粉芯丝材电弧喷涂的不稳定性分析

双丝电弧喷涂时,丝材顶端根据被加热状态可分为三个不同区域。其中,丝材顶端最外层被电弧直接加热（Ⅰ区）,这个区域的丝材完全熔化。由于传热效应使得相邻区域（丝材根部方向）的温度升高,从而产生软化区（Ⅱ区）。而与软化区相邻处（丝材根部方向）,传递的热量又软化了丝材并产生持续的变形区（Ⅲ区）,变形是由雾化气体所施加的动力产生的。采用高速摄像系统观察不同运行条件下丝材熔化、金属破碎并粒子形成的过程：Ⅰ区液态金属直接雾化成为很小的液滴,其尺寸是由熔化金属的特性和所施加的雾化气体压力所决定的,软化区是在阴阳极部位的金属薄片的源头。粉芯丝材的金属薄片要比实心丝材的尺寸小。受挤压作用形成的金属薄片形成二次引弧效应,因此增加了电弧喷涂过程的稳定性。本文分析了喷涂参数的影响、填充粉芯对熔化行为的影响、粒子形成以及喷涂过程的不稳定性,并将粉芯丝材和实心丝材的喷涂进行了对比。研究结果有利于提高双丝电弧喷涂模型的精度,并且可以通过优化喷管几何形状来增强金属液滴的雾化效果。     

Effect of evaporation and temperature-dependent material properties on weld pool development

This paper evaluates the effect of weld pool evaporation and thermophysical properties on the development of the weld pool. An existing computational model was modified to include vaporization and temperature-dependent thermophysical properties. Transient, convective heat transfer during gas tungsten arc (GTA) welding with and without vaporization effects and variable properties was studied. The present analysis differs from earlier studies that assumed no vaporization and constant values for all of the physical properties throughout the range of temperature of interest. The results indicate that consideration of weld pool vaporization effects and variable physical properties produce significantly different weld model predictions. The calculated results are consistent with previously published experimental findings.     

Increasing the output and the energy efficiency of electric arc furnaces with shaft heating of metal scrap

It is proposed to equip the electric arc furnaces with shaft heating of scrap that operate with continuous scrap melting in a liquid metal with powerful gas-oxygen burner units. When scrap is heated to 800°C at a gas flow rate of 15.5 m³/t and a bath is blown by oxygen at the slag-metal boundary, fuel-arc steelmaking units can successfully compete with the best modern electric arc furnaces in capacity and have ecological advantages.     

Hot model experiments of the metal bath spraying effect during the decarburization of Fe-C melts through oxygen top blowing

During recent years decarburization has been steadily gaining importance in converter steel metallurgy at the expense of refining reactions for other slag-forming companion elements. Because decarburization is currently a low-slag operation, the phase contact between gas and metal is critical. With the decrease in the amount of slag foam, more attention must be paid to the spraying of iron droplets during oxygen blowing. The experiments were carried out in a hot model reactor with a 50 kg capacity by oxygen top blowing upon Fe-C melts. The resulting spray of iron droplets was collected with the help of a special droplet sampler in the blowing converter. In the metal droplets a pronounced enhanced decarburization was found in comparison to the metal bath. The amount of metal spray was determined with respect to the oxygen blowing pressure, nozzle diameter, and distance between the lance and bath. Depending on the reactor contents, a high circulation rate of the droplets could be observed. At low blowing rates, FeO-slag is formed and sprayed along with the metal.     

Droplet formation,detachment, and impingement on the molten pool in gas metal arc welding

A mathematical model is developed to describe the globular transfer in gas metal arc welding (GMAW). This work is both theoretical and experimental. Using the volume-of-fluid (VOF) method, the fluid-flow and heat-transfer phenomena are dynamically studied during the following processes: droplet formation and detachment, impingement of a droplet on a solid substrate, impingement of multiple droplets on the molten pool, and solidification after the arc extinguishes. A He-Ne laser, in conjunction with the shadow graphing technique, is used to observe the metal transfer processes. Theoretical predictions and experimental results are in close agreement, suggesting that the theoretical treatment of the model is good.     

