The surface tension of liquid silicon at high temperature

The measurement of the surface tension of liquid silicon has a long history with many results but no general agreement between them. Two values at the melting temperature are cited in reviews (749 and 827 mN/m [N. Eustathopoulos, E. Ricci, B. Drevet, Note Technique DEM No. 97/58, CEA, 1997]) but there are few arguments to determine the correct one. In the present study, new data for the surface tension obtained with the analysis of characteristic frequencies of a levitating drop are presented. The effect of oxygen and nitrogen are also considered. These data are compared with former data obtained with contactless techniques. The most recent surface tension values obtained with drop weights ranging on two orders of magnitude and environments of different natures (argon, hydrogen and vacuum) show excellent agreement (within a 1.5% margin) at temperatures between 1350 K and 2400 K. The comparison of these data to others obtained with different techniques, reveal a good agreement, except those obtained with the sessile drop technique on some supports like BN, SiO₂ and MgO. However, these special cases may be connected with the reactivity of silicon with these supports.     

连铸坯微观及宏观偏析数学模型的研究进展

对连铸坯微观和宏观偏析模型及树枝晶间液相流动的研究进展进行了评述，采用近平衡凝固过程溶质再分配理论并结合连铸传热数学模型对连铸坯微观及宏观偏析的定量解析方法进行了分析。     

Numerical simulation of solidification kinetics in A356/SiCp composites for assessment of as-cast particle distribution

The present work is aimed at studying the effect of solidification rate on reinforcement clustering in particle reinforced metal matrix composites (PMMCs) through numerical simulations and experimental studies. A macrotransport-solidification kinetics (MTSK) model was used to simulate the solidification kinetics of the PMMCs. The experimental validation of the numerical model was achieved through the Newtonian and Fourier thermal analysis methods. Results reveal that the MTSK model can be successfully used to predict the local microstructural scales and to evaluate the risk of cluster formation in cast particle reinforced composites.     

Effect of silicon content on microstructure and electrochemical behavior of hypoeutectic Al-Si alloys

The correlation of mechanical properties and corrosion behavior with microstructure parameters can be very useful for planning solidification conditions in order to achieve a desired level of final properties. The present study aims to investigate the influence of silicon content on the microstructural pattern, i.e., dendrite spacings and distribution of interdendritic phases on the corrosion behavior of Al-Si alloys castings. The corrosion resistance was analyzed by both the electrochemical impedance spectroscopy (EIS) technique and Tafel's plots carried out in a 0.5 M NaCl test solution at 25 °C. The increase on silicon content has provided a dendritic refinement and a more extensive redistribution of the eutectic mixture which has provoked a decrease on the corrosion resistance.     

Thermal analysis and solidification pathways of Mg–Al–Ca system alloys

Mg–Al–Ca alloys are creep resistant magnesium alloys with high application potentials. The solidification pathways and microstructure formation in this alloy system are still under discussion. In this paper, the solidification behavior of AZ91 and AM50 with Ca addition (AZC91x and AMC50x alloys) was investigated by a computer-aided cooling curve analysis (CA-CCA) system. Microstructure and phase identification were carried out by SEM and EDX analysis. The results show that the Ca-containing phase formation mainly depends on Ca content and Ca/Al ratio. With increasing the Ca/Al ratio these phases transform from Al₂Ca to (Mg, Al)₂Ca and Mg₂Ca. Moreover, Ca addition decreases the liquidus temperature of Mg–Al alloys, but influences the solidus temperature in a more complex way. Increasing the Ca content also decreases the solid fraction at which dendrite coherency occurs. The relationship between solidification interval, dendrite coherency point, formation of Ca-containing phases and hot tearing is also discussed.     

Effects of growth rate and temperature gradient on the microstructure parameters in the directionally solidified succinonitrile–7.5 wt.% carbon tetrabromide alloy

Succinonitrile (SCN)–7.5 wt.% carbon tetrabromide (CTB) alloy was unidirectionally solidified with a constant growth rate (V = 33 μm/s) at five different temperature gradients (G = 4.1–7.6 K/mm) and with a constant temperature gradient (G = 7.6 K/mm) at five different growth rates (V = 7.2–116.7 μm/s). The primary dendrite arm spacings, secondary dendrite arm spacings, dendrite tip radius and mushy zone depths were measured. Theoretical models for the microstructure parameters have been compared with the experimental observations, and a comparison of our results with the current theoretical models and previous experimental results have also been made.     

Effect of Ce addition on macroscopic core-shell structure of Cu-Sn-Bi immiscible alloy

To investigate the evolution of bimetallic composite structure of immiscible alloy, a macroscopic core-shell type structure of 24Cu-16Sn-60Bi alloy was fabricated which consisted of (Cu₃Sn, Cu₁₀Sn₃) central core and Bi periphery. Ce addition was found to be capable of enhancing remarkably Marangoni motion of secondary droplets in liquid matrix. The role of Ce addition was attributed to change interfacial tension of droplet/matrix. The results show the possibility of modulating core-shell type structure in immiscible materials by doping rare earth elements.     

Rapid surface microstructuring of porous alumina ceramic using continuous wave Nd:YAG laser

Laser surface microstructuring of porous alumina ceramic is investigated using a continuous wave Nd:YAG laser. The surface microstructure development is based on the rapid surface melting and subsequent solidification resulting in the evolution of crystallographic and morphological textures. The results indicated that surface microstructure is greatly influenced by the laser processing parameters. Detailed investigations of the influence of laser fluence on the thermal history and microstructural parameters are presented. Such investigations are intended to facilitate the tailoring of surface microstructures by controlling the laser processing parameters.     

Investigation of solidification cracking susceptibility during laser beam welding using an in-situ observation technique

In recent years, laser beam welding has found wide applications in many industrial fields. Solidification cracks are one of the most frequently encountered welding defects that hinder obtaining a safe weld joint. Decades of research have shown that one of the main causes of such cracks are the strain and the strain rate. Obtaining meaningful measurements of these strains has always been a major challenge for scientists, because of the specific environment of the measurement range and the many obstacles, as well as the high temperature and the plasma plume. In this study, a special experimental setup with a high-speed camera was employed to measure the strain during the welding process. The hot cracking susceptibility was investigated for 1.4301 stainless steel, and the critical strain required for solidification crack formation was locally and globally determined.     

Developments in Processing of Functionally Gradient Metals and Metal–Ceramic Composites:A Review

Functionally gradient/graded materials(FGMs), an emerging new class of materials, are the outcome of the recent innovative concepts in materials technology. FGMs are in their early stages of evolution and expected to have a strong impact on the design and development of new components and structures with better performance. FGMs exhibit gradual transitions in the microstructure and/or the composition in a specific direction, the presence of which leads to variation in the functional performance within a part. The presence of gradual transitions in material composition in FGMs can reduce or eliminate the deleterious stress concentrations and result in a wide gradation of physical and/or chemical properties within the material. Functionally graded metal–ceramic composites are also getting the attention of the researchers. Among the fabrication routes for FGMs such as chemical vapour deposition, physical vapour deposition, the sol–gel technique, plasma spraying, molten metal infiltration, self propagating high temperature synthesis, spray forming,centrifugal casting, etc., the ones based on solidification route are preferred for FGMs because of their economics and capability to make large size products. The present paper discusses and compares various solidification processing techniques available for the fabrication of functionally gradient metals and metal–ceramic composites and lists their properties and possible applications. The other processing methods are briefly described.