Anti warping voltage detection of keyhole behavior

Anti warping voltage detection of keyhole behavior in plasma arc welding

Abstract: in the process of perforated plasma arc welding, keyhole stability is very critical to the stability of the welding process and high-quality weld formation. In this paper, the passive probe is used to conduct environmental experiments on the plasma anti warping (plasma cloud) voltage composed of plasma arc gradient voltage and plasma sheath voltage. The stress level (such as thermal stress, vibration stress, electrical stress, etc.) applied to the tested product by the equipment requires the same test specification. According to the plasma sheath theory, the anti warping voltage varying with the plasma anti warping length and angle can reflect the state information of the small hole and its formation and collapse process. The characteristics of the detection signal and the influence of material, probe position, welding current, plasma flow and other factors on the detection signal are discussed. Experiments show that this method can effectively predict the hole information and weld penetration

key words: plasma arc welding; Small hole; Anti warping voltage; Detection

0 preface

perforated plasma arc welding has the characteristics of high plasma arc energy density and large plasma flow force, and can realize single-sided welding and double-sided forming of medium thickness materials. However, the instability of welding holes directly affects the stability of plasma arc welding process and joint quality of small holes, which is the bottleneck of the application of this welding method [1]. Therefore, the research on the detection and real-time control of keyhole behavior is of great significance. At present, the detection of small hole behavior at home and abroad mainly adopts the tail flame conductivity method, the tail flame photoelectric method, the front arc detection method [2], the front image detection method [3], the sound sensing method [4], etc., but these

detection methods are difficult to be applied to actual production because of the harsh application conditions or the high cost

this paper proposes and studies the extraction of penetration perforation information from plasma arc back warping voltage signal, and designs a set of passive probe detection system. The system directly uses arc warping for sensing without adding complex instruments and equipment. It has the characteristics of accessibility, good reliability, practicality and low cost

1 anti warping voltage detection principle and method

perforated plasma arc welding is used for welding. During the formation of welding holes, a small arc from the molten pool will appear above the workpiece (opposite to the welding direction), which is called plasma arc anti warping (also known as plasma cloud [5]). With the increase of welding penetration, the size and inclination of anti warping will increase correspondingly, and the anti warping reaches the maximum at the moment when the small hole is about to form. In the process of forming and stable existence of small holes, small changes in welding conditions and the existence of surrounding interference will change the size of welding holes

at this time, the plasma warping also changes correspondingly with the size of the small hole. When the hole diameter increases, the arc tail flame increases and the anti warping decreases; On the contrary, the plasma warpage increases (as shown in Figure 1)

when the welding current is too large, the small hole collapses and the workpiece burns through, the plasma warping disappears. Therefore, through the detection of the existence and shape of plasma warping, the state of welding holes can be judged

use the passive probe detection system (as shown in Figure 2) to detect the state of anti warping. The principle is: due to the conductivity of the plasma, a sheath with negative potential will be formed at the contact between the probe and the surface of the plasma warping [6]. According to the sheath theory, sheath potential will be generated on the surface of the probe. After derivation, the sheath potential VW value is

, where: t is the plasma temperature; Ve is the average velocity of electrons; VI is the average velocity of ions; Me is the electronic quality; Mi is the ionic mass; K is Boltzmann constant. It can be seen that VW is only proportional to the plasma temperature T, related to the ratio of ion to electron mass, and independent of the shape of the probe and the length of the probe into the plasma. In addition, the detection voltage also includes the partial voltage caused by the potential gradient between the probe and the workpiece, and the value of this voltage is related to the potential gradient between the probe and the workpiece. With the change of the anti warping shape, the probe has made great progress and breakthrough from China's plastic extruder market, and the temperature of the daughter will also change periodically, resulting in the change of the detection voltage, so the change of the voltage can reflect the formation state of the small hole

2 test system and conditions

2.1 test system

the test system is mainly composed of the following parts: plasma arc welding (PAW) system, passive probe detection system, computer data acquisition system and FastCAM super 10k/10kc high-speed camera. Paw system is composed of plasma arc welding power supply, timing controller, welding gun, walking device, etc. The passive probe detection system is composed of probe, filter circuit and position scanning device. Computer data acquisition system is used for signal acquisition and processing

2.2 test conditions

the welding workpiece material is 1gr18ni9ti stainless steel, and the plate thickness is 5mm. Welding parameters: tungsten electrode shrink 3.0 mm, welding current 190 a, welding speed 5.6mm/s, ion gas flow 4.0L/min, protective gas flow 20 l/min, ion gas and protective gas are argon

