1,721,244 research outputs found
Rheological investigation of semisolid alsi7 alloy by means of oscillation experiments
No full textDynamic Mechanical Analysis (DMA) of semisolid aluminum alloys was performed in a rheometer of Searle type. DMA was applied on a binary AlSi7 alloy to demonstrate the advantages of the method for the investigation of the behavior of alloys in semi-solid state and it was compared to classical shear experiments. Frequency sweeps, amplitude sweeps as well as constant condition experiments (CCE) were performed. It became obvious that elastic properties are getting more dominant with increasing resting time without shearing. The shift from a more viscous to more elastic nature of the material can be quantified. Interestingly, it was found that the semi-empirically based Cox-Merz rule, usually applied for polymers, holds for the semisolid material as well. This allows investigating the shear viscosity under different relevant conditions-important to improve material models for die-filling simulation of semisolid alloys
Experimental investigations on the formation of rosettes during shear
No full textIn this work, the effect of stirring conditions on agglomeration and coalescence-coarsening in isothermal, globulitic, semisolid AlCu10%wt was investigated. It is shown that for the investigated system, a shear rate regime exists which promotes crystallographic alignment of the agglomerated grains, leading to a rapid coalescence process, thereby the formation of rosettes. It is also experimentally validated that for lower shear rates, the size and porosity of aggregates are increased compared to that at higher level of shear
Crystallization and ripening phenomena in semi-solid steels
No full textRheological properties, crystallization and ripening phenomena have been investigated for three different steel grades in the semi-solid state. For each steel grade detailed investigation has been performed via differential scanning calorimetry to quantify the solid fraction as a function of temperature. The experimental procedure for all samples has been as follows: the completely liquid samples were cooled with a constant rate under constant shearing to temperatures corresponding to solid fractions of 30 and 40%. Tracking the viscosity during this procedure allowed analyzing the crystallization kinetics and final ripening process, which differ for the different steel grades. Evaluating the viscosity data on the base of a combination of convection dominated ripening model and a modified Krieger-Dougherty equation gives valuable information of the microstructural evolution. This is validated by additionally quenching samples
Rheological Characterization of Semi-Solid Metals: A Review
No full textIn the present review, the main findings on the rheological characterization of semi-solid metals (SSM) are presented. Experimental results are a fundamental basis for the development of comprehensive and accurate mathematics used to design the process effectively. For this reason, the main experimental procedures for the rheological characterization of SSM are given, together with the models most widely used to fit experimental data. Subsequently, the material behavior under steady state condition is summarized. Also, non-viscous properties and transient conditions are discussed since they are especially relevant for the industrial semi-solid processing
Visco-Elastic Properties of Semi-Solid Alloys
No full textOscillation and creep experiments have been performed with Semi-Solid Material (SSM) AlSi7 with 35% solid fraction to investigate the early visco-elastic properties after shearing of the material in a Searle Rheometer. The preparation of the SSM has been done in situ using a standard procedure to guarantee for all experiments the same initial properties of the material. First, oscillation experiments at low amplitude allowed to study the evolution of material structure with time. Subsequently, creep experiments have been performed changing the resting period based on previous results. Creep experiments are characterized by exposing the material to a sudden increase of shear stress. The resolution in time has been 0.01 seconds, which allows observing the dynamics of the development of visco-elastic properties. The material exhibits viscoelastic properties that are becoming more pronounced with longer resting time. This is in accordance with previous experiments where the ratio between elastic and viscous properties increases with increasing resting time. The development of the elastic properties follows the increase of the yield stress due to the creation of an internal structure of the material, which starts immediately after stopping shearing. The investigation of the short-term response of SSM can be particularly relevant for industrial practice, where material deformation during die filling is very fast and the material flow does not take place in steady-state condition
Rheological properties of liquid metals and semisolid materials at low solid fraction
