**3. Methodology**

#### *3.1. Laser Welding*

Laser welding tests involved creating a bead of length 90 mm on each sample. A maximal power of laser of approximately 2.5 kW was set. Welding speeds of 600, 1600, and 2600 mm.min−<sup>1</sup> were tested. Three 316L samples were studied for each speed. 15CDV6 samples were also examined to study the weld pool formation according to the welded material. A comparison of the weld pool morphology of the two materials (316L and 15CDV6) was conducted.

As noted earlier, the aim here was to create a database (experimental benchmark) that enables the validating of numerical simulations of laser welding. To achieve this, we were interested in the weld pool morphology, temperature evolution, residual stresses, and distortions of the 316L samples. The length and width of the weld pool were measured from the images of the high-speed camera. However, to determine its depth and height, the approach proposed in Section 2.2.4 must be used. The infrared camera enabled us to obtain the temperature distribution. At the same time, the pyrometer enabled us to measure the brightness temperature for each color given by the infrared camera (points P1, P2, P3, and P4: Figure 5). Residual stresses related to the welding process were also measured at three points (1, 2, and 3) on 316L samples (Figure 9). The welding conditions and measurements performed for the two materials are given in Table 2.

**Figure 9.** Measurement points of residual stress.

**Table 2.** Machine parameters and measured quantities for both materials (316L and 15CDV6).


#### *3.2. Melting of a Powder Bed*

A single bead of 50 mm length was created by melting a zone of the powder layer. Similar to laser welding, the maximum power of the laser was used (2.5 kW). Scanning speeds (speed of laser movement) between 400 and 3600 mm.min−<sup>1</sup> were examined. Three beads were created for each speed.

#### **4. Results and Discussion**

In this section, the results of the laser welding process (molten pool morphology, temperature field, residual stresses, and distortions) are given and discussed. The authors are interested in the effect of the welding speed on these results. The results (formation of the molten pool) of the melting of a powder bed are then presented.

#### *4.1. Laser Welding*

#### 4.1.1. Weld Pool Morphology

Figure 10 shows the microscopic images of a cross-section of 316L and 15CDV6 weld beads for a speed of 1600 mm.min−1. These images enable us to determine the depth, height, and width of the weld bead. The length of the molten pool can be measured from images of the high-speed camera (the width can also be measured from these images: Figure 11). Tables 3 and 4 illustrate respectively the dimensions of the 316L and 15CDV6 weld beads for three speeds (600, 1600 and 2600 mm.min−1). Figure 12 shows the high-speed camera images of the weld pool of the two materials for a welding speed equal to 1600 mm.min−1.

**Figure 10.** Cross-section of the weld bead (microscopic images, v = 1600 mm.min−1): (**a**) 316L; (**b**) 15CDV6.

**Figure 11.** Morphology of the weld pool (316L): 1600 mm.min−1.

**Figure 12.** Morphology of the weld pools of both materials (1600 mm.min−1): (**a**) 316L; (**b**) 15CDV6.


**Table 3.** Morphology of the weld pool (316L).



The images captured by the optical microscope and the high-speed camera enabled us to build a database of 316L (Table 3) and 15CDV6 (Table 4) weld pool morphologies. Tables 3 and 4 indicate a significant effect of the welding speed on the weld pool morphology. As expected, the weld pool is longer and shallower for the speed of 2600 mm.min−1. In Figure 12, the weld pool, HAZ, and base metal can be clearly distinguished. Because 316L steel is austenitic, it does not undergo a major metallurgical transformation during welding (not HAZ). However, the HAZ is very remarkable for the 15CDV6 material (bainitic steel). Figure 12 also shows that the weld pool morphology is sensitive to the welded material. For example, using the same welding conditions, the 15CDV6 weld pool is longer compared to the 316L weld pool (Tables 3 and 4). This may be justified by the fact that the two materials do not have the same thermophysical properties (thermal conductivity, viscosity, surface tension, etc.).

