Spatter Formation Research Articles

Adjustable ring mode and single beam fiber lasers: A performance comparison

The welding performance of adjustable ring mode (ARM) and standard single beam fiber lasers was analyzed from a steel welding perspective. The ARM laser has the capability of adjusting the distribution of laser power across beams. Thus, the ARM laser enabled an increase of penetration depth and minimized spatter formation. Furthermore, at the same total power, the ARM laser with higher power in the center than in the ring beam increased the depth of penetration in comparison with a standard fiber laser. However, applying more power in the ring beam than in the center beam noticeably reduced the accumulated spatter on the weld bead and surrounding area.

Types of spatter and their features and formation mechanisms in laser powder bed fusion additive manufacturing process

Spatter causes defect formation, powder redistribution and contamination in laser powder bed fusion (LPBF) additive manufacturing process. It is critical to distinguish different types of spatter and understand their features and formation mechanisms. This work reveals the features and formation mechanisms of five unique types of spatter during the LPBF process by in-situ high-speed, high-energy x-ray imaging. Spatters observed during LPBF testing are quantified by their speed, size, and direction. Distinct quantifiable characteristics for each type of spatter are identified. Effects of the laser power, scan speed, and ambient pressure on spatter formation and features are unraveled. A spatter formation map for AlSi10Mg alloy is constructed.

Powder incorporation and spatter formation in high deposition rate blown powder directed energy deposition

A high deposition rate blown powder directed energy deposition process is presented. Clad tracks are deposited and the process is observed by high-speed imaging. An island of unmelted powder forms inside the melt pool, in the centre of the laser spot, which can be attributed to the highly focussed powder flow and the laser beam configuration used. On contact with the melt pool, the powder grains melt to join the melt pool, or they overcome surface tension and are engulfed by the melt. Powder grains can also incorporate into a mushy zone that may be present on the powder island. The powder island appears to rotate in the melt pool and incorporates relatively slowly. The speed of rotation is connected to the size of the island, which also depends on the energy density used. Spatter can form from the edges of the melt pool or from areas around the island when molten metal droplets burst. Frames from high-speed videos are presented and reasons for the various phenomena observed are discussed.

In situ capture of spatter signature of SLM process using maximum entropy double threshold image processing method based on genetic algorithm

Although selective laser melting (SLM) has the advancements of fabricating complex geometric, multi-material and multi-functional structures, several defects still affect the process stability, in particular spatter. The formation of spatter in SLM depends on the process parameters, and these can potentially be used to tune the process to obtain better product quality. However, there is still a lack of efficient methods for processing spatter images to allow in situ detection of the onset of spatter. In this paper, an in situ monitoring method for acquiring spatter images in SLM was presented. A maximum-entropy double-threshold image processing algorithm based on genetic algorithm (MEDTIA-GA) was proposed to recognize spatter from images, and its results were compared with three conventional threshold segmentation methods: Otsu’s method, Triangle threshold segmentation algorithm, and K-means clustering algorithm. Results show that MEDTIA-GA method was able to eliminate three types of errors: noise sensitivity, spatter conglutination, and spatter omission. In addition, the average processing time of 37 ms for MEDTIA-GA method was far shorter than those for other three conventional threshold segmentation methods. Finally, the relationship between the spatter area as well as the number and the laser energy density were analyzed

Spatter stability: constraining accumulation rates and temperature conditions with experimental bomb morphology

