Variable Grain Research Articles

Morphology and genesis of giant seafloor depressions on the southeastern continental shelf of the Korean Peninsula

We identify and describe five giant seafloor depressions from the southeastern continental shelf of the Korean Peninsula using multibeam bathymetry, sub-bottom profiler, and multi-channel seismic reflection data, supplemented by piston cores. Multibeam bathymetry data from the shelf show four crescent-shaped depressions (SD1 to SD4) and one near-circular depression (SD5) within a group of NW-SE trending depressions, the largest covering an area of about 7 km2 on the seafloor. The depressions reach up to ~4.5 km in width and ~2 km in length and have asymmetric cross-sections. Some have depths as large as 40 m below the surrounding seafloor with walls as steep as 45°. The depressions are confined to water depths between 130 and 170 m and bounded on the north by a large submarine channel that was plausibly formed by fluvial or tidal processes during the Last Glacial Maximum (LGM) sea-level lowstand. Multi-channel seismic and sub-bottom profiler data reveal truncated depression walls and the presence of sediment drift deposits within the depressions, indicating that both erosion and deposition are active processes. Flaser and lenticular bedding in the cored drift deposits along with variable grain size (ranging between ~2.6 phi and ~4.3 phi) are diagnostic features of the bottom currents influenced by tidal forces. Depressions SD1 to SD4 lack evidence of fluid or gas escape. In contrast, many features of depression SD5 are characteristic of gas escapes and blowouts, including acoustic anomalies, a 20-m-high carbonate mound or carbonate-encrusted mound, and mud dikes and mud patches in cores. Based on the SD5 example, we think it is likely that the other crescent-shaped seafloor depressions formed originally as pockmarks by gas/fluid venting, and have since become inactive. The pockmarks represent zones of weakened sediment that were eroded, expanded, and merged by bottom currents to form larger seafloor depressions. Modern currents are strong enough to transport shelf sediments, and these currents were probably much stronger at lower sea levels when the Korea Strait was a more restricted passage between the East China Sea and East Sea.

Characterization of the frequency dependence of the electrical properties of sandy soil with variable grain size and water content

The electrical properties of soil at low radio-frequencies are important for engineering applications. In this work the effects of excitation frequency (42 Hz-1 MHz) as well as soil characteristics and conditions on the electrical properties of soil are experimentally investigated using remolded soil samples. Several sandy soils were tested with variable water content (dry up to saturation) using guarded two-electrode arrangements. A method is proposed based on the Nyquist plot to estimate the critical frequency above which measurements of the electrical properties of wet soil are free from electrode polarization effects. The latter result in extremely high real permittivity values, which do not correspond to the response of soil. The critical frequency is higher for smaller grain size and higher water content and varies according to a power law with soil power-frequency conductivity. The real and (effective) imaginary relative permittivity decrease with increasing frequency whereas the effective conductivity increases; this is attributed to the distributed relaxation of the interfacial polarization mechanism associated with the particulate nature of soil. The values of soil electrical properties increase with increasing water content and decreasing grain size (increasing specific surface) as a consequence of enhanced polarization and conduction phenomena. The effective imaginary permittivity varies with frequency according to a power law with an exponent approximately −1; the multiplier coefficient of this law expresses losses and has been formulated as a function of the degree of saturation and soil specific surface.

Characterization of the frequency dependence of the electrical properties of sandy soil with variable grain size and water content

The electrical properties of soil at low radio-frequencies are important for engineering applications. In this work the effects of excitation frequency (42 Hz-1 MHz) as well as soil characteristics and conditions on the electrical properties of soil are experimentally investigated using remolded soil samples. Several sandy soils were tested with variable water content (dry up to saturation) using guarded two-electrode arrangements. A method is proposed based on the Nyquist plot to estimate the critical frequency above which measurements of the electrical properties of wet soil are free from electrode polarization effects. The latter result in extremely high real permittivity values, which do not correspond to the response of soil. The critical frequency is higher for smaller grain size and higher water content and varies according to a power law with soil power-frequency conductivity. The real and (effective) imaginary relative permittivity decrease with increasing frequency whereas the effective conductivity increases; this is attributed to the distributed relaxation of the interfacial polarization mechanism associated with the particulate nature of soil. The values of soil electrical properties increase with increasing water content and decreasing grain size (increasing specific surface) as a consequence of enhanced polarization and conduction phenomena. The effective imaginary permittivity varies with frequency according to a power law with an exponent approximately −1; the multiplier coefficient of this law expresses losses and has been formulated as a function of the degree of saturation and soil specific surface.

