Evaluation of spatter particles, metal vapour jets, and depressions considering influence of laser incident angle on melt pool behaviour

Building of practical parts involves the application of metal-based laser powder bed fusion using a laser beam (PBF-LB/M) owing to its high-precision manufacturing. However, the quality of the built parts obtained via the PBF-LB/M processes varies with the building conditions, and a thorough understanding of the building mechanism has not been achieved owing to the complex and interrelated process parameters involved. The incident angle of the laser beam, which changes on the platform during the laser beam scan owing to the designed three-dimensional data, is among the principal parameters that affects the building aspects. In this study, the melt pool in the singletrack formation during the PBF-LB/M processes was visualised using a high-speed camera, and the influence of the laser incident angle on the ejection characteristics of spatter particles formed around the laser-irradiated area was investigated. Consequently, the spatter particles and metal vapour jet behaviour varied with the laser incident angle. There was a reduction in number of spatter particles owing to the origin of the incident direction being from behind the laser irradiation area. In addition, the laser incident angle also affected the melt pool morphology because of the depression in the melting. Furthermore, the burial depth of the pores varied with the laser incident angle, and is related to the depth of the depression during the melt pool formation.

APPLICATIONS OF MOLECULAR MARKERS AND GENOMIC TOOLS IN PEARL MILLET [CENCHRUS AMERICANUS (L.) MORRONESYN. PENNISETUM GLAUCUM (L.) R. BR.]

Thisreviewsummarizesthemainmolecularmarkersandtheirapplicationson pearl milletaswellasasummaryof the discoveriesonitsreferencegenome.Molecularmarkers,unlikemorphologicalandbiochemicalmarkers,arehighlypolymorphicandneutral. Theirgreatliabilitycomesfromthefactthattheydirectlyconcern the DNA.Theyhavebeenwidelyusedonpearl millet,rangingfromlow andmedium-throughputtohigh-throughput markers, targetingspecificregionsorcharacterizinggermplasmat thegenomelevel. Many studiesrelatetomappingusingdifferentpopulationsandhaveidentifiedQTLslinkedtoimportantagronomictraits(floweringtime,tolerancetodrought,to mildew,phosphorus absorption),iron content...Studieshavealsobeenconductedondomesticationsyndromeandshowedtheir importance of genes flowfromwildmilletstocultivatedvarieties. Genotyping-by-Sequencing - a rapid, cost-effective and reduced representation sequencing method – has been used to assess genetic diversity, population structure, LD and heterotic pool formation in pearl millet. A draft genome sequence that can serve as a reference for further development of genomics-assisted breeding is now available. It is an important milestone in generating genomic resources for pearl millet. Annotation of 24,000 genes indicates that enrichment of wax biosynthesis genes providing potential genetic mechanisms for heat and drought tolerance. Althoughmolecularmarkersarewidelyappliedtomillet,geneticandgenomicresourcesarestilllimitedcomparedtootherimportantcereals.However,theavailabilityofacollectionofinbredlinesrepresentativeofgermplasmandareferencegenomeoffernewperspectivesintheimprovement of pearl millet.

Simulated and experimental analysis of laser beam energy profiles to improve efficiency in wire-fed laser deposition

Laser cladding is a well-established technique, with the majority of prior numerical modelling work focused on delivery and melt pool behaviour of powder-based processes. This research presents new investigations into optimised laser beam shaping for the unique characteristics of wire-based processes, where direct substrate heating, as well as heat transfer between the wire and substrate, is important. The value of this subject is the improved deposition rates and dense metallic structures that can be achieved by wire-based deposition processes compared to powder-based material delivery. The within-wire temperature distribution (AISI 316 stainless steel), the heat transfer and direct heating of the substrate (mild steel) are modelled via heat transfer simulations, with three laser beam irradiance distributions. This analysis identified the removal of localised high-temperature regions typically associated to standard Gaussian distributions, and the improved substrate heating that a uniform square beam profile can provide. Experiments using pre-placed wire and a 1.2 kW CO2 laser were analysed using cross-sectional optical microscopy to provide model validation and evidence of improved wire-substrate wetting, while maintaining favourable austenitic metallurgy in the clad material. A key finding of this work is a reduction, from 480 to 190 W/mm2, in the required irradiance for effective melt pool formation when changing from a Gaussian distribution to a uniform square distribution. This also provided a 50% reduction in total energy. The potential improvements to energy efficiency, cost reductions and sustainability improvements are recognised and discussed.

Numerical modeling of fluid flow, heat, and mass transfer for similar and dissimilar laser welding of Ti-6Al-4V and Inconel 718

Most of the researches published on the numerical modeling of laser welding are looking at similar welding, mainly due to the difficulty of simulating the mixing phenomenon that occurs in dissimilar welding. Furthermore, numerical modeling of dissimilar laser welding of titanium and nickel alloys has been rarely reported in the literature. In this study, a 3D finite volume numerical model is proposed to simulate fluid flow, heat, and mass transfer for similar and dissimilar laser welding of Ti-6Al-4V and Inconel 718. The laser source was simulated by volumetric heat distribution, which considers the effects of keyhole and heat transfer on the workpiece. The heat source parameters were calibrated through preliminary experiments, by comparing the simulated and experimental weld pool shapes and dimensions. The model was used to simulate both homogenous and dissimilar laser weldings of Ti-6Al-4V and Inconel 718, and a systematic comparison was carried out through a number of selected experiments. The effects of three distinct levels of laser power (1.25 kW, 1.5 kW, 2.5 kW) on temperature distribution and velocity field in the welds pool were analyzed. Results highlighted the effects of Marangoni forces in the weld pool formation. Furthermore, in order to analyze the mass transfer phenomenon in dissimilar welding, species transfer equations were considered, demonstrating the important role played by the mass mixture in the weld pool formation. Finally, a high level of agreement between simulations and experiments—in terms of weld pool shape and dimensions—was observed in all cases analyzed. This proves the ability of the proposed numerical model to properly simulate both the similar and dissimilar welding of Ti-6Al-4V and Inconel 718 alloys.

