Number Of Spatters Research Articles

Effect of Spatter Behavior on Mechanical Properties and Surface Roughness of Printed Parts during PBF-LM of 316L.

The spatter generated by the interaction between laser and powder during Powder Bed Fusion-Laser Melting (PBF-LM) can significantly affect the quality of printed parts. A high-speed camera is used to observe the dynamic process of spatter's behavior under different layer thickness and laser powers during the printing process, and to analyze the printed samples' surface roughness, microstructure, and mechanical properties. In terms of spatter image processing, employing an optical flow approach to track and quantify the number of spatters efficiently eliminates statistical redundancy and improves statistical correctness. It is found that under the same laser power, the number of spatters produced by the laser scan direction with the gas flow (LSD-W) is more than that by the laser scan direction against the gas flow (LSD-A), and the number of spatters produced increases with the increase of laser power. Analyzing the mechanical properties and surface roughness of the printed samples under different process parameters quantitatively reveals that differences in the spatter amount generated under different process parameters in the PBF-LM process is not the determining factor affecting the difference in tensile strength of printed parts. During LSD-W, the number of spatters generated at laser power of 170 W and layer thickness of 0.03 mm is 87, and the tensile strength of the printed sample is 618 MPa. During LSD-W, the number of spatters generated at laser power of 320 W and layer thickness of 0.05 mm is 211, and the tensile strength of the printed sample is 680 MPa. Instead, spatter generation has a more direct impact on the surface roughness of printed parts. The layer thickness is 0.03 mm, the laser power is 170 W, and (Ra = 2.372 μm) is the surface roughness of the sample. The layer thickness is 0.05 mm, the laser power is 320 W, and (Ra = 8.163 μm) is the surface roughness of the sample.

Interaction between Local Shielding Gas Supply and Laser Spot Size on Spatter Formation in Laser Beam Welding of AISI 304

Background. Spatter formation at melt pool swellings at the keyhole rear wall is a major issue for laser deep penetration welding at speeds beyond 8 m/min. A gas nozzle directed towards the keyhole, that supplies shielding gas locally, is advantageous in reducing spatter formation due to its simple utilization. However, the relationship between local gas flow, laser spot size, and the resulting effects on spatter formation at high welding speeds up to 16 m/min are not yet fully understood. Methods. The high-alloy steel AISI 304 (1.4301/X5CrNi18-10) was welded with laser spot sizes of 300 μm and 600 μm while using a specially designed gas nozzle directed to the keyhole. Constant welding depth was ensured by Optical Coherence Tomography (OCT). Spatter formation was evaluated by precision weighing of samples. Subsequent processing of high-speed images was used to evaluate spatter quantity, size, and velocity. The keyhole oscillation was determined by Fast Fourier Transform (FFT) analysis. Tracking the formation of melt pool swellings at the keyhole rear wall provided information on the upward melt flow velocity. Results. The local gas flow enabled a significant reduction in the number of spatters and loss of mass for both laser spot sizes and indicated an effect on surface tension by shielding the processing zone from the ambient atmosphere. The laser spot size affected the upward melt flow velocity and spatter velocity.

Off-axis high-speed camera-based real-time monitoring and simulation study for laser powder bed fusion of 316L stainless steel

In order to develop smart laser powder bed fusion (LPBF) devices that autonomously identify a defect and remove it during the process for first-time-right and zero-defect parts, it is important to develop reliable on-machine defect measuring capabilities. As defects in LPBF parts often occur below the layer that is being processed, capturing the information of a printing layer may not give information about physical phenomena that are occurring below this layer. Therefore, to predict volumetric features such as porosity only by looking at the layer being processed, the correlation between process signatures identified in-process and defects measured via post-process inspection methods (for example X-ray computed tomography) needs to be conducted. Hence, in situ monitoring and post-process metrology form a basis to better understand the fundamental physics involved in an LPBF process and ultimately to determine its stability. By utilizing high-speed imaging, various process signatures are produced during single-track formation of 316L stainless steel with various combinations of laser power and scan speed. In this study, we evaluate whether these signatures can be used to detect the onset of potential defects. To identify process signatures, image segmentation and feature detection are applied to the monitoring data along the line scans. The process signatures determined in the current study are mainly related to the features like the process zone length-to-width ratio, process zone area, process zone mean intensity, spatter speed and number of spatters. It is shown that the scan speed has a significant impact on the process stability and spatter formation during single-track fusion. Simulations with similar processing conditions were also performed to predict melt pool geometric features. Post-process characterization techniques such as X-ray computed tomography and 2.5-D surface topography measurement were carried out for a quality check of the line track. An attempt was made to correlate physics-based features with process-related defects and a correlation between the number of keyhole porosities, and the number of spatters was observed for the line tracks.

