3.1 Influence of magnetic field intensity on plasma electron temperature distribution
The laser welding is carried out in the atmospheric environment, the plasma generated in the welding process is in the local thermodynamic equilibrium state. The plasma temperature is approximately equal to the temperature of the electron in the excited state, so the plasma temperature can be obtained by calculating the temperature of the excited state electron.
In this paper, the excitation temperature is measured by the Boltzmann diagram method. Since the laser plasma core region system belongs to the local thermodynamic equilibrium (LTE), according to Boltzmann's law of distribution, when the spectral line transitions from energy level k to i, the number of excited plasma particles could be described by the Boltzmann function:
$${n}_{k}=\frac{{n}_{0}}{Z}{g}_{k}\text{exp}\left(\frac{-{E}_{k}}{KT}\right)$$
1
In formula (1), \(Z\) represents the ground state partition function. \({n}_{k}\) represents the number of particles in the excited state. \({n}_{0}\)represents the number of particles in the ground state. \({g}_{k}\) represents the statistical weight of the upper energy level. Ek represents the ionization energy of the k level. K represents the Boltzmann constant. T represents the electron temperature.
According to the principle of spectroscopy, when a particle transforms from a higher energy level k to a lower energy level i, the intensity of the characteristic spectral line \({I}_{ki}\)produced can be expressed as:
$${I}_{ki}={A}_{ki}h{V}_{ki}{n}_{k}$$
2
In formula (2), \({V}_{ki}\) is the transition frequency of the characteristic spectral line. \({A}_{ki}\) is the transition probability of the characteristic spectral line k-level to i-level, and h is the Planck constant.
Combine (1) and (2), substitute the elimination parameter for \({n}_{k}\), and take the logarithm of ln on both sides of the equation:
$$ln\left(\frac{{I}_{ki}{\lambda }_{ki}}{{A}_{ki}{g}_{k}}\right)={ln}\left(\frac{{n}_{0}hc}{Z}\right)-\frac{{E}_{k}}{KT}$$
3
In formula (3), c is the speed of light. \({\lambda }_{ki}\) is the wavelength of the characteristic spectral line. \({ln}\left({n}_{0}hc/Z\right)\) is a constant. Using the least square method, using computer software to linearly fit the \(ln\) {(\({I}_{ki}{\lambda }_{ki}\))/ (\({A}_{ki}{g}_{k}\))} and \({E}_{k}\) data of multiple spectral lines, the slope of the fitted straight line obtained is -1/\(KT\), and the slope is the temperature T of the plasma can be obtained.
The physical characteristics of the laser wire-filled welding plasma in the narrow gap groove under the action of the magnetic field are extracted by high-speed imaging and spectral analysis, and four characteristic lines of FeI are used to calculate the electron temperature, which are FeI 438.84nm, FeI 461.88nm, FeI 465.46nm, and FeI 520.23nm. The calibration of the corresponding characteristic peak spectral line is shown in Fig. 4.
Spectroscopic related parameters of FeI can be found in the NIST spectral database, which are shown in Table 3. According to the following parameters, the original spectral data can be calculated by the Boltzmann multi-line slope method, and the electron temperature of the plasma can be obtained, so as to get the distribution of the plasma temperature under different magnetic induction intensities.
Table 3
The spectral parameters of FeⅠ line used in Boltzmann temperature calculation (from NIST)
Element(λ/nm)
|
Aki(107s−1)
|
Ek(eV)
|
gk
|
eⅠ (438.84065)
|
1.03
|
6.42699835
|
7
|
FeⅠ (461.87572)
|
0.0136
|
5.63240396
|
9
|
FeⅠ (465.46052)
|
0.368
|
6.26546665
|
7
|
FeⅠ (520.23355)
|
0.0511
|
4.55852277
|
7
|
During the single-line collection of plasma spectroscopy diagnosis in this paper, a total of 8 collection points is set up, which are evenly distributed from bottom to top along the centerline of the groove. The #1 collection point is 0.4mm away from the bottom of the groove, and the distance between every two adjacent test points is 1.2mm, as shown in Fig. 5. In the test, the depth of the groove is 8mm, so there are seven collection points located inside the groove, and the remaining one (#8 collection point) located at outside of the groove, which is 0.9mm away from the top surface of the groove. The #8 collection point is within the range of the plasma’s core and is used to study the boundary electron temperature of the core region of the plasma. According to the Boltzmann multi-line slope method, the plasma electron temperature distribution law at the centerline of the narrow gap groove is calculated as shown in Fig. 6.
