Large Melt Pool Research Articles

The reduced alloy in Earth's upper mantle: Experimental constraints on Fe-Ni-S-C(-O) melt compositions and deep mantle oxygen fugacity (5–16 GPa)

High-pressure experiments conducted at upper mantle conditions reveal increased Fe3+ contents in majoritic garnet with increasing pressure. Consequently, at ∼8 GPa, Fe2+ disproportionation is expected to generate Fe0, leading to a higher metal/sulfur ratio of the initially present monosulfidic melt. This study experimentally investigates equilibrium systematics between the reduced Fe-Ni-S-C(-O) melt and mantle silicates at 5–16 GPa and near-adiabatic temperatures (1420–1590∘C), aiming to constrain the alloy melt composition in equilibrium with bulk silicate Earth-like (BSE) mantle silicates. As analysis of unquenchable liquid alloys requires large melt pools, experimental charges were designed towards a 50:50 ratio of alloy:silicates, with Fe/Ni ratios, sulfur and carbon scaled to correspond to a BSE with 200–250 ppm S and 100–150 ppm C.Observed phase assemblages consist of Fe-Ni-S-C(-O) melt (metal/(S+O): 0.9–3.5, Ni/(Fe+Ni): ∼0.3–0.6 and 0.2–1.6 wt.% oxygen) saturated with graphite/diamond and coexisting with olivine/wadsleyite (XMg: 0.76–0.91), garnet and cpx ± low-Ca pyroxene. Oxygen fugacities (fO2) range from -0.5 to +4.0 relative to the iron-wüstite (IW) buffer. In most runs, Fe0–Fe2+ redox equilibration affected silicate XMg's, resulting in values <0.90. Therefore, an empirical model was developed to predict KDNi-Fealloy-olivine, allowing to recalculate the experimental results for a primitive BSE mantle composition with XMg of 0.90.Modeling BSE, the alloy's metal/(S+O) increases from 1.1 to ∼7 while Ni/(Fe+Ni) decreases from 0.35 to 0.19 at 5–16 GPa. With BSE-like S concentrations, the abundance of the Fe-Ni-S-C(-O) melt increases from 600–750 ppm at 5 GPa to ∼3800 ppm at 16 GPa, 100–150 ppm bulk carbon lead to graphite/diamond saturation to ∼14 GPa. Finally, a method is presented to derive fO2 from modeled liquid alloy compositions, which act as a “redox-sensor”. For a BSE-like mantle with 250 ppm S, fO2 decreases from IW+2.6 at 5 GPa to IW at 8 GPa, further decreasing to IW-1.0 at 12–16 GPa.

Emplacement mechanism of ponded light plains on the Moon: Insight from topography roughness

Light plains on the Moon are distinctive smooth deposits formed by large impact basins. Featuring diverging morphology and frequent spatial association with large secondary craters, light plains were interpreted to be formed either by ground-hugging ejecta flows and/or debris flows triggered by landing of impact ejecta. Light plains located in closed depressions, term ponded light plains (PLPs) here, cover ∼3% of the lunar global surface. They lack diagnostic geomorphology that could be ascribed to secondary impacts or primary ejecta, suggesting a possibly different emplacement mechanism, which has not been investigated before. PLPs feature comparable morphology with both lunar mare that were mainly emplaced by effusive volcanism and large impact melt pools that were deposited by melt-rich ejecta in rims and terraces of large impact basins. For such smooth plain materials, topography roughness at small scales might be an indicative information for their different emplacement styles. This study analyses topography roughness for PLPs and similar-sized melt pools that were formed by the Orientale basin and also coeval mare units on the Moon. We find that mare units have smaller roughness than same-aged melt pools, because large entrained blocks are abundant in melt pools that likely yielded larger viscosity. We further notice that PLPs have a medium topography roughness between coeval mare units and large melt pools, and the topography roughness of PLPs is closer to that of mare units. Correlated observations using high-resolution images suggest that the smoother topography of PLPs than cogenetic melt pools are mainly due to the minor existence of entrained blocks in PLPs. This comparison suggests that while PLPs and melt pools are both basin-related smooth ejecta deposits, PLPs may feature a lower viscosity during the emplacement, and they are most consistent with being melt-rich primary ballistic ejecta that were accumulated in local depressions. Considering that the youngest multiple-ring basin on the Moon, the Orientale basin, is capable of forming numerous PLPs across most parts of the entire Moon, this work predicts that both disrupted and sandwiched melt-rich ejecta deposits should be widespread in the lunar highland crust due to earlier-formed impact basins.