Ripple dynamics over various microtopographical roughness elements and their implications for river management

AbstractSpecific problems in connection with the sediment regime have arisen in the granite and gneiss area in the northern part of Austria due to the increased sediment supply of coarse sand and fine gravel as a product of rock weathering. One of the physical characteristics of these sediments is their high mobility in form of ripple bedforms along the surface. In these rivers, the microtopographical structures of the bed surface are hydraulically active, especially in the initial stages when ripples have not overtopped the largest elements of the bed's roughness elements. There is a lack of detailed studies of the impacts of these microtopographical structures on ripple dynamics. Thus, the aim of the present study was to investigate the impact of various microtopographic roughness elements on ripple movement. This study was performed using a 1:1‐scaled physical laboratory experiment. The results showed, in all repetitions of the experiment with variable grain sizes, that the higher the microtopographical variability of the river substrate and turbulence was, the higher the measured transport velocities of the ripples were. The expected hidden impacts on sediment transport were not documented. Important implications for sediment management were discussed, as river sections with high accumulation rates of coarse sand and fine gravel are prone to additional degradation as downstream transport decreases over smooth ripple–dune morphologies. Additionally, structural elements such as exposed rocks may increase the downstream transport due to the increased variability in turbulence.

RETRACTED ARTICLE: The effect of deposition pressure on the material properties of pulsed laser deposited BaAl2O4:Eu2+, Dy3+ thin films

BaAl2O4:Eu2+, Dy3+ thin films have been successfully deposited by pulsed laser deposition (PLD) method. Formation of crystalline phase was confirmed by X-ray diffraction (XRD) measurement. The average crystallite size calculates using the Scherer’s formula was found to be between 15.2 and 30.5 nm. Scanning electron microscopy (SEM) micrograph revealed that, the films have non uniform morphology with variable grain size for all the deposition conditions. The surface morphology displays some tendency for agglomeration as a function of increasing deposition pressure, which may be due to the sintering or coalesce of small particles into clusters. Atomic force microscopy (AFM) measurements also show an increase in surface roughness as the oxygen deposition pressure is increased. The BaAl2O4:Eu2+, Dy3+ thin-film phosphors exhibit photoluminescence (PL) emission at 495 nm upon 254 nm excitation. This can be attributed to the electronic transition of Eu2+ ions from 4f65d1 to 4f. The present study recommends that this phosphor could be used as luminescent materials for light emitting diode applications.

Effects of grain size on the chemical weathering index: A case study of Neogene fluvial sediments in southwest Japan

Grain size variation of sediments and sedimentary rocks generally complicates the assessment of the degree of chemical weathering as measured by the Chemical Index of Alteration (CIA) value. Mineralogical and geochemical research was carried out on Miocene and Pliocene fluvial sediments in southwest Japan in order to evaluate the effects of grain size on the degree of chemical weathering. A strong positive linear trend is usually exhibited between Al2O3/SiO2 ratios and quartz intensities obtained by X-ray powder diffraction, suggesting that the Al2O3/SiO2 ratio may be suitable as a grain size index for semi-consolidated and unconsolidated sediments. Most of the fine-grained sediments, for which Al2O3/SiO2 ratios are over 0.4, have higher CIA values of over 90. In contrast, coarse-grained sediments, for which Al2O3/SiO2 ratios are lower, especially below 0.2, display a wide range of CIA values. The CIA values for intermediate grain sized samples, for which Al2O3/SiO2 ratios range from 0.2 to 0.4, reflect distinctive differences in the degree of chemical weathering between Miocene and Pliocene sediments. Within this range of Al2O3/SiO2 ratios, the Miocene sediments have higher CIA values (86–98) than Pliocene sediments (71–92). To compare the degree of chemical weathering among some sediments or sedimentary rocks with variable grain sizes, it is desirable to evaluate with the Al2O3/SiO2 ratios limited to the intermediate grain size range.

