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.     

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.     

Effect of submerged arc welding parameters on microstructure of SA516 steel weld metal

Abstract

The influence of submerged arc welding (SAW) process parameters on the microstructure of SA516 grade 70 steel weld metal (WM) was investigated in the present work. Steel plates of 17 mm thickness were submerged arc welded using welding currents of 700–850 A and welding speeds of 5·3–15·3 mm s^?1. The morphologies and volume fractions of the various ferrites in the WM were studied using optical microscopy and the morphologies and chemical compositions of the WM inclusions were examined using scanning electron microscopy and energy dispersive X-ray spectrometry. The results showed that the WM grain structure coarsened but the grain width of prior austenite grains decreased with increasing heat input. Also, the proportion of acicular ferrite (AF) in the WM increased initially, while the volume fractions of grain boundary ferrite and Widmanstätten ferrite decreased with increasing welding current. However, with further increasing the welding current above 800 A, less AF was produced. The weld nugget area decreased with increasing welding speed at all currents, but did not affect the amount of AF produced.

Au cours de ce travail, on a examiné l’influence des paramètres du procédé de soudage à l’arc sous flux en poudre (SAW) sur la microstructure du métal soudé (WM) de l’acier SA516, Gr. 70. On a soudé à l’arc sous flux en poudre des tôles d’acier de 17 mm d’épaisseur en utilisant des courants de soudage de 700 à 850 A et des vitesses de soudage de 5·3 à 15·3 mm s^?1. On a étudié la morphologie ainsi que la fraction volumique des diverses ferrites du WM en utilisant la microscopie optique et l’on a examiné la morphologie ainsi que la composition chimique des inclusions du WM en utilisant la microscopie électronique à balayage (SEM) et la spectroscopie à dispersion d’énergie (EDS). Les résultats ont montré que la structure de grain du WM grossissait mais que la largeur de grain des grains antérieurs d’austénite diminuait avec une augmentation de l’apport de chaleur. Également, la proportion de ferrite aciculaire (AF) dans le WM augmentait initialement, alors que les fractions volumiques de ferrite des joints de grain (GBF) et de ferrite de Widmanstatten (WF) diminuaient avec l’augmentation du courant de soudage. Cependant, avec une augmentation supplémentaire du courant de soudage au-dessus de 800 A, moins de ferrite aciculaire était produite. La région du noyau de soudure diminuait avec une augmentation de la vitesse de soudage à toutes les valeurs de courant, mais la quantité d’AF produite n’était pas affectée.     

高性能WNQ690钢气体保护焊试验研究

以WNQ690钢为研究对象,采用气保护焊接工艺,焊接线能量控制在15kJ／cm,WER70NH匹配WNQ690钢进行焊接,研究WNQ690钢焊接性能及焊接组织结构。接头抗拉强度为750MPa,冷弯性能优良,焊缝-40℃平均冲击功为137J,熔合线及热影响区冲击功均达到较高水平,焊接接头的HV10硬度在234-300之间,说明WNQ690钢淬硬倾向较小。WNQ690钢材的组织为回火贝氏体,焊接接头过热区组织为贝氏体,焊缝中含有较多的针状铁素体和少量的先共析铁素体,焊接热影响区的细小贝氏体组织使焊接热影响区具有较高的冲击韧性。     

Local properties of undermatched steel weld metal

Samples from two undermatched, multipass welds on 50.8-mm-thick HY-100 steel were tested using a novel microtensile test machine and the local material properties were investigated using a chemical analysis, metallography, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The microtensile test technique allowed samples from individual weld beads and weldmetal heat-affected zones to be tested in three orthogonal directions. Relationships between local microhardness and tensile properties were established. The filler metals for the two welds were MIL-70S and MIL-100S. The MIL-70S weld formed ferritic microstructures; the weld-metal heat-affected sites were predominantly polygonal ferrite, while the as-deposited regions were a mixture of lath and polygonal ferrite. This weld showed a large variation in properties from the central weld bead to the outer ones. The outermost site exhibited significant anisotropy in strength that was not revealed by microhardness measurements. The yield strength specification was 483 MPa, while the average at the center of the weld was 675 MPa and the outer sites had an average of 445 MPa. Elongation for the samples from the center was significantly lower as well, 5 pct as compared to 18 pct for the outer sites. The yield strength showed a strong correlation with the size of inclusions measured by TEM. Microprobe analysis found no dilution of the base metal alloying additions into the weld metal. The MIL-100S filler formed predominantly fine acicular ferrite throughout the weld. The strength was much more uniform; the yield strength specification was 690 MPa, while the center of the weld was 756 MPa and the outer sites had an average of 616 MPa. The inclusion size did not play an important role in the variation in mechanical properties.     

