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1.
TIG welding of high manganese stainless steels was conducted with different sulfur contents of 5 and 20 ppm. The penetration depth of the welding bead clearly increased even when the sulfur content of the sample was only very slightly increased from 5 to 20 ppm. In situ observation of the surface of the molten pool revealed that the increase in penetration depth of the welding bead could be attributed to an elevation of the average temperature at the center of the molten pool from 2070 to 2200 K due to the generation of an inward fluid flow in the pool. The results of precise measurements of the surface tension of molten high manganese stainless steels using the electromagnetic levitation (EML) technique, thoroughly explained that the inward flow in the molten pool of the sample containing a sulfur content of 20 ppm was induced by Marangoni convection driven by the boomerang shape temperature dependence of the surface tension of the molten sample. The experimental results of the variations in the temperature distribution and the fluid flow direction in the molten pool depending on the sulfur content were reproduced well by a numerical calculation considering the four dominant driving forces of plasma jet, buoyancy, electromagnetic forces, and Marangoni convection, which indicated that the fluid flow direction was dominantly controlled by Marangoni convection. 相似文献
2.
Nitrogen absorption by iron, Fe-20Cr-10Ni alloy, and SUS329J1 duplex stainless steel during CO 2 laser welding in an Ar-N 2 gas mixture was investigated and compared with equilibrium data predicted on Sieverts’ law and data on absorption during
arc and YAG laser welding. The nitrogen absorption during CO 2 laser welding is lower than that during arc welding, but higher than that during YAG laser welding. Compared with arc welding,
the lesser contact of monatomic nitrogen with the weld pool surface and the higher partial pressure of metal vapor in the
keyhole may result in the lower nitrogen absorption during CO 2 laser welding, while the very low density of monatomic nitrogen in the atmosphere during YAG laser welding due to the low-temperature
plume may lead to the lower nitrogen absorption during YAG laser welding than during CO 2 laser welding. 相似文献
3.
Nitrogen desorption by high-nitrogen steels (HNSs) containing 0.32 and 0.53 pct nitrogen during CO 2 laser welding in an Ar-N 2 gas mixture was investigated and the obtained data were compared with those for arc welding and at the equilibrium state
predicted by Sieverts’ Law. Although the nitrogen content in the weld metal during CO 2 laser welding was lower than that in the as-received base material in all conditions, the nitrogen desorption was larger
in the top part of the weld metal than in the keyhole region. The nitrogen desorption in the Ar atmosphere was less during
CO 2 laser welding than during arc welding. With the increase in nitrogen partial pressure, the nitrogen content in the weld metal
sharply increased during arc welding, but only slightly increased during CO 2 laser welding. The nitrogen absorption and desorption of the HNS weld metal were much smaller during CO 2 laser welding than during arc welding. 相似文献
4.
This article presents a mathematical model simulating the effects of surface tension (Maragoni effect) on weld pool fluid
flow and weld penetration in spot gas metal arc welding (GMAW). Filler droplets driven by gravity, electromagnetic force,
and plasma arc drag force, carrying mass, thermal energy, and momentum, periodically impinge onto the weld pool. Complicated
fluid flow in the weld pool is influenced by the droplet impinging momentum, electromagnetic force, and natural convection
due to temperature and concentration gradients, and by surface tension, which is a function of both temperature and concentration
of a surface active element (sulfur in the present study). Although the droplet impinging momentum creates a complex fluid
flow near the weld pool surface, the momentum is damped out by an “up-and-down” fluid motion. A numerical study has shown
that, depending upon the droplet’s sulfur content, which is different from that in the base metal, an inward or outward surface
flow of the weld pool may be created, leading to deep or shallow weld penetration. In other words, it is primarily the Marangoni
effect that contributes to weld penetration in spot GMAW. 相似文献
5.