Influences of channel morphology and large wood on bed sediment grain size characteristics along a headwater stream, southern Brazil

<p>Headwater streams are mainly responsible for providing sediments and nutrients to the downstream regions. In addition, the riparian vegetation of these streams is a source of large wood (LW) for the water channel. Sediments and LW play an ecologically-important role in maintaining aquatic habitat, meanwhile sometimes execute serious intensification of the impact of downstream large floods. The quantity and size distribution of sediments on the channel bed and LW certainly influence on channel hydraulics and stability as well as the quality of aquatic habitats. Hence, the characterization of sediments and LW supports the renaturalization and conservation of streams/rivers. Therefore, the objective of the present study was to evaluate the influence of the LW deposition and the channel morphology on the bed sediment grain size along a reach (700 m) inside a headwater catchment (15 km<sup>2</sup>), southern Brazil. The mean slope of the reach channel is 0.33% which can be considered low. This catchment is characterized with the Mixed Ombrophilous Forest, the climate is the Cfb according to Köppen classification and the average annual rainfall is 1881 mm. Hydrological (rainfall and discharge) and LW dynamics monitoring has been carried out in this catchment since 2017. The step, pool and cascade are observed as channel morphologies along the study reach, which were divided into three parts (upstream, middle, and downstream). Furthermore, 26 cross-sections of the stream were established for field survey, such as measurement of the diameter (axis -a, -b, and -c) of the bed sediments (D<sub>90</sub>, D<sub>84</sub>, D<sub>50</sub>, D<sub>16</sub>, D<sub>10</sub>, and D<sub>max</sub>) and LW presence verification. The definition of the sampling points sought to ensure a correct representation of the fluvial geomorphology. The present study reported the results obtained through the field survey carried out on August 8th, 2020, using caliper, tree caliper, metric tape and GPS. The grain sizes varied from 324.64 mm to 2.76 mm (particles smaller than 2 mm were not computed). The results showed that the presence of LW in the section induced a smaller value of the sediment grain size. In the pool formation a greater variation in the sediment granulometry was found than in cascades and steps. Furthermore, within the pool, the sediment sizes tended to increase from upstream to downstream. It is concluded that the LW acts as natural barriers for sediment deposition, avoiding the sediments exhaustion and generating low-velocity zones, favoring the maintenance of aquatic habitat. Also, the difference in the granulometry of the sediments within the pool formation may be a function of the flow velocity. In Brazil, the importance of fieldwork involving the monitoring of LW and bed sediments should be highlighted as a tool for understanding the hydrogeomorphic processes and for conserving the riparian zone, because such fieldworks are still few.</p>

Trace metals and Nd isotopes in 3.35 Ga old stromatolites of the Strelley Pool Formation (Australia) unravel the genesis of carbonates and chert

<p>Stromatolites are laminated, presumably microbial, structures, consisting largely of an authigenic precipitate, thus providing potential geochemical archives of early Earth aqueous environments and their habitability. In this study, we report trace element and Sm/Nd isotope data from Palaeoarchean stromatolites and adjacent cherts of the Strelley Pool Formation (NW Australia), obtained by ICP-MS and TIMS, to test their reliability as archives for palaeo-environmental reconstruction and to understand authigenic mineral formation. Stromatolitic carbonates plot together with the stratigraphically underlying Marble Bar cherts on a linear Sm-Nd regression line yielding an age of 3253 ±320 Ma. In contrast, associated crystal-fan carbonates yield 2718 ±220 Ma, suggesting that their Sm-Nd isotope system was altered after deposition. Geochronological information via Sm-Nd dating of black and white cherts is limited, probably due to a reset of the isotope system during an unknown Paleoproterozoic or younger alteration event. Carbonates, as well as white cherts, show shale-normalized rare earth element and yttrium patterns (REY<sub>SN</sub>; except for redox-sensitive Ce and Eu) parallel to those of modern seawater, indicating a seawater-derived origin. Positive Eu<sub>SN</sub> anomalies (2.1 - 2.4), combined with heterogeneous ɛNd<sub>3.35Ga</sub> values (-3.2 to +5.8) within alternating stromatolite laminae, support that seawater chemistry was affected by both continental weathering and high-temperature hydrothermal fluids that episodically delivered chemical elements from young mafic and older felsic rock sources into the stromatolite environment. In contrast, black cherts show REY<sub>SN</sub> patterns characteristic of a non-seawater source and significant amounts of elements leached from the surrounding rocks, overprinting the pristine geochemical composition of ancient seawater. In conclusion, Archaean stromatolites indeed preserve pristine authigenic phases at the mm-scale that contain signatures representative of the water chemistry prevailing in the depositional environment of the time.</p>