Unstable Keyhole Research Articles

Instability monitoring of molten pool in pure copper laser welding based on a multi-scale cascade model and spatial optical signals

During laser welding of pure copper, instabilities such as violent molten pool oscillation, large spatters, and melt ejection severely damage weld quality, which are caused by copper’s high reflectivity on commonly used infrared laser. The seriously unstable molten pool and keyhole and complicated laser-material interactions result in complex process signal emission waveforms, adding to the difficulties in process stability monitoring tasks. In this work, to break down different contents in the complex signals and deeply analyzing signal-process relation, combinative spatial optical sensor system was designed, and time-frequency signal analysis in multi-scale windows was performed. It was found that the infrared radiation at the front and end side of molten pool indicates the oscillation behavior of liquid metal surface, and the signal fluctuation patterns of visible radiation from different height of metal vapor varied when meeting severe instability like melt ejections. Signal features were extracted based on the understanding of process mechanism and signal behaviors. A cascade model combining Artificial Neural Network (ANN) and Support Vector Machine (SVM) was introduced to predict weld seam quality, where the ANN model focused on short-time stability status perception and the SVM model was used to decide macroscopic seam formation defects based on combining outputs of the ANN model in a long-term sampling window. Application results showed that the recognition accuracy of pit was 100 % and the accuracy of uneven toe reached 86.3 %. The multi-source signals of unstable molten pool recognized by the cascade model were summarized. The evolution process of copper molten pool ejection was revealed.

Towards implementation of alloy-specific thermo-fluid modelling for laser powder-bed fusion of Mg alloys

Multi-physics thermo-fluid modeling has been extensively used as an approach to understand melt pool dynamics and defect formation as well as optimizing the process-related parameters of laser powder-bed fusion (L-PBF). However, its capabilities for being implemented as a reliable tool for material design, where minor changes in material-related parameters must be accurately captured, is still in question. In the present research, first, a thermo-fluid computational fluid dynamics (CFD) model is developed and validated against experimental data. Considering the predicted material properties of the pure Mg and commercial ZK60 and WE43 Mg alloys, parametric studies are done attempting to elucidate how the difference in some of the material properties, i.e., saturated vapor pressure, viscosity, and solidification range, can influence the melt pool dynamics. It is found that a higher saturated vapor pressure, associated with the ZK60 alloy, leads to a deeper unstable keyhole, increasing the keyhole-induced porosity and evaporation mass loss. Higher viscosity and wider solidification range can increase the non-uniformity of temperature and velocity distribution on the keyhole walls, resulting in increased keyhole instability and formation of defects. Finally, the WE43 alloy showed the best behavior in terms of defect formation and evaporation mass loss, providing theoretical support to the extensive use of this alloy in L-PBF. In summary, this study suggests an approach to investigate the effect of materials-related parameters on L-PBF melting and solidification, which can be extremely helpful for future design of new alloys suitable for L-PBF.

A novel magnetic field assisted powder arc additive manufacturing for Ti60 titanium alloy: Method, microstructure and mechanical properties

Due to the rapid heating-cooling process and unstable keyhole, pore defects are easily formed during the high energy beam-powder additive manufacturing (AM) process, posing great challenges to the high-performance manufacturing of aerospace components. Using an arc with lower energy density as a heat source can effectively avoid these defects, but its matching raw materials are limited to wire. In this paper, a novel magnetic field assisted powder arc additive manufacturing (MFA-PAAM) method has been proposed, which combines arc heat source and powder raw material to achieve dense metal parts. A transverse magnetic field (TMF) was introduced to solve the powder spattering problem which impacts the stability of the PAAM process, by regulating the direction of the arc plasma flow and the arc pressure. The unique melting behavior of the powder under arc heating was captured by a high-speed camera, which can be summarized into two steps: balling and adsorption by the molten pool. And the well-formed and fully dense Ti60 high-temperature titanium alloy wall components were successfully manufactured using the self-developed equipment. The microstructure of the thin-wall component is dominated by basket-weave distributed α laths and few β sheets remaining at their boundaries. A layered periodic distribution along the building direction was observed, which is mainly distinguished from the content and morphology of the β phase. The average ultimate tensile strength (UTS) and elongation to failure in the travelling direction are 1003.2 MPa and 15.1%, while the average UTS and elongation along the building direction are 1009.3 MPa and 16.6%, respectively. The fracture morphologies in both directions are characterized by ductile fracture. The MFA-PAAM technique shows great potential for fabricating materials that are difficult to be drawn into wires with high efficiency and performance.

