Microporosity Formation Research Articles

Prediction of Microporosity in Complex Thin-Wall Castings with the Dimensionless Niyama Criterion

The dimensionless Niyama criterion was used to predict the formation of microporosity in nickel-based superalloy casting, which extended the model application from a simple plate casting to complex thin-wall superalloy casting. The physical characteristics of the superalloy were calculated by JMatPro software. The relation between the volume percentage of microporosity and the dimensionless Niyama values were constructed. Quantitative metallographic measurements of the microporosity of the practical thin-wall casting were carried out. The prediction agreed well with the experiment in general, except for some thick-wall sites in the casting.

Evaluation of Formation and Evolution of Microporosity in Anodic Copper Solidification Processes: Simulation and Experimental Validation

The current study analyzes the formation and evolution of microporosity during the solidification of anodic cooper. The aim of this study is to develop a thermofluid-formulation including microstructural evolution and to perform experiments to validate some measured variables with the respective numerical predictions. To this end, a set of experiments is carried out in copper testing primary and eutectic phase formation together with porosity evolution. To evaluate the formation of different microstructural phases and porosity, anodic copper (99.80 pct purity, approximately) is poured into different types of molds. The effect of heat extraction on the thermofluid-microstructural response is evaluated using graphite and steel molds to promote different cooling rates. The microporosity depends on the microstructural formation; hence the microstructure needs to be firstly described. The proposed microstructural model takes into account nucleation and grain growth laws based on thermal undercooling together with microstructural evolution. The primary phase evolution model is based on both solute diffusion at the grain scale and the dendrite tip growth kinetics, while the eutectic evolution is assumed proportional to the copper initial composition and eutectic undercooling. The microporosity model accounts for the partial pressures of gases and the solute distribution in the liquid and solid phases. The corresponding numerical formulation is solved in the framework of the finite element method. Finally, the computed temperature, solid, and liquid volumetric fractions, and pressure histories together with the final values for the radius, density, and pore volumetric fraction, are all compared and validated with the experimental measurements.

MODELING OF MICROPOROSITY FORMATION IN ANAl-7%Si ALLOY

The performance of castings is primarily dependent on the solidification microstruc- tures and defects. Gas porosity is one of the major casting defects existing in the castings of aluminium and magnesium alloys. In this work, a two-dimensional (2D) cellular automaton (CA) model is pro- posed to simulate dendrite and microporosity formation during solidification of alloys. The model involves three phases of liquid, gas and solid. The effect of liquid-solid phase transformation on the nucleation and growth of porosity, the redistribution and diffusion of solute and hydrogen, and the effects of surface tension and environmental pressure are taken into account. The growth of both den- drite and porosity is simulated using a CA approach. The diffusion of solute and hydrogen is calculated using the finite difference method (FDM). The simulations can reveal the coupling and competitive growth of dendrites and microporosities, as well as the microsegregation of solute and hydrogen. The model is applied to simulate the microporosity formation during solidification of an Al-7%Si (mass fraction) alloy. The effects of initial hydrogen concentration and cooling rate on microporosity forma- tion are investigated. The results show that the simulated pressure difference between the inside and outside of a porosity as a function of the reciprocal of porosity radius obeys the Laplace law. With the increase of initial hydrogen concentration, porosity volume fraction increases, and the incubation time of microporosity nucleation and growth decreases, while the porosity density does not increase obvi- ously. With cooling rate decreasing, porosity volume fraction and maximum porosity radius increase, as well as porosity nucleates and starts to grow at higher temperatures. However, the porosity density shows a decreasing trend with the decrease of cooling rate. The competitive growth between different microporosity and dendrites is observed. The porosity nuclei with larger size are able to grow prefer-

Wollastonite nanofiber–doped self-setting calcium phosphate bioactive cement for bone tissue regeneration

