Large Nugget Research Articles

Metamorphic origin of large nuggets of platinum-group metals: evidence from multiphase inclusions in Os-Ir-Ru alloys from the Adamsfield placer, Tasmania

Abstract Platinum-group elements (PGE) are most effectively concentrated via sulfide-silicate melt immiscibility; however, under sulfide-undersaturated conditions, PGE may exhibit highly siderophile behavior and occur as native metals and alloys. In this case, they can form micrometer-size inclusions within Cr-spinel, but also large nuggets (up to several kilograms), found in chromitites and related placer deposits. The exact formation mechanism of such large nuggets and accumulations of PGE unrelated to sulfide melts remains controversial due to mass balance issues. In this study of multiphase inclusions in Os-Ir-Ru nuggets from the Adamsfield placer district (Tasmania, Australia), we constrain their crystallization environment. Multiphase inclusions comprise variable proportions of hornblende, enstatite, quartz, anthophyllite, anorthite, chlorite and native iridium and coexist with single-phase olivine and Cr-spinel inclusions. The heterogeneity in phase and chemical composition of the inclusions indicates a complex origin from an inhomogeneous source media. This is corroborated by grain scale disequilibrium mineral assemblages, where forsterite and quartz are both included within a single osmium grain. Our proposed multi-stage origin of Adamsfield Os-Ir-Ru nuggets involved magmatic olivine-Cr-spinel-Os-Ir-Ru cumulates in peridotite bodies that were subsequently overprinted by various hydrothermal and metamorphic processes, including serpentinization or growth of Os-Ir-Ru from supercritical fluids. The final metamorphic stage resulted in the obliteration of Os-Ir-Ru zonation, culminating in the current assemblage of inclusions.

Mechanical behavior investigation for quenching and partitioning steel dissimilar resistance spot welds

Resistance spot welding is still the most common joining process for autobody structure assembly. It has many advantages and is simple to use, but the main challenge is controlling the process parameters that affect mechanical properties and weld quality. Advanced high-strength steels were developed to meet the demands of automakers such as lighter and stronger autobody. Quenching and partitioning (Q&P 980) steel is a third-generation advanced high strength steel with a combination of high strength and high ductility. In this study, effect of resistance spot welding parameters on the mechanical performance of dissimilar combination of Q&P980 steel and SPFC780Y high strength steel was investigated. Welding current, welding time, electrode pressure and holding time were chosen as the most important process parameters influencing the weld quality. A comprehensive investigation was conducted on geometrical attributes such as nugget size, fusion zone width, fusion zone area, electrode indentation and heat affected zone area and their correlations with peak load (Pmax), failure energy, fracture energy, and elongation at peak load (Lmax). In contrast the failure modes and fracture behavior were studied using scanning electron microscopy. It was observed that using high levels of welding current and welding time while maintaining low levels of electrode pressure and holding time result in a maximum peak load, energy absorption and maximum (Lmax) with pullout failure mode (PF) and this was demonstrated for large nugget size to a critical limit. The methods and findings of this research can be used as a basis for further research and development in the field of dissimilar resistance spot welding.

Spatial distribution of soil organic carbon in apple orchard soils of Kashmir Himalaya, India

Apple orchards constitute an important agro-ecosystem across Kashmir Himalayan Region (KHR), but estimates of their soil organic carbon (SOC) are unavailable. We investigated 174 apple orchards for estimating SOC spatial distribution across KHR at regional scale and evaluate accuracy of five interpolation methods. The selected orchards were representative of varied size, age, and management practices prevalent across KHR. Soil samples were collected from both tree rows and alleys between tree rows by digging pits at three depth intervals of 0–10; 10–20 and 20–30 cm. A total of 1044 soil samples were collected. The interpolation methods used included, ordinary kriging (OK), inverse distance weighing, empirical bayesian kriging, radial basis functions and local polynomial interpolation. Cross validation was used to assess the comparative performance of each method by measuring interpolation bias and accuracy. The soils were structurally less stony, with weak granular to moderate crumb at surface and sub-angular to angular blocky structure at sub-surface. The soils exhibited lowest co-efficient of variation for soil acidity and highest for soil electrical conductivity. With mean value of 12.33 ± 3.98 g kg−1 on concentration basis and 48.45 Mg C ha−1 on stock basis, the SOC declined with increasing soil depth, so much that near surface (0–10 cm) SOC content could explain well over 64% and 35% of second (20–30 cm) and third layer (20–30 cm) variation respectively. Exponential model best described SOC content across all depths. Semi-variograms of SOC at topsoil exhibited larger nugget effect while nugget sill ratios of 43% suggested moderate SOC spatial dependence. Cross validation exhibited better accuracy for OK in terms of indicating good match between observed and predicted SOC. This study exemplifies that apple orchards across KHR are significant carbon pool, improves our understanding of spatial distribution of SOC and help in evaluating soil health by providing site specific maps.

