Keyhole Stability Research Articles

Effect of vapor/plasma-liquid flow behavior on the keyhole oscillation in laser-MIG hybrid welding of Invar alloy

A coupled heat transfer-fluid flow-model is implemented to simulate the laser melt inert gas (MIG) hybrid welding (LMHW) for Invar alloy. The temperature profile and materials flow during the LMHW process are monitored and verified according to the experimental data. The effect of vapor/plasma-liquid flow behavior on the keyhole oscillation in LMHW of Invar alloy is deeply investigated and addressed. The stability of the keyhole is determined by changes in the mass and energy of the welding pool caused by droplet impact. This investigation clarifies the effect of metal vapor/plasma produced by heat source on the keyhole stability, contributing to an improved prediction accuracy of the state of flow behavior in the molten pool of Invar alloy. The simulation result is approximately consistent with the experimental result, which shows the coupled model is reliable.

Oxygen content and morphology of laser cleaned 5083 aluminum alloy and its influences on weld porosity

In this paper, laser cleaning of the 5083 alloy was used to remove the native oxide film and modify the morphological defects. The effects of oxygen content changes and surface morphology evolution on weld porosity after pretreatment with different energy densities (E) was studied. After laser cleaning, the oxygen element first decreases and then increases. With 3.5–35 J/cm2, the surface oxygen content drops by more than 60% compared with the untreated sample. It is the lowest with 17.5 J/cm2, a decrease of 75%. The main oxide are Al2O3 before and after laser cleaning. Untreated specimens have morphological defects such as scratches, superficial damage, and bulge. As E increased, the morphological defects were modified by the crater, gradually for fusion, and finally formed a striation morphology. The micromorphology after laser cleaning did not significantly affect the penetration and weld width. However, observation of the keyhole behavior reveals the micromorphology can affect the keyhole stability to regulate the process porosity. The change trend of the hydrogen porosity of the weld is consistent with the oxygen content. With 17.5 J/cm2, the hydrogen porosity of the weld is 0.9%.

Effects of sub-atmospheric pressure on keyhole dynamics and porosity in products fabricated by selective laser melting

A powder-scale multi-physics model is developed to investigate the keyhole dynamics and porosity in products fabricated by selective laser melting (SLM) under the sub-atmospheric pressure. The effects of the sub-atmospheric pressure on the plume attenuation of the laser beam, vaporization temperature and vapor recoil pressure are taken into account. The thermo-fluid behaviour in the melt pool under the atmospheric and sub-atmospheric ambient pressure is analysed in the SLM process. The simulation results suggest that the reduction of the sub-atmospheric pressure can increase the melt pool depth, whereas the effects are insignificant under the low sub-atmospheric pressure. As compared with the atmospheric pressure, the sub-atmospheric pressure leads to the decrease of the average temperature and the increase of the average recoil pressure on the keyhole surface, which is in favour of increasing keyhole stability and maintaining keyholes open. Consequently, the sub-atmospheric pressure can help to improve the keyhole stability and thus minimize pore defects in SLM-built products.

Effect of the welding position on weld quality when laser welding Inconel 617 Ni-based superalloy

Inconel 617 Ni-based superalloy sheets were joined using a laser-welding process. The welding experiments were conducted on four different welding positions (i.e., flat, horizontal, vertical-up, and vertical-down), using two sets of welding parameters with different heat inputs. Then, the geometry, internal defects, mechanical properties, and molten-pool dynamics of the laser joints were evaluated. The results showed that, when using high-heat input parameters, the welding position has an obvious effect on the laser weld quality. The top surface of the flat weld and the root surface of the vertical-down weld are both slightly concave. Moreover, the horizontal weld and vertical-down weld have serious keyhole-induced porosity defects, which lead to relatively worse mechanical properties. The analysis shows that the stability of keyhole differs for different welding positions; keyhole-induced porosity is easily generated in the horizontal and vertical-down positions. Moreover, solid metal blocks escape routes of the bubbles, resulting in more serious porosity for these welding positions. However, when using low-heat input parameters, the welding position has less influence on the laser weld quality because there is less molten metal, which makes the keyhole more stable in different welding positions.

