Variable Polarity Plasma Arc Welding Research Articles

Effect of Salt Spray Corrosion Performance on Variable Polarity Plasma Arc Welding of Aerospace Aluminum Alloy

Effect of Salt Spray Corrosion Performance on Variable Polarity Plasma Arc Welding of Aerospace Aluminum Alloy

A novel optimizing Pulsed Plasma Gas Variable Polarity Plasma Arc Welding (PPG-VPPAW) method for improving weld quality

To enhance the reliability of aluminum alloy welding quality, a novel Pulsed Plasma Gas Variable Polarity Plasma Arc Welding (PPG-VPPAW) system is proposed. This system enables independent control and rapid adjustment of the plasma gas, allowing for flexible modulation of arc pressure output under constant thermal conditions, thereby reducing porosity and refining grain structure. The research analyzed the impact of parameters of different plasma gas on arc characteristics, welding porosity, and welding microstructure. The results reveal that the periodic changes in pulsed plasma gas induce corresponding variations in arc temperature, arc shape, arc pressure, arc voltage, and the morphology of the weld pool keyhole. The magnitude of the difference between the base value and peak value of pulsed plasma gas determines the extent of arc pressure fluctuations and the behavior of the backside keyhole. The maximum oscillation amplitude of the backside keyhole can reach up to 4.5 mm². Additionally, the oscillation of pulsed plasma gas within the molten pool has been demonstrated to effectively reduce weld porosity and refine grain size. Under this process, the grain size of the weld metal can be reduced by up to approximately 25%. Current work provides an important direction for fabricating highly reliable aluminum alloy weldments.

Formation and strengthening mechanism of equiaxial cellular grain zone in AA2195-T8 Al–Cu–Li alloy twin-wire P-VPPA welded joint with Ti–Zr microalloying

A novel method involving twin-wire pulsed VPPA (P-VPPA) welding was combined with Ti–Zr microalloying to improve the mechanical properties of AA2195-T8 Al–Cu–Li alloy variable polarity plasma arc (VPPA) welded joints. This method aimed to increase the width of the equiaxial cellular grain zone (ECZ) by adding higher content of Ti–Zr and reducing heat input. The microstructure evolution process was established to understand the formation and strengthening mechanism of the ECZ. The results showed that the strength of the ECZ was higher than that of the dendrite in the weld zone (WZ). The twin-wire VPPA welding led to the formation of a wider ECZ near the fusion zone (FZ). This ECZ was continuously distributed throughout the thickness direction of the welded joint. Combined with higher Ti–Zr microalloying, the maximum width of ECZ increased to 2.5 mm and extended to the WZ. Consequently, the proportion of cracks passing through the ECZ during tensile fracture increased from 27.6% to 88.7%. Moreover, the ultimate tensile strength (UTS) of the welded joint exhibited a notable improvement, rising from 328 MPa to 370 MPa, while the elongation also increased to 5%. The UTS of the welded joint achieved 66% of the base metal. Additionally, the average grain size in the ECZ was 14 μm. The formation of the ECZ was primarily attributed to the nucleation precursors of Al3(Ti, Zr) and β′ phase, as well as less welding heat input near the FZ. The widening of the ECZ was mainly due to the presence of more Al3(Ti, Zr) phases. Moreover, the grain boundary of the ECZ exhibited the distribution of Al–Cu eutectic, θ(Al2Cu), Ω, T2(Al6CuLi3), and ω(Al7Cu2Fe) phases, while a small amount of T1(Al2CuLi) phase precipitated within the grain. The strengthening mechanism of ECZ is the result of multiple strengthening effects, including the reduction of element segregation and microcracks, the precipitation of the T1 phase, and the fine grain strengthening.

Stability mechanism of the molten pool in variable polarity plasma arc welding of medium thickness aluminum alloy

Stability mechanism of the molten pool in variable polarity plasma arc welding of medium thickness aluminum alloy

Microstructure evolution and mechanical properties of AA2195-T8 Al-Li alloy back gas-shielded VPPA weld

