Study on the Physical Characteristics of Plasma and Its Relationship with Pore Formation during Laser-Metal Active Gas Arc Hybrid Welding of 42CrMo Steel

Abstract

The automobile industry puts forward higher requirements for the design and manufacture of steel pistons. However, the welding of 42CrMo steel pistons still has unsolved technical problems, especially welding defects that cannot be directly detected, such as pores, which are easily generated inside the weld. A plasma experiment of laser-metal active gas arc (MAG) hybrid welding 42CrMo steel was conducted in this paper, and plasma signals inside and outside the keyhole were detected during the laser welding, leading laser laser-MAG hybrid welding, and leading arc laser-MAG hybrid welding of 42CrMo steel. The characteristic parameters such as electron temperature and electron density were calculated and analyzed to investigate the relationship between plasma behavior and the formation of weld porosity in the welding process of 42CrMo steel. Based on the fluctuations in plasma electron temperature and electron density, the prediction of pore formation in the weld of 42CrMo steel was made, aiming to provide guidance for achieving a stable and reliable laser-MAG hybrid welding process for 42CrMo steel.

1. Introduction

Quenched and tempered 42CrMo steel is one of the main manufacturing materials for automobile engine steel pistons [1]. This material has a high carbon content and alloy content, and the carbon equivalent (Ceq) of 42CrMo steel is approximately 0.87%, which indicates that 42CrMo steel has poor weldability and is prone to welding defects such as pores and cracks. Defects such as pores and cracks of 42CrMo steel generally appear inside the weld, which cannot be directly observed after welding but can only be found through ultrasonic or industrial CT detection. This is a thorny problem in the application of 42CrMo steel piston welding.

Plasma behavior during laser welding is closely related to welding quality. Scholars [2,3,4] observed the plasma inside and outside the keyhole in the process of 10 kW fiber laser welding stainless steel and studied the mechanical characteristics of the plasma. Li et al. [5] monitored the plasma during laser deep penetration welding of H340LA steel panels and learned that the plasma characteristics are closely related to the weld performance. Gong et al. [6] analyzed the relationship between laser energy transmission and plasma plume during laser welding of aluminum alloys and studied the scattering effect of plasma plume. Qiu et al. [7] proposed a spectral intensity method to compare the difference between heat conduction welding and laser welding in a YAG laser welding experiment on 304 stainless steel. Based on the detection results of laser plasma electrical signals, Huang et al. [8] proposed the formation mechanism of surface defects. Luo et al. [9] carried out real-time detection of acoustic emission signals during pulsed YAG laser welding and studied the characteristics of weld penetration and plasma plume according to the acoustic emission signals. Gao et al. [10] established a three-dimensional plasma model in fiber laser–arc hybrid welding and simulated and discussed the influence of the temperature and velocity of the jet steam and the distance between the arc and the laser. In order to detect the plasma plume during pulsed YAG laser welding in real time, Liu et al. [11] conducted an experimental study based on an asynchronous signal acquisition system. Tang et al. [12] studied the effects of different welding parameters on the plasma spectral characteristics and weld shape of aluminum alloy laser-MIG hybrid welding and explained the relationship between weld shape and plasma. Liu et al. [13] used spectral diagnostic technology to study the coupling effect between the arc and laser during the laser–arc hybrid welding of martensitic high-strength steel. Zhang et al. [14] conducted welding experiments on zinc “sandwich” samples, observed the behavior of keyholes, vapor and plasma and analyzed their effects, and studied the electron temperature distribution of zinc plasma. Liu et al. [15] collected plasma spectral information during laser–arc hybrid welding and explained the physical phenomena. Gao et al. [16] studied the effect of laser power on the plasma characteristics through spectral analysis, calculated the characteristic parameters of the plasma based on the spectral data, and analyzed the laser–matter interaction of aluminum alloys. Tenner et al. [17] studied the dynamic correlation between the keyhole and plasma plume at a high feed rate and high laser power and found that the keyhole stability at moderate laser power is mainly determined by the evaporation process, while at a high feed rate and high laser power, the keyhole stability is determined by the molten pool. Liu et al. [18] used pulsed YAG laser-TIG hybrid welding to conduct experiments on magnesium alloys and studied the changes in the electron density and electron temperature of the plasma during the welding process. The results show that the electron density increases and the arc temperature decreases after the laser is added; the main reason for this is the evaporation of the workpiece material caused by the laser. Li et al. [19] also obtained similar research results. They calculated the electron density and electron temperature of the plasma during the YAG laser-MIG hybrid welding of low carbon steel and found that the plasma temperature of hybrid welding is slightly lower than that of MIG welding, and the electron density is higher than that of MIG welding.

