The methods used to model thermal plasmas, including treatments of diffusion in arcs in gas mixtures, are reviewed. The influence of thermophysical properties on the parameters of tungsten–inert-gas (TIG) welding arcs, particularly those that affect the weld pool, is investigated using a two-dimensional model in which the arc, anode and cathode are included self-consistently. The effect of changing each of six thermophysical properties on the characteristics of an argon TIG arc is assessed. The influence of the product of specific heat and mass density is found to be particularly important in determining the arc constriction. By examining the influence of the different properties on the heat flux density, current density and shear stress at the anode, it is concluded that the weld pool depth can be increased by using shielding gases with high specific heat, thermal conductivity and viscosity. The effect of metal vapour on the arc and weld pool properties is assessed. The most important effect of the metal vapour is found to be the increased electrical conductivity at low temperatures, which leads to lower heat flux density and current density at the weld pool, implying a shallower weld pool.

https://iopscience.iop.org/article/10.1088/0022-3727/42/19/194006/pdf

Modelling of thermal plasmas for arc welding: the role of the shielding gas properties and of metal vapour

The durability of metals and ceramics in molten aluminum is a great concern in engineering applications such as die casting, containment of liquid metals and semi-solid processing This paper summarizes related work along with the experimental results from our laboratory Most of the important engineering materials are included Ceramics such as graphite, aluminosilicate refractories, AlN, Al2O3, Si3N4, sialons are characterized as inert in molten aluminum and its alloys The corrosion resistance of metals is generally poorer than that of inert ceramics, although the durability of titanium and niobium is relatively good Factors affecting the material durability in molten aluminum, including the interfacial layers, dynamic agitation, surface coatings and grain size are also discussed

Review Durability of materials in molten aluminum alloys

AlN powders (particle size = 0.44 ± 0.08 μm) containing no deliberate sintering additives were consolidated to near theoretical density in 5 min at 2003 K (1730 °C) using a Plasma Activated Sintering (PAS) process. PAS is a novel consolidation method that combines a very short time at high temperature with pressure application in a plasma environment. The in situ cleaning ability of powder particle during plasma activated densification leads to enhanced particle sinterability. The densities of undoped AlN specimens that were PAS consolidated at 2003 K for 5 min under 50 MPa pressure ranged from 97.5 to 99.3% of theoretical. The initial submicron particle size of AlN powders was retained in the final microstructure that consisted of polycrystalline grains with an average size of ≍0.77 ± 0.1 μm.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0884291400018409

Plasma activated sintering of additive-free AlN powders to near-theoretical density in 5 minutes

Effects of gas flow rate on the length of atmospheric pressure plasma jets have been investigated using a capillary dielectric barrier discharge configuration. For the discharge in only downstream region, three distinguishable modes of plasma jet length versus argon gas flow rate, namely, laminar, transition, and turbulent jet mode, have been identified. For the case of discharge in both downstream and upstream regions, the curve of length versus flow rate has significant “dent” in the laminar jet mode for pure helium, neon, and argon flow gas spraying into air ambient.

Effects of gas flow rate on the length of atmospheric pressure nonequilibrium plasma jets

High-purity aluminium nitride (AlN) powders of submicron size were consolidated to almost theoretical density in minutes at 1800°C using a plasma-activated sintering (PAS) process. The structure and microchemistry of grain boundaries in this polycrystalline material (average grain size, about 0.43 μm), sintered without additives commonly used in the densification of ceramics, was examined by high-resolution electron microscopy and high-spatial-resolution electron-energy-loss spectroscopy (EELS). The grain boundaries were ‘clean’ down to atomic-scale resolution and revealed AIN grain-to-grain contacts throughout the material; EELS data show no evidence for significant oxygen at grain boundaries or at grain triple junctions. A low density of oxygen-containing inversion domain boundaries were found in the grains, and they are thought to be formed to accommodate small amounts of oxygen (about 0.82 at.%) during the PAS consolidation process. The observed clean grain boundaries suggest that the plasma ...

Clean grain boundaries in aluminium nitride ceramics densified without additives by a plasma-activated sintering process

Long, laminar plasma jets at atmospheric pressure of pure argon and a mixture of argon and nitrogen with jet length up to 45 fi,Hes its diameter could be generated with a DC are torch by! restricting the movement of arc root in the torch channel. Effects of torch structure, gas feeding, and characteristics of power supply on the length of plasma jets were experimentally examined. Plasma jets of considerable length and excellent stability could be obtained by regulating the generating parameters, including are channel geometry gas flow I ate, and feeding methods, etc. Influence of flow turbulence at the torch,nozzle exit on the temperature distribution of plasma jets was numerically simulated. The analysis indicated that laminar flow plasma with very low initial turbulent kinetic energy will produce a long jet, with low axial temperature gradient. This kind of long laminar plasma jet could greatly improve the controllability for materials processing, compared with a short turbulent are let.

