A Review of Welding in Space and Related Technologies

Deployment of welding and additive manufacturing (AM) technologies in the space environment has the potential to revolutionize how orbiting platforms are designed, manufactured, and assembled. These technologies offer the option for repair of sustained damage to habitat structures on space missions, as astronauts would be able to manufacture new parts (using welding-derived AM processes suitable for use in the external space environment) and weld cracks. An added benefit is that required repairs can be achieved more economically, as new parts need not be shipped from Earth. With further maturation of in-space welding capabilities, astronauts could operate under given standards and weld damaged structures rather than rely on cargo resupply. This Technical Memorandum (TM) begins by reviewing the available literature relevant to welding in space, focusing on solidification, heat and mass transfer, and fluid flows in microgravity. This survey considers research on the effects of welding in microgravity on a material system. The various in-space welding devices that have been previously designed and tested are examined to determine their capabilities and shortcomings, with a focus on the results of their individual welding experiments. Safety measures are discussed to protect the orbiting International Space Station (ISS) and crew during welding operations. Finally, the state of the art is examined by focusing on current approaches to AM and on-orbit welding that are being developed by several companies in conjunction with NASA

Origin of fluids in the shallow geothermal environment of Savo, Solomon Islands.

Savo is a recently emergent volcano. An active geothermal system has been present for at least 50 years, expressed at the surface by numerous hot springs, fumaroles and steaming ground. Samples of water and steam were collected from geothermal features and non-thermal springs and wells, and representative samples of altered rocks and precipitates were collected from geothermal areas. Analysis of the waters for anion, cation and stable isotope composition shows that the waters discharging at the surface fall into two groups Reoka type fluids have the high sulfate, low pH, and enriched δ18O and δD values typical of steam heated acid sulfate waters, where shallow groundwater is heated by rising steam and gas. Isotopically light H2S is oxidised in the near surface environment to produce the sulfate content. Rembokola type fluids have chemistry distinct from the Reoka type fluids, despite the two being found within close proximity (<10 m). Rembokola Type fluids produce a carbonate sinter, so are assumed to be saturated with bicarbonate. The aqueous sulfate has heavy δ34S, suggesting that it is not exclusively produced by the oxidation of H2S in the near surface environment. We suggest that condensation of volcanic gases (including CO2 and isotopically heavy SO2) into meteoric-derived groundwater in the upper levels of the volcanic edifice produces these carbonate–sulfate waters. The presence of SO2 suggests that there is a degassing magma at depth, and potentially a high sulfidation-type epithermal system beneath the steam heated zone

An assessment of global resources of rocks as suitable raw materials for carbon capture and storage by mineralisation

Carbon capture and storage by mineralisation (CCSM) is a method proposed for capturing CO2 by reacting it with magnesium in ultramafic rocks to form carbonate minerals and silica. Large quantities of magnesium silicate rocks are required for this process and to demonstrate the feasibility, and adequately plan for the development and supply of mineral resources, their locations and quantities must be known. This study attempts to globally define the spatial extent and quantity of resources that could be used for the CCSM processes and to assess, if based on resources, this could be a viable, widely applicable CO2 sequestration process. It has been estimated that around 90 teratonnes of material is available. This is sufficient to capture global CO2 emissions for over 700 years at current levels of output and highlights the enormous resource. Even if only a small part is utilised, it could make a significant impact on CO2 reduction. The majority of the resource is contained within ophiolitic rocks. The study further attempts to split CCSM resources into altered (serpentine-rich rocks) and unaltered (olivine-rich rocks) due to the different processing requirements for these rock types. Carbon capture and storage by mineralisation is likely to be of most use in areas with no access to underground geological CO2 storage or for small operations where underground storage is not practical. This study demonstrates that substantial resources are available and their supply is unlikely to be a constraint

Patches in a side-by-side configuration: a description of the flow and deposition fields