The purpose of this study was to synthesize a self-setting bioactive cement by incorporation of wollastonite nanofibers (WNFs) into calcium phosphate cement (CPC). The composition, morphology, setting time, compressive strength, hydrophilicity, and degradation of WNF-doped CPC (wnf-CPC) were investigated. Scanning electron microscopy, Fourier transform infrared spectroscopy, X-ray diffraction, and inductively coupled plasma atomic emission spectroscopy were utilized. Additionally, methyl-thiazolyl-tetrazolium bromide assay, scanning electron microscopy, inductively coupled plasma atomic emission spectroscopy, and histological evaluation were used to study the cell and tissue responses to wnf-CPC, both in vitro and in vivo. The results confirmed that the addition of WNFs into CPC had no obvious effect on the setting time or the compressive strength of wnf-CPC, provided the WNF amount was not more than 10 wt%. However, the hydrophilicity and degradability of wnf-CPC were significantly improved by the addition of WNFs – this was because of the change of microstructure caused by the WNFs. The preferred dissolution of WNFs caused the formation of microporosity in wnf-CPC when soaked in tris hydrochloride solution. The microporosity enlarged the surface area of the wnf-CPC and so promoted degradation of the wnf-CPC when in contact with liquid. In addition, MG-63 cell attachment and proliferation on the wnf-CPC were superior to that on the CPC, indicating that incorporation of WNFs into CPC improved the biological properties for wnf-CPC. Following the implantation of wnf-CPC into bone defects of rabbits, histological evaluation showed that wnf-CPC enhanced the efficiency of new bone formation in comparison with CPC, indicating excellent biocompatibility and osteogenesis of wnf-CPC. In conclusion, wnf-CPC exhibited promising prospects in bone regeneration.

Factors Affecting the Nucleation Kinetics of Microporosity Formation in Aluminum Alloy A356

Metal cleanliness is one of the most critical parameters affecting microporosity formation in aluminum alloy castings. It is generally acknowledged that oxide inclusions in the melt promote microporosity formation by facilitating pore nucleation. In this study, microporosity formation under different casting conditions, which aimed to manipulate the tendency to form and entrain oxide films in small directionally cast A356 samples was investigated. Castings were prepared with and without the aid of argon gas shielding and with a varying pour surface area. Two alloy variants of A356 were tested in which the main difference was Sr content. Porous disc filtration analysis was used to assess the melt cleanliness and identify the inclusions in the castings. The porosity volume fraction and size distribution were measured using X-ray micro-tomography analysis. The measurements show a clear increment in the volume fraction, number density, and pore size in a manner consistent with an increasing tendency to form and entrain oxide films during casting. By fitting the experimental results with a comprehensive pore formation model, an estimate of the pore nucleation population has been made. The model predicts that increasing the tendency to form oxide films increases both the number of nucleation sites and reduces the supersaturation necessary for pore nucleation in A356 castings. Based on the model predictions, Sr modification impacts both the nucleation kinetics and the pore growth kinetics via grain structure.

Modeling of Microporosity Size Distribution in Aluminum Alloy A356

Porosity is one of the most common defects to degrade the mechanical properties of aluminum alloys. Prediction of pore size, therefore, is critical to optimize the quality of castings. Moreover, to the design engineer, knowledge of the inherent pore population in a casting is essential to avoid potential fatigue failure of the component. In this work, the size distribution of the porosity was modeled based on the assumptions that the hydrogen pores are nucleated heterogeneously and that the nucleation site distribution is a Gaussian function of hydrogen supersaturation in the melt. The pore growth is simulated as a hydrogen-diffusion-controlled process, which is driven by the hydrogen concentration gradient at the pore liquid interface. Directionally solidified A356 (Al-7Si-0.3Mg) alloy castings were used to evaluate the predictive capability of the proposed model. The cast pore volume fraction and size distributions were measured using X-ray microtomography (XMT). Comparison of the experimental and simulation results showed that good agreement could be obtained in terms of both porosity fraction and size distribution. The model can effectively evaluate the effect of hydrogen content, heterogeneous pore nucleation population, cooling conditions, and degassing time on microporosity formation.

