Pock Marks Research Articles

Near Dry Powder Mixed Electric Discharge Machining of AA7050 Hybrid Composites Utilizing Composite Tool Materials

Electric Discharge Machining (EDM) is a prominent technique for producing components with complex geometry and intricate shapes. However, productivity depends on the selection of optimal input variables. This research investigates EDM and Near-Dry EDM (NDEDM) on AA7050 hybrid composites, exploring the impact of tool materials, Al2O3 powder concentration, and dielectric fluids on Material Removal Rate (MRR) and Surface Roughness (Ra). NDEDM offers a higher MRR due to localized heating and energy concentration, while hydrocarbon oil EDM provides lower Ra due to cooling and lubrication. Tool material properties, thermal conductivity and bonding strength, affect MRR and TWR. Aluminum composite tools via stir casting display a strong bonding interface, leading to efficient energy transfer and higher MRR, while copper composite tools via friction stir processing exhibit weaker bonding, potentially reducing MRR. Hydrocarbon oil significantly lowers TWR due to effective lubrication and cooling, while near dry EDM results in slightly higher TWR. Al2O3 powder enhances MRR, with an optimal concentration of 1 g/l, while higher concentrations hinder MRR. The presence of Al2O3 powder enhanced the surface quality by reducing the size of the recast layers and producing a smoother surface. Globules, craters, microcracks, pock marks and re-casted layers are the noticeable features spotted on the surface. The results were optimized using the TOPSIS optimization technique and process parameters 32 A current, 30 µs Ton, 3 mm gap distance machined under 3 g/l Al2O3 incorporated hydrocarbon oil dielectric medium using copper composites tool proffers optimal machining performance.

Developing a steady state wear equation for AA7050 hybrid composites/steel interface at elevated temperature

ABSTRACT In this research work, an attempt was made to reinforce AA7050/5Gr composites with multi-walled carbon nanotube (MWCNT) of varying weight percentages processed through stir casting route. SEM with EDS mapping revealed that the particles were uniformly distributed over the composites. Hardness reduces with increasing MWCNT weight percentage owing to the inverse hall petch effect and increment in void content. A third-body abrasion, which happens when the CNT in the aluminium matrix material detaches from the surface and erodes material from the composites pin as well as the counter face, causes the wear resistance to rise with the addition of CNT particles. A mechanically mixed layer, which avoids direct metal-to-metal contact and thus increases wear resistance, was created at the abraded surface and at high temperature, where the reduction of wear rate was due to the development of oxide protective layer. A steady-state wear equation for the contacting surface at high temperature (R = 1/Y √(ln W/2X)) for AA7050 hybrid composites–steel interface was developed. The enhancement in wear resistance was directly proportional to the proportion of ferrous content present on the surface, which was confirmed on the elemental analysis. Pock marks, micropits, craters and cracks were the features observed on the worn surface morphology, whereas delamination and plasticisation were the observed modes of wear mechanism.

Clarifying the phenomenon of Ultrasonic Assisted Electric discharge machining

In this research work, an attempt was made to machine the Nimonic alloy utilizing the Ultrasonic Vibration assisted Electric Discharge Machining (UVEDM) Process and performance was accessed in terms of material removal rate, tool wear rate and surface roughness. The key finding includes, increase in MRR was achieved by the complete flushing of machined debris and the generation of an intensified plasma channel, while the decrease in Ra was induced by uniform heat distribution. With UVEDM, machined performance was improved due to the fact that UV facilitates the homogeneous heat distribution, which was corroborated by analyzing the surface topography of machined debris. Globules, pock marks, cracks, pits, material doom and craters were the distinct features observed on the surface topography. The shape of the machined debris was angular, erroneous circular, spherical and irregular for the UV frequency of 0,20,40 and 60 kHz respectively. The process parameters utilized for study was optimized using the MOORA-ELECTRE Integrated Optimization Technique (MEIOT).