Effects of gas flow parameters on droplet spatter features and dynamics during large-scale laser powder bed fusion

The droplet spatters generated by the laser induction directly affect the processing quality of the laser powder bed fusion (LPBF) process, gas parameters are a key factor in this process. In this work, the formation characteristics and dynamic behavior of process-by-products under different gas flow parameters were presented through high-speed imaging. A novel image processing method and spatters feature extraction algorithm were used to obtain the spatter number, the total spatter area, the spattering angle and the spattering velocity of 8 scenarios. With the comparative analysis, scan direction against the gas flow (SD-A) is proved to produce fewer droplet spatters than scan direction with the gas flow (SD-W), and the quantitative relationship between the gas flow parameters and the droplet spatter behavior is established for the first time. The maximum spattering velocity increases to 12.8 m/s as the gas flow velocity rise to 2.5 m/s. Finally, the mechanisms of the droplet spatter behavior influenced by gas flow parameters are discussed. The work provides a theoretically reference for the design and control of gas flow parameters during the large-scale LPBF.

CFD–DPM Simulation Study of the Effect of Powder Layer Thickness on the SLM Spatter Behavior

Selective Laser Melting (SLM) has significant advantages in manufacturing complex structural components and refining the alloy microstructure; however, spatter, as a phenomenon that accompanies the entire SLM forming process, is prone to problems such as inclusions, porosity, and low powder recovery quality. In this paper, a Computational Fluid Dynamics–Discrete Particle Method (CFD–DPM) simulation flow for predicting the SLM spatter behavior is established based on the open-source code OpenFOAM. Among them, the single-phase flow Navier–Stokes equation is used in the Eulerian framework to equivalently describe the effect of metal vapor and protective gas on the flow field of the forming cavity, and the DPM method is used in the Lagrangian framework to describe the metal particle motion, and the factors affecting the particle motion include particle–particle collision, particle–wall collision, fluid drag force, gravity, buoyancy force, and additional mass force. In addition, the equivalent volume force and fluid drag force are used to characterize the fluid–particle coupling interaction. For the spatter behavior and powder bed denudation phenomenon, the calculation results show that the spatter height and the drop location show a clear correlation, and the powder bed denudation phenomenon is caused by the high-speed gas flow, causing the surrounding gas to gather in the forming area, which in turn drives the motion of the powder bed particles. For the effect of powder layer thickness on spatter and powder bed denudation, the calculation results show that the effect of powder layer thickness on the number of spatters is large (when the thickness was increased from 50 μm to 100 μm, the number of spatters increased by 157%), but the effect on spatter height and drop location distribution is small. When the powder layer thickness is small, the width of the denudation zone is significantly larger, but when the powder layer reaches a certain thickness, the width of the denudation zone does not show significant changes. It should be noted that the presented model has not been directly validated by experiments so far due to the difficulty of tracking the large-scale motion of SLM spatter in real time by current experimental means.

