Variable Polarity Gas Tungsten Arc Research Articles

Welding stability evaluation of 2219 aluminum alloy by double-pulse variable polarity GTAW based on empirical mode decomposition and Hilbert-Huang transform

According to the current real-time quality inspection requirements for welding processes, the electrical signals in double-pulse variable polarity gas tungsten arc welding (VP-GTAW) welding process of 2219 aluminum alloy were collected. The characteristics of steady-state and non-steady-state signals of the welding process were studied. The Hilbert-Huang transform based on empirical mode decomposition was applied to analyze the stability of double-pulse VP-GTAW. Fast Fourier Transform (FFT) spectrum and Hilbert spectrum of the intrinsic mode functions were obtained, and the stability of the welding process was studied. The relationship between the welding signal of the welding process and the weld formation was established, as the welding process becomes unstable, the time-frequency entropy of the welding process increases.

Comparative study of solidification behaviors of weld pool through modelling of heat transfer and fluid flow during single- and multiplelayer deposits of 2319-aluminum alloy based on variable polarity gas tungsten arc welding

Comparative study of solidification behaviors of weld pool through modelling of heat transfer and fluid flow during single- and multiplelayer deposits of 2319-aluminum alloy based on variable polarity gas tungsten arc welding

Microstructure and Mechanical Properties of Wire\u2009+\u2009Arc Additively Manufactured 2050 Al\u2013Li Alloy Wall Deposits

Aluminum–Lithium (Al–Li) alloy is a topic of great interest owing to its high strength and light weight, but there are only a few applications of Al–Li alloy in wire + arc additive manufacturing (WAAM) process. To identify its feasibility in WAAM process, a special AA2050 Al–Li alloy wire was produced and employed in the production of straight-walled components, using a WAAM system based on variable polarity gas tungsten arc welding (VP-GTAW) process. The influence of post-deposited heat treatment on the microstructure and property of the deposit was investigated using optical micrographs (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD), hardness and tensile properties tests. Results revealed that the microstructures of AA2050 aluminum deposits varied with their location layers. The upper layers consisted of fine equiaxed grains, while the bottom layer exhibited a coarse columnar structure. Mechanical properties witnessed a significant improvement after post-deposited heat treatment, with the average micro-hardness reaching 141HV and the ultimate tensile strength exceeding 400 MPa. Fracture morphology exhibited a typical ductile fracture.

Effects of Welding Speed and Pulse Frequency on Surface Depression in Variable Polarity Gas Tungsten Arc Welding of Aluminum Alloy

A three-dimensional (3D) numerical model with the volume of fluid method is developed for high-speed variable polarity gas tungsten arc welding (VP-GTAW) of aluminum alloys. It predicts the thermal flow field in the weld pool, the weld pool surface deformation, and solidified bead geometry during VP-GTAW in successive welding passes. Verification of the numerical model was performed by comparing the calculated results with metallography of welded cross-sections. The prediction showed reasonable accuracy in predicting weld bead geometry. The prediction average relative errors of the bead width and depth of penetration are less than 7%. The deformed weld pool surface, the fluid flow in the weld pool, and maximum fluid temperature in the workpiece based on the developed model, are discussed in detail. The effects of welding speed and pulse frequency on surface depression are studied. The results show that the maximum fluid temperature is closely correlated to the welding speed and pulse frequency. Further, the upper and lower limit of maximum fluid temperature would provide a clue by which the surface depression and the pitch of humps may be recognized. An increase in welding speed will lead to the increase of the pitch of humps, but the reverse is true in the pulse frequency. These detailed physical insights facilitate the prediction of welding surface defects in the high-speed VP-GTAW of aluminum alloy.

Keyhole welding of AA2219 aluminum alloy with double-pulsed variable polarity gas tungsten arc welding

A novel ultrasonic frequency double-pulsed variable polarity gas tungsten arc welding (DP-VPTIG) process was developed in the keyhole welding of 7 mm AA2219 aluminum alloy. The stable keyhole welding process was successfully achieved through analyzing the effects of characteristic current parameters on the weld formation. In low-frequency pulse peak stage, the fully penetrated keyhole formed owing to the high penetrating arc. In low-frequency pulse base stage, the keyhole closed as the arc force sharply declined. Fully equiaxed weld zone grain structures were obtained due to the stirring effect of the double pulses. The distribution of microhardness in the welded joint was consistent with that of the microstructure. It is expected to effectively improve the welding efficiency in aluminum alloy fabrication using DP-VPTIG process.