Sea bottom characteristics and geochemistry of oil and gas seeps in the Gulf of Mexico

From 2015 to 2018, TGS conducted a comprehensive multiclient oil and gas seep mapping survey in the Gulf of Mexico (GOM). The basis for identifying seeps on the sea bottom was a high-resolution multibeam echosounder survey, mapping approximately 880,000 km2 of the sea bottom deeper than 750 m water depth, at a bathymetric resolution of 15 m, and a backscatter resolution of 5 m. We identified more than 5000 potential oil and/or gas seeps, and of those, we cored approximately 1500 for hydrocarbon geochemical analysis. The sea bottom features best related to hydrocarbon seepage in the GOM are high backscatter features, mud volcanoes, pock marks, brine pools, “popcorn” texture, fault traces, and anticlinal crests. We also tracked gas plumes in the water column back to the sea bottom to provide an additional criterion for hydrocarbon seepage. Cores from sea bottom targets recovered liquid oil, tar, and gas hydrates. Oil extract and gas analyses of samples from most target types produced values substantially higher than background in both oil and gas.

A novel Cu-Gr composite electrode development for electric discharge machining of Inconel 718 alloy

Requirement of processes with high productivity leads to the development of new tools and machining technique. One such attempt was made to develop a Copper Composite (CC) tool reinforced with graphite particles through the stir casting route. The CC tool was utilized to machine the Inconel 718 alloy using Electric Discharge Machining (EDM) technique by varying input variables viz tool material, discharge current, pulse on time, pulse off time and gap distance. Brass, Copper, Copper—5% Graphite (CG5), Copper—10% Graphite (CG10), Copper—15% Graphite (CG15) are the various used tool material whereas Material Removal Rate (MRR), Tool Wear Ratio (TWR) and Surface Roughness (Ra) were recorded as responses. The results revealed that using the CG10 and CG15 tools causes arcing and open circuit under some parametric conditions. CG5 offers best MRR and least TWR owing to the bridging effect, high thermal conductivity and lesser density which was suitable for roughing operation whereas brass offers least Ra which was recommended for finishing operation. The surface topography was analyzed with the aid of Scanning Electron Microscope (SEM). Micro crack, pock marks, globules and remelted layers are some of the defects observed in surface morphology. The process parameters was optimized using VIKOR technique.

Pojangmacha People

Pojangmacha People Jung Hae Chae I am thinking of the sad old men I knew. The salt on their drunken foreheads, a sorrow. Of souls stained deep with glutamine and guilt. O, the beauty of MSG. I'm thinking of comfort, the kind rising out of the bodies of blood soups and strangers. Of longing. Inside the tiny drinking tent, the haloed bodies of haejang-guk, the mother of all soups, the soup that chased away tremors and trauma and money troubles and time and time and more time, that soup. The bodies. The into-the-wee-hour-drinking bodies, the sitting-with-bottomstouching-is-just-fine bodies, the bodies sweating out their failing livers, those bodies. Tired and failed as they were, huddled together inside this tiny domed cathedral were their tiny lives, powered by a tiny light, the 1970s-oppressive-cum-underground-guerrilla light, powering the dim city and its people and their small dreams, past the curfew. O, how holy was this light and this coterie, the men lurching headlong into this bloodbath of comfort, the guk chasing away their daily hangovers and bad deeds, only to pass on their karmic debt to their only sons, pass down the bloody mess of their lives. How its biliary notes and fraternal bodies bosomed, not mocked, their chronic bad lives and livers. My father. I'm thinking of him, but not him. Of souls cursed and caressed inside the pojangmachas inside a village. No, not a village but a nation full of sad, sad, old men, with their jaundiced faces riddled with the pock marks of postwar trauma, with their deadly breaths, who lowered their sorry heads into guk after guk, who fawned over their women and children, even as they beat them senseless. The women. I'm thinking of my dead grandmother and my mother, their mothers' mothers, the women gathered at the hair salon next to my grandmother's house, the one with the outhouse. The outhouse. I'm thinking of a childhood lost to dreams, of dreams lost, then found scribbled on the walls of an outhouse, a shit hole brimming with no-good fathers and husbands and lovers, and later of sons. And the women who made them. [End Page 208] 1 When he drank, only when he drank, my father smeared the brown goo with his bare hands all over the wall next to his bed, the smell so terrible it'd wake everybody up, the women. It was a house full of women without husbands or fathers or keepers. The boorish drunk, the women must have thought, who defied even the worst of their imagination about the war they and their mother have had to endure and the bad men made worse by the war. Torn up by years of drinking, his weak constitution would make him go on the bed prepared specially for him by my grandmother. When he drank, and only when he drank, my father broke through a kind of barricade that he'd self-imposed between himself and the world that had become wholly unpredictable; he was enabled, albeit temporarily, to become just a little more authentically terrible than his usual, more guarded, terrible self. My father and his father and his father's father all died from an alcohol-induced liver failure in their mid-fifties. While my father was alive, I saw him all of five or six times, and though I couldn't tell whether he was, in fact, intoxicated each time I saw him, he might as well have been, since he was a most sullen human to be around. He cracked a smile not once. His face, a brown jaundiced earth with its deep grooves and cracked furrows, a once-rich gorge now gone dry, would not have tolerated one. He was unpleasant and awkward around people. More precisely, he was reliably capable of inducing the kind of discomfort felt deep in the bellies of people when confronted, say, with a foul-smelling wounded animal in a cage corner, inspiring, at once, empathy and terror. They want to help and free the hurt animal but aren't sure whether he is safe to approach...