Variable polarity plasma arc welding (VPPA) is an adaptable and efficient method for welding large Al-Li alloy components like rocket fuel tanks. This work investigated the microstructure and properties of AA2195-T8 Al-Li alloy weld using the back gas-shielded VPPA method. The process of microstructure evolution is established to reveal the softening mechanism of the weld. The results show that the mechanical properties of the weld are significantly reduced without back gas shielding, the back of the weld oxidizes to form Li2O with MgO, resulting in back hollows and pores. The addition of back gas shielding during welding solves the above problem. With the addition of back gas shielding, the increase of the strength and elongation for the weld with reinforcement is 33.6 % and 96.7 %, and for the weld without reinforcement is 6.9 % and 28.9 %. There is no significant effect on the microstructure of the weld. The second phases in the weld are mainly Al-Cu and Al-Cu-Mg-Ag eutectic, with a small amount of Al7Cu2Fe and separated eutectic θ phase. The matrix phase in the weld is mainly the composite phase composed of θ and Ω phases. The dissolution of the nanoscale precipitated phase, severe grain boundary segregation, burnout of alloying components, and microscopic defects are the main causes of the reduction in weld properties. In summary, this work provides practical suggestions for optimizing the VPPA welding process to improve the mechanical properties of Al-Li alloy welded joints.

Mechanisms of Gravitational Influence on Weld Pool Behavior and Weld Bead Performance in Variable Polarity Plasma Arc Welding across Different Welding Position.

This article comprehensively explores the cross-scale effects of gravity on macroscopic flow formation and weld bead formation in variable polarity plasma arc welding. Gravity-induced changes in welding direction were achieved through welding at different spatial positions. The properties of the weld bead were investigated at various spatial locations. Additionally, an elemental tracing technique was employed to study the internal flow behavior of molten metal. In the flat welding position, there is an observable trend of increasing grain size in the welded bead, accompanied by a significant expansion of the coarse grain zone. Consequently, the properties of the weld bead in the flat position are inferior to those achieved in the vertical welding position. This phenomenon can be attributed to the accumulation of molten metal at the exit side of the keyhole, resulting in temperature accumulation. Research indicates that the internal flow within the weld pool plays a critical role in causing this phenomenon. The study's findings reveal the presence of two distinct vortex flow patterns within the weld pool: one aligned with the welding direction and the other directed towards the interior of the weld pool. Particularly noteworthy is the substantial expansion of the flow channel area in the flat welding position, which significantly amplifies the impact of internal flow. This enhanced flow intensity inevitably leads to the increased buildup of molten metal at the keyhole exit side. These studies lay the groundwork for achieving high-quality and controllable spatial-position welding.

Analysis of molten metal fluid flow mechanisms in variable polarity plasma arc welding of aluminum alloys

The stability and quality of welding in variable polarity plasma arc welding are determined by the directional fluid flow within the molten pool. In this study, the particle tracking method was employed to obtain flow traces and velocity of the molten metal on the walls of the keyhole using a high-speed camera system. Additionally, a confocal laser scanning microscope was utilized to capture the 3D morphology of the keyhole, while the flow channels were obtained through sectioning. By correlating the flow traces and velocity with the 3D keyhole morphology, the experimental results revealed the presence of saddle points on both the front and rear walls of the keyhole. The front saddle point represents the region where the flow velocity in the depth direction is minimum, causing the keyhole front side to converge upward and downward. The depth and volume of fluid flow to the keyhole sidewall are determined by this front saddle point. On the other hand, the rear saddle point is located at the center of the molten pool, marking the entry and exit sides of the keyhole rear wall where the melting metal eventually reaches. The location of the post-settlement point will affect the weld state. During the welding process, the double saddle points of the molten pool remain dynamically stable, and their position is influenced by shear force, gravity, arc pressure, keyhole morphology, and flow channels. These findings contribute to controlling the process characteristics necessary for stable welding of the keyhole in the molten pool.

Cross-scale process quality control of variable polarity plasma arc welding based on predefined temperature field

This study investigates the underlying causes of Variable Polarity Plasma Arc Welding (VPPAW) spatial positional instability and its adverse impact on weld seam quality. Microstructural analysis reveals that the primary cause of poor transverse mechanical properties is the asymmetric distribution of grains. Through the study of heat and mass transfer in the molten pool, it was found that the asymmetric flow of molten metal under the influence of gravity is the main factor leading to uneven temperature distribution and asymmetric grain distribution in the weld pool. A novel approach based on a predefined temperature field is proposed to regulate the weld pool's stability and welding quality. The implementation of the predefined temperature field effectively improves VPPAW weld pool flow, leading to enhanced temperature distribution and grain size and distribution. These findings provide a theoretical foundation and valuable engineering recommendations for achieving high-quality spatial position welding in VPPAW.

The characteristics of dynamic stability in aluminium alloy keyhole weld pool flow

The keyhole welding process in Variable Polarity Plasma Arc Welding enhances weld quality significantly, yet it introduces pool instability. Dynamic stability is pursued by investigating molten metal flowing within keyholes and their characteristics. Weld pool flow patterns are obtained via particle tracking, while instability characteristics are captured employing a high-speed camera. Analysing directional molten metal flow unveils Convergence and Separation Points in weld pool flow. The latter’s displacement induces weld pool instability, resulting in an increase in keyhole area and a reduction in the pinch angle. The keyhole area signifies pool instability, while the rear wall angle reflects the welding state. These findings provide a theoretical basis for weld control and advance vision-based intelligent manufacturing welding techniques.