#### 4.1.2. Temperature Field

The temperature distribution during the laser welding of the 316L samples is shown in Figure 13. We note that an arbitrary unit (A.U.) was used in Figure 13 because the infrared camera was not calibrated (the scale does not represent temperature values). Figure 14 illustrates the evolution of the brightness temperature in the melted zone according to welding speed. The evolution of this temperature in different positions of the weld bead is given in Figure 15 (v = 1600 mm.min−1).

The images of the temperature distribution captured by the infrared camera (Figure 13) show that the welding speed affects the isotherms distribution (and thus the morphology of the weld pool). The pyrometer enabled us to determine the brightness temperature values (Figure 15) corresponding to the different colors captured by the infrared camera. This brightness temperature in the weld pool is not very sensitive to the welding speed (Figure 14).

We note that the true temperature restoration is the key problem of pyrometry because of the unknown emissivity and its possible variation during measurement. Different methods can be applied but no universal solution is found [27,31]. The brightness temperature of a non-blackbody target is defined as the temperature of a blackbody with the same monochromatic luminance. The relation between the two temperatures is presented in [48]. The true temperature can be measured during a future work by using other experimental approaches (bichromatic pyrometer, thermocouple, etc.).

**Figure 13.** Temperature field given by infrared camera (316L): (**a**) v = 600 mm.min−1; (**b**) v = 1600 mm.min−1; (**c**) v = 2600 mm.min−1.

**Figure 14.** Effect of the welding speed on the brightness temperature evolution (316L).

**Figure 15.** Measurement of the brightness temperature in different positions on the weld bead for v = 1600 mm.min−1: P1, P2, P3, and P4 (316L).

#### 4.1.3. Residual Stresses and Distortions

Axial and longitudinal residual stresses were measured on the 316L samples after welding using the DRX machine. As Figure 9 shows, three measurement points were chosen. We specify that the first point is tangent to the weld bead. Table 5 summarizes the results of these stresses for speeds of 600, 1600, and 2600 mm.min−1.

The vertical displacements of the welded samples were measured using the measuring arm. Vertical displacements of *Uz* of 0.411, 0.174, and 0.091 mm were measured respectively for the welding speeds of 600, 1600, and 2600 mm.min−1.


**Table 5.** Residual stresses of 316L samples (points 1, 2, and 3) after welding.

Residual stresses in the order of 100 MPa were observed before the welding of the 316L samples. These stresses increased after welding to reach approximately 688 MPa for a speed of 600 mm.min−1. This indicates that the residual stresses related to the laser welding process can be very important. As shown in Table 5, these stresses are very sensitive to the welding speed. For example, the longitudinal residual stresses decrease when the speed increases. The vertical displacements also decrease when the welding speed increases (*Uz* = 0.411 for v = 600 mm.min−<sup>1</sup> and *Uz* = 0.091 for v = 2600 mm.min−1). This can be justified by the effect of the welding speed on the molten pool morphology and thermal cycle.

The results of the laser welding process enabled us to conduct an experimental benchmark (molten pool morphology, temperature field, residual stresses, and distortions). This benchmark can assist in validating the different types of numerical simulations of this process. For example, the molten pool morphology and temperature field results can be used to validate thermo-fluid simulations. The molten pool morphology and especially the free surface shape can also indicate that the tension surface gradient is positive (for used welding conditions). The mechanical results of the numerical simulation can be validated using the residual stresses and distortion results.

#### *4.2. Morphology of the Molten Pool: Melting of a Powder Bed*

Figure 16 shows the formation of the molten pool during the melting of the spherical powder. The effect of the scanning speed on the molten pool morphology is given in Figures 17 and 18. The influence of the types of powder on the formation of the molten pool is presented in Figures 19 and 20. Tables 6 and 7 summarize the dimensions of the respective beads of the spherical and irregular powders.