Different welding parameters will make the welding process have different physical behavior, such as the plasma’s physical behavior, so the plasma’s physical behavior can indirectly determine whether the welding parameters are selected properly. At present, there are many studies on laser-induced plasma in the process of laser deep penetration welding. However, in order to ensure the intensity and clarity of the collected radiation spectrum signals and prevent the shielding gas from blowing away the laser-induced plasma, the relevant research did not add shielding gas during the welding process when the spectrum was collected. The research conclusions in this way are different from the plasma’s physical characteristics in the production practice and can only be used as a reference for theoretical research. In addition, there are few plasma studies on laser-MAG hybrid welding, and the plasma research on laser–arc hybrid welding is limited to the plasma outside the keyhole and does not involve the plasma inside the keyhole.

The physical characteristics of the plasma inside and outside the keyhole during the laser-MAG hybrid welding process of 42CrMo steel was investigated in this paper, and the relationship between the plasma’s physical behavior and pore defect formation was studied to guide the production application of the laser welding of 42CrMo steel pistons.

2. Experimental Procedure

The experimental setup is shown in Figure 1. A YLS-10000 fiber laser system and a Fronius TPS 5000 arc welding system were used in the experiment. A KUKA KR60HA robot was used to control the experimental laser welding process, and a SpectraPro-2356 spectrograph was applied to capture the plasma radiation.

Among them, the plasma inside and outside the keyhole was monitored using the “sandwich” experimental method, as shown in Figure 2. During the test, the fixture holding the “sandwich” sample was fixed on a numerical control two-dimensional electric displacement platform, and the relative positions of the laser beam, focusing lens, and optical fiber bundles were adjusted according to the relationship between the lens imaging positions. During the welding process, the laser welding head remains still, the displacement platform moves horizontally and linearly according to the set welding speed, and the spectrometer collects the spectral signals inside and outside the keyhole at fixed points and transmits them to the computer.

In order to obtain spectral signals at different positions inside and outside the keyhole at the same time, the spectrometer was set to eight channels, and eight optical fiber bundles were configured to collect at the same time to obtain multipoint and multiframe data of the plasma inside and outside the keyhole during the welding process. The data collected by each optical fiber bundle corresponded to the spectral signal at a certain position in the depth direction of the keyhole at different times. The eight channels set up in the spectrum test corresponded to different positions in the vertical direction inside and outside the keyhole of the “sandwich” sample, in which the first, second, and third channels corresponded to the outside of the keyhole, and the fourth, fifth, sixth, seventh, and eighth channels corresponded to the inside of the keyhole. The distance between each channel was about 1 mm, and the surface of the “sandwich” sample was in the middle of the third and fourth channels. If the surface coordinates of the sample are taken as the zero point, the direction outside the keyhole was positive, and the direction inside the keyhole was negative; then, the position of the first channel was +2.5 mm, the position of the second channel was +1.5 mm, the position of the third channel was +0.5 mm, the position of the fourth channel was −0.5 mm, the position of the fifth channel was −1.5 mm, the position of the sixth channel was −2.5 mm, the position of the seventh channel was −3.5 mm, and the position of the eighth channel was −4.5 mm.

According to the collected spectrogram data, the electron temperature of the plasma can be calculated using the Boltzmann diagram method. Afterward, the Fe I spectral line of 492.05 nm was selected for Lorentz linear fitting to obtain the full width at half maximum of the spectral line and calculate the electron density of the plasma. The data calculation results of plasma electron temperature and density at multiple times were averaged.

Welding experiments on 42CrMo steel plates (length: 100 mm; width: 30 mm; thickness: 7 mm) using the same welding parameters as the “sandwich” experiment were performed. Before welding, each specimen was carefully cleaned with acetone and then was preheated to 350 °C in a KSY-14-16 three-phase silicon-controlled temperature control box and retained at 350 °C for 3 h. Esab ER80S-G welding wires (diameter: 1.0 mm) were used in the experiment. A phoenix v/tome/x m 300 kV industrial CT system was used to nondestructively examine and count the pores.

Table 1. 42CrMo chemical composition (wt.%).

C	Si	Mn	S	P	Cr	Ni	Cu	Mo
0.44	0.23	0.76	0.017	0.011	0.99	0.042	0.04	0.17

presents the chemical composition (wt.%) of quenched and tempered 42CrMo steel.

