Spatter Deposition Research Articles

Spatter transport in a laser powder-bed fusion build chamber

The adverse effects of spatter particles are well known in laser powder-bed fusion (LPBF) additive manufacturing. To prevent the deposition of spatter particles, an inert gas flow is commonly used to transport these spatters away from the build plate. However, the inert gas flow does not remove all spatters due to varying spatter sizes and ejection angles. Therefore, it is essential to understand and predict spatter trajectories to achieve superior LPBF parts. The present study focuses on numerical modelling of spatter trajectories in Renishaw AM250 using an Eulerian-Lagrangian discrete phase model. The argon velocity profile and spatter trajectories with and against the flow are computed for various materials, sizes and ejection angles. The simulation results are validated with experimental results and show a presence of uneven flow due to inlet geometry along with varying flow profiles across the build height due to inlet location. Spatter analysis shows three methods which result in spatter deposition. Spatter particles either fall directly on the build plate, are transported by the airflow or are re-directed in the recirculation zone. The findings presented in this work indicate the importance of build chamber design along with material-based parameter optimization that results in maximum spatter removal.

Effects of spatter deposition and build location in laser powder bed fusion of maraging steel parts

Effects of spatter deposition and build location in laser powder bed fusion of maraging steel parts

Optimization of micropore fabrication on the surface of ultrathick polyimide film based on picosecond UV laser

Micropores fabricated on organic polymer films have a wide range of applications in fields such as microfiltration, new energy, and biomedical separation. The use of laser processing technology can complete the processing of micropores on the surface of ultrathin films with high precision, but there is still some difficulty in the processing of ultrathick films. In this paper, a picosecond ultraviolet (UV) laser was used to explore the high-precision manufacturing process of micropores on the surface of ultrathick polyimide (PI) films. The effects of laser power, laser frequency, and scanning speed on the cone angle and spatter deposition area of micropores’ fabrication on ultrathick PI were studied based on orthogonal experiments. The mechanism of processing micropores on ultrathick PI was analyzed by studying the deposition area and morphology of the spatter generated during the laser ablation process. It was found that high-quality micropores can be fabricated at low laser frequency and high power.

Efficient milling and cutting of borosilicate glasses through a thin flowing water film with a picosecond laser

Direct laser ablation allows high-quality processing, precision, and cutting of complex shapes. Unfortunately, this technology is usually not considered as the primary candidate for cutting and milling of glasses due to low processing speeds. Thus, laser-based rear side drilling and crack generation techniques are chosen instead. However, we report on the milling and cutting of borosilicate glass plates with high average power and high pulse repetition rate picosecond laser. We optimised laser processing parameters for the efficient direct material ablation and employed water-assisted ablation to improve the ablation efficiency and the cutting speed of borosilicate glasses. The laser process was realised through a thin flowing water film, which was formed by spraying water mist on the surface of the workpiece. Firstly, we characterised thin water film parameters, such as film thickness, flow velocity, and defined a working area with constant processing conditions. Next, we optimised laser processing parameters for both processing environments: ablation through the thin water film and in air. Under optimal parameters, ablation through the thin water film was much more effective, increasing the cutting speed of 420 μm thick glass plates nearly ten times – from 0.52 to 5 mm/s. Furthermore, the morphology of the cuts was improved as well: cut wall roughness and edge chipping were reduced, spatter deposition was avoided in the vicinity of the cut. We investigated the milling of 130 to 1700 μm wide and 50 to 500 μm deep grooves with a maximum depth-to-width ratio of 3.8. Findings revealed that the ablation efficiency increase provided by the water layer did not depend on the depth or the width of the groove and remained constant. Therefore, it became possible to predict the ablation efficiency through the thin water film by knowing the ablation efficiency in air and an absolute ablation efficiency difference between two processing environments. • Efficient borosilicate glass milling and cutting in the air and through the water film was conducted with a picosecond laser. • Optimal laser fluence for efficient glass ablation was 40% higher than in air. • An absolute change in ablation efficiency between two environments did not depend on the transverse dimensions of the groove. • Water film improved ablation efficiency significantly, resulting in nearly 10 times higher cutting speed than in air.

Characterization of micro-holes drilled in alumina ceramic by the combined pulse laser technique.