As can be seen from Fig. 6, the electronic temperature distribution in the vertical direction inside the groove has a very similar trend with the distance increase from the bottom of the groove in spite of the different magnetic fields. In the same group, as shown in Fig. 6(a), except for an "inflection point" at position #2, the overall trend of electron temperature of #1~#8 plasmas decrease with increasing distance from the bottom of the groove. The laser induced plasma is ejected out of the hole and enters the air. With the decay of the electron energy, the kinetic energy of the electron gradually decreases, so the overall trend of the curve conforms to the law of energy conservation. The sudden decrease of electron temperature in point # 2 (inflection point) is due to that point is the intersection of light and wire. The energy changes at the "inflection point", with the wire end melting and dripping. Meanwhile, the wire absorbs metal vapor and plasma thermal radiation energy, which induce the decreasing of the total kinetic energy of the electrons in the plasma. As a result, the temperature drops quickly from # 1 to # 2. However, due to the space limitation of the narrow gap structure, the lower plasma continues to be ejected upward, bypass the droplet at the end of the welding wire, and enter into the upper space position of the droplet and reassemble, so the electronic temperature rose. In addition, the electron temperature gradient of these collection points is slight, which fully reflects the extrusion and confinement effect of the narrow gap groove on the plasma, and improve the utilization rate of the laser energy.
As the magnetic induction intensity gradually increases from 0mT to 120mT, as shown in Fig. 6(b), the electron temperature at positions #1~#6 shows an increasing trend, and position #1 is the closest point to the bottom of the groove. The electron temperature increases from 4909K to 5069K with an increasement of 160K. This is because the plasma is ejected from the small hole, but the direction of electron movement is not completely parallel to the magnetic induction line of the longitudinal magnetic field. Some high-speed moving electrons cut the magnetic induction line and are subjected to the Lorentz force in the magnetic field. As a result, the electrons obtain additional kinetic energies, which cause the rise of electron temperature. The electron temperature at positions #7 and #8 first decrease and then increase. When the magnetic induction intensities are 60mT and 90mT, the electron temperatures are similar, and the minimum temperature value is at 60mT, which are 4741K and 4677K, respectively. On one hand, it shows that under the action of an external magnetic field, the plasma is compressed as a whole, which causes the decay of the kinetic energy of the plasma electrons outside the groove. On the other hand, it shows that the magnetic field has a more significant effect on the plasma when the magnetic induction intensity lies in the range between 60mT and 90mT. Under this condition, the energy concentrates inside the groove and melts the welding wire and base metal.
As shown in Fig. 5(b), the multi-line acquisition method is used to diagnose the plasma in the narrow gap groove. 24 locations are collected, and all the spectral calculation results are drawn in the form of cloud diagrams. The plasma electron temperature distributions at different magnetic induction intensities are shown in Fig. 7. The plasma high temperature center appears at the bottom of the groove and near the intersection of the light and wire, both of which are the positions directly irradiated by the laser beam. Half of the laser spot is projected on the end of the welding wire, and the other half is projected on the surface of the base metal. The laser beam acts on the metal vapor above the irradiated point to excite a large number of electrons. Therefore, the electron temperature at these two places is relatively high, and the plasma electron temperature decreases from the center to the surroundings. In addition, the plasma electron temperature in the range of 60mT ~ 90mT is more concentrated in the groove. The electron temperature in the groove ranges from 4781K to 5033K, and the electron temperature outside the groove is lower than 4682K.
