*2.6. Dilution*

DED involves the printing of subsequent layers of material to build a final part. There is heat accumulation due to multi-track and multi-layer deposition, and that will change the microstructure and final properties of the printed parts. A minimum level of metallurgical bonding is required between subsequent layers, which can be quantified using a dimensionless parameter called dilution. Dilution can be defined as:

$$D = \frac{d}{h+d} \tag{7}$$

where *d* is the depth of the melt pool below the substrate level, and *h* is the height of the material deposited above the substrate level. A high value of *h* correlates to a higher powder flow rate or lower energy input (corresponding to low dilution and lack of fusion between different layers), whereas a high value of *d* is due to a lower powder feed rate or higher energy input (corresponding to high dilution and keyholing phenomenon). There is an optimum value for dilution, which results in a good metallurgical bonding [83]. Figure 5b shows the optimal dilution level in a metal-substrate system generally between 10% and 30%, which is the standard value adopted by several researchers [84], and Figure 5a,c shows the extreme cases of dilution (<10% and >30%, respectively).

**Figure 5.** (**a**) Zero penetration (*d* = 0) or low penetration, meaning a lack of fusion with the substrate; (**b**) optimal level of dilution (generally between 10% and 30%); (**c**) high *d* leads to keyholing.

#### **3. Common Defects**

Defects may arise in the material during manufacturing. It is important to know the cause of such defects and take effective measures to counteract them. The most common defects arising during DED are discussed in the following sections.