Mesoscale Simulation of Laser Powder Bed Fusion with an Increased Layer Thickness for AlSi10Mg Alloy

Low performance is considered one of the main drawbacks of laser powder bed fusion (LPBF) technology. In the present work, the effect of the AlSi10Mg powder layer thickness on the laser melting process was investigated to improve the LPBF building rate. A high-fidelity simulation of the melt pool formation was performed for different thicknesses of the powder bed using the Kintech Simulation Software for Additive Manufacturing (KiSSAM, version cd8e01d) developed by the authors. The powder bed after the recoating operation was obtained by the discrete element method. The laser energy deposition on the powder particles and the substrate was simulated by ray tracing. For the validation of the model, an experimental analysis of single tracks was performed on two types of substrates. The first substrate was manufactured directly with LPBF technology, while the second was cast. The simulation was carried out for various combinations of process parameters, predominantly with a high energy input, which provided a sufficient remelting depth. The calculations revealed the unstable keyhole mode appearance associated with the low absorptivity of the aluminum alloy at a scanning speed of 300 mm/s for all levels of the laser power (325–375 W). The results allowed formulating the criteria for the lack of fusion emerging during LPBF with an increased layer thickness. This work is expected to provide a scientific basis for the analysis of the maximum layer thickness via simulation to increase the performance of the technology.

Harmonizing sound and light: X-ray imaging unveils acoustic signatures of stochastic inter-regime instabilities during laser melting

Laser powder bed fusion (LPBF) is a metal additive manufacturing technique involving complex interplays between vapor, liquid, and solid phases. Despite LPBF’s advantageous capabilities compared to conventional manufacturing methods, the underlying physical phenomena can result in inter-regime instabilities followed by transitions between conduction and keyhole melting regimes — leading to defects. We investigate these issues through operando synchrotron X-ray imaging synchronized with acoustic emission recording, during the remelting processes of LPBF-produced thin walls, monitoring regime changes occurring under constant laser processing parameters. The collected data show an increment in acoustic signal amplitude when switching from conduction to keyhole regime, which we correlate to changes in laser absorptivity. Moreover, a full correlation between X-ray imaging and the acoustic signals permits the design of a simple filtering algorithm to predict the melting regimes. As a result, conduction, stable keyhole, and unstable keyhole regimes are identified with a time resolution of 100 µs, even under rapid transitions, providing a straightforward method to accurately detect undesired processing regimes without the use of artificial intelligence.

Vacuum laser beam welding of AZ31 magnesium alloy: Weld formability, microstructure and mechanical properties

Laser welding of magnesium alloy is still challenging because of the frequently occurred defects. In this work, a promising vacuum laser welding technique was used to join AZ31 magnesium alloy. The effect of ambient pressure on weld surface appearances and weld fusion cross sections was symmetrically investigated. It was found that the penetration depth increased and the weld width became narrower with the descending of ambient pressure from 101 kPa to 10 kPa. However, the penetration depth did not increase and even decreased when the ambient pressure further decreased to 1 kPa and 0.1 kPa. In addition, hump defects occurred at lower ambient pressure of 1 kPa and 0.1 kPa. The weld pool and keyhole behaviors were in-site observed using a high-speed camera at various ambient pressures. The results showed that unstable keyhole and turbulent flow were found in the molten pool at 101 kPa. The keyhole became more stable as the ambient pressure decreased from 101 kPa to 10 kPa. When the ambient pressure further decreased to 1 kPa and 0.1 kPa, the liquid metal thickness between keyhole wall and weld pool periphery became very narrow, which sometimes resulted in unstable absorption of the laser energy. Finally, a 10 mm-thick bottom-locking weld was made under vacuum. The average grain diameter of the weld in vacuum is 28.4 μm, which is much lower than 46 μm in atmosphere. The tensile results showed that the average ultimate tensile strength and the elongation of the joints welded in vacuum at 10 kPa was 8.8 % and 44.0 % higher than that of the joints welded in atmosphere respectively. The enhanced mechanical properties were attributed to the reduced porosity and refined microstrustures of the weld joint made under vacuum.