3. Results and Discussion

3.1. Calculation Results and Analysis of Plasma’s Physical Parameters in Laser-MAG Hybrid Welding

When the laser power is 5200 W, the defocusing distance is +7 mm, the welding speed is 0.75 m/min, and the shielding gas flow rate is 15 L/min. The radiation spectrum signals of 42CrMo steel during laser welding were collected. The results show that under the action of the shielding gas, the spectrometer did not collect obvious plasma radiation lines, and most of the collected spectra are shown in Figure 3. It can be seen from Figure 3 that no obvious line spectrum can be found on the spectrogram, and there is no data available for the calculation of the plasma’s physical parameters. Analyzing Figure 3, it can be considered that when the lower laser power is used, due to the blowing effect of the paraxial shielding gas, the relatively thin plasma is blown away, resulting in the failure to collect the plasma radiation line signal.

When the laser power is 4200 W, the defocusing distance is +2 mm, the welding speed is 0.75 m/min, the shielding gas flow rate is 15 L/min, the laser–arc distance is 3.5 mm, and the wire feed rate is 5 m/min. The radiation spectrum signals of 42CrMo steel at a certain moment during the leading laser laser-MAG hybrid welding process were collected, respectively. The spectrum of the plasma collected by the spectrometer at a wavelength between 410 nm and 550 nm is shown in Figure 4.

According to the collected spectrogram data at each moment, the electron temperature of the plasma can be calculated according to the Boltzmann diagram method. Six Fe I spectral lines were selected for Boltzmann plotting to calculate the electron temperature of the plasma. The parameters of the selected six Fe I spectral lines are shown in

Table 2. Fe I spectral line parameters.

Wavelength ${\mathit{\lambda }}_{\mathit{p}\mathit{q}}$ (nm)	Transition Probability ${\mathit{A}}_{\mathit{p}\mathit{q}}$ (s−1)	Energy of the Upper Level ${\mathit{E}}_{\mathit{p}}$ (eV)	Statistical Weight ${\mathit{g}}_{\mathit{p}}$
492.05	3.58 × 10−7	5.3516	9
495.76	4.22 × 10−7	5.3085	11
516.75	2.72 × 10−6	3.8835	7
522.72	2.89 × 10−6	3.9286	5
532.80	1.15 × 10−6	3.2410	7
537.15	1.05 × 10−6	3.2657	5

[4].

According to the relative intensity of the plasma spectral signal collected in the experiment, a Boltzmann diagram can be drawn, as shown in Figure 5, which is the Boltzmann diagram of a certain frame of spectrum in the first channel of the leading laser laser-MAG hybrid welding. It can be seen from Figure 5 that the linear fitting slope −1/kT_e is −2.3852 eV⁻¹, and the Boltzmann constant $k$ is 8629.0656 eV/K.

Substitution Equation (1) [20] is:

$$\ln \left(\frac{I_{pq}{\lambda }_{pq}}{g_p{A}_{pq}}\right)=\ln \left(\frac{hcN}{4\pi Z}\right)-\frac{E_P}{k{T}_e}$$

(1)

where $I_{pq}$ is the measured intensity of the spectral line for the transition $p\to q$; $A_{pq}$ is the transition probability for the transition $p\to q$; $g_p$ is the statistical weight of the upper level; ${\lambda }_{pq}$ is the nominal wavelength; $h$ is Planck’s constant; $c$ is the speed of light; $E_p$ is the energy of the upper state $p$; $n$ is the population density; $Z$ is the partition function; $k$ is the Boltzmann constant; and $T_e$ is the electron temperature.

The plasma electron temperature was calculated to be 4865.41 K. From this, the electron temperature of each frame can be obtained, and the average value is 4718.22 K. The calculation method of other channels is similar to this.

As the left and right of the Fe I spectral line with a wavelength of 492.05 nm does not overlap with other spectral lines, its line broadening will not be affected by other spectral lines, which can ensure the accuracy and reliability of the plasma electron density calculation results. Therefore, this article selects the Fe I spectral line of 492.05 nm for Lorentz linear fitting, as shown in Figure 6, to obtain the half-width of the spectral line, calculate the electron density of the plasma at each moment, and obtain the average value.

The final calculation results are shown in

Table 3. Calculation results of plasma’s physical parameters of leading laser hybrid welding.