Development and performance evaluation of sintered tool tip while EDMing of hardened steel

In Electrical discharge machining operations, tool cost has the maximum contribution in the final cost. Copper is chiefly used tool material for electrical discharge machining process. Although, copper has very high thermal and electrical conductivity but it disintegrates more, especially during the machining of hardened materials due to its lower hardness. Therefore, an effort has been made to fabricate a sintered tool for electrical discharge machining operations and further evaluated it for wear characteristics. This article describes a study on electrical discharge machining using a sintered tool tip to improve wear resistance. In this work, three batches of tool tips using copper, titanium carbide, silicon carbide and graphite are fabricated by powder metallurgy. Initial estimation of the electrodes has been done by evaluating electrical and mechanical properties. Depending upon the desirable properties for the electrode, a best tool tip has been suggested and utilized for electrical discharge machining on hardened EN31 steel. Electrode wear rate, change in tool shape, volume of material removed, surface roughness and surface integrity has been evaluated in this study. The proficiency of the cermet tool tip has been compared with copper tool tip. It has been found that during machining with the cermet tool tip, the electrode wear rate and change in roundness of tool tip decreased up to 14.82% and 20% respectively as compared to copper tool tip. The decrease in change in roundness during machining also signifies the better shape retention of cermet tool tip. The volume of material removal is higher when cermet tool tip is used for the same set of process parameters. From the study of SEM micrographs, it can be observed that the recast layer is less and there are fewer surface cracks on the surface machined by cermet tooltip. The formation of micropores and pock marks are more predominant on workpieces machined by cermet tooltip. The width of cracks is lesser when cermet tool tip is used in comparison to copper tool tip.

The Effects of Hydrate on the Strength and Stiffness of Some Sands

AbstractGas hydrates can form more or less at the same time as seafloor sediment. They can have the effect of significantly stiffening and strengthening deep‐ocean sediments. Subsequent increases in situ temperature or decreases in pressure may trigger hydrate dissociation, leading to large reductions in the strength and stiffness of the sediment and possible seafloor instability. Gas hydrate dissociation not only removes cementing. It also releases freshwater and significant amounts of trapped gas that are dependent on multiple factors such as type of sediment, available pore space, hydrate morphology, and hydrate saturation. The presence of pock marks in areas of known seabed instability suggests that hydrate dissociation may have been a factor in triggering failure at these locations. Having reviewed the mechanisms by which the strength and stiffness of seabed sediment may be changed during dissociation, this paper reports the results of laboratory testing to evaluate the effects of loss of hydrate cement on strength and stiffness, for a range of sand‐sized materials with differing particle size, specific surface area, and particle shape, using a laboratory gas hydrate triaxial apparatus. The results suggest that both the strength and the stiffness of hydrate‐cemented granular materials are affected significantly by the specific surface available for hydrate cementation and, to a certain extent, by the particle shape. Uniform coarse granular sediments of lower specific surface area can suffer significant loss of stiffness and strength upon hydrate dissociation, changing the sediment from dilative to contractive. Finer‐grained sediments appear less affected by dissociation.