One camera-based laser keyhole welding monitoring system using deep learning

The laser-beam absorptance changes dynamically during laser keyhole welding due to unstable keyhole movements, and monitoring the absorptance can provide a deep understanding of the process. Recently, Kim et al. [1,2] developed a deep-learning-based method to monitor the absorptance by detecting the top and bottom keyhole apertures and estimating the absorptance from the reconstructed keyhole shape based on the detected apertures. However, this method was limited in that it required simultaneously observing the top and bottom keyhole apertures using two cameras. In this study, we proposed a novel deep-learning-based method to monitor the laser-beam absorptance in a keyhole using only one camera during laser keyhole welding of Al 5052-H32 alloy. In this method, both the top and bottom keyhole apertures were simultaneously detected from the images coaxially obtained from the top side. Although part of the bottom apertures may be sometimes obscured when viewed from above, this study demonstrated that the predicted absorptance was accurate enough and sufficient for monitoring laser welding processes of aluminum alloys. Using the developed method, changes in welding mode and generation of welding defects were successfully detected.

High Reflectivity and Thermal Conductivity Ag–Cu Multi-Material Structures Fabricated via Laser Powder Bed Fusion: Formation Mechanisms, Interfacial Characteristics, and Molten Pool Behavior

Ag and Cu have different advantages and are widely used in key fields due to their typical highly electrical and thermal conductive (HETC) properties. Laser powder bed fusion (LPBF), an innovative technology for manufacturing metallic multi-material components with high accuracy, has expanded the application of Ag-Cu in emerging high-tech fields. In this study, the multi-material sandwich structures of Ag7.5Cu/Cu10Sn/Ag7.5Cu were printed using LPBF, and the formation mechanism, interface characteristics, and molten pool behavior of the Ag7.5Cu/Cu10Sn (A/C) and Cu10Sn/Ag7.5Cu (C/A) interfaces were studied to reveal the influence of different building strategies. At the A/C interface, pre-printed Ag7.5Cu promoted Marangoni turbulence at a relatively low energy density (EA/C = 125 J/mm3). Due to the recoil pressure, the molten pool at the A/C interface transformed from a stable keyhole mode to an unstable keyhole mode. These phenomena promoted the extensive migration of elements, forming a wider diffusion zone and reduced thermal cracking. At the C/A interface, the molten pool was rationed from the conduction mode with more pores to the transition mode with fewer defects due to the high energy density (EC/A = 187.5 J/mm3). This work offers a theoretical reference for the fabrication of HETC multi-material structures via LPBF under similar conditions.

The Effect of Process-Induced Porosity on Fatigue Properties of Ti6Al4V Alloy via High-Power Direct Energy Deposition

Titanium alloy is widely used in the aviation sector and has become the most important structural material in aircraft manufacturing. However, manufacturing a large-scale titanium component owns a high buy-to-fly ratio due to its poor machinability and expensive price. Over the last decade, the additive manufacturing (AM) technology has developed rapidly and has become a promising processing method for titanium alloys. In the future, in order to enhance processing efficiency and material utilization, a higher laser energy source is supposed to be applied in AM processes. Nevertheless, porosity within the AM fabricated part is the most important issue that restricts the application of AM technology. In the present work, two bulks with different porosities were fabricated using high-power direct energy deposition (HP-DED), and the high cycle fatigue (HCF) performance of the as-build part was tested and compared. The result shows that a lack of fusion (LOF), spherical pores and un-melted particles are the main porosity defects in the as-build part. The shape, size and location of the defect will have a synthetic effect on HCF performance. In addition, the unstable key-hole during the process will facilitate the formation of a pore, which consequently increases the porosity. Online monitoring and closed-loop feedback systems should be established for enhancing the process stability.

