Spatter Formation Research Articles

A study on the effect of energy input on spatter particles creation during selective laser melting process

Selective laser melting (SLM) is a promising manufacturing technique for the production of complex metallic components. One of the crucial factors influencing the mechanical properties of the final product is spatter particles formation during the process. In this study, high- speed photography is utilized to record the formation mechanisms and the dynamic behavior of spatter particles. An image processing analysis framework is utilized to assess the distribution of spatter particles under various energy inputs. It is found that changing the laser scan velocity has more influences on spatter formation in comparison with the energy input. The relationship between the numbers of created spatter particles, induced unmelted regions and density variability are interpreted and discussed based on other observations, such as microscopic examination and density analysis of SLM parts. The obtained results could be used to enhance the current manufacturing process parameters optimization methods in SLM process.

Numerical study of spatter formation during fiber laser welding of aluminum alloy

The weld convection and spatter formation in aluminum alloy fiber laser welding are investigated based on numerical simulation and high speed photography results. It is found that a clockwise flow pattern exists in the rear part of the keyhole. The molten metal at the top surface of the rear part flows to the edge of the weld pool, owing to the surface tension gradient. The spatters generate mainly around the keyhole. The spatter formation sequence is that the formation of the swelling around the keyhole caused by the upward flowing molten metal, the increasing of the swelling size caused by the accumulation of vertical momentum, and the spatter ejection from the keyhole when the momentum of the molten metal is sufficient to overcome the surface tension. The recoil pressure caused by evaporation and the shear stress caused by metallic vapor flow are responsible for the spatter formation.

Numerical simulation of spatter formation during fiber laser welding of 5083 aluminum alloy at full penetration condition

The droplet escape condition in laser welding is established in this paper. A three-dimensional numerical model is developed to study the weld pool convection and spatter formation at full penetration during the fiber laser welding of 5083 aluminum alloy. It is found that when laser power is 9 kW, the bottom of the keyhole is dynamically opened and closed. When the bottom of the keyhole is closed, the molten metal at the bottom of the back keyhole wall flows upwards along the fusion line. When the bottom of the keyhole is opened, few spatters can be seen around the keyhole at the top surface, two flow patterns exists in the rear part of the keyhole: a portion of molten metal flows upwards along the fusion line, other portion of molten metal flows to the bottom of the keyhole, which promote the spatter formation at the bottom of the keyhole rear wall.

MECHANICAL PROPERTIES OF AA5754 SHEETS WELDED BY COLD METAL TRANSFER METHOD

Cold Metal Transfer (CMT) is a modified gas metal arc welding method. This method is different from conventional gas metal arc welding methods. In this process the heat input and the formation of spatter are reduced with the control of the wire movement and arc. So, CMT welding became indispensable in the welding of thin sheets. The importance of lightweight structures in vehicles is increasing day by day, so the use of thin sheet aluminum alloys such as AA5754 is becoming increasingly widespread. In present study, 2mm thick AA5754 sheets were joined with cold metal transfer welding using ER5356 (AlMg5) filler wire. The effect of heat input on the weld quality was investigated. Microstructural examinations were executed by optical microscope and SEM. Mechanical properties of joints were determined by tensile and bending tests. It was found that increasing heat input did not have a significant effect on tensile strength of AA5754-AA5754 joints, while it corrupted the bending strength.

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.

An octree-based adaptive mesh refinement method for three-dimensional modeling of keyhole mode laser welding

Numerical simulations of laser welding could provide unprecedented insight into the physical details of the welding process; however, it remains a central challenge to understand laser welding with a high-efficiency and high-resolution method. This paper reports an octree-based adaptive mesh refinement method for efficient process modeling of laser welding in three dimensions. Based on multiresolution analysis algorithm, we also propose a hybrid adaptive mesh refinement strategy depending on local temperature, fluid velocity, and volume of fluid (VOF) value of weld pool of laser welding. Using the proposed method, the predicted heat transfer, and fluid flow behaviors of weld pool are consistent with experimental and previous theoretical results. Spatter formation at the beginning stage of laser welding is observed with a grid resolution in tens of microns. Under certain circumstances, the liquid jet can be produced when the fluid speed near the top surface of the weld pool is 1m/s. The time for breakup of a liquid jet into spatters is about 0.065ms. Furthermore, using the method, we just need 7hours to complete a simulation of a 40-ms real laser welding process on a small workstation (2.60GHz CPU, 16 cores), comparing with 120hours needed using uniform grids. In a word, this method shows great potential for efficient computer simulation-based process optimization of laser material processing.

