Globular Grains Research Articles

Addition of Swarf to Produce a Semi-Solid Slurry during High Pressure Die-Casting of AS9U3 Aluminum Alloy

An internal cooling agent is used in rapid slurry forming (RSF) process to produce a high solid fraction slurry for a short period of time. In the process used in this research, the swarf which is known to be a low enthalpy material was added to the melt as the internal cooling agent. During the process, the swarf started to melt and a semi-solid slurry with a relatively high solid fraction was formed. This slurry was formed by exchanging the enthalpies between the low and high enthalpy materials. A commercial Al-Si-Cu alloy, i.e. AS9U3 Aluminum alloy, was used in this investigation. The microscopic examination showed that the Al-Si eutectic colonies start to melt during the melting process of swarf material resulting in the formation of globular Alpha-Al grains due to the multiplication of secondary dendrites arms. The fracture of dendrites arms and the subsequent spheroidization were suggested to be the origin of non-dendritic globular grains in the final microstructure. The amount of primary globular Alpha-phase was measured by the image analysis software. The results showed that during high pressure die-casting of AS9U3 Aluminum alloy using 4 mm thick samples, around 35 percent solid has been formed at the temperature of 580 oC.

Using the Phase Field Method to Investigate Microstructural Evolution of Semi-Solid 357.0 Slurries

Semi-solid aluminum slurries have globular grains, different from traditional dendritic solidification microstructures. The mechanism responsible for the formation of these globular grains is still in dispute. Some researchers suggest that the globular grains are formed by fractured dendrites, while others report it is due to copious nucleation. This study will model the growth of the α-Al phase during the production of semi-solid slurries using the swirled equilibrium enthalpy device (SEED) process, where liquid alloy with a low superheat is poured into a steel crucible and swirled within the semi-solid region. Experimental observations have shown that the final microstructure of the slurry produced by the SEED process is relatively non-uniform, with large dendrites at surface and fine spherical particles at the center. Open source code for the phase field method has been adopted to simulate the different microstructural evolution for semi-solid alloy 357.0. The effect of localized temperatures under isothermal condition on the morphology of the α-Al particles has been investigated according to the special conditions of the SEED process. In addition, suggestions for modifying the slurry production to achieve more uniform microstructure are discussed.

Microstructure Evolution of TiB<sub>2</sub>/7075 Composites in Semi-Solid State near Liquids within Holding Time

Novel reactive technique has been employed in synthesis of 7075 Al matrix composites, and effect of TiB2 particles on sedimentation and microstructure of 7075 Al Alloy in near liquidus was investigated. Researchers have shown that 4.5%TiB2/7075 composites had the smallest grains and the minimum liquidus and solidus temperatures in 7075 Al matrix composites. 4.5%TiB2/7075 composites holding for 21 hours appeared stratification phenomenon, the globular grains were 61μm and 214μm, 250μm and 283μm with the increase of holding time respectively. The microstructures evolution of TiB2 /7075 composites holding for different time at 635°C was a two-stage mechanism that the globular grains was formed firstly and secondly sedimentation and layering were began after a period of time of Ostwald ripening. The globular grain continued to increase during the settling process.

Microstructure and mechanical properties of Mg–Zn–RE–Zr alloy after thixoforming

Magnesium is particularly challenging material, when formed from liquid phase because of high flammability risk. An alternative process for casting, which eliminates above mentioned disadvantage, is thixoforming, which involves a lower temperature of process and operation in the partially solidified state. Influence of semi-solid metal processing on EZ33A magnesium alloy (Mg–Zn–RE–Zr) microstructure and mechanical properties was studied. Ingot microstructure revealed globular grains with coarse eutectic mixture consisting of Mg7Zn3RE, T-phase – RE(Mg,Zn)11 and α(Mg). Heterogeneous nucleation of magnesium solid solution allowed obtaining structure appropriate for thixoforming. Using differential scanning calorimetry, temperature of process was determined to be 622°C, which corresponded to about 30% of the liquid phase. Thixo-cast microstructure consisted of α(Mg) globular grains with a size of 76±1.1 surrounded by fine eutectic mixture in a volume of 35%. T6 heat treatment (solution at 500°C for 6h and ageing at 190°C for 33h) caused increase of grain size to 92μm and the precipitation of two kinds of phases within the α(Mg): β′1 and β′2 responsible for the increase of yield strength to 135MPa, compression strength to 383MPa and hardness to 73HV5.