Study of porosity suppression in oscillating laser-MIG hybrid welding of AA6082 aluminum alloy

The AA6082 aluminum alloy was welded by oscillating laser-MIG hybrid welding. By using beam oscillation, the porosity can be effectively suppressed in the range of 100−300 A arc current, especially at low arc current level. Percent porosity decreased greatly with the increase of the oscillating amplitude and frequency. The threshold frequency of porosity-free at different amplitudes were achieved. The effect of beam oscillation on porosity formation and suppression was studied by observing keyhole behavior via a “sandwich” high-speed video method. It was found that the porosity suppression depended on three reasons. Firstly, high-frequency oscillating keyhole can “capture” the bubbles or cavities in the molten pool and solidification front. Secondly, high-frequency oscillating keyhole can enlarge the diameter of keyhole and improve keyhole stability. Thirdly, beam oscillation can form stirring effect in molten pool, which can produce turbulence to reduce possibility of bubbles moving to the solidification front.

Novel measures for spatter prediction in laser welding of thin-gage zinc-coated steel

This paper studies full-penetration remote laser stitch welding of thin-gage zinc-coated steel sheets in a lap joint condition. The spatter was classified as either Type A or Type B based on their characteristics, corresponding to low and high linear energies, respectively. The Type A spatter was mainly affected by the zinc vapor outgassing through the keyhole openings, while Type B was more influenced by the vulnerability of the keyhole rear wall to the impact of the high-pressure zinc vapor jet originating from the faying interface. Two measures, Zout and Zp, are proposed to predict the occurrence of Type A and Type B spatter. A thermo-fluid numerical model was developed to investigate the physics in the remote laser welding of zinc-coated steels, such as molten pool dimensions, keyhole stability, and zinc coating evaporation and to calculate Zout and Zp in the welding process. The effects of welding parameters, such as laser power and feed speed, on spatter occurrence were studied numerically and verified against experimental observations. It was found that the laser linear energy could directly affect values of Zout and Zp. A medium linear energy, striking a balance between Zout and Zp, could generate welds with much less spatter compared to a relatively low or high linear energy.

Transient dynamics and stability of keyhole at threshold in laser powder bed fusion regime investigated by finite element modeling

A Finite element model is developed with a commercial code to investigate the keyhole dynamics and stability at keyhole threshold, a fusion regime characteristic to laser microwelding or to Laser Powder Bed Fusion. The model includes relevant physics to treat the hydrodynamic problems—surface tension, Marangoni stress, and recoil pressure—as well as a self-consistent ray-tracing algorithm to account for the “beam-trapping” effect. Implemented in both static and scanning laser configurations, the model successfully reproduces some key features that most recent x-ray images have exhibited. The dynamics of the liquid/gas interface is analyzed, in line with the distribution of the absorbed intensity as well as with the increase of the keyhole energy coupling. Based on these results, new elements are provided to discuss our current understanding of the keyhole formation and stability at threshold.

Influence of oscillating magnetic field on the keyhole stability in deep penetration laser beam welding

The stability of the keyhole decreases for deep penetrated high-power laser beam welding. The keyhole tends to collapse with increasing laser power and e.g. keyhole induced porosity can occur. This study deals with the observation of the keyhole during high-power laser beam welding in partial penetration mode by means of a high-speed camera. A butt configuration of 25 mm thick structural steel and transparent quartz glass was used for the experiments. An oscillating magnetic field was applied perpendicular to the welding direction on the root side of the steel plate. The keyhole was highlighted with a coaxial diode laser. It was ascertained that the stability of the keyhole and the weld penetration depth were increased by applying an oscillating magnetic field with an oscillating frequency of 1.2 kHz and a magnetic flux density of 50 mT.

Laser-arc combined welding of aa5754 alloy

Laser-arc combined welding of 5754 magnesium aluminium alloy was performed with the CO2 heat source and MIG torch. The effects of welding conditions on penetration depth, geometry, porosity and crack susceptibility were investigated. Concerning porosity suppression and key-hole stability, optimal process parameters were identified. The grain boundary wetting phenomenon was observed in the fused zone.

