Powder Melt Pool Coupled Active Tig Welding: How Much Do You Know?

Abstract: Powder pool coupled active TIG welding is a new efficient welding method. By selecting the corresponding active agent powder, almost all metals can be welded.

Aiming at the AC powder pool coupled active TIG welding arc using MnCl2 as the activating agent, the plasma spectrum was collected, and the changing rule of the temperature of the arc plasma with time was analyzed by Boltzmann drawing method.

Combined with the changing rule of the arc voltage, the influence of MnCl2 on the AC arc was studied by comparing with the traditional AC TIG arc.

The results show that for AC TIG arc, the arc spectral intensity in EN period is higher than that in EP period, the arc voltage in EN period is lower than that in EP period, and the arc temperature in EN period is lower than that in EP period.

However, due to the introduction of active agent MnCl2, the center temperature and arc voltage in EN and EP periods of AC powder melt pool coupled active TIG arc are higher than those of traditional AC TIG arc, and the weld penetration is significantly increased compared with traditional AC TIG welding.

Related reading: MIG vs TIG Welding

Preface

Since Barton Welding Research Institute first proposed the argon arc welding technology on the flux layer, active welding has attracted extensive attention because it can significantly increase penetration, among which active TIG welding has the most relevant research.

By selecting appropriate process parameters and active flux, it can significantly improve the welding efficiency and maintain the high quality of TIG welding.

However, the coating of active agent is usually completed by manual brushing or spraying, which is difficult to ensure the coating quality and reduce the production efficiency.

At the same time, for aluminum, magnesium and other active metals, the previous AA-TIG welding (Advanced A-TIG Welding), AA-TIG welding (Arc assisted Activating TIG welding) and GPCA-TIG welding (Gas Pool Coupled Activating TIG Welding) and other methods of introducing active element O through active gas are not applicable.

In view of the above problems, Lanzhou University of Technology proposed PPCA-TIG (Powder Pool Coupled Activating TIG Welding), which uses double-layer gas for welding.

The inner layer uses inert gas to protect tungsten electrode and molten pool metal, and the outer layer uses an automatic powder feeding device to send the active agent powder with the shielding gas into the arc molten pool coupling system.

The interaction between active flux and arc molten pool can significantly increase the penetration, greatly improve the welding efficiency, and is easy to realize mechanized and automatic welding.

Spectral analysis is widely used in plasma research because of its rich information, high sensitivity and accurate temperature measurement.

Tanaka et al. measured the composition of active TIG arc by spectrum, and analyzed the influence of active agent on TIG arc;

Chai Guoming et al. analyzed the temperature distribution of A-TIG welding arc by spectrum.

The periodic change of AC TIG arc is its most basic feature.

Although some scholars have analyzed the change process of arc electron density with time by spectral method, the change process of arc temperature is rarely reported.

For AC TIG arc and AC PPCA-TIG arc using MnCl2 as activator, as shown in Fig. 1, Boltzmann mapping method is used in combination with arc voltage analysis to study the change process of arc temperature with time, and analyze the influence of MnCl2 activator powder entering the arc with the outer gas on the arc characteristics.

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Fig.1 PPCA-TIG welding schematic

1. Test method

1.1 Diagnosis principle of Boltzmann mapping method

Boltzmann mapping method is used to measure the plasma temperature by measuring the relative intensity of multiple spectral lines in the plasma.

If the plasma is in local thermodynamic equilibrium, the radiation coefficient of the spectral line in the plasma can be expressed as

Powder Melt Pool Coupled Active Tig Welding: How Much Do You Know? 2

Calculate the logarithm of both sides of equation (1) to get

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Where K=ln [h/4 π (n/z)], independent of the type of spectral line.

When calculating the temperature, select multiple spectral lines of the same particle (atom or ion), query the relevant parameters of the corresponding spectral lines (excited state energy E, transition probability A and statistical weight g), take ln[εL/νAjgj] and E as the ordinate and abscissa respectively, and make each point, and use the least square method to fit each point.

The slope of the fitted line is (-1/kBT), so as to solve the plasma excitation temperature T.

Boltzmann plot method is used to solve the excitation temperature of plasma.

The plasma does not need to strictly meet the local thermodynamic equilibrium conditions, and it has the advantages of high measurement accuracy, simple and convenient calculation, etc.

