Large Spatter Research Articles

Microstructure and Mechanical Properties of a Weld Seam from Magnetron High-Current CO2 Welding

External magnetic field (EMF)-assisted high-current CO2 welding is beneficial for improving the large spatter and poor performance of the welding heat-affected zone for mild steels under high-current welding specifications. In this paper, the droplet transfer behaviors were determined using a high-speed camera on a self-developed magnetically controlled CO2 welding system. Based on these welding specifications, a three-dimensional, transient, multi-energy field coupling welding system model to investigate the mechanism of the droplet and molten pool in EMF-assisted welding was developed. The microstructure and mechanical properties of the welded joint were systematically studied. The results show that the Lorentz force applied by the EMF to twist the droplet decreases the accumulated energy in the short-circuited liquid bridge and changes the liquid metal flow condition, both of which reduce the spatter by 7% but increase the welded joint hardness by 10% and tensile strength by 8%.

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.

The Formation Mechanism and Suppression Strategies of Spatter in Pulsed Gas Metal Arc Welding for Titanium Alloy

The GMAW process for titanium alloy is not commonly applied within the industry due to the occurrence of severe spatter. This research endeavors to elucidate the mechanism underlying spatter formation and explore efficacious strategies to suppress spatter. The experimental results demonstrated the existence of two distinct spatter types: large and small spatter particles. The high- speed images and synchronous electrical signals were utilized for determining the spatter formation mechanism, with force analysis serving to mutually validate the inferences. The large spatter particles originated from the whole transitional molten droplet as it descended within the arc space, while the small spatter particles were formed by the partial transitional molten droplet as it contacted the weld pool. The cathode jet force accounted for the formation of large spatter particles, whereas the electromagnetic force was responsible for the small spatter particles. To suppress spatter, increasing detachment current and decreasing pulsing frequency were employed. Consequently, the spatter rate witnessed a remarkable decrease from 14.00% to 3.33% with a progressive increment in detachment current from 100 A to 300 A, and a corresponding decline from 12.67% to 1.33% upon decrementing the pulsing frequency from 90 Hz to 50 Hz. This research suggests that a judicious increase in the detachment current can effectively decrease the spatter rate while concurrently preserving welding efficiency.

Improvement in Microstructure and Properties of 304 Steel Wire Arc Additive Manufacturing by the Micro-Control Deposition Trajectory.

In this study, the GMAW welding torch was controlled by a stepping motor to achieve a periodic swing. By controlling the swing speed, a micro-variable deposition path was obtained, which was called the micro-control deposition trajectory. The influence of the micro-control deposition trajectory on the arc characteristics, microstructure, and mechanical properties of 304 steel wire arc additive manufacturing was studied. The results showed that the micro-control deposition process was affected by the swing arc and the deposition trajectory and that the arc force was dispersed over the whole deposition layer, which effectively reduced the welding heat input. However, the arc centrifugal force increased with the increase in the swing speed, which easily caused instability of the arc and large spatter. Compared with common thin-walled deposition, the deposition width of micro-control thin-walled deposition components was increased. In addition, the swinging arc had a certain stirring effect on the molten pool, which was conducive to the escape of the molten pool gas and refinement of the microstructure. Below, the interface of the deposition layer, the microstructure of the common thin-walled deposition components, and the micro-control thin-walled deposition components were composed of lathy ferrite and austenite. Compared with the common deposition, when the swing speed increased to 800 °/s, the microstructure consisted of vermicular ferrite and austenite. The tensile strength and elongation of the micro-control thin-walled deposition components are higher than those of the common thin-walled deposition components. The tensile fracture mechanism of the common thin-walled deposition components and the micro-control thin-walled deposition components was the ductile fracture mechanism.