Channel	Position Inside and Outside the Keyhole (mm)	Electron Temperature (${\mathit{T}}_{\mathit{e}}$, K)	Electron Density (${\mathit{N}}_{\mathit{e}}$, 1016 cm−3)
first channel	+2.5	4718.22	24.515
second channel	+1.5	4919.44	27.857
third channel	+0.5	4903.44	31.602
fourth channel	−0.5	5089.90	29.138
fifth channel	−1.5	4917.36	26.834
sixth channel	−2.5	5067.67	21.511

and

Table 4. Calculation results of plasma’s physical parameters for leading arc hybrid welding.

Channel	Position Inside and Outside the Keyhole (mm)	Electron Temperature (${\mathit{T}}_{\mathit{e}}$, K)	Electron Density (${\mathit{N}}_{\mathit{e}}$, 1016 cm−3)
first channel	+2.5	5251.12	27.071
second channel	+1.5	5745.03	30.652
third channel	+0.5	5799.73	32.480
fourth channel	−0.5	5688.71	30.733

. Table 3 shows the calculation results of the plasma temperature and density obtained by the leading laser laser-MAG hybrid welding process, and Table 4 shows the calculation results of the plasma electron temperature and density obtained by the leading arc laser-MAG hybrid welding process. The results show that in the process of the leading laser hybrid welding, the first to sixth channels collected obvious plasma spectra, and the seventh and eighth channels did not collect spectral signals. And in the leading arc hybrid welding process, the first to fourth channels collected plasma spectral signals, but the fifth to eighth channels did not collect data for the calculation of the plasma’s physical parameters. From the calculation results in Table 3 and Table 4, it can be seen that the plasma temperature of each channel of the leading laser laser-MAG hybrid welding is generally lower than that of the leading arc laser-MAG hybrid welding; the average temperature is about 700K lower, and the electron density of the leading arc hybrid welding is also slightly higher. Compared with the calculation results of the electron temperature and density of the plasma inside and outside the keyhole when the 10kW fiber laser is used to weld stainless steel in the literature [4], the electron temperature of hybrid welding is not much different from it, but the electron density is much higher. At the same time, it can be seen that the highest plasma electron density of the two hybrid welding processes is near the opening of the keyhole.

A comprehensive analysis of the plasma spectrum collection results of laser welding and two kinds of laser-MAG hybrid welding shows that under the action of the paraxial shielding gas, the laser-induced plasma generated by the laser welding of 42CrMo steel is blown away by the shielding gas and disappears or the amount is so small that it cannot be detected. However, laser-MAG hybrid welding can still detect the plasma spectrum, indicating that when there is paraxial blowing, the plasma in hybrid welding is mainly arc plasma. At the same time, due to the addition of the MAG arc, more gas ions in the hybrid welding arc atmosphere during the welding process are in an excited state, which improves the transition probability of particles between different energy levels. Thus, the bremsstrahlung, composite radiation, and excitation radiation of the hybrid welding arc are strengthened, and the ionization degree of the composite arc is promoted. Moreover, the addition of the MAG arc intensifies the collision of particles inside the hybrid welding arc, which increases the temperature and density of the plasma electrons. The plasma spectral signal of the leading laser hybrid welding appears deeper in the keyhole than the spectral signal of the leading arc hybrid welding, indicating that when the laser is in front, the effect of the fiber laser on the arc ignition and arc compression of the MAG arc is more obvious. The arc is attracted to the deeper keyhole, and the Fe ion vapor in the laser-induced plasma forms an ionization channel, which makes the arc expand along the ionization channel, which can widen the range of the arc and play a role in stabilizing the arc. The higher plasma electron temperature and density of the leading arc hybrid welding shows that when the arc is in the front, as the MAG arc first melts the base material, and when the laser is irradiated on the molten liquid base material, the absorption rate of the base material to the laser is improved, thereby generating more metal vapor and plasma. From the results that the highest plasma electron density is near the opening of the keyhole, it can be known that after the fiber laser recombines with the MAG arc, the arc plasma energy center intensity area is closer to the molten pool, and the arc shifts to the laser recombination area.

3.2. Relationship between the Plasma’s Physical Behavior and Pore Formation in Laser-MAG Hybrid Welding

The research in the previous section is the result of the change in the average value of the plasma’s physical parameters with the depth of the keyhole within a certain period of time, which cannot reflect the plasma’s physical behavior of the entire laser welding process. This section investigates the fluctuation in the plasma’s physical parameters over time at multiple times and their relationship with the generation of pores.

According to the CT detection results, a group of welding parameters with more pores and less pores were selected for plasma experiments. The welding results of the two groups of parameters are shown in

Table 5. Welding results for the first and the second group of parameters.