Influence of the metal flow in the keyhole molten pool on the molten pool stability in continuous variable polarity plasma arc keyhole vertical-up welding

The metal flow of the keyhole molten pool affects weld pool stability and weld formation vitally during continuous variable polarity plasma arc welding (VPPAW). A novel “keyhole inside alternating oscillation” (KIAO) thermal-mechanical coupling model was developed to study the influence of keyhole molten pool metal flow on molten pool stability. This model can achieve the oscillation effect of heat-force on the molten pool during the electrode negative (EN) and electrode positive (EP) phase. A heat source, with keyhole channel backward bending, was used to create a keyhole backward deviation along the workpiece thickness. An asymmetric arc pressure source was adopted to ensure asymmetric distribution of the arc pressure. The metal flow of the stable and cut molten pool are calculated in continuous VPPAW process, and the conditions needed for the stable existence of the molten pool are obtained. The molten pool must have sufficient liquid metal which needs to flow as follows: in the front of the upper molten pool, a part of the metal flows in the form of “two-way rotating confluence” along the keyhole side wall to the rear of the lower molten pool, and part of the metal flows backwards to the rear of the upper molten pool. The metal at the rear of the lower molten pool flows upwards along the keyhole side wall. Under the combined action of these flow behaviors, the expansion speed of the leading edge of the keyhole is approximately equal to the shrinkage speed of the keyhole trailing edge, and the keyhole molten pool is stable. The stability of the bottom molten pool has a significant impact on the overall molten pool stability. The calculated fusion line and the offset distance between the central axis of the keyhole exit and the welding torch corresponded with the measured results.

Physical mechanisms of fluid flow and joint inhomogeneity in variable-polarity plasma arc welding of thick aluminum alloy plates

A variable-polarity plasma arc (VPPA) is an effective energy source for the welding of thick aluminum alloy plates. However, the mechanisms influencing the fluid flow and the inhomogeneous distribution associated with thick-plate VPPA welding remain unclear, restricting the application of this technology in welding of thick aluminum alloys. Here, the relationship between the microstructure of the weld bead, energy transfer, and fluid flow is clarified by combining in situ three-dimensional x-ray imaging and multi-physics modeling. We find that heat conduction at the keyhole wall is the main factor influencing the morphology of the weld pool. The plasma arc pressure hinders the upward flow of liquid metal, while shear forces promote this flow. This causes the metal close to the weld pool surface to flow slowly, while that inside the weld pool has much higher velocity. It is also concluded that the large crystal size observed in the lower layer of the weld is partly caused by heat treatment from the upper layer of the thick plate. An eddy with a high flow velocity to the rear of the weld pool destroys the crystal-growth process, and this is considered to be one of the reasons for fine crystals appearing in the upper part of the weld. The mechanisms revealed here will help us to guide the use of VPPA technology in the production of stable, high-quality welding of thick aluminum alloys.

In situ monitoring and penetration prediction of plasma arc welding based on welder intelligence-enhanced deep random forest fusion

Online process monitoring and quality control has been a long-standing challenge for variable polarity plasma arc welding (VPPAW) due to the inherent instability and fluctuation of the keyhole molten pool. This work developed an innovative welder intelligence-enhanced deep random forest fusion (WI-DRFF) approach, aiming to describe the dynamics of front-side molten pool and accurately predict the weld penetration. Based on the human welder’s prior knowledge, we firstly proposed an image processing algorithm to extract the low-level handcrafted features, which could quantitatively describe the geometrical appearance of the keyhole. Afterwards, we constructed a convolutional neural network (CNN) to learn the high-level discriminative features of weld pool and interpret the physical characteristics of the deep features with visualization. Finally, we incorporated the handcrafted keyhole features and deep features to concatenate a multi-level feature vector for predicting the weld penetration based on random forest (RF) classifier. Extensive experiments demonstrate that our proposed approach yields a remarkable classification performance comparing with state-of-the-art machine learning algorithms even with limited training data. This approach is a new paradigm in the digitization and intelligence of welding process and can be exploited to provide a feedback in an adaptive quality control system.