SO2 emissions from the Timanfaya eruption (1730–36 CE), Lanzarote, Canary Islands

The 1730–36 CE eruption of Lanzarote (Canary Islands) is the third largest basaltic fissure eruption known to have occurred in the last 1000 years, after the Icelandic events of Laki (1783–84 CE) and Eldgja (939 CE). Approximately 5.2 km3 (1.1 × 1012 kg) of alkalic basalt (sensu lato) lava and pyroclastic rocks were emplaced on land, with a further unknown amount entering the sea, The activity occurred in five eruptive phases from vents situated along a 15-km long, E-W trending fissure. Each phase began with Strombolian fire fountain activity, building large spatter and scoria cones, accompanied and followed by effusive ‘a'ā and pāhoehoe lava flow emplacement. Phases I and II were the most explosive and occurred between 1st September 1730 and June 1731. Samples of scoria and lava surfaces were collected from all phases and the major elements, S, Cl and F contents of matrix glasses and glass inclusions in olivine (the dominant phenocryst phase) were analysed by electron microprobe. Water contents of selected glasses were measured on polished wafers by Fourier Transform Infrared spectroscopy. Glass compositions mimic the range of published whole rock compositions, with Phase I being predominantly basanitic and later phases alkali basalt. Whole rock variations in incompatible trace element ratios are mimicked by ratios involving TiO2, K2O and P2O5 in glass. Matrix and glass inclusion analyses in a given sample are similar, indicating that the inclusions formed synchronously with eruption and that most phenocryst crystallization occurred during earlier evolution of the magmas. Matrix glasses in scoria and surface lava samples have 80–300 ppm S and 260–750 ppm H2O, whereas glass inclusions in olivine have 420–2650 ppm S and 0.1 to 0.5 wt% H2O. Glass inclusions with low sulfur contents were trapped during degassing-induced crystallization as the magma approached the surface. The inclusions with the highest sulfur contents, at given P2O5/TiO2 ratio, are inferred to represent un-degassed melt, and their S content and S/K2O ratios correlate very strongly with incompatible element concentrations and P2O5/TiO2 ratios. The original S contents of 131 matrix glasses were calculated from their P2O5/TiO2 ratios and compared to their measured degassed S contents. The mass of S released per unit mass of magma ranged from about 0.25% in Phase I Episode 1to 0.17% in later phases, with propagated uncertainties of about 2 to 5%. About 40 Mt. of SO2 was released overall, with ca. 25 Mt. being released during the first 5 months of the eruption (Phase I). Estimates of average magma discharge rates imply that plumes remained in the troposphere, with the possible exception of those associated with extreme peak discharge rates during Phase I Episode 1 which may have just reached the upper troposphere – lower stratosphere. Lower eruption rates during the later phases would have confined SO2 emissions to the troposphere. Although acid and sulfate peaks dated to 1730 and 1731 have been reported from some Greenland ice cores, the evidence for stratospheric impact, such as cooling at the Earth's surface and increased stratospheric turbidity, is minimal.

Image Processing and Feature Extraction for Hull Structure GMAW Based on Weld Pool Visual Sensing

Image processing and feature information extraction based on the visual perception of the weld pool are considered essential components of intelligent welding quality monitoring of hull structure gas metal arc welding (GMAW). The unstable characteristics, such as large spatter, much smoke, and strong arc light during hull structure GMAW, lead to the blurring of image acquisition and the difficulty of contour extraction of the weld pool. The present study is aimed at addressing the practical issues from two perspectives, i.e., a spectrum‐visual‐sensing acquisition system and an image‐processing and feature extraction algorithm. First of all, by analyzing the light energy distribution law and acquiring the optical parameters relevant to the cut‐off composite dimming and near‐infrared narrowband filtering, spectral sensing is employed in establishing models of arc light radiation to detect the strength of continuously distinctive spectral lines. Besides, an appropriate high‐speed charge‐coupled device (CCD) camera is selected to build a visual acquisition system, which can reduce the external interference of the arc light on the image acquisition of the weld pool. Afterwards, the implementation of an image‐processing fusion model based on the spatial information fuzzy C‐means (FCM) clustering analysis and Sobel edge detection operator accompanies the investigation of the geometric aspects of the weld pool image. In terms of clear segmentation of the interest region, the edge detection and accurate extraction of the target contour are successfully obtained. In the subsequent section, the Hough transform analysis is adopted to establish the geometric feature extraction model of the weld pool, with corner detection, conversion, and camera calibration as the core technology. Additionally, the left and right views of the image contour are calibrated to achieve the lossless conversion of corner pixels and physical coordinates. Finally, three other distinct image‐processing methods are designed to compare the segmentation effect of the edge contour with the fusion model, and then, the extraction accuracy of the geometric features of the weld pool is verified. The interference of the arc light and smoke has been demonstrated to be substantially diminished, which is attributable to the visual‐sensing system during image acquisition of the weld pool. The results of edge fusion of the weld pool image show that based on the GMAW using the FCM‐Sobel fusion method, a superior extraction accuracy of geometric features characterized by smoothness, continuousness, no breakpoints, and less noise has fulfilled the engineering requirements.