Influence of the wrinkle surface structures on the vapor flow and keyhole stability in 20 kW high power laser welding

The stability of the extremely narrow keyhole is still the biggest challenge for the 20 kW or higher-level laser welding. The underlying causes of keyhole instability, including the wrinkle structures-induced thermal instability and vapor-induced dynamic instability, are studied in this work. A multiphase flow model integrated with a high-efficient free surface reconstruction algorithm and a high-accuracy wrinkle surface structures observation platform are built to study the morphology evolution and thermodynamic behaviors on the extremely narrow keyhole. It is found that the wrinkle structures (humps) that exist on the front keyhole wall could significantly regulate the energy distribution, and change the pressure condition in keyhole civility. The excessive energy accumulation, which is aroused by the abnormal hump distribution, is the reason for the unstable keyhole wall fluctuation. By increasing the welding speed, the intermittent-contact behavior between the laser beam and the keyhole opening is suppressed, which helps reduce the hump generation. By increasing the laser power, the drilling effect on the solid-liquid interface could be avoided, which helps to improve the downward flow efficiency of the melting layer on the front keyhole wall. An optimizing strategy based on the welding speed and laser power adjusting is offered to regulate wrinkle morphology. The experiment results show that keyhole stability is significantly improved under the optimized wrinkle surface structures.

Effect of zinc vapor forces on spattering in partial penetration laser welding of zinc-coated steels

A three-dimensional thermal-fluid numerical model considering zinc vapor interaction with the molten pool was developed to study the occurrence of zinc vapor-induced spatter in partial penetration laser overlap welding of zinc-coated steels. The zinc vapor effect was represented by two forces: a jet pressure force acting on the keyhole rear wall as the vapor bursts into the keyhole and a drag force on the upper keyhole wall as the vapor escapes upwards. The numerical model was calibrated by comparing the predicted keyhole shape with the keyhole shape observed by high-speed X-ray imaging and applied for various weld schedules. The study showed that large jet pressure forces induced violent fluctuations of the keyhole rear wall, resulting in an unstable keyhole and turbulent melt flow. A large drag force pushed the melt adjacent to the keyhole surface upward and accelerated the movement of the melt whose velocities reached 1 m/s or even higher, potentially inducing spatter. Increased heat input facilitated the occurrence of large droplets of spatter, which agreed with experimental observations captured by high-speed camera.

Time-Resolved Geometric Feature Tracking Elucidates Laser-Induced Keyhole Dynamics

During laser melting of metals, localized metal evaporation resulting in the formation of a keyhole shaped cavity can occur if processing conditions are chosen with high power density. An unstable keyhole can have deleterious effects in certain applications (e.g., laser powder bed fusion) as it increases the likelihood of producing defects such as porosity. In this work, we propose a pipeline that enables complete segmentation and extraction of various geometric features in keyholing conditions. In situ synchrotron high-speed X-ray visualization at the Advanced Photon Source provides large datasets of experimental images with a high spatio-temporal resolution across a range of laser parameters for Ti-6Al-4V. Computer vision image processing techniques were used to extract time-resolved quantitative geometric features (e.g., depth, width, front wall angle) throughout keyhole evolution which were subsequently analyzed to understand the relationship between the variation of local keyhole geometry and processing conditions. This analysis is the first to employ a data-driven approach to further our understanding of the keyholing process regime.