An Experimental Investigation of the Effects of Grain Size and Pore Network on the Durability of Vicenza Stone

The textural properties of a rock and the characteristics of its pore network are intrinsic variables to consider when investigating the resistance of rock to weathering. To establish the influence of these variables on the effectiveness and progression of weathering processes, two lithotypes of a limestone were analyzed with respect to their texture and porosity, and to their performance during artificial weathering experiments. The studied rock is Vicenza Stone, a bioclastic limestone with heterogeneous texture, variable grain size from fine to coarse, and high levels of open porosity (27–28%). Two sets of samples were obtained from a quarry, defining coarse-grained (CGV) and fine-grained (FGV) lithotypes of Vicenza Stone. Multiple techniques (optical microscopy, scanning electron microscopy, image analysis, mercury intrusion porosimetry, hygroscopicity, ultrasound, and water absorption) were used to characterize the texture and porosity of the limestone. The sample sets were subjected to freeze–thaw (UNI 11186:2008, Cultural heritage—Natural and artificial stone—Methodology for exposure to freeze-thawing cycles, 2008) and salt-crystallization (partial continuous immersion method with a Na2SO4-saturated saline solution) weathering experiments to investigate the processes and progression of mechanical and chemical weathering. Differences in freeze–thaw-induced micro-cracks and in the deposition of secondary micrite between the lithotypes show that the CGV has an overall higher susceptibility to freeze–thaw weathering than FGV. Salt-weathering processes induced micro-crack formation, material dissolution, and secondary precipitation, suggesting the simultaneous occurrence of mechanical stress and chemical dissolution–precipitation processes. FGV is more prone to salt-induced weathering than CGV, whereas CGV is susceptible to the wide thermal excursions of freeze–thaw weathering.

Microstructure and formation mechanisms of δ-hydrides in variable grain size Zircaloy-4 studied by electron backscatter diffraction

Microstructure and crystallography of δ phase hydrides in as-received fine grain and ‘blocky alpha’ large grain Zircaloy-4 (average grain size ∼11 μm and >200 μm, respectively) were examined using electron backscatter diffraction (EBSD). Results suggest that the matrix-hydride orientation relationship is {0001}α||{111}δ;<112¯0>α||<110>δ for all the cases studied. The habit plane of intragranular hydrides and some intergranular hydrides has been found to be {101¯7} of the surrounding matrix. The morphology of intergranular hydrides can vary depending upon the angle between the grain boundary and the hydride habit plane. The misfit strain between α-Zr and δ-hydride is accommodated mainly by high density of dislocations and twin structures in the hydrides, and a mechanism of twin formation in the hydrides has been proposed. The growth of hydrides across grain boundaries is achieved through an auto-catalytic manner similar to the growth pattern of intragranular hydrides. Easy collective shear along <11¯00> makes it possible for hydride nucleation at any grain boundaries, while the process seems to favour grain boundaries with low (<40°) and high (>80°) c-axis misorientation angles. Moreover, the angle between the grain boundary and the adjacent basal planes does not influence the propensity for hydride nucleation.

Geochemistry, mineralogy, and acid-generating behaviour of efflorescent sulphate salts in underground mines in Nevada, USA

Geochemical characterization of metal-bearing sulphate salts from underground mine complexes in Nevada, USA, illustrates a variety of compositional and paragenetic trends. The most common metal species observed were Fe, Al, and Mg; and Al-bearing salts were the most commonly observed minerals. Mineralogical investigation shows that salts have variable grain sizes, are inter-grown, and display relationships that differ from those observed at many sites with similar minerals. Determinations for samples collected from one area show the presence of both paragenetically-early and -late minerals. These complex relationships may be the result of multiple generations of mineral growth. Simple dissolution testing indicates the propensity for each of the six distinct mineral phases observed to rapidly produce acidic solutions. Speciation and mineral equilibrium calculations using two different thermodynamic databases (Pitzer and Minteq.v4) show that different solution properties and mineral solubilities would be predicted depending on the database used. Additionally, some speciation results display unreasonable charge imbalances that would need to be rectified for the results to be utilized in predictive modeling. These results highlight the importance of considering various modeling codes and databases when simulating the environmental impact of concentrated solutions resulting from the oxidation of sulphides and formation of secondary minerals.

Geodynamic setting of Upper Miocene to Quaternary alkaline basalts from Harrat al ‘Uwayrid (NW Saudi Arabia): Constraints from K[sbnd]Ar dating, chemical and Sr-Nd-Pb isotope compositions, and petrological modeling

