Keyhole Evolution Research Articles

A study on keyhole evolution and weld ripple formation in dissimilar welding under pulsed laser

ABSTRACTA three-transient multiphase model is developed to study the dissimilar metal welding of pure niobium plate to titanium alloy Ti–6Al–4V sheet under pulsed laser. The physical process of dissimilar metal welding involves melting, resolidification, mass transfer, self-consistent keyhole, and weld bead formation. The major physical factors, such as recoil pressure induced by vapourisation, surface tension, heat transfer, fluid flow, Marangoni shear stress, buoyancy force, and their coupling are considered. The results show that the keyhole mainly occurs on the Ti–6Al–4V side due to the differences in physical properties of the materials. The effects of pulse overlapping factor on the weld bead are studied. It is found that the pulsed laser has a significant influence on the weld bead formation. The mixing of materials mostly occurred in the upper part of the molten pool. The simulated weld bead profile and the phase distribution agree well with the experimental results.

Numerical analysis of thermal fluid transport behavior during electron beam welding of 2219 aluminum alloy plate

A two-dimensional mathematical model based on volume-of-fluid method is proposed to investigate the heat transfer, fluid flow and keyhole dynamics during electron beam welding (EBW) on 20 mm-thick 2219 aluminum alloy plate. In the model, an adaptive heat source model tracking keyhole depth is employed to simulate the heating process of electron beam. Heat and mass transport of different vortexes induced by surface tension, thermo-capillary force, recoil pressure, hydrostatic pressure and thermal buoyancy is coupled with keyhole evolution. A series of physical phenomena involving keyhole drilling, collapse, reopening, quasi-stability, backfilling and the coupled thermal field are analyzed systematically. The results indicate that the decreased heat flux of beam in depth can decelerate the keyholing velocity of recoil pressure and promote the quasi-steady state. Before and close to this state, the keyhole collapses and complicates the fluid transport of vortexes. Finally, all simulation results are validated against experiments.

Sustaining the open keyhole in slow-falling current edge during K-TIG process: Principle and parameters

Saving heat input from the arc is important to guarantee quality weld in keyhole mode Tungsten Inert Gas (K-TIG) Welding process. To this end, slowly falling edge was introduced in the current waveform. The fully-penetrated keyhole evolution behaviour during the current falling stage was visualized by a fast imaging system in real-time. It was found that, compared with the square current welding process, keyhole exit is closing at a slowly speed when the current is gradually decreasing and still opens even though the welding current is far below the threshold point. Based on the forces evolution analysis, Marangoni force in the weld pool should be the major factor to slow down the keyhole closing process during the current falling stage. Future experimental observation reflects that, the primary thermal state of the weld pool and the decreasing speed of the current can be used to control the keyhole closing behaviour. The degree of the saved energy input is related to the keyhole open period time in the slowly falling current edge. The research results lay foundation to deep understand the coupling behaviour between the “arc - weld pool - keyhole” system in K-TIG welding process and to further optimize the welding current waveform to stabilize the keyhole welding with low heat input.

Thermal Process Model of Plasma Arc Welding with Considering the Backside Keyhole Deviation

When the plasma torch and the work-piece move relatively in keyhole plasma arc welding(PAW), it is found that there is a keyhole exit deviation distance between the keyhole center point and the welding torch axis. Considering the keyhole deviation, studying the interaction of weld pool and keyhole is of great theoretical as well as practical significance to obtain the high-quality welds. In addition, the research can also provide guidelines to optimize the control method, on the basis of taking the backside keyhole deviation distance as the controlled variable. A combined volumetric heat source model is established to reveal the dynamic interaction between heat transfer process and keyhole evolution. Along the thickness of the test plate, a deviation parameter of the volumetric heat source center is set in the model, which describes the thermal effect of the keyhole deviation in the PAW. In the keyhole shape equation, the backward pushing force of plasma jet is taken into account, which reflects the influence the plasma jet on the weld pool. The thermal conduction process and keyhole evolution in PAW are numerically simulated. The predicted and measured keyhole exit sizes and keyhole establishment time agree with each other.

