting and to provide a basis for the process optimization. The laser melting process of the AlSi10Mg powder at different scan times was shown in Figure 9a. In order to assess the effect of the LED on heating of the powder, the peak temperature and the interfacial velocity at the edge of the molten pool was rescored as the center of the laser beam moved to the point of X = 0.36 mm and Y = 0.1 mm. As shown in Figure 9b, the peak temperature and the interfacial velocity increased from 1810 K and 1.2 m/s to 2831 K and 5.1 m/s as the applied LED increased from 1.071 to 3 J/cm, respectively. At a relatively low LED, the insufficient laser energy result in insufficient melting of the powder. As

#### Figure 9.

(a) Temperature field and surface melt velocity obtained by simulation at condition of P = 180 W and v = 1000 mm/s. (b) Influence of line energy density on the peak temperature and surface melt velocity of molten pool. (c) Influence of line energy density on the width and depth of molten pool.

## Heat and Mass Transfer - Advances in Science and Technology Applications

shown in Figure 9c, the depth of the molten pool is about 20 μm at LED of 1.071 J/cm. This implies that the low applied LED can lead to the insufficient melting of the powder and the resultant interlayer pores. In addition, the balling phenomena was severely occurred in this condition. On the contrary, at relatively high LED, the high temperature of the molten pool leads to a strong perturbation within the molten pool, thereby resulting in the instability of the free surface of the molten pool. In this case, a large amount of the pores was generated by the instability of the scan track [24].

When the laser moves to the point of X = 0.36 mm along scanning direction, the molten pool was chosen in this case.

#### 2.3.3 Effect of hatching spacing

Figure 10 shows that at an interconnect pores will be formed in the overlap if the large interconnected gap has not been completely filled by re-melting liquid because of the high hatching spacing. An overlap in the multi-tracks is necessary to have continuity between two adjacent scan tracks leading to a dense solidification. Due to the overlap between the successive adjacent scans, the hatching spacing is always less than the laser beam radius. Hatch spacing is another parameter, which is highly affects the pore formation.

As revealed in Figure 11, at laser power of 180 W and scan speed of 1000 mm/s, the number of the inter-track pores was apparently reduced as the hatch spacing

#### Figure 10.

The effect of hatching spacing on the formation of porosity in overlap between adjacent scan tracks.

#### Figure 11.

Typical microstructure of the SLM-processed AlSi10Mg samples at hatch spacing of (a) 70 μm, (b) 60 μm, and (c) 50 μm. (d) Effect of the hatch spacing on the porosity of the as-built sample. The laser power of P = 180 W, scan speed of v = 1000 mm/s and layer thickness of d = 35 μm were fixed in these experiments.

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

decreased from 70 to 50 μm. As a result, the porosity of the SLM-processed sample was accordingly reduced from 6.1 to 0.3% (Figure 11d). Since at a relatively larger hatch spacing, the portion of the re-melted material was reduced, leading to insufficient overlap between two adjacent scan tracks.

As the case in Figure 12, the insufficient overlap will increase the possibility of the inter-track pores formation. In general, reducing the hatch spacing is beneficial for the reduction of the inter-track pores [3]. However, as the relatively low hatch spacing applied during SLM, although a fine bonding between two adjacent scan tracks can be obtained, the build efficiency was decreased under this condition. So it is important to choose a reasonable hatch spacing during SLM.