Controlling process instability for defect lean metal additive manufacturing

The process instabilities intrinsic to the localized laser-powder bed interaction cause the formation of various defects in laser powder bed fusion (LPBF) additive manufacturing process. Particularly, the stochastic formation of large spatters leads to unpredictable defects in the as-printed parts. Here we report the elimination of large spatters through controlling laser-powder bed interaction instabilities by using nanoparticles. The elimination of large spatters results in 3D printing of defect lean sample with good consistency and enhanced properties. We reveal that two mechanisms work synergistically to eliminate all types of large spatters: (1) nanoparticle-enabled control of molten pool fluctuation eliminates the liquid breakup induced large spatters; (2) nanoparticle-enabled control of the liquid droplet coalescence eliminates liquid droplet colliding induced large spatters. The nanoparticle-enabled simultaneous stabilization of molten pool fluctuation and prevention of liquid droplet coalescence discovered here provide a potential way to achieve defect lean metal additive manufacturing.

Effects of He content in shielding gases on high-efficient hybrid laser arc welding with C-276 filler metal

High-efficient welding technology with rotating arc assisted by laser under shielding gases with different He content was investigated for the application of 9% Ni steel in LNG storage. High-speed image and electrical signal acquisition were adopted to investigate arc stability and metal transfer behavior during hybrid laser arc welding. The results indicated that with increasing content of He, the weld formation became better first and then worse. Ar+30 %He improved the formation of welding seam. The He content had a significant influence on the electric signal waveforms. With the addition of 30 %He, the waveforms had the minimal fluctuation. It was rotating spray transfer with pure Ar, Ar+30 %He and Ar+50 %He. The difference was that with the He content increase, transfer mode shifted from unstable rotating spray transfer to stable rotating spray transfer, to irregular rotating spray transfer. With pure He, it was large droplet transfer, resulting in the formation of large spatters. The research of rotating spray transfer trajectory showed that the rotation velocity was relatively constant and one rotation time was 3.19 ms in one pulse period under Ar+30 %He. The trajectory equation was obtained for describing the rotating spray transfer. This indicated that the addition of 30 %He improved the coupling effect of laser and arc, enhanced the matching degree between rotating spray transfer and arc pulse period. It was meaningful to understand the effects of shielding gases on 9% Ni steel hybrid laser arc welding, providing theoretical basis for LNG storage efficient and reliable manufacturing.

Characterization and mitigation of aerosols and spatters from ultrasonic scalers

BackgroundDental procedures often produce aerosols and spatter, which have the potential to transmit pathogens such as severe acute respiratory syndrome coronavirus 2. The existing literature is limited. MethodsAerosols and spatter were generated from an ultrasonic scaling procedure on a dental manikin and characterized via 2 optical imaging methods: digital inline holography and laser sheet imaging. Capture efficiencies of various aerosol mitigation devices were evaluated and compared. ResultsThe ultrasonic scaling procedure generated a wide size range of aerosols (up to a few hundred μm) and occasional large spatter, which emit at low velocity (mostly < 3 m/s). Use of a saliva ejector and high-volume evacuator (HVE) resulted in overall reductions of 63% and 88%, respectively, whereas an extraoral local extractor (ELE) resulted in a reduction of 96% at the nominal design flow setting. ConclusionsThe study results showed that the use of ELE or HVE significantly reduced aerosol and spatter emission. The use of HVE generally requires an additional person to assist a dental hygienist, whereas an ELE can be operated hands free when a dental hygienist is performing ultrasonic scaling and other operations. Practical ImplicationsAn ELE aids in the reduction of aerosols and spatters during ultrasonic scaling procedures, potentially reducing transmission of oral or respiratory pathogens like severe acute respiratory syndrome coronavirus 2. Position and airflow of the device are important to effective aerosol mitigation.