Numerical simulation of plasma arc welding with keyhole-dependent heat source and arc pressure distribution

A three-dimensional and transient model with keyhole geometry-dependent heat source and arc pressure distribution is developed for plasma arc welding process. The influence of the keyhole evolution on heat flux and arc pressure distribution on the keyhole wall is taken into consideration. The dynamic variation of the temperature field and fluid flow in the weld pool as well as the keyhole shape and size are quantitatively analyzed. The experimentally observed phenomena, i.e., the backward curved keyhole channel and the very thin layer of molten metal at the front edge of weld pool, are first demonstrated with numerical simulation. The predicted establishment time of open keyhole, displacement of keyhole exit, and fusion line generally agree with the experimental ones.

Improving the Prediction Accuracy of Keyhole Establishment Time in Plasma Arc Welding

Accurate prediction of both the keyhole establishment time and dimensions is of great significance for optimizing the process parameters in plasma arc welding. In this study, appropriate modifications of both heat source and plasma arc pressure models are made to improve the calculation accuracy of the keyhole evolution. The partition fractions of the combined heat source and distribution of the plasma arc pressure on the keyhole wall are transiently adjusted with the variation of the keyhole depth at each time step. A vision sensor is employed to detect the key-holing process for validating the simulation results. The prediction accuracy of the keyhole establishment time is greatly improved.

Multi-physical Simulation of Laser Welding

Abstract Laser welding is a highly demanded technology for manufacturing of body parts in the automotive industry. Application of powerful multi-physical simulation models permits detailed investigation of the laser process avoiding intricate experimental setups and procedures. Features like the degree of power coupling, keyhole evolution or currents inside the melt pool can be analyzed easily. The implementation of complex physical phenomena, like multi-reflection absorption provides insight into process characteristics under selectable conditions and yields essential information concerning the driving mechanisms. The implementation of additional physical models e. g. for diffusion discloses new potential for investigating welding of dissimilar materials. In this paper we present a computational study of laser welding for different conditions. Applied to a real case model predictions show good agreement with experimental results. Initial tests including species diffusion during welding of dissimilar materials are also presented.

Energy propagation in plasma arc welding with keyhole tracking

A three dimensional mathematical model was developed to research the dynamic interaction between energy propagation and keyhole evolution in PAW (Plasma Arc Welding). Particularly, a keyhole-tracking heat source model was proposed to reflect the energy propagation in the wake of keyhole evolution, which was tracked by the VOF (Volume of Fluid) technique. The model consists of a Gaussian heat flux on top surface and a lower developing conical heat source related to the dynamic keyhole growth. In addition, a dynamic energy distribution coefficient was established, bound up with the keyholing process for the first time. The design of this heat transfer model introduces the analogy to actual energy propagation in experiment. Evolution of the dynamic energy density distribution concerning keyhole effect was analyzed in details, and the corresponding temperature field was calculated and displayed to reveal the mechanism of thermal process in the workpiece. Furthermore, the keyholing process and molten metal flow in the weld pool were investigated to exhibit how the keyhole promotes the deep penetration welding. Finally, the experiments were carried out on the stainless steel plate, and the measured weld bead geometry, keyhole size and time for workpiece completely penetrated through were all close to simulation results.

A DYNAMIC HEAT SOURCE MODEL WITH RESPECT TOKEYHOLE EVOLUTION IN PLASMA ARC WELDING

Most of the familiar objects in modern society,from buildings and bridges,to vehicles, computers,and medical devices,could not be produced without the use of welding.Especially, with the rising development of advanced manufacturing industry,such as aircraft and aerospace industries, shipbuilding and marine industries and automotive industries,cost-effective high-efficiency high-quality welding processes are being progressively required for increasing performance requirements and enhancements in product quality.Thus,the plasma arc welding(PAW) provides a means for these process demands by using a high power density heat source.The keyhole effect is commonly recognized as the primary attribute to the deep-penetration welding.Compared to electron beam welding and laser welding,PAW is more cost effective and more tolerant of joint gaps and misalignment. However,the mechanism of keyhole formation in PAW process differs from that in other high power density welding processes.In PAW the keyhole is produced and maintained mainly by the pressure of the plasma arc,rather than by the recoil pressure of the evaporating metal in electron beam and laser welding.Considerable research has been focused on keyhole tracking and effective heat source models for PAW process.However,the existing models rarely can present the transient influences of the keyhole evolution on heat transfer and fluid flow in the weld pool.In this work,a three dimensional PAW model was established with the interaction between heat source and keyhole evolution considered. A combined heat source model was proposed to account for the transient energy propagation. It consists of a Gaussian heat flux model on the top surface and below a dynamic developing conical heat source,which continues rising in the wake of the keyhole growth.Volume of Fluid(VOF) method was applied to track the dynamic keyhole shapes,and the transient height of heat source model was simultaneously updated with the increasing keyhole depth.The transient evolution of heat density distribution concerning the keyholing effect was analyzed in details,and the corresponding temperature field was calculated and displayed to reveal the mechanism of heat transfer in the weld pool.The keyhole process and molten metal flow in the weld pool was also investigated.Finally,experiment was carried out on a stainless steel plate with thickness 6 mm,and the calculated results showed good agreement with the experimental data.It validated the mathematical model and the applicability of the dynamic heat source,which provides an insight into the understanding of the thermal process in the keyhole of PAW.