Understanding of spatter formation in fiber laser welding of 5083 aluminum alloy

The droplet escape condition is established, a three-dimensional numerical model is built to investigate the behavior of the weld pool in fiber laser welding of 5083 aluminum alloy, the shear stress caused by the metallic vapor flow that has great influence on spatter formation is considered. A high speed photography system is used to capture the transient images of the weld pool, the spatter formation is discussed. The numerical and experimental results show that the spatters generate mainly around the keyhole. Two factors are responsible for the spatter formation: the surface tension of the molten metal around the keyhole is low. The recoil pressure caused by evaporation and the shear stress caused by metallic vapor flow accelerate the upward moving melt around the keyhole. Increasing the welding speed and placing the nozzle at the trailing position can decrease the upward momentum of the molten metal around the keyhole, and stabilize the keyhole, which are help to suppress the formation of spatter.

Spatter formation in selective laser melting process using multi-laser technology

This study demonstrates the significant role of recoil pressure in selective laser melting (SLM) process using multi-laser technology. High-speed photography is utilized to observe the formation mechanism, and also the behavior of spatter particles during SLM fabrication. A computational image analysis framework is developed to assess the size and the number of induced spatters. The morphology and the composition of spatters and their influence on the surface of the fabricated parts are determined. Unmelted regions, resulting from spatter deposition into the powder or the solidified layer, are found to be a detrimental type of defects on mechanical properties of SLM parts. This is followed by a discussion on demand for developing a meaningful process parameters optimization to enhance the mechanical properties of SLM products.

Investigations of Corrosion Resistance of Laser Separated Open Cell Metal

Open cell metal foam contributes promising solutions to the light weight design, battery applications, and renewable energy. Still, challenges are present concerning the cutting into a defined shape. Mechanical processes like milling and grinding create surface smearing on the foam, which leads to a loss of their open cell behavior. Laser remote cutting offers a promising solution in order to overcome those challenges. Their investigations consider that this technique has a high potential concerning cutting speed, which is increased by more than 500% compared to state of the art laser separation techniques. Nevertheless, quality improvements regarding spatter formation and fouling or corrosion resistance has to be investigated. This paper offers insight into the viability of remote laser cutting and how quality aspects can be improved.

Material properties of CMT—metal additive manufactured duplex stainless steel blade-like geometries

Impeller blades are often individual and complex-shaped components made of challenging metals. As the manufacturing of such blades is highly sophisticated, only a few companies worldwide possess the necessary processing knowledge and that is why long production times have to be accepted by customers. To overcome this economic disadvantage, manufacturing technologies are permanent under supervision and it seems that metal additive manufacturing could thereby play an important role in future. In this paper, wire arc additive manufacturing (WAAM) based on gas metal arc welding (GMAW) is considered. Shape-giving GMAW is well known in industrial manufacturing, but its application is limited due to restrictions by the welding process itself: For thinner wall thicknesses, a significant reduction of the weld process energy is required which increases the risk of process instabilities and spatter formation. Extensive welding process-related efforts have been undertaken to overcome this fact and a new GMAW process, called CMT (Cold Metal Transfer) was introduced. CMT is based on a high-frequency forward and backward movement of the welding wire electrode and provides an almost spatter-free and absolute precise, periodic detachment of accurately defined droplets from the filler wire at very low process energies. In combination with an accurate robotic movement of the CMT welding torch, geometries with minimum thicknesses in the range of 2–4 mm can be build up layer by layer. Additionally, a broad range of different, well established and third party-approved GMAW filler metals for joining is available. In this work, an impeller blade-like geometry out of duplex stainless steel has been manufactured by CMT using a filler wire type G 22 9 3 N L. The investigations have shown that the achieved surface roughness is comparable to sand casting and the microstructure is without any evidence for porosity and lack of fusion. Furthermore, an austenite/δ-ferrite weld microstructure with partly preferred grain orientations and a δ-ferrite content of around 30FN exists. The measured mechanical properties, especially strength and toughness, are comparable to data provided by the filler metal data sheet.

Keyhole stability during laser welding—Part II: process pores and spatters

Spatters and pores that occur during laser deep penetration welding are unwanted defects that make post processing necessary. Therefore, these defects need to be avoided. The origin of pores and spatters are assumed to be in the process dynamics, especially the highly dynamic vapor channel. The keyhole is assumed to be responsible for producing these defects. Based on keyhole calculations from Part I, in this paper, calculated keyhole properties are correlated to pore and spatter formation observed during experiments. Spatters are recorded using a high speed camera and a spatter detection routine for measuring spatter speed, size and number per time. Pore number per time and pore sizes are detected using X-ray imaging after welding. Temporal spatter and pore characteristics can be correlated to dynamic values of the keyhole. The analytical model can predict spatter and pore formation depending on local frequencies and amplitudes in the keyhole. When using different beam profiles, extremely high local frequencies of the Top Hat beam at smaller keyhole dimensions seem to lead to an increased spatter and pore formation. The spring coefficient can be used as an indicator to predict spatter and pore characteristics.

