Time-resolved Visualization of Laser Beam Melting of Silica Glass Powder

AbstractSilica glass is an inorganic dielectric material that can be used for laser beam melting without cracking. However, the extremely high viscosity makes consolidation of powder very slow. To study the dynamics of consolidation, a 10.6μm laser beam was directed on the powder layer deposited on the solid substrate of the same material. The laser-interaction zone was lighted with green laser and filmed with a high-speed camera at 6000 fps. The process develops steadily. Neither fluctuation nor droplets are observed. An expanding consolidation zone is observed. Viscous merging of softened powder particles is supposed to be the principal mechanism of consolidation. Mathematical model based on this mechanism confirms formation of the consolidated zone in the center. Both the experiment and the model indicate that consolidation looks like propagation of a sharp front. Comparison of the experiments and the calculations estimates the consolidation front temperature of about 1800-1900K

Experimental Study of Residual Stresses in Metal Parts Obtained by Selective Laser Melting

AbstractHigh local temperature gradients occur at additive manufacturing by selective laser melting of powder. This gives rise to undesirable residual stresses, deformations, and cracks. To understand how to control the formation of the residual stresses, a reliable method is necessary for measuring their distribution in the fabricated part. It is proposed to cut the part into thin plates and to reconstruct the residual stresses from the measured deformation of the plates. This method is tested on beams with square cross-section built from stainless steel. The beams were cut by electrical discharge machining and chemically etched. The obtained stress profile in vertical transversal direction slightly increases from the top to the bottom of the beam. This dependency is confirmed by numerical modeling. The measured stress profile agrees with the known results by other authors

Thermodynamic model of hardness: Particular case of boron-rich solids

A number of successful theoretical models of hardness have been developed recently. A thermodynamic model of hardness, which supposes the intrinsic character of correlation between hardness and thermodynamic properties of solids, allows one to predict hardness of known or even hypothetical solids from the data on Gibbs energy of atomization of the elements, which implicitly determine the energy density per chemical bonding. The only structural data needed is the coordination number of the atoms in a lattice. Using this approach, the hardness of known and hypothetical polymorphs of pure boron and a number of boron-rich solids has been calculated. The thermodynamic interpretation of the bonding energy allows one to predict the hardness as a function of thermodynamic parameters. In particular, the excellent agreement between experimental and calculated values has been observed not only for the room- temperature values of the Vickers hardness of stoichiometric compounds, but also for its temperature and concentration dependencies

Land-use/-cover changes and their effect on soil erosion and river suspended sediment load in different landscape zones of european russia during 1970–2017

Contemporary trends in cultivated land and their influence on soil/gully erosion and river suspended sediment load were analyzed by various landscape zones within the most populated and agriculturally developed part of European Russia, covering 2,222,390 km2 . Based on official statistics from the Russian Federation and the former Soviet Union, this study showed that after the collapse of the Soviet Union in 1991, there was a steady downward trend in cultivated land throughout the study region. From 1970–1987 to 2005–2017, the region lost about 39% of its croplands. Moreover, the most significant relative reduction in cultivated land was noted in the forest zone (south taiga, mixed and broadleaf forests) and the dry steppes and the semi-desert of the Caspian Lowland—about 53% and 65%, respectively. These territories are with climatically risky agriculture and less fertile soils. There was also a widespread reduction in agricultural machinery on croplands and livestock on pastures of the region. A decrease in soil/gully erosion rates over the past decades was also revealed based on state hydrological monitoring data on river suspended sediment load as one of the indicators of the temporal variability of erosion intensity in river basins and the published results of some field research in various parts of the studied landscape zones. The most significant reduction in the intensity of erosion and the load of river suspended sediment was found in European Russia’s forest-steppe zone. This was presumably due to a favorable combination of the above changes in land cover/use and climate change

Modelling of Selective Laser Melting Process of Quartz Glass at Elevated Temperatures

Selective laser melting (SLM) to date is the method of additive manufacturing allowing fabricating products from powder layer-by-layer according to a 3D model. However, when applying this method to fragile materials, parts crack while fabricating due to high temperatures. Quartz glass is a promising material for fabricating products by SLM without cracks due to a low thermal expansion. However, quality of fabricated material differs from the fused cast ones. This article aims to test the method of SLM with preheating to improve the material quality. Experiments on single track formation in SLM are analysed by modelling the coupled processes of heat transfer and powder consolidation in the laser-interaction zone. The mathematical model is validated by the experiments. It is shown that the preheating can improve the material quality and increase the process productivity but overheating may result in undesirable crystallization