Influencing mechanism of an external magnetic field on fluid flow, heat transfer and microstructure in aluminum resistance spot welding

ABSTRACT The magnetically assisted resistance spot welding (MA-RSW) method provides a promising approach for improving weld performance. However, the MA-RSW process involves complex multiphysics fields that make numerical modeling very challenging. Thus, most resistance spot welding (RSW) models adopt electrothermal-mechanical coupling and ignore the influence of fluid flows caused by electromagnetic forces on nugget growth. In this paper, a magnetohydrodynamic (MHD) model, which couples the electric, thermal, magnetic and flow fields, is developed to explore the influencing mechanism of an external magnetic field (EMF) on the flow patterns and microstructures of aluminum resistance spot welds. Numerical results show that the application of an EMF can lead to the formation of Lorentz force in the liquid nugget, which transforms the flow pattern from an in-plane flow to a combined in-plane and out-of-plane flow. The flow velocity of molten metal in the MA-RSW process is 5 times higher than that of the RSW process, and the maximum temperature of the MA-RSW model is reduced from 774°C in the MHD RSW model to 725°C. Therefore, the MA-RSW joint exhibits a 25.5% larger nugget diameter, a more than 20% finer grain structure, and fewer defects than the RSW joint in the experimental results.

Gold nuggets: the inside story

Gold nuggets have long captured the imagination of geologists, prospectors and the public alike, but their origin remains disputed. Supporting a supergene origin, most gold nuggets in Australia have been found at or near the soil surface. Many are intimately associated with, and even appear to enclose, soil materials and weathered rock. Even large nuggets (e.g., >2 kg) have surface features and/or gross morphologies that suggest chemical reworking in the regolith. Conversely, other nuggets have been found at depths of ten metres or more in the regolith, and large masses of gold have been encountered at considerable depth, in completely unweathered, hypogene environments. Nuggets and particulate gold from many deposits in Australia, New Guinea, SE Asia and Brazil have been examined by optical and electron-optical techniques to determine characteristics that may indicate their genesis and stability in the regolith. The specimens have been collected at or close to the surface but all nuggets (mass range ~1 gm to >8 kg) and many smaller grains appear to be hypogene. They have nearly homogeneous Ag contents, mostly in the range 3 to 20 wt%, although some have no detectable Ag. One specimen also contains up to 3 wt% Hg, but no other alloyed metals > 0.1 wt% were detected. Enclosed minerals are rare – with only galena, Bi sulphide, galenobismutite and complex Ag-Hg tellurides in a few samples. The internal structure of the nuggets comprises nearly equigranular, randomly-oriented crystal domains. Many crystals display coherent twins and/or short incoherent twins that terminate within the crystal, all typical of thermal annealing at temperatures >250°C. Some small nuggets from SE Asia, also with annealing fabrics are possibly the product of hydrothermal remobilization and re-precipitation. In comparisons, some specimens from New Guinea contain 10->30 wt% Ag and have internal structures such as zoning and ‘fern-like’ crystal habits. These are derived from epithermal deposits and have not been deformed or recrystallized since initial deposition. Even the largest nuggets have internal evidence of weathering. Many have secondary minerals such as Fe oxides, clays and calcite within them, but none of these is fully enclosed. Rather, they are all open to the outer margin of the nuggets, situated in interconnecting voids along crystal boundaries. These boundaries also exhibit Ag depletion, similar to the depletion rims on the external surface. EBSD analysis shows there is no variation in crystallographic orientation across, or into, the depletion zones. These characteristics show that nuggets are dissolving in the surface environment, not forming, with weathering reactions initiated on the external surface and, internally, along crystal boundaries.

