Anping Dong

Continuous separation of Fe–Al–Zn dross phase from hot dip galvanised melt using alternating magnetic field

Abstract Experiments to continuously separate Fe–Al–Zn dross phase from hot dip galvanising zinc melt were conducted on a laboratory scale apparatus by using high frequency alternating magnetic field. Effects of processing time (t) on separation efficiency were investigated. The experimental results show that using the electromagnetic repulsive force resulting from the electrical conductivity difference between zinc melt and Fe–Al–Zn dross phase, the deleterious zinc dross particles causing surface defects of galvanising steel sheets can be continuously separated from the zinc bath under alternating magnetic field, and the separation efficiency increases with the increase in processing time. When the magnetic frequency is 17·5 kHz, the effective magnetic flux intensity is 0·1 T, the cross-section of the ceramic square pipe is 10 × 10 mm, and the processing time is 0·6–2·5 s, the separation efficiency of zinc dross varies from 43·76 to 85·71%, and the experimental results are in reasonable agreement with the theoretical results.

Formation Mechanism of Spherical TiC in Ni-Ti-C System during Combustion Synthesis

The formation mechanism of TiC particles in a Ni-Ti-C system were revealed by using differential thermal analysis (DTA), XRD, and SEM to identify the reaction products in different temperature ranges. The results indicated that the synthesis mechanism of TiC in Ni-Ti-C system was complex; several reactions were involved in the combustion synthesis of TiC-Ni composite. The Ni-Ti intermediate phases play important roles during the formation of TiC. Moreover, the influence of heating rate on the size range of TiC was also discussed.

Freckle Defect Formation near the Casting Interfaces of Directionally Solidified Superalloys

Freckle defects usually appear on the surface of castings and industrial ingots during the directional solidification process and most of them are located near the interface between the shell mold and superalloys. Ceramic cores create more interfaces in the directionally solidified (DS) and single crystal (SX) hollow turbine blades. In order to investigate the location of freckle occurrence in superalloys, superalloy CM247 LC was directionally solidified in an industrial-sized Bridgman furnace. Instead of ceramic cores, Alumina tubes were used inside of the casting specimens. It was found that freckles occur not only on the casting external surfaces, but also appear near the internal interfaces between the ceramic core and superalloys. Meanwhile, the size, initial position, and area of freckle were investigated in various diameters of the specimens. The initial position of the freckle chain reduces when the diameter of the rods increase. Freckle area follows a linear relationship in various diameters and the average freckle fraction is 1.1% of cross sectional area of casting specimens. The flow of liquid metal near the interfaces was stronger than that in the interdendritic region in the mushy zone, and explained why freckle tends to occur on the outer or inner surfaces of castings. This new phenomenon suggests that freckles are more likely to occur on the outer or inner surfaces of the hollow turbine blades.

Prediction of Cavitation Depth in an Al-Cu Alloy Melt with Bubble Characteristics Based on Synchrotron X-ray Radiography

The size of cavitation region is a key parameter to estimate the metallurgical effect of ultrasonic melt treatment (UST) on preferential structure refinement. We present a simple numerical model to predict the characteristic length of the cavitation region, termed cavitation depth, in a metal melt. The model is based on wave propagation with acoustic attenuation caused by cavitation bubbles which are dependent on bubble characteristics and ultrasonic intensity. In situ synchrotron X-ray imaging of cavitation bubbles has been made to quantitatively measure the size of cavitation region and volume fraction and size distribution of cavitation bubbles in an Al-Cu melt. The results show that cavitation bubbles maintain a log-normal size distribution, and the volume fraction of cavitation bubbles obeys a tanh function with the applied ultrasonic intensity. Using the experimental values of bubble characteristics as input, the predicted cavitation depth agrees well with observations except for a slight deviation at higher acoustic intensities. Further analysis shows that the increase of bubble volume and bubble size both leads to higher attenuation by cavitation bubbles, and hence, smaller cavitation depth. The current model offers a guideline to implement UST, especially for structural refinement.

In Situ Observation of the Zr Poisoning Effect in Al Alloys Inoculated by Al-Ti-B

In situ synchrotron X-ray radiography observations of the Zr-poisoning phenomenon of an Al-20 wt pct Zn alloy inoculated by Al-5Ti-1B were carried out. The effects of Zr addition on heterogeneous nucleation, grain growth, and final grain size were quantitatively studied and further analyzed using the interdependence model. The experimental results show that the undercooling needed for nucleation increases and the nucleation rate decreases at the early stage of solidification with Zr addition, resulting in fast grain growth, a high solidification rate and increased severity of solute segregation in the solid–liquid coexistence regions. At the same time, the poisoning is a progressive process that is enhanced with the increasing Zr content, holding temperature, and holding time. The nucleation-free zone of the Zr-containing sample, either measured from the radiographs or calculated by the interdependency theory, is larger than that of the Zr-free sample. Our analysis shows that both the increase in the nucleation-free zone and the average interparticle spacing of the most potent available nucleation particles contribute to the increase of the grain size caused by Zr poisoning.

