The study of surface active element on weld pool development in A-TIG welding

Surface-active element transport and its effect on liquid metal flow in laser-assisted additive manufacturing

Marangoni convection and weld shape variation in A-TIG welding process

Although numerical calculations of heat transfer and fluid flow can provide detailed insights into welding processes and welded materials, these calculations are complex and unsuitable in situations where rapid calculations are needed. A recourse is to train and validate a neural network, using results from a well tested heat and fluid flow model to significantly expedite calculations and ensure that the computed results conform to the basic laws of conservation of mass, momentum and energy. Seven feedforward neural networks were developed for gas metal arc (GMA) fillet welding, one each for predicting penetration, leg length, throat, weld pool length, cooling time between 800°C and 500°C, maximum velocity and peak temperature in the weld pool. Each model considered 22 inputs that included all the welding variables, such as current, voltage, welding speed, wire radius, wire feed rate, arc efficiency, arc radius, power distribution, and material properties such as thermal conductivity, specific heat and temperature coefficient of surface tension. The weights in the neural network models were calculated using the conjugate gradient (CG) method and by a hybrid optimisation scheme involving the CG method and a genetic algorithm (GA). The neural network produced by the hybrid optimisation model produced better results than the networks based on the CG method with various sets of randomised initial weights. The CG method alone was unable to find the best optimal weights for achieving low errors. The hybrid optimisation scheme helped in finding optimal weights through a global search, as evidenced by good agreement between all the outputs from the neural networks and the corresponding results from the heat and fluid flow model.

Surface ripple of resolidified stainless steel after disruption load

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.

Numerical simulation on interaction between TIG welding arc and weld pool

In this two-part article, the weldabilities of AA 1100 aluminum and AISI 409 stainless steel by the pulsed Nd:YAG laser welding process have been examined experimentally and compared. The effects of laser pulse time and power density on laser spot weld characteristics, such as weld diameter, penetration, melt area, melting ratio, porosity, and surface cratering, have been studied and explained qualitatively in relation to material-dependent variables such as absorptivity and thermophysical properties. The weldability of AISI 409 stainless steel was reported in Part I of this article. In the present article, the weldability of AA 1100 aluminum is reported and compared to that of AISI 409 stainless steel. Weld pool shapes in aluminum were found to be influenced by the mean power density of the laser beam and the laser pulse time. Both conduction-mode and keyhole-mode welding were observed in aluminum. Unlike stainless steel, however, drilling was not observed. Conduction-mode welds were produced in aluminum at power densities ranging from 3.2 to 10 GW/m2. The power density required for melting aluminum was approximately 4.5 times greater than stainless steel. The initial transient in weld pool development in aluminum occurred within 2 ms, and the aspect ratios (depth/width) of the steady-state conduction-mode weld pools were approximately 0.2. These values are about half those observed in stainless steel. The transition from conduction- to keyhole-mode welding occurred in aluminum at a power density of about 10 GW/m2, compared to about 4 GW/m2 for stainless steel. Weld defects such as porosity and cratering were observed in both aluminum and stainless steel spot welds. In both materials, there was an increased propensity for large occluded vapor pores near the root of keyhole-mode welds with increasing power density. In aluminum, pores were observed close to the fusion boundary. These could be eliminated by surface milling and vacuum annealing the specimens, suggesting that such pores were due to hydrogen. Finally, excellent agreement was obtained between experimental data from both alloys and an existing analytical model for conduction-mode laser spot welding. Two nondimensional parameters, the Fourier number and a nondimensional incident heat flux parameter, were derived and shown to completely characterize weld pool development in conduction-mode welds made in both materials.

The Sharma parameterS o which is characteristic of molten alkali halides and polymers has also been shown to be characteristic of a wide variety of liquids. Calculated data using the volume expansivity of the liquid establish the constancy of theS o-parameter which retains, on an average, a constant value of 1·11 for quasi-spherical molecular liquids. It is shown thatS o-parameter is also related to volume expansivity of the surface layer of the liquid, temperature coefficient of surface tension of liquids and describes the temperature and volume or pressure dependence of thermodynamic Gruneisen parameter and isochoric heat capacity with significant contribution for influencing the thermoacoustic and surface tension properties of liquids.