Comparison of mechanical properties and the nugget formation of composite ceramic-centered annular welding and traditional resistance spot welding

A resistance welding method that uses a composite ceramic-centered annular electrode (RAW) was adopted to improve the weldability of high-strength steel. The design idea was to increase the strength of welded joints by the formation of a large nugget with low indentation depth. The strength and the nugget formation of joints produced by the RAW, and the traditional spot welding (RSW) were comparatively investigated by the microstructural, artificial neural network, and finite element model analyses. The highest failure load reached 22.3 kN in the RAW, and the strongest influence on the strength was welding time, followed by welding current, and electrode force at the end. The interaction between these welding parameters significantly influences the strength of joints in the RAW but not in the RSW. Based on the ANN models, a detailed relationship between the parameters and the strength in RAW were obtained, which also determined that the optimal window of welding parameters in the RAW was larger than that in the RSW. The FEM model analysis showed that the annular nugget was generated first, then expanded outward and inward in RAW, and finally completely fused in the nugget center. The mode of heat transfer and the nugget formation could contribute to better dissipation of the resistance heat generated in the RSW. This “soft” heat distribution yields a large nugget with low indentation depth.

Climate variability in space-time variogram models of annual rainfall in arid regions

Global climate change affects weather patterns around the world. Rainfall variability, especially in arid regions, can increase or decrease drastically in the space and time domains. To determine the spatial and temporal continuity of rainfall phenomena, geostatistical models and variograms are key tools for estimating and modeling the space and time dependence in a data set. The basic steps in any regional modeling method involve obtaining the experimental variogram and fitting a model to the experimental variogram. Theoretical matching is performed for the experimental variogram considering the sill, nugget, and other features, which reflect the regional characteristics of the phenomena concerned. In this study, a product-sum space-time-standardized variogram procedure is proposed and used to identify the spatial and temporal structures of annual rainfall in the western part of Saudi Arabia. The standardized variogram is, in fact, the reduced scale of the original form without the loss of the characteristics of the underlying phenomena. The results of the space-time-standardized experimental and variogram models indicate that the average annual spatial variograms have a continuous structure based on an exponential model with a small nugget. The average annual temporal variograms have a continuous structure based on an exponential model with a large nugget. These results confirm the high variation in the rainfall pattern in the time domain and low variation in the spatial domain.

Effect of process parameters on the joint strength in ultrasonic welding of Cu and Ni foils

ABSTRACTManufacturing of battery packs for electric vehicles requires joining at different levels such as cell level, unit level, and module level, which include the joining of highly conductive and dissimilar layers of thin metal foils. Conventional welding techniques such as fusion welding, resistance spot welding, and laser beam welding are not appropriate for the joining of multiple layers of metal foils with a comparatively thicker tab because they face difficulties in producing large weld nuggets and generate weld defects such as brittle phases. Ultrasonic welding, a solid-state technique, has been found to be the most successful joining process for battery pack manufacturing. Several studies focused on the joining of two layers of dissimilar materials while very few have successfully characterized the welding performance of multi-layer ultrasonic metal welding. In this study, a set of weld attributes was introduced, and weld parameters have been optimized for sound quality joints using optical microscopy and nanoindentation based on those attributes. Different failure types were associated with weld quality based on microscopic images of fractured specimens. A correlation between weld parameters and weld attributes (i.e., bond density and post weld thickness) was developed to provide some insights into their interdependencies.

Resistance ceramic-filled annular welding of thin steel sheets

To meet the requirements of automobiles for high-strength steel welding and thin-sheet welding, a composite ceramic-filled annular electrode (A-electrode) was designed by controlling welding energy distribution. By embedding a circular non-conductive ceramic rod in the center of a conventional copper electrode, the welding heat changed from centralization in the center of the nugget of traditional spot welding to a more dispersed annular distribution. As a result, an annular nugget that corresponds to the annular copper end face was formed. Given the energy dispersion, a higher current was required to bond the sheets by the A-electrode than that by a traditional electrode. However, the indentation depth on the surface of the nugget decreased effectively. The stress distribution indicated that the stress was concentrated at the edge of the nugget under the load, and the unconnected part in the middle did not affect the strength. The size of the annular nugget was designed by adjusting the structure of the A-electrode. The strength of the nugget could be improved by forming a large nugget area.

