**3. Generation of defects**

The two major phenomena that contributes to the generation of defects in powder bed fusion are **Keyhole** and **Spattering**. In this section, these modes are discussed to understand the underlying physics of why and how a defect may occur in a layer or whole printed part. In a typical laser processing case, two modes of heat transfer participate in defect generation: namely, conduction mode and keyhole mode [32]. Conduction is melting is controlled by heat conduction. In keyhole mode, input power density is sufficient or high enough to vaporize the metal powder. It then creates a much deeper hole or cavity compared to conduction mode. Collapse of such cavities can leave voids in printed parts [33], hence, conduction mode of heat transfer is desired in laser additive manufacturing.

### **3.1 Keyhole mode**

As mentioned earlier, keyhole is a major cause of forming pores and voids. Basic criterion of identifying the keyhole mode is shown in Eq. (1)

$$\frac{2\text{d}}{\text{w}} \ge 1\tag{1}$$

where *d* is the remelted depth, *w* is the melt pool width, and the quantity, *d=w*, is the normalized depth [34–36]. There are few other empirical formulae and methods proposed by researchers [37]. These enable the designers to identify the preferred conduction mode in L-PBF conveniently. By experimental methods, it was found that normalized depth is proportionally related to product of power density and square root of laser interaction time [38]. It obeys Eq. (2) as following:

$$\frac{d}{w} \propto \frac{P}{\pi \sigma^2} \times \sqrt{\frac{2\sigma}{w}} \tag{2}$$

where P is laser power, σ is laser spot size, and ν is laser scan speed.

*Multiscale Modeling Framework for Defect Generation in Metal Powder Bed Fusion Process… DOI: http://dx.doi.org/10.5772/intechopen.104493*

**Figure 2.**

*Pressure and time dependent spattering mechanism.*

#### **3.2 Spattering phenomenon**

Spattering is a physical phenomenon in laser powder bed fusion process observed from experiments [39, 40]. It is considered to be the major cause of the structural defects in the printed products. This is a complex physical phenomenon that requires experimental procedures to observe and challenging to model properly in a FEA computer numerical analysis.

From previous experimental tests, it is found that some spatters have a propensity to merge together and form larger particles. Through their comparison study between multi-laser and single laser scanning, Andani et al. [41] found that high number of working lasers can induce higher recoil pressure above melting pool and as a result more spatters are ejected for molten pool. In case of stationary laser impulse, as time goes by, vaporization occurs after melting and generates intensive vapor jet that ejects metal powders. With the surrounding pressure of inert gas decreasing, it forces the particles surrounding the molten pool to move forward. In this manner, metal powders are ejected with a large divergence angle as vapor can expand freely. This physical event can be observed as "Spattering" by using high-speed X-ray monitoring. **Figure 2** [42] shows a schematic of pressure and time dependent spattering mechanism.