Influence of Rare Earth Additions on the Microstructure and Mechanical Properties of Al7Si0.3Mg Alloys Processed by Semi-Solid Die Casting and Gravity Die Casting

Microstructures with fine globular grains and refined eutectic structures are important to enhance the mechanical properties of A356 alloys processed by semi-solid and gravity die casting. Rare earth (RE) additions have been shown to be capable of refining both the α-Al particles as well as modify the eutectic phase of alloys. In semi-solid die casting, Al7Si0.3Mg alloys with RE concentrations (0, 0.1 and 0.4 wt.%) were used to prepare semi-solid slurries using the SEED (Swirling Enthalpy Equilibrium Device) method, and subsequently semi-solid die cast. The same compositions of alloys were also applied to gravity die casting. The microstructure and mechanical properties of castings in two processes have been characterized. Compared to the grains produced in gravity die casting, globular grains with small size (260 μm) in the semi-solid die casting significantly enhance the UTS and elongation of alloys. Although the size of grains had no change with increasing RE concentrations in alloys. The Al-Si eutectics were changed to refined morphology with the 0.1 wt.% RE addition, which enhanced the ductility of alloys in two processes. When increasing the RE addition to 0.4 wt.%, the RE-rich phases precipitated at grain boundaries, which decreased the UTS and elongation of alloys.

Chemical routes for synthesis of polypyrrole–based nanocomposites incorporating graphene platelets from natural Shungite

Polypyrrole (PPy)-based nanocomposites are synthesized by a micro-emulsion oxidative chemical polymerization of pyrrole monomer. Graphene platelets (GP), extracted from Shungite mineral powders through innovative procedures, are used as nanoinclusions to enhance the interesting properties of the pure polymer. SEM and HR-TEM analyses evidence that diverse PPy nanostructures are obtained in the presence of different dopant surfactants: globular grains with sodium dodecyl sulfate (SDS) anionic surfactant, and tubular features with hexadecyltrimethylammonium bromide (CTAB) cationic surfactant. Raman spectroscopy reveals that the polymer chains are in a semi-oxidized state. Functional characterizations evidence that the PPy-based nanocomposites doped with SDS are characterized by improved charge transport properties.

Effect of Nano-SiC on Microstructure and Mechanical Properties of AZ91 Magnesium Alloy Processed by Thixomolding

Magnesium injection molding technology was used to produce Mg based nanocomposites. AZ91 chips were mixed with 5 wt.% of β-SiC nanoparticles in solid state and fed to pre-heated cylinder of prototype injection molding machine. Using screw rotation, granules were transferred to nozzle area at simultaneous intensive shearing and mixing of slurry containing reinforcement phases. Injection process was conducted at 595 °C, which corresponded to about 90% liquid phase and cast to steel die preheated to 150 °C. Detailed characterization of microstructure was performed using SEM and TEM microscopes. Composite microstructure consisted of α(Mg) globular grains with size of about 60 μm and volume of 7-10% surrounded by mixture of proeutectic magnesium solid solution with irregular shape and average size of 12 μm as well as fine eutectic mixture (α(Mg) + β-Mg17Al12). Additionally, TEM-BF image showed β-SiC nanoparticles with size of 20-50 nm in the area of eutectic. Hardness and compression strength of AZ91 nanocomposites increased from 58 to 75 HV and from 200 to 235 MPa, respectively.