Post-Modern Feminist Ideology in Nayantara Sahgul Select Novels-A Critical Study

Literature is the mirror of life as a statement which reflects of the society and the characters are the images of writers. The novels of Kiran Desai and Anita Desai deals with the men and women, specifically women struggling against oppression and injustice heaped upon them in the name of tradition and culture. This paper has been discussing about Nayantara Saghul novels here it portrays the inalienable right of the characters in her novels. As a writer with feminist concerns, Nayantara Sahgal is a progeny of the tradition where in power it is defined as goddess ‘Saki’ a female symbols. Her fictional world is busy by political leaders, business tycoons, foreign advisors, upper class people, journalists and highly qualified persons like ambassadors and ministers. A political theme is often combined to the theme of man-woman relationship their marital problems, their temperamental incompatility and the problems arising out of their submissiveness and finally the place of a woman in Indian society when she opts to dissolve a seventeen years old marriage. A divorced women is stigmatized forever and she is curiously watched by others as if divorce where “a disease that left pock marks”. Nayantara’s main concern is with self expression within marriage. She describes marriage as a “life-long damage” if the other partner is not sensitive enough to communicate. Marriage is the unwritten law of most societies and very few do not subscribe to in the post modern world. Sahgal presents couples from three generations and details their antithetical relationships in postmodern feminism.

Surface crack density and recast layer thickness analysis in WEDM process through response surface methodology

ABSTRACTIn this work, quantitative assessment of surface damage in terms of parameters like surface crack density and recast layer thickness in wire electrical discharge machining (WEDM) process has been undertaken. The effect of processing conditions on crack formation is studied using scanning electron microscope. Surface crack density and recast layer thickness analysis in terms of machining parameters such as pulse on time, pulse off time, peak current, spark gap voltage significantly deteriorate the microstructure of machined samples, which produces the deeper, wider overlapping craters, pock marks, globules of debris and micro cracks. The microstructure analysis of WEDM surface was based upon the theory of electrical discharge phase and metallurgical physics. It is found that the pulse on time, pulse off time and peak current are the most dominating parameters for both surface crack density and recast layer thickness.

MICROSTRUCTURE ANALYSIS AND MATERIAL TRANSFORMATION OF PURE TITANIUM AND TOOL WEAR SURFACE AFTER WIRE ELECTRIC DISCHARGE MACHINING PROCESS

The wire electric discharge machining process is an alternative for machining of the high strength temperature resistant materials. The present research work is mainly focused on the investigation of integrity of the work surface and wire electrode surface after machining with WEDM. Experimental results showed that pulse-on time, pulse-off time and peak current significantly affected the surface integrity with the formation of deep-wide overlapping craters, pock marks, debris, micro-cracks and recast layer. Both carbides and oxides were formed either in free form and/or in compound form due to decomposition of de-ionized water, machined samples and wire material. The compounds like titanium dioxide (rutile) (TiO2), (TiO0.325), Ti2O3 , Ilmenite (Fe2Ti4O), titanium carbides (TiC) and copper titanium dioxide (Cu3TiO4) were formed due to phase transformations that were analyzed through X-ray diffraction and energy dispersive X-ray method. The effect of process parameters on the wear of wire surface has also been considered.