Chloritization in Proterozoic granite from the Äspö Laboratory, southeastern Sweden: record of hydrothermal alterations and implications for nuclear waste storage

AbstractHydrothermal alteration of Proterozoic granitic rocks in the Äspö underground laboratory, southestern Sweden, resulted in the formation of chlorite with large variations in textural and chemical characteristics, which reflect differences in formation temperatures, fluid composition, and reaction mechanisms. The mineral assemblage associated with chlorite, including Ca-Al silicates (prehnite, pumpellyite, epidote, and titanite), Fe-oxides, calcite, albite and K-feldspar, suggests that chloritization occurred at temperatures of between 200–350°C during various hydrothermal events primarily linked to magmatism and rock deformation. Petrographic and electron microprobe analyses revealed that chlorite replaced biotite, amphibole and magnetite, and hydrothermal chlorite phases filled fractures and vugs in the granitic rocks. While fracture-filling chlorite reduces fracture permeability, chloritization reactions in the host granite resulted in the formation of new localized microporosity that should thus be taken into consideration when evaluating the safety of the granitic basement rocks as a repository for nuclear waste. It is also important to take into account that similar alteration reactions may occur at the site of stored nuclear waste where temperatures in excess of 100°C might be encountered.

Physics of microporosity formation in casting alloys – sensitivity analysis for Al–Si alloys

The problem of microporosity (microshrinkage) formation in casting alloys continues to be of interest in spite of the many computational models proposed. This is because of the complexity of the problem that involves many material and process variables intertwined in complex physics. This paper introduces two models for the growth of a gas pore in the mushy zone of a solidifying alloy. While the models ignore important contributors to the problem, such as fluid transport and stress, they capture the physics that connects the main variables of the process: initial gas content, temperature/fraction solid, cooling rate, temperature gradient, applied pressure and melt purity (pore nucleation). The models have been used to conduct a sensitivity analysis of the process for the particular case of Al–Si alloys.

Influence of solidification variables on the microporosity formation on Al–Cu (4.5 wt%) alloy with axial heat processing

The effects of solidification rate, hydrogen concentration, and level of convection on porosity formation in Al–Cu (4.5 wt%) alloys were investigated using Axial Heat Processing (AHP). This processing technique is similar to the conventional directional solidification (DS) technique, except that it utilizes a graphite baffle immersed near the solidification interface to control the shape of the interface and impart an axial temperature gradient. It was found that the samples produced by AHP contained 20–40% less microporosity than similar samples produced by conventional DS. The reduction was also more pronounced with decreasing a cooling rate and increasing an initial hydrogen concentration in the melt. These differences are attributed to the solute accumulation that is due to the confinement of the liquid below the baffle and the concomitant reduction in the convection level near the interface.

Microporous polyacrylate matrix containing hydrogen bonded nanotubular assemblies

We report the straightforward photo-polymerization of polyacrylate films containing bis-urea based self-assembled nanotubes. The obtained materials are characterized by gas adsorption measurements, 129Xe NMR spectroscopy and WAXS. The presence of the bis-ureas is shown by butane adsorption (at 273 K and ambient pressure) to be responsible for the formation of a significant microporosity. This porosity is however not detected by the classical argon adsorption procedure (at 77 K and low pressure). This effect is attributed to the contraction of the material at low temperature and pressure, and may be of general concern for other organic porous materials. One of the potential advantages of the present materials is that the porosity results from the self-assembled nanotubes and should therefore be independent of the matrix mechanical properties. It should in particular be possible to adjust the flexibility of the matrix by changing the monomer composition.

Numerical and experimental investigation of microporosity formation in a ternary Al–Cu–Si alloy

Macrosegregation and porosity formation are investigated by both a numerical model and transient directional solidification experiments. The macrosegregation pattern, and the theoretical and apparent densities are presented as a function of the casting length. X-ray fluorescence spectrometry was used to determine the experimental macrosegregation profiles. The measurement of microporosity was performed by a pyknometry procedure. The local composition along an Al–6 wt%Cu–1 wt%Si casting length is used as an input parameter for simulations of microporosity evolution. The results show that the addition of 1 wt% silicon to the Al–Cu alloy composition increases significantly the volumetric fraction of pores as compared with the corresponding porosity exhibited by an Al–6 wt%Cu alloy casting. It is also shown that the use of a carbon steel chill mold induced an abnormal increase in the fraction of pores close to the casting cooled surface which was caused by a higher Fe concentration provoked by the diffusive flux of iron from the chill.