Parametric study of melt pool geometry in hybrid plasma arc-laser melting process for additive manufacturing application

Wire-based hybrid arc-laser additive manufacturing is suitable for producing large metallic parts (metres in scale) with high deposition rates and near-net-shape. In this process, the surface quality and dimensional accuracy of the deposited parts are determined by the melt pool geometry. However, how to control the melt pool in the hybrid process is complex due to the multiple parameters that can be used. In this study, control of melt pool geometry by investigating different process parameters, including laser power, travel direction, arc-laser separation distance, laser beam size, and arc current in the hybrid plasma transferred arc (PTA)-laser process, was studied systematically. It was found that a larger melt pool was achieved with the PTA-leading configuration compared to that with the laser-leading configuration due to a higher laser absorption occurred with the former configuration. The melt pool was enlarged by either increasing the laser power or arc current due to the increased energy input. However, if the laser power density is high enough to determine the melt pool depth, the increasing arc current has very little effect on melt pool depth but only increases the melt pool width. In addition, the melt pool became shallower and wider when using a larger laser beam. The arc-laser separation distance had a minor effect on the melt pool geometry due to the fixed energy input used in the studied cases. The results of this study provide a reference for melt pool control in wire-based hybrid arc-laser additive manufacturing.

Registration and fusion of large-scale melt pool temperature and morphology monitoring data demonstrated for surface topography prediction in LPBF

In-situ monitoring technologies for laser powder bed fusion (LPBF) additive manufacturing often face one key challenge, extracting the ultrafast melt pool (MP) signatures for understanding the localized part properties. Further, the spatial information of each monitored MP signature is essential for correlating the MP – part property. This spatial information is often unavailable especially from commercial LPBF printers. Many MP monitoring methods have been reported and utilized. However, very few of these have the MP’s spatial information. To overcome this challenge, in this work we report a method for spatially registering the key MP signatures (MP intensity, temperature, and area) to the monitored print parts. The MP signatures are obtained from our coaxial high-speed single-camera based two-wavelength imaging pyrometry (STWIP) system and the MP spatial information is obtained from an off-axis camera system. A machine learning aided image analysis method is employed to retrieve the spatial distribution of MPs within the corresponding part’s coordinates system. Then, the MP signature maps (MPSMs) are reconstructed by mapping the STWIP measured MP signatures to the registered MP coordinates. Further, a long short-term memory (LSTM) neural network is developed for estimating the layer surface topography from the registered MPSMs. The obtained results indicate that the layer surface topography can be more accurately estimated by using MP temperature signature rather than MP intensity and/or area signatures as in common practice. Our developed methods for MP monitoring, registration, and MP-surface topography prediction offer advanced capabilities for the online detection of process anomalies and part defects. • Multi-modal in-situ melt pool (MP) monitoring. • Data registration framework for large scale multilayer MP data. • In-situ MP signature significance, quantification, and analysis. • Accurate surface topography prediction using the registered MP data. • Applicability in in-situ part qualification.

Effect of Preheating on the Residual Stress and Material Properties of Inconel 939 Processed by Laser Powder Bed Fusion.

