Keyhole Plasma Research Articles

Relationship between the keyhole laser welding and the plasma

In this paper we investigated the behaviours of the laser beam keyhole welding with different laser beam sources, CO2 and a solid state Nd: YAG. We showed the differences between the types of laser welding, the geometry and method. Today the most used laser welding technology is the laser beam keyhole welding. In our research we investigated the heating effect of photons and the absorption method on the substrate surface. The photons can significantly absorbed better in case of 106 W/cm2 or more because of the plasma state in the welding material. The quality of the keyhole plasma depend on the free electrons of the materials and working medium. A theory of the creation of plasma phase and cloud were investigated. There is a difference working inside the metal vapour and working gas plasma cloud using CO2 and Nd: YAG. The solid state laser beam source shows better quality of the joint in keyhole welding.

Effect of Melt-In and Key-Hole Modes on the Structure and Mechanical Properties of AISI 430 Steel Welded Using Plasma Transfer Arc Welding

In this study, AISI 430 stainless steel couple 10 mm thick were welded in the butt position by melt-in and key-hole plasma transfer arc welding (PTAW) processes without using any filler wire addition and pretreatment. Welded joints were manufactured choosing a constant nozzle diameter (2.4 mm), welding speed (0.01 m/min), shielding gas flow rate (25 L/min), two different plasma gas flow rates (0.8 and 1.2 L/min), three different welding currents in melt-in PTAW process (80, 85 and 90 A), and three current intensities of the key-hole PTAW process (120, 125, and 130 A). In order to determine the microstructural changes that occurred, the interface regions of the welded samples were examined by scanning electron microscopy (SEM), optic microscopy (OM), X-ray diffraction (XRD) after PTAW. Microhardness and V-notch impact tests were applied to determine the mechanical properties of the welded samples. In addition, fracture surface were examined by SEM after impact tests.

Stereo analysis on the keyhole and weld pool behaviors in K-PAW with triple CCD cameras

A synchronous visual sensing system was established to observe the topside weld pool, keyhole entrance and keyhole exit at the same time during keyhole plasma arc welding (K-PAW). Complete topside weld pool was captured by a common CCD camera perpendicularly to the welding direction. Meanwhile another two cameras were employed to monitor the keyhole entrance from the rear view and to capture the keyhole exit from the substrate backside, respectively. The evolution process of the weld pool and keyhole behaviors and shapes along with the heat input accumulation were observed. The mechanism of the interactions were discussed and analyzed. Image registration was conducted fusing the weld pool, keyhole entrance and keyhole exit in the same coordinate system. The geometrical dimensions were measured and could reflect the interior intense interactions. Comprehensive investigation of the complicated physical process provided some fundamental and profound understandings.

Numerical analysis of plasma arc physical characteristics under additional constraint of keyhole**Project supported by the National Natural Science Foundation of China (Grant No. 51505008) and the National Science and Technology Major Project of China (Grant No. 2014ZX04001-171).

The physical characteristics of a plasma arc affect the stability of the keyhole and weld pool directly during keyhole plasma arc welding (KPAW). There will be significant change for these characteristics because of the interaction between the keyhole weld pool and plasma arc after penetration. Therefore, in order to obtain the temperature field, flow field, and arc pressure of a plasma arc under the reaction of the keyhole, the physical model of a plasma arc with a pre-set keyhole was established. In addition, the tungsten and base metal were established into the calculated domain, which can reflect the effect of plasma arc to weld pool further. Based on magneto hydrodynamics and Maxwell equations, a two-dimensional steady state mathematical model was established. Considering the heat production of anode and cathode, the distribution of temperature field, flow field, welding current density, and plasma arc pressure were solved out by the finite difference method. From the calculated results, it is found that the plasma arc was compressed a second time by the keyhole. This additional constraint results in an obvious rise of the plasma arc pressure and flow velocity at the minimum diameter place of the keyhole, while the temperature field is impacted slightly. Finally, the observational and metallographic experiments are conducted, and the shapes of plasma arc and fusion line agree with the simulated results generally.

Behavior of Exit Keyhole Diameter during Switch off Period in Plasma Keyhole Arc Welding

The purpose of this investigation is to clarify the behavior of exit keyhole diameter during switch off (cutting arc) period in case of Plasma keyhole arc welding (PKAW). During switch off period, the keyhole and weld pool are visualized from backside via a high speed video camera (HSVC). The result showed that keyhole diameter on the backside is unstable. The keyhole size is much changed in X-axis. Firstly, it is increased and then it is decreased to a stationary value at t = 0.05 s after cutting arc. Meanwhile, the size in Y-axis is not changed approximately from t = 0.01 s after cutting arc. The results can be considered to control this process more efficiency.