3 test results and analysis

3.1 the anti warping voltage signal collected under the test conditions Figure 3 is the anti warping voltage signal of the stable hole process collected after the optimal probe position is determined by scanning (curve 1) and the synchronous verification voltage signal collected by the tail flame conduction method (curve 2). Through the comparison between the anti warping voltage signal and the tail flame voltage verification signal, it can be seen that the two waveforms maintain a good corresponding relationship: in the process of small hole growth, the plasma arc tail flame becomes larger and the anti warping becomes smaller; The corresponding tail flame voltage becomes larger and the anti warping voltage becomes smaller; In the process of small hole shrinkage, the tail flame becomes smaller and the anti warping becomes larger; Therefore, the tail flame voltage becomes smaller and the anti warping voltage becomes larger. The test results show that the anti warping voltage detection method can accurately collect the status information of welding holes from the front of the workpiece

Figure 4 shows the collected anti warping voltage signal when welding with varying welding current. It can be seen from the figure that before point a, the welding current is small, the workpiece is not penetrated, the welding hole is not formed, and the reverse warping voltage is small. Between a and B, the welding penetration increases with the increase of welding current, and finally a small hole is formed at point B; In this process, the size angle of plasma warping increases continuously, and reaches the maximum value at the moment of impending penetration (point B) (which can be used to judge whether a small hole is formed). b. C is the stage of stable existence of small holes. With the increase of welding current, the diameter of welding small holes increases as a whole. Therefore, the plasma anti warping gradually decreases, and the corresponding anti warping voltage signal also decreases. After point C, the workpiece burns through, the anti warping disappears, and the anti warping voltage signal is zero. At the moment before the workpiece is burned through, due to the change of the shape of the small hole, the anti warping angle vibrates periodically, forming a negative crest

3.2 main factors affecting the anti warping voltage signal

3.2.1 materials

the influence of stainless steel (1Cr18Ni9Ti) and ordinary low carbon steel (Q235) on the anti warping voltage is studied in this paper. It was found that under normal welding conditions, due to the large differences in the thermal conductivity, linear expansion coefficient and chemical composition of the two materials, there was a great difference in the shape of the molten pool, resulting in significant differences in the maximum angle of plasma warping and the size of plasma warping (as shown in figure 5), resulting in different detection voltage of plasma warping under the same conditions. When different materials are used for welding, the detection position of the probe must be readjusted

3.2.2 probe position

the temperature distribution of plasma arc back warping is uneven, and the temperature gradually decreases from the center to both ends. Therefore, when the probe moves from the center of the anti warping to the outside, the absolute value of the sheath voltage decreases with the decrease of temperature, so the detected anti warping voltage amplitude also decreases; When the probe is outside the plasma, although the temperature is very high, there is no plasma, so the detection voltage is zero in terms of energy saving. In addition, when the probe is too close to the plasma anti warping jet center, the probe will be burned due to the high anti warping temperature, so it cannot be detected. In short, it is very critical to determine the reasonable position of the probe in the warping

3.2.3 welding speed

under the condition that other welding process parameters remain unchanged, when the welding speed changes, the heat input changes accordingly, resulting in different shapes of welding pool. Plasma anti warping is ejected at the maximum tangent of the weld pool in the opposite direction of welding, so its maximum anti warping angle will change accordingly. With the increase of welding speed, the maximum anti warping angle decreases; The welding speed decreases and the maximum anti warping angle increases. Therefore, the temperature of the plasma sheath where the fixed position probe is located will change with the change of the anti warping morphology, which will affect the value of the anti warping voltage and the growth and development of crops

3.2.4 welding current

the change of welding current will also affect the value of detection voltage (as shown in Figure 4). Before the formation of the small hole, the welding current increases, the penetration depth of the molten pool increases, the anti warping angle increases, and the detection voltage increases, reaching the maximum value when the small hole is about to be formed; When the small hole exists stably, the welding current increases, the diameter of the small hole in the molten pool increases, the maximum anti warping angle and height decrease correspondingly, and the maximum value of the detection voltage becomes smaller, which may even cause the probe to separate from the plasma anti warping, and the voltage signal cannot be detected

4 conclusion

(1) plasma reverse warping voltage signal can well reflect the existence and diameter of small holes, which is very suitable for penetration control of perforation welding

(2) material, probe position, welding speed and welding current have a great influence on the plasma anti warping voltage value. Only selecting the appropriate detection position can we get the ideal detection voltage signal

(3) the passive probe system is used to directly measure the voltage of reverse warping in perforated plasma arc welding. The detection circuit has a good isolation effect from the main welding circuit, which improves the reliability and safety, and the circuit is simple and practical. (end)