Investigation into the optical emission of features for powder-bed fusion AM process monitoring

Process monitoring and control is an essential approach to improve additive manufacturing (AM) built quality. For the development of powder bed fusion (PBF) AM monitoring system, sensing process optical emission is a popular approach. This is because it provides rich information on melt pool condition which directly determines final part quality. However, the optical emission information is convoluted. And lack of full understanding of it limits the further development of an optimal monitoring system. Therefore, the aim of this study is to explore the correlations between the optical emission and the processing condition to help enhance PBF process monitoring. A high-speed camera was used to acquire the images of the optical emission in the waveband of 800–1,000 nm. Several typical features were extracted and analyzed with the increase of laser power. The K-means clustering method was used to identify the hidden patterns of these features. Five hidden patterns have been identified, and therefore the collected dataset was partitioned into five subsets. The extracted features in each subset were characterized. It is found that (1) plume area and plume orientation are the two most crucial features for processing condition monitoring; (2) number of spatters and spatter dispersion index are sensitive to some minor process vibrations which have little effect on built quality. Additionally, the SVM model was built for process quality identification. It is found that (3) the time sequence information of the features can help improve the quality identification performance.

Keyhole dynamic status and spatter behavior during welding of stainless steel with adjustable-ring mode laser beam

A novel adjustable-ring mode laser (ARM) welding of SUS304L stainless steel sheet was investigated in this work. The ARM laser consists of two parts: ring laser beam as outer part; circular laser beam as center part. A Phantom high-speed camera with image processing algorithm was used to monitor the keyhole dynamic behavior. The spatter ejection, molten pool, and keyhole status were observed from the top view position. The characteristics of spatters were monitored from the side position of the camera. Experimental results showed that the pure circular laser beam (regular Gaussian beam) generated a periodic opening and collapsing keyhole with plenty of spatters in the welding process. However, the pure ring laser beam generated a continuously opening keyhole-inlet in the welding process. Because of the ring laser on welding, the dual-mode ring/circular laser (ARM laser) maintained a relatively opening keyhole with a smaller number of spatters comparing to pure circular laser welding. This work provided a full understanding of the process mechanism regarding the welding with shaped laser beam. The adjustable-ring mode laser is expected to be applied in a high-quality industrial manufacturing process.

Power density effect on the laser beam-induced eruption of spatters in fiber laser keyhole welding

Violent eruption of spatters is one of the major problems restricting fiber laser keyhole welding technology development. In this study, the fiber laser power density variation was attained by altering the laser power and laser spot diameter for analyzing its impact on the eruption of spatters. The severity of spatters’ eruption can be indicated by the number of spatters and the mass loss of the weld seam. The correlation between the number of spatters and laser power density can be positive (if the latter is adjusted by changing the laser power) or negative (if the laser power density is varied by changing the laser spot diameter). When the latter diameter is changed, it has a strong impact on the vapor induced by the laser on the front keyhole wall (FKW), the FKW inclination angle, and the molten pool surface tension. The number of spatters also positively correlates with the laser-induced vapor on the FKW or the molten pool width and negatively correlates with the FKW inclination angle. Reducing the laser spot diameter can increase the laser power density, enhancing the welding process stability and suppressing the eruption of spatters to a certain extent.

Online monitoring of an additive manufacturing environment using a time-of-flight mass spectrometer

Laser-based Directed Energy Deposition (L-DED) is a promising additive manufacturing technology, which has broad application prospects and practical value in many fields. The nondestructive testing technology of L-DED work piece quality monitoring requires high accuracy and capability for real-time process. This work developed an online monitoring system based on a miniature electron impact ion source time-of-flight mass spectrometer (EI-TOF) and a high-speed camera, which monitored the atmosphere above the molten pool area. The number of spatters was measured using a high-speed camera. The composition of the samples and spatters was measured by using element analyzer, inductively coupled plasma mass spectrometry, and electron spectroscopy. The variation in the atmosphere was measured using EI-TOF, and the results confirmed that the O2, N2 and H2O content decreased during the L-DED process. At the laser power of 400 W, the oxygen consumptions were 92.5% and 86.4% compared with those at the laser power of 500 W and 600 W, respectively. During the L-DED process, the number of spatters decreased when the laser power was increased from 400 W to 600 W. And at the laser power of 600 W, the average number of spatters was reduced to 59.3% compared with the laser power of 400 W. It can be found that dust contained a large amount of virgin powder and little spatters. It also can be found that at the laser power of 400 W, 500 W and 600 W, the amount of oxygen in the dust was 0.544%, 0.242% and 0.159%, respectively. The relative content of oxygen in the cross section of the seam was 35.58%, 43.79% and 44.30%, respectively. The deviation of the monitoring results (H2O, N2, O2) are significant at the same power when different materials was used as the substrate.