The volcanic field of Harrat ar Rahah-’Uwayrid (NW Saudi Arabia) consists of an older plateau basalt sequence that overlies Cambrian sandstone and younger cinder cones with smaller flows that are concentrated in the central part of this field. Petrographic and whole rock geochemical data indicate that the older plateau basalts are alkali olivine basalts, while the younger volcanic products are basanites and tephrites with some phonotephrites and tephriphonolites. KAr model ages on 22 bulk-rock samples were obtained for variable grain size fractions (125–250, 250–500 μm). These dates range from 8.2 to 0.34 Ma, whereby three phases of volcanic activity during the Upper Miocene, Pliocene and Quaternary can be distinguished. Sr-Nd-Pb isotopic ratios are similar for all age groups with 87Sr/86Sr = 0.70307–0.70324, 143Nd/144Nd = 0.512912–0.512957, 206Pb/204Pb = 19.360–19.717, 207Pb/204Pb = 15.603–15.633 and 208Pb/204Pb = 39.083–39.521 (present-day ratios are indistinguishable from calculated initial ratios) suggesting that chemical differences among the lavas were probably produced by different degrees of partial melting rather than by different source compositions. Trace element ratios indicate an origin of all volcanics by small degrees of partial melting of amphibole-spinel-garnet peridotite. Geochemical and isotopic data of the Harrat ar Rahah-’Uwayrid are similar to those from the small volcanic fields of Harrat Kura (about 200 km SSE of Harrat al ‘Uwayrid) and of Wadi Jizan in SW Saudi Arabia, but clearly more enriched than those of the volcanic fields that are located in the more central parts of the eastern shoulder of the Red Sea Rift. The lithospheric thickness underneath Harrat ar Rahah-’Uwayrid is estimated to about 60 km based on published results from seismic tomography. Our data support the model that the metasomatized lithospheric mantle was the primary source of the magmas, while the asthenospheric mantle or a plume-type component played an unsignificant role. The composition of the lithospheric mantle source was similar to prevalent mantle (PREMA). There is no need to postulate the participation of a HIMU component and/or enriched mantle components (EM-1, EM-2) in the source of the investigated magmas.

Electron density fluctuations in collisional dusty plasma with variable grain charge

The kinetic theory of electric fluctuations in a collisional weakly ionized dusty plasma is formulated with due regard to the grain charging dynamics. The correlation functions of electron and ion density are obtained by considering their collisions with neutrals described within the Bhatnagar-Gross-Krook model. The electron density correlation spectra in isothermal and nonisothermal plasma are calculated for various values of grain density, grain size, and ion collisionality.

Depositional, diagenetic and mineralogical controls on porosity development in the Ungani Field, Canning Basin

A 75 m-thick section of Early Carboniferous Tournaisian Ungani Dolomite reservoir was continuously cored in Ungani Far West 1, an appraisal well 3 km west of the Ungani field located on the southern flank of the Fitzroy Tough in the Canning Basin. The objective of this coring program was to better understand the pore systems, mineralogy, and diagenetic history of the reservoir to allow calibration and extrapolation of the petrophysical evaluation of the reservoir at the Ungani field. Petrography, stable isotopic, XRF, hyperspectral logging, grain density and CT scan studies were conducted on the core. The core consists of a 12m overlying sealing shale and 63 m of vuggy, fractured and dolomitised reservoir. The reservoir is commonly bioclastic-rich but pervasive dolomitisation hinders recognition of earlier depositional features. The upper carbonate facies is interpreted as shallow to moderate depth marine ramp-type deposits while the lower carbonate facies is suggestive of shallow platform top settings with ‘reefal’ constructing organisms. Bio-mouldic, vuggy, fracture, cavern and inter-crystalline porosity resulting from multistage brecciation, fracturing, dolomitisation and dissolution events are all present and critical to reservoir development. Based on hyperspectral logging and thin section petrography, the reservoir is predominantly dolomite with late phase cements comprising of quartz, calcite, gypsum, anhydrite, chalcedony, dickite and pyrite. Variable grain densities that correspond with porosity have been noted throughout the core but no obvious relationship between the mineralogy and porosity can be established. Shallow to moderate burial and marine or evaporative reflux fluids are likely responsible for the pervasive dolomitisation. Subsequent leaching of calcite is also key to reservoir development.

Effects of variable grain hydration during steeping on the content and physicochemical properties of non-starch polysaccharides in malt and wort

• Worts from highly-hydrated malts contain more arabinoxylans (AX) than β-glucans (BG). • Increasing steep-out moisture has opposite effects on the levels of BG and AX in wort. • The weight average molar mass (Mw) of wort NSP is affected by steeping conditions. • The average Mw of wort AX is higher than that of wort BG. • Shorter steeping may be adequate for barley genotypes with high enzyme potentials.