Numerical analysis of metal transfer in gas metal arc welding

The present article describes a numerical procedure to simulate metal transfer and the model will be used to analyze the transport processes involved in gas metal arc welding (GMAW). Advanced Computational fluid dynamics (CFD) techniques used in this model include a two-step projection method for solving the incompressible fluid flow; a volume of fluid (VOF) method for capturing free surface; and a continuum surface force (CSF) model for calculating surface tension. The electromagnetic force due to the welding current is estimated by assuming several different types of current density distribution on the free surface of the drop. The simulations based on the assumption of Gaussian current density distribution show that the transition from globular to spray transfer mode occurs over a narrow current range and the size of detached drops is nonuniform in this transition zone. The analysis of the calculation results gives a better understanding of this physical procedure. Comparisons between calculated results and experimental results are presented. It is found that the results computed from the Gaussian assumption agree well with those observed in experiments.     

Mathematical modeling of the dynamic behavior of gas tungsten arc weld pools

The dynamic behavior of stationary fully penetrated gas tungsten arc weld pools was investigated through numerical simulation. The effects of arc pressure, electromagnetic force, and surface tension gradients on surface depression, convection, and temperature distribution were calculated. The top surfaces of fully penetrated pools were easily depressed since they were only supported by surface tension. Circulatory convection patterns were generated by electromagnetic forces and surface tension gradients and were significantly affected by the vertical velocity component produced by pool oscillation. The temperature distribution in and around the pool was influenced by pool convection. During pool formation and growth, the fully penetrated molten pool sagged dramatically when the bottom pool diameter approached the top diameter. The sagged pool oscillated with higher magnitude and lower frequency than partially penetrated or fully penetrated pools before sagging occurred. The dynamic behavior and the amount of material lost during melt-through were affected by the pool size and the magnitude of arc pressure.     

Elemental partitioning and microstructural development in duplex stainless steel weld metal

A thermodynamic analysis which is capable of estimating the austenite/ferrite equilibria in duplex stainless steels has been carried out using the sublattice thermodynamic model. The partitioning of alloying elements between the austenite and ferrite phases has been calculated as a function of temperature. The results showed that chromium partitioning was not influenced significantly by the temperature. The molybdenum, on the other hand, was found to partition preferentially into ferrite phase as the temperature decreases. A strong partitioning of nickel into the austenite was observed to decrease gradually with increasing temperature. Among the alloying elements, average nitrogen concentration was found to have the most profound effect on the phase balance and the partitioning of nitrogen into the austenite. The partitioning coefficient of nitrogen (the ratio of the mole fraction of nitrogen in the austenite to that in the ferrite) was found to be as high as 7.0 around 1300 K. Consequently, the volume fraction of austenite was influenced by relatively small additions of nitrogen. The results are compared with the experimentally observed data in a duplex stainless steel weld metal in conjunction with the solid state δ → δ + γ phase transformation. Particular attention was given to the morphological instability of grain boundary austenite allotriomorphs. A compariso between the experimental results and calculations indicated that the instability associated with irregular austenite perturbations results from the high degree of undercooling. The results suggest that the model can be used successfully to understand the development of the microstructure in duplex stainless steel weld metals.     

Reducing metal losses when smelting steel in high-power arc furnaces

The heat transfer in arc furnaces with high input power is considered, in the case of induced slag foaming. Most of the power is consumed in evaporation of the metal and slag at hot spots under the electrodes and in zones of oxygen injection.     

Heat and metal transfer in gas metal arc welding using argon and helium

This article describes a theoretical investigation on the arc parameters and metal transfer in gas metal arc welding (GMAW) of mild steel using argon and helium shielding gases. Major differences in the predicted arc parameters were determined to be due to large differences in thermophysical properties. Various findings from the study include that an arc cannot be struck in a pure helium atmosphere without the assistance of metal vapor, that a strong electromagnetic cathode force affects the fluid flow and heat transfer in the helium arc, providing a possible explanation for the experimentally observed globular transfer mode and that the tapering of the electrode in an argon arc is caused by electron condensation on the side of the electrode. Formerly Graduate Student, Massachusetts Institute of Technology     

Low temperature aging behavior of type 308 stainless steel weld metal

The aging behavior of welded type 308 stainless steel was evaluated by mechanical property testing and microstructural examination. Aging was carried out at 475°C for up to 20,000 h. The initial material consisted of austenite with approximately 10% ferrite. Upon aging, the ferrite hardness increased up to 100%. This hardening was accompanied by a noticeable increase in the ductile—brittle transition temperature and a drop in the upper shelf energy, as measured by Charpy impact tests, and a degradation in fracture toughness, as determined by J-integral test. Tensile properties did not change significantly with aging. Microstructural analysis indicated that the ferrite decomposed spinodally into iron-rich α and chromium-enriched α′. In addition, abundant precipitation of nickel- and silicon-rich G-phase was found within the ferrite and M₂₃C₆ carbide formed along the austenite-ferrite interface. These effects are similar to the aging behavior of cast stainless steels. Occasionally, large G-phase or α precipitates were also found along the austenite-ferrite interface after aging more than 1000 h. After comparison of the mechanical property changes with the microstructural features, it was concluded that both spinodal decomposition as well as G-phase formation contribute to ferrite hardening. Spinodal decomposition results in embrittlement of the weld insofar as the ductile-brittle transition temperature is raised. G-phase formation and carbide precipitation are associated with a degradation in the ductile fracture properties, as shown by a drop in the upper shelf energy and a decrease in the fracture toughness.     

