Crystallization in additive manufacturing of metallic glass

Metallic glasses are non-crystalline metals that are obtained by rapid cooling of the melt to bypass crystallization. The amorphous atomic structure shows enhanced properties relative to the crystalline counterpart. For example, enhanced mechanical properties, improved corrosion resistance, as well as excellent soft magnetic properties. The unique properties of metallic glasses make them promising for a wide range of applications, e.g. spring materials,structural components, electrical motors, and biomedical implants. One drawback is the cooling rate required for glass formation, which limits the thickness of cast components to only a few millimeters. As a solution, additive manufacturing (AM) shows promising potential to produce large-scale metallic glass components. In AM, the solidification process is short and confined to a small volume that is repetitively added. Despite the highheating and cooling rates in AM, control of crystallization is still an issue and a complete understanding of the interplay between the thermal process and crystallization is missing. This thesis presents numerical simulations and experimental analyses related to the formation and growth of crystals in a Zr-based bulk metallic glass. The aim is to provide a better understanding of crystallization in metallic glasses during non-isothermal processing, with special emphasis on AM by laser powder bed fusion (LPBF). The experimental investigations involved in-situ small-angle neutron scattering measurements of nucleation and growth of crystals in a Zr-based metallic glass processed by LPBF and suction casting. It is concluded that crystals form at a higher rate in the material processed by LPBF as a result of the increased oxygen content. Further, the crystallization mechanisms were identified as rapid nucleation followed by diffusion-controlled growth in both materials. The numerical simulations are based on phase-field and classical nucleation and growth theory, which were developed to study the nucleation, growth, and dissolution of crystals in metallic glasses. The models have been used to predict time-temperature-transformation and continuous-heating/cooling-transformation diagrams, but also to simulate the crystallization process during LPBF by utilizing thermal finite element simulations of the laser-material interaction. The simulation results demonstrate several important aspects of crystallization in the LPBF process, such as the effect of rapid heating and cooling on the nucleation rate, the importance of the growth mode during cyclic reheating as well as the resulting gradients in particle size and density arising from localized laser processing. In particular, the results emphasize that numerical models that track the evolution of the particle size distribution are well suited for modeling crystallization in LPBF processing of metallic glass

Efficient Modeling of a Flexible Beam in Dymola using Coupled Substructures in a Floating Frame of Reference Formulation

In this Master Thesis a three dimensional Euler-Bernoulli beam model was implemented in the simulation software Dymola. The beam model is based on the Floating Frame of Reference formulation combined with the Craig-Bampton method. The theory is developed with the scope to capture the dynamic and static responses of a beam model in a compact and computer efficient implementation. The Thesis includes derivation of kinematic description, mass matrix, stiffness matrix and force vectors of an Euler-Bernoulli beam in three-dimensional space. Two one-dimensional models have been derived as well. The implementation in Dymola is described together with validation of the model, discussion and conclusions. The validation of the model shows great accuracy in static loading both in elongation, torsion and bending. Excitation of eigenfrequencies is possible but the results slightly differs from the analytical solutions. Dynamic tests of the beam model shows realistic responses but further testing on this subject is recommended. Compatibility with other components in Dymola works fine. However there are some minor issues that should be solved to enhance the efficiency. Overall the static and dynamic responses of the beam model works sufficiently well

Towards a Theory of Software Development Expertise

Software development includes diverse tasks such as implementing new features, analyzing requirements, and fixing bugs. Being an expert in those tasks requires a certain set of skills, knowledge, and experience. Several studies investigated individual aspects of software development expertise, but what is missing is a comprehensive theory. We present a first conceptual theory of software development expertise that is grounded in data from a mixed-methods survey with 335 software developers and in literature on expertise and expert performance. Our theory currently focuses on programming, but already provides valuable insights for researchers, developers, and employers. The theory describes important properties of software development expertise and which factors foster or hinder its formation, including how developers' performance may decline over time. Moreover, our quantitative results show that developers' expertise self-assessments are context-dependent and that experience is not necessarily related to expertise.Comment: 14 pages, 5 figures, 26th ACM Joint European Software Engineering Conference and Symposium on the Foundations of Software Engineering (ESEC/FSE 2018), ACM, 201

Climate impact of willow grown for bioenergy in Sweden

Short-rotation coppice willow (SRCW) is a fast-growing and potentially high-yielding energy crop. Transition to bioenergy has been identified in Sweden as one strategy to mitigate climate change and decrease the current dependency on fossil fuel. In this study, life cycle assessment was used to evaluate and compare the climate impacts of SRCW systems, for the purpose of evaluating key factors influencing the climate change mitigation potential of SRCW grown on agricultural land in Sweden. Seven different scenarios were defined and analysed to identify the factors with the most influence on the climate. A carbon balance model was used to model carbon fluxes between soil, biomass and atmosphere under Swedish growing conditions. The results indicated that SRCW can act as a temporary carbon sink and therefore has a mitigating effect on climate change. The most important factor in obtaining a high climate change-mitigating effect was shown to be high yield. Low yield gave the worst mitigating effect of the seven scenarios, but it was still better than the effect of the reference systems, district heating produced from coal or natural gas