Dependence of the surface tension of molten silicon on temperature, the oxygen partial pressure in ambient argon atmosphere have been determined at temperatures ranging from 1693 to 1873K and in the range of PO2 from 10-25 to 10-14 MPa using a high purity BN substrate with the sessile drop method. Change of the surface tension with the oxygen partial pressure is very small in the range of PO2≤10-22 MPa. The surface tension decreases remarkably with increasing the oxygen partial pressure in the range of PO2 from 10-22 to 10-20 MPa, and increases slightly with increasing the oxygen partial pressure in the range of PO2>PO2, sat. The temperature coefficient of surface tension, δσ/δT, is minus and increases with increasing PO2, δσ/δT at PO2=10-25 MPa is equal to about -0.74 mN·m-1·K-1 and then δσ/δT gradually increase with increasing the oxygen partial pressure up to 10-22 MPa. δσ/δT steeply increases with increasing the oxygen partial pressure in the range of PO2 from 10-22 to 10-20 MPa. And again gradually increases with further increases of PO2 up to 10-15 MPa, where δσ/δT is equal to -0.15 mN·m-1·K-1. It is found that the surface tension of molten silicon and its temperature coefficient are sensitive to the oxygen partial pressure of argon atmosphere in the measuring system.

Over time, the demand for high-strength linepipe has increased significantly. One of the challenges for developing higher strength linepipe has been maintaining an appropriate level of fracture toughness, yield to tensile strength ratio, and weldability. Fortunately, significant progress has been made in the production of high strength line pipe steel. A major improvement in steel making has been the utilization of secondary steel treatments to refine the steel and accurately control alloy additions to achieve a higher level of steel cleanliness. In particular, these refining treatments have enabled the achievement of extremely low sulfur levels. For most purposes, restricting sulfur content is desirable to help prevent a reduction in mechanical properties such as fracture toughness of the steel. Fortunately, steelmaking and desulfurization technologies have advanced to the point where pipeline steel with sulfur contents less than the requirements by API 5L are available on a large scale. Extremely low sulfur contents, however, can lead to other problems when welding steels. These weldability problems are related to the fact that sulfur is a known surface active agent for steels. Low sulfur concentrations lead to a reversal of the Marangoni convection in the weld pool, which is responsible for the large differences in weld penetration on otherwise identical steels. Additionally, when welding heats of unmatched sulfur concentrations, the arc will tend to deviate towards the low sulfur heat and axially shift the root of the weld if one of the heats was below a critical value for the sulfur content and the other was above this value. Although this phenomenon has been primarily observed in stainless steels, the increasing ability to produce linepipe steel with extremely low sulfur contents has led to the possibility that this phenomenon could also occur in low carbon pipeline steels. One pipeline system utilizing cellulosic consumables for shielded metal arc welding (SMAW) of X70 steel with sulfur contents an order of magnitude below that permitted by API 5L and with widely varying sulfur contents shows evidence of this effect. The profiles of the welds in this system exhibited a tendency for lack of penetration, asymmetric weld roots, and concave welds. One approach to ameliorate this would be the specification of a lower permissible amount of sulfur.

Stability of the weld pool during laser keyhole welding depends on a number of interrelated physical phenomena. Understanding these phenomena is essential for good control of the welding process. A considerable body of theoretical work has been published concerning identification of the keyhole and weld pool dynamics in penetration laser welding of thin steel sheets. The present paper describes a method for obtaining information about weld bead formation. This technique provides data about the three dimensional shape of the weld pool and has been used to reconstruct the weld pool in 0.75 and 1.2 mm sheet steel. The method has been used to identify different keyhole penetration modes and reveal differences in weld pool solidification for specified thicknesses. Solidification rates in different regions of the weld pool have also been calculated and presented. This study provides a methodology that can be used to support modelling of weld pool formation during laser keyhole welding.Stability of the weld pool during laser keyhole welding depends on a number of interrelated physical phenomena. Understanding these phenomena is essential for good control of the welding process. A considerable body of theoretical work has been published concerning identification of the keyhole and weld pool dynamics in penetration laser welding of thin steel sheets. The present paper describes a method for obtaining information about weld bead formation. This technique provides data about the three dimensional shape of the weld pool and has been used to reconstruct the weld pool in 0.75 and 1.2 mm sheet steel. The method has been used to identify different keyhole penetration modes and reveal differences in weld pool solidification for specified thicknesses. Solidification rates in different regions of the weld pool have also been calculated and presented. This study provides a methodology that can be used to support modelling of weld pool formation during laser keyhole welding.