Nowadays, combined pulse laser (CPL) technology has shown obvious advantages in alumina ceramic drilling. However, the characterization of micro-holes drilled by the CPL is not clear. In this paper, micro-holes drilled by ns-ms and ns/ms CPLs are systematically compared from the aspects of hole diameter, cracks, spatter deposition, recast layer, re-solidified particles, grain size, and chemical composition. The results show that due to the synchronous output of the nanosecond laser, the ns/ms CPL can eject more melt through expelling of the plasma shock wave; thus, the recast layer, re-solidified particles, and oxygen vacancies are decreased, while the spatter deposition is increased. On the other hand, due to the higher temperature and larger temperature gradient introduced by the nanosecond laser, the hole diameter, cracks, and grain size are increased. Therefore, an ideal CPL method to optimize the drilling performance is proposed. The research results have important guidance for improving the processing quality of the CPL, especially for alumina ceramic laser processing.

Implications of two backward blood spatter models based on fluid dynamics for bloodstain pattern analysis

Bloodstain pattern analysis (BPA) is an integral part of crime scene investigation. For violent crimes involving gunshots, standard practice in police departments worldwide have some physical limitations. For instance, the effect of gravity and air drag on trajectories of blood droplets are neglected using current reconstruction methods, which results in a well-known overestimation of the height of the source of blood. As a consequence, more sophisticated models for blood spatter trajectory reconstruction are being developed, two of which are highlighted in the present work. They allow the prediction of bloodstain patterns produced from backward spattered blood droplets from blunt and sharp bullets. Our recent models attribute the splashing of blood to the Rayleigh–Taylor instability which arises when blood is accelerated towards lighter air. This physically-based description comes with the powerful predictive capability to correlate features of bloodstain patterns with the specific bullet and gun that produced them, as well as with the body position. The results of the numerical models were compared with four experiments simulating blood spatter deposition on a vertical wall through the number of stains produced, average stain area, and average impact angle at the surface, and the agreement found is fairly good. Moreover, further insight is obtained by probing and explaining the influence of observable parameters on the resulting spatter pattern, with the goal of aiding BPA experts evaluating a crime scene.

Quality Evaluation of Micro Drilled Hole Using Pulsed Nd:YAG Laser: a Case Study on AISI 316

Laser drilling is increasingly becoming a suitable choice for performing micro-drilling on advanced engineering and bio-compatible materials. In laser drilling operation, two types of major defects like spatter deposition and taper are generally encountered. In order to reduce the drilling defects, understanding the influence of process parameters on quality of hole is highly desirable. The influence of process parameters such as laser energy, pulse repetition rate, pulse width and gas pressure on the quality characteristics viz. circularity at entry and exit, taper and spatter area is studied in this work using Taguchi’s L27 orthogonal array. Analysis of variance (ANOVA) indicates that pulse width is most influencing parameter having percentage contribution of 43.981, 51.93, 88.28 and 39.94 for circularity at entry, circularity at exit, taper and spatter area respectively. It is observed that spatter area and heat affected zone (HAZ) increases with increase in laser energy and pulse width. This is due to the fact that heat input increases causing more material to melt but sufficient time is not allowed to completely flush away the molten material. As a result, heat is not dissipated adequately resulting in spatter formation and heat affected zone. The analysis also reveals that increase in pulse repetition rate and gas pressure leads to decrease in spatter area. Increase in pulse repetition rate causes increase in laser power leading to formation of laser supported absorption (LSA) wave which may cause the blockage of input energy. Hence, material ejection becomes less and scope for formation of spatter turns out to be less. Micrographs of drilled surface are analyzed using scanning electron microscope (SEM). The micrograph study suggests that increase in laser energy leads to increase in HAZ and circularity. The study also reveals that increase in pulse repetition rate leads to increase in burr formation in the hole periphery. Empirical models have been proposed to predict the quality characteristics of drilled hole. Mean relative error within 10% between the predicted value from empirical relations and experimental value for quality characteristics of hole indicates the validity of the proposed models. In order to obtain optimum parametric setting for all the quality characteristics, a well-known evolutionary approach known as genetic algorithm (GA) is used. The suggested best parametric conditions are definitely useful for the practitioners to avoid trial-and-error process of setting the process parameters for obtaining laser drilled holes of desired quality.

Experimental Investigation Of Quality Characteristics In Nd:YAG Laser Drilling Of Stainless Steel (AISI 316)