Dynamics of solid-liquid interface and porosity formation determined through x-ray phase-contrast in laser welding of pure Al

Through X-ray phase contrast method, solid-liquid interface, keyhole, and porosity in laser welding of pure Al could be clearly observed, and the shapes could be quantitatively evaluated. Almost no molten metal occurred at the keyhole bottom, and the temperature gradient and cooling rate there were estimated to be enormously large as compared to other locations in the weld pool. The flow of molten metal around the keyhole bottom was about twice as fast as in the upper part of the weld pool, suggesting that the amount of heat transport around the bottom was larger than in the upper part. Porosity formation is determined by keyhole behavior. In the case of a stable keyhole, obtained with a fan to remove the laser-induced plume, the porosity rate was around 6%, and the gas in the voids consisted entirely of hydrogen. In the case of an unstable keyhole, obtained without fan usage, the porosity rate was around 18%, and the gas included a component of air (nitrogen) in addition to hydrogen.

Stable keyhole welding process with K-TIG

Keyhole welding process was successfully achieved by using a self-made K-TIG torch. High quality welds were produced in 8mm-thick stainless steel plates. To evaluate the keyhole stability in the novel manufacturing process, keyhole behavior was observed by using a vision system from backside of the workpiece. Efflux plasma interference was eliminated by using a filter glass. Keyhole exit behavior was imaged in real time during the welding process, keyhole size and position were extracted from the keyhole image sequence. Keyhole behavior parameters, including keyhole length, width, length/width, and deviation distance were measured. It was found that keyhole exit was deviated away from the torch axis even though the arc current was very high in K-TIG welding process, keyhole exit had an oval shape. Continuous open keyhole welding were easily achieved in K-TIG process, without unstable keyhole stage when the keyhole firstly opens. Keyhole size, shape and position were all related to welding current. The observation results lay solid foundation to understand the thermal-physical behavior in K-TIG, and for the further optimizing of the welding torch.

Fiber laser-MIG hybrid welding of 5 mm 5083 aluminum alloy

Aluminum and its alloys are difficult to weld due to their specific characteristics. New joining methods such as high power fiber laser-MIG can provide higher overall productivity compared to the arc welding or laser beam welding. There is huge lack of information on how the specific welding parameters in laser-arc hybrid welding affect quality of welds such as torch arrangement, distance between heat sources and shielding gas composition while other parameters were kept at constant. Laser-arc hybrid welds with short separation distance between sources produced severe porosity in one pass 5mm thick aluminum alloy sheets welding due to unstable interactions and keyhole frequent collapses. Higher process stability and lower porosity level can be achieved by applying trailing torch arrangement. Addition of significantly more expensive helium to the shielding gas did not provide any benefits in terms of porosity decrease, process stability and mechanical properties of welds overall as was expected with selected welding parameters.

Numerical modeling on the formation process of keyhole-induced porosity for laser welding steel with T-joint

A mathematical model of laser welding steel with a T-joint was developed in this paper to investigate the formation process of keyhole-induced porosity, helping to understand the mechanism of porosity formation in laser welding steels with heavy section. Solidification model and adiabatic bubble model were coupled in this model, which could more approximately reflect the formation process of bubble and its evolution into porosity. The volume-of-fluid (VOF) method was taken to track free surfaces of keyhole and porosity. The numerical results showed that the unstable keyhole during the laser welding process induced the collapse of keyhole and then resulted in bubbles in the molten pool. These bubbles moved following with the fluid flow in the molten pool, where some bubbles could escape out of molten pool under the competition of flow and solidification speed. But some bubbles captured by a solidified wall during the migration process in the molten pool would evolve into porosities. It was also found that some bubbles formed adjacent to a fusion line were easier to be captured by a solidification surface, which could give explanation for some porosities occurring close to the fusion line. A good agreement of simulation and experimental results proved the reliability of this mathematical model, while the mechanism of porosity formation was better clarified with this model.