Test Number	Weld Appearance	CT Photograph	Number of Pores
1			44
2			3

(The CT photograph only shows the number of pores in one section of the specimen).

The first group of parameters for the leading arc welding are as follows: the laser power is 4200 W, the defocusing distance is +2 mm, the welding speed is 0.75 m/min, the shielding gas flow rate is 15 L/min, the laser–arc distance is 3.5 mm, and the wire feed rate is 5 m/min.

The second group of parameters for the leading laser welding is as follows: the laser power is 4300 W, the defocusing distance is +5 mm, the welding speed is 0.75 m/min, the shielding gas flow rate is 15 L/min, the laser–arc distance is 3.5 mm, and the wire feed rate is 5 m/min.

The plasma outside the keyhole was analyzed first, and the calculation results of the plasma electron temperature and density of the third channel during the welding process of the two groups of parameters at different times were taken for comparative analysis. The change in electron temperature with time is shown in Figure 7, and the change in electron density with time is shown in Figure 8. The time interval of the abscissa in the figure is 200 ms, and the plasma signal acquisition time period basically corresponds to the formation time period of the weld seam taken in Table 5.

Then, the plasma inside the keyhole was analyzed, and the calculation results of the plasma electron temperature and density of the fourth channel during the welding process of the two groups of parameters at different times were obtained for comparative analysis. The change in electron temperature with time is shown in Figure 9, and the change in electron density with time is shown in Figure 10.

As shown in Figure 7, Figure 8, Figure 9 and Figure 10, no matter outside the keyhole or inside the keyhole, the fluctuations in plasma temperature and density over time in the welding process of the first group of parameters are relatively large, while the fluctuations in plasma temperature and density over time in the welding process of the second group of parameters are relatively stable. The electronic temperature signal of the plasma is closely related to the stability of the welding process [13], and the interaction between the gas flow stability in the keyhole and the fluctuation in the molten pool is also an important factor for the generation of porosity [21,22,23]. Combining the weld CT detection results of the two groups of parameters, it can be known that during the laser-MAG hybrid welding process of 42CrMo steel, when the plasma physical characteristics fluctuate greatly with time, the stability of the molten pool and keyholes is poor; the solidification rate of the molten pool goes beyond the filling rate of the collapsed keyhole by the molten material, and it is easy to form pores in the weld.

42CrMo steel easily produces pores during welding, and most of the pores are formed inside the weld, which can only be detected with the help of expensive CT equipment. Based on the fact that the instability of the molten pool and pores is closely related to the physical characteristics of the plasma, it is of positive significance to judge whether a large number of pores are generated in the weld of 42CrMo steel by monitoring the fluctuation in the physical characteristics of the plasma over time.

4. Conclusions

In this article, a spectrometer was used to conduct welding experiments on 42CrMo steel, and the plasma spectra and physical parameters of laser welding, leading laser laser-MAG hybrid welding, and leading arc laser-MAG hybrid welding were collected and analyzed. The main conclusions are as follows:

(1)

When there is paraxial shielding gas blowing during the welding process, the laser welding of 42CrMo steel does not detect the plasma spectrum, but the hybrid welding can still detect the plasma spectrum. Regarding the welding parameters in this paper, the average plasma electron temperature and plasma electron density of the leading laser laser-MAG hybrid welding are, respectively, 4936 K and 26.9 × 10¹⁶ cm⁻³, and the leading arc laser-MAG hybrid welding are, respectively, 5621 K and 30.2 × 10¹⁶ cm⁻³;

(2)

The plasma electron temperature of each channel of the leading laser laser-MAG hybrid welding is generally lower than that of the leading arc laser-MAG hybrid welding; the average temperature is about 700 K lower, and the plasma electron density of the leading arc hybrid welding is also slightly higher;

(3)

The highest plasma electron density of both hybrid welding processes is in the third channel. Among them, the value of the leading laser laser-MAG hybrid welding process is 31.602 × 10¹⁶ cm⁻³, and the value of the leading arc laser-MAG hybrid welding process is 32.480 × 10¹⁶ cm⁻³;

(4)

During the laser-MAG hybrid welding of 42CrMo steel, the stability of the plasma’s physical characteristics is closely related to the formation of pores. When the physical characteristics of the plasma fluctuate greatly over time, a large number of pores will be generated, and when the physical characteristics of the plasma are relatively stable, there will be fewer pores in the welding. For 42CrMo steel, which is prone to welding porosity that is difficult to detect, the porosity generation in the weld bead of 42CrMo steel can be monitored online by identifying the fluctuation in the plasma electron temperature and electron density.