Numerical and experimental investigations of variable polarity gas tungsten arc welding

Variable polarity gas tungsten arc welding (VP-GTAW) has been suggested as a feasible method for welding aluminum alloys due to the enhanced surface cleaning of the oxide film. However, the principles governing the correlations between the electrode positive (EP) ratio, weld geometry, and weld microstructure are not well understood. Therefore, experiments with different EP ratios but same welding current were conducted. A three-dimensional heat transfer and fluid flow model of VP-GTAW was developed with validation against experimental results. Solidification parameters such as the temperature gradient, solidification growth rate, and cooling rate were computed from the model. The results indicate that VP-GTAW produces nearly defect-free joints. Both the top weld width and bottom weld width decrease with increasing EP ratio due to the decreased heat input. The bottom weld width is greater than the minimum weld profile width in the middle of the plate, which indicates that the Marangoni stress has a significant effect on the convective heat transfer and weld geometry. Variable grain dimensions are produced by different EP ratios at the same welding current. Weld pool oscillation originated from the alternating arc force plays a dominant role in the grain dimension.

Arc characteristics in double-pulsed VP-GTAW for aluminum alloy

This paper studied the variation in arc profile and arc macroscopic electromagnetic force with double-pulsed variable polarity gas tungsten arc welding (DPVP-GTAW) for aluminum alloy, using a synchronous acquisition and analysis system. The arc shapes were regionalized into arc edge region and arc core region by image processing method based on the gray level of the arc images. The arc characteristic size such as arc electrode end diameter DE, arc workspace end diameter DB and arc length L were measured and analyzed in the arc core region. In variable polarity pulse phase, DE and DB decreased, while L increased when welding current switched from negative polarity to positive polarity. DE, DB and L were all much larger in low-frequency pulse on tp compared with those in low-frequency pulse off tb. The size of arc profile changed periodically between tp and tb throughout the welding process. The arc macroscopic electromagnetic force Fz and arc pressure Pz oscillated in cycles with the variation of arc profile, inducing the refinement of weld zone grain structure.

Optimization of Hybrid Ultrasonic Frequency Pulsed VP-GTAW Process Parameters on Tensile Properties of AA 5456 Alloy

Tensile properties optimization of AA 5456 aluminum alloy was carried out with hybrid ultrasonic frequency pulsed variable polarity gas tungsten arc wending (HPVP-GTAW). An orthogonal method was employed to conduct the experiments, and the tensile properties of AA 5456 aluminum alloy welded joints were measured and analyzed. Regression models were developed based on the least square estimation by taking tensile strength, yield strength, percent elongation, and ratio of reduction in area as response functions of variable polarity frequency fL, pulse frequency fH and a dimensionless parameter ψ, which were calculated by background current Ib, peak current Ip, and pulse duration δ, respectively. The developed regression equations were checked for validity by coefficient of correlation r2 and confirmatory experiments. Optimum parameters of HPVP-GTAW were achieved through the discussion on response surfaces and contour plots drawn using the regression equations.

Simulation and analysis of heat transfer and fluid flow characteristics of variable polarity GTAW process based on a tungsten–arc-specimen coupled model

A tungsten–arc-specimen coupled model was set up for variable polarity gas tungsten arc welding process of aluminum. The numerical simulated arc voltage is consistent with experimental results, and the calculated arc temperature is in agreement with the results of the study conducted by Tashiro et al. under the same experimental conditions. The research found that: the highest arc temperature remains almost unchanged but the arc shape is more dispersed in direct current electrode positive (DCEP) phase than that in direct current electrode negative (DCEN) phase. The highest tungsten temperature in DCEP phase is much higher than that in DCEN phase and occurs near the tungsten tip. The Marangoni force played an important role under the welding current of AC150A, whereas the Marangoni force and electromagnetic force both contribute to the flow in the molten pool under the welding current of AC250A. This indicates that electromagnetic force plays a strong role in the flow in the molten pool under a large welding current. The heat transferred to the specimen is mainly composed of thermal conduction and electron heating in DCEN phase, whereas thermal conduction contributes to most of the heat transfer in DCEP phase. The calculated heat transfer efficiency in DCEN phase is much higher than that in DCEP phase. This is mainly because of the lack of electron heating in DCEP phase.

Galvanic Weld Metal-Base Metal Corrosion in AZ31 Magnesium Weldments

Arc welds were made on wrought AZ31 magnesium plate using a variable polarity gas-tungsten arc process with AZ61 filler wire. The aluminum content of the weld metal was systematically varied using inserts pre-placed in the weld joint, machined from magnesium die-cast alloys AM20, AM50, AM60, and AZ91. When immersed in 3.5% NaCl + Mg(OH)2 solution at room temperature, pitting corrosion was found to initiate in the weld heat affected zone, spread briefly to the weld metal, and then concentrate in the base metal. Galvanic couples were examined between wrought AZ31 and die-cast alloys to further examine galvanic behavior between wrought and cast materials. Results showed that the cast material was anodic relative to the wrought material over long periods of time, unlike what was observed for weldments. Details of this study will be presented and results will be related to alloy aluminum content and the ability to passivate.