**Figure 16.** Formation of the molten pool during melting of a 316L spherical powder bed (v = 1600 mm.min−1).

**Figure 17.** Effect of scanning speed on molten pool morphology (spherical powder): high-speed camera images.

**Figure 18.** Effect of scanning speed on molten pool morphology (spherical powder): infrared camera images.

**Figure 19.** Effect of powder distribution on molten pool morphology (high-speed camera images, v = 400 mm.min−1): (**a**) Spherical powder; (**b**) Irregular powder.

**Figure 20.** Molten pool morphology (microscopic images, v = 400 mm.min−1): (**a**) Spherical powder; (**b**) Irregular powder.


**Table 6.** Molten pool morphology of spherical powder (20–50 μm).

**Table 7.** Molten pool morphology of irregular powder (100–150 μm).


Figure 16 shows the formation of the molten pool during the melting of a spherical powder layer. In this figure, the molten pool, the bead (consolidated material), and the unfused powder can be distinguished. A denuded zone can also be distinguished; it appears between the bead and the unfused powder. The videos captured by the high-speed camera can explain the causes of the appearance of this zone. Indeed, the molten pool absorbs the powder around it. This absorption can be justified by the mechanical flow of the powder which moves it towards the molten pool. This flow is probably less important for low layer thicknesses.

Figure 17 shows the effect of the scanning speed on the formation of the molten pool. The latter is longer and narrower for the higher velocities. The "balling" phenomenon appears from a speed of 3600 mm.min−<sup>1</sup> (Figures 17 and 18). Figure 19 shows the molten pool formation of both types of powder (spherical and irregular powders). We note that the type of powder has a direct effect on the formation and morphology of the bead. This can be justified by the influence of the size and the granulometry of the powder on its mechanical flow. A standardized flowability test (ASTM) was used to measure the rate of this flow. It involves timing the flow of 50 *g* of powder through a hole with a diameter of 2.54 mm. The spherical powder flowed after 16 *seconds* while 38 *seconds* was required for the flow of the irregular powder. These results confirm that the flowability of the spherical powder is greater than that of the irregular powder.

The cross-section of the beads of the spherical and irregular powders are given in Figure 20. We observed that the morphology of the bead is sensitive to the types of powder. For example, gaps of 12 and 24% were observed respectively for the width (L1) and height (H) for the speed of 600 mm.min−1. This difference can also be justified by the flowability of the two powders. We have observed that the height of the spherical powder bead is greater than the initial thickness of the powder layer (2 mm) for speeds of 400, 600, 800, and 1000 mm.min−1. This can be explained by the flowability of the spherical powder which facilitates its absorption by the molten pool. This absorption facilitates the increase in the volume of the bead. For these velocities, the phenomenon of shrinkage of the layer powder (shrinkage related to the variation of the density between the powder bed and the molten pool) is, therefore, less visible. However, it begins to be observed from a speed of 1200 mm.min−1. Indeed, the powder quantity absorbed is less important for high velocities. The flowability of the irregular powder and therefore its absorption by the molten pool are less important. For this reason, the height (H) of the irregular powder bead is less than 2 mm, even for low velocities.

The study of the melting of a powder bed enabled us to analyze and understand this process better. As the first remark, we have observed that the molten pool absorbs the unfused powder around it. This phenomenon was very visible especially for the spherical powder, which has a significant flowability. The absorption of the powder by the molten pool leads to the formation of a denuded zone between the bead and the unfused powder. It also facilitates the increase in the bead volume, especially for the spherical powder and at low velocities. Therefore, the shrinkage phenomenon simulated by several studies [46,49] cannot always occur during the melting of a powder bed. However, considering the material addition (related to the mechanical flow of the powder around the molten pool) during the numerical simulation may be necessary according to manufacturing conditions. Considering the fluid flows in the molten pool (related to the absorption of the powder by the molten pool) will also be necessary. The formation of an interface between the molten zone and the unfused powder can also be simulated.