Numerical and experimental investigation into gas flow field and spattering phenomena in laser powder bed fusion processing of Inconel 718

Accumulated spatters not only deteriorate the quality of the built parts in terms of surface roughness and/or density but also sabotage the whole fabrication process via damaging the powder re-coater. A cross-flow of inert gas inside the processing chamber is by far the only effective mean of removing the spouted spatters from the build platform. Despite the essential role, there are few efforts to investigate the interactions between the cross-flow dynamics and the spouted spatters, and hence, to optimize the design and settings of the cross-flow. Therefore, in this study, a coupled computational fluid dynamics (CFD) – discreate element method (DEM) simulation framework is proposed to study the interactions between the flow field and the spouted spatters during the single scan process. The simulation results for the maximum ejection distance and maximum ejection height of the spatters were shown to deviate from the numerical results reported in the literature by no more than 15%. Moreover, the simulation results for the distribution of the landing sites of the spatters were consistent with the experimental observations, which showed that a large number of large spatters landed near the scan track, while a small number of large spatters landed at a greater distance from the scan track. To the best of the authors’ knowledge, this study represents the first reported attempt to use a coupled CFD-DEM simulation framework to investigate the spatter dynamics within an LPBF chamber and to validate the results with experimental observations and published data. The proposed model is an important tool to optimize and calibrate the cross-flow inside the L-PBF processing chamber.

Denoising and feature extraction of weld seam profiles by stacked denoising autoencoder

Active vision sensing is widely used in intelligent robotic welding for bead detection and tracking. Disturbed by welding noise such as arc light and spatter, it is a hard work to extract the laser stripe and feature values. This paper presents a method for denoising and feature extraction of weld seam profiles with strong welding noise in gas metal arc welding (GMAW) process by using stacked denoising autoencoder (SDAE). This algorithm encodes the images of various butt joints with strong welding noise to several useful intermediate representations, which can be decoded to the image of pure laser stripe in 1-pixel width. The results show little deviations when there are large spatters across the laser stripe. A back propagation neural network (BPNN) is developed to verify the reliability of the intermediate representations gotten from the encoder, in which the intermediate representations are input neurons and the weld seam width is output neuron. The average width error in training dataset and testing dataset is 0.042 mm and 0.061 mm. The results show that this algorithm can extract the weld seam profiles with strong welding noise and extract feature values accurately.

High-power laser-matter interaction during laser powder bed fusion

Laser powder bed fusion (LPBF) additive manufacturing (AM) is developing with the goal of fabricating parts with high performance and high efficiency. Laser power is the key factor to the efficiency, microstructure and performance in LPBF. However, there are limited reports regarding the laser-matter interaction in LPBF under high-power conditions. In this work, the molten pool characteristics and spatter behavior in LPBF with a high power and a wide process window (from 350 W to 1550 W) are studied based on high-speed high-resolution imaging. The results show that the molten pool characteristics and spatter behavior depend on the laser input energy. The average ejection velocity and ejection angle increase with the laser power. The droplet column ejection and large spatters are prone to occur with a high-power laser. Furthermore, the times at which the vapor depression and the protrusion in the molten pool first occur decrease dramatically with an increase in the laser input energy. When the laser mode and spot size are kept constant, the laser power determines the amount of time required for melting, the vapor depression and the protrusion in LPBF to occur, while the laser scan velocity determines whether the laser dwell time is sufficient for these phenomena to form.

Effects of shielding gases on process stability of 10CrNi3MoV steel in hybrid laser-arc welding

Hybrid laser-arc welding (HLAW) of 10CrNi3MoV steel was carried out with different shielding gas mixtures including 100% Ar, 5% CO2 + Ar, 10% CO2 + Ar, 15% CO2 + Ar, 20% CO2 + Ar, 25% CO2 + Ar and 100% CO2. A CMOS high speed camera and real-time electric signal data were utilized to obtain the high-speed images (HSI) and electric signals of the droplet transfer and weld pool dynamics in real time during the HLAW process. The results showed that the increase of CO2 content increased the synergetic effect, especially the energy density of laser, and improved the penetration. The electric signal and HSI results revealed that, at low CO2 content (5% CO2 + Ar, 10% CO2 + Ar), the process was unstable with spatters and the metal transfer mode was globular mode. But at medium CO2 content (15% CO2 + Ar, 20% CO2 + Ar), the process was stable with less spatters and the metal transfer mode was spray mode. Stable process and accepted sound weld were obtained using 20% CO2 + Ar, which corresponded to the welding current 200 A and welding voltage 29 V. As CO2 content in the shielding gas increased, the increase in the weld current and welding voltage deteriorated the stability of the welding process, and the transfer mode shifted to repelled transfer, leading to the formation of large spatters.