Generation of long, laminar plasma jets at atmospheric pressure and effects of flow turbulence

Argon DC plasma jets in stable laminar flow were generated at atmospheric pressure with a specially designed torch under carefully balanced generating conditions. Compared with turbulent jets of short length with expanded radial appearance and high working noise, the laminar jet could be 550 mm in length with almost unchanged diameter along the whole length and very low noise. At gas feeding rate of 120 cm3/s, the jet length increases with increasing arc current in the range of 70–200 A, and thermal efficiency decreases slightly at first and then leveled off. With increasing gas flow rate, thermal efficiency of the laminar jets increases and could reach about 40%, when the arc current is kept at 200 A. Gauge pressure distributions of the jets impinging on a flat plate were measured. The maximum gauge pressure value of a laminar jet at low gas feeding rate is much lower than that of a turbulent jet. The low pressure acting on the material surface is favorable for surface cladding of metals, whereas the high pressure associated with turbulent jets will break down the melt pool.

Characteristics of Argon Laminar DC Plasma Jet at Atmospheric Pressure

Nontransferred DC laminar plasma jets of stable flow and low impinging pressure acting on the substrate were used to heat W–Mo–Cu cast iron for phase transfer hardening of the surface layer. Substrates were heated in multipass with or without overlapping or heated with only single-pass. Surface morphologies of the molten trace and microstructure of the cross-section were observed, and the hardness distribution of the treated surface layer was examined. The surface layer of single-pass-heated specimen has an average hardness of about 900 HV0.1, while the specimen treated with multipass shows an average hardness of about 700 HV0.1, because of the heat effect from the neighboring pass treating, compared with the substrate hardness of about 300 HV0.1. The results demonstrate the stable and favorably controlled heating of the laminar plasma jet on the substrate surface and feasibility of using it as a tool for surface hardening of cast iron.

/pdf/feasibility-of-laminar-plasma-jet-hardening-of-cast-iron-2ef5xi38zi.pdf

Feasibility of laminar plasma-jet hardening of cast iron surface

Modeling results are presented to compare the characteristics of laminar and tur- bulent argon thermal plasma jets issuing into ambient air. The combined-diffusion-coefficient method and the turbulence-enhanced combined-diffusion-coefficient method are employed to treat the diffusion of ambient air into the laminar and turbulent argon plasma jects, respec- tively. It is shown that since only the molecular diffusion mechanism is involved in the laminar plasma jet, the mass flow rate of ambient air entrained into the laminar plasma jet is compar- atively small and less dependent on the jet inlet velocity. On the other hand, since turbulent transport mechanism is dominant in the turbulent plasma jet, the entrainment rate of ambient air into the turbulent plasma jet is about one order of magnitude larger and almost directly proportional to the jet inlet velocity. As a result, the characteristics of laminar plasma jets are quite different from those of turbulent plasma jets. The length of the high-temperature region of the laminar plasma jet is much longer and increases notably with increasing jet inlet velocity or inlet temperature, while the length of the high-temperature region of the turbu- lent plasma jet is short and less influenced by the jet inlet velocity or inlet temperature. The predicted results are reasonably consistent with available experimental observation by using a DC arc plasma torch at arc currents 80-250 A and argon flow rates (1.8-7.0)×10 −4 kg/s.

/pdf/comparison-of-laminar-and-turbulent-thermal-plasma-jet-36jur52puq.pdf

Comparison of Laminar and Turbulent Thermal Plasma Jet Characteristics — A Modeling Study

The generation, jet length and flow-regime change characteristics of argon plasma issuing into ambient air have been experimentally examined. Different torch structures have been used in the tests. Laminar plasma jets can be generated within a rather wide range of working-gas flow rates, and an unsteady transitional flow state exists between the laminar and turbulent flow regimes. The high-temperature region length of the laminar plasma jet can be over an order longer than that of the turbulent plasma jet and increases with increasing argon flow rate or arc current, while the jet length of the turbulent plasma is less influenced by the generating parameters. The flow field of the plasma jet has very high radial gradients of plasma parameters, and a Reynolds number alone calculated in the ordinary manner may not adequately serve as a criterion for transition. The laminar plasma jet can have a higher velocity than that of an unsteady or turbulent jet. The long laminar plasma jet has good stiffness to withstand the impact of laterally injected cold gas and particulate matter. It could be used as a rather ideal object for fundamental studies and be applied to novel materials processing due to its attractive stable and adjustable properties.

/pdf/experimental-study-on-the-thermal-argon-plasma-generation-47a2m7zyrn.pdf

Wenxia Pan

Papers

Generation of long, laminar plasma jets at atmospheric pressure and effects of flow turbulence

Characteristics of Argon Laminar DC Plasma Jet at Atmospheric Pressure

Feasibility of laminar plasma-jet hardening of cast iron surface

Comparison of Laminar and Turbulent Thermal Plasma Jet Characteristics — A Modeling Study

Experimental Study on the Thermal Argon Plasma Generation and Jet Length Change Characteristics at Atmospheric Pressure