Effects of Secondary Dendrite Arm Spacing on Microporosity of A357 Alloy

Microporosity in cast aluminum alloys formed during solidification of castings can be due to the evolution of dissolved hydrogen gas from the liquid metal, the inability of liquid metal to feed through interdendritic channels, or a combination of both. The secondary dendrite arm spacing (SDAS) of the casting may affect either the evolution of hydrogen gas or the ability of feeding. In this investigation, a method based on the Nearest-Neighbor-Distance (NND for short) cluster analysis of image analysis data was used for distinguishing the gas and shrinkage pores in A357 alloy, in order to study the effects of SDAS on the microporosity formation. It shows that the shrinkage pores are prone to form in the specimens with small SDAS. The discrete, isolated gas microporosity is prone to form in the specimens with large SDAS.

Functionalization of carbon nanofibers through electron beam irradiation

Carbon nanofibers were oxidized in air at 350°C under the influence of a 3MeV electron beam at doses of 1000 and 3500kGy. XPS analysis showed that oxygen was readily incorporated on the surface: the ratio O 1s/C 1s increased approximately by a factor of three when the carbon nanofibers were irradiated at 3500kGy. The oxidized nanofibers exhibited better dispersion than as-received nanofibers when mixed with water/methanol (50% v/v). Raman spectroscopy revealed that the ID/IG ratios were statistically unchanged for all samples because the damage on the nanofiber surface was highly localized and did not lead to modifications on the bulk carbon nanofiber structure. On the other hand, SEM, TEM, and AFM images illustrated that cutting, welding, and collapse may occur on the structure of irradiated nanofibers. BET analysis showed that the surface area of irradiated samples did not significantly increase, probably because the radiation process may lead to micro-porosity formation. The temperature during irradiation had no effect on the nanofibers’ surface and the thermal stability of the irradiated samples was determined to be lower.

Microstructural Development in a Ternary Al-Cu-Si Alloy during Transient Solidification

Macrosegregation and microporosity formation numerical models are dependent on microstructural parameters, such as primary and secondary arm spacings to provide the permeability coefficient in the mushy zone. As can be observed in the literature, growth models for ternary alloys for unsteady solidification are rarely found. In this paper, the primary (λ1) dendrite arm spacing was measured along the length of an Al-Cu-Si alloy casting and correlated with transient solidification thermal variables. A combined theoretical and experimental approach has been carried out to quantitatively determine such thermal variables, i.e., transient metal/mold heat transfer coefficient, liquidus isotherm velocity and cooling rate ahead the liquidus front.

Hierarchically microporous/macroporous scaffold of magnesium–calcium phosphate for bone tissue regeneration

Hierarchically 3D microporous/macroporous magnesium–calcium phosphate (micro/ma-MCP) scaffolds containing magnesium ammonium phosphate hexahydrate [NH 4MgPO 4·6H 2O] and hydroxyapatite [Ca 10(PO 4) 6(OH) 2] were fabricated from cement utilizing leaching method in the presence of sodium chloride (NaCl) particles and NaCl saturated water solution. NaCl particles produced macropororosity, and NaCl solution acted as both cement liquid and porogens, inducing the formation of microporosity. The micro/ma-MCP scaffolds with porosities varied from 52 to 78% showed well interconnected and open macropores with the sizes of 400–500 μm, and degradation of the scaffolds was significantly enhanced in Tris–HCl solution compared with macroporous MCP (ma-MCP) and corresponding calcium phosphate cement (CPC) scaffolds. Cell attachment and proliferation of MG 63 on micro/ma-MCP were significantly better than ma-MCP and CPC scaffolds because of the presence of microporosity, which enhanced the surface area of the scaffolds. Moreover, the alkaline phosphatase (ALP) activity of the MG 63 cells on micro/ma-MCP was significantly higher than ma-MCP and CPC scaffolds at 7 days, and the MG 63 cells with normal phenotype spread well and formed confluent layers across the macroporous walls of the micro/ma-MCP scaffolds. Histological evaluation confirmed that the micro/ma-MCP scaffolds improved the efficiency of new bone regeneration, and exhibited excellent biocompatibility, biodegradability and faster and more effective osteogenesis in vivo.