Strong noise image processing for vision-based seam tracking in robotic gas metal arc welding

The robustness of the image processing algorithm is very important based on vision sensor in robotic seam tracking, which will directly affect the accuracy of weld seam shaping quality. Especially in GMAW (Gas Metal Arc Welding), there is a lot of strong noise image. This paper studies an algorithm for the several weld seam images with strong noise in robotic GMAW, such as the atypical weld seam, the strong arc light and the large spatter. Based on a purpose-built visual sensing system, the fast image segmentation, the feature area recognition of the convolutional neural network (CNN), and the feature search technique are used to identify the weld seam features accurately in the algorithm. The selection range of the threshold is increased from 0.5 × 107 to 0.9 × 107 by using the proposed algorithm, which reduces the difficulty of parameter adjustment and increases the stability of seam tracking system. And, the accuracy of the CNN model was 98.0% for the atypical weld seam identification. To evaluate the robustness of the proposed algorithm, the accuracy is verified using experiments on two typical strong noise images. The experiments show that the average error of feature extraction accuracy is 0.26 mm and 0.29 mm. The results show that the proposed algorithm can extract the feature of weld seam image with strong noise accurately and effectively.

Effects of coupling between the laser plasma and two arcs on metal transfer in CO2 laser double-wire MIG hybrid welding

CO2 laser double-wire MIG hybrid welding with direct current is a kind of process with high metal deposition rate and large weld penetration. Microscopic interaction model between the laser plasma and two arcs is proposed. Forces acting on the laser plasma and droplet were analyzed. A high speed camera system and an electrical signal acquisition system were used to investigate the coupling and metal transfer. The theoretical analysis and experimental results reveal that when the electromagnetic force of the leading arc on the laser plasma (FLP) is generally equal to the electromagnetic force of the trailing arc on the laser plasma (FTP), there is higher electron density in two lower sides of the laser plasma. Therefore, despite the great electromagnetic force, the two arcs do not deflect from the wire axis. A stable, spatter-free spray transfer can be achieved. When FLP > FTP, there is higher electron density in the upper left side and the lower right side of the laser plasma. Consequently, the trailing arc will deviate from the wire axis under the influence of the electromagnetic force, resulting in an unstable metal transfer with large spatter. Similarly, when FLP < FTP, there is an unstable welding process in the leading arc.

Experimental analysis of spatter generation and melt-pool behavior during the powder bed laser beam melting process

The experimental analysis of spatter formation was carried out on an instrumented SLM set-up allowing the quantification of spatter ejections and possible correlation with melt-pool behavior. Considering nearly similar SLM conditions than those carried out on SLM machines, an increase of large spatters (>80μm) with volume energy density (VED) was clearly demonstrated on a 316L stainless steel, which was attributed to the recoil pressure applied on the melt-pool by the metal vaporization and the resulting high velocity vapor plume. In a second step, much lower spattering was shown on Al-12Si powder beds than on 316L ones. Fast camera analysis of powder beds indicated that droplet formation was mostly initiated in the powder-bed near the melt-pool interface. On Al-12 Si alloys, such droplets were directly incorporated in the MP without being ejected upwards as spatters like on 316L. Last, it was shown that a strong reduction of spattering was possible even on 316L, with the use of low VED combined with larger spots (≈0.5mm), allowing to melt sufficiently deep layers in conduction regime and ensure adequate dilution between layers.