One of the main limitations of laser powder bed fusion technology is the residual stress (RS) introduced into the material by the local heating of the laser beam. RS restricts the processability of some materials and causes shape distortions in the process. Powder bed preheating is a commonly used technique for RS mitigation. Therefore, the objective of this study was to investigate the effect of powder bed preheating in the range of room temperature to 400 °C on RS, macrostructure, microstructure, mechanical properties, and properties of the unfused powder of the nickel-based superalloy Inconel 939. The effect of base plate preheating on RS was determined by an indirect method using deformation of the bridge-shaped specimens. Inconel 939 behaved differently than titanium and aluminum alloys when preheated at high temperatures. Preheating at high temperatures resulted in higher RS, higher 0.2% proof stress and ultimate strength, lower elongation at brake, and higher material hardness. The increased RSs and the change in mechanical properties are attributed to changes in the microstructure. Preheating resulted in a larger melt pool, increased the width of columnar grains, and led to evolution of the carbide phase. The most significant microstructure change was in the increase of the size and occurrence of the carbide phase when higher preheating was applied. Furthermore, it was detected that the evolution of the carbide phase strongly corresponds to the build time when high-temperature preheating is applied. Rapid oxidation of the unfused powder was not detected by EDX or XRD analyses.

In-situ alloying of a fine grained fully equiaxed Ti-based alloy via electron beam powder bed fusion additive manufacturing process

Alloy design using Additive Manufacturing (AM) methods is an interesting research area attracting the attention of researchers across the world. Formation of strong texture, columnar grains, and chemical inhomogeneity in AM-designed alloys are among the serious challenges dealt with in various research works. This paper addresses these challenges for the first time for a Ti6Al4V–7Cu alloy produced via Electron Beam Powder Bed Fusion (EB-PBF) method following an in-situ alloying approach. In this work, a mixture of Ti6Al4V and elemental Cu powders was used as the feedstock. The results showed a relatively homogenous Cu distribution in the product due to the large melt pool created during the EB-PBF process, a development indicating the method's applicability for in-situ alloying and alloy design. The prior β columnar grains were effectively converted into equiaxed grains due to a constitutionally supercooled zone created by the rejection of Cu solutes in front of the solidification front and a relatively lower thermal gradient during EB-PBF compared with other AM processes. The addition of Cu enhanced the microhardness of Ti6Al4V, which could be considered an outcome of equiaxed grain formation, solid solution strengthening mechanism, and Ti2Cu intermetallic precipitation.

Sensitivity of laser powder bed fusion additive manufactured HAYNES230 to composition and print parameters

We developed a coupled CALPHAD and finite element-based computational model of the Laser Powder Bed Fusion (LPBF) process for HAYNES230, considering the feedstock composition and packing density. We further used this model to investigate the effect of variation in feedstock composition and print parameters on the quality of the final printed part. Sensitivity of the maximum reached temperature to variations in characteristics of the laser source is also studied considering a single-track laser scan on a layer of metal powder. We analyzed temperature evolution in the powder bed and melt pool geometry along the path of the laser. Our results indicate that the LPBF process of HAYNES230 alloy requires a powder layer thickness of ∼20 μm and laser spot size ∼30 μm radius compared to other alloys. It is essential to achieve sufficient melt pool depth necessary for cohesion with the substrate while avoiding large melt pool width that adversely affects the formation of cracks and residual stresses. We also revealed that reducing the laser power or increasing scan speed drastically reduces peak temperature while less susceptible to solute composition.

Process characterization and analysis of ceramic powder bed fusion

Powder bed fusion (PBF) of ceramics is often limited because of the low absorptance of ceramic powders and lack of process understanding. These challenges have been addressed through a co-development of customized ceramic powders and laser process capabilities. The starting powder is made of a mix of pure alumina powder and alumina granules, to which a metal oxide dopant is added to increase absorptance. The performance of different granules and process parameters depends on a large number of influencing factors. In this study, two methods for characterizing and analyzing the PBF process are presented and used to assess which dopant is the most suitable for the process. The first method allows one to analyze the absorptance of the laser during the melting of a single track using an integrating sphere. The second one relies on in-situ video imaging using a high-speed camera and an external laser illumination. The absorption behavior of the laser power during the melting of both single tracks and full layers is proven to be a non-linear and extremely dynamic process. While for a single track, the manganese oxide doped powder delivers higher and more stable absorptance. When a full layer is analyzed, iron oxide-doped powder is leading to higher absorptance and a larger melt pool. Both dopants allow the generation of a stable melt-pool, which would be impossible with granules made of pure alumina. In addition, the present study sheds light on several phenomena related to powder and melt-pool dynamics, such as the change of melt-pool shape and dimension over time and powder denudation effects.