Effect of processing variables and bulk composition on the surface composition of spray dried powders of a model food system

The surface compositions of food powders created from spray drying solutions containing various ratios of sodium caseinate, maltodextrin and soya oil have been analysed by Electron Spectroscopy for Chemical Analysis. The results show significant enrichment of oil at the surface of particles compared to the bulk phase and, when the non-oil components only are considered, a significant surface enrichment of sodium caseinate also. The degree of surface enrichment of both oil and sodium caseinate was found to increase with decreasing bulk levels of the respective components. Surface enrichment of oil was also affected by processing conditions (emulsion drop size and drying temperature), but surface enrichment of sodium caseinate was relatively insensitive to these. The presence of “pock marks” on the particle surfaces strongly suggests that the surface oil was caused by rupturing of emulsion droplets at the surface as the surrounding matrix contracts and hardens.

Generalist Predators and Predation of Black CutwormAgrotis ipsilonLarvae in Close Mown Creeping Bentgrass

Predators play a significant role in regulating pest outbreaks in turfgrass (Potter 2001). The most common predaceous arthropods recorded from turfgrass are spiders, predatory beetles (Carabidae and Staphylinidae) (Kunkel et al. 1999; Rothwell & Smitley 1999), predatory hemipterans, and ants (Lopez & Potter 2003; Frank & Shrewsbury 2004a). Natural enemies and their effects can be monitored by pitfall traps (e.g., Kunkel et al. 1999; Frank & Shrewsbury 2004a) or by exposing prey overnight without direct observation (Kunkel et al. 1999; Lopez & Potter 2000; Frank & Shrewsbury 2004a). Furthermore, most studies are conducted in roughs or fairways, which have more biodiversity (Lopez & Potter 2000), greater mowing height, and are less intensely managed than greens. Predation and predator abundance can be significantly different based on mowing heights (Rothwell & Smitley 1999). A better understanding of ecosystem services in putting greens may provide insights into reducing pesticide inputs. The objectives of this study were to determine the types of nocturnal predators in putting greens and determine which taxa are involved in predation of black cutworm larvae (Agrotis ipsilon Hufnagel) in close-mown bentgrass. Black cutworms provide a model system to investigate predation in turfgrass. Black cutworm larvae feed nocturnally on creeping bentgrass ( Agrostis stolonifera L.) greens causing pock marks that interfere with ball roll and aesthetics (Potter 1998). Predation trials were conducted at the Turfgrass Research Unit, Auburn, Alabama and the Grand National Golf Course, Opelika, Alabama. Plots or greens had not been sprayed with insecticides for at least 2 wk prior to each experiment. At the Turfgrass Research Unit, 2 plots of creeping bentgrass were used. A 706 m2 plot was used for larval predation trials, and a 929 m2 plot for predator surveys. Turfgrass in these plots was maintained at 0.33 cm with surrounding turf 2.54 cm high. Putting greens have 'Dominant' creeping bentgrass, whereas 'El Toro* zoysiagrass ( Zoysia japonica Steud), and 'Tifway' bermudagrass (Cynodon dactylon L. Pers Cynodon transvaalensis Burtt-Davy) were established in collars. Greens and collars are maintained at 0.36 and 1.27 cm, respectively. Larvae were obtained from a commercial in-

Variation in Elytral Macular Forms of Coccinella septempunctata L. (Coleoptera: Coccinellidae) in North America

Coccinella septempunctata L. is a widespread Palearctic species that has been introduced several times into North America within the last century. It is generally considered monomorphic with seven elytral maculae in North America. In this paper, we document variation in elytral macular patterns of C. septempunctata based on examination of 674 (567 North American and 107 Palearctic) specimens of C. septempunctata and 382 digital submissions of North American C. septempunctata. Two beetles lacked apical maculae, and 18 beetles had paired humeral markings. Humeral markings were consistent in position and typically had less-dense black pigmentation than maculae. Sixteen of the 18 beetles with paired humeral markings were from 11 states in the United States, one was from Alberta, Canada, and one was from Italy. Some specimens had asymmetrical elytral markings that included pock marks, irregular fuscous markings, or both. Nineteen specimens of C. septempunctata (19 North American, 1 Palearctic) and 16 C. septempunctata in digital images had asymmetrical markings. Some beetles with paired humeral markings also had dark pock marks or irregular fuscous markings. We recommend examining additional C. septempunctata from various locations to accurately estimate frequency and pattern of variation in elytral macular forms. Possible causes and implications of variation in the elytral maculae of C. septempunctata are discussed.