Analysis and improvement of laser wire filling welding process stability with beam wobble

The reasonable selection of laser welding modes, namely keyhole mode and heat conduction mode, is based on the practical application requirements. The results show that the heat flux rapidly decreases and beam moving velocity increases with the beam wobble amplitude of 2.0 mm and frequency of 150 Hz, promoting the transformation of keyhole mode to heat conduction mode. Wider and shallower weld morphology is obtained. The improvement of keyhole stability and modification of liquid metal fluidity promote the overflow of bubbles. In case of beam wobble welding, the movement and turbulence of weld pool driven by the beam wobble help to break the columnar crystals. Welding of 40-mm thick plates without incomplete fusion and pore defects is achieved by suitable beam wobble parameters.

Influence of waveforms on Laser-MIG hybrid welding characteristics of 5052 aluminum alloy assisted by magnetic field

Laser-MIG hybrid welding of 5052 aluminum alloy assisted by magnetic field with three waveforms was performed. The results showed that compared to that without a magnetic field (NMF), the weld depth was increased by 46.3%, 45.1% and 22.4%, respectively, with the sequence of the static magnetic field (SMF), Triangular-wave magnetic field (TWMF) and Sine-wave magnetic field (SWMF), due to different dispersing effect of magnetic fields on welding plasmas. The similar effect of magnetic field with three waveforms on stirring molten pool during solidification led to almost the same refinement of grain size and randomness of grain orientation. Additionally, magnetic fields were found to improve stability of keyhole, prolong solidification time of molten pool and provide more escaping channels of bubbles, the porosity rate of weld was thus reduced from 7.80% to 3.46%, 2.11% and 0.82% when SMF, TWMF and SWMF were employed, respectively. As a result, the hardness and tensile properties of fusion zone under magnetic field with three waveforms were significantly improved and the highest tensile strength was obtained in the case of SWMF. Among the three types of magnetic fields, SWMF was the best in terms of reducing the weld porosity and increasing the tensile strength.

Effect of Local Gas Flow in Full Penetration Laser Beam Welding with High Welding Speeds

Spatter formation is a major issue in deep penetration welding with solid-state lasers at high welding speeds above 8 m/min. In order to limit spatter formation, the use of local gas flows represents a technically feasible solution. By using the gas flow, the pressure balance inside the keyhole, and therefore the keyhole stability, is affected. Existing investigations demonstrate a reduction in spatter and pore formation for partial penetration welding up to a welding speed of 5 m/min. However, the effect of the gas flow is not yet clarified for full penetration welding at welding speeds above 8 m/min. By using a precisely adjustable shielding gas supply, the effect of a local gas flow of argon was characterized by welding stainless steel AISI304 (1.4301/X5CrNi18-10). The influence of the gas flow on the melt pool dynamics and spatter formation was recorded by means of high-speed videography and subsequently analyzed by image processing. Schlieren videography was used to visualize the forming flow flied. By the use of the gas, a change in melt pool dynamics and gas flow conditions was observed, correlating to a reduction in loss of mass up to 70%. Based on the investigations, a model of the acting effect mechanism was given.

Depth Dependence and Keyhole Stability at Threshold, for Different Laser Welding Regimes

Depending of the laser operating parameters, several characteristic regimes of laser welding can be observed. At low welding speeds, the aspect ratio of the keyhole can be rather large with a rather vertical cylindrical shape, whereas at high welding speeds, low aspect ratios result, where only the keyhole front is mainly irradiated. For these different regimes, the dependence of the keyhole (KH) depth or the keyhole threshold, as a function of the operating parameters and material properties, is derived and their resulting scaling laws are surprisingly very similar. This approach allows us to analyze the keyhole behavior for these welding regimes, around their keyhole generation thresholds. Specific experiments confirm the occurrence and the behavior of these unstable keyholes for these conditions. Furthermore, recent experimental results can be analyzed using these approaches. Finally, this analysis allows us to define the aspect ratio range for the occurrence of this unstable behavior and to highlight the importance of laser absorptivity for this mechanism. Consequently, the use of a short wavelength laser for the reduction of these keyhole stability issues and the corresponding improvement of weld seam quality is emphasized.