Microporosity in cast alloys: simple considerations on its formation

A few examples are chosen on the basis of the significance of the effects of microporosity on the solidification or post-solidification behaviour and properties of cast products. The paper begins with a short survey of the main casting variables that affect the formation of microporosity in different types of cast alloys: aluminium-based alloys, nickel-based alloys in investment casting, and spheroidal graphite cast iron. The second part summarises the most useful and comprehensive physical ideas that allow understanding of the formation of microporosity for various cast alloys and casting processes. Three main causes of microporosity formation are pointed out, i.e. local chemistry of liquid, local dendritic microstructure, and local liquid pressure within the mushy zone. In the last part, emphasis is put on the influence of another important cause of microporosity: the mechanical behaviour of the solid in the mushy zone during solidification. Similarities and differences between microporosity and hot tearing are tentatively drawn.

X-Ray Microtomographic Characterization of Porosity in Aluminum Alloy A356

The presence of microporosity can, in certain circumstances, lead to a significant reduction in the mechanical properties of cast aluminum alloys. The size of the microporosity is especially crucial to the performance of castings in fatigue applications. The aim of this study is to investigate the effect of cooling rate and degassing time on the formation and evolution of microporosity in aluminum alloy A356 (Al-7Si-0.3Mg) castings using traditional metallography and X-ray microtomography (XMT) methods. The final results clearly confirm that increasing the cooling rate and degassing time yield lower microporosity within the microstructure. The XMT analysis enabled the calculation of the volume fraction and size distribution of microporosity. The XMT analysis revealed a bimodel distribution of pores with one population of small pores and a second population of large pores. The large population has been attributed to hydrogen gas solubility, whereas the smaller population is proposed to be linked to the localized entrapment of liquid metal occurring at the end of mass feeding.

Titanium Coatings on AISI 316L Stainless Steel Formed by Thermal Decomposition of TiH2 in Vacuum

ABSTRACTTi-coated AISI 316L stainless steel, for potential biomedical applications, is obtained by thermal decomposition of TiH2 under vacuum. The presence of hydrogen in the coating material facilitates the sintering process of Ti particles, with simultaneous formation of several inter-diffusion layers at the substrate/coating interface, whose thickness and chemical composition depend mainly on the treatment temperature. Coatings prepared at 1100°C exhibit formation of a wide zone at the substrate/coating interface, which is associated with the appearance of cracks, and which consists of a mixture of λ + χ + α-Fe phases. Formation of abundant microporosity is also observed in this region, which is attributed to the Kinkerdall effect.

Investigation of microporosity formation mechanisms in A356 aluminium alloy castings

The problem of microporosity formation during solidification of aluminum alloy castings has great commercial importance and engineering interest due to its detrimental effects on mechanical properties. The present study concentrates on the mechanisms of microporosity formation in aluminium alloy castings. The microstructural characterisation of the produced samples was carried out with image analysis and SEM. It was observed that formation of shrinkage induced interdendritic porosity depends on solidification mode of the sample. The precipitation of intermetallic s-Al5FeSi platelets in the interdendritic liquid were also found to promote the interdendritic porosity formation by blocking interdendritic feeding. The results reveal that the oxide inclusions have a significant effect on formation of hydrogen precipitation induced regular micropores.

Numerical Simulation on the Dendritic Spacing and Microporosity in A356 Alloy Ingot

A stochastic mathematic model contained the effects of dendrite morphology, solidification shrinkage and dissolved gases was formed to simulate microporosity formation and growth. Microporosities appear in the interspaces of primary dendrites as well as secondary dendrites from microscopic view of A356 aluminum alloy experimental ingot with a metal mold. In the past literatures it took the volumetric fraction of microporosities as a function of the local density. In the present work a single pore size and distribution were predicted concerning the combination of shrinkage and dissolved gases and dendritic spacing. The dendritic spacing is a main parameter to decide the pore pattern. For shrinkage and dissolved gases causes, the favorable one is determined by dendritic spacing, also the local cool rate and tip growth rate. The dense degree of the experimental ingots in different casting conditions was discussed. The variations of dens degree from the measured values in different casting conditions are similar to that of porosity volume fraction from the predicted results.