In the last few decades, a lot of research attention has been paid to flow-vegetation interactions. Starting with the description of the flow field around uniform macrophyte stands, research has evolved more recently to the description of flow fields around individual, distinct patches. However, in the field, vegetation patches almost never occur in isolation. As such, patches will influence each other during their development and interacting, complex flow fields can be expected. In this study, two emergent patches of the same diameter (D = 22 cm) and a solid volume fraction of 10% were placed in a side-by-side configuration in a lab flume. The patches were built as an array of wooden cylinders, and the distance between the patches (gap width Delta) was varied between Delta = 0 and 14 cm. Flow measurements were performed by a 3D Vectrino Velocimeter (Nortek AS) at mid-depth of the flow. Deposition experiments of suspended solids were performed for selected gap widths. Directly behind each patch, the wake evolved in a manner identical to that of a single, isolated patch. On the centerline between the patches, the maximum velocity U-max was found to be independent of the gap width Delta. However, the length over which this maximum velocity persists, the potential core L-j, increased linearly as the gap width increased. After the merging of the wakes, the centerline velocity reaches a minimum value U-min. The minimum centerline velocity decreased in magnitude as the gap width decreased. The velocity pattern within the wake is reflected in the deposition patterns. An erosion zone occurs on the centerline between the patches, where the velocity is elevated. Deposition occurs in the low velocity zones directly behind each patch and also downstream of the patches, along the centerline between the patches at the point of local velocity minimum. This downstream deposition zone, a result of the interaction of neighbouring patch wakes, may facilitate the establishment of new vegetation, which may eventually inhibit flow between the upstream patches and facilitate patch merger

From continent to intra-oceanic arc: zircon xenocrysts record the crustal evolution of the Solomon island arc

The first U-Pb ages from a ca. 26–24 Ma pluton on Guadalcanal, in the intra-oceanic Solomon island arc (southwest Pacific Ocean), reveal Eocene- to Archean-aged zircon xenocrysts. Xenocryst populations at ca. 39–33 Ma and ca. 71–63 Ma correlate with previously obtained ages of supra-subduction magmatism within the arc. A ca. 96 Ma zircon population may be derived from Cretaceous ophiolite basement crust or region-wide continental rift-related magmatism. Xenocryst age populations alternate with periods of oceanic basin formation that fragmented the East Gondwana margin. Early Cretaceous to Archean zircon xenocryst ages imply continental origins and a cryptic source within the arc crust; they may have been introduced by Eocene interaction of a continental fragment with the arc, and concealed by ophiolite obduction. The data demonstrate that continentally derived zircons may be transported thousands of kilometers from their source and added to intra-oceanic arc magmas, a process likely facilitated by cyclical subduction zone advance and retreat. The findings highlight the continuum of arcs that occurs between continental and oceanic end members, and the caution with which zircons should be used to determine the provenance and setting of ancient arc terranes accreted to the continental crust

Hydrothermal alteration and fluid pH in alkaline-hosted epithermal systems

Epithermal gold mineralisation is found in a wide compositional range of host lithologies, but despite the diversity the alteration mineral assemblages are often similar between deposits. Notable exceptions are those gold deposits hosted in alkaline host rocks. Alkaline-hosted epithermal deposits are rare, but important, as they include some of the world’s largest known epithermal deposits by contained metal (e.g. Ladolam, Cripple Creek, Porgera). As well as the exceptional gold contents, the alkaline-hosted systems tend to exhibit different alteration mineral assemblages, with less quartz and widespread silicification than sub-alkaline-hosted equivalents, and greater enrichments in tellurium, and a scarcity of acid alteration (advanced argillic) types. In this study, geochemical modelling is used to demonstrate that 300 °C hydrothermal fluids in equilibrium with alkali, silica-undersaturated host rocks at low water/rock ratios reach significantly higher pH than equivalents in sub-alkaline lithologies. A maximum, near-neutral pH (5.5–6) is buffered by reactions involving quartz in silica-saturated alkaline and calc-alkaline lithologies. In silica-undersaturated, alkaline host rocks, quartz is exhausted by progressive water-rock interaction, and pH increases to 7 and above. Both tellurium and gold solubility are favoured by neutral to high fluid pH, and thus there is a clear mechanism within these hydrothermal systems that can lead to effective transport and concentration to produce gold telluride ore deposits in alkaline igneous hosts. This modelling demonstrates that alkaline rocks can still be altered to advanced argillic assemblages; the paucity of this alteration type in alkaline hosts instead points to NaCl ≫ HCl in magmatic volatile phases at the initiation of hydrothermal alteration

Two state model for critical points and the negative slope of the melting-curve

We present a thermodynamic model which explains the presence of a negative slope in the melt curve, as observed in systems as diverse as the alkali metals and molecular hydrogen at high pressure. We assume that components of the system can be in one of two well defined states - one associated with low energy, the other with low volume. The model exhibits a number of measurable features which are also observed in these systems and are therefore expected to be associated with all negative Clapeyron-slope systems: first order phase transitions, thermodynamic anomalies along Widom lines. The melt curve maximum is a feature of the model, but appears well below the pressures where the change in state occurs in the solid: the solid-solid transition is related to the melt line minimum. An example of the model fitted to the electride transition in potassium is discussed