Control of Slag Formation, Foaming, Slopping, and Chaos in BOF

Decarburization, slag formation, foaming, and slopping in basic oxygen furnace (BOF) can now be partly understood, and to some extent predicted with the help of several on-line measurements and on-line control models. The principal reaction in BOF is decarburization. The bulk of the decarburization takes place in the turbulent region of jet impact irrespective of the fact whether the slag is solid, liquid or foamy. Metal droplets are ejected from the jet impact zone but it is difficult to distinguish the decarburization occurring in the bulk metal from that occurring in the droplets. Slag in BOF is heterogeneous and always contains some entrained gas bubbles and solid material (either un-dissolved or precipitated). At no stage the slag is 100 % liquid. A significant part of the metal droplets fall back and travel through the semi liquid slag. Through this mechanism the droplets can cause slag foaming and slopping in the BOF. Phenomenon of slag foaming and slopping can be monitored and controlled by following, dynamically, the audio signals, gas flow rate and composition, by tracking ‘Chaos’ in chemical reactions, by manipulating the chaotic attractors, and by monitoring the lance water temperature and weight. Relative stability potential (RSP) diagrams are found to be a good indicator of the dynamics of process inside the BOF and help in advance prediction of the impending chaos.     

A model of post-combustion in iron-bath reactors,part 4: results for post-combustion with preheated air

This paper describes calculated results of post-combustion with pre-heated air, heat transfer from gas to the melt via a mixture of metal and slag droplets, reoxidation at the metal droplets, CO evolution rate from the melt, production rate of the reactor, and carbon consumption of various process modes of a 15-t reactor with or without oxygen blowing through the vessel bottom. Using pre-heated air, the achievable post-combustion degree is higher than using pure oxygen. Nevertheless, slag droplets as heat carrier are necessary. With 50 % slag droplets the real post-combustion degree can be higher than 45 %. With 90 % slag droplets it can be even higher than 80 %. The operating point of the reactor is discussed.     

Kinetics of oxidation of carbon in liquid iron-carbon-silicon-manganese-sulfur alloys by carbon dioxide in nitrogen

The oxidation of carbon with the simultaneous oxidation of silicon, manganese, and iron of liquid alloys by carbon dioxide in nitrogen and the absorption of oxygen by the alloys from the gas were studied using 1-g liquid iron droplets levitated in a stream of the gas at 1575 °C to 1715 °C. Oxidation of carbon was favored over oxidation of silicon and manganese when cast iron (3.35 pct C, 2.0 pct Si, 0.36 pct Mn, and 0.05 pct S) reacted with CO₂/N₂ gas at 1635 °C. An increase in the flow rate of CO₂/N₂ gas increased the decarburization rate of cast iron. The rate of carbon oxidation by this gas mixture was found to be independent of temperature and alloying element concentrations (in the range of silicon = 0 to 2.0 pct manganese = 0 to 0.36 pct and sulfur = 0 to 0.5 pct) within the temperature range of the present study. Based on the results of a kinetic analysis, diffusion of CO₂ in the boundary layer of the gas phase was found to be the rate-limiting step for the reactions during the earlier period of the reaction when the contents of carbon, silicon, and manganese are higher. However, the limiting step changed to diffusion of the elements in the metal phase during the middle period of the reaction and then to the diffusion of CO in the gas phase during the later period of the reaction when the content of the elements in the metal were relatively low. For the simultaneous oxidation reactions of several elements in the metal, however, the diffusion of CO₂ in the gas phase is the primary limiting step of the reaction rate for the oxidation of carbon during the later period of reaction. Formerly Visiting Assistant Research Scientist, Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109     

A simplified model for the oxidation of disintegrated steel streams

A model is suggested for calculation of oxidation of disintegrated steel streans by mass transfer of oxygen to the droplets. The mass transfer of oxygen into the cylindrical space occupied by droplets and gas occurs by bulk flow and diffusion. Driving force for the bulk flow of air is the suction developed by the consumption of oxygen molecules and by acceleration. The differential equations describing the vertical gas velocity and the oxygen concentration of the gas phase are solved simultaneously. The increase of oxygen content of the metal phase is computed for a steel stream disintegrated into droplets of various sizes.     