Die Design for Main Bearing Cap of Engine Block Based Semi-Solid Die Casting Process and the Comparison Analysis with Squeeze Casting Process

There are two new processes to development automobile structural components which have certain thickness. In the present paper, taking a main bearing cap product as an example, analyses die design by comparing the experimental and computational numerical simulation results. For the main bearing cap, product structure and mold design were designed to be suitable for characters of SSM die casting and squeeze process. Semi-solid slurry has significantly higher viscosity than liquid metal. This character of fluidity and solid fraction phase make the flow condition more laminar than liquid squeeze casting with the partial fill experiment. And compared with squeeze casting process, the globular shape grain size is smaller than dendritic structure. And mechanical property result shows that the elongation of SSM die casting can achieve more than twice than squeeze casting.

Investigations of nano-crystalline Sr0.5Ba0.5Nb2O6 and bulk ceramics synthesized by a polymerization method using PEG400

Nano-crystalline Sr0.5Ba0.5Nb2O6 was synthesized by a one-pot method using PEG400 and citric acid. Calcination of the (Sr,Ba,Nb)-gel at 600 °C leads to Sr0.5Ba0.5Nb2O6 with a crystallite size of 24(2) nm and a specific surface area of 38.5(10) m2 g−1. Sintering up to 1325 °C leads to ceramics with globular or irregular-shaped grains and average grain sizes between 1.3 and 2.4 μm, whereas higher temperatures lead to a rod-like microstructure. The indirect allowed optical band gap varies between 3.70(5) and 3.29(5) eV. Dielectric measurements show a diffuse phase transition and weak relaxor properties. The maximum of the permittivity occurs between 116 and 147 °C. The frequency dependence of the impedance can be well described by one or two RC-circuits depending on sintering temperature. The melting temperature is determined as 1506(7) °C with ΔHf = 140(20) kJ mol−1. The average linear thermal expansion coefficient is found to be 10.5(5)⋅10−6 K−1.

Effect of processing routes on structure-property correlationship of ADC 12 Al alloy

The present work focuses primarily on the effect of processing routes on structure-property correlationship of ADC 12 Al alloy. Both conventional processing (Gravity cast and pressure die cast) and Semi Solid processing techniques (cooling slope and Rheo-Pressure Die Cast; RPDC) were attempted to bring out the correlationship between their microstructures and mechanical properties of ADC 12 Al alloy. Gravity cast experiment was done on ingots of ADC12 Al melted in an resistance furnace and poured at temperature 585 °C in a copper mould. In the Pressure Die-Casting process, ingots of ADC12 Al alloy were melted in an resistance furnace and the liquid metal was poured in the mould of the pressure die casting system. Semi-solid metal processing was carried out using cooling slope experiments in a developed in-house set up and effect of rheocasting on the semi solid microstructure was studied. Rheo pressure die cast experiments were carried out in an integrated set-up of cooling slope and pressure die-cast system. The obtained result of gravity cast material reveals dendritic morphology of primary Al phase and elongated eutectic phase; upon using pressure in a pressure die cast system, the structure of primary Al phase changes to large size dendrites along with rosette shaped morphology. The cooling slope technique to generate semi-solid structure yields nearly globular and fine grains; whereas the rheo pressure die casting experiments reveal nearly globular shape primary Al exhibit that distribute relatively uniformly in the matrix compared to the all three above the processing routes. From The best mechanical properties are also obtained in Rheo-pressure die cast system with tensile strength (223 MPa), elongation (6.5%), and hardness (87 VH) compared to all other processing routes. Explanations of fracture micro mechanisms are supplement to the investigation.