Albedo on cropland: Field-scale effects of current agricultural practices in Northern Europe

Agricultural land use and management affect land surface albedo and thus the climate. Increasing the albedo of cropland could enhance reflection of solar radiation, counteracting the radiative forcing (RF) of greenhouse gases (GHGs) and local warming. However, knowledge is lacking on how agricultural practices affect albedo under local conditions, and on the benefits of individual practices. In this study, field measurements were made in 15 paired plots at a site in Northern Europe to determine albedo, net shortwave irradiance and RF impacts under various common crops, cultivation intensities and tillage practices. Field data for 2019-2020 were compared with satellite-based albedo for the surrounding region in 2010-2020. At regional level, different combinations of soil type, yearly weather and agricultural practices led to great variability in the albedo of individual crops, despite similar pedo-climatic conditions. At field level within years, albedo differences were determined mainly by crop type, species-specific phenology and post-harvest management. Annual albedo was higher with perennial ley (0.20-0.22) and winter-sown crops (0.18-0.22) than with spring-sown crops (0.16-0.18) and bare soil (0.13). Barley had the highest albedo among winter and spring cereals. In summer, when increased albedo could alleviate local heat stress, oats reduced net shortwave irradiance at the surface by 0.8-5.8 Wm(-2) compared with other cereals, ley, peas or rapeseed. Delayed or reduced tillage gave high local cooling potential (up to-13.6 Wm(-2)) in late summer. Potential benefits for global mean climate as GWP(100 )per hectare and year reached-980 kg CO(2)e for avoiding black fallow,-578 kg CO(2)e for growing a winter-sown variety and-288 kg CO(2)e for delayed tillage. Thus realistic albedo increases on cropland could have important effects on local temperatures and offset a substantial proportion of the RF deriving from field-scale GHG emissions on short time-scales

Albedo impacts of current agricultural land use: Crop-specific albedo from MODIS data and inclusion in LCA of crop production

Agricultural land use and management practices affect the global climate due to greenhouse gas (GHG) fluxes and changes in land surface properties. Increased albedo has the potential to counteract the radiative forcing and warming effect of emitted GHGs. Thus considering albedo could be important to evaluate and improve agricultural systems in light of climate change, but the albedo of individual practices is usually not known. This study quantified the albedo of individual crops under regional conditions, and evaluated the importance of albedo change for the climate impact of current crop production using life cycle assessment (LCA). Seven major crops in southern Sweden were assessed relative to a land reference without cultivation, represented by semi-natural grassland. Crop-specific albedo data were obtained from a MODIS product (MCD43A1 v6), by combining its spatial response pattern with geodata on agricultural land use 2011–2020. Fluxes of GHGs were estimated using regional data and models, including production of inputs, field operations, and soil nitrogen and carbon balances. Ten-year mean albedo was 6–11% higher under the different crops than under the reference. Crop-specific albedo varied between years due to weather fluctuations, but differences between crops were largely consistent. Increased albedo countered the GHG impact from production of inputs and field operations by 17–47% measured in GWP100, and the total climate impact was warming. Using a time-dependent metric, all crops had a net cooling impact on global mean surface temperature on shorter timescales due to albedo (3–12 years under different crops), but a net warming impact on longer timescales due to GHG emissions. The methods and data presented in this study could support increasingly comprehensive assessments of agricultural systems. Further research is needed to integrate climatic effects of land use on different spatial and temporal scales, and direct and indirect consequences from a systems perspective

Climate impact and energy efficiency from electricity generation through anaerobic digestion or direct combustion of short rotation coppice willow

Short rotation coppice willow is an energy crop used in Sweden to produce electricity and heat in combined heat and power plants. Recent laboratory-scale experiments have shown that SRC willow can also be used for biogas production in anaerobic digestion processes. Here, life cycle assessment is used to compare the climate impact and energy efficiency of electricity and heat generated by these measures. All energy inputs and greenhouse gas emissions, including soil organic carbon fluxes were included in the life cycle assessment. The climate impact was determined using time-dependent life cycle assessment methodology. Both systems showed a positive net energy balance, but the direct combustion system delivered ninefold more energy than the biogas system. Both systems had a cooling effect on the global mean surface temperature change. The cooling impact per hectare from the biogas system was ninefold higher due to the carbon returned to soil with the digestate. Compensating the lower energy production of the biogas system with external energy sources had a large impact on the result, effectively determining whether the biogas scenario had a net warming or cooling contribution to the global mean temperature change per kWh of electricity. In all cases, the contribution to global warming was lowered by the inclusion of willow in the energy system. The use of time-dependent climate impact methodology shows that extended use of short rotation coppice willow can contribute to counteract global warming