Surface void suppression for pure copper by high-speed laser scanner welding

Welding of copper was conducted using a single-mode fiber laser rotated at a high speed by a galvano scanner. The rotation diameter and frequency were changed to investigate the effect of the beam rotation on the welding quality. The rotation diameter changed from 0.3 to 1.0mm and the rotation frequency changed from 77 to 285Hz. The weld depth obtained with scanning is smaller than that obtained without scanning. The weld depth decreases as rotation frequency increases, regardless of the rotation diameter. The bead width obtained with scanning is larger than that obtained without scanning because of the larger laser radiation area. In the welding there is an unscanned area, based on the relationship between laser scanning conditions and welding speed, that makes the weld depth periodic. It was found that welding geometry can be controlled by adjusting scanning parameters. Spatter and surface voids decrease as rotation frequency and diameter increase. It was confirmed a scanning speed over 500mm/s suppressed large spatter and surface voids.

Development of a low-spatter CO2 arc welding process with a high-frequency pulse current

In CO2 arc welding of solid wire, metal transfer phenomena and spatter generation are investigated with rectangular pulse current, and a low spatter CO2 arc welding process with high frequency pulse current is developed. The optimal conditions of high frequency pulse CO2 arc welding are in the range of peak current: 450–550 A and pulse frequency: 450–750 Hz. These high frequency pulse currents have the effect of droplet oscillation due to resonance between applied pulse frequency and the natural frequency of the droplet. The droplet is transferred consistently every 9–11 pulses and the average interval of metal transfer is about 16 ms which is reduced to half that of conventional CO2 arc welding. This average droplet weight is 34 mg, showing a large reduction in comparison with that of the conventional method. As a result, total spatter weight is reduced by 70% in comparison with the conventional method, and especially, large spatters more than 0.5 mm in diameter are reduced from 1.5 to 0.2 g/min.

高周波パルスを用いた低スパッタCO2アーク溶接技術の開発

In CO2 arc welding of solid wire, metal transfer phenomena and spatter generation are investigated by with rectangular pulse current, and low spatter CO2 arc welding process with high frequency pulse current is developed. The optimal conditions of high frequency pulse CO2 arc welding are in the range of peak current : 450-550A and pulse frequency: 450-750Hz. These high frequency pulse currents have the effect of the droplet oscillation due to resonance between applied pulse frequency and the natural frequency of the droplet. The droplet is transferred consistently every 9-11 pulses and the average interval of metal transfer is about 16ms which is reduced to half than that of conventional CO2 arc welding. This average droplet weight is 34mg, showing a large reduction in comparison with that of the conventional method. As a result, total spatter weight is reduced by 70% in comparison with the conventional method, and especially, large spatters more than 0.5mm in diameter are reduced from 1.5g/min to 0.2g/min.

ステンレス鋼の高出力ファイバーレーザ溶接とその特徴的な溶接部の形成機構の解明

A high-power fiber laser can produce an ultra-high peak power density of MW/mm2 level corresponding to a focused electron beam, and is promising as one of the desirable heat sources for high-speed and deep-penetration welding. In this study, therefore, fiber laser welding and simultaneous observation with high-speed video cameras and X-ray transmission real-time imaging system were performed with the objective of obtaining a fundamental knowledge of welding phenomena and defect formation mechanisms in producing deep and narrow weld beads in Type 304 stainless steel. A 6 kW fiber laser beam through φ 100 μm feeding fiber or φ 300 μm process fiber could be focused at 1.1 MW/mm2 or 120 kW/mm2 peak power density, respectively. Consequently, deeply penetrated weld beads with narrow width were formed, and fully penetrated welds in 8 mm thick plate could be produced at less than 2 and 4.5 m/min for 120 kW/mm2 and 1.1 MW/mm2, respectively. In the case of the ultra-high power density, humping weld beads were formed at more than 6 m/min. This reason was attributed to the intermittent melt ejection from a small keyhole inlet and the subsequent melt flows on the narrow molten pool surface. On the other hand, in the case of the high power density, weld fusion zones with porosity, sound partial penetration welds and underfilled weld beads with large spatters were obtained at the welding speeds below 3 m/min, between 4.5 to 8 m/min and above 10 m/min, respectively. These formation mechanisms were interpreted by considering bubbles formation from a keyhole tip leading to pores captured by the solidifying front, the formation of a long molten pool suppressing and accommodating spattering, and the production of a narrow, short molten pool causing easy generation of spattering, respectively.