The free-surface shape and temperature distribution produced in liquid metal droplets by heating coil pulses in the TEMPUS electromagnetic levitation facility

We present the results of analytical and numerical calculations of the free-surface shape and temperature distribution produced in liquid metal droplets processed in the TEMPUS electromagnetic levitation facility. The mathematical models were developed to predict the behavior of liquid metal droplets in containerless experiments used to measure thermophysical properties aboard the Space Shuttle Columbia during the IML-2 mission in July 1994. A normal stress balance model was used to numerically calculate the equilibrium free-surface shapes for various samples produced by a number of induction coil voltages. Analytical and numerical calculations were performed to model the heat transfer in the liquid metal droplets during and following the heating coil pulses. The work illustrates the use of mathematical modeling in the design of microgravity experiments and is applicable to industrial processes such as casting and skull melting.     

电弧炉炼钢中碳氧二次燃烧的影响因素和应用

电弧炉炼钢时，废气与熔池和渣子接触，CO气体的温度接近钢水温度，整个过程基本由反应动力学条件控制，凭借吹氧可以在熔池上方进行二次燃烧。1台60t电弧炉2000多炉的二次燃烧的应用结果表明，当吹入氧气操作得当，二次燃烧对水冷炉壁、炉顶、电极寿命、3角区耐火材料寿命、金属烧损率均无影响，氧耗增加7．93m^3/t，电耗降低38kWh/t，冶炼周期可稳定缩短4min。     

Studies on metal build upon atomisers during free fall gas atomisation of liquid metals

Abstract

In free fall gas atomisation of liquid metals, the atomised droplets mainly move downward, while some of them fly in an upward direction. Under certain conditions the upward moving droplets will deposit on the surface of the gas nozzle and metal delivery tube of an atomiser, which may cause a hindrance to the flow of liquid metal and gas. The effect of atomisation parameters, such as gas pressure, focal length, apex angle and diameter of gas nozzle, on the metal buildup on the atomiser have been studied during the free fall gas atomisation of lead, zinc and aluminium. The plenum pressure of gas at which deposition of atomised droplets on the surface of the gas nozzle and metal delivery tube takes place has been termed as limiting plenum pressure, and the corresponding gas velocity at the impingement point as limiting gas velocity. It has been shown that the limiting plenum pressure is different for different metals in the same atomiser. The limiting plenum pressure has been found to increase with free fall distance or specific gravity of liquid metals, and with a decrease in apex angle of atomiser. A correlation is proposed to determine the limiting gas velocity for free fall gas atomisation of metals.     

Calculation of weld metal composition change in high-power conduction mode carbon dioxide laser-welded stainless steels

The use of high-power density laser beam for welding of many important alloys often leads to appreciable changes in the composition and properties of the weld metal. The main difficulties in the estimation of laser-induced vaporization rates and the resulting composition changes are the determination of the vapor condensation rates and the incorporation of the effect of the welding plasma in suppressing vaporization rates. In this article, a model is presented to predict the weld metal composition change during laser welding. The velocity and temperature fields in the weld pool are simulated through numerical solution of the Navier-Stokes equation and the equation of conservation of energy. The computed temperature fields are coupled with ve-locity distribution functions of the vapor molecules and the equations of conservation of mass, momentum, and the translational kinetic energy in the gas phase for the calculation of the evap-oration and the condensation rates. Results of carefully controlled physical modeling experi-ments are utilized to include the effect of plasma on the metal vaporization rate. The predicted area of cross section and the rates of vaporization are then used to compute the resulting com-position change. The calculated vaporization rates and the weld metal composition change for the welding of high-manganese 201 stainless steels are found to be in fair agreement with the corresponding experimental results.     