The Research on YAG Laser Welding Porosity of Al-Li Alloy

The laser welding of Aluminum-Lithium alloy (Al-Li) alloy were conducted to investigate the weld porosity features in this paper. The results show that there are two kinds of weld porosity existing for laser welding Aluminum-Lithium alloy sheet. They are can be divided into metallurgical porosity and unstable keyhole porosity according to their different reason causing them. The mechanism of unstable keyhole porosity occurring was discussed according to the results of YAG laser welding processing. The methods how to reduce weld porosity for laser welding of Aluminum-Lithium alloy was also described.

A three-dimensional sharp interface model for self-consistent keyhole and weld pool dynamics in deep penetration laser welding

A three-dimensional sharp interface model is proposed to investigate the self-consistent keyhole and weld pool dynamics in deep penetration laser welding. The coupling of three-dimensional heat transfer, fluid flow and keyhole free surface evolutions in the welding process is simulated. It is theoretically confirmed that under certain low heat input welding conditions deep penetration laser welding with a collapsing free keyhole could be obtained and the flow directions near the keyhole wall are upwards and approximately parallel to the keyhole wall. However, significantly different weld pool dynamics in a welding process with an unstable keyhole are numerically found. Many flow patterns in the welding process with an unstable keyhole, verified by x-ray transmission experiments, were successfully simulated and analysed. Periodical keyhole collapsing and bubble formation processes are also successfully simulated and believed to be in good agreement with experiments. The mechanisms of keyhole instability are found to be closely associated with the behaviour of humps on the keyhole wall, and it is found that the welding speed and surface tension are closely related to the formation of humps on the keyhole wall. It is also shown that the weld pool dynamics in laser welding with an unstable keyhole are closely associated with the transient keyhole instability and therefore modelling keyhole and weld pool in a self-consistent way is significant to understand the physics of laser welding.

Weld strength improvement for Al alloy by using laser weaving method

Laser welding of aluminum alloys usually produces linear welds that have shear-tensile strength lower than that of the base metal. This is usually due to porosity resulting from an unstable keyhole and cracking which is related to the high crack sensitivity of many of these alloys. In this work laser weaving is used to improve the joint strength of laser welds. In this research, lap welds and bead-on-plate welds were produced on 1 mm thick aluminum alloy sheets using various weaving schemes. When the weaving width was 4 mm, crack propagation was reduced, compared with a weaving width of 2mm. No cracks occurred when the weaving frequency was 5 Hz for weaving widths of 2 and 4 mm. The shear-tensile strengths were measured according to the weaving parameters such as weaving width and frequency. The laser weaving increased the fusion line length, therefore improving the joint strength.

Influence of surface preparation and process parameters on the porosity generation in aluminum alloys

The 4 kW cw Nd:yttrium–aluminum–garnet laser welding of solid solution hardened A5083 (5% Mg) and precipitation-hardened A356 (7% Si) alloys, sensitive to porosity formation was carried out at different welding speeds, under He gas shielding, and for partial penetration conditions, on bead on plate configuration. The porosity formation was examined through observation and evaluation of weld beads, based on x-ray radiography inspection coupled with image analysis, and quantification of gas volume percentage by densitometry. The results show that both surface preparation and dual-beam configuration can have a beneficial effect on the porosity content: (1) After polishing, laser cleaning, sand blasting, or degreasing of the surface, hydrogen and oxygen sources are decreased or removed. The resulting effect is a reduction from 15% to less than 2% of the porosity rate on A356 alloy, with better results, in the case of laser cleaning. (2) The use of dual beam conditions ensures a larger weld pool, facilitates the porosity’s ejection, and stabilizes the keyhole as compared with single beam configurations. For instance, a longitudinal bi-spot configuration reduces the porosity rate from 15% to less than 0.5% on A356 alloy. In A5083 welds, most of the macroporosities remain in the beads, due to an unstable keyhole (Mg vapors) which can collapse and entrap millimeter-like blow holes.