It is experimentally shown that a thin layer of silica flux leads to an increased depth of weld penetration during activated TIG (=A-TIG) welding of Armco iron. The oxygen-content is found higher in the solidified weld metal and it is linked to the increased depth of penetration through the reversed Marangoni convection. It is theoretically shown for the first time that the basic reason of the reversed Marangoni convection is the phenomenon called ??surface phase transition?? (SPT), leading to the formation of a nano-thin FeO layer on the surface of liquid iron. It is shown that the ratio of dissolved oxygen in liquid iron to the O-content of the silica flux is determined by the wettability of silica particles by liquid iron. It is theoretically shown that when the silica flux surface density is higher than 15 µg/mm 2, reversed Marangoni flow will take place along more than 50 pct of the melted surface. Comparing the SPT line with the dissociation curves of a number of oxides, they can be positioned in the following order of their ability to serve as a flux for A-TIG welding of steel: anatase-TiO 2 (best)-rutile-TiO 2 (very good)-silica-SiO 2 (good)-alumina-Al 2O 3 (does not work). Anatase (and partly rutile) are self-regulating fluxes, as they provide at any temperature just as much dissolved oxygen as needed for the reversed Marangoni convection, and not more. On the other hand, oxygen can be over-dosed if silica, and other, less stable oxides (such as iron oxides) are used. 相似文献
6.
Five single oxide fluxes—Cu 2O, NiO, SiO 2, CaO, and Al 2O 3—were used to investigate the effect of active flux on the depth/width ratio in SUS304 stainless steel. The flux quantity,
stability, and particlesize effect on the weld-pool shape and oxygen content in the weld after welding was studied systematically.
The results showed that the weld depth/width ratio initially increased, followed by a decrease with the increasing flux quantity
of the Cu 2O, NiO, and SiO 2 fluxes. The depth/width ratio is not sensitive to the CaO flux when the quantity is over 80×10 −5 mol on the 5×0.1×50 mm slot. The Al 2O 3 flux has no effect on the penetration. The oxygen content dissolved in the weld plays an important role in altering the liquid-pool
surface-tension gradient and the weld penetration. The effective range of oxygen in the weld is between 70 and 300 ppm. A
too-high or too-low oxygen content in the weld pool does not increase the depth/width ratio. The decomposition of the flux
significantly depends on the flux stability and the particle size. Cu 2O has a narrow effective flux-quantity range for the deep penetration, while the Al 2O 3 flux has no effect. The SiO 2 flux with a small particle size (0.8 or 4 μm) is a highly recommended active flux for deep penetration in actual gas tungsten arc welding (GTAW) applications. 相似文献
7.
The shape and depth of the area molten during a welding process is of immense technical importance. This study investigates
how the melt pool shape during laser welding is influenced by Marangoni convection and tries to establish general qualitative
rules of melt pool dynamics. A parameter study shows how different welding powers lead to extremely different pool shapes.
Special attention is paid to transient effects that occur during the melting process as well as after switching off the laser
source. It is shown that the final pool shape can depend strongly on the welding duration. The authors use an axisymmetric
two-dimensional (2-D) control-volume-method (CVM) code based on the volume-averaged two-phase model of alloy solidification
by Ni and Beckermann [1] and the SIMPLER algorithm by Patankar. [2] They calculate the transient distribution of temperatures, phase fractions, flow velocities, pressures, and concentrations
of alloying elements in the melt and two solid phases (peritectic solidification) for a stationary laser welding process.
Marangoni flow is described using a semiempirical model for the temperature-dependent surface tension gradient. The software
was parallelized using the shared memory standard OpenMP. 相似文献
9.
