Mechanical and corrosion properties of full liquid phase sintered WE43 magnesium alloy specimens fabricated via binder jetting additive manufacturing

This study investigates full liquid phase sintering as a process of fabrication parts from WE43(Mg-4wt.%Y-3wt.%RE-0.7wt.%Zr)alloy using binder jetting additive manufacturing(BJAM).This fabrication process is being developed for use in producing structural or biomedical devices.Specifically,this study focused on achieving a near-dense microstructure with WE43 Mg alloy while substantially reducing the duration of sintering post-processing after BJAM part rendering.The optimal process resulted in microstructure with 2.5%porosity and significantly reduced sintering time.The improved sintering can be explained by the presence of Y_(2)O_(3)and Nd_(2)O_(3)oxide layers,which form spontaneously on the surface of WE43 powder used in BJAM.These layers appear to be crucial in preventing shape distortion of the resulting samples and in enabling the development of sintering necks,particularly under sintering conditions exceeding the liquidus temperature of WE43 alloy.Sintered WE43 specimens rendered by BJAM achieved significant improvement in both corrosion resistance and mechanical properties through reduced porosity levels related to the sintering time.

Constitutive behavior and processing maps of a new wrought magnesium alloy ZE20 (Mg-2Zn-0.2Ce)

ZE20(Mg-2Zn-0.2Ce)^2 is a new wrought magnesium alloy with improved extrudability and mechanical properties[1].To understand the constitutive behavior and workability of this new alloy,Gleeble thermomechanical testing has been carried out in this study.The flow stress behavior of ZE20 was investigated between 250℃–450℃ and 10^–3 s^–1–1.0 s^–1 in isothermal compression.Constitutive descriptions of the flow stress are provided.A new general approach at application of the extended Ludwik equation is demonstrated and was found to be more accurate than the hyperbolic sine Arrhenius model while having a similar number of model constants.Processing maps were developed based on the experimental results and are verified with microstructural investigation.A region of safe processing with non-basal texture and high activity of dynamic recrystallization(DRX)was found between 375℃ and 450℃,from 10^–1 s^–1 to 10^–2.5 s^–1.A region of potentially safe processing with annealing that is associated with shear band nucleation of non-basal grains was identified for temperatures as low as 300℃ and rates as high as 10^–1 s^–1.

Phase equilibria and microstructure investigation of Mg-Gd-Y-Zn alloy system

In order to develop high strength Mg-Gd-Y-Zn alloys,key experiments coupled with CALPHAD(CALculation of PHAse Diagrams)calculations were carried out in the current work to provide critical understanding of this important alloy system.Three Mg-10 Gd-xY-yZn(x=4 or 5,y=3 or 5,wt.%) were mapped on Mg-Gd-Y-Zn phase diagrams for phase equilibria and microstructure investigation.Electron microscopy was performed for phase identification and phase fraction determination in as-cast and solution treated conditions.In all three alloys,the major phases were Mg-matrix and long period stacking order(LPSO) 14 H phase.With ST at 400 and 500℃,the phase fraction of LPSO 14 H increased,particularly the fine lamellar morphology in the Mg matrix.The as-cast and 400℃ Mg10 Gd5 Y3 Zn samples had Mg(Gd,Y) present.At 500℃,Mg(Gd,Y) is not stable and transforms into LPSO 14 H.The Mg 10 Gd5 Y5 Zn alloy included the WPhase,which showed a reduction in phase fraction with solution treatment.These experimental results were used to validate and improve the thermodynamic database of the Mg-Gd-Y-Zn system.Thermodynamic calculations using the improved database can well describe the available experimental results and make accurate predictions to guide the development of promising high-strength Mg-Gd-Y-Zn alloys.