Fluids and mineralisation in the Scottish Dalradian

Fluid inclusion studies of orogenic vein-type gold deposits show a strong genetic association with high-temperature, low-salinity, and volatile-rich fluids. Although fluid models for this type of mineralisation are well established, they commonly ignore several important facts that indicate a role for low temperature fluids. Visible gold is invariably fracture-controlled and is hence related to fluid processes represented by later (secondary) fluid inclusions. In the limited number of cases where these have been studied, a multiple fluid history involving low-temperature brines (<250ºC) is commonly observed. Gold is often associated with a suite of minerals characterised by sulphosalts, tellurides, sulpho-tellurides and rare gold alloys. Some of these are only stable below 275ºC, indicating an association with low-temperature processes. This study collates fluid inclusion and mineralogical data representing the major styles of mineralisation (e.g. Cononish and Calliachar Burn [Au–quartz]; Lagalochan [porphyry–Cu±Au]; Tomnadashan [igneous related Cu]; Tyndrum, Castleton and Inverneil [Pb–Zn]; Stronchullin and Corrie Buie [Pb–Zn±Au]) and regional fluids (e.g. Loch Lomond) in the Scottish Dalradian. Six fluid types are identified: 1. High-salinity (halite-bearing: NaCl>35 wt %) and high-temperature (>300ºC) fluid inclusions. These are typical of porphyry copper deposits world-wide and were recorded from samples at Lagalochan, Tomnadashan and Comrie. 2. High-temperature (250–400ºC), volatile-rich (major CO2+CH4+N2: 15–25 wt % NaCl eq) and moderate salinity (7–10 wt % NaCl eq) fluids inclusions. In the Dalradian, this fluid is ubiquitous. It occurs in veins and breccias, associated with igneous intrusions. It is also one of the major fluid-types in regional metamorphic quartz-veins and is recorded at nearly all mineralised localities. Elsewhere, this fluid-type is associated with orogenic gold mineralisation. However, in the Scottish Dalradian, its presence is not indicative of gold mineralisation. 3. Moderate to high-temperature (200–350ºC) and moderate salinity (7–10 wt % NaCl eq) fluids containing volatiles (minor CO2+CH4+N2: 10–15 wt %% NaCl eq). These have the same distribution and associations as Fluid 2. 4. Low to moderate-temperature (150–250ºC) low-salinity brines (<10 wt % NaCl eq) with little or no volatile component. This is analogous to fluids associated with epithermal gold mineralisation. It occurs in both igneous and metasedimentary rock-hosted mineralisation, and is present in a number of metamorphic quartz veins. Its presence as primary fluid inclusions in sphalerite, at Stronchullin, shows it plays a significant role in mineralisation. 5. Low-temperature (<150ºC) high-salinity (c. 20 wt % NaCl eq) brines. This fluid is typical of Mississippi Valley Type Pb–Zn deposits world-wide. It is present in most gold mineralised localities, but inclusions are low in abundance. Its role in Au metallogenesis is unclear, but a similar type of fluid is associated with gold-free base metal mineralisation (e.g. Tyndrum Pb–Zn). 6. A low temperature (monophase) aqueous fluid. This could be a low temperature equivalent of either Fluid 4 or Fluid 5. This fluid is sparsely distributed over a wide area. Types 1 and 2 and possibly 3 represent prograde fluids, which have a deep crustal (magmatic and metamorphic) origin, and are probably responsible for introduction of metals into the system. Then initiation of extensional tectonics permitted a major ingress of meteoric–basinal fluids (Types 4, 5 and 6). Fluid 4 and/or 5 remobilises earlier Fe–Cu–(Mo)–As–Au–S mineralisation and results in a base metal–gold overprint at many of the localities. Late-stage (Devonian-Carboniferous?) basin development is a possible source for the high-salinity low-temperature brines (Type 5 fluid). Fluid 6 could be responsible for the localised dickite–kaolinite mineralisation in the Highland Boundary Fault Zone and supergene alteration seen in mineralised localities (e.g. Calliachar Burn). Although the volatile-rich fluids play a major role in metallogenesis in the Scottish Dalradian, it is clear that low-temperature brines played a significant role in gold mineralisation. In terms of understanding the deposit-scale distribution of gold, it is of prime importance to know how these late-stage fluids interacted with pre-existing mineralised structures. Also, for exploration, there is a need to develop new technologies to predict where and how these fluids have acted, as they are probably responsible for the erratic distribution of gold-grades that characterise many orogenic gold deposits