Micro-machining of advanced engineering materials such as stainless steels, titanium alloys, nickel alloys, and ceramics is a challenging task due to properties like high strength to weight ratio, toughness, slenderness ratio, and low thermal conductivity. Stainless steel of grade AISI 316 has wide applications in medical science, automobile engineering, aviation, and aerospace industry due to their favourable material properties such as low thermal conductivity, high corrosion resistance, and high strength to weight ratio. Micro-machining of AISI 316 with high aspect ratio is a difficult task due to excessive heat generation, micro-structural changes, and tool breakage. Laser beam machining (LBM) process is a suitable alternative machining process, since it localizes the heat source to control the micro-structural changes. The difficulty arises during laser drilling because of spatter area and heat affected zone (HAZ), which adversely affects the quality of laser drilled holes. The present study attempts to experimentally investigate the effect of laser process parameters such as pulse width, laser energy, pulse frequency, and flushing pressure on the performance measures such as spatter deposition and heat affected zone (HAZ) during drilling operation. To reduce the total number of experimental run and obtain maximum information for the experimental trials, Taguchi L27 has been adopted. Analysis of variance (ANOVA) is performed to identify significant laser parameters influencing both the performance measures. From the study, it is revealed that pulse width is the most significant parameter in the formation of spatter and HAZ. From the results, it is identified that spatter area initially increases with increase in pulse frequency and then decreases for any level of pulse width. As pulse frequency increases, laser power increases resulting in more heat input into the material. This causes increase in vapour pressure inside the laser drilled hole, more material ejection, and increase in spatter area. However, laser supported absorption (LSA) waves developed at higher value of pulse frequency brings about blockage of laser energy from the material surface as an engrossing plasma, which results in reduction of material ejection and reduction in spatter area. It is observed that HAZ increases with increase in pulse width and pulse frequency. It may be due to higher average power of the laser beam, which is directly proportional to pulse width and pulse frequency. Higher the value of pulse width, higher will be the laser thermal energy and higher HAZ.

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.

Assessment of drilling characteristics of alumina coated on aluminium using CO2 laser

Abstract This paper reports experimental investigation of effect of laser parameters on the drilling responses of alumina coated aluminium substrates using 1.8 kW CO 2 pulsed mode laser. Alumina was deposited on 4 mm thick Al 6065 plates by plasma spray technique. The main influencing factors were: Laser power on spatter deposition and Heat Affected Zone; Pulse frequency on entrance and exit circularities; Piercing time on taper, heat affected zone and exit circularity. Genetic algorithm optimization results indicated simultaneous improvement of 5.2% in hole quality at 416.6 W laser power, 1.4 kHz pulse frequency and 0.4 s piercing time. Response Surface Methodology indicated minimum spatter deposition area of 11.57 mm 2 , Heat Affected Zone 0.589 mm, taper 2.35° and maximum circularity at entrance 0.88 and exit 0.905 at 400 W laser power, 1.2 kHz frequency and 0.48 s piercing time. These results were in close agreement with those of Genetic algorithm. The optimum values were validated through confirmatory experiments.

Effects of material combination on laser beam drilling of a metal foil using a cover plate

ABSTRACTThis research investigated the microdrilling characteristics of metal foils depending on the materials of the cover plates and metal foils in the cover plate-laser beam machining (c-LBM) process, which is a method to achieve better quality in metal foil machining with a given piece of equipment. Laser beam drilling using a nanosecond pulsed laser was carried out on 10-µm-thick stainless steel 304 (STS304), nickel, and copper foils with 100-µm-thick cover plates of each material. Consequently, STS304 was found to be an effective cover plate material for reducing the hole diameter and spatter deposition on metal foils. Compared to the results without using a cover plate, the average hole diameter and the area of spatter deposition decreased by up to 77% and 96%, respectively, by using the STS304 cover plate. Meanwhile, the thermal deformation of the STS304 and nickel foils was prevented by using a cover plate, while the copper foil was barely deformed even without a cover plate. Lastly, it was remarkable that the copper foil was drilled with approximately 67% lower pulse energy than the effective minimum pulse energy required to drill it by using the STS304 cover plate, resulting in a smaller hole with little spatter.

Precise micromachining of yttria-tetragonal zirconia polycrystal ceramic using 532 nm nanosecond laser

High quality micro-sized steps and blind hole structures without microcracks, chips or spatter deposition were machined on yttria-tetragonal zirconia polycrystal (Y-TZP, 3mol% yttria) by nanosecond laser (wavelength=532nm, pulse width ~6ns). The diameter of blind hole is 500μm and each step is 500±10μm wide and 100±5μm deep. The 1.35mm3/min removal rate and the smooth machined surface with Ra=2.824μm roughness depicting the high precise and efficient processing were achieved. The ablation characteristics of nanosecond laser process of Y-TZP ceramic were also studied. Based on the study, a reasonable design of the processing path for micromachining of a finer embedded step with 24±2μm width (smaller than the 60μm focused spot size) around the inner-wall of a 2×2mm2 cavity was developed. These results and discussion offer new possibilities in the manufacturing of bio-ceramic products by nanosecond laser with high processing quality and efficient.