From mine to part: directed energy deposition of iron ore

Purpose This paper aims to gain an understanding of the behaviour of iron ore when melted by a laser beam in a continuous manner. This fundamental knowledge is essential to further develop additive manufacturing routes such as production of low cost parts and in-situ reduction of the ore during processing. Design/methodology/approach Blown powder directed energy deposition was used as the processing method. The process was observed through high-speed imaging, and computed tomography was used to analyse the specimens. Findings The experimental trials give preliminary results showing potential for the processability of iron ore for additive manufacturing. A large and stable melt pool is formed in spite of the inhomogeneous material used. Single and multilayer tracks could be deposited. Although smooth and even on the surface, the single layer tracks displayed porosity. In case of multilayered tracks, delamination from the substrate material and deformation can be seen. High-speed videos of the process reveal various process phenomena such as melting of ore powder during feeding, cloud formation, melt pool size, melt flow and spatter formation. Originality/value Very little literature is available that studies the possible use of ore in additive manufacturing. Although the process studied here is not industrially useable as is, it is a step towards processing cheap unprocessed material with a laser beam.

Silicic microinclusions in a metasomatized eclogite from Roberts Victor mine, South Africa

We report results of a systematic search for fluid/melt microinclusions (<1 μm) in “dusty” garnets from a heavily metasomatized Type-I kyanite-bearing eclogite from the Roberts Victor mine, South Africa. FTIR analyses of “dusty” zones indicate the presence of molecular water, CO2 and carbonate in the inclusions and of hydroxyl groups in the garnet. EPMA analysis of 136 microinclusions reveals the bulk composition of the microinclusions. Relative to the host garnet, they are enriched in TiO2, FeO, CaO, Na2O and K2O and depleted in SiO2, Al2O3 and MgO. Most of the elements form compositional mixing arrays of microinclusion + clear garnet. The arrays trend away from the compositions of large melt pools or secondary minerals found in the xenolith. They point, with some notable differences, towards the array of silicic to low-Mg carbonatitic high-density fluids trapped in diamonds. The main differences are the presence of molecular CO2 in addition to carbonate, the lower concentration of incompatible major constituents such as water, K2O and TiO2 and higher levels of Al2O3 and FeO. Still, the resemblance is close, indicating the role of diamond forming fluids in mantle metasomatism in general. The trapped melts are secondary and penetrated late in the history of the eclogite. They did not take part in the main metamorphic or metasomatic reactions, however they can account for introduction of Al2O3, FeO and incompatible trace elements into the ecologies.

Resolving the porosity-unmelted inclusion dilemma during in-situ alloying of Ti34Nb via laser powder bed fusion

A parametric experimental study on the role played by the power (P), velocity (VL) and profile (top-hat or Gaussian) of the laser on the porosity, inclusion content and microstructural evolution of in-situ alloyed laser powder bed fusion (LPBF) manufactured Ti34Nb was conducted. For this, alloys were printed with three sets of processing parameters, in which the above-mentioned parameters were varied but the energy density was held constant. A detailed tomographic and microstructural investigation of these alloys was followed by tensile tests. Observations of melt pools in the single tracks and single stripes were complemented by thermal finite element method (FEM) simulations that studied their evolution. Results show that a top-hat laser profile combined with high P (> 650 W), high VL (> 650 mm/s) and short stripe width scanning strategy forms a large single stripe melt pool that moves slowly and has a low aspect ratio. The large and slow melt pool allows Nb to melt efficiently and minimizes its unmelted content while the low melt pool aspect ratio prevents keyholing that can otherwise result in porosity. These melt pool attributes also favor the formation of a columnar β-Ti microstructure with a strong {100} texture, which has the ideal combination of strength and elastic modulus. The mechanisms of microstructural evolution were explained and the scope of this parameter optimization principle was discussed in the context of further improving the properties of this alloy.