Analysis-effective induction efficiency of laser in pulse laser-GTA welding of titanium alloy

This study aims to explain the effective induction efficiency of laser in the laser-gas tungsten arc (GTA) hybrid welding process by arc plasma characteristics and keyhole behaviors analysis during the process experiment, the real-time monitoring, and the theoretical analysis. The emission spectrum of the arc plasma was acquired by a spectrometer, and the dynamic keyhole behaviors were synchronously observed using a high-speed camera. The electron temperature and the ratio of resistivity of the arc plasma were calculated. Results indicate that defocusing distance greatly affects the effective induction efficiency of the laser in hybrid welding, and an appropriate defocusing distance can maximize the effective induction efficiency of the laser. The main reason is that the defocusing distance influences arc plasma characteristics and keyhole behaviors, and these determine the coupling discharge of laser-induced plasma and GTA plasma. Finally, by comparing the effective induction efficiency of the laser at different defocusing distances, it was found that when the laser focuses inside the keyhole, the induction efficiency is more effective.

Experimental investigation on the weld pool formation process in plasma keyhole arc welding

This paper seeks to clarify the weld pool formation process in plasma keyhole arc welding (PKAW). We adopted, for the first time, the measurement of the 3D convection inside the weld pool in PKAW by stereo synchronous imaging of tungsten tracer particles using two sets of x-ray transmission systems. The 2D convection on the weld pool surface was also measured using zirconia tracer particles. Through these measurements, the convection in a wide range of weld pools from the vicinity of the keyhole to the rear region was successfully visualized. In order to discuss the heat transport process in a weld pool, the 2D temperature distribution on the weld pool surface was also measured by two-color pyrometry. The results of the comprehensive experimental measurement indicate that the shear force due to plasma flow is found to be the dominant driving force in the weld pool formation process in PKAW. Thus, heat transport in a weld pool is considered to be governed by two large convective patterns near the keyhole: (1) eddy pairs on the surface (perpendicular to the torch axis), and (2) eddy pairs on the bulk of the weld pool (on the plane of the torch). They are formed with an equal velocity of approximately 0.35 m s−1 and are mainly driven by shear force. Furthermore, the flow velocity of the weld pool convection becomes considerably higher than that of other welding processes, such as TIG welding and GMA welding, due to larger plasma flow velocity.

An evolutionary keyhole-mode heat transfer model in continuous plasma arc welding

Keyhole plasma arc welding is superior to other ordinary arc welding technologies in deep penetration welding. Energy can be transferred to bottom metal by hot plasma arc in keyhole rather than merely by heat conduction through whole thickness. Hence, an evolutionary keyhole-mode heat transfer model was developed to demonstrate this thermo-physical process. In view of the relative movement of plasma torch and workpiece, a double-elliptical heat source was set up on top surface and a developing conical heat source below, which is closely related to keyhole depth by Volume of Fluid method. Based on analogy to practical welding process, a dynamic energy distribution coefficient was established and an adjustment factor was introduced to account for pressure variation. According to numerical simulation, evolutionary energy propagation process, corresponding temperature field and molten pool geometry were demonstrated simultaneously. The dynamical keyholing process and the consequent keyhole-mode heat transfer and fluid flow were revealed in continuous keyhole plasma arc welding. Experiment was conducted on an 8 mm-thick stainless steel plate, and the measured weld geometry, keyhole size and keyhole exit deviation show reasonable agreement with calculated results.

Application of Pulsed Laser-TIG Hybrid Heat Source in Root Welding of Thick Plate Titanium Alloys

Laser-TIG (tungsten inert gas) hybrid welding process is applied to produce stable back beads in the first layer during one-side multilayer welding. To explain why stable back beads can be formed, a comparison is conducted between single TIG and laser-TIG hybrid welding. Results indicate that during laser action the arc column is enhanced and deflected to the keyhole, which changes the spatial distribution of plasma by strong attraction. The keyhole plasma possesses extreme electric conductivity. After laser action, the enhanced arc is restituted, the low energy density arc acts on the molten pool, and the bottom of the molten pool is solidified to prevent its collapse. The heat and force are redistributed over the top of molten pool under the alternative action of the enhanced arc and original arc. In this case the thermal distribution and mechanical situation in the molten pool must be more balanced and reasonable, and the back bead appearance is continuous, stable and uniform. This process gives high quality and efficient root welding of the thick plate.