Coupled mechanisms of the keyhole, energy transfer and compositional change associated with the variable polarity plasma arc process

A three-dimensional numerical model that coupled the arc, keyhole and weld pool was developed to investigate the keyhole behavior and energy transfer of an aluminum alloy in a variable polarity plasma arc welding process. The convective pattern in the weld pool was measured using an advanced x-ray transmission system. The elemental distribution of the bead was determined using an electron-probe microanalyzer. The coupled mechanisms of the keyhole, energy transfer, and compositional change were revealed. With a keyhole, both the arc energy loss and energy input increase, while the total energy transported from the arc to the weld pool decreases a bit, so the arc energy efficiency decreases. The heat flux at the keyhole is low and decentralized, resulting in low keyhole temperature; thus, the loss of elements with low boiling points due to evaporation can be ignored. Elements can be sufficiently mixed and distributed uniformly throughout the bead.

Effect of Asymmetric Material Flow on the Microstructure and Mechanical Properties of 5A06 Al-Alloy Welded Joint by VPPA Welding

The microstructure, texture, and mechanical properties of the asymmetric welded joint in variable polarity plasma arc (VPPA) welding were studied and discussed in this paper. The asymmetric welded joint was obtained through horizontal welding, where the effect of gravity caused asymmetric material flow. The results showed that the grain size and low angle grain boundary (LAGB) at both sides of the obtained welded joint were asymmetric; the grain size differed by a factor of 1.3. The average grain size of the Base Metal (BM), Lower-weld zone (WZ) and Upper-WZ were 25.73 ± 1.25, 37.87 ± 1.89 and 49.92 ± 2.49 µm, respectively. There is discrepancy between the main textures in both sides of the welded joint. However, the effect of asymmetric metal flow on the weld texture was not significant. The micro-hardness distribution was inhomogeneous, the lowest hardness was observed in regions with larger grain sizes and smaller low angle grain boundary. During tensile strength tests, the specimens fractured at the position with the lowest hardness although it has reached 89.2% of the strength of the BM. Furthermore, the effect of asymmetric metal flow and underlying causes of asymmetric weld properties in VPPA horizontal welding have been discussed and analyzed.

Physical mechanism of material flow in variable polarity plasma arc keyhole welding revealed by in situ x-ray imaging

Lightweight manufacturing reduces energy consumption and protects the environment; therefore, it offers an important direction of development in engineering. Variable polarity plasma arc (VPPA) welding is a light gauge aluminum and magnesium alloy joining technology capable of the highly efficient welding of medium-thickness metals via the keyhole effect, and without the complex pre-processing required by traditional methods. However, the stability of the keyhole weld pool is easily lost, resulting in a much narrower parameter window than that of conventional methods. Here, through keyhole morphology analysis and x-ray in situ imaging experiments, we reveal the material flow behavior, the mechanism behind the stability of the keyhole weld pool, and the reason for the very narrow process parameter window in VPPA welding processes. We discovered that the polarity pressure difference of the plasma arc, which is induced by the keyhole boundary shape and plasma arc intensity, drives the flow pattern to the top side of the keyhole, which is beneficial for keyhole filling and the formation of a stable weld bead. The influence of the difference in plasma arc pressure and the keyhole boundary on the flow field revealed in this study may guide the optimization process of light metal joining to achieve the highly efficient and defect-free manufacturing of large and complex structures.

Development in plasma arc welding process: A review

Plasma arc welding process has found wide application for joining of metals in manufacturing industries in the recent times. It has become the preferable welding process over other arc welding processes since it possesses increased welding speed, produces better weld quality, and has less impact of process variability on the process performance. A lot of advancement has taken place to enhance the stability of the plasma arc welding process, increase the depth of penetration and weld quality in recent years. The process is more preferred for welding of similar materials owing to the formation of stable, controlled, and constricted arc with low current requirements. It gives longer electrode life for high production. Variable polarity plasma arc welding (VPPAW) process, Pulsed plasma gas welding (PPGW), Double pulsed plasma arc welding (DPPAW) processes are widely used for joining the mid-thick steels (9.5 mm–12.5 mm). Pulsed plasma gas welding process helps to improve the process stability and the weld quality than other VPPAW process. Apart from these process Laser assisted Plasma arc welding, PAW-MIG hybrid welding, plasma spot welding, soft plasma arc welding process are some of the advancements to the PAW processes which are used for special industrial application. The present review paper focuses on the evolution and the development of the plasma arc welding process, its types, and the suitability of the process towards various applications in the manufacturing industries.