Spatter reduction by multi-beam illumination in laser powder-bed fusion

Modulation of laser radiation is a promising approach to increase the process speed in laser powder-bed fusion. However, with an increased buildup rate, the preservation of product quality must be guaranteed. For this purpose, we investigated spatter generation with single-beam and multi-beam distributions for single tracks and printed objects. The investigation with a coaxial high-speed camera to capture thermal radiation of spatters and an off-axis high-speed camera for a qualitative observation of the melt pool led to a better understanding of spatter generation. The results showed dependency of power and scan velocity on the number of spatters. Furthermore, we showed that the application of multi-beams preserves the number of spatters per track at higher applied power. Due to the wide melt pools and fewer required hatches, the total number of spatters in a generated part was significantly reduced. In addition, by means of high-speed recordings, it was shown that melt pools with high volume have a higher stability against impurities of the powder bed and spatters. Finally, the application of multi-beam illumination led to a higher buildup rate with reduced spatter particles.

Dual-beam laser-matter interaction at overlap region during multi-laser powder bed fusion manufacturing

To meet the urgent demand of large-scale parts fabrication in aerospace and energy fields, laser powder bed fusion (LPBF) additive manufacturing is developing towards multi-laser powder bed fusion (ML-PBF). However, defects such as the surface quality degradation and lack of fusion are more prone to appear at the overlap region of the deposit printed by multiple laser beams, which is detrimental to the consistency and uniformity of parts formed using ML-PBF. Here, based on a high-speed, high-resolution imaging technology and our ML-PBF equipment, the dual-beam laser-matter interaction at the overlap region in ML-PBF was investigated. At the overlap region, the collision and interaction between two molten pools influences the flow pattern of the liquid metal, accompanied by the large-sized droplet spatter expelling out. Moreover, the collision and accumulation of the liquid metal and spatter are responsible for the formation of the surface and internal structure defects of overlap samples. Furthermore, we reveal multiple transitions of the dominant mechanisms of the spatter formation in ML-PBF, which can be interchanged from the vapor-induced recoil pressure dominant stage to the vapor-induced entrainment dominant stage. We also propose the growth rate of spatter number r s , which can be well correlated with the transient transition of spatter formation mechanism. This work is expected to provide the scientific basis for ML-PBF to achieve consistency and uniformity.

Investigations on the influence of adapted metal-based alloys on the process of laser beam melting

Additive manufacturing technologies are characterized by complex process interrelations. Consequently, specifically adapted alloys are required to enable a robust building process. In particular, laser beam melting (LBM) is increasingly used for the fabrication of sophisticated functional parts for various applications in numerous industrial sectors, such as automotive and aerospace. However, process stability and repeatability are major challenges for industrializing LBM. This paper presents a comprehensive investigation of the influence of AlSi10Mg additives in a 316L stainless steel powder during LBM. A two-stage experimental approach was applied, during which the temperature field around the molten track and the number of spatters during the LBM process were determined by means of high-speed thermographic imaging. Furthermore, the microstructure of the additively manufactured specimens, the modified 316L stainless steel powder, and the respective raw materials was characterized by scanning electron microscopy. The experimental study described in this paper aimed to obtain correlations between the additive content (input), the temperature field of the molten track, and the microstructure (outputs). It was found that the cooling rate decreases with a higher amount of AlSi10Mg in the powder. Furthermore, the microstructure analysis demonstrated an increasing formation of the body-centered cubic phase with a higher fraction of AlSi10Mg. The conclusion is that additives in the powder considerably affect important key characteristics of the LBM process.