Mathematical modeling of three-dimensional heat and fluid flow in a moving gas metal arc weld pool

Mathematical models capable of accurate prediction of the weld bead and weld pool geometry in gas metal arc (GMA) welding processes would be valuable for rapid development of welding procedures and empirical equations for control algorithms in automated welding applications. This article introduces a three-dimensional (3-D) model for heat and fluid flow in a moving GMA weld pool. The model takes the mass, momentum, and heat transfer of filler metal droplets into consideration and quantitatively analyzes their effects on the weld bead shape and weld pool geometry. The algorithm for calculating the weld reinforcement and weld pool surface deformation has been proved to be effective. Difficulties associated with the irregular shape of the weld bead and weld pool surface have been successfully overcome by adopting a boundary-fitted nonorthogonal coordinate system. It is found that the size and profile of the weld pool are strongly influenced by the volume of molten wire, impact of droplets, and heat content of droplets. Good agreement is demonstrated between predicted weld dimensions and experimently measured ones for bead-on-plate GMA welds on mild steel plate.     

Theory of Initial Growth of a Microcavity in a Liquid Metal Around a Gas-Releasing Particle. Part 1. Physical and Mathematical Models

Certain inhomogeneities in a liquid can act as bubble nucleation centers. If such a center is a source of gas on a considerable scale, the bubble can grow rapidly to an appreciable size. The following model is proposed for analyzing this process: a solid sphere (compound containing a gas-forming element such as hydrogen) is surrounded by a liquid metal. The initial equations are as follows: the Navier-Stokes equation, in which there are terms containing the concentration of the gas component; the equation of continuity; and the equation for the convective diffusion of the gas component in the liquid metal. The growth of the bubble obeys an integrodifferential equation derived here, which reflects the effects of the hydrodynamic, diffusion, and capillary factors.     

Isothermal solidification kinetics of diffusion brazing

Diffusion brazing (DB) is a process that produces interface-free joints that approach the bulk properties of the material that is to be joined. Solid-state diffusion of the melting point depressant (MPD) element into the joint metal causes the solid/liquid (S/L) interface to advance until the joint is solidified. The time required to complete this isothermal solidification stage was modeled using a moving boundary analysis. Precision measurements of the interlayer width as a function of time were made on the copper-silver system. The bonding apparatus consisted of a suspended load feedback system that prevented leakage of joint metal due to extrusion or surface wetting, thus preserving mass balance. The interlayer width vs bonding time measurements obtained were found to be within a factor of 4 of the theoretical model. The difference was ascribed to the difficulty in accurately measuring the actual liquid width and loss of silver due to vaporization. Large spherical protrusions grow during bonding, which further roughens the interface. However, experimental and predicted concentration profiles of silver were found to be in complete agreement. As the liquid remaining in the grain boundary grooves is diffused away, porosity can develop due to volume changes. By holding the joint at temperature for extended periods, complete solidification followed by homogenization will occur. Selecting the appropriate bonding temperature to achieve a specified maximum concentration of braze metal at the joint is dependent on both the isothermal solidification and homogenization kinetics.     

A model of post-combustion in iron-bath reactors,part 3: theoretical basis for post-combustion with pre-heated air

This paper describes the post-combustion of carbon monoxide with pre-heated air, heat transfer from gas to melt via a mixture of metal and slag droplets, the microkinetics of reoxidation of metal droplets in the gas consisting of CO₂, CO and N₂, and the total heat and mass balance in the gas space and in the iron-bath. The computer program presented here Is flexible and can simulate various process modes of iron-bath reactors. The production rate of the reactor and the carbon consumption can be calculated herewith.     

Effects of surface active elements on weld pool fluid flow and weld penetration in gas metal arc welding

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.     

Theory of Initial Growth of a Microcavity in a Liquid Metal Around a Gas-Releasing Particle. Part 1. Physical and Mathematical Models

Certain inhomogeneities in a liquid can act as bubble nucleation centers. If such a center is a source of gas on a considerable scale, the bubble can grow rapidly to an appreciable size. The following model is proposed for analyzing this process: a solid sphere (compound containing a gas-forming element such as hydrogen) is surrounded by a liquid metal. The initial equations are as follows: the Navier-Stokes equation, in which there are terms containing the concentration of the gas component; the equation of continuity; and the equation for the convective diffusion of the gas component in the liquid metal. The growth of the bubble obeys an integrodifferential equation derived here, which reflects the effects of the hydrodynamic, diffusion, and capillary factors.