Elucidation of the effect of welding speed on melt flows in high-brightness and high-power laser welding of stainless steel on basis of three-dimensional X-ray transmission in situ observation

This research was performed with the objective of clarifying the effect of welding speed on melt flows during melt-run welding of SUS304 stainless steel plates with a 6-kW power laser beam on the basis of three-dimensional X-ray transmission in situ observation. As welding speed increased from 25 to 250 mm/s, three kinds of welds characterized by porosity formation and no defects or underfilling due to spatters were produced. The average and the maximum values of measured melt flow velocity were 3 and 10 times higher than the welding speed, respectively. Two kinds of circulation flows at the inlet or the tip of a keyhole were confirmed to control heat transfer in a molten pool. It was found that the circulation flows were so sensitive to the welding speed that bubbles resulting in porosity or spatters were often formed. According to X-ray observation of the formation of spatters with tungsten carbide (WC) tracers, as the melt flow rose along the keyhole wall, the velocity was accelerated from 0.24–0.54 m/s near the keyhole inlet. Consequently, the melt flows made the convex surface behind the keyhole grow higher, resulting in spattering.

Modeling for GMAW process with a current waveform control method

Aimed at reduced spattering and better weld formation, we adopted a refined method of current waveform control, and reported a model for GMAW comprising four modules that were inverter power supply, arc load, wire feeder system and waveform control system. SIMULINK tools of MATLAB were implemented to establish this model, and the simulation results were compared with our experiments to comprehensively evaluate the influence of the large constant current and the small constant current of arc stage on the welding outcomes. In the short circuit transfer process, the base short-circuit current duration and the peak short-circuit current had great impacts on spatter generation whereas the effect of the current surge rate was relatively small within a certain range. Parameters of waveform control method we obtained and optimized based on this model were confirmed effective and efficient for a high stability of welding process.

Influence of different zinc coatings on laser brazing of galvanized steel

The use of hot-dip galvanized steel sheets as base material for laser brazing led to the formation of spatter and a wavy appearance of the seam. High-speed imaging revealed that spatter formation occurs mainly close to the seam edges. Thermography of the brazing process showed that the type of zinc coating affects the resulting temperature distribution and thereby changes the conditions for the zinc evaporation. Higher temperatures were observed at the seam edges on electro galvanized and phosphated substrates. The cross sections and the process analysis indicate that the zinc evaporation on electro galvanized substrates takes place prior to the wetting with the molten filler material. On the other hand a merely partial evaporation of the zinc coating and the accumulation of zinc at the seam edges takes place during the brazing of hot-dip galvanized substrates. The formation of spatter is caused by the zinc dissolution and the subsequent evaporation of the zinc rich alloy at the seam edges. The optical properties of the base material are considered to be a crucial factor for the temperature distribution and the resulting zinc evaporation. Brazing trials on hot-dip galvanized and phosphated substrates, which show a reflectance similar to electro galvanized and phosphated specimens, confirm this assumption, as no spatter formation at the seam edge was observed.

Microstructure and Mechanical Properties of Hybrid Welded Joints with Laser and CO2-Shielded Arc

With the objective of reducing the operating costs, argon-rich shielding gas was replaced by 100% CO2 gas in hybrid laser-arc welding of shipbuilding steel. The welding parameters were optimized to obtain buried-arc transfer in order to mitigate spatter formation. Sound butt joints could be successfully produced for plates of 14 and 17 mm thickness in one welding pass. Subsequently, the welded joints were subjected to different tests to evaluate the influence of CO2 shielding gas on the mechanical properties of the welded joints. All tensile-tested specimens failed in the base material, indicating the higher strength of the welded joints. The impact toughness of the welded joints, measured at −20 °C, reached approximately 76% of that of the base material, which was well above the limit set by the relevant standard. The microstructure of the fusion zone consisted of grain boundary ferrite and acicular ferrite uniformly over the plate thickness except for the joint root where the microstructure was chiefly ferrite with an aligned second phase. This resulted in higher hardness in the root region compared with the top and middle parts of the fusion zone.

Analysis of spatters in laser welding with beam oscillation: A machine vision approach

Spatter formation is still an issue in welding, particularly in laser welding. This research takes up the topic and proposes a machine vision approach for spatter tracking in high speed image series. After a description of the multi hypothesis tracking method, in which a Kalman-filter is used for optimal object state estimation, the tracking is applied on high speed images from an experimental series on laser welding with beam oscillation using stainless steel. On the basis of its results three different spatter formation mechanisms could be identified and described. Those were spatter formation by material ablation, by periodic re-entry of the laser spot into the melt pool and by melt pool dynamics.