Influence of UVB-Irradiation on the Structures and Solid Particle Erosion Resistance for CF/PC Composites

The service life and properties of Carbon fiber reinforced polycarbonate (CF/PC) composites are seriously affected by ultraviolet radiation from outdoor exposure during aging. In this work, the changes of structure and solid particle erosion resistance for CF/PC composites after ultraviolet irradiation were studied. It was shown that ultraviolet irradiation causes photo-oxygen aging and photo-fries re-arrangement of the composite, and the result was confirmed by FTIR. We correlated the solid particle erosion resistance with aging time, and found that the solid particle erosion resistance of CF/PC composites greatly decreased by UVB irradiation during 15 hours. Furthermore, the eroded material surface was analyzed using scanning electron microscope (SEM). It suggests that ultraviolet aging leads to plasticization and degradation, resulting in reduction of erosion resistance of the composite.

Magnesium casting technology for structural applications

This paper summarizes the melting and casting processes for magnesium alloys.It also reviews the historical development of magnesium castings and their structural uses in the western world since 1921 when Dow began producing magnesium pistons.Magnesium casting technology was well developed during and after World War II,both in gravity sand and permanent mold casting as well as high-pressure die casting,for aerospace,defense and automotive applications.In the last 20 years,most of the development has been focused on thin-wall die casting applications in the automotive industry,taking advantages of the excellent castability of modern magnesium alloys.Recently,the continued expansion of magnesium casting applications into automotive,defense,aerospace,electronics and power tools has led to the diversification of casting processes into vacuum die casting,low-pressure die casting,squeeze casting,lost foam casting,ablation casting as well as semi-solid casting.This paper will also review the historical,current and potential structural use of magnesium with a focus on automotive applications.The technical challenges of magnesium structural applications are also discussed.Increasing worldwide energy demand,environment protection and government regulations will stimulate more applications of lightweight magnesium castings in the next few decades.The development of use of Integrated Computational Materials Engineering(ICME)tools will accelerate the applications of magnesium castings in structural applications.

Towards high strength cast Mg-RE based alloys:Phase diagrams and strengthening mechanisms

Mg-rare earth(RE)based systems provide several important commercial alloys and many alloy development opportunities for high strength applications,especially in aerospace and defense industries.The phase diagrams,microstructure,and strengthening mechanisms of these multicomponent systems are very complex and often not well understood in literature.We have calculated phase diagrams of important binary,ternary,and multicomponent RE-containing alloy systems,using CALPHAD(CALculation of PHAse Diagrams).Based on these phase diagrams,this paper offers a critical overview on phase equilibria and strengthening mechanisms in these alloy systems,including precipitation,long period stacking order(LPSO),and other intermetallic phases.This review also summarized several promising Mg-RE based cast alloys in comparison with commercial WE54 and WE43 alloys;and explored new strategies for future alloy development for high strength applications.It is pointed out that the combination of precipitation and LPSO phases can lead to superior strength and ductility in Mg-RE based cast alloys.The precipitates and LPSO phases can form a complex three-dimensional network that effectively impedes dislocation motion on the basal and non-basal planes.The LPSO phases can also prevent the coarsening of precipitates when they interact,thus providing good thermal stability at elevated temperatures.Future research is needed to determine how the combination of these two types of phases can be used in alloy design and industrial scale applications.

Magnesium research and applications: Past, present and future

As the lightest structural metal and one of the most abundant metallic elements on earth, magnesium(Mg) has been used as an "industrial metal" for lightweighting in the transportation and electronics industries, in addition to other traditional applications in aluminum alloying,steel desulfurization and protective anodes. In recent years, research has shown significant potential for Mg to become a "technology metal"in a variety of new applications from energy storage/battery to biomedical products. However, global Mg production has shown steady but moderate growth in the last three decades. Mg applications as an industry metal are still limited due to some sustainability concerns of primary Mg production, as well as a number of technical issues related to the structural and corrosion performance of commercial Mg alloys.New Mg applications as an industrial or technology metal face tremendous technical challenges, which have been reflected in the intensified global research efforts in the last twenty years. This paper will review some past and present applications, and discuss future opportunities and challenges for Mg research and applications for the global Mg community.