Investigation on microstructure, mechanical properties and corrosion behavior of VPPA welded Al–Mg–Mn–Sc–Zr alloy

Variable Polarity Plasma Arc Welding (VPPAW) can be used for the excellent welding of a 6 mm thick Al–Mg–Mn–Sc–Zr alloy, promising for the aerospace applications, due to its favorable welding quality, high production efficiency and small welding deformation. At present, investigations on the corrosion behavior of this type of welded joints are still insufficient. Here, the microstructure, mechanical properties and corrosion behavior of VPPAW-based Al–Mg–Mn–Sc–Zr alloy were systematically studied. The results show that the lowest value of the hardness of the welded joint occurs in the center of the weld, the fracture position of the welded joint is located in the welded region, and the bond zone (BZ) has the poorest corrosion resistance. The ultimate tensile strength (UTS) and welding coefficient of the welded joint are 327 MPa and 74.5 %, respectively. The low strength of the welded joint is mainly due to the combustion of the Mg element, the growth of grain size and the dissolution of second phase particles, resulting in the weakening effects of solid solution strengthening, precipitation strengthening, and grain boundary strengthening. The poor corrosion resistance of the BZ is attributed to the higher density of Fe-rich intermetallic particles and the weaker passivation film in this region.

Control of gravity effects on weld porosity distribution during variable polarity plasma arc welding of aluminum alloys

Abstract In contrast to other welding processes of aluminum alloys, variable polarity plasma arc (VPPA) nonvertical-up welding produces pores in specific positions (on the upper side of the welded joint) rather than throughout the whole welded joint due to the effect of gravity. In horizontal welding, the porosity area percentage on the upper side of the welded joint reached about ∼20%. Asymmetric metal flow in the keyhole molten pool caused either side of the weld to have different cooling rates. This difference in cooling caused the metal on this side of the weld to rapid crystallization, leading to higher concentrations of copper and manganese around the weld that are conducive to higher porosity. The Cu content reached ∼50% around the pores but was only ∼7% in the filler wire and ∼5% in the base metal. The root reason for porosity is the asymmetric metal flow. To correct this, the free point was found according to the force balance on the leading side of the keyhole. A proposed method to produce symmetric metal flow was used, and the resulting welds were determined to be of high quality via X-ray phase-contrast imaging.

Study on the Decoupled Transfer of Heat and Mass in Wire Variable Polarity Plasma Arc Welding.

A hybrid arc-wire welding method based on the variable polarity plasma arc (VPPA) and variable polarity pulse metal inert-gas (VP-PMIG) was proposed for manufacturing aluminum alloys. This paper aims to clarify the decoupling control process of heat and mass transfer in the hybrid welding process. To understand the arc physics and analyze the mass transfer behavior, the hybrid arc shape and droplet cross-sectional area with different parameters were obtained by high speed video photography. Further, the melting area of the base metal was analyzed by macro-metallography of the weld bead cross-section to study the heat transfer. It is found that the hybrid arc shape changes with time. The VPPA main arc is deflected to one side by the VP-PMIG, making the temperature distribution asymmetric, and during the VP-PMIG pulse necking occurs. The cross-sectional area of the droplet is more obviously affected by the VP-PMIG current than the VPPA current. Meanwhile, the VPPA current dominates the melting area of the base metal. Therefore, we conclude that heat transfer to the base metal is from the VPPA, while droplet transfer is mainly controlled by the VP-PMIG arc. These conclusions are confirmed by analyzing the decoupling degree of heat and mass transfer of the base metal by the VPPA and VP-PMIG arc.

Prediction of the Weld Pool Stability by Material Flow Behavior of the Perforated Weld Pool.

This article presents the application of a computational fluid dynamics (CFD) finite volume method (FVM) model for a thermo-mechanical coupling simulation of the weld pool used in variable polarity plasma arc welding (VPPAW). Based on the mechanism of the additional pressure produced through self-magnetic arc compression and the jet generated from mechanical plasma arc compression, and considering the influence of arc height and keyhole secondary compression on arc pressure, a three-dimensional transient model of variable polarity plasma arc (VPPA) arc pressure was established. The material flow behaviors of the perforated weld pools were studied. The results show that three kinds of flow behavior existed in the perforation weld pools and it is feasible to predict the weld pool stability by the material flow behaviors of the perforated weld pools. The weld pools can exist stably if the material flow in the bottom of the perforated weld pools can form confluences with moderate flow velocities of 0.45 m/s, 0.55 m/s and 0.60 m/s. The weld pools were cut when the material flowed downward and outward with the maximum velocity of 0.70 m/s, 0.80 m/s. When the maximum material flow velocity was 0.40 m/s, the weld pool collapsed downward under the action of larger gravity. The thermo-mechanical coupling model was verified by the comparison of the simulation and experimental results.