Fabrication of Al7075–Al2O3np-based metal matrix composites with a high solid fraction for thixoforming

Abstract

Semi-solid state mixing of Mg-Zn-RE alloys—microstructure and mechanical properties

Mixing two kinds of alloys in semi-solid temperature range allows obtaining new one with superior properties. Critical challenges facing the process are determination of mutual interaction of slurries, selection of appropriate temperature process, and designing optimal heat treatment conditions. Two kinds of magnesium chips E21 and WE43B containing different compositions of rare earth elements and similar solidus-liquidus temperature ranges were mixed in equal weight/volume ratio using thixoforming technology. Granules were hot compressed at 400 °C/5 min/200 MPa and thixoformed at 625 °C, which corresponded to about 25% of the liquid phase. Structure of E21WE43B thixo-cast consisted of globular grains of average size 59 μm. They were surrounded by the eutectic mixture coming from both alloys. X-ray analysis confirmed the presence of magnesium solid solution as well as intermetallics Mg3Nd and Mg24Y5 phases located in interglobular areas. T6 heat treatment consisted of solutioning at 525 °C for 8 h and aging at 190 °C for 120 h leading to increase of hardness from 57 to 85 HV. High resolution transmission electron microscopy studies allowed identifying β’ planar precipitates, approximately 8 x 2 nm in size and coherent with matrix, homogeneously distributed in the α(Mg) in the T6 treated sample. They were responsible for strengthening of E21WE43B thixo-cast leading in consequence to yield strength 190 MPa at compression strength 475 MPa at plasticity 13.1%.

Formation of a Multilevel Hierarchical Mesosubstructure by Cross Rolling and Its Influence on the Mechanical Behavior of Austenitic Steel

A multilevel hierarchical mesosubstructure is experimentally shown to form in metastable austenitic steel subjected to multipass cross rolling in the temperature range 950-75№C. A multilayer mesosubstructure with highly refined grains, changed grain geometry and microhardness builds up between the macro- and microlevels. Its outer layer has a finely dispersed structure and the highest microhardness Hμ = 3400 MPa. The two underlying layers are composed of globular grains 0.9 μm in diameter and exhibit the respective close microhardnesses 3100 and 3000 MPa. The near-axial layer has elongated grains of a fiber-band structure with nanosized width. Between the micro- and nanolevels, a nanosized hierarchical mesosubstructure is formed in all band structures, with the mesosubstructure of nanosized carbides and cardonitrides precipitated at low-angle boundaries. Despite a certain decrease in the steel plasticity in uniaxial tension, such a multilevel mesosubstructure causes a multiple increase in its fatigue life. The formation of hierarchical mesosubstructures is associated with the mechanism of plastic distortion under the conditions of the crystal lattice curvature, when bifurcational structural states arise in its interstices. Multilevel hierarchical mesosubstructures have a strong positive effect on the mechanical behavior of solids, by increasing their strength, wear resistance, and fatigue life.

Evolution and distribution of Al 2 Sm phase in as-extruded AZ61– x Sm magnesium alloys during semi-solid isothermal heat-treatment

Abstract The evolution and distribution of Al2Sm phase in as-extruded AZ61–xSm (x=0, 1.5, 2.0 and 2.5, mass fraction, %) magnesium alloys during semi-solid isothermal heat treatment were investigated. The results showed that when as-extruded AZ61 magnesium alloys were modified with Sm, the smaller and rounder grains were obtained during semi-solid isothermal heat treatment. When the Sm content is 2.0% (mass fraction), the average size of the globular grains reached the smallest value of 90 μm. Although a few Al2Sm particles existed in the α-Mg grains, most of Al2Sm particles solidified at the edge of the globular grains with the width of ~20 μm. These phenomena are mainly attributed to the forces acting on Al2Sm particles in front of the solid–liquid interface, leading to Al2Sm particles accumulating at the solid–liquid interface and then solidifying at the edge of the globular grains in the quenching process.