Bimodal grain structure formation and strengthening mechanisms in Mg-Mn-Al-Ca extrusion alloys

The effects of small additions of calcium (0.1%and 0.5%~1) on the dynamic recrystallization behavior and mechanical properties of asextruded Mg-1Mn-0.5Al alloys were investigated.Calcium microalloying led to the formation of Al_(2)Ca in as-cast Mg-1Mn-0.5Al-0.1Ca alloy and both Mg_(2)Ca and Al_(2)Ca phases in Mg-1Mn-0.5Al-0.5Ca alloy.The formed Al_(2)Ca particles were fractured during extrusion process and distributed at grain boundary along extrusion direction (ED).The Mg_(2)Ca phase was dynamically precipitated during extrusion process,hindering dislocation movement and reducing dislocation accumulation in low angle grain boundaries (LAGBs) and hindering the transformation of high density of LAGBs into high angle grain boundaries (HAGBs).Therefore,a bimodal structure composed of fine dynamically recrystallized (DRXed) grains and coarse un DRXed regions was formed in Ca-microalloyed Mg-1Mn-0.5Al alloys.The bimodal structure resulted in effective hetero-deformation-induced (HDI) strengthening.Additionally,the fine grains in DRXed regions and the coarse grains in un DRXed regions and the dynamically precipitated Mg_(2)Ca phase significantly enhanced the tensile yield strength from 224 MPa in Mg-1Mn-0.5Al to335 MPa and 352 MPa in Mg-1Mn-0.5Al-0.1Ca and Mg-1Mn-0.5Al-0.5Ca,respectively.Finally,a yield point phenomenon was observed in as-extruded Mg-1Mn-0.5Al-x Ca alloys,more profound with 0.5%Ca addition,which was due to the formation of (■) extension twins in un DRXed regions.

Reducing Refractive Index Variations in Compression Molded Lenses by Annealing

Compression molding of glass optics is gradually becoming a viable fabrication technique for high precision optical lenses. However, refractive index variation was observed in compression molded glass lenses, which would contribute to image quality degradation. In this research, annealing experiments were applied to control the refractive index variation in molded glass lenses. The refractive index variations pre and post annealing experiment in molded lenses were measured by an experiment setup based on Mach-Zehnder interferometer. The experimental results showed that the refractive index variation can be controlled providing that a proper cooling process is applied during cooling.

Storing Freshwater Versus Storing Electricity in Power Systems with High Freshwater Electric Demand

We consider a power system whose electric demand pertaining to freshwater production is high(high freshwater electric demand),as in the Middle East,and investigate the tradeoff of storing freshwater in tanks versus storing electricity in batteries at the day-ahead operation stage.Both storing freshwater and storing electricity increase the actual electric demand at valley hours and decrease it at peak hours,which is generally beneficial in term of cost and reliability.But,to what extent?We analyze this question considering three power systems with different generation-mix configurations,i.e.,a thermal-dominated mix,a renewable-dominated one,and a fully renewable one.These generation-mix configurations are inspired by how power systems may evolve in different countries in the Middle East.Renewable production uncertainty is compactly modeled using chance constraints.We draw conclusions on how both storage facilities(freshwater and electricity)complement each other to render an optimal operation of the power system.

Three-dimensional visualization and quantification of microporosity in aluminum castings by X-ray micro-computed tomography

Porosity is a major issue in solidification processing of metallic materials.In this work,wedge die casting experiments were designed to investigate the effect of cooling rate on microporosity in an aluminum alloy A356.Microstructure information including dendrites and porosity were measured and observed by optical microscopy and X-ray micro-computed tomography(XMCT).The effects of cooling rate on secondary dendrite arm spacing(SDAS)and porosity were discussed.The relationship between SDAS and cooling rate was established and validated using a mathematical model.Three-dimensional(3-D)porosity information,including porosity percentage,pore volume,and pore number,was determined by XMCT.With the cooling rate decreasing from a lower to a higher position of the wedge die,the observed pore number decreases,the porosity percentage increases,and the equivalent pore radius increases.Sphericity of the pores was discussed as an empirical criterion to distinguish the types of porosity.For different cooling rates,the larger the equivalent pore radius is,the lower the sphericity of the pores.This research suggests that XMCT is a useful tool to provide critical 3-D porosity information for integrated computational materials engineering(ICME)design and process optimization of solidification products.