Micro-dimple array fabricated on surface of Ti6Al4V with a masked laser ablation method in air and water

Abstract A method called masked laser ablation (MLA) was proposed to fabricate a micro-dimple array on a substrate surface with a pulsed Nd:YAG laser. Micro-dimple arrays were successfully fabricated on the surface of Ti6Al4V samples both in air and water. The aim of this paper was to investigate the influence of processing parameters on the morphology of micro-dimples, spatter deposition, micro-dimple depth and ablation rate. Contrast experiments were conducted to evaluate the effects of masks. Results showed that the shape of micro-dimples in air could be affected by the overlapping of spatter between the adjacent micro-dimples, while in water it is nearly copied from the mask. The bottom of the micro-dimple fabricated in water was coarse but the periphery was clean. The spatter deposition extremely depended on the laser intensity, pulse numbers and surroundings. Discussions about the existence of masks were carried on.

Laser Microdrilling of Thermal Barrier Coatings

Parametric study of laser microdrilling of Ytrria stabilized Zirconia coated titanium plates employing 12kW CO2 laser was conducted in order to investigate the influence of laser power, frequency and assist gas pressure on quality characteristics such as spatter deposition area, entrance circularity, entrance and exit diameter and taper. Spatter deposition area was found significantly influenced by laser power. Entrance circularity and entrance diameter was influenced by assist gas pressure. Exit diameter was influenced by both laser power and assist gas pressure. Taper angle was influenced by assist gas pressure. Multi objective optimization using Grey Relational Analysis indicated that minimum spatter deposition (19.1152mm2), nominal entrance circularity (0.8709), entrance hole diameter (1mm) and exit hole diameter (0.5709) and minimum taper angle (2.1421°) corresponded to 2000W laser power, 10kHz frequency and 5bar assist gas pressure.

An experimental and numerical study on laser percussion drilling of thick-section alumina

A major challenge in laser percussion drilling of thick-section ceramics is to obtain a low taper and low spatter deposition hole leading to high quality post-processing. In order to achieve the fine hole drilling, it is important to understand the mechanism of laser percussion drilling. In this paper, an experimental and numerical study on laser percussion drilling was carried out. A two-dimension (2D) axisymmetric finite element (FE) model for simulation of temperature field and proceeding of hole formation during percussion drilling was developed. The FE model was validated by the corresponding experiment. Furthermore, a theoretical model for evaluation of temperature at melt front and velocity of melt ejection was presented in order to further validate the FE model and study the spatter deposition. The effects of laser peak power, pulse duty cycle and pulse repetition rate on hole diameter and spatter deposition were investigated by the developed models and experiments, in which the simulated results were in good agreement with the experiments. The study indicated that the size and temperature of the melt front significantly affected the hole diameter formation and spatter deposition during laser percussion drilling. The characteristic of melt front was mainly determined by the employed laser peak power, pulse repetition rate and pulse duty cycle. Based on the experimental and numerical study, the process parameters were optimised and a drilled-hole with low taper and low spatter deposition was obtained using a 3.5kW CO2 laser. A microstructural and element compositional study was also performed in this work, by which the characteristics of microstructure and element composition in HAZ around laser drilled hole were revealed.

Micro-machining of silicon wafer in air and under water

Laser ablation micro-machining tests are conducted on silicon wafer, both in air and under flowing water stream, with the use of 355 nm-X AVIA laser. Effects of laser pulse frequency, power level, scan velocity and focal plane position on the associated laser spatter deposition (in air), irradiated areas (under flowing water film) and taper are investigated. It shows that low frequency, i.e. 30–40 kHz, and high peak power result in smaller spatter and irradiated areas, and the hole taper decreases with increase in pulse frequency. Increase in the laser fluence broadens both the areas and increases the hole taper. Both areas enlarge with the increase of scanning velocity of more than 3 mm s −1. The scan velocity has no effect on hole taper in air environment but inconsistent hole taper is obtained under flowing water stream. Furthermore, moving the focal plane position below the workpiece surface contributes relatively smaller areas of spatter deposition, irradiation and taper in comparison to zero focal plane position. Finally, the differences between laser ablation in air and under water are identified. The reduction in the spatter deposition and irradiated areas around the perimeter of the ablated hole and a smaller taper with the use of laser trepan drilling method in air and under water machining are investigated in this paper.