3.2 The influence of magnetic induction intensity on the flow behavior of molten pool
In the process of narrow-gap laser welding with filler wire, when other technological parameters are constant, the molten pool height increases with the increase of magnetic induction intensity, which indicates that the stability of molten pool flow is strengthened. Under the action of Lorentz force, the magnetic field interacts with the plasma with high energy density in the laser keyhole, and the magnetic field can stir the molten pool and stabilize the flow state of the molten pool liquid surface by controlling the swing of the keyhole. The evolution of the stability of the molten pool level under the action of a magnetic field is shown in Fig. 8. h in the figure is the distance from the bottom of the groove to the liquid level of the molten pool.
Figure 8 (a) represents the state of the molten pool with no magnetic field, and (b) ~ (d) represent the state of the molten pool under different applied magnetic fields (B1 < B2 < B3). An appropriate magnetic field can relieve the violent shock of the molten pool during the droplet transition, and keep the liquid level relatively calm. The liquid level increases from h0 to h2 with the magnetic field from B0 to B2. However, with the further increase of the magnetic field to B3, the liquid level decreases to h3. The stirring effect of the magnetic field on the molten pool accelerates the movement of the fluid in the molten pool, causing the liquid level to become unstable, resulting in h2 > h3.
Duplex stainless-steel contains ferritic structure and therefore has weak ferromagnetism. Under the action of magnetic field, molten pool metal in high temperature melting state is magnetized to form magnetic fluid. Magnetic fluid and laser hole in molten pool interact with magnetic field together, and the dynamic behavior of the hole has great influence on molten pool metal flow. High-speed cameras are used to take fixed shots of the molten pool of magnetic field assisted laser self-fusion welding on the surface of the test plate to characterize the flow behavior of the molten pool in the laser welding process under the action of magnetic field, as shown in Table 4.
Table 4 High-speed images molten pool flow in different magnetic inductive intensity
After a short time of laser focusing on the surface of the plate, a photoinduced hole appears in the center of the molten pool, and the hole remains relatively stable in the welding process until the end of the welding. Taking the laser hole as the reference position, it can be found that the liquid metal in the molten pool has different flow behavior under different magnetic induction intensity. From (a) you can see that without magnetic field, melt liquid metal forward together in the direction of welding holes on the front, front metal volume increases gradually, then under the action of gravity level to reduce the flow backwards, at this point, a large number of cutting-edge metals instantly back contact with laser holes caused wild shocks occur in the molten pool. (b) shows that when the magnetic induction intensity is 60mT, there is no accumulation and backflow of the metal at the front of the small hole in the welding process, and the liquid level of the molten pool remains calm and the welding process is very stable. In (c), the magnetic induction intensity is 90mT, and obvious ripples can be seen on the molten pool metal liquid surface of the molten pool first. The convergence and backflow behavior of the metal at the front of the hole appears again, but the shock degree of the molten pool caused by the backflow of the metal at the front becomes significantly smaller. It can be seen from (d) that when the magnetic induction intensity is 120mT, the molten pool liquid flows too fast towards the front of the laser hole, resulting in a large amount of metal spatter, and the welding process is extremely unstable. When a magnetic field is applied, the plasma electron temperature rises under the action of Lorentz force, as shown in Fig. 7. The high temperature center is concentrated near the bottom of the groove and the intersection of the light and wire intersection, which makes the metal droplet transition instantly cause violent vibrations on the surface and inside of the molten pool, and the molten pool liquid level becomes more up and down. The distance between the lowest position of the molten pool and the upper surface of the bottom of the groove gradually increases with the magnetic field increases, and the range of the high-temperature central area in the groove gradually increases. The vibration of the molten pool becomes more intense, and the height of the molten pool increases accordingly. When the magnetic field intensity increases above the critical value, the high-temperature central area becomes too large to keep the surface of the molten pool stable, which leads to the thickness of molten pool decreasing.