Numerical simulation of heat transfer and fluid flow during vacuum electron beam welding of 2219 aluminium girth joints

In this paper, a three-dimensional model, which is combined with the volume of fluid (VOF) model, is established to study the dynamic behaviour and keyhole stability of circumferential weld pools in vacuum electron beam welding (EBW). The influences of gravity, the Marangoni effect and recoil pressure are taken into account during the simulation process. For partial penetration welding, the humps on the keyhole wall cause a redistribution of the surface tension; furthermore, there is little difference in the keyhole evolution during flat welding and girth welding due to the supporting effect of the base metal. After entering the full penetration stage, a tail forms at the circumferential weld pool surface under the action of the Marangoni effect and the recoil pressure. Remarkably, the fluctuations in the temperature and flow velocity are more intense at the lower weld pool surface than that at the upper surface due to the interaction between the electron beam and the keyhole wall. In addition, the flow field on a three-dimensional scale is analysed. Moreover, corresponding experiments are carried out to verify the accuracy of the models adopted in this study. The simulation results are consistent with the experimental data.

Experimental research on formation mechanism of porosity in magnetic field assisted laser welding of steel

The dynamic keyhole behaviors are directly observed in laser welding of AH36 steel/glass, and Zirconia particle is used to observe the convective pattern. The weld bead formation, keyhole instability and porosity formation are analyzed. Moreover, the differences in keyhole instability and porosity formation between no magnetic field assisted and magnetic field assisted laser welding are analyzed. With the magnetic field is added, the weld width, small concave, the formation of the keyhole collapse and the convective pattern are similar in laser welding of steel, and the generated Lorentz force hinders the flow of liquid metal. Widening of bottom weld width without undercut defect can be obtained with magnetic field assisted. The lower fluctuation of the weld depth, the longer time of plasma plume above the weld pool, fewer undercuts and spatter defects contributes to the increase of the keyhole stability, and thus restrains the porosity formation. Besides, an easier keyhole open, the larger bubble escape velocity and the lower solidification rate also promote the reduction of the porosity formation.

Influence of welding angle on the weld morphology and porosity in laser-arc hybrid welding of AA2219 aluminum alloy

The influence of welding angle on weld morphologies and porosity characteristics was studied in laser-arc hybrid welding of AA2219 aluminum (Al) alloy. The upward sloping and vertical weld (USVW) was performed better than the downward sloping and vertical weld (DSVW). The high-speed camera was used to observe the welding process. It is found that the welding process becomes stable when the transitional position of the droplet was 2 mm away from the keyhole. By analyzing the force act on keyhole and characteristics of the pool, it is found that the weld morphologies and porosity characteristics were associated with gravity-laser angle (β), which affects the keyhole stability and characteristics of the pool. The results showed that the flow velocity was accelerated when the β increased, which was a benefit for decreasing porosity. This is because the gravity in the direction of the welding accelerates the pool flow. Due to the increasing of the β, the static pressure and the vapor pressure in the keyhole were increased, which improved the stability of the keyhole and decreased the porosity.

Process stability and parameters optimization of narrow-gap laser vertical welding with hot wire for thick stainless steel in nuclear power plant

Incomplete fusion, pore and melt sagging are common defects for multi-layer narrow-gap laser welding with filler wire in vertical positions for large and heavy structures. This investigation showed that vertical-up laser welding had a lower sensitivity to pores than vertical-down welding, whereas increasing welding speed could improve welding stability and reduce the formation possibility of pores. For laser vertical welding with hot wire, surface tension transfer mode could effectively guarantee pre-heating of filler wire and achieve a stable welding process. The keyhole stability and melt flow varied with welding directions. The melt sagging defect was usually observed and mainly caused by the concentrated laser energy, which easily led to discontinuous weld bead. Laser beam oscillation technology increased laser melting area and promoted wetting behavior of filler wire with base metal. The weld width increased and concave weld metal surface was obtained with suitable process parameters. Narrow-gap welding process could be carried out at a small defocusing position with a lower laser power, further decreasing tendency of melt slagging. The 20-mm defect-free joint was obtained by vertical-up and vertical-down narrow-gap laser oscillation welding with hot wire.

