Grain count in castings: theoretical background and experimental verification

In this work, a theoretical model is proposed for heterogeneous nucleation on substrates whose size distributions can be described by the Weibull statistics. In particular, the proposed model suggests that the size distribution for the various nucleation sites is exponential in nature. Measurements of grain count were carried out on experimental Al–1·3Si and Al–5·0Cu single phase alloys inoculated using an Al–5Ti–1B master alloy. In addition, experimental nodular and flake graphite iron castings were processed under various metallurgical conditions. In single phase alloys, the area of the equiaxed dendritic grain count was estimated from the electron backscattering diffraction analysis, whereas the graphite nodule and graphite eutectic cell count were estimated on polished cast iron surface sections by stereological means. In addition, maximum undercoolings were determined by thermal analysis. The experimental outcome indicates that the grain count can be properly described by a proposed exponential function of the maximum undercooling at the onset of alloy solidification. Finally, the magnitudes of the nucleation parameters were experimentally determined in this work.     

Bubble Growth Rates and Nucleation Site Densities in Saturated Pool Boiling of Water at High Pressures

Experiments were carried out to observe boiling behaviors of water on horizontal and vertical surfaces at pressures from 0.35 to 5 MPa. The growth curves of the primary bubbles are well described by the t^1/2 variation over the whole range of pressures. The growth rates of primary bubbles are proportional to the square root of the Jakob number, and agree with the correlation by Labuntsov with the arbitrary constant β = 3. The conventional correlations of bubble growth rates, which are directly proportional to the Jakob number, predict slower growth rates at higher pressures. The coalescence behaviors of the primary bubbles were also measured on the vertical surface at 3.66 MPa. The coalesced bubbles, which were formed by the coalescence of two primary bubbles, grow at rates similar to the rates of the primary bubbles. The nucleation site densities measured on the vertical surface at pressures up to 5 MPa increase in proportion to about the 1.5th power of the pressure under equivalent heat flux conditions. The dependence of the nucleation site densities on the heat flux is very similar to the results obtained near atmospheric pressure where the nucleation site density is proportional to the 1.5th power of the heat flux. The nucleation site densities measured in the range of pressures of 0.35 to 5 MPa and at heat fluxes of 0.05 to 0.35MW/m² agree fairly well with the available correlations.     

Concurrent nucleation, formation and growth of two intermetallic compounds (Cu6Sn5 and Cu3Sn) during the early stages of lead-free soldering

This study investigates the concurrent nucleation, formation and growth of two intermetallic compounds (IMCs), Cu₆Sn₅ (η) and Cu₃Sn (ε), during the early stages of soldering in the Cu-Sn system. The nucleation, formation and growth of the IMC layers is simulated through a multiphase-field model [1] and [2] in which the concurrent nucleation of both IMC phases is considered to be a stochastic Poisson process with nucleation rates calculated from classical nucleation theory [3]. CALPHAD thermodynamic models are used to calculate the local contributions to the free energy of the system and the driving forces for precipitation of the IMC phases. The nucleation parameters of the η phase are estimated from experimental results [4] and those of the ε phase are assumed to be similar. A parametric investigation of the effects of model parameters (e.g. grain boundary (GB) diffusion rates, interfacial and GB energies) on morphological evolution and IMC layer growth rate is presented and compared with previous works in which nucleation was ignored [5]. In addition, the resulting growth rates are compared with the available literature and it is found that, for a certain range in the model parameters, the agreement is quite satisfactory. This work provides valuable insight into the dominant mechanisms for mass transport as well as morphological evolution and growth of IMC layers during early stages of Pb-free soldering.     

Deformation and failure in nodular cast iron

Ductile failure in nodular cast iron is explored through uniaxial tension and notched tension experiments. Specimens obtained through tests interrupted at various stages of deformation and failure evolution were examined through quantitative microscopy to discern the mechanisms of failure and to quantitatively evaluate the local strain evolution. Fractographic observations were used to identify the onset and evolution of damage processes during the deformation and failure of nodular cast iron. These tests and observations reveal that void growth and coalescence occurred only within a narrow localized band, whose size is comparable with the size of the graphite nodules; no statistically significant changes in the porosity were observed outside this zone.     