We have developed the first experimental methodology to create a volcanic spatter pile using molten basalt. This method permits reproduction of thermal conditions that yield the wide variety of spatter morphologies observed in nature. The morphology of the clasts is most strongly controlled by the time the clast spends above the glass transition temperature, which is in turn affected by the rate of accumulation and cooling of the deposit. Also, spatter piles that remain hotter over longer durations experience increased fusion between clasts, less void space between clasts, and generally larger aspect ratios. Our experimental method successfully replicated natural microcrystal textures, rheology, and clast size. Work is still therefore required to achieve realistic vesicle distribution and deposit void space. Based on presented experimental work, we estimate emplacement conditions of Southern Idaho spatter vents to have been ~ 850–900 °C, with eruption temperatures closer to 1000–1100 °C. The rapid decrease from eruption temperature to effective emplacement temperature is the result of clast flight as well as equilibrating with the cooler surrounding material. The morphology of the natural clasts matches experiments that have accumulation rates of 2.5–4.5 m/h, which also is consistent with the few measurements made at active eruptions. Finally, we provide a constraint on the temperatures and accumulation rates that can lead to the construction of fused spatter features, as well as provide the steps for future experiments to investigate other aspects (such as compression, impact, and larger sizes) of spatter formation by adapting our methodology.

Controlling interdependent meso-nanosecond dynamics and defect generation in metal 3D printing.

State-of-the-art metal 3D printers promise to revolutionize manufacturing, yet they have not reached optimal operational reliability. The challenge is to control complex laser-powder-melt pool interdependency (dependent upon each other) dynamics. We used high-fidelity simulations, coupled with synchrotron experiments, to capture fast multitransient dynamics at the meso-nanosecond scale and discovered new spatter-induced defect formation mechanisms that depend on the scan strategy and a competition between laser shadowing and expulsion. We derived criteria to stabilize the melt pool dynamics and minimize defects. This will help improve build reliability.

In-situ Synchrotron imaging of keyhole mode multi-layer laser powder bed fusion additive manufacturing

The keyhole mode in laser powder bed fusion (LPBF) additive manufacturing can be associated with excessive porosity and spatter, however, the underlying physics in multilayer build conditions remain unclear. Here, we used ultra-fast synchrotron X-ray imaging to reveal this phenomena. We in investigated melt pool dynamics, keyhole porosity and spatter formation mechanisms and their impact in all layers of the build. We observed that the transient melt pool dynamics associated with the keyhole include: (I) keyhole initiation, (II) keyhole development, and (III) melt pool recovery. Porosity and spatter were associated with stages (II) and (III). We also discovered that droplet spatter can form due to the collapse of the keyhole recoil zone, causing molten particle agglomeration and ejection during stage (III). Our results clarify the transient dynamics behind the keyhole mode in a multi-layer LBPF process and can be used to guide the reduction in porosity and spatter in additive manufacturing.

Effects of Reduced Ambient Pressure and Beam Oscillation on Gap Bridging Ability during Solid-State Laser Beam Welding

In recent decades, beam welding processes have been set up as a key technology for joining applications in automotive engineering and particularly in gearbox manufacturing. Due to their high beam quality, energy efficiency, reliability as well as flexible beam guidance, modern solid-state lasers offer numerous advantages, but also pose increased requirements on the production and positional accuracy of the components for the joining process. In particular, small-focus diameters present a challenge for components with process-induced tolerances, i.e., disc carriers in automatic transitions. Furthermore, welding processes utilizing solid-state lasers show an increased spatter formation during welding at high welding speeds. Accordingly, the primary objective of the presented work consists in extending the current areas of application for solid-state laser beam welding in gearbox manufacturing through an improved process reliability regarding tolerance compensation and spatter formation. Therefore, this experimental study aimed to describe the effects of a dynamic beam oscillation in combination with a reduced ambient pressure in the process environment on both gap bridging ability and spatter formation during the laser beam welding of case hardening steel. For basic process evaluations, laser beam welding at reduced ambient pressure and laser beam welding with dynamic beam oscillation were initially studied separately. Following a basic process evaluation, samples for 2 mm full-penetration-welds with varying gap sizes were analyzed in terms of weld seam geometry and weld spatter formation.