To improve the accuracy, the following requirements shall be met when selecting spectral lines:

① Avoid selecting the spectral line corresponding to the energy level with less particle number density near the ground state energy level;

② Select the spectral line in the smallest wavelength range;

③ The emission coefficient of at least 5 spectral lines shall be measured;

④ During the measurement, the temperature difference of the plasma emission source shall not be too large.

1.2 Test conditions

The test objects are respectively traditional AC TIG arc and AC PPCA-TIG arc.

The shielding gas of TIG arc and the internal and external shielding gas of PPCA-TIG arc are argon with a purity of 99.9%.

The chloride activator can remove the oxide film on the surface of aluminum alloy, and MnCl2 can significantly increase the penetration in chloride.

Therefore, typical chloride MnCl2 is selected as the activator powder with a particle size of 100-200 mesh. 

The 8 mm 3003 aluminum alloy plate is used as the welding base metal.

The arc spectrum information acquisition system is shown in Fig. 2.

The spectrometer is AvaSpec-ULS3648-10-USB2, an optical fiber digital spectrometer of Avantes.

The spectrum information acquisition location is shown in Fig. 3.

The distance from the tungsten electrode tip is y=3mm.

The acquisition method is fixed-point acquisition.

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Fig.2 Schematic diagram of spectral test system

Before the test, wipe the aluminum alloy surface with acetone to remove the oil stain on the surface, and then use a grinder to remove the oxide film on the aluminum alloy surface.

At the same time, dry and heat the active agent to remove the crystal water and absorbed water from the active agent itself.

During the test, firstly open the gas cylinder and cooling water circuit, start the air flow meter controlling the outer air circuit and the motor controlling the powder feeder while striking the arc, start to transport the powder, collect the arc spectrum information after the arc is stably burned, and collect the arc voltage with the USB-6215 data acquisition card.

The acquisition position of the arc voltage is shown in Fig. 3.

After the test, extinguish the arc and stop feeding powder.

The process parameters of PPCA-TIG welding test are: welding current 160 A, arc length 4 mm, inner layer argon flow 12 L/min, outer layer argon flow 8 L/min, powder feeder motor speed 30 r/min.

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Fig.3 Acquisition position

2. Test results and analysis

2.1 Arc spectrum information

According to Boltzmann’s mapping method, select 6 spectral lines of Ar Ⅱ within 445~480 nm, query the transition probability A, statistical weight g and excited state energy E of the selected spectral lines in NIST database, and calibrate the selected spectral lines, as shown in Fig. 4a.

In addition, the spectral information of PPCA-TIG-MnCl2 arc within 445~480 nm is shown in Fig. 4b.

Compared with Fig. 4a, it is found that in addition to the corresponding Ar II spectral lines, Mn I (475.40 nm, 478.34 nm), Mn II (449.88 nm, 450.22 nm) and Cl II (476.86 nm) also appear, which indicates that after MnCl2 enters the arc with the outer gas, melting, evaporation, dissociation and ionization take place under the action of high arc temperature and strong electric field.

The particles Mn, Mn+, Cl and Cl – are produced.

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Fig.4 Arc spectral information

2.2 Arc temperature measurement and calculation

Take traditional AC TIG welding as an example to process the data.

The characteristic intensity of Ar Ⅱ spectral line at a certain time in Fig. 4a is shown in Table 1.

The data in Table 1 are analyzed by Boltzmann mapping method, and the linear relationship of ln(εL/νAg)-E is obtained by fitting with relevant software, as shown in Fig. 5.

The fitting calculation result is y=a+bx, a=-31.935 7 ± 2.105, b=-0.719 97 ± 0.104 26, where R2 is 0.922 61.

According to the slope (-1/kBT), the temperature at this point is 16 113 K, which is basically consistent with the traditional DC TIG arc temperature measured previously.

Table 1 strength of selected Ar II Lines

No.Spectral line  Characteristic strength
1457.93952.81
2458.98974.44
3460.951102.13
4465.791084.69
5472.681396.38
6473.591557.44
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2.3 Effect of MnCl2 introduction on AC PPCA-TIG welding arc

The periodic change of AC arc is its most basic feature.

In order to make the spectral sampling time interval characterize the change process of AC TIG arc with time, the current and voltage waveforms of the welding power source were measured.

The results are shown in Fig. 6.