The Evaluation of Laser Weldability of the Third-Generation Advanced High Strength Steel

To meet the demands of vehicular safety and greenhouse gas emission reduction, the automotive industry is increasingly using advanced high strength steels (AHSS) in the production of the components. With the development of the new generation of AHSS, it is essential to study their behavior towards manufacturing processes used in the automotive industry. For this purpose, the welding capability of newly developed third-generation Gen3 980T steel was investigated using the Nd:YAG (Neodymium:Yittrium Aluminum Garnet) laser-welding with different parameter conditions. The analysis was made by uniaxial tensile tests, micro-hardness, Scanning Electron Microscopy (SEM) and X-Ray Diffraction (XRD). The criteria used to evaluate the quality of the weld were the distance between the fracture and the weld bead and the surface finish. A relationship between the quality of the weld and the energy density was observed, expressed by a partial penetration for values below the optimal, and by irregularities in the weld bead and a high number of spatters for the values above the optimal.

Effect of Parameters Change on the Weld Appearance in Stainless Steel Underwater Wet Welding with Flux-Cored Wire

The underwater wet welding (UWW) technology is rapidly developing as a crucial method in the maintenance work of marine equipment and offshore platform. The rapid development of UWW technology has also exposed the problems to be solved urgently. Therefore, the influence of welding parameters on the weld appearance and welding spatters was investigated in this paper. The main welding parameters used in the study are welding current, arc voltage, welding speed and the contact tip-to-work distance (CTWD). Through the orthogonal test, it is found that, as each welding parameter increases within a certain range, the amounts of welding spatter decreases first and then increases, and the weld forming effect first becomes better and then deteriorates. The amount of wet welding spatter is mainly affected by the welding speed. When the welding speed is low, the splash is more, and the change of the welding current and the arc voltage has a little effect on the number of spatters. When the welding speed is large, the spatter is most with a small welding current and a large arc voltage. After evaluating the weld morphology obtained by welding under various parameters, a set of optimal parameters was obtained. The best parameters for the underwater wet welding of stainless steel with self-shielded flux-cored wire are determined to be 200 A-29 V-2.0 mm/s-15 mm (CTWD).

Role of welding speed on keyhole-induced porosity formation based on experimental and numerical study in fiber laser welding of Al alloy

The keyhole behaviors were observed directly by laser welding experiment with quartz glass. Based on Fresnel absorption of laser beam and multi-reflection combined with volume of fluid (VOF) method, a three-dimensional mathematical model was established to study the role of welding speed on keyhole behaviors and keyhole-induced porosity formation in laser welding of aluminum alloy. The keyhole behaviors and weld pool fluid flow were discussed, and the result shows that, although the welding speed varies, the mechanism of keyhole collapse was similar. However, the keyhole stability at higher welding speed was improved due to reduced weld depth fluctuations, spatter number, keyhole depth to width ratio, and keyhole collapse frequency. Furthermore, the improved keyhole stability impeded the keyhole collapse, and the lower weld depth and solidification rate facilitated bubble escaping from the weld pool easily, which are the main factors for suppression of porosity at higher welding speed.

Impact of multi-focus beam shaping on the process stability

Unwanted process defects occur during laser deep penetration welding. The formation mechanisms of the main process defects like spatters and pores are not completely identified yet. Several methods like beam oscillation or laser power modulation are known to help reducing the occurrence of defects. These methods change the temporal energy input. It can be assumed that also beam shaping influences the energy input into the keyhole and has the potential to influence process defects as well. Due to the adapted energy input keyhole instabilities, that are assumed to influence the formation of the defects, can be avoided. A newly developed multi-focus beam shaping optic that creates multiple beam waists in axial direction is used in this work. The additional areas with increased intensities are used to modify the energy input in the keyhole in order to explain the mechanism of spatter formation and evaluate the potential of the axial beam shaping for defect suppression during laser deep penetration welding. It is found that the number of spatters can be reduced when the middle sections of the keyhole are illuminated with a high intensity. It can be concluded that spatters can be reduced when a keyhole collapse is avoided and a high enough energy input is guaranteed in the upper keyhole sections.