Topographic gradients and lode gold sourcing recorded by placer gold morphology, geochemistry, and mineral inclusions in the east fork San Gabriel River, California, U.S.A

Topography plays a major role in the morphology, geochemistry, and mineral inclusion profiles of placer gold from the East Fork of the San Gabriel River. These trends are consistent with a significant change in topographic gradient over a short distance, and coincident with the break in slope along the range-front of the San Gabriel Mountains. Placer gold from above this topographic transition is more rounded and 3-dimensional, with elevated silver in the alloy of the native gold, little evidence of gold enrichment rims, and abundant inclusions of brittle and reactive minerals. Below the topographic transition, placer gold assumes a distinctly flattened form, with lower silver alloy content, thin gold enrichment rims, and rare inclusions of relatively inert minerals. This relationship is observed for placer gold shed from two inferred lode sources within the river catchment, with placer gold from each of these sources being traceable as a unique population over at least 8 km of the river. Significantly, these placer gold properties are distinctive enough to fingerprint gold from specific portions of the placer deposit. These identifying characteristics, with virtually identical morphology and chemical trends, are observed in the large nuggets of the Mel Fisher collection of purported San Gabriel gold. These similarities appear to confirm the provenance of the collection, including identification of the specific locations where each nugget originated.

The 21 cm absorption line and the axion quark nugget dark matter model

We argue that dark matter in the form of macroscopically large nuggets of standard model quarks and antiquarks can help to alleviate the tension between standard model cosmology and the recent EDGES observation of a stronger than anticipated 21 cm absorption feature. The effect occurs as a result of the thermal emission from quark nugget dark matter at early times and at energies well below the peak of the CMB. Similar radiation at early times may also contribute a fraction of the GHz range excess observed by ARCADE2.

Optimal interpolation and isarithmic mapping of soil properties: I The semi‐variogram and punctual kriging

SummaryKriging is a means of spatial prediction that can be used for soil properties. It is a form of weighted local averaging. It is optimal in the sense that it provides estimates of values at unrecorded places without bias and with minimum and known variance. Isarithmic maps made by kriging are alternatives to conventional soil maps where properties can be measured at close spacings.Kriging depends on first computing an accurate semi‐variogram, which measures the nature of spatial dependence for the property. Estimates of semi‐variance are then used to determine the weights applied to the data when computing the averages, and are presented in the kriging equations.The method is applied to three sets of data from detailed soil surveys in Central Wales and Norfolk. Sodium content at Plas Gogerddan was shown to vary isotropically with a linear semi‐variogram. Ordinary punctual kriging produced a map with intricate isarithms and fairly large estimation variance, attributed to a large nugget effect. Stoniness on the same land varied anisotropically with a linear semi‐variogram, and again the estimation error of punctual kriging was fairly large. At Hole Farm, Norfolk, the thickness of cover loam varied isotropically, but with a spherical semi‐variogram. Its parameters were estimated and used to krige point values and produce a map showing substantial short‐range variation.

Influencing factors on nugget formation during multipoint welding by single-side resistance spot welding

ABSTRACTRecently, weight reduction and improvement of crashworthiness of auto bodies have become important issues. At the same time, stiffness of auto bodies is also needed to ensure a smooth ride. Using hollow parts, such as bended pipes and hydroformed parts, is one of the solutions to the demand for both rigidity and light weight.To weld hollow parts and sheet panels together, welding methods which allow us to access from one side are required. Single-side resistance spot welding (single-side RSW) process is one of those, and has recently been attracting attention.However, because of the long current path and small electrode force, it is difficult to concentrate the electric current in the welding spot compared with conventional direct resistance spot welding (direct RSW). Furthermore, in multipoint welding, shunt current will occur easily, and the nugget formation will be inhibited.To obtain a guideline for making sound nuggets, influencing factors for shunt current were investigated. In addition, a numerical study was carried out to discuss the difference between direct RSW and single-side RSW.According to the CAE analysis, the shunt current of single-side RSW will be higher rate than direct RSW. The rate of shunt current was influenced by the electrical resistance of its current path. For this reason, with shorter distance between welding points, or with lower electrical resistance of material, it is difficult to get large nuggets. By enhancing the electrical resistance of shunt current path, shunt current could be reduced and a larger nugget would be obtained.

