3.1 Monitoring of laser keyhole and keyhole-induced porosity formation
The fluctuation of the molten pool at different shielding gas could be observed by high-speed camera as shown in Fig. 3. The high-speed camera images show that in the case of using pure argon as a shielding gas, the keyhole front is irregular in shape with the appearance of some spatter, which indicates the keyhole instability, while the situation changes when adding nitrogen, where the keyhole front became more stable, and if only nitrogen is used, the front of the keyhole is close to a regular oval.
The low stability of the molten pool with argon shielding gas gives chance for the trapped gas bubbles to create pores in the solidified weld metal. Whereas when nitrogen gas is used, the keyhole becomes so stable that it suppresses or prevents the formation of bubbles, thereby reducing porosity or lack of porosity.
X-ray transmission imaging snapshots through CW fiber laser welding of 304 stainless steel are shown in Fig. 4, welding was performed with 5 kW power and 2 m/min speed in shielding gases of 100%Ar, 50%Ar + 50%N2, and 100%N2, respectively. The results of monitoring X-ray transmission confirm that bubbles are generated from the tip of the keyhole to form porosity. In the case of using pure argon as a shielding gas, bubbles that lead into pores were started to form from the keyhole tip, then moved with the anticlockwise vortex to the rear part along with the molten pool bottom. Afterwards, bubbles are captured by the solidification front due to the high solidification rate of laser welding. By adding nitrogen in the shielding gases, the keyhole becomes more stable, both the number and the volume of bubbles were reduced with nitrogen increasing in the shielding gas. In the utilization case of pure nitrogen, the keyhole was stable and no significant bubbles were formed.
The previous investigation was assured by a radiographic test. The results of the X-Ray radiographic examination shown in Fig. 5 reveal the presence of a few pores at laser welding with argon shielding gas as well as argon with a low percentage of nitrogen, but at a higher percentage of nitrogen, pores were disappeared. It is confirmed that the cause for lesser or no porosity in welds shielded with nitrogen gas is imputed to the difficulty of bubbles forming in liquid metal nature. [15]. In addition, the ability to absorption of nitrogen in the molten pool of austenitic stainless steel may be another reason for the porosity reduction when using nitrogen gas [11].
3.2. Weld metal nitrogen content
The nitrogen content of weld metal was measured in a variety of samples and the average results are represented graphically based on the nitrogen percentage in shielding gas in Fig. 6. The results show that the measured base metal nitrogen content was 360 ppm, while it was slightly reduced to be 340 ppm when applying pure argon as a shielding gas.
Also, the results showed that an increase of nitrogen percentage in argon -nitrogen mixture from 25% and more increases the N content of solidified weld metals. In the case of 75% N2 or more as a shielding gas, only a slight increase in nitrogen content. It seems that it reaches the solubility limit of 490 ppm in the solidified weld metal.
As the high temperature of the laser in the welding area leads to the dissociation of nitrogen from polyatomic molecular nitrogen gas N2 to the monatomic nitrogen gas N [20], as shown in Eq. 1. Thus, facilitating the absorption of nitrogen in molten iron.
N2 (shielding gas) = > 2N (plasma) = > 2N (% Weld pool) (1)
At a given temperature, Sievert's law is the one that governed the equilibrium solubility of the nitrogen in the molten weld metal. According to Sievert, the concentration of nitrogen in the molten weld metal is proportional to the square root of the diatomic nitrogen partial pressure above the welding bath ‘as indicated in Equation. 2’ [21, 22].
$${N}_{eq}={K}_{eq} \sqrt{{P}_{{N}_{2}}}$$
2
Where:
\({N}_{eq}\) , is the molten weld metals nitrogen concentration at equilibrium with diatomic nitrogen (wt-%).
\({K}_{eq}\) , is the equilibrium constant.
\({P}_{{N}_{2}}\) , is the nitrogen Partial pressure in the shielding gases (atm).
This means that the solubility limit of nitrogen in molten stainless steel can be increased by raising the partial pressure of the diatomic gas above the melting point. Nevertheless, there are many doubts about Sievert's law's applicability when describing diatomic gas dissolution in molten weld metal in the presence of plasma [20, 21, 23].
To elaborate more on the solidified weld metal nitrogen content, it was imposed that the final nitrogen content is the result of complex processes input and output of nitrogen atoms in the molten weld pool during laser welding.
As for the nitrogen input and absorption in the molten metal from two sources:
In addition, the process of removing dissolved nitrogen from the weld pool is done by recombining the nitrogen atoms to form nitrogen molecules (N2) that may escape into the atmosphere, plus escaping of some atoms close to the surface during and immediately after solidification as represented by Eq. 2.
2N (% Weld pool) = > N2 (gas) (2)
From this standpoint, increasing the turbulence of the weld pool increases the amount of escaping nitrogen due to the increase in the surface area of the weld pool. Also, in the case of using only argon as a shielding gas, means that no input of nitrogen atoms in the weld pool, this allows some of the nitrogen atoms already existing in the weld pool to combine to compose nitrogen molecules (N2) that can getaway into surrounding atmosphere. This hypothesis is schematically illustrated in Fig. 7
In laser welds with pure argon as the shielding gas, a significant amount of delta ferrite is observed in re-solidified fusion zones (Fig. 8a). As nitrogen is added to the shielding gas, ferrite percentages decrease, whereas austenite percentages increase in the fusion zone. As illustrated in Fig. 8b, the fusion zone microstructure can be observed when pure nitrogen is used as the shielding gas. Scanning electron microscopy (SEM) examination gives deep information on the microstructure of re-solidified fusion zones of the laser welds (Fig. 9). SEM imaging shows the diversity in the delta ferrite such as primary delta ferrite, skeletal delta ferrite and lathy delta ferrite in the matrix of austenite.
It is noteworthy that the researcher concluded in previous studies [2, 25] that weld metal's microstructure is significantly affected by nitrogen, as it limits the formation of delta ferrites and increases austenite.