Measurement of the keyhole entrance and topside weld pool geometries in keyhole plasma arc welding with dual CCD cameras

Dual cameras were employed to observe the complete topside weld pool from the lateral side and to capture the keyhole entrance from the rear view in plasma arc welding. By image registration, the images were fused into a cohesive whole in the same physical coordinate system. Weld pool and keyhole behaviors were studied under different welding parameters. Possible interaction mechanisms between plasma arc, gas flow, liquid metal and keyhole were discussed. Instant and significant influences on the keyhole entrance shape and geometries originated in the changed forces applied on the surrounded liquid metal.

Visual sensing of the weld pool geometry from the topside view in keyhole plasma arc welding

Weld pool and keyhole behaviors are the key factors to determine the process stability and weld joint quality in keyhole plasma arc welding. In this study visual sensing of the topside weld pool based on a low-cost CCD camera was realized. A visual sensing system with special dimmer glass and bandpass filtering was developed for solving the problems like strong arc disturbances and large radiation intensity gradient. Boundaries of the complete topside weld pool were acquired through image processing of the captured raw images. Weld pool dimensions were then calculated and calibrated. The relationships between the dimensions and welding current, welding speed and plasma gas flow rate were investigated. Three stages, including blind keyhole, unsteady keyhole and steady keyhole, were defined to describe the weld pool behaviors along with the keyhole’s evolution. The variation trends of weld pool shapes were discussed to study the possible mechanism of the complex physical processes. For keyhole plasma arc welding, this provides the industry a prospective low-cost way to predict the penetrations by sensing weld pool and keyhole from topside.

Influence of Pilot Gas Composition on Convective Pattern of Weld Pool Surface in Plasma Keyhole Arc Welding

Influence of Pilot Gas Composition on Convective Pattern of Weld Pool Surface in Plasma Keyhole Arc Welding

Correlation of keyhole exit deviation distance and weld pool thermo-state in plasma arc welding process

Keyhole behavior is believed to determine the process stability in keyhole plasma arc welding, and thermal state of weld pool is one of the most critical factors to affect keyhole stability. To evaluate the correlation of the keyhole behavior to the thermal state of weld pool, vision observation system is employed to image the keyhole exit. Keyhole exit deviation distance is proposed to characterize the thermal state of weld pool. In the normal fully penetrated keyhole welding process, backside keyhole exit deviates a distance away from the torch axis. When the thermal energy in the weld pool and keyhole is increased by increasing the welding current, decreasing the welding speed or slowing the heat dissipation speed, the keyhole exit deviation distance decreases; when the weld pool thermal energy is decreased, the deviation distance increases. Dynamic performance of the new proposed thermal state factor is sound. Correlation mechanism between the keyhole exit deviation distance and the thermal state is established in view of the dependent relationship of the keyhole leading wall (leading melting side) to the keyhole exit deviation distance. The observation results lay solid foundation for deeply understanding of the keyhole behavior and further control of the keyhole process in plasma arc welding process.

Numerical analysis of the heat transfer and material flow during keyhole plasma arc welding using a fully coupled tungsten–plasma–anode model

A coupled tungsten–plasma–anode mathematical model is first proposed in this paper to analyze the heat transfer and fluid flow in stationary keyhole plasma arc welding (PAW). The calculated plasma arc pressure is validated by a water-cooled copper based arc pressure acquisition experiment. The plasma arc temperature calculated by spectrum experiment in radial direction is used to verify the arc temperature obtained by numerical simulation. The numerical results show that: the evolution of temperature and fluid flow behavior as well as the keyhole formation in the PAW process offers an invaluable insight into the understanding of the physical essence, revealing how the keyhole promotes the deep penetration welding. Besides, the driving forces are quantitatively analyzed to explain the material flow phenomenon in the molten pool. Moreover, the energy input, transformation, transfer, and dissipation due to different physical processes in the PAW process at 0.4 s after arc ignition are calculated and analyzed to understand the energy structure and flow in the system. The findings from the study provide guidance for engineers in designing plasma arc welding schedules to achieve quality welds.

A prediction model for keyhole geometry and acoustic signatures during variable polarity plasma arc welding based on extreme learning machine

Purpose The purpose of this paper is to investigate the relationship between the keyhole geometry and acoustic signatures from the backside of a workpiece. It lays a solid foundation for monitoring the penetration state in variable polarity keyhole plasma arc welding. Design/methodology/approach The experiment system is conducted on 6-mm-thick aluminum alloy plates based on a dual-sensor system including a sound sensor and a charge coupled device (CCD) camera. The first step is to extract the keyhole boundary from the acquired keyhole images based on median filtering and edge extraction. The second step is to process the acquired acoustic signal to obtain some typical time domain features. Finally, a prediction model based on the extreme learning machine (ELM) technique is built to recognize different keyhole geometries through the acoustic signatures and then identify the welding penetration status according to the recognition results. Findings The keyhole geometry and acoustic features after processing can be closely related to dynamic change information of keyhole. These acoustic features can predict the keyhole geometry accurately based on the ELM model. Meanwhile, the predict results also can identify different welding penetration status. Originality/value This paper tries to make a foundation work to achieve the monitoring of keyhole condition and penetration status through image and acoustic signals. A useful model, ELM, is built based on these features for predicting the keyhole geometry. Compared with back-propagating neural network and support vector machine, this proposed model is faster and has better generalization performance in the case studied in this paper.