Thermal sensitivity of CO2 and CH4 emissions varies with streambed sediment properties

Globally, rivers and streams are important sources of carbon dioxide and methane, with small rivers contributing disproportionately relative to their size. Previous research on greenhouse gas (GHG) emissions from surface water lacks mechanistic understanding of contributions from streambed sediments. We hypothesise that streambeds, as known biogeochemical hotspots, significantly contribute to the production of GHGs. With global climate change, there is a pressing need to understand how increasing streambed temperatures will affect current and future GHG production. Current global estimates assume linear relationships between temperature and GHG emissions from surface water. Here we show non-linearity and threshold responses of streambed GHG production to warming. We reveal that temperature sensitivity varies with substrate (of variable grain size), organic matter (OM) content and geological origin. Our results confirm that streambeds, with their non-linear response to projected warming, are integral to estimating freshwater ecosystem contributions to current and future global GHG emissions.

Grain number and grain filling of two-row malting barley in response to variation in post-anthesis radiation: Analysis by grain position on the ear and its implications for yield improvement and quality

Grain weight is reported to be a relatively well conserved characteristic across spring barley (Hordeum vulgare L.) crops that vary in grain number m−2. Understanding the mechanisms that promote stability in grain weight is important to ensure that efforts to increase grain number beyond current high levels successfully increase yield without compromising grain quality. The aims of this study were to establish 1) whether post-anthesis grain abortion contributes to the stability of grain weight by helping match grain numbers to post-anthesis assimilate supply and 2) whether variations in post-anthesis assimilation per unit grain number affect the heterogeneity of grain weight. Field experiments were conducted in a high-yield potential environment for spring barley in 2011 and 2012. Crops were either shaded post anthesis (a 59% reduction in radiation incident on the crop) to reduce net carbon assimilation or grown unshaded. Grain growth was measured at different spikelet locations on the ear and on different shoots (main shoot and tillers) of the same plant. Shading crops from 14 days after anthesis until harvest maturity reduced yield by 19–20%, mean grain weight (MGW) by 12–16% and harvest index by 5–6%, but did not significantly affect grain number in either year. The magnitudes of these effects were considerably lower than the reduction in radiation imposed by shading suggesting some compensatory adjustment in radiation use efficiency or dry matter partitioning to grain after shading. The rate of grain filling was higher for grains in central spikelets than grains at distal or basal locations on the ear. Shading reduced the rate of grain filling to a similar extent (23–27%) at most locations evaluated on the ear, but had no effect on the duration of grain filling. In spite of the comparable effects of shading on grain growth across different spikelet positions and hierarchy of shoots, crops harvested after shading tended to have a more variable individual grain weight (larger interquartile range and coefficient of variation) than crops that were unshaded. The results show that post-anthesis grain abortion does not contribute to the stability of MGW in spring barley. Moreover, low levels of post-anthesis radiation in crops of large grain number m−2 (sink capacity) can increase heterogeneity of grain weight, which may have negative consequences for grain quality.

Direct Correlations of Grain Boundary Potentials to Chemical States and Dielectric Properties of Doped CaCu3Ti4O12 Thin Films

Colossal dielectric constant CaCu3Ti4O12 has been recognized as one of the rare materials having intrinsic interfacial polarization and thus unusual dielectric characteristics, in which the electrical state of the grain boundary is critical. Here, the direct correlation between the grain boundary potential and relative permittivity is proposed for the CaCu3Ti4O12 thin films doped with Zn, Ga, Mn, and Ag as characterized by Kelvin probe force microscopy. The dopants are intended to provide the examples of variable grain boundary potentials that are driven by chemical states including Cu+, Ti3+, and oxygen vacancy. Grain boundary potential is nearly linearly proportional to the dielectric constant. This effect is attributed to the increased charge accumulation near the grain boundary, depending on the choice of the dopant. As an example, 1 mol % Ag-doped CaCu3Ti4O12 thin films demonstrate the best relative permittivity as associated with a higher grain boundary potential of 120.3 mV compared with 82.6 mV for the reference film. The chemical states across grain boundaries were further verified by using spherical aberration-corrected scanning transmission electron microscopy with the simultaneous electron energy loss spectroscopy.

Variations in agronomic and grain quality traits of rice grown under irrigated lowland conditions in West Africa.