No full textRheological properties of liquid metals are difficult to investigate experimentally because of the extreme border conditions to consider. One difficulty is related to the low viscosity of liquid metals. Surface tension effects can cause forces that can be considerably higher than the viscous forces in the liquid metals. Evaluating the experimental data without considering these effects leads to an apparent shear thinning behavior of the material. In the present study, experiments were performed by means of a Searle rheometer changing the dimension of the measuring system with metals of high surface tension, as mercury and tin. It became evident that surface tension plays a significant role in the effects that falsify measurements at low shear rate. Conclusions can be drawn to what extent measurements of semi-solid metals are affected
Numerical method for modelling spray quenching of cylindrical forgings
No full textNowadays, in steel industries, spray quenching has been used as a cooling method for the microstructure optimization after a forging process. In comparison with other cooling systems, there are many process parameters involved in spray quenching, which make it versatile, effective and able to adapt the quenching upon different cross sections of heavy parts. Simulation can represent a useful tool in properly designing the spray process for a specific forging and its reliability depends on the proper definition of input data, in particular of the heat transfer coefficient between the water and the part. Volumetric spray flux has been proposed as one of the preeminent factors on determining the heat transfer coefficient during spray quenching, in industrial processes, the volumetric spray flux is affected by the overlap of sprays on the forging surface due to the use of multi-nozzles systems. The present paper is aimed at developing a method able to take into account, by using a defined amplification factor, the effect of overlapping of sprays on the heat transfer coefficient to be applied in simulation of quenching processes
Untersuchungen zur katalytischen Oxidation von submikronen Kohlenstoffpartikeln aus motorischen Abgasen
No full textThe combustion of fossil fuel (e.g. coal, oil, wood) produces gaseous pollutants (e.g. nitrogen oxide, carbon monoxide and hydrocarbons) and a multitude of submicron carbon particles and particles on the micrometer scale (soot) simultaneously. In general, the number of nanoparticles per cubic metre exhaust predominate the number of particles in the micrometer range clearly. For example, modern diesel engines emit a few 10 000 soot particles per cm3 exhaust of size above several 100 nm compared to up to 107 carbon particles in the range between 10 and 200 nm. One promising approach to eliminate the soot particles in the submicron range from engine exhaust is the usage of a catalytic coated electrically heatable SiC-filter following the so- called Jülicher concept. This work is the first step in the development of such an filter. Several catalysts were investigated. They were chosen because of their anticipated applicability. A laboratory reactor was therefore developed with an externally heated reactor tube and a model soot generator. The model soot was produced with an spark discharge generator in a nitrogen carrier gas stream. After adding an oxidation media (oxygen or nitrogendioxide), the aerosol was fed into the reactor, which contains the catalyst coated onto alumina oxide spheres with 1.5 to 1.7 mm in diameter. The oxidation product was recorded in dependence of the granular bed temperature. Furthermore the particle phase was mainly measured before and behind the reactor with a Scanning Mobility Particle Sizer (SMPS) and a Photoelectric Aerosol Sensor (PAS). The contact between soot particles and the catalyst was investigated with the before mentioned equipment taking a theoretical determination of the particle deposition on the catalyst bed based on the Happel-Model into account. The theoretical model includes the deposition mechanisms: particle diffusion, interception, impaction, gravitational settling and thermophoresis together with the mechanisms of tempering and thermal coagulation (agglomeration), which both affect the morphology of the particles. A good agreement between calculation and measurement was achieved with deviations of +/- 20% at the most. Investigations of the catalytic particle oxidation with oxygen pointed out that precious metal as well as non-precious metal catalysts show a significant activity below 200°C. A two-step oxidation process was observed during oxidation of the model soot and could be described with a double Arrhenius equation. The variation of the dwell time in the reactor mainly affected only the soot particle deposition in the granular bed and in conclusion the amount of particle mass accumulated under the chosen experimental conditions. An oxygen content of more than 10 vol.-% had no influence of the oxidation of the deposited particles, whereas a dependence on oxygen concentrations up to 20 vol.-% was observed concerning the transmitted particle phase above 625°C . Compared with oxygen, the usage of nitrogendioxid as oxidation medium lowers the onset of the particle oxidation down to about 100°C approximately. Furthermore a higher turnover was measured in the lower temperature region. Final tests with dry diesel soot and oxygen were performed to compare the diesel soot oxidation with the results obtained with the model soot. A two-step oxidation process with oxygen as oxidation medium was observed as well. The oxidation onset temperature increases by 75°C at the most, when using the diesel soot in stead of the model soot
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
- …