The results show that the current waveform of the welding power source is a standard square wave, with a period of about 16.7 ms, and EN: EP ≈ 12.06: 4.64.

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Fig.6 Welding currentvoltage wave form

Set the sampling interval of the spectrum to 2 ms, collect the spectral information of the traditional AC TIG arc and AC PPCA-TIG-MnCl2 arc, and extract the six Ar Ⅱ spectral lines selected for temperature calculation for a period. The results are shown in Fig. 7.

For these two arcs, the spectral intensity in EN period is higher than that in EP period.

In the EN period, the arc shrinkage is high, the arc is concentrated and the arc light is strong;

In EP period, the arc is scattered and distributed widely, and the arc light is weak.

The reason for different arc shapes is that during EP period, the arc is attached to the cathode spot, and the cathode spot tends to look for the part of the molten pool that has oxides, while the oxides in the center of the molten pool have almost been cleaned, so the cathode spot will move to the edge of the molten pool, and the arc attached to the cathode spot will also expand.

At the same time, the current in EP period is the same as that in EN period, so the arc light weakens while the distribution range becomes larger.

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Fig.7 Change of spectral line intensity

The periodic temperature of traditional AC TIG arc and AC PPCA-TIG-MnCl2 arc calculated by Boltzmann drawing method is shown in Fig. 8.

For traditional AC TIG arc, the average temperature in EN period is 16031 K, and that in EP period is 16723 K.

If the average temperature in EN period is lower than that in EP period, the difference is 692 K.

As the thermal power of the arc P=UI, combined with the current and voltage in Fig. 6, it can be seen that the current values in the EN period and EP period are the same, and the voltage value in the EN period (17.9 V) is less than that in the EP period (26.2 V), making the heat generation of the arc in the EP period greater than that in the EN period.

The reason for the different voltage values is that in the EN period, the tungsten electrode, as a hot cathode material, emits electrons in the way of thermal emission.

When the temperature reaches a certain condition, it is easier to emit electrons.

In EP period, aluminum alloy, as a cold cathode material, emits electrons in the way of field emission, which requires higher voltage to complete emission;

In addition, the temperature is positively correlated with the electron density.

The average electron density measured in the literature in the EN period is less than that in the EP period, which is the same as the rule obtained in this experiment.

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Fig.8 Temperature variation

For AC PPCA-TIG-MnCl2 arc, the average temperature in EN period is 16 460 K, 429 K higher than that in traditional AC TIG arc, and the average temperature in EP period is 17 056 K, 333 K higher than that in traditional AC TIG arc.

The reasons for the temperature rise are:

On the one hand, MnCl2 active powder enters the arc through the outer gas. Under the action of the high temperature and strong electric field of the arc, the dissociated Cl has a large electron affinity.

It absorbs electrons at the periphery of the arc, making the conductive channel of the arc center narrow, the arc shrinks, and the current density of the plasma in the arc center increases, and the temperature rises;

On the other hand, the evaporation and dissociation of the active agent powder and the flow of cold outer gas will consume a lot of arc heat.

According to the principle of minimum voltage, the arc will further shrink, and the temperature of the arc center will further increase.

The arc voltage of TIG arc and PPCA-TIG-MnCl2 is shown in Fig. 9.

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Fig.9 Arc voltage

On the basis of the aforementioned PPCA-TIG welding process parameters, the welding test was carried out with a welding speed of 100 mm/min, and the weld surface formation and weld penetration of traditional AC TIG welding and AC PPCA-TIG welding using MnCl2 active agent powder were compared.

As shown in Fig. 10, the penetration of AC PPCA-TIG-MnCl2 welding reached 2.4 times of that of traditional AC TIG welding, and good weld surface formation could be guaranteed at the same time.

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Fig.10 Weld surface forming and penetration

3. Conclusion

(1) For AC TIG arc, the arc light in EN is stronger than that in EP, the arc voltage in EN is lower than that in EP, and the arc temperature in EN is lower than that in EP.

(2) When MnCl2 is used as the active agent powder in AC PPCA-TIG welding, the active agent component melts and evaporates into the process of arc dissociation and ionization.

Compared with traditional TIG welding, the arc center temperature and arc voltage in EN section and EP section are increased.

(3) When MnCl2 is used as the active agent powder in AC PPCA-TIG welding, the penetration is significantly increased compared with traditional AC TIG welding, and good weld surface formation can be ensured at the same time.

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