In-situ monitoring of laser-based PBF via off-axis vision and image processing approaches

With the development of powder bed fusion (PBF) additive manufacturing technique for functional parts production, process monitoring and diagnosis is highly demanded to ensure its process reliability and repeatability. An off-axis vision monitoring method using high-speed camera is proposed in this paper. An optical filter with 350 nm–800 nm cut-off was used to enhance the image contrast between the plume and the melt pool. A new image processing method was designed to extract features from the melt pool, plume and spatters, respectively. Kalman filter tracking was used to pinpoint the exact melt pool position, and image segmentation algorithm was developed to segment the melt pool, plume and spatters from each other; a new tracking method was utilized to remove the spatters generated in the previous frame. After image processing, the features of melt pool intensity, plume area, plume orientation, spatter number, spatter area, spatter orientation and spatter velocity were extracted and their correlations with the scanning quality were investigated. The results indicated that these features were potential indicators for scanning quality assessment. The proposed method could be used to further study the characteristics of plume and spatter and to explore the diagnosis performance based on the fusion of melt pool, plume and spatter information. It provides a promising means for in-situ monitoring and control of PBF process.

Ultra high power (100 kW) fiber laser welding of steel.

A 100kW fiber laser was first used to weld steel. Speeds at the range between 0.3 and 5.0m/min were tested, and the maximum weld bead depth of 70mm was achieved by single pass welding. Solidification cracking and porosity occurred when the welding speed was lower than 0.5m/min, while undercut appeared when the welding speed was higher than 3.0m/min. Both the ratio of depth to width and the cross section area of the weld bead had a positively linear relationship with the welding speed. A high speed camera was used to observe the characteristics of the keyhole and molten pool. The average number of spatters increased with the welding speed, while the keyhole diameter and the length of the molten pool in front of the keyhole decreased with the welding speed. This Letter validates the application potential of a 100kW ultra high power fiber laser in manufacturing, e.g., welding, cutting, and additive manufacturing.

Localized boiling-induced spatters in the high-power laser welding of stainless steel: three-dimensional visualization and physical understanding

Serious spatter defects occur with the intensive interaction between a high-power density laser beam (> 107 W/cm2) and the material used in high-power (HP) laser welding. In this study, we developed a three-dimensional comprehensive model to physically simulate the keyhole and weld pool dynamics in HP laser welding. Our recent proposed adaptive mesh-refinement method was adopted to solve the model. An interesting phenomenon was directly observed: the intense localized boiling on the mesoscale keyhole wall induced many small spatters (LBiS), sized hundreds of microns, during 10 kW HP laser welding of 304 stainless steel. Experimental studies of parameters were also conducted to validate the model and characterize the spatter behaviors outside the keyhole wall during welding. The theoretical predictions of LBiS behaviors agreed well with the experimental observations. Moreover, the flying directions of LBiS generally approached vertical and were relatively insensitive to welding speed, even when the speed was increased to a very high value of 15 m/min. This finding shows that LBiS are very different from the commonly observed spatter defects produced by the intense friction effect of a vapor plume toward the keyhole, and this finding demonstrates the variety of the spatters. Furthermore, an energy conservation theory was proposed to characterize LBiS, and a novel dimensionless spatter number was formulated to calculate their formation threshold. The spatter number relates to the Reynold and Weber numbers of the molten flow of the weld pool and shows the relative importance of an inertia flow driven by the recoil pressure and the resistance from viscous dissipation, surface tension and gravity. The spatter number was less than 1 when LBiS was formed. The proposed theory was validated against the numerical predictions, and good agreements were obtained.

Analysis of welding process stability and weld quality by droplet transfer and explosion in MAG-laser hybrid welding process

The metallic vapor and molten pool include plenty of welding information in the manufacturing process of MAG (Metal Active Gas Arc Welding)-laser hybrid welding. In this work, double high speed cameras are used to take the real-time images of the top-vapor and bottom-molten pool in the hybrid welding process. In order to monitor welding process status and weld quality, this paper proposes a method which combines the period of droplet transfer and droplet explosion. In time domain, the morphological features include the area of top-vapor, area of spatters, number of spatters and the average gray value of the bottom-molten pool, those features are used to research the stability of short circuiting transfer as well as the mechanism of the welding process. Besides, the welding process stability and weld quality are analyzed by features including the period of droplet transfer and droplet explosion in time and frequency domain. The experimental results show the period of droplet transfer and droplet explosion can effectively reflect welding process and weld quality. It provides a basis for monitoring and controlling MAG-laser hybrid welding process status.