Selection of electrode tips for the resistance spot welding of dissimilar stainless steels

This research sought to resolve the issue of electrode tip selection used in the resistance spot welding for two different stainless steel grades under otherwise identical welding parameters: SUS316 and SUS425. The study used a full fractional design in order to perform the analysis of the variables involved, while the quality of the weld was assessed by considering the indentation depth, penetration, and nugget diameter. The results of the study described the nugget diameter and penetration for all welds with the various different electrode tips which met the specified requirements of the customers. A single type of upper R30 and lower R30 electrodes achieved acceptable indentation depth. Ideally, good weld quality would demand a large nugget diameter, shallow indentation, and good penetration, all contribution to an acceptable overall appearance. Both R30 electrode tips are able to achieve the desired quality requirements in the spot welding process.

Effect of nugget size and notch geometry on the high cycle fatigue performance of resistance spot welded DP590 steel sheets

The fatigue property of resistance spot welded DP steels in relation to weld nugget size, microstructures and notch geometry was evaluated. High heat input welding produces large nuggets with a ‘tongue’ forming dual notches, while low heat input produces smaller nuggets with a sharp notch and a partially bonded interface ahead of the notch. In the high load regime, the joint performance can be directly correlated to the nugget size while in the low load regime it is primarily controlled by the notch geometry. Under cyclic loading, kinked cracks are initiated directly at the tongue of the larger nugget followed by through thickness propagation. Contrarily, in case of small nugget the crack propagation is initially interfacial followed by kinked cracking. Hence, fatigue performance under low load is better in joints with smaller nuggets, primarily because of delayed kinked crack initiation as opposed to accelerated kinked crack growth in larger nuggets with dual notches.

Proper sampling, total measurement uncertainty, variographic analysis & fit-for-purpose acceptance levels for pharmaceutical mixing monitoring

Author Summary: Process monitoring in technology and industry in general, in pharmaceutical batch and continuous manufacturing in particular, is incomplete without full understanding of all sources of variation. Pharmaceutical mixture heterogeneity interacts with the particular sampling process involved (by physical extraction or by Process Analytical Technology (PAT) signal acquisition) potentially creating four Incorrect Sampling Errors (ISE), two Correct Sampling Errors (CSE) in addition to the Total Analytical Error (TAE). In the highly regulated pharmaceutical production context it is essential to eliminate, or reduce maximally, all unnecessary contributions to the Total Sampling Error (TSE) to the Measurement Uncertainty (MUtotal) in order to be able to meet stringent regulatory blend and dose uniformity requirements. Current problems mainly stem from inadequate understanding of the challenges regarding sampling of powder blends. In this endeavor the Theory of Sampling (TOS) forms the only reliable scientific framework from which to seek resolution. We here present the variographic approach with an aim to conduct TSE error variance identification and to show how to develop fit-for- purpose acceptance levels in critical powder blending process monitoring. The key issue regards the nugget effect, which contains all non-optimised [ISE, CSE] plus TAE contributions to MUtotal. A large nugget effect w.r.t. the sill is a warning that the measurement system is far from fit-for-purpose, and must be improved. Regulatory guidances have hitherto called for physical sampling from within blenders, leading to significant ISE associated with the insertion of sample thieves (sampling spears). Instead of self- crippling spear sampling we here call for a paradigm shift, very much from the TOS regimen, in the form of alternative on-line variographic characterisation of 1-D blender outflow streams. Practical illustrations and case histories are described in parallel contributions to WCSB7.

Weld Growth Mechanisms and Failure Behavior of Three-Sheet Resistance Spot Welds Made of 5052 Aluminum Alloy

This paper investigates the weld nugget formation in three-sheet aluminum alloy resistance spot welding. The nugget formation process in three equal thickness sheets and three unequal thickness sheets of 5052 aluminum alloy were studied. The results showed that the nugget was initially formed at the workpiece/workpiece interfaces (i.e., both upper interface and lower interface). The two small nuggets then grew along the radial direction and axial direction (welding direction) as the welding time increased. Eventually, the two nuggets fused into one large nugget. During the welding process, the Peltier effect between the Cu-Al caused the shift of the nugget in the welding direction. In addition, the mechanical strength and fracture mode of the weld nuggets at the upper and lower interfaces were also studied using tensile shear specimen configuration. Three failure modes were identified, namely interfacial, mixed, and pullout. The critical welding time and critical nugget diameter corresponding to the transitions of these modes were investigated. Finally, an empirical failure load formula for three-sheet weld similar to two-sheet spot weld was developed.