A mathematical model has been developed to describe the transient heat and fluid flow fields in gas tungsten arc welding (GTAW). The transient development and diminution of the weld pool at two periods after the arc ignites and extinguishes are analysed quantitatively. The data for the weld pool configurations under different welding conditions from the transient state to the quasi-steady state are obtained. The time for the weld pool shape to reach the quasi-steady state and the time for the weld pool to solidify completely are predicted. GTAW experiments show that the predictions of the weld pool shape based on the model are in agreement with the measured values. The numerical results of the dynamic development and diminution of weld pool configurations could be used to correlate the transient characteristics of weld pool behaviour with the occurrence of weld formation defects.

The use of dual or multiple laser beams is necessary for welding thick-section metals, especially for Nd:Yag lasers which are limited to relatively low power as compared to CO2 lasers. It was also reported that the use of dual laser beams for welding can increase keyhole stability leading to a better weld quality. So far, the development of dual-beam laser welding technologies has been in the experimental stage. The objective of this paper is to develop mathematical models and the associated numerical techniques to calculate the transient heat transfer and fluid flow in the weld pool and to study weld pool dynamics during the dual-beam laser welding process. The simulation was conducted for a three-dimensional stationary dual-beam laser welding. A very interesting change of the top-surface view of the weld pool was predicted. During the welding process, the top-view shape of the weld pool changes, starting from an oval-shape with the long-axis connecting the centers of the two laser beams, to a circle, and finally to an oval-shape with the short-axis connecting the centers of the two laser beams. Although a direct comparison with published experimental observation is impossible (due to the lack of detailed experimental data), the predicted weld pool shape is similar to that observed from experiments. The dynamical change of the weld pool shape can be well explained by the predicted fluid flow field.

Short-circuiting gas metal arc welding (GMAW), a main joining process for light-weight steel structures, is a complex, strongly non-linear, dynamic, and multi input/output process. It is essential to implement real-time monitoring and control of this process in order to ensure product quality and service reliability of the welded structures. In this paper, a low-cost and compact vision system is developed to observe the weld pool surface profiles in short-circuiting GMAW under different levels of welding current, welding speed, and contact tube-to-workpiece distance. Appropriate technical means are employed to lower the arc light interference and to synchronize the image grabber with the short-circuiting stage. Clear and complete weld pool surface images are captured by Observation from the Front of the Pool (OFP) and Observation from the Rear of the Pool (ORP), and the weld pool shape and size are determined. The two viewing methods (OFP and ORP) can detect the pool width reliably, but the detected pool length has a larger error. A modification method is proposed. The weld pool images are captured by Observation from the Side Position (OSP), and the tilted slope of the pool surface and the reinforcement height are detected in this way. Through modification, the measurement accuracy of the pool length is improved. This lays a foundation for the process control of short-circuiting GMAW, but further investigation is still required as a next step.

Laser Metal Deposition (LMD) has been developed for repair of damaged components and for the direct forming of three-dimensional structures. From the aspect of productivity, shape accuracy of the deposit must be improved to shorten the machining process after LMD. The objective of this work is to control build-up shape by adjusting weld pool size. Configurations of deposits made with various LMD process conditions were measured by cross sectional observation. In addition, signal intensity of heat radiation, plume emission and laser reflection from weld pool were monitored by photodiodes integrated in a laser processing head during the LMD process. The results showed strong correlation between weld pool size and signal intensity of heat radiation. An adaptive shape control system, which consists of an in-process monitoring system and Proportional-Integral-Derivative (PID) controller using that correlation, was developed for controlling build-up shape. Due to the linear relationship with weld pool size, laser power was selected as a PID output parameter adjusted by heat radiation signal. Since the target value of heat radiation signal can be changed flexibly during LMD process in our PID control system, deposit shape is successfully controlled as required with sufficient profile accuracy of build-up layers.Laser Metal Deposition (LMD) has been developed for repair of damaged components and for the direct forming of three-dimensional structures. From the aspect of productivity, shape accuracy of the deposit must be improved to shorten the machining process after LMD. The objective of this work is to control build-up shape by adjusting weld pool size. Configurations of deposits made with various LMD process conditions were measured by cross sectional observation. In addition, signal intensity of heat radiation, plume emission and laser reflection from weld pool were monitored by photodiodes integrated in a laser processing head during the LMD process. The results showed strong correlation between weld pool size and signal intensity of heat radiation. An adaptive shape control system, which consists of an in-process monitoring system and Proportional-Integral-Derivative (PID) controller using that correlation, was developed for controlling build-up shape. Due to the linear relationship with weld pool size, las...