A smiting-god-figurine found in the Sanctuary of Poseidon at Kalaureia

In 2007 a Reshef figurine was found in a secondary context southeast of the Temple of Poseidon at Kalaureia. This article discusses its origin in the Syro-Palestinian area in the thirteenth century BC and suggests it arrived at Kalaureia towards the end of the Late Bronze Age and was deposited in a sacral context. As Reshef in later history was identified with Apollo in the Greek environment, the author speculates on there being perhaps a kernel of truth in the later myth of Apollo and Poseidon having exchanged dwelling places in the hoary past. The peculiar surface of the piece called for a technical analysis, which was carried out by Andreas Karydas from the Institute of Nuclear Physics, Demokritos, Athens. It clarified that the “pock marks” on the surface stem from the manufacturing process and are not the result of corrosion.

Some study on electrical discharge machining of ({WC+TiC+TaC/NbC}–Co) cemented carbide

Electrical discharge machining process is a potential method of shaping ({WC+TiC+TaC/NbC}–Co) cemented carbide known for its superior hardness and compressive strength at high temperature and resistance to diffusion wear. Yet, detailed study on electrical discharge machining of this material is lacking in the literature. In the present investigation, therefore, mathematical models are developed for material removal rate, wear ratio, and surface roughness in electrical discharge machining of this cemented carbide using the procedure of statistical design of experiments. Current setting, pulse on time, and pulse off time are chosen as input parameters. Based on available machine settings, a face-centered central composite design is selected for meaningful experimentations. The procedure may be extended to develop a data bank for such type of materials. Further, to reveal the attributes behind the removal of material from the work-piece surface, scanning electron micrographs are studied. It appears that sufficient superheating of work-piece material and subsurface boiling is essential for efficient material removal and that formation of pock marks due to burst of blisters and associated crack formation may be controlled by choosing a proper dielectric.

Venerating vaccines

The eradication of smallpox, announced by WHO in 1979 after 30 years of vaccination campaigns, is one of the greatest achievements of medical science. A vile disease that killed 300–500 million people during the 20th century (and left many lucky survivors facially disfigured by pock marks) had been defeated by means of a potent vaccine, efficiently delivered in the field through a range of modern technologies.

Petroleum systems and structures offshore central West Greenland: implications for hydrocarbon prospectivity

A detailed geophysical mapping project has been carried out by the Geological Survey of Denmark and Greenland (GEUS) in the offshore region south-west and west of Disko and Nuussuaq, central West Greenland as part of the preparations for the Disko West Licensing Round in 2006 (Fig. 1). The main purpose of the study was to evaluate the prospectivity of this almost 100 000 km2 large region, and to increase knowledge of basin evolution and the structural development. Results of the work, including a new structural elements map of the region and highlights of particular interest for hydrocarbon exploration of this area, are summarised below. Evidence of live petroleum systems has been recognised in the onshore areas since the beginning of the 1990s when seeps of five different oil types were demonstrated (Bojesen-Koefoed et al. 1999). Oil seeps suggesting widely distributed marine source rocks of Mesozoic age are particularly promising for the exploration potential (Bojesen-Koefoed et al. 2004, 2007). Furthermore, possible DHIs (Direct Hydro carbon Indicators) such as gas-clouds, pock marks, bright spots and flat events have been interpreted in the offshore region (Skaarup et al. 2000; Gregersen & Bidstrup in press). The evaluation of the region (Fig. 1) is based on all public and proprietary seismic data together with public domainmagnetic and gravity data. The seismic data (a total of c. 28 000 line km) are tied to the two existing offshore exploration wells in the region (Hellefisk-1 and Ikermiut-1). The study also incorporates information on sediments and volcanic rocks from onshore Disko and Nuussuaq (Fig. 2). Ten seismic horizons ranging from ‘mid-Cretaceous’ to ‘Base Quaternary’ (Fig. 2) have been interpreted regionally. Large correlation distances to wells, varying data quality and a thick cover of basalt in the north-eastern part of the region, add uncertainty in the regional interpretation, especially for the deeper horizons such as the ‘mid-Cretaceous’ equivalent to Santonian sandstone interval drilled in Qulleq-1 far south. Based on the seismic interpretation (Fig. 3) structural elements maps, horizon-depth maps and isopach maps have been produced; these maps, together with general stratigraphic knowledge on potential reservoirs, seals and source rocks (Fig. 2), provide important information for discussions of critical play elements including kitchens and structures.The existence of many large structures combined with the evidence of live petroleum systems has spurred the recent major interest for hydrocarbon exploration in the region.