Numerical and experimental study on nugget formation process in resistance spot welding of aluminum alloy

The weld nugget formation in the resistance spot welding (RSW) of aluminum alloy was investigated in the present study. The nugget formation process was directly observed by using a digital high-speed camera. Numerical simulation was also employed to investigate the nugget formation process. The results showed that for the RSW of two aluminum alloy sheets, a nugget was first formed in the workpiece/workpiece (W/W) interface and grew along the radial direction and axial direction of the sheets, and then it became a large elliptical nugget. For the RSW of three aluminum alloy sheets, two small nuggets were firstly formed in two W/W interfaces and grew along the axial direction and radial direction; finally they fused into one nugget. Besides, there existed a critical welding time, after which the nugget size remained nearly unchanged. This indicates that a long welding time is unnecessary for the RSW of aluminum alloy. In addition, the calculated nugget radius was compared with the experimental results, which showed that the simulation results agreed well with the experimental results.

Improvement of aquifer transmissivity assessment in crystalline and metamorphic fractured rocks in Bondoukou area, North-Eastern Côte d’Ivoire

This study is performed in the Bondoukou area located in north-eastern Cote d’Ivoire. The aim of this study was to improve the assessment of transmissivity of the highly fractured aquifers of Bondoukou area using kriging techniques. A previous estimation of the transmissivity was conducted with aquifer transmissivity of crystalline rocks (n = 62). These transmissivities were later supplemented by 78 samples of aquifer transmissivity of metamorphic rocks. The results of this study were compared with those obtained in crystalline rock aquifers. In both cases, the variogram exhibits a large nugget effect representing respectively 63 and 70% of total dispersion. The nugget effect of this study is less important than the previous study. The range is equal to 12 km and is larger than that obtained in the previous study. The structuring of the variogram is enhanced when we consider the aquifer transmissivity of metamorphic rocks. Transmissivity estimated in both studies varies between 4.00E-06 and 1.72 E-06 E-05 m 2 .s -1 and between 3.86 E-06 and 1.84 E-05 m 2 .s -1 respectively in the previous study and in this study. In both cases, the observed and estimated transmissivities values are comparable and are spread over two orders of magnitude. Kriging maps showed that the estimation of transmissivity in Bondoukou area is better in this study. The results obtained in this study are interesting because they showed improved estimate of the transmisssivity in the Bondoukou area. Key-words: crystalline basement, fractured aquifers, Bondoukou, transmissivity, variogram, kriging.

Resistance Spot Welding Of Magnesium Alloy Sheets with Cover Plates

Lightweight materials such as high-strength steels and aluminum alloys are widely used in automotive components for the purposes of weight reduction and consequent higher fuel efficiency. As new alloys have been developed, magnesium alloys have become attractive for the production of automotive components due to their unique properties, such as high specific strength, damping capacity, and recyclability. In the assembly process of these materials, resistance spot welding (RSW) is used to construct thin sheet structures. Although RSW of steel sheets with high weld quality has already been established, the following problems remain with regard to RSW of magnesium alloys: high welding current, degeneration of electrodes, and blowholes and expulsion in or around the nugget. In this study, RSW with cover plates was applied to magnesium alloy sheets to investigate the performance of spot-welded joints. The weld quality was evaluated based on the tensile shear strength of the joints, the shape and the size of the nugget, and the appearance of blowholes and expulsion. Since RSW with cover plates is similar to conductive heat resistance seam welding (CHRSMW), the results showed that RSW with cover plates enabled the spot welding of a magnesium alloy with a large nugget and high tensile shear strength in the joint. It was also shown that blowhole-free spot welds with a large nugget and high tensile shear strength were obtained for welding currents above 8 000 A with a long down-slope time of welding current and a high electrode force.