Visualizing the influence of the process parameters on the keyhole dimensions in plasma arc welding

The keyhole status and its dimensions are critical information determining both the process quality and weld quality in plasma arc welding (PAW). It is of great significance to measure the keyhole shape and size and to correlate them with the main process parameters. In this study, a low-cost vision system is developed to visualize the keyhole at the backside of the test-pieces in PAW. Three stages of keyhole evolution, i.e. initial blind stage (non-penetrated keyhole), unstable stage with momentarily disappeared keyhole and quasi-steady open keyhole stage (fully-penetrated keyhole), are measured in real-time during the PAW tests on stainless steel test-pieces of thickness 8 mm. Based on the captured images of keyhole under different welding conditions, the correlations of the main welding process parameters (welding current, welding speed, plasma gas flow rate) with the keyhole length, width and area are visualized through vision measurement. It lays a solid foundation for implementing keyhole stability control and process optimization in keyhole PAW.

Experimental Study and Numerical Modeling of Keyhole Behavior during CO2 Laser Welding

The present paper describes the results of high speed photography, acoustic emission (AE) detection and plasma light emission (LE) measurement during CO₂ laser welding of 304 stainless steel in different processing conditions. Video images with high spatial and temporal resolution allowed to observe the melt dynamics and keyhole evolution. The existence of keyhole was confirmed by the slag motion on the weld pool. The characteristic frequencies of flow instability and keyhole fluctuations at different welding speed were measured and compared with the results of Fourier analyses of temporal AE and LE spectra. The experimental results were compared with the newly developed numerical model of keyhole dynamics. The model is based on the assumption that the propagation of front part of keyhole into material is due to the melt ejection driven by laser induced surface evaporation. The calculations predict that a high speed melt flow is induced at the front part of keyhole when the sample travel speed exceeds several 10 ㎜/s. The numerical analysis also shows the hump formation on the front keyhole wall surface. Experimentally observed melt behavior and transformation of the AE and LE spectra with variation of welding speed are qualitatively in good agreement with the model predictions.

Modeling of laser keyhole welding: Part II. simulation of keyhole evolution, velocity, temperature profile, and experimental verification

This article presents the simulation results of a three-dimensional mathematical model using the level set method for laser-keyhole welding. The details of the model are presented in Part I.[4] The effects of keyhole formation on the liquid melt pool and, in turn, on the weld bead are investigated in detail. The influence of process parameters, such as laser power and scanning speed is analyzed. This simulation shows very interesting features in the weld pool, such as intrinsic instability of keyholes, role of recoil pressure, and effect of beam scanning.

Multiple reflection and its influence on keyhole evolution

In laser drilling and keyhole welding, multiple reflection phenomena determine how the energy is transferred from the laser beam to the workpiece, and, most importantly, all other physics such as fluid flow, heat transfer, and the cavity shape itself depend on these phenomena. In this study, a multiple reflection model inside a self-consistent (or self-evolving) cavity has been developed based on the level set method and ray tracing technique. In the case of drilling, it is observed that the laser energy tends to concentrate near the center, where the effective intensity reaches a value two orders of magnitude higher than the original distribution. In keyhole welding, however, the maximum laser intensity is only around five times higher than the original during the entire process. Combined with the strong keyhole fluctuation, the redistributed intensity patterns are very dynamic. The intensity fluctuation drives the keyhole fluctuation, and the keyhole fluctuation, in turn, affects the intensity fluctuation. This study demonstrates that drill holes are highly efficient surfaces to focus a large amount of energy in a tiny area while keyholes have the capacity to evenly distribute the energy in a large area. It is also shown that multiple reflection phenomena are highly geometry dependent and a preassumed hole shape as adopted in many prior studies may lead to an inappropriate result.