A Study on the Influences of Welding Position on the Keyhole and Molten Pool Behavior in Laser Welding of a Titanium Alloy

Various welding positions need be used in laser welding of structures with complex configurations. Therefore, it is necessary to gain knowledge of how the welding positions can influence the keyhole and weld pool behavior in order to better control the laser weld quality. In the present study, a computational fluid mechanics (CFD) model was constructed to simulate the laser-welding process of the titanium alloy Ti6Al4V, with which the keyhole stability and the fluid flow characteristics in weld pool were studied for four welding positions, i.e., flat welding, horizontal welding, vertical-up welding, and vertical-down welding. Results showed that the stability of the keyhole was the best in flat welding, the worst in horizontal welding, and moderate in vertical welding positions. Increasing heat input (the ratio of laser power to welding speed) could increase the keyhole stability. When the small heat input was used, the dimensions and flow patterns of weld pools were similar for different welding positions. When the heat input was increased, the weld pool size was increased, and the fluid flow in the weld pool became turbulent. The influences of gravity became significant when a large heat input was used, especially for laser welding with vertical positions. Too high a heat input in vertical-up laser welding would lead to oscillation and separation of molten metal around the keyhole, and in turn result in burn-through holes in the laser weld. Based on the present study, moderate heat input was suggested in positional laser welding to generate a stable keyhole and, meanwhile, to guarantee good weld quality.

Numerical and experimental investigation of thermo-fluid flow and element transport in electromagnetic stirring enhanced wire feed laser beam welding

The introduction of electromagnetic stirring to laser beam welding can bring several beneficial effects e.g. element homogenization and grain refinement. However, the underlying physics has not been fully explored due to the absence of quantitative data of heat and mass transfer in the molten pool. In this paper, the influence of electromagnetic stirring on the thermo-fluid flow and element transport in the wire feed laser beam welding is studied numerically and experimentally. A three-dimensional transient heat transfer and fluid flow model coupled with dynamic keyhole, magnetic induction and element transport is developed for the first time. The results suggest that the Lorentz force produced by an oscillating magnetic field and its induced eddy current shows an important influence on the thermo-fluid flow and the keyhole stability. The melt flow velocity is increased by the electromagnetic stirring at the rear and lower regions of molten pool. The keyhole collapses more frequently at the upper part. The additional elements from the filler wire are significantly homogenized because of the enhanced forward and downward flow. The model is well verified by fusion line shape, high-speed images of molten pool and measured element distribution. This work provides a deeper understanding of the transport phenomena in the laser beam welding with magnetic field.

Simulation and experimental studies of keyhole induced porosity in laser-MIG hybrid fillet welding of aluminum alloy in the horizontal position

The keyhole induced porosity in laser-MIG hybrid fillet welding of aluminum alloy in the horizontal position was investigated with simulation and experimental methods. A three dimensional model is developed, which takes into account geometric feature of horizontal filled joint and the coupling of keyhole, droplet and molten pool. Meanwhile, the major forces are also incorporated in the model. The inclination of heat source is dealt with through rotation of coordinate system. This model is able to calculate the keyhole dynamic behavior and the formation process of gas bubble as well as keyhole induced porosity directly. Also, it can describe the merging and disappearing of bubbles. The weld porosity was examined by the X-ray non-destructive testing. Large weld pool size provides more time for bubble to escape from the molten pool, which is the major factor responsible for the reduction in hybrid welding. Besides, the forward flow caused by the clockwise vortex is weakened, which also plays a positive role in improving the stability of back keyhole wall. The keyhole collapses at the middle part easily in horizontal fillet welding, raising the possibility of forming large pore. Bubble is easy to be captured by the upper weld pool boundary when floating up. With enhancing the welding current, the stability of keyhole is enhanced to some extent and the keyhole induced porosity is reduced.