Study on the burning loss of magnesium in fiber laser welding of an Al-Mg alloy by optical emission spectroscopy

In order to study the burning loss of magnesium, a specially designed device was used to capture the spectrum emitted from the keyhole plasma in deep penetration laser welding of aluminum alloy 5052. The content of magnesium in the weld bead was measured by an electron probe micro-analyzer (EPMA). The effect of the welding parameters on the spectral intensity of magnesium plasma was examined, and the distributions of the spectral intensity of magnesium plasma in the radial and depth directions of the keyhole were investigated. Finally, the mechanism of the magnesium loss was analyzed. The results indicate that burning loss of magnesium can be monitored by spectral analysis technique. The welding parameters have a great effect on the burning loss of magnesium. The influence of welding speed on the burning loss of magnesium on the top surface is dependent on the penetration regime. The distribution of magnesium in the weld is not uniform. In the radial direction of the weld bead, the content of magnesium increases from the center to the edge of the keyhole. In the depth direction of the weld bead, the content of magnesium decreases firstly and then increases from the top to bottom. The maximum burning loss of magnesium occurs at the middle of the weld bead.

Comparative study on interactions between laser and arc plasma during laser-GTA welding and laser-GMA welding

This paper describes an investigation on differences in interactions between laser and arc plasma during laser-gas tungsten arc (LT) welding and laser-gas metal arc (LM) welding. The characteristics of LT heat source and LM heat source, such as plasma behavior, heat penetration ability and spectral information were comparably studied. Based on the plasma discharge theory, the interactions during plasma discharge were modeled and analyzed. Results show that in both LT and LM welding, coupling discharge between the laser keyhole plasma and arc happens, which strongly enhance the arc. But, the enhancing effect in LT welding is much more sensitive than that in LM welding when parameters are adjusted.

NUMERICAL SIMULATION OF DYNAMIC BEHAVIOR OF KEYHOLE AND MOLTEN POOL AT K-PAW QUASI STEADY PROCESS

The keyhole plasma arc welding (K-PAW) is widely applied in engineering project now as a high energy beam welding with its advantages of low-cost and easy operation. However, the arc instability may arise and welding defects will be produced in K-PAW due to the high current and strong plasma penetrating force when medium thickness plates are welded, finally weakening the efficiency of K-PAW. Furthermore, it is found that the flow field of liquid metal in the molten pool and the stability of keyhole have a critical influence on welding quality. Therefore, modeling and simulating molten pool, keyhole and flow field in the K-PAW quasi steady process except for arc starting and ending phases are helpful to understand the welding process theory completely and promote its application further. But to date, there is little study on the coupled analysis of molten pool and keyhole in the quasi steady welding process due to the difficulty to make keyhole stable. In this work, based on the principles of fluid dynamics with considering arc pressure, surface tension, electromagnetic force, buoyancy and gravity, a *国家自然科学基金资助项目51205176 收到初稿日期: 2015-10-07,收到修改稿日期: 2015-12-23 作者简介:徐斌,男, 1988年生,博士生 DOI: 10.11900/0412.1961.2015.00518 第804-810页 pp.804-810

Studies on microstructure and mechanical properties of keyhole mode Nd:YAG laser welded Inconel 625 and duplex stainless steel, SAF 2205

Abstract

Dynamic variation of keyhole exit and its inclination in plasma arc welding

The dynamic behavior of keyhole in plasma arc welding determines the weld quality. In this study, the keyhole images are observed during the process, and both the keyhole geometry and its position relative to the torch axis are determined. The deviation distance of the keyhole exit at bottom side of the workpiece from the torch centerline is used to describe the extent of keyhole inclination and is measured based on the captured sequential images of the keyhole exit. Experiments are conducted to study the dynamic variation of keyhole exit and its inclination extent. The interaction between the plasma arc and the molten pool and its influence on the keyhole inclination are analyzed. The results lay solid foundation for monitoring and controlling the thermal process in keyhole plasma arc welding.