Sulfur – induced embrittlement in high-purity, polycrystalline copper

Tensile tests were carried out in high-purity, polycrystalline copper alloys with three concentrations of sulfur impurities (14, 27 and 7920 at ppm) at temperatures between 20 °C and 400 °C. The ductility drops with increasing sulfur concentration and temperature while the ultimate tensile strength increases. The alloys exhibit a grain size of several millimeters and contain mostly random grain boundaries (GBs). The microstructure and composition is investigated by transmission electron microscopy (TEM) and atom probe tomography (APT). The microstructure of the samples with sulfur contents of 14 and 27 ppm consists of globular grains and neither of the microanalytical techniques employed reveals the formation of Cu-sulfides or sulfur segregation to GBs. Even after annealing at 500 °C, no sulfide formation or sulfur segregation to GBs was detected. In the alloy with a sulfur content of 7920 ppm, a dendritic structure is observed and in the interdendritic region monoclinic Cu2S precipitates with a size range from 5 nm to several μm are observed at GBs and also within the grains. The influence of S on the ductility is discussed considering the TEM and APT results.

Lactobacillus kefiranofaciens, the sole dominant and stable bacterial species, exhibits distinct morphotypes upon colonization in Tibetan kefir grains

Tibetan kefir grains (TKGs), natural starters for milk fermentation, are believed to comprise diverse microflora of lactic acid and acetic acid bacteria. In order to better understand the bacterial community in TKGs, TKGs that had been cultured continuously either naturally or aseptically for 10 months were subject to analysis using both culture-dependent and various culture-independent methods. Results of DGGE, metagenomics, FISH, qPCR and isolation all demonstrated that Lactobacillus kefiranofaciens is the only dominant and stable bacterial species in TKGs regardless of culture conditions and time. FISH and SEM showed that L. kefiranofaciens exhibited two distinct morphotypes of short rod (3.0 μm in length) and long rod (10.0 μm in length) upon colonization of either the outer surface or inner component of TKGs, providing evidence for its trophic adaptation to the hollow globular grain structure of TKGs. These findings pave ways for further study of the specific symbiotic interaction between L. kefiranofaciens and the dominant Saccharomyces cerevisiae yeast in TKGs in vivo.

Hydrogen-Induced Phase Transformation and Microstructure Evolution for Ti-6Al-4V Parts Produced by Electron Beam Melting

In this paper, phase transitions and microstructure evolution in titanium Ti-6Al-4V alloy parts produced by electron beam melting (EBM) under hydrogenation was investigated. Hydrogenation was carried out at the temperature of 650 °C to the absolute hydrogen concentrations in the samples of 0.29, 0.58, and 0.90 wt. %. Comparative analysis of microstructure changes in Ti-6Al-4V alloy parts was performed using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD). Furthermore, in-situ XRD was used to investigate the phase transitions in the samples during hydrogenation. The structure of Ti-6Al-4V parts produced by EBM is represented by the α phase plates with the transverse length of 0.2 μm, the β phase both in the form of plates and globular grains, and metastable α″ and ω phases. Hydrogenation to the concentration of 0.29 wt. % leads to the formation of intermetallic Ti3Al phase. The dimensions of intermetallic Ti3Al plates and their volume fraction increase significantly with hydrogen concentration up to 0.58 wt. % along with precipitation of nano-sized crystals of titanium δ hydrides. Individual Ti3Al plates decay into nanocrystals with increasing hydrogen concentration up to 0.9 wt. % accompanied by the increase of proportion and size of hydride plates. Hardness of EBM Ti-6Al-4V alloy decreases with hydrogen content.