Rice breeding in West Africa has been largely skewed toward yield enhancement and stress tolerance. This has led to the variable grain quality of locally produced rice in the region. This study sought to assess variations in the agronomic and grain quality traits of some rice varieties grown in this region, with a view to identifying sources of high grain yield and quality that could serve as potential donors in their breeding programs. Forty‐five varieties were grown under irrigated conditions in Benin and Senegal with two trials in each country. There were wide variations in agronomic and grain quality traits among the varieties across the trials. Cluster analysis using paddy yield, head rice yield, and chalkiness revealed that 68% of the total variation could be explained by five varietal groupings. One group comprising seven varieties (Afrihikari, BG90‐2, IR64, Sahel 108, WAT311‐WAS‐B‐B‐23‐7‐1, WAT339‐TGR‐5‐2, and WITA 10) had high head rice yield and low chalkiness. Of the varieties in this group, Sahel 108 had the highest paddy yield in three of the four trials. IR64 and Afrihikari had intermediate and low amylose content, respectively, with the rest being high‐amylose varieties. Another group of varieties consisting of B6144F‐MR‐6‐0‐0, C74, IR31851‐96‐2‐3‐2‐1, ITA222, Jaya, Sahel 305, WITA 1, and WITA 2 had high paddy yield but poor head rice yield and chalkiness. The use of materials from these two groups of varieties could accelerate breeding for high yielding rice varieties with better grain quality for local production in West Africa.

Circular electrodes to reduce the current variation of OTFTs with the drop-casted semiconducting layer

Circular organic thin film transistor (OTFT) structures are proposed to reduce the impact of variable grain alignment on the drive current of the polycrystalline organic thin film transistor (OTFT). As the circular structure is planar symmetric, the orientation of the grain cannot affect the drive current of the circular OTFT. Thus, circular electrodes expected to provide a lower variation. Top-gate, bottom-contact circular and conventional OTFTs with drop-casted polycrystalline 6,13-Bis(triisopropyl-silylethynyl) (TIPS)-Pentacene organic semiconducting layer (OSC) are fabricated to verify the theoretical variation reduction. The relative standard deviation (RSD), defined as the ratio of standard deviation and the average of drive current is used as the degree of variations in different structures. According to our fabrication result, circular transistors have a significantly lower variation (20% RSD), compared to the variation of conventional OTFTs (61% RSD).

Effect of thermomechanical parameters on mechanical properties of base metal and heat affected zone of X65 pipeline steel weld in the presence of hydrogen

In this study, the effect of different microstructures resulting from thermochemical processes of X65 pipeline steel on mechanical properties of welding in the presence of hydrogen was investigated. According to studies conducted, due to the charge of hydrogen, the hardness of base metal (BM) increased by 10%, the hardness of heat affected zone (HAZ) samples increased by about 13%, the yield strength of base metal increased by an average of 5% and the yield strength of HAZ increased by about 7%, indicating hardening. Elongation was reduced by 32–70% and the percentage of ductile fracture decreased by an average 46% which shows that the fracture of samples in the presence of hydrogen is brittle. In the study of hydrogen defect and concentration of hydrogen permeated to the material it was determined that, the ferrite-pearlite banded microstructure of base metal has a higher hydrogen defect sensitivity than HAZ with an acicular ferrite microstructure and hard phase. In base metal samples under various rolling conditions, the two-pass rolled and the final rolling temperature of 800 °C sample (2BM800), with a smaller grain microstructure and higher interface, had the most diffusive hydrogen content and therefore had the highest hydrogen defects. In the case of HAZ samples, the effect of hydrogen on the increase in hardness and strength properties is greater than that of the base metal samples due to the different microstructure (of the acicular ferrite) and the variable grain size (i.e. the presence of two coarse-grained and fine grained areas). Overall, the results of this study showed that the microstructure has a significant effect on mechanical properties, the amount of diffusive hydrogen and hydrogen defects in welds.

Numerical and experimental investigations of variable polarity gas tungsten arc welding

Variable polarity gas tungsten arc welding (VP-GTAW) has been suggested as a feasible method for welding aluminum alloys due to the enhanced surface cleaning of the oxide film. However, the principles governing the correlations between the electrode positive (EP) ratio, weld geometry, and weld microstructure are not well understood. Therefore, experiments with different EP ratios but same welding current were conducted. A three-dimensional heat transfer and fluid flow model of VP-GTAW was developed with validation against experimental results. Solidification parameters such as the temperature gradient, solidification growth rate, and cooling rate were computed from the model. The results indicate that VP-GTAW produces nearly defect-free joints. Both the top weld width and bottom weld width decrease with increasing EP ratio due to the decreased heat input. The bottom weld width is greater than the minimum weld profile width in the middle of the plate, which indicates that the Marangoni stress has a significant effect on the convective heat transfer and weld geometry. Variable grain dimensions are produced by different EP ratios at the same welding current. Weld pool oscillation originated from the alternating arc force plays a dominant role in the grain dimension.