Real-time observation of cathodic cleaning during variable-polarity gas tungsten arc welding of aluminium alloy

Online observation is expected to provide a better understanding of the cathodic cleaning of oxides from the molten pool during variable-polarity gas tungsten arc welding (VP GTAW) of aluminium alloys. In this paper, a machine-vision system with appropriate illumination and filtering is used to monitor in real time the effect of different process parameters on the cleaning of oxides from the molten pool during VP GTAW of Al 6061. Based on the observations, the process conditions under which a clean molten pool can be achieved are determined. In addition, the control of the welding process to maintain the consistency of cathodic cleaning is discussed. The results showed that in order to have an oxide-free molten pool, the solid surface in front of the molten pool should be cleaned from oxides by the electric arc. The choice of process parameters to satisfy this condition has been discussed. It was found that the percentage of direct current electrode positive (DCEP) polarity in the cycle of current has the highest impact on the cathodic cleaning, with the arc current having less influence, and the welding speed showing the least effect. Furthermore, in order to keep the consistency of oxide cleaning, process parameters should be set or controlled to maintain the cleaned zone larger than the molten pool.

Rapid prototyping of 4043 Al-alloy parts by VP-GTAW

This paper presents an investigation on the microstructure and property of 4043 Al-alloy parts built using a novel layer-deposition technique based on a variable polarity gas tungsten arc welding (VP-GTAW) process. Processing parameters strongly affect the geometry, microstructures, roughness, and hardness of a deposited part. A hollow cylindrical part with 120 layers is built up with a good surface quality. The deposited layers have a fine dendrite structure at the top layer, and coarse columnar/cellular grain structures in the middle and at the bottom of a deposited wall. The deposited wall has some fine equiaxed grains at the surface layers of the sidewalls. The precipitates are primarily distributed at the interdendrite regions and grain boundaries. The bonding zone exhibits an epitaxial and columnar grain growth from the original grains of a substrate. Precipitates are mainly located at the interior of columnar grains in the bonding zone. The deposited wall exhibits slight increasing trend in hardness from the bottom and middle layers to the top layer. The surface roughness resulting from the fine interlayer striations is found to be in the order of 5 μm, while the verticality of sidewalls mainly depending on the surface ripples is less than 5%.

Rapid prototyping of 5356-aluminum alloy based on variable polarity gas tungsten arc welding: process control and microstructure

This paper concentrates on rapid prototyping of a 5356-aluminum alloy based on a new deposition process of variable polarity gas tungsten arc welding (VPGTAW), and describes the microstructure and geometrical properties of the deposited layers. The wettability and distortion tendency of the deposited layers is effectively improved by preheating the substrate up to 118°C, monitoring the arc-length, and adjusting the arc current during the deposition process. The relationships between the geometry of the deposited layers and the welding parameters are developed. The surface roughness of the deposited parts is found to be in the order of 2 μm. The deposited layers exhibit equiaxed dendrites at the top layer, fine equiaxed grains at the middle, and bottom of a deposited wall together with some precipitates distributed at the grain boundary regions, and coarse columnar grains at the bonding zone between the deposited wall and the substrate. The residual microstructure such as grain size and distribution of precipitates is highly dependent on the related locations in the deposited wall. The deposited samples possess a maximum hardness at the top layer and exhibit a slight decreasing trend towards the middle and bottom of the fabricated part due to the heat effects of the material that occurs during the deposition. By understanding these relationships between parameters and their effect on the process output, the process can be used more effectively and the quality be improved as well.

Rapid prototyping based on variable polarity gas tungsten arc welding for a 5356 aluminium alloy

This paper presents a new deposition process for directly building cylindrical parts of the 5356 aluminium alloy with variable polarity gas tungsten arc welding (VPGTAW). The relationship between the geometric sizes of the deposited layer and the welding parameters is investigated. A machine vision sensor is used to monitor and control the arc length that is a key welding parameter in the achievement of a uniform deposition. By optimizing the depositing speed and the thickness of the depositing layer, there is no need for a cooling system to cool the part. Three-dimensional parts with different wall thicknesses and different shapes are successfully obtained. The surfaces of the deposited parts are smooth and uniform.