Wind-integration benefits of controlled plug-in electric vehicle charging

Flexibility in plug-in electric vehicle(PEV) charging can reduce the ancillary cost effects of wind variability and uncertainty on electric power systems. In this paper, we study these benefits of PEV charging, demonstrating that controlled PEV charging can reduce costs associated with wind uncertainty and variability. Interestingly, we show that the system does not require complete control of PEV-charging loads to mitigate the negative cost impacts of wind variability and uncertainty. Rather, PEV owners giving the system a two-hour window of flexibility in which to recharge their vehicles provides much of the benefits that giving full charging control does.

Benefits of Stochastic Optimization for Scheduling Energy Storage in Wholesale Electricity Markets

We propose a two-stage stochastic model for optimizing the operation of energy storage. The model captures two important features: uncertain real-time prices when day-ahead operational commitments are made;and the price impact of charging and discharging energy storage. We demonstrate that if energy storage has full flexibility to make real-time adjustments to its day-ahead commitment and market prices do not respond to charging and discharging decisions, there is no value in using a stochastic modeling framework, i.e., the value of stochastic solution is always zero. This is because in such a case the energy storage behaves purely as a financial arbitrageur day ahead, which can be captured using a deterministic model.We show also that prices responding to its operation can make it profitable for energy storage to "waste" energy, for instance by charging and discharging simultaneously, which is normally sub-optimal. We demonstrate our model and how to calibrate the price-response functions from historical data with a practical case study.

Expansion Planning for Renewable Integration in Power System of Regions with Very High Solar Irradiation

In this paper,we address the long-term generation and transmission expansion planning for power systems of regions with very high solar irradiation.We target the power systems that currently rely mainly on thermal generators and that aim to adopt high shares of renewable sources.We propose a stochastic programming model with expansion alternatives including transmission lines,solar power plants(photovoltaic and concentrated solar),wind farms,energy storage,and flexible combined cycle gas turbines.The model represents the longterm uncertainty to characterize the demand growth,and the short-term uncertainty to characterize daily solar,wind,and demand patterns.We use the Saudi Arabian power system to illustrate the functioning of the proposed model for several cases with different renewable integration targets.The results show that a strong dependence on solar power for high shares of renewable sources requires high generation capacity and storage to meet the night demand.

Using Interim Recommitment to Reduce the Operational-cost Impacts of Wind Uncertainty

Using wind-availability forecasts in day-ahead unit commitment can require expensive real-time operational adjustments.We examine the benefit of conducting interim recommitment between day-ahead unit commitment and real-time dispatch.Using a simple stylized example and a case study that is based on ISO New England,we compare system-operation costs with and without interim recommitment.We find an important tradeoff—later recommitment provides better wind-availability forecasts,but the system has less flexibility due to operating constraints.Of the time windows that we examine,hour-20 recommitment provides the greatest operational-cost reduction.

Sensitivity-based Vulnerability Assessment of State Estimation

We propose a technique to assess the vulnerability of the power system state estimation. We aim at identifying the measurements that have a high potential of being the target of false data injection attacks. From the perspective of the adversary, such measurements have the following characteristics: ① being influential on the variable estimates;② corrupting their measured values is likely to be undetected. Additionally, such characteristics should not change significantly with the system operation condition. The proposed technique provides a systematic way of identifying the measurements with such characteristics. We illustrate our methodology on a 4-bus system, the New England 39-bus system, and the IEEE 118-bus test system, respectively.