The microstructural evolution and hardness of the equiatomic CoCrCuFeNi high-entropy alloy in the semi-solid state

The semi-solid slurries of the CoCrCuFeNi high entropy alloy (HEA) were fabricated through the recrystallization and partial melting (RAP) process by cold-rolling and partial remelting. Based on Kim's theory, the solidus-liquidus temperature range and the fraction of the liquid phase as a function of temperature were examined from the Differential Scanning Calorimetry (DSC) curve. The effect of isothermal temperature and soaking time on the microstructural evolution of the CoCrCuFeNi HEA in a semi-solid state were analyzed. The results showed that the globular structure was obtained without the new phase formed for the semi-solid slurries, containing two face-centered-cubic (FCC) solid-solution phases. The average grain size of the globular grain and the globular degree increased with increasing the isothermal temperature and soaking time. The study of the coarsening kinetics indicated that the coarsening process could be described by the modified liquid film migration model. The sluggish diffusion effect of the current HEA leaded to the low coarsening rate, compared with other conventional alloys. However, the existence of the liquid phase in the semi-solid state resulted in a higher grain growth rate for the CoCrCuFeNi HEA with a lower activation energy of 206.9 kJ/mol, compared with the diffusion couple and the grain growth process for the other HEAs at low temperatures. To evaluate the mechanical properties, Vickers hardness was measured to represent the strength of the CoCrCuFeNi HEA. The CoCrCuFeNi semi-solid slurries showed a Hall-Petch type relationship of the average grain size dependence on Vickers hardness.

Growth of carbon nanotubes in arc plasma treated graphite disc: microstructural characterization and electrical conductivity study

Circular graphite discs were treated in arc plasma by varying arcing time. Analysis of the plasma treated discs by field emission scanning electron microscope revealed globular grain morphologies on the surfaces, but when the same were observed at higher magnification and higher resolution under transmission electron microscope, growth of multiwall carbon nanotubes of around 2 nm diameter was clearly seen. In situ growth of carbon nanotube bundles/bunches consisting of around 0.7 nm tube diameter was marked in the case of 6 min treated disc surface. Both the untreated and the plasma treated graphite discs were characterized by X-ray diffraction, energy dispersive spectra of X-ray, X-ray photoelectron spectroscopy, transmission electron microscopy, micro Raman spectroscopy and BET surface area measurement. From Raman spectra, BET surface area and microstructure observed in transmission electron microscope, growth of several layers of graphene was identified. Four-point probe measurements for electrical resistivity/conductivity of the graphite discs treated under different plasma conditions showed significant increase in conductivity values over that of untreated graphite conductivity value and the best result, i.e., around eightfold increase in conductivity, was observed in the case of 6 min plasma treated sample exhibiting carbon nanotube bundles/bunches grown on disc surface. By comparing the microstructures of the untreated and plasma treated graphite discs, the electrical conductivity increase in graphite disc is attributed to carbon nanotubes (including bundles/bunches) growth on disc surface by plasma treatment.

Impact of Ce-base mischmetal on the microstructure and mechanical behavior of ZK60 magnesium casting alloys

Magnesium alloys are important alternatives for structural weight reduction due to their low density and good specific mechanical strength. Among the magnesium alloys, the ZK type exhibits the highest mechanical strength. However, it has limitations on hot working or welding owing to the presence of intermetallics with low melting point and consequently susceptibility to hot crack formation. The addition of rare earths tends to form intermetallics with higher melting points, thus inhibiting the formation of hot cracks, as well as building up thin and dense surface films that improve corrosion resistance. This work studies the addition of 1.5% wt. of mischmetal (Mm) to the ZK60 alloy, and then demonstrates the effects of the casting process with mechanical mixing in the semi-solid state. The alloys produced were: ZK60 and ZK60–1.5 wt%Mm manufactured by conventional casting and rheocast ZK60–1.5 wt%Mm produced with mechanical mixing in the semi-solid state. The casting and cooling methods result in defect-free and chemically homogeneous materials, and the mechanical mixing provides a homogeneous microstructure with globular grains. The mechanical strength was though higher for the ZK60 alloy due to its increased solute content within the Mg-matrix and the smaller quantity of intermetallics that builds up an intermittent network. The Mg7Zn3 intermetallic, which is the main precipitate for ZK60 alloy, has hardness 20% higher than the MgZn2Ce intermetallic that is precipitated with the mischmetal addition and partly removes solute from the Mg-solid solution.