Contemporary long-term trends in water discharge, suspended sediment load, and erosion intensity in river basins of the north caucasus region, sw russia

For the first time, contemporary trends in water discharge, suspended sediment load, and the intensity of overall erosion in the river basins of the North Caucasus region, as one of Russia’s most agriculturally developed geographic areas, were identified. The study was carried out using monitoring data of the Federal Service for Hydrometeorology and Environmental Monitoring of the country for 21 rivers by comparing two periods: 1963–1980 and 2008–2017. According to the study’s results, trends of an increase in the mean annual water discharge (by 2–97%) and the essential reduction in its intra-annual variability have been found in most of the studied rivers. On the contrary, the trends of reduction in annual suspended sediment load and the intensity of erosion in the river basins were identified in most of the study region. Their most essential and statistically significant decreases (by 47–94%) were recorded within the Stavropol Upland, which several decades ago was considered one of the most erosion-dangerous territories of the entire country, as well as in some river basins of the central part of the Greater Caucasus’s northern slope (by 17–94%). The changes in climate (reducing the depth of soil freezing and meltwater runoff on the soil) and land use/cover (reduction of acreage and load (pressure) of agricultural machinery on the soil, reducing livestock on pastures, and the transfer of water from the neighboring, more full-flowing rivers) are considered the leading causes of the aforementioned trends. The findings will contribute to solving some economic and environmental problems of both the region and adjacent territories and water areas

Modelling of Selective Laser Melting Process of Quartz Glass at Elevated Temperatures

Selective laser melting (SLM) to date is the method of additive manufacturing allowing fabricating products from powder layer-by-layer according to a 3D model. However, when applying this method to fragile materials, parts crack while fabricating due to high temperatures. Quartz glass is a promising material for fabricating products by SLM without cracks due to a low thermal expansion. However, quality of fabricated material differs from the fused cast ones. This article aims to test the method of SLM with preheating to improve the material quality. Experiments on single track formation in SLM are analysed by modelling the coupled processes of heat transfer and powder consolidation in the laser-interaction zone. The mathematical model is validated by the experiments. It is shown that the preheating can improve the material quality and increase the process productivity but overheating may result in undesirable crystallization

Contemporary trends in river flow, suspended sediment load, and soil/gully erosion in the south of the boreal forest zone of european russia: The vyatka river basin

Recent decades in the north of the East European Plain have been characterized by significant changes in climate and land use/cover, especially after the collapse of the USSR in 1991. At the same time, the hydrological consequences of these changes, especially changes in erosion processes and river sediment load, have been studied insufficiently. This paper partially covers this existing knowledge gap using the example of the Vyatka River basin. Draining an area of 129,000 km2, the Vyatka River is among the largest rivers in the boreal forest zone of European Russia. Cultivated land occupies about one-fifth of the river basin area; about three-fourths is covered by taiga forest vegetation. The results of state long-term hydrometeorological monitoring and information on land use/cover made it possible to reveal contemporary (since the 1960s) hydrological and erosion-intensity trends and their drivers within the greater (96%) part of the river basin. There has been a statistically insignificant increase in water discharge in the Vyatka River basin during recent decades. This is due to a statistically insignificant increase (for the entire basin studied) in the spring snowmelt-induced floodwater flow and a statistically significant rise in the discharge in the year’s warm and cold seasons. The main reason for the detected trends is increased precipitation, including heavy rainfall during the warm season. In contrast to this, the total annual suspended sediment load of the river (especially that which was snowmelt-induced) and, consequently, soil/gully erosion intensity have experienced a significant decrease in recent decades (up to 58% between 1960–1980 and 2010–2018). Land-use/-cover changes (a reduction of cultivated land area and agricultural machinery, a decline of livestock in pastures) following the collapse of the Soviet Union are considered the main reasons for this decrease. The most noticeable changes in water discharge, suspended sediment load, and erosion intensity were observed in the most agriculturally developed southwest and south parts of the Vyatka River basin. All the above trends may be considered with a high probability to be representative for the south sector of the taiga zone of the East European Plain