Experimental research on heat transfer behavior of large scale two-layer salt melt pool based on COPRA facility

During a core meltdown accident, the significant decay heat within the corium pool will continually impose thermal load on the vessel wall and challenge the integrity of reactor pressure vessel (RPV). External Reactor Vessel Cooling (ERVC) is considered to be an effective strategy to reduce the potential of vessel failure. After long-term cooling, the initially uniform melt pool can reach a final steady state with stable two-layer or three-layer configuration. Based on the COPRA facility, a series of tests were performed to study the natural convection heat transfer phenomenon of stratified corium pools. The test section was a full-scale 2D slice, simulating the hemispherical lower head of the HPR1000 reactor design. Direct electrical heating was applied in the lower part of vessel to simulate the homogenously heated ceramic pool, while no heat was generated in the upper part to model the metallic layer. A binary mixture of 20 mol% NaNO3–80 mol% KNO3 was selected as simulant material due to the similar phase-change characteristics with the prototypical melt. During the experiment, effects of heating power, top boundary condition, corium pool height, decay heat distribution and simulant material on corium pool heat transfer behavior were analyzed. The results implied that the top cooling and decay heat distribution slightly influenced the temperature and heat flux of ceramic pool, but strongly impacted the characteristics of metallic layer. The comparison between the dimensionless parameters of water tests and salt tests also demonstrated the key role of simulant material. Furthermore, the downward heat transfer Nudn and the upward heat transfer Nuup obtained from COPRA two-layer experiment were in accordance with the previous data or empirical correlations. Note that using the same simulant material for oxide layer and metal layer fails to repeat the boundary conditions in the reactor case, however, the current study pays more attention to the qualitative analysis of different influence on natural convection heat transfer behavior. More valuable information about the crust characteristics and stratification effect can be obtained by the following CFD simulation of these tests.

On microstructure and corrosion behaviour of AlSi10Mg alloy with low surface roughness fabricated by direct metal laser sintering

In this study, in-depth microstructure and electrochemical properties of highly smoothed AlSi10Mg parts fabricated through direct-metal-laser-sintering are reported. The samples with the lowest surface roughness were characterized by having a periodic large and small melt pools (MPs) pattern in their upskin layers. Consequently, a noticeably coarser eutectic-Si network, Al-dendrite and grain size formed along the larger MPs’ borders, rationalized by their slower solidification rate. Such microstructural features were found to control/deteriorate the electrochemical performance of the as-printed samples than their surface roughness. An improved electrochemical stability for the samples that experience the fastest solidification rate along their MP-boundaries was revealed.

Yakutites from the Popigai meteorite crater

For the first time, 60 large diamond aggregates were found inside the Popigai meteorite crater during washing of alluvial deposits along the Dogoi river crossing the crater. These aggregates are similar in appearance to yakutites from the placers of Northern Yakutia (YPY), and we regard them as yakutites from the Popigai crater (YPC). The structure and optical properties of Popigai impact diamonds from the impact melt rocks (tagamites) in the crater (PIDT) and yakutites YPC/YPY were compared in detail. In all these cases, a polycrystalline structure consisting of nanoscale grains of cubic and twinned cubic diamond (lonsdaleite) was found. This is the result of a solid-phase graphite-diamond transition due to an impact event 35 million years ago. The diamond aggregates show the following features: a red shift of the short-wave edge of the transmission, broadening of the diamond Raman peaks, signals from other diamond polytypes and numerous inclusions of other minerals in the Raman spectra, and a dominant broadband photoluminescence (PL). PL in the N3 system associated with N3V centers in PIDT diamonds indicates a high-temperature annealing of these aggregates with resulting aggregation of impurities during the prolonged cooling of large impact melt pockets and pools. It is assumed that some of the impact diamonds were ejected from the crater during the impact event and experienced rapid cooling. Some of these diamonds fell back into the crater (YPC yakutites), others have been deposited outside the crater and displaced during erosion (YPY yakutites). Difference in size and shape between the PIDTs and yakutites YPC/YPY is due to the difference in size of original graphite flakes or aggregates and/or due to the fundamentally different technologies of diamond extraction.