INVESTIGATION OF THE EFFECT OF ASSIST GAS PRESSURE AND PULSE FREQUENCY ON SELECTED HOLE CHARACTERISTICS IN Nd:YAG LASER PERCUSSION DRILLING

Laser drilling has been widely used in many applications especially in aerospace industry for drilling cooling holes in gas turbine engine components. Spatter formation is one of the inherent defects associated with laser drilling. It is formed around the hole rim due to the deposition of the molten or vaporized material that is expulsed from the workpiece during drilling. Hole taper is another problem facing laser drilling. The manufacturers usually use a gas jet (assist gas) in tandem with laser beam when material removal by laser. In this study an investigation of the effect of oxygen pressure -as assist gas- on the spatter deposition area, inlet diameter, and taper of holes drilled on superalloy is carried out. The effect of laser pulse frequency on the same hole characteristics is also presented. A trial to find the effect of assist gas pressure and pulse frequency on the total volume removed from the laser-drilled holes is also done. The drilling operation was carried out by a fiber-optic delivered 400 W pulsed Nd:YAG laser system. Scanning electron microscope (SEM) was used to capture images of the laser-drilled holes at the workpiece surface. An image processing software was employed to measure both spatter deposition area and inlet hole diameter. Exit hole diameter was measured by digital optical microscope. It was found that the increase of assist gas pressure could increase the spatter deposition area, inlet hole diameter, and hole taper. The increase of pulse frequency may lead to the same results. Continuous increase of gas pressure and pulse frequency could decrease spatter deposition area.

THE INFLUENCE OF PULSE SHAPE ON HOLE GEOMETRY DURING Nd:YAG LASER DRILLING OF STAINLESS STEEL SHEETS

Laser drilling has been used for making large number of cooling holes in gas turbine engine industry due to its benefits in saving cost and time. In single pulse laser drilling, a hole can be formed in thin metallic sheets by one laser pulse with enough energy, in which the hole making may take just few milliseconds. Pulse shaping is one of the important factors that could affect the hole characteristics. In the present study; three types of pulse shapes; rectangular, ramping up, and ramping down, with pulse width of 2 ms were used to drill through holes in 1 rnm thick stainless steel sheets. Scanning electron microscope (SEM) was used to capture the necessary images of the laser-drilled holes at the different pulse shapes. An image processing technique was applied to identify the spatter around the drilled holes. It was also used to calculate the inlet hole diameter and its associated spatter deposition area. Optical microscope was utilized to measure the exit hole diameter. The hole taper was evaluated by calculating the inlet to exit hole diameters ratio. Finally, the prescribed holes are reproduced using two pulses having the same total energy as the one pulse used before. A comparison between the two drilling techniques was established. It was found that the pulse shaping could have a remarkable effect on the measured hole configurations when the two types of laser drilling were applied on the used stainless steel sheets. Also, the two drilling techniques produced holes with different final geometry.

Spatter-free laser drilling of alumina ceramics based on gelcasting technology

Spatter and microcracks are inherent defects commonly associated with holes produced with laser drilling. In order to prevent spatter deposition and formation of microcacks this work reports a novel laser drilling method based on the gelcasting technology. The work presents a new laser drilling method, through which the irregular spatter at the exit of the hole and microcracks produced by the traditional laser drilling method can be effectively prevented by direct laser drilling of gelcast green body and compact micro holes with much more regular shapes without microcracks can be successfully obtained.

Welding and rheomorphism of phonolitic fallout deposits from the Las Cañadas caldera, Tenerife, Canary Islands

The Las Canadas caldera is a nested collapse caldera formed by the successive migration and collapse of shallow magmatic chambers. Among the pyroclastic products of this caldera are phonolitic fallout deposits that crop out in the caldera wall and on the extracaldera slopes. These deposits exhibit an uninterrupted facies gradation from nonwelded to lava-like and record continuous volcanic deposition. Densely welded and lava-like facies result from the extreme attenuation and complete homogenization of juvenile clasts that destroy original clast outlines and any evidence of fallout deposition. Agglutination contributes significantly to the final degree of flattening observed in the welded facies. After deposition, rheomorphic flowage occurs. Emplacement temperatures for one of the welding sequences are calculated from magmatic temperatures and a model of tephra cooling during fallout. Results are 486 °C for the nonwelded facies and 740 °C for the moderately welded facies. For the same welding sequence, a cooling time between 25 and 54 days is estimated from published experimental and computational data as the possible duration of welding and rheomorphism. Following deposition and agglutination, the lava-like pyroclastic facies had the rheological properties of viscous lavas and flowed down the outer slopes away from the caldera. Some lava-like masses detached from proximal areas to more distal regions. During deposition, the eruptive style evolved from Plinian fallout to fountain-fed spatter deposition. This evolution was accompanied by a decrease in explosive power and a lower height of the eruptive column, which produce higher emplacement temperatures and more effective heat retention of pyroclasts.