Molecular and morphological characterization of the association between bacterial endosymbionts and the marine nematode Astomonema sp. from the Bahamas

Marine nematode worms without a mouth or functional gut are found worldwide in intertidal sandflats, deep-sea muds and methane-rich pock marks, and morphological studies show that they are associated with endosymbiotic bacteria. While it has been hypothesized that the symbionts are chemoautotrophic sulfur oxidizers, to date nothing is known about the phylogeny or function of endosymbionts from marine nematodes. In this study, we characterized the association between bacterial endosymbionts and the marine nematode Astomonema sp. from coral reef sediments in the Bahamas. Phylogenetic analysis of the host based on its 18S rRNA gene showed that Astomonema sp. is most closely related to non-symbiotic nematodes of the families Linhomoeidae and Axonolaimidae and is not closely related to marine stilbonematinid nematodes with ectosymbiotic sulfur-oxidizing bacteria. In contrast, phylogenetic analyses of the symbionts of Astomonema sp. using comparative 16S rRNA gene sequence analysis revealed that these are closely related to the stilbonematinid ectosymbionts (95-96% sequence similarity) as well as to the sulfur-oxidizing endosymbionts from gutless marine oligochaetes. The closest free-living relatives of these gammaproteobacterial symbionts are sulfur-oxidizing bacteria from the family Chromatiaceae. Transmission electron microscopy and fluorescence in situ hybridization showed that the bacterial symbionts completely fill the gut lumen of Astomonema sp., suggesting that these are their main source of nutrition. The close phylogenetic relationship of the Astomonema sp. symbionts to known sulfur-oxidizing bacteria as well as the presence of the aprA gene, typically found in sulfur-oxidizing bacteria, indicates that the Astomonema sp. symbionts use reduced sulfur compounds as an energy source to provide their hosts with nutrition.

Near-mirror-finish phenomenon in EDM using powder-mixed dielectric

This paper presents a study of the near-mirror-finish phenomenon in electrical discharge machining (EDM) when fine powder is introduced into the dielectric fluid as a suspension at the tool–workpiece or inter-electrode gap during machining. For this study, the dielectric flushing system of a conventional die-sinking EDM machine was specially modified to inject and distribute the powder into the dielectric fluid, especially at the gap between the tool and the workpiece. Machining was performed on various types of steel with different types of powder suspensions at a peak current of around 1 A. Particular combinations of powder-mixed dielectric and workpiece have been found to produce mirror-finish or glossy machined surfaces. Close scrutiny of the mirror-finish surfaces reveals shallow overlapping re-solidified discs with smooth rims, unlike typical EDMed surfaces, which are typically covered with deep craters, pock marks and globules. The various factors affecting the generation of the mirror-like surfaces are discussed. The appropriate settings of electrode polarity and pulse parameters and the correct combination of workpiece material and powder characteristics have a significant influences on the mirror-finish condition. The use of negative electrode polarity (i.e. with the tool as the negative electrode, which is a condition normally used for finishing EDM) is necessary to achieve the mirror-finish condition. Other features of the powder-mixed dielectric EDM are shorter machining time, more uniform dispersion of the electrical discharges, and stable machining. Based on the results of the experimental investigation, the types of material composition, powder properties and machine setting in bringing about near-mirror conditions are discussed.