Build-up strategies for temperature control using laser metal deposition for additive manufacturing

The track geometry created with laser metal deposition (LMD) is influenced by various parameters. In this case, the laser power has an influence on the width of the track because of an increasing energy input. A larger melt pool is caused by a rising temperature. In the case of a longer welding process, there is also a rise in temperature, resulting in a change of the track geometry. This paper deals with the temperature profiles of different zigzag strategies and spiral strategies for additive manufacturing. A two-color pyrometer is used for temperature measurement on the component surface near the melt pool. Thermocouples measure the temperatures in deeper regions of a component. The welds are located in the center and in the edge area on a test part to investigate the temperature evolution under different boundary conditions. The experiments are carried out on substrates made from mild steel 1.0038 and with the filler material 316L. The investigations show an influence on the temperature evolution by the travel path strategy as well as the position on the part. This shows the necessity for the development and selection of build-up strategies for different part geometries in additive manufacturing by LMD.

Experimental insights into Stannern‐trend eucrite petrogenesis

AbstractThe incompatible trace element‐enriched Stannern‐trend eucrites have long been recognized as requiring a distinct petrogenesis from the Main Group‐Nuevo Laredo (MGNL) eucrites. Barrat et al. () proposed that Stannern‐trend eucrites formed via assimilation of crustal partial melts by a MGNL‐trend magma. Previous experimental studies of low‐degree partial melting of eucrites did not produce sufficiently large melt pools for both major and trace element analyses. Low‐degree partial melts produced near the solidus are potentially the best analog to the assimilated crustal melts. We partially melted the unbrecciated, unequilibrated MGNL‐trend eucrite NWA 8562 in a 1 atm gas‐mixing furnace, at IW‐0.5, and at temperatures between 1050 and 1200 °C. We found that low‐degree partial melts formed at 1050 °C are incompatible trace element enriched, although the experimental melts did not reach equilibrium at all temperatures. Using our experimental melt compositions and binary mixing modeling, the FeO/MgO trend of the resultant magmas coincides with the range of known Stannern‐trend eucrites when a primary magma is contaminated by crustal partial melts. When experimental major element compositions for eucritic crustal partial melts are combined with trace element concentrations determined by previous modeling (Barrat et al. ), the Stannern‐trend can be replicated with respect to both major, minor, and trace element concentrations.

Experimental temperature cycling as a powerful tool to enlarge melt pools and crystals at magma storage conditions

Experiments in high silica systems at temperatures close to the solidus often produce crystals and melt pools that are too small for in situ analysis. Oscillating the temperature during an experimental run speeds up recrystallization of magma by dissolving small and increasing the size of larger crystals, dramatically changing the crystal size distribution. This principle of periodic heating and cooling, caused for example by repeated injection of hot magma, is also a potential acceleration for the formation of phenocrystic textures in natural rocks. Here we show that temperature cycling has the potential to significantly enlarge melt pools and crystals in a fluid saturated dacitic system. Using a natural dacite dredged from the Pacific-Antarctic Rise as starting material, we performed crystallization experiments applying temperature cycling systematically for two different temperatures and different water activities at 200 MPa. For experiments at 950 °C (with a H2O ~1, ~0.3, and <0.1) an internally heated pressure vessel was used, experiments at 800 °C (with a H2O ~1, ~0.5) were performed in a cold-seal pressure vessel. Comparative experiments at equilibrium conditions with constant temperature were performed for both approaches. For all other experiments temperature was cycled with amplitudes of 20 K for different time intervals but constant total run duration after initial equilibration at constant temperature. Additionally, for one experiment at 800 °C, the temperature was increased several times by 50 K to study the potential of dissolving tiny crystals in the matrix. As a result of the temperature cycling, tiny crystals in the matrix were preferentially dissolved, leading to large melt pools with only rare mineral inclusions enabling microprobe analysis using a defocused beam. With regard to the area of the 10 largest crystals of each cycling experiment, clinopyroxene crystals were up to 19 times larger, and plagioclase crystals even up to 69 times when comparing to experiments performed at constant temperature. Grain sizes of FeTi-oxide phases are less influenced by this technique. Essential requirements for applying temperature cycling routinely are identical phase relations and compositions in runs with constant and cycled temperature. For all studied temperatures and water activities, the phase assemblage was the same and compositions of all phases are identical within the analytical error. Thus, the temperature cycling technique opens interesting perspectives, especially in facilitating in situ analysis in near solidus systems.