Cellular automaton simulation and experimental validation of eutectic transformation during solidification of Al-Si alloys

The morphology of eutectic silicon in solidification microstructure is critical to the performance of Al-Si-based alloys.Simulating eutectic Si phase formation has been a challenge in ICME (integrated computational materials engineering) based design and manufacturing of solidification products of Al-Si-based alloys.In this study,our previous three-dimensional (3-D) cellular automaton (CA) model for α-Al dendritic growth was extended to include eutectic (α-Al + Si) transformation in multi-dendrite domains,providing a complete solidification simulation of critically important Al-Si based alloys.The quantitative results of the Si phase in the eutectic microstructure were experimentally validated using scanning electron microscopy and deep etching techniques.The simulation results show a good agreement with the experimental observations and calculations by the Scheil model and lever rule.This 3-D CA model is useful for predicting and optimizing the solidification microstructure including eutectic transformation during solidification processing such as casting,potentially welding,and additive manufacturing.

Predicting gas and shrinkage porosity in solidification microstructure:A coupled three-dimensional cellular automaton model

Porosity formation during solidification of aluminum-based alloys,due to hydrogen gas and alloy shrinkage,has been a major issue adversely affecting the performance of solidification products such as castings,welds or additively manufactured components.A three-dimensional cellular automaton(CA)model has been developed,for the first time,to couple the predictions of hydrogen-induced gas porosity and shrinkage porosity during solidification microstructure evolution of a binary Al-Si alloy.The CA simulation results are validated under various cooling rates by porosity measurements in an experimental wedge die casting using X-ray micro computed tomography(XMCT)technique.This validated porosity moel provides a critical link in integrated computation materials engineering(ICME)design and manufacturing of solidification products.

A vector autoregression weather model for electricity supply and demand modeling

Weather forecasting is crucial to both the demand and supply sides of electricity systems. Temperature has a great effect on the demand side. Moreover, solar and wind are very promising renewable energy sources and are, thus, important on the supply side. In this paper, a large vector autoregression(VAR) model is built to forecast three important weather variables for 61 cities around the United States. The three variables at all locations are modeled as response variables. Lag terms are used to capture the relationship between observations in adjacent periods and daily and annual seasonality are modeled to consider the correlation between the same periods in adjacent days and years. We estimate the VAR model with16 years of hourly historical data and use two additional years of data for out-of-sample validation. Forecasts of up to six-hours-ahead are generated with good forecasting performance based on mean absolute error, root mean square error, relative root mean square error, and skill scores. Our VAR model gives forecasts with skill scoresthat are more than double the skill scores of other forecasting models in the literature. Our model also provides forecasts that outperform persistence forecasts by between6% and 80% in terms of mean absolute error. Our results show that the proposed time series approach is appropriate for very short-term forecasting of hourly solar radiation,temperature, and wind speed.

Equilibria in Interdependent Natural-gas and Electric Power Markets: an Analytical Approach

Natural-gas and electric power systems and their corresponding markets have evolved over time independently. However, both systems are increasingly interdependent since combined cycle gas turbines that use natural gas to produce electricity increasingly couple them together. Therefore, suitable analysis techniques are most needed to comprehend the consequences on market outcomes of an increasing level of integration of both systems. There is a vast literature on integrated natural-gas and electric power markets assuming that the two markets are operated centrally by a single operator. This assumption is often untrue in the real world, which necessitates developing models for these interdependent yet independent markets. In this vein, this paper addresses the gap in the literature and provides analytical Nash-Cournot equilibrium models to represent the joint operation of natural-gas and electric power markets with the assumption that the market participants in each market make their own decisions independently seeking the maximum profits, as often is the case in the real world. We develop an analytical equilibrium model and apply the Karush-Kuhn-Tucker (KKT) approach to obtain Nash-Cournot equilibria for the interdependent natural-gas and electric power markets. We use a double-duopoly case to study the interaction of both markets and to derive insightful analytical results. Moreover, we derive closed-form analytical expressions for spot-market equilibria in both natural-gas and electric power markets, which are relevant and of practical significance for decision makers. We complement the double-duopoly study with a detailed sensitivity analysis.