国内热轧焊接气瓶钢的质量现状

介绍了国内焊瓶制造业和钢材生产厂家对热轧焊瓶钢板的质量需求与生产供应状况,分析了国内焊瓶钢的采标、质量控制和质量改进的进展.     

Analysis of inclusions and precipitates in tough weld metal of X80 steel

Submerged arc welding(SAW)and gas metal arc welding(GMAW)experiments of Nb-bearing X80 steel were conducted with high-toughness wires.The inclusions in weld metals were analyzed in terms of their types and sizes.In GMAW,the inclusions are primarily Ti,Ca,Si,Al,and Mg compounds with no Nb and are generally less than 0.8 pm in size,whereas,in SAW weld,the inclusions are larger,mostly approximately 2-5 μm in size,and are cored with Ca and Ti,exhibiting obvious oxidation metallurgical features.The SAW joint was hot-deformed,and Nb-bearing nano precipitates were newly found in the weld metal through transmission electron microscopy,and Nb-free core-shell inclusion was found through scanning electron microscopy.The inclusions and precipitates were dispersed in or on the boundaries of acicular ferrite,contributing to acicular ferrite nucleation and grain refinement.     

X80钢高韧性焊缝中夹杂物及析出物分析

采用高韧性焊丝进行了含Nb X80钢气体保护焊和埋弧焊,分析了焊缝金属冲击断口中的夹杂物种类及尺寸.焊缝中的夹杂物主要含Ti、Ca、Si、Al及Mg等,气体保护焊缝夹杂物尺寸一般小于0.8 μm,而埋弧焊缝的则明显增大,均不含Nb,焊缝表现出氧化物冶金特性.对埋弧焊接头进行热变处理,新发现焊缝中形成了含Nb纳米级析出物及个别不含Nb的核壳夹杂物.这些杂夹物、析出物弥散分布于针状铁素体晶内及晶界,发挥了晶粒细化作用.     

Direction of grain-boundary migration in the weld metal of an austenitic stainless steel

The moving direction of the grain boundary (GB), after solidification in the weld metal of AISI310S stainless steel, was examined through a computer simulation technique using the vertex dynamics model and by observing the microstructure. The results are as follows. (1) The grain-growth exponent in the vertex model was fitted to describe the experimental data. (2) The vertex dynamics model can predict the moving direction of a grain boundary in the weld metal after solidification.     

Gas metal arc welding of duplex stainless clad steel plates

The 2205 duplex stainless + DH36 clad steel plate was welded by gas metal arc welding (GMAW),and the welding performance of the clad steel plate was investigated.The results show that the adaptability of the welding procedure for the base metal of carbon steel,the transition layer,and the cladding material is excellent.The test results indicate that the phase proportion and component dilution of the GMAW-welded joints of clad steel plate can be effectively controlled to yield joints with good mechanical properties and corrosion resistance.     

Mechanism governing nitrogen absorption by steel weld metal during laser welding

The existence of monatomic nitrogen in the plasma just over the keyhole during CO₂ laser welding was confirmed by the monochromatic image of a specific spectrum line emitted by monatomic nitrogen. The smaller reaction area of the molten pool with monatomic nitrogen is considered to lead to less nitrogen absorption during CO₂ laser welding than that during arc welding. The effect of the penetration mode shows that the nitrogen absorption during CO₂ laser welding mainly occurs on the upper surface of the molten pool. The nitrogen content in a reduced-pressure nitrogen atmosphere during CO₂ laser welding is in good agreement with that obtained during yttrium aluminum garnet (YAG) laser welding within the range of low nitrogen (partial) pressures. This result supports the supposition that the different behaviors of nitrogen absorption between CO₂ laser welding and YAG laser welding can be reasonably attributed to the lesser amount of monatomic nitrogen during YAG laser welding.     

Numerical analysis of metal transfer in gas metal arc welding under modified pulsed current conditions

A method has been proposed to pulsate current in gas metal arc welding (GMAW) to achieve a specific type of desirable and repeatable metal transfer mode, i.e., one drop per pulse (ODPP) mode. This method uses a peak current lower than the transition current to prevent accidental detachment and takes advantage of the downward momentum of the droplet oscillation to enhance the detachment. A numerical model with advanced computational fluid dynamics (CFD) techniques, such as a two-step projection method, volume of fluid (VOF) method, and continuum surface force (CSF) model, was used to carry out the simulation for the metal transfer process. The Gauss-type current density distribution was assumed as the boundary condition for the calculation of the electromagnetic force. The calculations were conducted to demonstrate the effectiveness of the proposed method in achieving the desired metal transfer process in comparison with conventional pulsed current GMAW. Also, the critical conditions for effective use of this proposed method were identified by the numerical simulation. Comparison showed good agreement between calculation and experimental results.