A study of micro-channel size and spatter dispersion for laser beam micro-milling

ABSTRACTDuring micromachining the accuracy of micro-feature is always a major concern. Using laser beam micro-milling (LBMM) the produced size is generally larger than the input design size. Another important drawback of LBMM is the formation of melt spatter around the feature geometry. The spatter adheres within the premises of the machined feature and influences the machining results, especially the geometry of previously machined features by adhering inside or around the feature. Determination of spatter size is very important when the objective is to produce an array of micro-features such as micro-channels. Thus, the minimum distance between two consecutive micro-features is necessary to be known in order to achieve safe geometry for which the size of spatter plays a significant role. The knowledge of spatter scattering and pre-calculated distance of dispersing spatter could certainly assist the machinist in estimating the safe distance between adjacent micro-channels. In this study, the micro-channel size and average spatter size are studied during the micro-milling of Ti-6AL-4V by Nd:YAG laser beam machining (LBM). Response surface methodology (RSM) has been utilized to plan a set of experiments and mathematical models are proposed to pre-calculate the channel size and spatter dispersion. Finally, the models are validated through confirmatory tests showing an acceptable range of error.

Studies on the spatter behaviour when welding AA5083 with a Yb-fibre laser

The processing advantages of welding with high brightness and 1 μm wavelength laser sources have been well reported. However, it is also reported that such high brightness laser sources, whilst providing the necessary penetration depth and welding speed, may result in unacceptable levels of weld spatter. So far, the effects of process parameters on spatter formation have not been well studied on aluminium alloys. In this paper, systematic experimental trials were carried out on the AA5083 aluminium alloy plates to study the effects of different process parameters on the spatter generated. High speed imaging and numerical modelling (using the computational fluid dynamic, CFD) were used to understand the formation mechanisms of the spatter. Results show that the process parameters having the most influence on spatter formation are laser power, welding speed × focal length of the focusing unit, laser power squared × welding speed, laser power squared × focal length of the focusing unit. The welding process in aluminium alloy AA5083 aluminium alloy is quite unstable, the occurrence of spatter ejections is closely related to the turbulence in the melt pool, and full penetration welds tend to produce less spatter than partial penetration welds. To minimise spatter ejection, it is recommended that a high welding speed is used with the laser power just sufficient to achieve full penetration the joint geometry, and the focus position of the laser beam should be appropriately away from the top surface of the workpiece.

Employment of fiber laser technology to weld austenitic stainless steel 304 l with aluminum alloy 5083 using pre-placed activating flux

Abstract The overlapping welding was carried out in keyhole mode between austenitic stainless steel 304 l and aluminum alloy 5083 using a low power fiber laser in continuous irradiation. The significant content of magnesium as the alloying element with low boiling point and high vapor pressure inside the AA 5083 matrix can induce the spatter formation and depression on surface of the weld beads upon laser beam absorption and temperature growth which can deteriorate the mechanical properties and appearance of the joints. To reduce these defects, a variety of single and multi-components activating fluxes including oxide-based TiO2 and halide-based CaF2 flux powders were pre-placed on the surface of welding material prior to laser welding. The EDX and XRD analyses in addition to microhardness and shear tests were carried out to characterize the joints. The obtained results showed that, the oxide and halide activating fluxes can significantly improve the joints' strength up to 1.48 and 1.85 times in average respectively compared with autogenous joint. It was deduced that the simultaneous effect of significant decrease in joints' surface depression leading to welds' geometry improvement in addition to less formation of interfacial Fe–Al intermetallics, were the major causes for considerable strength improvements.

Characteristics of the effect of swirling gas jet assisted laser percussion drilling based on machine vision

In this paper, a machine vision-based system is used for in-situ monitoring and characterization of the effect of swirling gas jet assisted laser percussion drilling. Feasibility of laser percussion drilling on highly reflective materials, i.e., Al5052 with the implementation of assisting swirling gas jet, was explored. The influence of the gas jet type on the mechanism of material removal and resulting spatter is reported. The authors conclude that the swirling gas jet method greatly reduces spatter formation around the hole exit during the laser-drilling process. Consequently, in order to understand and monitor the gas jet effects in laser drilling, real-time in situ metrology is employed to observe the details. Compared with the results obtained when using a straight gas jet, laser drilling with a swirling gas jet reduces the drilled hole debris by 0.69 and increases the drilled hole efficiency by a factor of up to 1.5. Experimental results show that both the drilling depth and machining quality can be greatly and simultaneously improved. Effects of the swirling flow on the drilling efficiency and spatter thickness of the holes are investigated.