From an analytical and theoretical study of flat and out-of-position gas metal arc (GMA) C-Mn steel welds containing varying
additions of silicon and manganese, we conclude that the buoyancy effect (flotation obeying Stokes’ law) does not play a significant
role in the separation of oxide inclusions during weld metal deoxidation. Consequently, the separation rate of the particles
is controlled solely by the fluid flow pattern in the weld pool. A proposed two-step model for the weld metal deoxidation
reactions suggests that inclusions formed in the hot, turbulent-flow region of the weld pool are rapidly brought to the upper
surface behind the arc because of the high-velocity flow fields set up within the liquid metal. In contrast, those formed
in the cooler, less-turbulent flow regions of the weld pool are to a large extent trapped in the weld metal as finely dispersed
particles as a result of inadequate melt stirring. The boundary between “hot” and “cold” parts for possible inclusion removal
is not well defined, but depends on the applied welding parameters, flux, and shielding gas composition. As a result of the
intricate mechanism of inclusion separation, the final weld metal oxygen content depends on complex interactions among the
following three main factors: (1) the operational conditions applied, (2) the total amount of silicon and manganese present,
and (3) the resulting manganeseto-silicon ratio. The combined effect of the latter two contributions has been included in
a new deoxidation parameter, ([pct Si][pct Mn]) −0.25. The small, negative exponent in the deoxidation parameter indicates that control of the weld metal oxygen concentrations
through additions of silicon and manganese is limited and that choice of operational conditions in many instances is the primary
factor in determining the final degree of deoxidation to be achieved. 相似文献
11.
Understanding the viscous behavior of copper smelting slags is essential in increasing the process efficiency and obtaining
the discrete separation between the matte and the slag. The viscosity of the FeO t-SiO 2-Al 2O 3 copper smelting slags was measured in the current study using the rotating spindle method. The viscosity at a fixed Al 2O 3 concentration decreased with increasing Fe/SiO 2 ratio because of the depolymerization of the molten slag by the network-modifying free oxygen ions (O 2−) supplied by FeO. The Fourier transform infrared (FTIR) analyses of the slag samples with increasing Fe/SiO 2 ratio revealed that the amount of large silicate sheets decreased, whereas the amount of simpler silicate structures increased.
Al 2O 3 additions to the ternary FeO t-SiO 2-Al 2O 3 slag system at a fixed Fe/SiO 2 ratio showed a characteristic V-shaped pattern, where initial additions decreased the viscosity, reached a minimum, and increased
subsequently with higher Al 2O 3 content. The effect of Al 2O 3 was considered to be related to the amphoteric behavior of Al 2O 3, where Al 2O 3 initially behaves as a basic oxide and changes to an acidic oxide with variation in slag composition. Furthermore, Al 2O 3 additions also resulted in the high temperature phase change between fayalite/hercynite and the modification of the liquidus
temperature with Al 2O 3 additions affecting the viscosity of the copper smelting slag. 相似文献
12.
Continuous CO 2 laser welding of an Fe-Cu dissimilar couple in a butt-weld geometry at different process conditions is studied. The process
conditions are varied to identify and characterize the microstructural features that are independent of the welding mode.
The study presents a characterization of the microstructure and mechanical properties of the welds. Detailed microstructural
analysis of the weld/base-metal interface shows features that are different on the two sides of the weld. The iron side can
grow into the weld with a local change in length scale, whereas the interface on the copper side indicates a barrier to growth.
The interface is jagged, and a banded microstructure consisting of iron-rich layers could be observed next to the weld/Cu
interface. The observations suggest that solidification initiates inside the melt, where iron and copper are mixed due to
convective flow. The transmission electron microscopy (TEM) of the weld region also indicates the occasional presence of droplets
of iron and copper. The microstructural observations are rationalized using arguments drawn from a thermodynamic analysis
of the Fe-Cu system. 相似文献
13.
Recent computer calculations of heat transfer and fluid flow in welding were intended to provide useful insight about weldment
geometry for certain specific welding conditions and alloys joined. However, no generally applicable correlation for the joining
of all materials under various welding conditions was sought in previous work. To address this difficulty, computer models
of fluid flow and heat transfer were used for the prediction of weld pool geometry in materials with diverse properties, such
as gallium, pure aluminum, aluminum alloy 5182, pure iron, steel, titanium, and sodium nitrate under various welding conditions.