Development of an alternative for corrosive resistant galvanized steel compatible for laser welding

Corrosion is the most challenging issue for any manufacturing industry, particularly for automotive and shipbuilding. Although, galvanized steel has some attractive features like long life, toughest coating, and automatic protection for damaged areas. It is quite difficult to weld the galvanized steel because the formation of spatter cannot be ignored during the joining of galvanized steel. The major reason for the formation of spatter is due to the large difference of melting point between Fe (1538°C) and, Zn (906°C). Therefore, to develop an alternative of galvanized steel which will be more susceptible to joining of galvanized steel is a more challenging task. To full fill the requirement of manufacturing industries, in this research we have suggested how to develop such alloy which will be a strong corrosive resistant keeping intake the properties of galvanized steel as well as easy to do laser welding.

Influence of hydrogen, oxygen, nitrogen, and water vapor on the formation of pores at welding of copper using laser light at 515 nm wavelength

Due to increased absorptivity of laser light at 515 nm wavelength in copper materials, welds with low formation of spatters and homogeneous weld depths can be performed.1 However, depending on the used process parameters, a process regime with strong formation of pores is observed. These pores are not resulting from instabilities of the capillary (process pores), but they are observed to result from the interaction of ambient gas with the melted material in the welding zone. Different gaseous elements such as nitrogen, hydrogen, oxygen, and water vapor, therefore, were applied to the welding process to investigate their influence on the resulting weld seam porosity. For this purpose, all welds were performed in a welding chamber. It is found that the reaction of ambient nitrogen with ambient oxygen during the welding process has a strong influence on the resulting weld seam porosity. Furthermore, pores are observed in the weld seam when the raw copper material contains an increased amount of oxygen. Depending on which copper material is used, strategies are derived to perform welds with low or no formation of pores.

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

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

Vapor plume and melted zone behavior during dissimilar laser welding of titanium to aluminum alloy

The present study deals with continuous Yb:YAG laser welding of pure titanium to aluminum alloy A5754 performed with different beam offsets to the joint line. Spectroscopic and morphological characterization of vapor plume exiting the keyhole was combined with post-mortem observation and energy-dispersive X-ray spectroscopy (EDX) analysis of the welds. The laser beam centered on the joint line resulted in periodic transversal inclination of a vapor jet on the aluminum side associated with a local increase of melt width and an intense spatter formation. Such behavior can be attributed to the instability of the keyhole wall from the aluminum side. The beam offset on the titanium side led to the stabilization of vapor plume, the attenuation of spattering, and a minimal mixing between the two liquid metals. On the contrary, the beam offset on the aluminum side produced a violent formation of spatters and a highly unstable, diffuse vapor jet. In this case, the liquid metals underwent a violent mixing that was progressively reduced along with the decrease in quantity of melted Ti. The observed spectral regions contained the emission peaks of neutral Ti and Mn. Very few emission lines of Al were observed, because they require higher excitation energies compared to Ti and Mn. Boltzmann linearization using Ti emission peaks allowed the estimation of vapor temperature to be between 5000 and 6000 K, except for the welds with the beam offset on Al.

Effect of reduced ambient pressure and atmospheric composition on material removal mechanisms of steel and aluminum by means of high-speed laser processing

Balancing processes require highly precise mass corrections especially in case of high-speed turning rotors. Material removal by means of cw-mode laser radiation represents a novel approach for industrial balancing applications in order to achieve sufficient removal rates. Thereby, spatter formation was identified as primary removal mechanism. In this study, the effect of reduced ambient pressures and atmospheres with varying concentrations of argon, nitrogen and oxygen on spatter formation and loss of mass were investigated for AISI304 (1.4301, X5CrNi18-10) and EN AW-2618 (DIN 3.1924) under the use of a 400W single mode-fiber laser.