汽车隔热板双色注射模设计

对汽车隔热板进行了工艺分析,结合客户需求和制品特点,确定了制品的分型面和注塑设备,并对成型零件、浇注系统、推出机构等进行了设计,模具运行可靠,生产的制品达到了使用要求。     

Study of different types of monomer emulsion feedings to semibatch emulsion polymerization reactors

Semibatch emulsion polymerization processes with a monomer emulsion feed are of great importance in both academia and industry. Monomer emulsion feeds can be applied to semibatch reactors using either a stream of an emulsified monomer or two streams of a neat monomer feed and an aqueous solution of an emulsifier. The effect of the feeding policy on the rate of polymerization and on the secondary particle formation was studied for a seeded semibatch emulsion polymerization of styrene. When a single-stream monomer emulsion feed is applied to a semibatch process, the monomer-swollen micelles formed in the feed might become the locus of initiation upon entering the reaction vessel. Under the conditions of this study, the application of monomer emulsion feed in either one stream or two streams did not result in secondary particle formation. The incoming monomer-swollen micelles were disintegrated to supply emulsifier molecules for the stability of growing particles, before they can capture radicals and become polymer particles. The rate of polymerization was found to be independent of the way that the monomer emulsion feed is added. In the absence of nitrogen, the rate of polymerization decreased more appreciably for the monomer emulsion feed, due to the oxygen dissolved in the emulsified monomer. The number of particles, however, was not affected by the purging policy. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 2472–2477, 2001     

Thermodynamic mechanism of free heme action on sickle cell hemoglobin polymerization

For insights into the mechanisms of heme action on the rate of sickle cell hemoglobin polymerization, we determine the erythrocytic concentration of free heme using a novel method based on enzymatic catalysis and luminescence. We find in sickle cell patients 44 ± 10 µM, in sickle trait individuals, 33 ± 4 µM, and in healthy adults, 21 ± 2 µM. We test the applicability of two mechanisms of heme action: a kinetic one, whereby heme aggregates serve as heterogeneous nucleation centers, and a thermodynamic pathway, in which free heme enhances the attraction between sickle hemoglobin (HbS) molecules in solution. We show that the latter mechanism exclusively operates. The enhanced attraction leads to increase of the total volume of a population of dense liquid clusters by about two orders of magnitude. As the dense liquid clusters serve as locations and precursors to the formation of the HbS polymer nuclei, their increased volume directly leads to faster polymer nucleation. © 2015 American Institute of Chemical Engineers AIChE J, 61: 2861–2870, 2015     

纳米金刚石膜的成核及生长特性研究

采用直流等离子体喷射化学气相沉积(DC arc pla sma jet)系统制备了纳米金刚石膜(NCD),研究了不同的金刚石成核剂溶液对NCD 膜的成核及生长的影响。研究表明,在成核剂溶液中添加二甲基亚砜(DMSO)后成核密度明显 得到提高,而 且制备的NCD膜晶粒分布均匀、致密。当金刚石粉体作为成核剂时,随着其粒径 的增大,NCD膜成核密度下降,晶粒的尺寸均匀性也变差,而粒径为5nm的金刚石纳米粒 子作为成核剂时,NCD膜晶粒间结合致密、颗粒分布均匀。最后,选择5nm的金刚石纳米 粒子和丙酮/DMSO配制的分散液作为成核剂配方,经过60min的生长 ,制备了粒径为50～70 nm的结晶性和品质良好的NCD膜,适用于高频声表面波(SAW)器件及各种光学窗口的研制。     

Identification of cell‐nucleation mechanism in foam injection molding with gas‐counter pressure via mold visualization

The mechanisms of cell nucleation and growth are investigated in foam injection molding (FIM) using gas‐counter pressure (GCP). An in‐situ mold visualization technique is employed. The application of GCP suppresses cell nucleation, and prevents the blowing agent from escaping during mold‐filling. The inherent structural heterogeneity in the regular FIM can be improved because of the uniform cavity pressure when employing GCP. The cavity pressure profiles show much faster pressure‐drop rates using GCP, because the single‐phase polymer/gas mixture has a lower compressibility than the two‐phase polymer/bubble mixture. Therefore, both the cell nucleation and growth rates are significantly increased through a higher pressure‐drop rate on the removal of the GCP. The effect of GCP magnitude on the cell morphology is explored. When the GCP is lower than the solubility pressure, bimodal foaming occurs. As the GCP increases above the solubility pressure, the cell density increases because of the higher pressure‐drop rate. © 2016 American Institute of Chemical Engineers AIChE J, 62: 4035–4046, 2016