Effect of Mg on the Microstructure and Corrosion Resistance of the Continuously Hot-Dip Galvanizing Zn-Mg Coating

The microstructure of continuously hot-dip galvanizing Zn-Mg coating was investigated in order to obtain the mechanism of the effects of Mg on the corrosion resistance. In this paper, the vertical section of the Zn-0.20 wt % Al-Mg ternary phase diagram near the Al-low corner was calculated. The results indicates that the phase composition of the Zn-0.20 wt % Al-Mg ternary phase diagram near the Al-low corner is the same as Zn-Mg binary phase diagram, suggesting Al in the Zn-Mg (ZM) coatings mainly concentrates on the interfacial layer between the coating and steel substrate. The microstructure of continuously hot-dip galvanizing ZM coatings with 0.20 wt % Al containing 1.0–3.0 wt % Mg was investigated using tunneling electron microscopy (TEM). The morphology of Zn in the coating changes from bulk to strip and finally to mesh-like, and the MgZn2 changes from rod-like to mesh-like with the Mg content increasing. Al in the ZM coatings mainly segregates at the Fe2Al5 inhibition layer and the Mg added to the Zn bath makes this inhibition layer thinner and uneven. Compared to GI coating, the time of the first red rust appears increases by more than two-fold and expansion rate of red rust reduces by more than four-fold in terms of salt spray experiment. The ZM coating containing 2.0 wt % Mg has the best corrosion resistance. The enhanced corrosion resistance of ZM coatings mainly depends on different corrosion products.

Investigation of Thin‐Walled IN718 Castings by Counter‐Gravity Investment Casting

Adjusted pressure casting is one kind of counter-gravity precision forming method in which the filling processes are at low counter-pressure while the solidification process are at high pressure. It aims at increasing the filling capacity for complex thin-walled castings and also provides an opportunity of introducing automatic control into the process by changing the pressure under which the metal is forced into the mold cavity. The filling capacity and solidification microstructure of complex thin-walled IN718 castings has been studied by experimental and numerical simulation. The results show that the liquid metal can flow through the mold cavity easily by imposing very low pressure. The average grain size of the 200mm×200mm×1mm thin walled IN718 casting piece varies from about 500μm to 800μm under different crystallization pressures. The average grain sizes are much smaller than those in the gravity casting process. Therefore, adjusted pressure casting is the promising technology for producing the complex thin-walled superalloy castings.

Non-metallic particles in Solar Grade Silicon (SoG-Si)

This study investigated the non-metallic inclusions in Solar Grade Silicon (SoG-Si), especially the distribution of inclusions in the top 15mm layer of multicrystalline silicon ingot. The SoG-Si ingot produced from directional solidification process usually pushes the impurities to the top and finally cut off and discarded, which leads to material loss. The hard inclusions lead to wire breakages during the cutting of the ingot into wafers. The main kinds of inclusions found in top-cut silicon scraps from two manufacturers have been investigated using acid extraction, automated feature analysis techniques and SEM-EDS and optical Microscope: they are needle-like Si3N4 and lumpy SiC inclusions. Surface observations of the scraps before polishing revealed that, Si3N4 inclusions are usually bigger and in some cases can be about a few millimeters. SiC inclusions are usually smaller, ∼200µm but can be ∼500µm in some cases. For the directional solidified silicon ingot, it was determined that an approximate distance of ∼10mm is a good enough cutoff thickness.

Purification of solar grade silicon using electromagnetic field

Non-metallic particles and the metallic impurity elements in the solar cell silicon have a strong detrimental effect on the conversion efficiency of the solar cell. Removing these impurities is one of the important tasks for silicon refining. The current paper proposed a new approach to purify silicon - electromagnetic (EM) separation. Since the non-metallic particles and the metallic impurity elements are non- or less-conductive while the molten silicon is well conductive, under EM field, the Lorenz force will push the particles to the boundary layer, thus separate these inclusions. In the current study, a high frequency EM field was imposed on the silicon melt in laboratory scale experiments with a frequency of 60 kHz and 15.0 A current, the non-conductive SiC particles were successfully pushed to the boundary layer close to the crucible wall.