Laser welding with side-gas application and its impact on spatter formation and weld seam shape

Spattering is a quality issue within deep penetration laser welding processes resulting in time-consuming and cost-intensive post-processing. In this study, the formation of unconventional weld shapes, e.g. with bulge formation was observed in a lap-joint, while using a side-gas jet to enlargen the capillary opening. During quasi-stationary conditions, spattering could not be detected. It was found that in addition to a side-gas jet, the implementation of conventional center-gas is required. However, the type of shielding gas (N2, Ar, He) used for both, the center- as well as the side-gas does not impact on the formation of the characteristic bulge shape. Furthermore, bulge characteristics strongly depend on the welding speed. Therefore, the formation of latter is attributed to surface tension driven melt flows. In conclusion, spattering can be virtually non-existent if the capillary enlargement resulting from the side-gas jet is kept above a certain threshold.

Numerical Simulation of Melt Hydrodynamics in Laser Micro Processing of Metals

Laser material processing at micro-meter domain can be used for a plethora of applications such as micro drilling, micro welding, micro cutting, micro-texturing as well as micro polishing. In laser micro-cutting, welding and drilling, a high power laser (Intensity ~1010 W/m2) is used to melt and vaporize the material to produce high penetration welds, kerf cuts, and high aspect ratio micro holes. Whereas, in the case of laser texturing, conduction mode welding and polishing, the laser power density is sufficiently low to allow only melting of the material. With the irradiance of high-intensity laser, generation of highly dynamic melt pool occurs due to the presence of vaporization induced recoil pressure, Young Laplace surface tension force, and thermo-capillary shear stress. The melt pool convection results in geometrical defects such as recast layer, porosity, spatter formation, humping, protuberances and melt ripples. These defects pose a severe challenge towards industry utilization of laser micro processing, where precision and repeatability is of utmost importance. Therefore, a competent numerical model considering three co-existing phases (solid-liquid-gas) is developed to predict defects and characterize different regimes through insights on micro-scale melt hydrodynamics. This paper gives an overview of characteristic melt pool hydrodynamics and associated defects in laser micro-processing operations. The developed model has been validated using experiments obtained with 1.07 μm SPI fiber laser.

Acoustic process monitoring in laser beam welding

Structure-borne acoustic emission (AE) measurement shows major advantages regarding quality assurance and process control in industrial applications. In this paper, laser beam welding of steel and aluminum was carried out under varying process parameters (welding speed, focal position) in order to provide data by means of structure-borne AE and simultaneously high-speed video recordings. The analysis is based on conventionally (e.g. filtering, autocorrelation, spectrograms) as well as machine learning methods (convolutional neural nets) and showed promising results with respect to the use of structure-borne AE for process monitoring using the example of spatter formation.

Feasibility of Fillet Edge Weld Using Laser Wobble Technique

In many industrial applications, especially for automotive, aerospace and electronics, laser welding is the preferred joining process due to its high flexibility, short process time and high precision. In conventional single-mode laser welding, the laser beam is fine (typical spot diameter 10 – 40 µm) producing a very narrow weld profile. Wobbling or oscillating the laser beam ensures the weld width and depth of penetration can be achieved in a controlled manner. The wobble technique is most useful for highly conductive and reflective materials as it helps to reduce the spatter formation, back-reflection and instability with the laser. Joining with an overlap configuration created using the wobble technique have been most commonly investigated. But the fillet edge weld is considered more useful for sealing applications without the need for additional filler material.This paper investigates the feasibility of producing fillet edge welds, using the wobble technique, to join 0.3 mm thin aluminium to 1.5 mm thick aluminium sheet. A 1.0 kW CW IR laser was used to produce the fillet edge welds aiming for both joining and sealing applications. The effects of process parameters including laser power, welding speed, wobble amplitude and wobble frequency were studied to evaluate the weld quality. Joint strength and weld microstructural features including penetration depth and interface width were evaluated as indicators of weld quality. The relations between the input laser process parameters and output quality indicators were identified and the weld quality was classified into under-weld, good-weld and over-weld categories. In the case of the good-weld condition, average lap shear strength was around 418.64 N. The results suggested that preferred penetration depth and interface width can be obtained by controlling the laser power, welding speed and wobble amplitude. Overall, this paper demonstrates the feasibility of obtaining good fillet edge joint using laser wobble technique.