From the results, a generally applicable relationship was developed between Peclet (Pe) and Marangoni (Ma) numbers. For a
given material, Ma and Pe increased with the increase in laser power and decrease in beam radius. For materials with high
Prandtl number (Pr), such as sodium nitrate, the Pe and Ma were high, and heat was transported primarily by convection within
the weld pool. The resulting welds were shallow and wide. For low Pr number materials, like aluminum, the Pe and Ma were low
in most cases, and low Pe made the weld pool deep and narrow. The cross-sectional areas of stationary and low speed welds
could be correlated with welding conditions and material properties using dimensionless numbers proposed in this article. 相似文献
14.
By combining a mathematical model of the welding arc and of the weld pool, calculations are presented to describe the free
surface temperature of weld pools for spot welding operations. The novel aspects of the treatment include the calculation
of the heat and current fluxes falling on the free weld pool surface from first principles, a realistic allowance for heat
losses due to vaporization, and a realistic allowance for the temperature dependence of the surface tension. The most important
finding reported in this article is that the free surface temperature of weld pools appears to be limited by Marangoni convection,
rather than heat losses due to vaporiza-tion. Furthermore, it was found that once thermocapillary flow can produce high enough
surface velocities (>25 cm/s), the precise nature of the relationship between temperature and surface tension will become
less important. 相似文献
15.
Measurements of the rate of interfacial reaction between CO 2-CO mixtures and CaO-SiO 2-FeOx slags have been made using the 13CO 2-CO isotope exchange technique. Ranges of slag compositions from 0 to 100 wt pct ‘FeO’ and CaO/SiO 2 between 0.3 and 2.0 were examined in the experiments. For each slag, the dependence of the apparent rate constant on temperature
and equilibrium oxygen potential was studied. The relationship between the rate constant and oxygen potential was found to
be in the form k
a= k
a
o
(a o) -α. The parameter a, with values between 0.5 and 0.9, was dependent on the slag composition. The activation energy of the reaction was independent
of iron oxide content and dependent on slag basicity. 相似文献
16.
The oxidation mechanism of liquid Fe-V alloys with V content from 5 to 20 mass pct under different oxygen partial pressures
using CO 2-O 2 mixtures with CO 2 varying from 80 pct to 100 pct was investigated by thermogravimetric analysis between 1823 K and 1923 K (1550 °C and 1650 °C).
The products after oxidation were identified by scanning electron microscopy energy-dispersive spectrograph and X-ray diffraction.
The results indicate that the oxidation process can be divided into the following steps: an apparent incubation period, followed
by a chemical reaction step with a transition step before the reaction, and diffusion as the last stage. At the initial stage,
a period of slow mass increase was observed that could be attributed to possible oxygen dissolution in the liquid iron-vanadium
coupled with the vaporization of V 2O. The length of this period increased with increasing temperature as well as vanadium content in the melt and decreased with
increasing oxygen partial pressure of the oxidant gas. This analysis was followed by a region of chemical oxidation. The oxidation
rate increased with the increase of the O 2 ratio in the CO 2-O 2 gas mixtures. During the final stage, the oxidation seemed to proceed with the diffusion of oxygen through the product layer
to the reaction front. The Arrhenius activation energies for chemical reaction and diffusion were calculated, and kinetic
equations for various steps were setup to describe the experimental results. The transition from one reaction mechanism to
the next was described mathematically as mixed-control equations. Thus, uniform kinetic equations have been setup that could
simulate the experimental results with good precision. 相似文献
17.