Nanosecond laser processing of Zr41.2Ti13.8Cu12.5Ni10Be22.5 with single pulses

In addition to their attractive mechanical properties, the amorphous structure of bulk metallic glasses (BMGs) leads to favourable conditions for their processing using micro machining operations. At the same time, the generally high hardness and strength of such amorphous metals make short or ultra-short pulsed laser ablation a fabrication technology of interest for generating micro scale features on BMG workpieces in comparison with mechanical material removal means. In spite of this, relatively little research has been reported on the prediction and observation of the thermal phenomena that take place when processing BMGs with pulsed laser irradiation for a range of delivered fluence values and pulse lengths. Such investigations are important however as they underpin reliable laser processing operations, which in turn lead to more predicable material removal at micro scale. In this context, this paper reports complementary theoretical and experimental single pulse laser irradiation analyses conducted in the nanosecond (ns) regime for possibly the most prominent BMG material due to its relatively high glass forming ability, namely Zr41.2Ti13.8Cu12.5Ni10Be22.5, which is also known as Vitreloy 1. To achieve this, different pulse lengths comprised between 15ns and 140ns and varied fluence values were considered when delivering single pulses on a Vitreloy 1 substrate using a Yb fibre laser system. A simple thermal model of the laser material interaction process for single pulses was also developed to support the observations and interpretations of the experimental data obtained. One of the main conclusions from this research, with respect to potential micro machining applications, is that shorter pulses, i.e. 25ns and less, could lead to the formation of relatively clean craters. For higher pulse lengths, the low thermal conductivity and melt temperature of this BMG substrate mean that laser irradiation easily leads to the formation of a relatively large melt pool and thus to the re-solidification of material ejected outside craters.

Imaging of the Dynamic Melt Movement Induced by a Pulsed Laser

A special case of an interrupted superheated process was developed, governed by the ablation pressure of a boiling melt, induced by a pulsed Nd:YAG-laser. A kind of cutting process was carried out, but at very low speed to generate a large melt pool that can be well studied. The interaction zone was observed by high speed imaging, with and without illumination. When switching the laser pulse on or off, different dynamic phenomena can be clearly observed, like drilling into a bulk of melt, breaking of a melt bridge, controlled pushing of the melt pool, waves running down, all driven by boiling. After the pulse, the melt smoothens and oscillates and is dragged back upwards by the surface tension forces from the melt shape curvature, ending in a torus-like equilibrium shape. The fundamental understanding that was generated could be applied to improve processes like keyhole laser welding, laser remote fusion cutting or laser drilling

Silicon nanodot formation and self‐ordering under bombardment with heavy Bi3 ions

AbstractSi nanodots of high density and hexagonal short‐range order are observed upon normal‐incidence bombardment of hot, crystalline Si with Bi3+ ions having a kinetic energy of a few tens of keV. The heights of nanodots are comparable to their widths of ∼20 nm. The implanted Bi accumulates in tiny Bi nanocrystals in a thin Si top layer which is amorphous due to implantation damage. Light and heavy ions up to Xe cause smoothing of surfaces, but Bi3+ ions considered here have a much higher mass. Atomistic simulations prove that each Bi3+impact deposits an extremely high energy density resulting in a several nanometer large melt pool, which resolidifies within a few hundreds of picoseconds. Experiments confirm that dot patterns form only if the deposited energy density exceeds the threshold for melting. Comparing monatomic and polyatomic Bi ion irradiation, Bi–Si phase separation and preferential ion erosion are ruled out as driving forces of pattern formation. A model based on capillary forces in the melt pool explains the pattern formation consistently.magnified imageHigh‐density Si nanodots are formed by polyatomic Bi ion irradiation of hot Si surfaces. Each impact causes local transient melt pools smaller than the dots. Hexagonally ordered patterns evolve by self‐organization driven by repeated ion‐induced melting of tiny volumes. Homogeneously distributed Bi nanocrystals are found in the a‐Si film. These nanocrystals are related to particularities of the Si–Bi phase diagram. (© 2013 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)