Characterization of laser spatter and condensate generated during the selective laser melting of 304L stainless steel powder

The selective laser melting process, commonly referred to as laser powder-bed fusion (L-PBF), is an Additive Manufacturing (AM) technique that uses a laser to fuse successive layers of powder into near fully dense components. Due to the large energy input from the laser during processing, vaporization causes instabilities in the melt pool leading to the formation of laser spatter and condensate, collectively known as heat-affected powder. Since heat-affected powder settles into the powder bed, the properties of the unconsolidated powder may be altered compromising its reusability. In this study, characterization of 304 L heat-affected powder was performed through particle size and shape distribution measurements, energy-dispersive spectroscopy, Raman spectroscopy, inert gas fusion, metallography, and x-ray diffraction. The results show morphological, chemical, and microstructural differences between the virgin powder and heat-affected powder formed during processing which aid in the understanding of laser spatter and condensate that form in the L-PBF process.

Influence of spatter particles contamination on densification behavior and tensile properties of CoCrW manufactured by selective laser melting

Due to spattering particles forming during Selective Laser Melting, powder pollution may severely affect the quality of the fabricated components and cause defects inside the microstructure to degrade the mechanical properties. Therefore, investigations into the change in powder characteristics and related microstructure defects, tensile properties are needed. In this paper, the influence of powder re-using time on Selective Laser Melting-based fabrication of CoCrW alloy was studied. The spattering process and the powder pollution caused by spatter particles were observed via high-speed imaging. The densification mechanism and the evolution of the microstructure of the CoCrW printed components were analyzed. The mechanical properties of the components printed using powders polluted at a varying degree by the spatter were tested at different stages of the life cycle. Results indicate that the Selective Laser Melting-generated spatter particles look more complex than the virgin powder particles. Compared with laser irradiating on the substrate, much more spatter escaped from the molten pool as the laser irradiated on the powder; the powder showed a trend towards increased particle size diameter through repeated recycling; residual pores distribution state and size applied with the various powder usage times were analyzed, pores with the average size of 300 μm could be achieved when powder recycled for the sixth time. At last, it is concluded that the spatter severely affects the mechanical properties of the fabricated components, as the tensile strength, yield strength and elongation of CoCrW alloy was reduced from 1284 MPa, 855 MPa and 11.15% to 874 MPa, 689 MPa and 3.82%, respectively. This gives insights into the influence of powder features on Selective Laser Melting-fabricated components and into improvement in stability of Selective Laser Melting manufacturing processes by controlling spatter formation.

Numerical Analysis of Molten Pool Behavior and Spatter Formation with Evaporation During Selective Laser Melting of 316L Stainless Steel

During the selective laser-melting process, material evaporation and resultant spatter are common phenomena that bring about many defects. However, the underlying physical phenomena such as molten pool behavior and spatter formation, as evaporation occurred, are sparsely understood and difficult to observe during the process. Thus, a three–dimensional powder-scale model was established to investigate the thermal and flow behavior of the molten pool, the morphology evolution of the molten pool and keyhole, and the spatter formation with evaporation in the selective laser-melting processing of 316L stainless steel. Phase transitions and variations in interfacial force were taken into account in this model. The modified phase-field method was applied to trace the melt–gas interface. The results showed that keyhole formed in molten pool under recoil pressure, and that there were some differences in the temperature distribution and flow behavior inside and outside the keyhole. In addition, the dimension and surface morphology of the molten pool and keyhole depth altered and gradually stabilized during the process. Moreover, droplet spatter formation at rear of molten pool proceeded roughly as follows: the backward ejection of melt vapor would induce a depression in molten pool and a bump on the molten pool surface, the bump moved backward and subsequently a liquid column formed, then the liquid column formed a neck and gradually pinched off, resulting in the droplet spatter. Furthermore, the characteristic morphology of the scan track and dimension of the molten pool were obtained through experiments, which showed good agreement with the simulation results.