Kinetics of the reaction of lead slags (PbO-CaO-SiO 2-FeO x -MgO) with CO-CO 2 gas mixtures was studied by monitoring the changes in the slag composition when a stream of CO-CO 2 gas mixture was blown on the surface of thin layers of slags (3 to 10 mm) at temperatures in the range of 1453 K to 1593 K (1180 °C to 1320 °C). These measurements were carried out under conditions where mass transfer in the gas phase was not the rate-limiting step and the reduction rates were insensitive to factors affecting mass transfer in the slag phase. The results show simultaneous reduction of PbO and Fe 2O 3 in the slag. The measured specific rate of oxygen removal from the melts varied from about 1 × 10 ?6 to 4 × 10 ?5 mol O cm ?2 s ?1 and was strongly dependent on the slag chemistry and its oxidation state, partial pressure of CO in the reaction gas mixture, and temperature. The deduced apparent first-order rate constant increased with increasing iron oxide content, oxidation state of the slag, and temperature. The results indicate that under the employed experimental conditions, the rate of formation of CO 2 at the gas-slag interface is likely to be the rate-limiting step. 相似文献
18.
Mathematical models of the gas metal arc (GMA) welding process may be used to study the influence of various welding parameters
on weld dimensions, to assist in the development of welding procedures, and to aid in the generation of process control algorithms
for automated applications. In this work, a three-dimensional (3-D), steady-state thermal model of the GMA welding process
has been formulated for a moving coordinate framework and solved using the finite-element method. The model includes temperature-dependent
material properties, a new finite-element formulation for the inclusion of latent heat of fusion, a Gaussian distribution
of heat flux from the arc, plus the effects of mass convection into the weld pool from the melted filler wire. The influence
of weld pool convection on the pool shape was approximated using anisotropically enhanced thermal conductivity for the liquid
phase. Weld bead width and reinforcement height were predicted using a unique iterative technique developed for this purpose.
In this paper, the numerical model is shown to be capable of predicting GMA weld dimensions for individual welds, including
those with finger penetration. Also, good agreement is demonstrated between predicted weld dimensions and experimentally derived
relations that describe the effects of process variables and their influence on average weld dimensions for bead-onplate GMA
welds on steel plate.
E. PARDO, formerly Postdoctoral Fellow, Department of Mechanical Engineering, University of Waterloo, Waterloo, ON, Canada
N2L 3G1, 相似文献
19.
In this study, AA5083 samples were butt welded under a conduction regime with high-power diode laser (HPDL). Various mixtures composed of Ar and CO 2 were used as a shielding gas. The influence of the shielding gas composition on the microstructure and on the properties of laser welds was analyzed. The weld beads were deeply characterized by metallographic/microstructural studies, X-ray diffraction (XRD), X-ray energy dispersive spectrometry (X-EDS) chemical analyses, X-ray photoelectron spectra (XPS), microhardness, and tensile strength. The corrosion resistance of laser-remelted surfaces with different CO 2/Ar ratios was also estimated by means of electrochemical tests. The addition of CO 2 to the shielding gas results in a better weld penetration and oxidizes the weld pool surface. This addition also promotes the migration of Mg toward the surface of weld beads and induces the formation of magnesium aluminates spinel on the welds. The best corrosion resistance result is achieved with 20 pct CO 2. The overall results indicate that the addition of small percentage of CO 2 to Ar leads to improvements of the mechanical and corrosion properties of the aluminum welds. 相似文献
20.
A three-dimensional transient model to solve heat transfer, fluid flow, and species conservation during laser welding of dissimilar
metals is presented. The model is based on a control volume formulation with an enthalpy-porosity technique to handle phase
change and a mixture model to simulate mixing of molten metals. Weld pool development, solidified weld pool shape, and composition
profiles are presented for both stationary as well as continuous laser welding in conduction mode. Salient features of a dissimilar
Cu-Ni weld are summarized and thermal transport arguments are employed to successfully explain the observations. It is found
that the weld pool shape becomes asymmetric even when the heat source is symmetrically applied on the two metals forming the
couple. It is also observed that convection plays an important role in the development of weld pool shape and composition
profiles. As the weld pool develops, the side melting first (nickel) is found to experience more convection and better mixing.
Results from the case studies of computation are compared with corresponding experimental observations, showing good qualitative
agreement between the two. 相似文献
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